"Tubular recipient with controlled atmospheres"

Texto completo

(1)“TUBULAR RETORT WITH CONTROLLED ATMOS PHERES ”. AUTHOR: JARRETT A. S MITH M. Code: 200126260. PROJECT ADVISOR: JAIRO ES COBAR M Sc, Dr Ing.. UNIVERS ITY D E LOS ANDES MECHANIC AL ENGIN EERING D EPARTMENT BOGOTA, JUNE 2005.

(2) IM - 2005 - I - 39 TABLE OF CONTENTS 1. INTRODUCTION..............................................¡ERROR! MARCADOR NO DEFINIDO.. 2. OBJECTIVE .....................................................¡ERROR! MARCADOR NO DEFINIDO.. 3. REVISION OF BIBLIOGRAPHY........................¡ERROR! MARCADOR NO DEFINIDO. 3.1 OVERVIEW OF POWDER METALLURGY......................¡E RROR! MARCADOR NO DEFINIDO. 3.2 OVERVIEW OF S INTERING......................................¡E RROR! MARCADOR NO DEFINIDO. 3.2.1 Physical Phenomenon - Diffusion..............................¡Error! Marcador no definido. 3.2.2 Schematic of creating a sintering cycle.......................¡Error! Marcador no definido. 3.2.2.1 3.2.2.2 3.2.2.3 3.2.2.4 3.2.2.5 3.2.2.6. 3.2.3. Delubing stage..............................................................................¡ Error! P reheating stage / P resintering..................................................¡ Error! Hot Stage.......................................................................................¡ Error! Cooling Stage...............................................................................¡ Error! Slow cooling stage.......................................................................¡ Error! P urging, a used as needed stage................................................¡ Error!. Parameters ...........................................................¡Error! Marcador no definido.. 3.2.3.1 Temperature – Heat.....................................................................¡ Error! 3.2.3.2 Gases..............................................................................................¡ Error! 3.2.3.2.1 Classification of Gases.........................................................¡ Error! 3.2.3.2.2 Combustion (Explosion) of gases.......................................¡ Error! 3.2.3.2.3 Ignition (Auto-ignition) of gases........................................¡ Error! 3.2.3.2.4 Gases for the atmosphere.....................................................¡ Error!. 3.2.4. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido.. Sintering Equipment ...............................................¡Error! Marcador no definido.. 3.2.4.1 3.2.4.2 3.2.4.3 3.2.4.4 3.2.4.5 3.2.4.6. Batch Furnaces.............................................................................¡ Error! Continuous Furnaces...................................................................¡ Error! Heating Units................................................................................¡ Error! Gas P ressure and Flow Equipment...........................................¡ Error! Additional Comparisons.............................................................¡ Error! Thermocouples.............................................................................¡ Error!. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido. Marcador no definido.. 4. METHODOLOGY.............................................¡ERROR! MARCADOR NO DEFINIDO.. 5. RESULTS AND ANALYSIS................................¡ERROR! MARCADOR NO DEFINIDO. 5.1 DETAILS OF C REATING THE R ETORT........................¡E RROR! MARCADOR NO DEFINIDO. 5.1.1 Six Channel Data Acquisition...................................¡Error! Marcador no definido. 5.1.1.1 5.1.1.2 5.1.1.3. General Architecture of the System.........................................¡ Error! Marcador no definido. Block Architecture of Te mperature Sensors..........................¡ Error! Marcador no definido. Implementation............................................................................¡ Error! Marcador no definido.. 5.1.2 Temperature Measurement (Thermocouple) ................¡Error! Marcador no definido. 5.1.3 Adequate Pressure and Flow for Mixing the Processing Gas....... ¡Error! Marcador no definido. 5.1.4 Retort construction – Shell.......................................¡Error! Marcador no definido. 5.1.5 Flange of the retort ................................................¡Error! Marcador no definido. 5.1.6 Seal of the Retort ...................................................¡Error! Marcador no definido. 5.1.7 Work load support..................................................¡Error! Marcador no definido. 5.2 C HARACTERIZATION ............................................¡E RROR! MARCADOR NO DEFINIDO. 5.2.1 Characterization Protocol .......................................¡Error! Marcador no definido. 5.2.2 Temperature characterizations .................................¡Error! Marcador no definido. 5.2.3 Fluid Flow Characterization....................................¡Error! Marcador no definido. 6. CONCLUSIONS ................................................¡ERROR! MARCADOR NO DEFINIDO.. 7 SUGGESTIONS FOR FURTHER DEVELOPMENT.............. ¡ERROR! MARCADOR NO DEFINIDO. 8. BIBLIOGRAPHY ..............................................¡ERROR! MARCADOR NO DEFINIDO..

(3) IM - 2005 - I - 39. Listing of Figures Figure 1: The steps in diffusion bonding [Ref. 1].......¡Error! Marcador no definido. Figure 2: Diffusion of atoms to points of contact [Ref. 1].......... ¡Error! Marcador no definido. Figure 3: Generalization of activation energy.............¡Error! Marcador no definido. Figure 4: Schematic of an Arbitrary Sintering Cycle .¡Error! Marcador no definido. Figure 5: Stress-strain curves for 304 stainless steel at vary in hydrogen concentration (given in wt%) [Ref. 8] ...............................................¡Error! Marcador no definido. Figure 6: Furnace Equipment [Ref. 19] ......................¡Error! Marcador no definido. Figure 8: Tubular Recipient Development M odel......¡Error! Marcador no definido. Figure 9: Prototype......................................................¡Error! Marcador no definido. Figure 10: Data Acquisition Card ...............................¡Error! Marcador no definido. Figure 11: General Architecture of System ................¡Error! Marcador no definido. Figure 12: Block Architecture of a Temperature Sensor ............ ¡Error! Marcador no definido. Figure 13: Graphical User Interface............................¡Error! Marcador no definido. Figure 14: Format of Results Displayed with Excel ...¡Error! Marcador no definido. Figure 15: Schematic of Configuration for Forming Gas ........... ¡Error! Marcador no definido. Figure 16: A Reducing Sintering Operation Sequence ............... ¡Error! Marcador no definido. Figure 17: M aterial Candidates for the retort..............¡Error! Marcador no definido. Figure 18: Operating Limits of Various Alloys in a Hydrogen Environment (Nelson Curves) [Ref. 9]...........................................................¡Error! Marcador no definido. Figure 19: Effect of temperature on metal loss from scaling for several carbon and alloy steels in air [Ref. 9] ............................................¡Error! Marcador no definido. Figure 20: General Comparison of the Hot-Strength Amongst Stainless Steels [Ref. 10] ...............................................................................¡Error! Marcador no definido. Figure 21: Time-temperature curves showing effect of carbon content on carbide precipitation, which forms in the areas to the right of the various carbon-content curves.[Ref. 6].............................................................¡Error! Marcador no definido. Figure 22: Family relationships for standard austenitic stainless steels .............¡Error! Marcador no definido. Figure 23: Cyclic O xidation Resistance [Ref. 10] ......¡Error! Marcador no definido. Figure 24: Effect of Acrylonitrile Content on Permeability of ButadieneAcrylonitrile Copolymers at 25°C..............................¡Error! Marcador no definido. Figure 25: O-ring compression force [Ref. 2].............¡Error! Marcador no definido. Figure 26: Linear Expansion % vs. Volume Swell %............... ¡Error! Marcador no definido..

(4) IM - 2005 - I - 39 Listing of Tables Table 1: Conforming Methods for Green Pieces.¡Error! Marcador no definido. Table 2: Classification of Some Common Gases¡Error! Marcador no definido. Table 3: Limits of Hydrogen by Volume Percentage .......... ¡Error! Marcador no definido. Table 4: Gas Cylinder Specifications ....................¡Error! Marcador no definido. Table 5: Types of Batch Furnaces.........................¡Error! Marcador no definido. Table 6: Types of Continuous Furnaces..............¡Error! Marcador no definido. Table 7: Accuracy of Variable Area Flowmeters.¡Error! Marcador no definido. Table 9: Selected American Standard Pipe.........¡Error! Marcador no definido. Table 10: Maximum service temperatures in dry air, based on scaling resistance [Ref: 6] ....................................................¡Error! Marcador no definido..

(5) 1 Introduction In this modern world, industries are always searching for a manufacturing process capable of supplying large quantities of exceptionally high quality pieces at reduced costs. metallurgy.. One of the answers of the search is powder. With it recognized as a useful technology, efforts are. continuously exerted in transferring production of parts from the competing manufacturing technologies of machining wrought steel, forging, and casting to powder metallurgy. Most of the transfer has been possible due to the associated developments in increased mechanical performance, better dimensional tolerances, and more cost efficient production of the powder metallurgy products; in other word, more knowledge and better equipment. The great potential of powder metallurgy is achieved by following a series of steps. The first is to identify the piece fit for this manufacturing process. A piece is fit based on characteristics such as geometry, material, and size. Also, a larger quantity of pieces justifies the production of the piece based on economic feasibility. Once the preliminary requirements are met, the design of the production process follows. One traditional route of transforming powder into a piece is by conforming a green piece and then sintering it. The design for this route will identify needs such as necessary equipment.. For sintering, the equipment used are. furnaces; they must be able to supply conditions which are met depending on the materials of the green piece and the end piece properties, for example density. Several types of furnaces can provide distinct sintering conditions, but they all share common conditions within that guarantee a large amount of success of sintering which are usually atmospheric composition, time dependent temperature cycles, and pressures. Only equipment capable of. 1.

(6) controlling conditions produce parts as expected. Therefore, the advantages sought in the powder metallurgy manufacturing process relies heavily on the capabilities of the equipment available.. 2 Objective Powder metallurgy is a manufacturing process with a complex and vast quantity of knowledge. Therefore, tackling of all equally important aspects of powder metallurgy is a team effort. And amongst many of the key aspects is the availability of reliable and versatile equipment because without this equipment all existent knowledge and intentions to build upon that knowledge is “fiction”.. Hence, this project is a consequence of a particular laboratory. equipment needed for developing and building upon powder metallurgy 1 knowledge with the objective of:. Design, construct, and characterize a laboratory scale retort.. 3 Revision of Bibliography Before designing the retort, a revision of bibliography is needed. It will define what possibilities and limitations the equipment should posses. With this information the parameters of the aspects that are deduced as important will be given a solution during the design stage.. 1. Powder metallurgy requires a variety of equipment which are not limited just to retorts. And various designs are available for each piece of equipment.. 2.

(7) 3.1 Overview of powder metallurgy. Powder metallurgy is a manufacturing process. used for production. components from powders followed generally by a heat treatment to produce a denser piece.. The components may be small, of high complexity, of. controlled properties, for high production quantities, of high melting 2 3 temperatures , of low ductility , and with close dimensional tolerances.. Depending on the component type, one of the five basic processes of powder metallurgy can be used: •. traditional conventional powder metallurgy (P/M) - compacting. •. metal injection molding (MIM). • powder forging (P/F) • hot isostatic pressing (HIP) • and cold isostatic pressing (CIP) Generally, any of the fi ve types of processes are composed of five subprocesses before obtaining the final product. A brief description of the five possible sub-processes follows. [Ref. 16]. The first is obtaining and preparing the raw materials. The powder must be obtained according to the metal. A few of the powder obtaining methods are atomization by gas and water, chemical methods, or milling of brittle materials. Then, the powder must be characterized because its property will define other sub-processes.. 2. Production of metals with high melting temperatures can be expensive due to di ffi culty of m elting and casting with other conventional manufacturing processes such as micro-casting. 3 When dealing with materi als of low ductility the production technique can be limited to P/M since machining and other manufacturing processes are extremely di fficult to apply.. 3.

(8) Mi xing comes after obtaining and preparing the raw materials. This is when the additives and/or lubricants are added or when blending and premixing of metal powders is done.. The third process is forming. Forming consists of giving the powders a functional shape and temporary properties. At this stage the piece formed is given the name of a green piece. Some of the conforming methods for green pieces are given in the table of Conforming Methods for Green Pieces. Table 1: Conforming Methods for Green Pieces Hot Compaction. Warm Compaction. Cold Compaction. Isostatic. Die Compacting. Die Compacting. Extrusion. Injection Molding. Isostatic. Die Compacting. Rolling. Spray ing. Injection Molding. Pressureless-sintering. Slip Casting Cold Forming. A green piece, when the application requires it, must then be sintered. Sintering is the fourth stage. Because the main goal of this project is to develop a recipient for use in the sintering process, sintering is left to a further, more in depth discussion which defines the design parameters. Finally, optional operations precede achieving the final product. This stage is required only when the properties of the “as sintered” do not suffice requirement specifications. Examples of possible, additional steps are repressing, re-sintering, metal infiltration by lubricants, and superficial treatments. All of the steps added during this stage give added values to the powder metallurgical part. The final products created with powder metallurgy may have geometrical complexities, functional properties, and level of quality that no other. 4.

(9) manufacturing process can feasibly match; but the powder metallurgy process can only produce the high quality pieces by an adequate assessment of the previous five processes.. 3.2 Overview of Sintering. A green piece, or a pre-form otherwise called, must be sintered before becoming a final product. Therefore, sintering is the process where the metal particles are given strength by the bonds between powder surfaces developed below the melting point of the mayor constituents. The rate of sintering, the rate at which bonds develop, depends on the temperature, the activation energy and diffusion coefficient for diffusion, and the original size of the particles.. Heat is the energy source for the atomic transport events, diffusion, of sintering. Thus, how the heat reaches the P/M piece must be understood. Heat transfer is possible via the following three means: radiation, conduction, and convection. Conduction and convection require the presence of a means to occur. Gas, for e xample, is a suitable means. In the absence of a gas, i.e. a vacuum, radiation is the only effective method of heat transfer. Briefly summarizing, the powders of P/M must follow a sequence such as that described by the figure: The steps in diffusion bonding.. 5.

(10) Figure 1: The steps in diffusion bonding [Ref. 1]. Part a) of the schematic shows a section of a single powder loosely resting on another section of a single powder.. The numerous sectors are grains.. Densification of the powders is seen in part b), part c), and part d). In part b), a force is applied to increase contact area of the two powders. An increased amount of contact area will increase the diffusion phenomenon. Grain growth is seen in part c). Grain growth by diffusion lowers the energy levels of the particles. Finally, part d) shows a reduction of pores left by vacancy diffusion. Parts c) and d) are performed during sintering. A more in depth discussion on diffusion is found later in this document.. Sintering must also be accompanied with appropriate atmospheres. When the atmosphere of sintering is referred to, it is more than likely to explain the gaseous composition surrounding the pieces.. In effect, atmosphere of. sintering refers to the chemical interaction between the atmosphere and the piece and not about the heat transfer capacity of the atmosphere. Some common interactions. include reduction of the piece, oxidation, and. carburization. These reactions proceed when temperature, pressure, and time conditions are met. A popular source of information is found in figures such as those that show the standard free energy of formation of metal oxides. type, sometimes. referred to as. the Ellingham. 6. In a figure of such. Diagram, a powder.

(11) metallurgical practitioner can find information pertaining to whether a piece will reduce or oxidize depending upon partial pressures of the gaseous constitutes of the atmosphere at different temperatures.. Finally, sintering, as will be explained in the section: Schematic of creating a sintering cycle, is dynamic. The conditions surrounding the green pieces during sintering will change.. The sub-stages of sintering are applied as. necessary. This too is the explained in the same section.. 3.2.1 Physical Phenomenon - Diffusion A simple and straight forward description of diffusion is stated by Askeland [Ref. 1] as: “Diff usion is the movement of atoms within a material. Atoms move in a predictable fashion to eliminate concentration differences and produce a homogeneous, uniform composition.”. Diffusion occurs in the retort as some of the gas penetrates the walls and as migration of atoms. of the material during the sintering operation.. Indisputably, diffusion must be understood for the selection of the material of the retort. Metallic diffusion mechanisms are dominated by the vacancy diffusion model and by the interstitial diffusion model. In both of these models, for any atom to migrate there must be an empty adjacent site and the atom must have sufficient energy to break bonds with neighboring atoms and then cause some lattice distortion during the displacement.. 7.

(12) A vacancy is a point defect normally due to an atom missing. Vacancy diffusion requires vacant lattice sites and is a function of the amount of vacant lattice sites available.. According to William D. Callister [Ref. 4]:. “The equilibrium number of v acancies N v f or a giv en quantity of material depends on and increases with temperature according to:. N v = Ne. ⎛ − Qv ⎞ ⎜ ⎟ ⎝ kT ⎠. In this expression, N is the total number of atomic sites, Qv is the energy required for the f ormation of a v acancy, T is the absolute temperature in kelv ins, and k is the gas or Boltsmann’s constant.”. It is apparent that the number of vacancies increases exponentially with an increase in temperature; being so, diffusion rate via the vacancy model increases. With self-diffusion and inter-diffusion occur, the vacancy model changes vacancies for host atoms. And in both cases, with vacancies and with host atoms, the atom that takes a step and the destination point, whether a vacancy or host atom, move in opposite directions to replace each other. Interstitial diffusion involves the migration of an atom into an interstitial site. Thus, the migrating atom must be small enough to fit in the interstitial sites left by the host atom’s lattice. Usually the impurities that diffuse via interstitial diffusion are hydrogen, carbon, nitrogen, and oxygen due to their small sizes when compared to host atoms such as iron. Finally, interstitial diffusion, in most metal alloys, occur more rapidly because of the more numerous amounts of interstitial positions compared to vacant sites.. An example of vacancy diffusion is sintering of metal powders. The migration of atoms is quite visible from the figure: Diffusion of atoms to points of contact extracted from reference [Ref. 1]. The figure demonstrates with a before and. 8.

(13) after the step like diffusion of atoms. In the first figure, the red atoms with an arrow describe the present location of the migrating atoms and the arrow points to the location they will assume. The second figure shows the atoms already in place.. Figure 2: Diffusion of atoms to points of contact [Ref. 1]. The situation describes interfacial diffusion. Interfacial diffusion first occurs at the contact area available between external surfaces of the particles. At the boundary of the surfaces the crystal structures terminate.. Because the. surface atoms are not bonded, they are at a higher energy state than the atoms at the interior positions giving rise to surface energy. Atoms at the surface want to reduce this energy but can not do so because they are mechanically rigid. In a similar fashion, atoms along grain boundaries are bonded less regularly than the interior atoms (though, more bounds are obviously present along grain boundaries than external surfaces).. Therefore, there is also. interfacial/grain boundary energy. And at elevated temperatures grains grow to reduce the total boundary energy. The additional amount of energy that must be added for any migration activity previously described is called activation energy. In sintering the energy is. 9.

(14) provided by heat. The energy is used to squeeze through the path migration. The figure: Generalization of activation energy [Ref. 1] shows schematically the typical situation. It is usually the case that higher activation energy is required for vacancy diffusion compared to interstitial diffusion.. Figure 3: Generalization of activation energy. 3.2.2 Schematic of creating a sintering cycle A basic sintering cycle schematic is composed of typically five stages. The five stages are the following: •. Delubing (or burn-off zone). •. Preheating. •. Hot. •. Cooling. •. Slow cooling. In each stage a different atmosphere is contained inside the retort. Each particular atmosphere has a function, a composition, and is within a temperature range (see figure: Schematic of an Arbitrary Sintering Cycle). The figure shows an arbitrary schematic of the sintering cycle with. 10.

(15) proportions roughly held. Each stage is performed at within a given time and temperature or at a rate of change of these parameters. By briefly stating what these are, the requirements that must be met by the tubular recipient are deduced.. Figure 4: Schematic of an Arbitrary Sintering Cycle. 3.2.2.1 Delubing stage The delubing stage is a consequence of the lubricant or binder required during conformation of the powder metallurgy part, but neither the lubricant or binder are desired as part of the piece’s composition during sintering due to negative consequences of the final properties. Therefore, if hydrocarbons are 4 used, the function of the delubing stage is to burn and remove hydrocarbons .. The organic material surrounds the metal powders in a homogeneous distribution within the piece. Organic material will be removed via the paths that develop from the outside inwards because the organic material on the outside will be the first to react and evaporate. The canals left serve as 4. Delubing also assists in clearing undesired surface contaminants.. 11.

(16) evacuation routes for the lubrication contained deeper in the piece. Since the organic material exits as gas, care must be taken to avoid entrapping an excessive amount due to insufficient evacuation rates because an excessive amount of gas will cause an internal explosion rupturing the piece. Finally, if all goes well, the evacuated organic material leaves behind a network of pores where organic material once assisted with the green strength of the compact. The previous occurs only when the proper control of holding times, temperatures, and flow rates are achieved. At this moment, between the delubing stage and the preheating stage, the extraction of surface contaminants and processing organics can be verified, tentatively, via thermogravimetric data on weight loss. Delubing temperature is governed by the type of lubricant used.. Four. commonly employed compacting lubricants are Acrawax, lithium stearate (LiC18H35O2), paraffin (C22H46 to C27H56), and zinc stearate (Zn(C18H35O 2)2). Their melting points respectively are: 140-143°C, 221°C, 40-60°C, and 130°C. So an acceptable range for common lubricants can be considered between 400°C to 650°C since a gaseous phase is desired and not a liquid 5 phase . A gaseous phase facilitates extraction of the organic material in a. scenario of increasing difficulty of removal when green compact density increases. Make note, e xplicitly, that the acceptable range of temperatures imply that the transition from melting ranges to temperatures in evaporation ranges cause part of the organic material to be driven out in a liquid phase. The extraction of a liquid lubrication continues until reaching the selected delubing temperature where the rest and most of the material leaves in a gaseous phase. 5. This might pose a problem since at this temperature the stainless steel recipient is susceptible to being sensitized and thus leading towards stress corrosion cracking.. 12.

(17) It is desired to be at the verge of the upper limit of the permissible temperature range due to reduced time requirements. But the upper limit temperature for any particular organic material is a compromise of the maximum temperature delubing can be performed at without the thermal degradation of the lubricant or binder. If the chemical degradation occurs, the organic material can react undesirably with the piece.. Another possible. danger of operating at the upper temperature limit is the rupture of the piece induced by stresses involved in accelerated volumetric changes (thermal stress rupturing) when the hydrocarbon leaves too promptly.. Delubing also includes the removal of the burnt material. The removal of the material may require assistance from the atmosphere at this stage. Small amounts of oxidants (e.g.: water, carbon dioxide, or an atmosphere with high dew points) are used. Therefore, a reaction of the following type (equation [1]) is expected with oxidants in the atmosphere: C x H y + H 2 O → CO + CO2 + H 2 [1]. Atmospheres leading to the previous reaction permit the hydrogen and carbon to leave the piece as gases. When oxidants are denied, for example, carbon would possibly remain as a sooty product. Once the transition to a gaseous product is achieved, the residue must be channeled out of the tubular recipient before proceeding to the next stages of the sintering process. If it is concluded that delubing is successfully performed, the now even more fragile, porous green piece is ready to proceed further in the sintering process.. 13.

(18) 3.2.2.2 Preheating stage / Presintering The preheating stage, also referred to as the presintering stage, prepares the piece for the diffusion activity that will occur during the hot stage. Diffusion requires atomic migration. So, for example, oxides in steels can impede drastically diffusion activity. Therefore, the preheating stage may be used for removal of surface oxides on the powder. Another consideration pertaining to the preheat stage is the rate of temperature change of the piece. The piece must reach the final hot stage temperature starting from the lower delubing temperature. It is obvious that the piece will expand during the transition.. If the expansion generates. tension stresses higher than the combination of the low resistance strength offered by the weak bonds / links left by the capillary channels and the new, scarce bonds forming during diffusion at this stage, the piece will rupture. Thus, it is key that the preheat stage consider the rate of the incremental temperature transition.. 3.2.2.3 Hot Stage Temperatures may be around 60% of the melting point of the metal. At this stage, the diffusion activity is at its highest rate. Thus, it is at this stage that most of the densification occurs. And since the temperature is at the highest of all of the sintering cycle, there will be more available energy for reactions to proceed.. The high energy available may lead to undesired reactions of. oxidation or reduction of the piece and the equipment. practitioner must apply a controlled atmosphere as needed.. 14. Therefore, the.

(19) 3.2.2.4 Cooling Stage The main problem associated with the cooling stage is time consumption. Time consumption, relatively speaking, is put into perspective when viewed from the length of the cooling zone compared to the sintering zone in a continuous furnace. The proportion may be of up to two and a half times. Reduction of these extended times are possible by managing cooling rates. Cooling rates are manageable when assisted by forced methods. Methods such as heat transfer by convection with circulating gases from the atmosphere around the piece easily reduce lengths in the cooling zone by 50%. And if these cooling rates are high enough, a sinter hardening process may even become available if required. Additional considerations of the cooling stage is the cooling rate uniformity of the load and the safe discharge of the load into standard atmospheric conditions. Cooling rate uniformity throughout the load depends on the size, shape, quantity, and distribution of what is being processed. Therefore, the placement of parts for cooling inside the retort, determine how well the part cools or heats. And secondly, before the piece can be discharged into an atmosphere with air, the temperature needs to be low enough to prevent reaction with the air.. 3.2.2.5 Slow cooling stage At the end of the sintering cycle is the slow cooling stage. Basically, it consists of letting the pieces to cool without assistance (just normal atmospheric conditions) until reaching manipulation temperatures.. It is. important to open the recipient commencing the slow cooling stage ONLY if. 15.

(20) the atmosphere inside is safe to mingle with the exterior conditions and the properties of the pieces are uncompromised.. 3.2.2.6 Purging, a used as needed stage Purging is an intermediate stage. It is sometimes required before beginning the sintering process or between any of the stages when there is a change of atmosphere. For example, purging of air from the retort before the furnace is heated to temperature above 150°C may help prevent oxidizing the interior of the retort or the pieces.. Yet, the possibility of an intentionally oxidizing. atmosphere does exist. In other words, purging is not a rigorous stage that is always performed.. Purging can be performed simultaneously with a temperature change and it can be done as quickly as a practical flow rate allows. Mainly, a “safe” atmosphere is sought with purging before reaching temperatures that permit proceeding of undesired thermo-chemical reactions.. The atmosphere is. “safe”, as a general rule of thumb, by replacing the contents of the control volume by five times. Purging, particularly concerning this project, is used to eliminate the presence of air, oxygen, or other oxidizers prior to admitting hydrogen into the systems; and, inversely, the system is purged of hydrogen before opening the system to the atmosphere. Purging should be done to prevent the formation of flammable mixtures and can be accomplished in several ways. The system should be “inerted” by suitable purging, for example, with nitrogen. Exactl y how purging is achieved is left to the designer of the procedure for sintering based upon requirements. Roughly, the designer must pick set-. 16.

(21) points of time and temperature to initiate and terminate purging.. The. designer must additionally consider safety hazards, purging techniques, and know consequences of the selections.. 3.2.3 Parameters Certain variables need to be set according to the sintering process needed by the green parts. Adverse effects are the outcome of adjusting temperature profiles and adjusting chemical composition of the processing gas. Therefore, some knowledge must be known about these parameters.. 3.2.3.1 Temperature – Heat The interaction of the retort with the external heating system defines not only temperature gradients within but also the type of forced and particular response to external inputs. One of the mayor goals of the characterization is obtaining the transitional and stable response in time of the retort system. Basically, the system’s homogeneous response to the inputs of heat at the external surface of the retort is likely a first order type. Yet, this prediction may be wrong.. Heat flow is one of the variables that must be comprehended. There are three places where heat transfer are a concern. The first is the heat transfer from the furnace to the retort; the second is the heat losses which increase power consumption of the furnace; the third is the heat transfer to the green pieces. But even though the need of understanding heat has been identified as important, a realistic approach must be taken because theoretical models can be very complicated, as the following example will show.. After the. explanation is given, the alternative empirical method will be developed.. 17.

(22) Take for instance the heat transfer to the green pieces. How does it get there? The green piece is placed on top of a ceramic support to keep it from welding to any structure. So the support is nonconductive; therefore the piece bottom will heat mostly by conduction from exposed sides of the piece to the bottom. A little will be accomplished by the gas interaction which effuses through the pores of the support.. By taking a step back and going to the more important heat transfer to the walls of the piece, an additional complication is identified. At first most heat is transferred via convection. Convection depends on numerous fluid properties such as density, viscosity, thermal conductivity, specific heat, surface geometry, and the flow conditions. This multiplicity of independent variables result from the fact that convection transfer is determined by two boundary layers that develop on the surface. The first being the velocity boundary layer and the second is the thermal boundary layer. Both boundaries consider the average properties.. For example, the velocity of travel of the fluid is. independent whether the motion is turbulent or laminar. Convection heat transfer, consequent of a thermal boundary layer, only occurs if there is a difference between a surface and free stream temperature. And this could go on and on. Matter of fact, I recommend the text book “Introduction to Heat Transfer” because convection is only relevant at lower temperatures. At higher temperatures radiation contributes most of the heat transfer. As a consequence of the complicated extent of the theoretical approach to evaluate heat, deductions will be based on empirical values. Thermoelectric means (i.e. thermocouples) based entirely on empirical calibrations is suitable for temperature measurement.. The empirical. calibrations are accompanied by the application of so-called thermoelectric. 18.

(23) “laws” which experience has shown to hold. Please see the annexed section that includes the Table: Summary of Thermocouples . With the use of properly selected thermocouples, in this case type K, temperatures will be recorded with a SIX CHANNEL DATA ADQUISITION CIRCUIT. A direct, brief description is given of the how the data acquisition works in the section: Six Channel Data Acquisition.. 3.2.3.2 Gases Gases present during the sintering cycle are an essential part of the atmosphere because the selection of the gases for use inside the retort is directly responsible for the proper and safe operation of the equipment and the quality of the powder metallurgy products.. This means that a prior. understanding of gases is necessary to continue the development of this project.. 3.2.3.2.1 Classification of Gases Gases are classified as oxidizers, inert, or flammable. The manner in which they contribute in combustion defines which category they belong to. Oxidizers, in effect, will contribute to combustion as an oxidant, but they are not flammable by themselves. Inert gases do not participate in combustion processes since they do not react with other materials.. Instead of. contributing in combustion, inert gas can limit a combustion process in a limited space by reducing the amount of – for say – oxygen. Flammable gases, together with air or oxygen in the right concentration, if ignited will burn or even explode. The following table classifies some common gases under their respective types:. 19.

(24) Table 2: Classification of Some Common Gases Oxidizers Air Chlorine Fluorine Nitric Oxide Nitrogen Dioxide Oxy gen. Inert Gases Argon Carbon Dioxide Helium Neon Nitrogen Xenon. Flammable Acety lene Ammonia Arsine Butane Carbon Monoxide Cyclopropane Ethane Ethy lene Ethy l Chloride Hy drogen Isobutane Methane Methy l Chloride Propane Propy lene. 3.2.3.2.2 Combustion (Explosion) of gases Combustion or explosion only occurs if the mixture is not too lean or too rich. The limits of a lean mixture and a rich mixture are the limiting concentrations commonly called the “Lower Explosive Limit” (LEL) and the “Upper Explosive 6 Limit” (UEL) , respectively. Within this range, a gas or vapor concentration. will burn or explode if an ignition source is introduced. Below the explosive or flammable limit the mixture is too poor to burn and above the upper explosive or flammable limit it is too rich to burn. Therefore, even if a gas is considered flammable, mixtures with oxidants will only burn if the fuel concentration lies within sharply defined LEL and UEL. Outside of these limits ignition and flame propagation can not be initiated by the application of an external stimulus. Even should a reaction mixture lie within its flammability limits ignition requires the input of sufficient energy in a suitable form.. 6. Alternative names for the “ Lower Explosive Limit” and the “ Upper Explosive Limit” are “ Lower Flammable Limit” and the “ Upper Flammable Limit” respectively.. 20.

(25) It should be noted that the phenomenon of flammability is distinct from, although related to, auto-ignition. The auto-ignition temperature is the lowest temperature at which the spontaneous ignition will occur of a mixture between a flammable with an oxidant without an ignition source.. 3.2.3.2.3 Ignition (Auto-ignition) of gases A process where a mixture does not ignite by itself but by a local ignition source is called induced ignition. In induced ignition, energy is deposited locally leading to a temperature rise in a small volume of the mixture. Then, from then on, auto ignition may take place generating more radicals. Flame propagation continues setting the remaining mixture on fire.. For safety reasons, the ranges of temperature, pressure and composition a mixture can auto-ignite should be known. Two possibilities exist within the ranges. The first, a mixture will ignite spontaneously. The second possibility, only a slow reaction is observed. When a flammable-oxidant mixture is supplied with sufficient energy, it still will not ignite until an induction time (ignition delay time) has passed. This ignition delay time can be as long as several hours or as short as microseconds and is characteristic for radical-chain explosions. During this time span, the radical population increases exponentially. These chemical reactions, radical formations, do consume fuel but the temperature remains nearly constant. As soon as the radical pool has grown enough to consume a significant fraction of the fuel, ignition occurs and the temperature starts to rise. In contrast, in a purely thermal ignition process there is no induction time, and the temperature increases immediately.. 21.

(26) Combustion processes involve radical chain reactions. Chain initiation steps start the reaction. In chain propagation reactions, the number of radicals does not change. It is the chain branching reactions that lead to an exponential increase in the radical pool. Chain termination can occur in a homogeneous or inhomogeneous manner. Examples of these types of chain reactions are:. - chain initiation:. H2 + O2. = 2OH. - chain propagation:. OH + H2. = H2O + H. - chain branching:. H + O2. = OH + O. - chain branching:. O + H2. = OH + H. - chain termination (heterogeneous):. ½ (H+ H). = ½ H2. - chain termination (homogeneous):. H + O2 + M = HO2 + M. 3.2.3.2.4 Gases for the atmosphere The gases chosen for the different stages of sintering are according to the assisting functions of the interaction amongst the atmosphere with the piece. In each stage the needs differ, thus the vapor phase surrounding the piece as the sintering process progressed must be modified.. Examples of some common roles of the atmosphere of particular interest for this recipient include: •. Accelerate removal of the lubricants or organic bonders at the low temperature stage of delubing.. 22.

(27) •. Guarantee the removal of organic products from the recipient in a gaseous phase; extraction of organic products in a liquid or solid phase are considered a nuisance and unachievable.. •. Reduce unnecessary oxides of the compact which impede proper sinter bonding in the preheat stage.. •. Provide inert and neutral atmospheres; uncontrolled amounts of oxygen and air will produce undesired reactions during the hot stage.. •. Allow the necessary partial pressures for the selected combination of atmospheres throughout any sintering stage.. •. Hydrogen and nitrogen are the two particular gases initially in mind for the process.. The properties and any pertinent information will be. described here after. And keep in mind throughout the development of this project how the design is based on implications of the gases used.. 3.2.3.2.4.1 Hydrogen. Hydrogen is commonly described as: “Hy drogen (H 2) is a colorless, odorless, flammable gas which may ignite spontaneously and burn with a colorless flame. Hydrogen is the lightest gas known and it is normally compressed and shipped at high pressure.” [Ref. 23]. Operating precautions are necessary because hydrogen is flammable according to the classification of gases. Therefore, operating precautions with hydrogen might address combustion hazards, pressure hazards, low temperature hazards, hydrogen embrittlement hazards, purging capability of the equipment, and health hazards. Pressure and low temperature hazards are obviated since the conditions of this project never place hydrogen in a cryogenic/liquid state.. In effect, the lowest expected temperature is. approximately 5 °C when hydrogen is stored as supplied in the high pressure. 23.

(28) cylinder. Consequently, only a brief consideration of combustion hazards and hydrogen embrittlement hazards are presented.. Safety hazards can be. reviewed in the hydrogen MSDS.. If the reader desires to access basic but sufficient information for handling hydrogen he or she should refer to the MSDS (Material Safety Data Sheet) included as the file “Hydrogen MSDS” under the references folder of the annexed compact disc.. Furthermore, reading of the presentation of. “HYDROGEN HANDLING SHORT COURSE”, prepared by Stephen S. Woods and found in the file HSCWoodsGeneral, is highly suggested.. But in general, due to hydrogen, the following safety precautions have been applied: •. The retort should never be opened with the presence of hydrogen above 150°C.. •. Purge prior, as required, if the necessity of opening the retort arises.. •. Vents should be located to prevent hydrogen from impinging on ventilation ducts or other equipment.. 3.2.3.2.4.1.1. Combustion Hazards. Hydrogen is only flammable and even explosive with the presence of an oxidant.. Hydrogen is dangerous when certain compositions by volume. percentage with an oxidant are met. The limits are summarized in the table that follows. Table 3: Limits of Hydrogen by Volume Percentage Flammable Limits @ 1 [atm] In Air In oxygen Lower % Upper % Lower % Upper % 4.00 74.2 4.65 93.9. 24. Detonable Limits @ 1 [atm] In Air In oxygen Lower % Upper % Lower % Upper % 18.2 58.9 15 90.

(29) Even if the atmosphere composition contains other constituents besides hydrogen and the oxidant, it is only necessary to compare the volumes of the hydrogen and the oxidant. But even when hydrogen content does fall within the explosive range, the possibility of explosion is present only if an energy source is present. An energy source can be a spark or heat. Therefore, all equipment when operating in presence of hydrogen should be grounded. When the energy source is of the type of a spark, an extremely small amount of power is needed to start a reaction (i.e. the minimum amount is 0.017 7 [mJ] ). The amount of power needed is comparable to the power available. from static electricity. And without a spark, auto-ignition of hydrogen within an oxidant can also occur. For example, auto-ignition in air is commonly taken as 500 °C.. 3.2.3.2.4.1.2. Hydrogen Damage. This section could be placed with or after the Retort Construction section since hydrogen damage should be studied according to the retort material. But at this point, it must be admitted that this section was modified and created taking advantage of the feedback loops described in the flowchart of the Tubular Recipient Development Model figure. The material of the retort has already been selected as a stainless steel 304.. Particular hydrogen effects on this stainless steel had to be considered. Since hydrogen is the smallest atom and at high temperatures is in its monatomic structure, the possibility of hydrogen attack and hydrogen embrittlement are considered.. Fortunately, those two particular forms of. damage caused by h ydrogen are mostly associated with low-alloy or carbon steels. Yet a description is included for general knowledge. 7. Higher power demands for ignition are required at different percentage volume compositions and conditions, but for safety’s sake, the minimum is considered.. 25.

(30) Hydrogen attack refers to the mechanism of damage of hydrogen when at temperatures above 220 °C penetrates the structure. The hydrogen then reacts with reducible species such as an iron carbide phase. The product of the reaction is methane which is then trapped inside the structure because the size of the molecule is too large to diffuse back out. Pressure builds as a consequence of the methane gas trapped inside the structure. The pressure builds until fissuring the steel. Once again, hydrogen attack does not occur in austenitic stainless steels. [Ref. 9] Hydrogen embrittlement, on the contrary of hydrogen attack, does affect stainless steel 304. But due to the nature of the function of the retort being static with low stresses, embrittlement is not a problem.. Furthermore,. depending upon hydrogen exposure of the stainless steel 304, tensile strength decreases and the yield strength increases (as seen in the immediate figure).. If it does become a concern, a bake-out cycle with. temperatures between 175 °C and 205 °C allows the hydrogen to escape the metal. [Ref. 9]. 26.

(31) Figure 5: Stress-strain curves for 304 stainless steel at vary in hydrogen concentration (given in wt%) [Ref. 8]. What is of great importance to austenitic stainless steels is hydrogen stress cracking. The explanation of this mechanism is left for the section Retort Construction – Shell.. 3.2.3.2.4.1.3. Commercial availability of hydrogen. Hydrogen is provided commercially in Bogotá with purity levels over 99.9%. The hydrogen used for this project is supplied by OXIGENOS DE COLOMBIA PRAXAIR INC. with a minimum purity of 99.99% and with a price of 35,000 3 [COP/mt ] std. for students. For non students the price is 42,000 COP if the. right person is contacted.. Thus, the hydrogen of interest is the following: Grade:. 4. Minimum purity:. 99.99%. Levels of impurities:. H 2O < 10 ppm; O2 < 5 ppm. Total Hydrocarbons. < 0.5 ppm. 27.

(32) OXIGENOS DE COLOMBIA PRAXAIR INC. facilitates rental services to their clients.. They offer the following high pressure gas cylinder suitable for. containing hydrogen with the following specifications:. Table 4: Gas Cylinder Specifications Model 40.219.150 MeMn. Operation Pressure bar kpsi. Hy draulic Capacity liters cu in. 150. 40.00. 2.176. External Diameter mm in. 2,440.80. 219.00. Total Length. 8.62. Weight. mm. in. Kg. lbs. 1,320.00. 51.96. 51.00. 112.46. The line out port for the particular hydrogen cylinder has a CGA 350 connection.. Additionally, the exterior of the cylinder is painted for. identification with bright red. Some pertinent information about safety for clients using high pressure cylinders is available under the Gases folder included in the reference folder of the compact disc with the file name “BOLETIN DE SEGURIDAD PAR A CLIENTES”.. Rental fee for the cylinder is 400 pesos daily. A transportation fee of 4000 pesos must also be paid.. 3.2.3.2.4.2 Nitrogen. Nitrogen is used for purging and as a “filler gas” due to its inertness.. A “filler. gas” is simply present during operation as part of the process mixing gas to eliminate the potential hazards of hydrogen.. Nitrogen should not affect. negatively the equipment. If it does diffuse into the structure, it might even help impede sensitization of the stainless steel.. 28.

(33) The nitrogen of interest is the following: Grade:. 4.6. Minimum purity:. 99.996%. Levels of impurities:. H 2O < 5 ppm; O2 < 5 ppm. 3.2.4 Sintering Equipment Selection of the appropriate furnace for the needs of the powder metallurgist can be based on several types of criteria. Possible criteria are: initial cost of equipment, production cost per weight – quantity, physical characteristics of the green pieces (e.g. weight), maximum operating temperature, and capability of filling and containing controlled volumes with the necessary atmosphere. But in general, the broadest two categories furnaces can be divided into are continuous and batch.. The continuous furnaces are for larger productions and hence the pieces are conveyed through the furnace at constant rates. The batch furnaces are for smaller productions and are usually placed manually or in a discrete manner. Both of the type of furnaces are subdivided into subcategories and they all posses relative advantages and relative disadvantages. But even if there are subdivisions, one characteristic holds for all subdivisions of the two categories. All continuous furnaces have separate chambers for each stage of the sintering process.. The loads are convoyed through each chamber to. complete the process. For the batch furnace category, all of the process is performed in a single multipurpose chamber.. And of course, there are. many combinations of sub-systems in all sintering equipment.. 29.

(34) Table 5: Types of Batch Furnaces Type Description Bell “The base is permanently installed on the floor. Work to be processed is loaded onto the base and then covered with a heatresistant alloy retort to contain the protective atmosphere. The furnace bell then is lifted and placed over the retort and base.” [Ref. 5] Elevator “It has the furnace bell above the mill floor on a fixed structure. The base and work load are covered with a retort and rolled on tracks under the furnace. An elevator system then raises the base, work, and retort into the furnace.” [Ref. 5] Vacuum “A vacuum furnace operates in the absence of an internal atmosphere.” “Generally, a batch-type vacuum furnace consists of an outer vacuum-tight cylindrical casing that contains a furnace with radiation shields or other types of insulation, work support, and heating elements. This casing is fitted with roughing and diffusion pumps to achieve the desired vacuum levels.”[Ref. 5]. 3.2.4.1 Batch Furnaces This type of furnace may be used when low quantities are produced and when the sintering process requires various heat treating conditions.. It. basically consists of a controlled volume (also called a retort), a heating unit, a base, a work support, a fluid flow unit, and a bell-shaped or cylindrical furnace.. The three common and standard furnaces are the bell, elevator, and batchvacuum furnace. The descriptions are as given in the table: Types of Batch Furnaces.. 3.2.4.2 Continuous Furnaces Continuous furnaces are usually used in productions of large quantities. They tend to be more automated than the batch furnaces. Also, they tend to. 30.

(35) be more expensive than the batch furnaces. Following is a brief description of the main types of continuous furnaces.. 31.

(36) IM - 2005 - I - 39 Table 6: Types of Continuous Furnaces Type Description Mesh-belt Conveyer “Mesh-belt conveyor furnaces consist of a charge and belt-driven table, a slow cooling zone, a final cooling zone, and a discharge table.” “These are the most commonly used equipment for the sintering of P/M compacts. They provide a continuous, reproducible time-temperature-atmosphere (thermal) profile.” [Ref. 5] Humpback “This is adaptation of the mesh-belt conveyor are used when high atmosphere purity and low atmosphere consumption are desired…” “A long, inclined entry section, which is gas tight, carries the belt and work from the charge table up to the sintering zone.” [Ref. 5] Roller-Hearth “In a roller-hearth furnace, parts are carried in trays through the furnace on driven rolls. Generally, these furnaces are capable of heavier loading on the hearth than mesh-belt furnaces.” [Ref. 5] Walking-beam “These are particularly well suited for applications in which sintering temperatures are above the limitations of the mesh-belt conveyor and roller-hearth furnaces.” [Ref. 5] For movement, a beam pivoted as a four bar mechanism creates rectilinear motion and displaces the pieces forward discretely. Pusher Furnaces “In a pusher furnace, parts to be processed are loaded on trays or ceramic plates that are pushed through the stationary hearth furnace.” “The pusher mechanism may operate intermittently or continuously.” [Ref. 5] Vacuum “The furnace is comprised of an external loading table, followed by an atmosphere delubrication chamber, a transfer station, and an “atmosphere to vacuum” vestibule section, followed by heating chamber, vacuum cooling chamber, a combination fan cooling and “vacuum to atmosphere” vestibule, and an unloading table. The operation of the furnace is completely automatic, under the supervision of an operator.” [Ref. 5]. 32.

(37) IM - 2005 - I - 39. 3.2.4.3 Heating Units For both continuous and batch furnaces, the heating units are the traditional heating units.. They are the direct-fired gas systems, radiant tubes for. indirect-fired gas systems, and the electrically heated with resistance elements. The direct-fired systems are usually the cheapest system to fuel. The fuel of the equipment can be natural gas, straight propane, a propane air mix, or any grade of oil that can be atomized. Since the products are exposed directly to the products of combustion they are referred to as flue products. These flue products are often not in their final stage. For example, if oxides are created superficially, it might not be a problem because the part might proceed into an additional stage of final sizing.. At the final sizing, the scales could be. removed.. The disadvantage possessed by the direct-fired systems are the need of a pressure-control system due to the flue created from combustion. The flue produced is difficult to control and has adverse consequences on the products.. Therefore, only certain materials or types of products can be. sintered in direct-fired furnaces.. The indirect-fired gas systems heat the pieces mostly by radiation. The tubes are heated from the inside either by gas combustion, oil combustion, or electrically heat produced from resistances.. Then the heat travels by. conduction through the tube and is radiated outwards. If the tubes use gas or oil, the tubes protect the pieces by containing combustion within protective tubes. Thus, the products of combustion never interact with the pieces and there can be controlled atmospheres around the work as dictated by the sintering process. If the tubes are heated electrically, more then likely the. 33.

(38) IM - 2005 - I - 39 tubes are in place to protect the resistance elements from the furnace atmospheres. The electrically heated furnace equipment are common in all temperature ranges. Like it was stated previously, they can be placed within tubes and be considered indirect or they can be exposed and be considered direct. Factors that influence the decision of direct or indirect depend a whole lot on the interaction between the elements and the furnace atmosphere.. For. example, the furnace atmosphere can cause mechanical damage to the elements. In any case, part of selecting an electrically heated furnace is choosing the element’s material (metallic or non-metallic), the configuration of the resistance (e.g. wire diameter or pitch between coils), and an electronic controlling device. Clearly the advantages of an electrically heated furnace over direct-fired or indirect-fired equipment are the system’s cleanness, ease of controlling temperature cycles automatically and consistently, and uniform heat distribution.. 3.2.4.4 Gas Pressure and Flow Equipment Pressure is the first variable to set in the fluid flow system’s lines of the processing gas that enters the retorts. The second variable to set is flow rate. And for a flow to e xist through a flow metering device, there must be a pressure difference either upstream the metering device or at the metering device itself. But if the pressure at the entrance of the metering device drops continuously, a constant, downstream flow cannot be kept unless the flow metering device is choked. Thus, a pressure difference is created upstream with pressure regulators and then the flow is regulated. A pressure regulator’s primary function consists in reducing high-pressure gas in a cylinder or process line to a lower, usable level as it passes to other. 34.

(39) IM - 2005 - I - 39 equipment.. They also maintain pressure within a system.. However, a. regulator is not a flow control device. Its only function is controlling delivery pressure.. There are four main types of gas pressure regulators determined by their specific application function.. These are line regulators, general-purpose. regulators, high purity regulators, and special service regulators.. A. combination of a line regulator in series with a high pressure, general purpose regulator is of particular interest. Typically, a line regulator is the point-of-use regulator serving in low pressure lines after the pressure is reduced by a general purpose regulator to an inlet pressure adequate for the line regulator. The scenario for this project is gas that exits from the cylinder source initially at approximately 2000 p.s.i... As the source is depleted, there will be a. continuous pressure drop until the pressure inside the cylinder is equal to the downstream pressure. And the flow of the gas will cease when the pressure equilibrium is reached. This means that a constant pressure difference must exist between two particular points. Say point A is located at the exit valve of the cylinder and B is just before the entrance of the fluid metering flow device. After point A, the general purpose regulator reduces most of the pressure to an intermediate point between point A and B. And then, the gas flows into the line regulator. The pressure drop across the line regulator is approximately a tenth of the pressure drop across the general purpose regulator. The reduced pressure drop difference across the line regulator lets it make fine adjustments when pressure varies downstream. Therefore, the line regulator tries to correct its outlet pressure when fluctuations downstream occur.. This might prove of extreme. usefulness as the gas inside the recipient might experience an increase in pressure due to the temperature rise.. 35.

(40) IM - 2005 - I - 39. There must be only a slight pressure difference between the inlet line of the retort and atmospheric pressure. This pressure must be set such that the metering flow device is calibrated against it.. And the amount of flow is. determined by situations, for example, such as those needed to evacuate water vapor formed during the reduction reaction between the hydrogen and oxides (or air) of the sintering cycle.. There should always be evidence that in fact there is a flow. If the flow ceases, the equipment could be damaged by oxidation; or the worst case scenario, if the flow of nitrogen ceases and the hydrogen flow does not, a combustion hazard would exist. So a variable area flowmeter is installed after the metering flow device. The accuracy standards are given in the table below.. Table 7: Accuracy of Variable Area Flowmeters. Flow rate %. 0.4 1.0 1.6 Accuracy class Total error % Measured Full-scale Measured Full-scale Measured Full-scale 100 0.400 0.400 1.000 1.000 1.600 1.600 90 0.411 0.370 1.028 0.925 1.644 1.480 80 0.425 0.340 1.063 0.850 1.700 1.360 70 0.443 0.310 1.107 0.775 1.771 1.240 60 50 40 30. 0.467 0.500 0.550 0.633. 0.280 0.250 0.220 0.190. 1.167 1.250 1.375 1.583. 0.700 0.625 0.550 0.475. 1.867 2.000 2.200 2.533. 1.120 1.000 0.880 0.760. 20 10. 0.800 1.300. 0.160 0.130. 2.000 3.250. 0.400 0.325. 3.200 5.200. 0.640 0.520. 36.

(41) IM - 2005 - I - 39 2.5. Accuracy class. Flow rate %. Total error % 100. Measured. 4.0 Full-scale Measured. 6.0. Full-scale Measured. Full-scale. 2.500. 2.500. 4.000. 4.000. 6.000. 6.000. 90. 2.569. 2.313. 4.111. 3.700. 6.167. 5.550. 80. 2.656. 2.125. 4.250. 3.400. 6.375. 5.100. 70. 2.768. 1.938. 4.429. 3.100. 6.643. 4.650. 60. 2.917. 1.750. 4.667. 2.800. 7.000. 4.200. 50. 3.125. 1.563. 5.000. 2.500. 7.500. 3.750. 40. 3.438. 1.375. 5.500. 2.200. 8.250. 3.300. 30. 3.958. 1.188. 6.333. 1.900. 9.500. 2.850. 20. 5.000. 1.000. 8.000. 1.600. 12.000. 2.400. 10. 8.125. 0.813. 13.000. 1.300. 19.500. 1.950. 3.2.4.5 Additional Comparisons As stated initially, there are several ways on the selection of furnaces. Here are some direct comparisons.. Table 8: Operating Characteristics and Capital Cost of Sintering Furnaces [Ref. 17] Type Maximum Capital Pounds Parts Operating Cost Produced Temperature °F $1,000’s Per Hour* M esh Belt 2100 $275 450 Ceramic Belt 2400 $350 250 Roller Hearth, 36” 2200 ** 2000 Pusher >2400 $375 250 Walking Beam >2800 $750 500 Continuous Vacuum >2400 $850 350 * Production rate depends on the ability of parts to be packed closely enough to yield this rate. ** Capital cost is about 50% more than. 3.2.4.6 Thermocouples Temperature can be measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic. Six common types are: thermocouples, resistive temperature devices (RTDs and thermistors), infrared. 37.

(42) IM - 2005 - I - 39 radiators, bimetallic devices, liquid expansion devices, and change-of-state devices. For this project, thermocouples will be used.. Any two dissimilar metals joined together is a thermocouple, but there are seven standard thermocouples. Of the seven, a type K is selected. A type K thermocouple is suitable for sintering because it functions within a temperature from –200 °C up to 1250 °C.. It consists of two wires joined together at a junction. One wire is an alloy of nickelchromium and the other wire joined is an alloy of nickel-aluminum. The given limits of error of standard type K thermocouples are 2.2 °C or 0.75% above 0 °C and 2.2 °C or 2.0% below 0 °C.. Furthermore, this thermocouple cannot be exposed to a. reducing atmosphere. Revised thermocouple reference tables are found in the Omega website.. The tables contain the values of thermoelectric voltage in millivolts. corresponding to temperatures.. ALL INFORMATION PERTANENT TO TEM PERATURE M EASUREM ENTE HAS BEEN OBTAINED FROM [Ref. 22].. 4 Methodology After reading this chapter, an orderly development of this project should be sensed. So as the reader advances, he or she should understand how each part of this document contributes and fits within the global contents for accomplishing the objective. A tubular recipient, which is the shape given to the retort, is an equipment within which green samples created with powders are placed so they can be sintered.. The energy for sintering comes from the furnace where the. 38.

(43) IM - 2005 - I - 39 recipient is placed. Thus a furnace is needed for this project. It has opted to only design the recipient and not the furnace. So this project utilizes what is already available at the University of Los Andes. This implies that the retort functions within the furnace described below.. Thermolyne Type 6000 Furnace Volume Interior Depth Interior Height Interior Width Power. 864 in3 10.0 in 6.8 in 12.8 in 3905 watts. Figure 6: Furnace Equipment [Ref. 19]. This furnace is used as a batch furnace. Though another project could actually design a recipient with movement into and out of the furnace making it a continuous furnace system. Additional to the physical properties given, the furnace has been adapted with a Eurotherm Controller/Programmer Type 818 and the lid has been replaced with a permanent, thermally isolated lid with a opening large enough for the retort.. The recipient designed is sealed, has a fluid flow system, and a measurement system for temperature. By referencing to the figure: Virtual Model, it is seen how the sintering system is composed of a 3 inch nominal diameter, schedule 40 tube which slides into the furnace.. 39.

(44) IM - 2005 - I - 39. Figure 7: Virtual M odel One end of the recipient is sealed permanently and the other end uses a blind flange as a lid (not shown). The open end is used for loading and unloading the green powder metallurgy parts that are placed on ceramic supports (shown as a yellow cylinder). The ceramic supports slide on the gray tray. The two purple tubes are the lines in and out of the processing gas mixture. Once the pieces are loaded, the lid not shown is fitted flush against the flange and held in place with a sanitary clamp (none of which is shown). Also, the lid has fitted the measurement thermocouples within sheaths. The whole apparatus slides into the furnace for the sintering cycle. [Ref. 12], [Ref. 14], [Ref. 5], [Ref. 11] To reach the preliminary model, a knowledge of requirements encompassed the starting point.. The knowledge included a general understanding of. powder metallurgy.. Thus, the sections Overview of Powder Metallurgy,. Overview of Sintering, and Schematic of Creating a Sintering Cycle are included as answers to requirement that must be met by the retort. From those sections it is concluded that if the development of the recipient permits control and monitoring of time, temperature, and atmospheres, which are the. 40.

(45) IM - 2005 - I - 39 parameters of the sintering process, a high quality powder metallurgy product is resultant and the project is a success. [Ref. 12], [Ref. 14], [Ref. 15], [Ref. 11]. With the requirements defined, three distinct sub-systems of the retort system (consider the retort equipment as a system) are identified:. the physical. system, fluid flow system, and thermal system. Each of these are separately developed. In all three systems, key components and variables are discretely considered for future, individual design.. This is the motive behind the. sections of Gases and Diffusion.. The section of Details of Creating a Controlled Atmosphere then explicitly unites the solutions given to the three systems. At this point a design for a measurement system is required for evaluation of the solutions given. The measurement system is a combination of mechanical and electronic data acquisition.. And this would conclude the first part of the objective as the. preliminary design stage. Logically, the construction stage proceeds, and with all of the previous completed, design and construction, tests and interpretation and characterization of the results follow. It is important to note that all times there is feedback, but the causes of the feedback are not included. Instead, the results are given. Finally, the development of the project is given below in figure 3 as a flow diagram for clarification.. 41.

(46) IM - 2005 - I - 39. Figure 8: Tubular Recipient Development Model. 5 Results and Analysis Design of the sintering equipment became possible with all of the background information. The results of the different areas of the equipment are given in this chapter. Afterwards, a brief analysis is based on the results of the characterization experiments.. 5.1 Details of Creating the Retort. The concept of a controlled atmosphere consists of being able to measure and control the parameters within the retort. Temperature is one of the parameters.. Quantity and composition of the processing gas are other. parameters. These three parameters must be measurable at all times. And the retort system must also allow adjustment of the parameters.. 42.

(47) IM - 2005 - I - 39 The figures displayed under Figure: Prototype are pictures of the equipment.. Figure 9: Prototype. 5.1.1 Six Channel Data Acquisition A Six Channel Data Acquisition electronic equipment was designed for particular use of collecting data for the retort. Basically, it is designed to collect information from transducers which can be for pressure, flow, or temperature.. Initially, the data collection of this project is limited to the. voltage signals given by the thermocouples and converting to degrees Celsius via computer software. The figure Data Acquisition Card shows a picture of the result.. 43.

(48) IM - 2005 - I - 39. Figure 10: Data Acquisition Card. 5.1.1.1 General Architecture of the System Input:. -Temperature of environment -Mode of operation selected by the user: Start Data Acquisition, End Data Acquisition, Display report with Excel, Sampling Time. Output:. -Led indicator when the state is ON -Display of temperature of each thermocouple -Report obtained data on Excel. Figure 11: General Architecture of System. 44.

(49) IM - 2005 - I - 39. 5.1.1.2 Block Architecture of Temperature Sensors Inputs :. -Signal from six type K thermocouples tipo K -Command of Start Data Acquisition and number of thermocouple of the desired thermocouple.. Outputs :. -Digitalized value of selected channel. Figure 12: Block Architecture of a Temperature Sensor. 5.1.1.3 Implementation General. Control. Block,. Control. of. Temperature. Sensors,. and. Multiplexing: After revising data acquisition possibilities, it was concluded that the opt technology for implementation is through microcontrollers. Then a choice amongst the microcontrollers available was based on the following selection criteria: •. A minimum of at least six channels. •. At least a 10 bit converter. •. An acceptable conversion time below half a second. 45.

(50) IM - 2005 - I - 39 •. Simplified programming. •. Commercial availability in Bogotá. •. Of low cost.. The criteria lead to the selection of a PIC18F452 microcontroller fabricated by Microchip. The datasheet is included in the Reference folder under the Data Acquisition Folder. Block of Amplification Signal: This block is implemented with an op-amp in a non-inverting configuration. The amplification is approximately of 100x. The component used is a LF353 with the necessary amount of precision in amplification. Resistors of 100 [Kohms] and 1 [Kohms] are implemented.. Block of Analog to Digital Conversion: The ADC of the PIC18F452 is used.. Communication Interface with the PC: A DLP-USB245M fabricated by FTDI chip permits Parallel-USB communication. Transmission velocities of up to 1 [Mb yte/second] can be achieved. All of the transmission control protocol is contained by the chip. The control signals handle logic signals of 3.3 [V] and 5 [V].. Communication Interface between the PC and the End-User:. The. software of the Data Acquisition Card was created with Visual Basic 6. The software allows the user to: Start Data Acquisition, End Data Acquisition, change the Sampling Rate, view the temperature of each of the six channels when there are updates according to the Sampling Rate, and store the data which the End-User can view by pressing the View Excel Report.. 46.

(51) IM - 2005 - I - 39. Figure 13: Graphical User Interface. Figure 14: Format of Results Displayed with Excel. 47.

(52) IM - 2005 - I - 39. 5.1.2 Temperature Measurement (Thermocouple) Type K thermocouples will be used as previously stated in the section of Thermocouples in the chapter of. Revision of Bibliography.. The. thermocouples must be compatible with the rest of the equipment and intended use of the equipment. Therefore, compatibility mostly concerns geometry and chemical resistance of the thermocouple or sheath.. Because the equipment is exposed to reducing atmospheres, it is necessary to place the thermocouples in a protective sheath. In protective sheaths junctions are either grounded, ungrounded or exposed. The junctions placed inside the retort must be ungrounded. They cannot be exposed because the atmospheres would damage the thermocouples and they also cannot be grounded because any possible electric discharge could cause an explosion hazard. Independent of the fact that an ungrounded thermocouple responds the slowest, it is the only possibility. Initially, there are two thermocouples inside the retort. Both are placed within stainless steel 304 sheaths. One thermocouple is fixed and the other slides axially. The fixed thermocouple serves as a reference and the other scans temperatures axially to discover gradients and response time.. The thermocouples are rated up to 1250 °C, yet the recommended maximum temperature of operation is 900 °C due to the sheath material.. When. operating with temperatures exceeding 1000 °C, type K thermocouples suffer corrosion which consumes the wire. The higher the temperature and the longer the exposure, the shorter the life of the element. The green scale that forms during exposure should be scraped off with a sharp tool until the bright metal is exposed. The thermocouple should be replace when the wire is less. 48.