Título del gráfico
CRECIMIENTO POBLACIONAL HISTORICO
Lauralice C.F. Canale, University of Sa˜o Paulo Xin Yao, Portland State University
G.E. Totten, Associac¸a˜o Instituto Internacional de Cieˆncia and Portland State University
MANY COMPONENTS, such as fasteners, crankshafts, camshafts, bearings, and others, re-quire a differentiated response of the surface and core to external loading. This can be accom-plished by surface (case) hardening methods such as induction and flame hardening or by surface diffusion processes such as carburizing and carbonitriding. Raja et al. have reported that case carburizing is one of the most common heat treatments for steel, accounting for 50% of all surface treatments (Ref 1). Case carburizing involves the creation of a gradient that exhibits high hardness, brittleness, and strength in the surface and greater toughness and ductility in the softer core in order to provide optimal (Ref 2):
Wear resistance
Resistance to scoring
Bending and/or torsional fatigue strength
Rolling-contact fatigue strength
These properties are optimized by maximiz-ing surface compressive stresses, and carburiz-ing is one of the most effective and commonly used methods to impart compressive stresses to the surface of a component (Ref 3). The focus of this chapter is on carburized and carbonitrided materials.
Gas carburizing, which is the most widely used carburizing process, is a surface diffusion process where the carbon concentration in a surface layer (case) of a steel matrix that is predominantly iron, chromium, and nickel is increased by heating the component at approxi-mately 850 to 950C with endothermic gas (Endogas), which is a blend of carbon mon-oxide, hydrogen, and nitrogen (with smaller
amounts of carbon dioxide, water vapor, and methane). Endogas is produced by reacting a hydrocarbon gas, such as natural gas (methane), propane, or butane, with air.
After the diffusion process is completed, the component may be quenched from the carbur-izing temperature or reheated to austenitize the steel, and then quenched. Bainite formation in the case is strongly inhibited by the presence of molybdenum and chromium. Since the surface contains higher carbon content than the core, it is harder than the softer core. Core hardness is most strongly affected by the presence of molyb-denum and manganese. Chromium exhibits a moderate effect, and nickel exhibits a weak effect (Ref 3). Core hardness is strongly affected by the quenchant selection and quenching tem-perature.
In addition to strengthening the case, the increased carbon content also provides desirable increased compressive stresses that will inhibit fatigue crack initiation. The lower carbon con-tent in the core also will produce improved fatigue strength.
Carbonitriding is similar to carburizing in that it is a diffusion process that involves the simul-taneous diffusion of carbon and nitrogen (from ammonia) into the steel surface. To obtain maximum strength, the carbonitriding process produces a surface that is enriched in nitrogen and carbon in the form of an epsilon (e)-carbonitride layer and a diffusion zone containing chromium-iron carbide, (Cr,Fe)7C3; chromium carbide nitride, Cr62C3 5N0.3; chro-mium nitride, (Cr2N) or [Cr, Fe(2Ni . . . x)]; and Fe2N phases (Ref 4, 5). Typical case thicknesses range from 50 to 200mm with a hardness
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Failure Analysis of Heat Treated Steel Components
L.C.F. Canale, R.A. Mesquita, and G.E. Totten, editors, p 177-240 DOI: 10.1361/faht2008p177
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between 750 and 900 HV. Like carburizing, the case depth of carbonitrided steel is dependent on both the carbonitriding diffusion time and temperature, as illustrated in Fig. 1 (Ref 2, 6).
Deeper case hardnesses may be obtained by first precarburizing prior to carbonitriding. Karamis¸
showed that carbonitrided AISI 5115 steel exhibited greater surface hardness and wear re-sistance than carburized AISI 5115 steel (Ref 4).
Carbonitriding processes are typically con-ducted in either a gas (ammonia) or a salt bath based on trade names such as Tufftride, Nitrotec, and Nitrox. Alternatively, a plasma nitriding process may be conducted. A brief summary comparison of carburizing and carbonitriding processes is provided in Table 1 (Ref 2).
Carter has reported that failures of carburized gears are primarily due to service-related causes, such as misalignment, poor lubrication, and overloading, which constitute the greatest source of all gear failures, as shown in Table 2
(Ref 7). Heat treatment was the second most often cited cause for failure. However, it is often difficult to detect the root cause of a specific failure under the conditions in which the failure occurred, and many of the service-related fail-ures could have been reduced with more atten-tion to the other potential causes of failure shown, since they are often interrelated.
Palaniradja et al. reported that 10 to 12% of carburized parts are rejected due to various process-related defects (Ref 8). To examine this in more detail, they conducted a Taguchi ana-lysis of gas carburization of AISI 8620 and 3310 steels, and their results showed that relative contribution to surface hardness was holding time (20%), carbon potential (20%), carburizing temperature (0%), and quenching time (60%).
Similarly, they also studied the effects of process variables on case depth and found: holding time (60%), carbon potential (9%), carburizing temperature (14%), and quenching time (10%) (Ref 8). These results show that an adequate understanding of failure analysis of carburizing and, by implication, carbonitriding must be accompanied by understanding the contribution of process parameters on resulting potential failures.
Some of the most common contributors to failure of carburized gears include surface finish, microstructure, excessive or inadequate case depth, incorrect case and/or core hardness, improper carbon concentration and hardness gradients, undesirable surface carbon content, excessive retained austenite, large amounts of globular and network carbides, intergranular oxidation, internal oxidation, residual stress, extremely coarse case or core grain structure, untransformed core with free ferrite, quenching and grinding cracks, surface decarburization, excessive heating during grinding, excessive removal of the case during grinding, micro-cracking, and so on (Ref 9, 10).
This chapter provides an overview of various contributors to failure of carburized and carbo-nitrided components, with the primary focus on carburized components.
Fig. 1 Correlation of case depth of carbonitrided steels with varying diffusion times and temperatures
Table 1 Comparison of carburizing and carbonitriding processes
Process Comments
Carburizing Hard, highly wear-resistant surface (medium case depths), excellent contact load potential, good bending fatigue strength, good seizure resistance, excellent quench cracking resistance, low-to-medium-cost steels required, high capital investment
Carbonitriding Hard, highly wear-resistant surface (shallow case depths), fair contact load potential, good bending fatigue strength, good seizure resistance, good dimensional control, excellent quench cracking resistance, low-cast steels usually satisfactory, medium capital investment
Table 2 Survey summary of sources of gear failures
Cause of gear failure %
Material quality and forming 0.8
Design 6.9
Service-related causes 74.7
Manufacturing 1.4
Heat treatment 16.2
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Design
Component design may contribute directly or indirectly to component failure. Deficiencies such as insufficient radii or sudden changes in section size are significant contributors to fail-ure. In addition, the presence of stress raisers, such as those shown in Fig. 2, are among the most common design contributors to quench cracking and fatigue failure.
A more comprehensive insight into design is provided by Kuehmann et al., who developed a systems analysis flow chart to describe the effects of case-core hardening in designing a carburizing process/metallurgical structure/
resulting properties and performance for the production of gears produced by three routes:
conventional forging, near-net shape casting, and powder metal processing (Fig. 3) (Ref 11).
To properly design a component, it is neces-sary to estimate surface loading, distortion after heat treatment, case depth and carbon profile,
case and core hardness, and core strength. As an estimate, for hardnesses within the range of 30 to 45 RC, the required case depth can be calculated from (Ref 2):
Case depth to 50 HRC=(1:2 · 107 W)=F where W is the force in pounds pressing the surfaces together, and F is the length of the line contact (inches).
Carter has recommended the following gen-eral design criteria (Ref 7):
If a component is carburized from both sides, the case depth should not be greater than 20% of the wall thickness.
At the base of gear teeth, 30% of the core material should remain uncarburized.
Shallow case depths usually require higher case hardness.
Case depths should be five times the accep-table wear limit.
Fig. 2 Effected of stress raisers on stress concentration and distribution of stress at several changes of form in components. (a) to (c) Progressive increases in stress with decreasing fillet radii. (d) to (f) Relative magnitude and distribution of stress resulting from uniform loading. (g) Stress caused by the presence of an integral collar of considerable width. (h) Decrease in stress concentration that accompanies a decrease in collar width. (i) Stress flow at the junction of a bolt head and a shank. (j) Effect of a single sharp notch. (k) Effect of a continuous thread. (l) Effect of a groove or gauge. Source: ASM Handbook, Volume 11, 2002, p 715
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Although machining is proportional to the case depth, it should be minimal.
Kern and Suess have recommended the fol-lowing general guidelines for heat treatment of gas-carburized gears (Ref 2):
For forgings, normalize or anneal (as re-quired by the alloy being heat treated) from a temperature at least 28C (50F) above the carburizing temperature.
Assure that the gears are machined prior to heat treatment.
Bring the gear to the carburizing temperature with sufficient circulation of a neutral atmosphere, and then introduce the gas used for carburizing.
For deep cases (41.5 mm, or 0.060 in.), adjust the carburizing atmosphere and time to produce uniform carbon diffusion from the surface to the core. A decrease of 0.15 to 0.20%/0.25 mm (0.010 in.) of depth is
nearly ideal. The gradient may be steeper for shallower case depths.
The recommended surface carbon is 0.90 to 1.10% for 4300, 4600, 8600, 8800, and 9400 carburizing steels. Although the same case depth is generally acceptable for grades such as 4800, they are preferably reheated for hardening. The recommended surface car-bon is 0.65 to 0.85% for high-nickel steels such as the 4800 series, which is usually direct quenched.
To minimize cost and distortion, use direct quenching whenever possible.
To assure optimal dimensional control, properly maintain quenching dies and plugs.
Quench as rapidly and uniformly as prac-tical, and use spray impingement fixtures on large, solid pinions that are four pitch and coarser.
Use hot oil quenching on fine pitch gears.
Fig. 3 Kuehmann et al. flow chart to summarize design elements of a carburizing process/metallurgical structure/resulting properties and performance comparison of gas carburizing gears produced by conventional forging, near-net shape casting, and powder metal processing
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Case hardness of the finished gear should be 60 HRC or greater.
If possible, test each gear for partial dec-arburization and/or upper transformation products.
To minimize distortion and to permit quieter operation, the surface carbon content should be uniform throughout the production cycle.
Steel Selection and Hardenability Steels typically used for case hardening con-tain carbon contents of less than approximately 0.25%. The carbon content of the case is usually controlled to between 0.8 and 1% C. The actual surface carbon content is generally limited to 0.9%, because excessively high carbon content may lead to the presence of unacceptably high retained austenite and brittle martensite. Some of the most commonly used AISI grades of steel used for carburizing are shown in Table 3 (Ref 12).
Plain carbon steels may be carburized; how-ever, relatively poor hardenability due to the lack of alloying elements reduces the carburiz-ing response of the case. Because of the stabi-lizing effect of the nitrogen relative to austenite, carbonitriding provides greater hardenability than attainable with carburizing. Therefore, plain carbon steels respond well to carboni-triding.
Proper steel selection is a critically important process to provide the desired case depth and microstructure and the required core properties.
Typically, the case structure should be fully martensitic, with the exception of allowing for required application design limits on retained austenite content. For example, the steel must
possess sufficient hardenability to provide the desired hardness and microstructure in both the case and the core. After carburizing, the com-ponent must possess sufficient toughness with-out exhibiting brittle failure.
Most steels that are carburized are deoxidized by the addition of aluminum (commonly desig-nated as killed steels). Deoxidation will provide finer grain sizes to temperatures of approxi-mately 1040C. Coarser grained steels may be carburized if grain refinement by double quench-ing is possible. Double quenchquench-ing typically in-volves direct quenching followed by reheating to a lower-temperature quenching a second time (Ref 13).
Selection of proper hardenability of steels for both carburizing and carbonitriding is critically important, both of the core and the case, since improper hardenability design can lead to un-desirable nonmartensitic transformation pro-ducts in the case, leading to a potential reduction in static and dynamic fatigue strength of up to 30% and a reduction of impact fatigue of up to a factor of 2.5 times (Ref 14). The hardenability gradient of the case and the core is dependent on a number of factors, including cooling rate during quenching, variability of the chemical composition (alloy content, carbon and nitro-gen) of the case, and the carburizing or carbo-nitriding method being used.
Core hardenability is being used increasingly to specify alloy steels used for case hardening where the hardenability of both case and core must be considered. Details for the traditional approach for the experimental determination of hardenability of carburizing steels are provided in Ref 15. Jominy curves for a number of car-burizing steel alloys with varying hardenability are shown in Fig. 4 (Ref 16).
Procedures have also been described for determining ideal diameter (DI) values and hardenability of carburizing steels from Jominy data using regression equations for composition and grain size (Ref 17, 18). The ideal diameter is defined as the diameter of a cylindrical steel bar that will form 50% martensite at the center when subjected to an “ideal” quench. Hardenability differences may be substantially greater for some case-hardening steel grades relative to others due to the difference in carbon content in the case and core. This is more critical for heavy-sectioned components that are reheated and quenched.
The hardness gradient through the case is due to the relationship between the thermal gradient Table 3 Common carburizing grades of steel
and their relative processing features
AISI steel
grade Note
4620 Lower-cost, chrome/nickel/molybdenum steel where only nominal hardenability and core response is required 8620 Most commonly specified grade. Excellent carburizing
response, with good hardenability for most section sizes 4320 Higher hardenability for improved core response in heavier
sections
4820 Increased nickel content for improved core toughness; slower response results in longer process times
9310 Maximum nickel content for maximum core toughness;
slower response results in longer process times
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and the carbon gradient during quenching.
Therefore, an increase in case hardenability required to produce greater amounts of marten-site for a given carbon content will result in an increased case depth. In such cases, a reduced (shallower) carbon profile and shorter carburiz-ing times will be necessary to obtain the desired hardness profile in the carburized component.
SAE J1975 standard “Case Hardenability of Carburized Steels” summarizes characteristics of carburized steels and factors involved in controlling hardness, microstructure, and
residual stress. Methods of determining case hardenability are also provided.
Parrish reported the following scheme that was developed to classify the case hardenability of steels (Ref 19):
Level 1: Surface carbon contents 40.8% C are martensitic.
Level 2: All carbon contents from the surface to 50% C are martensitic.
Level 3: All carbon contents from the surface to 0.27% C are martensitic.
Level 4: A martensitic case occurs at all carbon levels, including the core material just beneath the case.
Figure 5 illustrates the core hardenabilities for a number of carburizing steels (Ref 19). This figure is used by estimating the equivalent dia-meter for the critically stressed section of the component of interest, and then the expected level of case hardenability of that steel is deter-mined. Figure 5 indicates that level 4 is attain-able only for small section sizes of more alloyed steels, and level 3, depending on the section size, is more readily attainable for most of the steels shown. Level 2 is more typical of the more common case-hardened parts and should repre-sent a minimum target to be attained.
Case hardenability may vary widely even for steels with equivalent core hardenabilities. Kern
Fig. 5 Case hardenabilities of a number of carburizing steels with oil quenching
Fig. 4 Jominy hardenability data for a number of carburizing steels
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and Suess provided the following guidelines (Ref 2):
Steel grades in which the case hardenability is due to carbide-forming metals such as chromium (8600 series) are sensitive to microcracking, especially when direct quen-ched from the carburizing temperature. This can be controlled by restricting the carbon content in the case to 0.9%.
Steel grades with relatively high nickel content, for example, 4800 and 9300 series, may form excessive (430%) amounts of retained austenite when direct quenched unless the carbon content of the case is maintained at 50.75%.
Carburizing round section sizes greater than 76 mm (3.0 in.) may lead to difficulty in achieving the desired case and core micro-structures when quenching in oil. In such situations, consider induction hardening or nitriding or using a highly alloyed steel grade such as AISI 9310.
Some standard grades of steel exhibit nar-rower core hardenability bands than other grades. For example, 8620H exhibits a hardenability band spread of 14 HRC at J 4, and 9310 exhibits only 8 HRC spread at the same J-value. This provides a greater amount of distortion control in addition to some possible application-dependent prop-erty advantages as well.
One problem that can arise during the steel-making process or that may be observed as a mill-to-mill variant is the presence of segregation effects through the section of the steel billet during a continuous casting process, which re-sults in the presence of a white band (Ref 20).
White band is a type of negative segregation often observed in electromagnetically stirred continuous castings. The white banding pro-duces a significant hardness gradient across the billet. After subsequent rolling and forging or machining to produce a component, the resulting grainflow can produce nonuniform hardenability and/or soft spots that can significantly affect distortion.
In addition to proper hardenability selection, to achieve maximum core toughness, proper austenitization and quenching to martensite is necessary. These topics are discussed subse-quently.
Case Depth. The case of a carburized (or carbonitrided) steel alloy is that portion
extending inward from the surface, where the hardness is greater than that of the core. The total case depth is the distance or thickness of the carbon-enriched surface layer. The effective case is the point where 0.4 to 0.5% C (percent is called points in the industry) is present if the part is hardened to 50 HRC (510 HIV). The depth of the case is a function of carburizing time and carbon (carbon potential) at the surface. Genel and Demirkol have reported that the following equation model can be used to predict effective case depth (Ref 21):
Effective case depth (mm)= 0:41 ½Carburizing time (h)1=2 The carbon potential of a furnace atmosphere
Effective case depth (mm)= 0:41 ½Carburizing time (h)1=2 The carbon potential of a furnace atmosphere