ANÁLISIS DE LOS RESULTADOS

The determination of the optimum processing conditions for the HIPping of high strength chromium cupronickel (NES 824) was carried out using artificial porosity specimens which contained either internal porosity or surface-connected pores. The pore geometries included squat pores of large diameter and elongated pores of small diameter (Figure 63).

The specimens that contained surface-connected porosity required encapsulation prior to HIPping. The capsule was evacuated, sealed and together with the specimens

containing internal porosity subjected to X-ray penetration examination prior to HIPping. The HIP experiments were conducted within the laboratory HIP system. The

specimens were heated at 15°C/minute to the treatment temperature under an argon gas pressure that increased progressively during heating to reach a maximum in the

range 41-145MPa. The treatment temperatures selected lay in the range 850-1025°C. Previous work carried out by HIP Limited (173,213) had shown that a temperature of 950°C provided the best combination of strength and ductility for 70/30 cupronickel castings whilst inhibiting grain growth associated with higher HIPping temperatures.

Investigations were undertaken to substantiate the above findings and to determine the optimum HIP temperature with regard to improved mechanical properties. The experiments involved HIPping cast NES 824 material for a period of 4 hours at temperatures of 850°C, 950°C and 1025°C with argon gas pressures of 103MPa. The HIP pressure and sustain time were based on previous experience gained by H.I.P Limited on the HIPping of steel (214). In addition, a comparison was made between the rates of pore closure for three types of treatment at what proved to be the optimum temperature of 950°C:

(i) Type I - Successive 45 Minute HIP Cycles

In type I, pore closure of both internal and surface- connected pores as a consequence of repeated applications of a HIP cycle that involved holding the specimen for 45 minutes at 950°C with a gas pressure of 103MPa. This treatment was designed to allow observation of the stages of pore closure and identification of the densification mechanisms involved. It was thought that increasing the HIP time further might result in complete consolidation of the pores during a single HIP cycle and prevent any

meaningful investigations.

The specimens which contained surface-connected porosity were encapsulated, heated and pressurised to the sustain conditions, cooled and removed from their capsule and re­ encapsulated with fresh pressure transmitting medium before reapplying the temperature and pressure at the start of the next cycle. Thus each cycle included a heating/

pressurisation, a sustain and a cooling stage as illustrated in Figure 68.

(ii) Type II - Continuous HIP Cycle

In type II, pore closure of surface-connected pores

occurred when the same temperature and pressure as in (i) were applied as a single continuous cycle for various times from 45 minutes to 12 hours, thus involving only one

heating and one cooling stage as shown typically for a 3 hour sustain in Figure 69.

(iii) Type III - Successive Short HIP Cycles Without Renewed Encapsulation

To separate the effects of the encapsulation process

on densification from the effect of the initial heating and holding at temperature and pressure during the first HIP cycle, specimens which contained surface-connected

porosity were subjected to repeated cycles as in (i) but without re-encapsulation at the end of each cycle.

In addition to the three types of treatment at 950°C a type IV treatment was applied at 850°C, 950°C and 1025°C as discussed below.

(iv) Type IV - Zero Sustain Time HIP Cycles

The period for which the HIP conditions were held constant is termed the sustain period, and the experiments carried out in (i) indicated that the majority of densification occurred during the first cycle (or within 45 minutes at the sustain conditions). Thus, it was necessary to

estimate the amount of densification due to instantaneous plastic yielding and that due to time dependent mechanisms

(ie. creep and diffusion). Densification by plastic yielding takes place once the HIP pressure exceeds the yield stress of the material, and should therefore be complete by the time the HIP conditions are reached. Experiments to substantiate this assumption were carried out at temperatures of 850°C, 950°C and 1025°C with argon gas pressures of 103MPa for zero sustain times. An

extension of the sustain time to 45 minutes allowed the increase in densification due to time dependent mechanisms

to be identified.

On completion of the constant temperature and pressure stage the specimen was cooled at 10°C/minute until

ambient was reached. The dimensions of the pore profile after HIPping were determined by X-ray penetration

examination. Pore closure as a result of HIPping was

determined by measuring the absolute porosity remaining in the specimen. Quantitative evaluation of the extent of pore closure taking place during each HIP cycle was carried out using Seescan Image Analysis by tracing the pores

before and after HIPping, and measuring the change in

cross-sectional area of the pore. It was assumed that the change in area of a pore of known volume during HIPping could be equated to the reduced volume, since the reduction in pore size is equal across its radius because the HIPping pressure acts equally and instantaneously on all surfaces. The next stage of the HIP experiments was to examine the effect of the HIP process on densification. Varying the HIP pressure within the range 41-145MPa (6000-21000psi) allowed the prediction of the optimum processing conditions for maximum densification of NES 824 castings containing internal and surface-connected porosity at a processing temperature of 950°C.

The change in flow stress with temperature within the range 300-1025°C was determined for as-cast 70/30 cupronickel material in order to predict the minimum temperature at which plastic yielding would occur.

applied HIP pressure during the heating/pressurisation stage of the HIP cycle, allowed the minimum temperature at which plastic yielding would occur in practice to be

estimated.

3.4.3 ENCAPSULATION TECHNIQUES

The HIP experiments have indicated that for both un­

encapsulated and encapsulated NES 824 castings the initial rate of densification was very rapid (Figure 94 and

Figure 85), after which subsequent densification occurred at a much lower rate once the sustain conditions had been reached and held for time periods which exceeded 45

minutes (Figure 87).

Other HIP experiments conducted on encapsulated specimens which contained surface-connected porosity subjected to

successive (type III) HIP cycles of short duration without renewal of the packing material prior to each cycle, also showed a similar decline in the densification rate after the first HIP cycle (Figure 92). This indicated that the standard method of encapsulation which uses silica sand as the packing material was in some way impeding pore closure. This reduction in the densification rate needed to be

examined further and consideration was given to the

possibility that the sand particles were sintering together thus, options for a change in the type of refractory for the densification of 10% and 16% surface-connected pores were examined.

Eleven types of pressure transmitting media were used during encapsulation to identify the optimum refractory PTM, and to ascertain the effect of particle shape and size

on the densification process. The particle sizes of the various refractory materials were determined by sieve analysis, the results of which together with the melting points of the refractory materials are shown in Table 28. The chemical analyses of the refractory sands used as a PTM are shown in Table 29. The refractory pressure

transmitting media used in the HIP experiments included:

(i) High Purity Silica Sand

Silica sand is the standard PTM used for the encapsulation of castings prior to HIP processing (200). However, it tends to undergo mechanical locking and partial sintering at the temperatures and pressures associated with HIPping. This desert-based sand is of high purity as shown in

Table 29, and is predominantly of a narrow size fraction (~150pm). The particles are angular in shape as indicated in Figures 103 and 104.

(ii) Close Fitting Steel Capsule

This type of encapsulation without a refractory packing medium is only suitable for castings of simple geometry.

It may be considered as close to the ideal, because once the capsule is sealed it resembles a casting which contains internal pores and behaves in a similar manner during

densification by HIPping. Densification can not be

interfered with by sintering of refractory particles since there are none present.

(iii) Coarse Tabular Alumina Granules

The high melting point of alumina (2015°C) suggests that sintering will not take place. However, the coarse tabular granules (>355/jm but <1700^m) as shown in Figure 101, were expected to reach a maximum packing density in shorter times for equivalent HIP conditions than fine particles, and thus to impair densification.

(iv) Fine Calcined Alumina Powder

It was thought that the fine calcined powder containing particle sizes within the range 53-75-106pm might improve the densification rate, due to their greater resistance to sintering and/or mechanical locking, or by enhanced ease of particle flow under the HIP conditions.

(v) Calcined Alumina And Boron Nitride

Mixtures of these refractory materials in weight ratios of A1203:BN of 10:1 and 10:3 were used during HIP experiments. The boron nitride had a very fine particle size (<55jum) and tended to grind to dust during HIPping. The boron nitride was expected to act as a lubricant and enhance the ability of the refractory particles to transmit the applied load/ pressure during the HIP process.

(vi) Zirconium Oxide

Zirconia has a high melting point of 2715°C and would therefore not be expected to undergo sintering at the HIP conditions. The zirconia particles were angular in nature and had particle sizes within the range 75-150jum. This oxide material was chosen in order to investigate the effect of changing the type of refractory used as the PTM

on the rate of densification of encapsulated castings.

(vii) Boron Carbide

Boron carbide has a diamond structure of great hardness and a high melting point of 2450°C. This very fine powder

(<53-75jL/m) was used to investigate the effect of particle size and change of refractory on the densification rate of encapsulated castings. Boron carbide is hygroscopic and required heating to expel moisture prior to encapsulation. Mixtures of boron carbide and silicon carbide (weight ratio

of 3:10) were also examined as a potential refractory PTM.

(viii) Glass

Two types of glass were investigated, a low melting point soda glass (400°C) and a borosilicate glass known

commercially as Pyrex which melts around 1680°C. Fragments of glass were packed around the specimen and partially fused in a conventional muffle heat treatment furnace at 1000°C prior to HIPping. In addition investigations were carried out to determine whether evacuation of the capsules which contained fused Pyrex glass was beneficial in regards to densification, and also to provide an indication of the method by which the HIP pressure was transmitted to the test specimen.

(ix) Zone-M-Concrete Sand

This is a water table, 90% quartz aggregate sand chosen because of its particle characteristics. Its particle

shape was less angular than the desert sands, as is shown by comparison of Figures 105 and 106 with Figures 103 and 104. The particle size distribution of the zone-M

concrete sand was determined by sieve analysis to be in the range 106-1700pm. The particles greater than 355pm were of a shell-like nature and were removed. Zone-M-concrete sand was used as a PTM in two forms firstly as an aggregate of particle sizes (106-355pm) and secondly as a single

particle size sand (150pm); in order to investigate the effect of particle shape and size distribution on the rate of densification of encapsulated castings.

(x) Leighton Buzzard Sand

This is a siliceous based (>96% Si02) water table sand of single particle size (>355pm but <1700pm), the particles being of a rounded nature as shown in Figure 109 and 110. This sand was chosen as a comparison against the zone-M- concrete sand in order to compare the effect of large and small rounded particles on the rate of densification of encapsulated castings.

3.4.4 EFFECT OF THICKNESS OF SAND LAYER ON THE RATE