Precipitation Hardening Is One Type Engineering Essay

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Precipitation hardening is one type of heat treatment process. It is sometimes referred to as age hardening as the material hardens along time. It is a process in which hardening and strengthening of a metal alloy is performed through uniformly dispersed particles which precipitate from a supersaturated solid solution. During this heat treatment, a phase transformation is achieved. A second phase is obtained having formed extremely small uniform dispersed particles termed precipitates, from the original matrix form [1]. This enhances both the strength and hardness of some metal alloys.

Alloys such as Al-4.5% Cu, Al-6% Zn-2.5%, Mg will show an increase in hardness as time progresses at room temperatures or at a slightly elevated temperature [2]. In addition to this there are also magnesium-aluminium, copper-tin and some ferrous alloys that harden through precipitation.

The process of age hardening is achieved by two different heat treatments. During the first treatment, every solute atom is dissolved to form a single phase solid solution. The procedure consists of heating an alloy to a temperature which is inside the alpha phase area and it is left at that temperature until the beta phase which may have been present is completely dissolved. Thus the composition consists of only the alpha phase. Either rapid cooling or quenching follows afterwards; this will expose the alloy to a temperature which is for most room temperature preventing the formation of the beta phase. An alloy is in a none equilibrium state where an alpha phase solid solution supersaturated with B atoms as shown below is available. At this state the alloy is weak and relatively soft.

Fig.

For most alloys the diffusion rate to room temperature is extremely slow so that the single phase of alpha particles is retained more for longer periods.

The second heat treatment is the precipitation heat treatment, where the supersaturated alpha solid solution is heated to an intermediate temperature that is in the two phase region of both alpha and beta. This can be seen from the above diagram. The latter phase starts to form fine dispersed particles. After leaving the alloy for an appropriate amount of time at the intermediate temperature denoted by T2, the alloy is cooled back to room temperature.

From the below illustration both heat treatments are presented with temperature along time. The characteristics of the beta particles depend on both the precipitation temperature and time for aging. This will determine the type of strength and hardness of the alloy.

Plot fig 11.2

Aging may occur instantly at room temperature for some alloysas long the time period is extended. The behaviour of an alloy that underwent precipitation hardening can be seen through the following plot, where strength is analysed as time is extended further.

Plot

It can be seen that as time increases, the hardness also increases to reach a peak value, after which it starts to decline. The reduction in the strength is due to over aging which occurs after a long period which all properties are also affected by temperature variation [1].

The material that was selected for this research is 17-4PH sometimes referred to as stainless steel grade 630. This is a stainless steel which is common to heat treat through precipitation hardening. Precipitation hardening stainless steels can either be chromium or nickel that contains steel. This setup provides the ultimate properties of martensitic and austenitic grades. Thus the stainless steel will be able to show high strength gained from heat treatment and resistance to corrosion which reflects the austenitic grade. The heat treatment process leads to precipitation hardening having a martensitic or austenitic matrix. The addition of elements namely copper, aluminium, titanium, niobium and molybdenum in the stainless steel will result in hardening. Basically the material 17-4PH denotes an addition of 17% chromium and 4% nickel. It also contains 4% copper and 0.3%niobium [3]. This stainless steel combines high strength and hardness after heat treatment having an excellent resistance to corrosion. From several tests that have been carried out it was found that 17-4PH is superior to grades such as 420,431 and 410. The resistance to corrosion is similar to stainless steel grade 304. This alloy of stainless steel is composed of chromium-copper, has good machine-ability and the heat treatment can be done at low temperatures for a short time. Since heating is carried out for a relatively short period, this helps to reduce warping and scaling. Comparing high nickel non-ferrous alloy to chromium-copper stainless steel, the latter is found to be more cost effective [4].

The application of hardening stainless steels is mainly concerned where good corrosion resistance and high strengths are needed as well as where high fatigue strengths are needed. Earlier it was noted that during heat treatment there is a low distortion which enables the steel to be suitable for intricate parts that require machining and welding in which freedom of distortions is a necessity [5]. The stainless steel grade 630 can be applied to several applications including chemical processing components, gears, hydraulic actuators, jet engines, shafts, rocket/missile components, valve stems and wear rings [6]. The stainless steel 630 cannot be deteriorated. This material can be subjected to corrosive environments and it still lasts without breaking down or frequent maintenance. The material is also durable [7].

Machine-ability of the metal is quite easily achieved. The requirements are similar to that of austenitic steels. When 17-4PH is machined it forms gummy chips, although is it soft and ductile in the annealed state its hardness is still high with the application of age hardening. The stainless steel is available in readily forged, machined, welded, and brazed. An interesting fact is that the alloy can be fusion welded with any of the normal processes using 17-4PH filler metal without preheat. After welding has occurred, welded areas should be aged or treated through solution treatment process and then hardened through precipitation. Castings of 17-4PH are produced in sand moulds and by centrifugal casting. Since the material shows good cast-ability it is subjected to hot-tearing thus changes in section size should be avoided. Micro-shrinking is another factor that has to be considered as this decreases ductility but does not have impact on the yield strength. During the heat treatment process care must be taken to avoid contamination from the furnace atmosphere such as carbon or nitrogen. Scaling on the alloy can be minimised by applying air or using vacuum and dry argon. Moreover, if some oxides appear on the surface these can be removed by grit blasting or by abrasive tumbling [8]. As soon as the casts for 17-4PH are complete from the foundry, these should be inspected for any possible defects. The processing through the stages of the foundry should be able to ensure that correct melting and casting procedures are followed in order to prevent absorption of excessive hydrogen by the casts. This gives rise to a case in which the alloy 17-4PH was present in an air wing flap and it failed through fracture because there was presence of hydrogen. Remedies which could avoid this failure include also the inspection procedures to detect any cracks during the plating or painting process of the alloy whilst performing a further test through magnetic particle inspection which should be able to detect the cracks. It is evident that if the flakes where not present in the flap supports of the wing the alloy could have withstood the operating stresses [9]. Alas this misfortune in the aerospace industry, the unique combination of properties within 17-4PH makes the alloy one of the best solutions to many design and production problems [4].

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