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for castings and thick forgings to reduce quenching thermal stresses.


In general, as the temperature of water is raised, the stability of the vapour phase increases, and the onset of nucleate boiling in a stagnant fluid is suppressed. The maximum rate of cooling is decreased, and the overall rate of cooling is also decreased.


Figure 1. Distortion of an F/A-18 wing spar due to improper quenching and racking technique.


An extreme case of distortion is shown in Figure 1.


Aluminium is extremely prone to distortion. During solution heat treatment, temperatures are used that are very close to the liquidus temperature. This requires close control over the furnace temperatures, and the uniformity of temperatures within the furnace. This is because aluminium has very high plasticity, and low strength at typical solution heat treating temperatures [2]


.


In addition to the poor strength at elevated temperatures, aluminium also has a large coefficient of linear expansion. This results in large growth of aluminium during solution heat treatment and contraction during quenching. If the part is constrained, then high strains and stresses are developed in the part. If these stresses exceed the yield strength at temperature, then permanent setting of the part could occur, resulting in distortion. This indicates that racking and constraint of the parts are important to control the distortion of the part.


While aluminium has a high thermal diffusivity, the large quench rates necessary to achieve properties result in large thermal gradients centre to surface and surface to surface. These gradients, with different thermal properties contribute to aluminium distortion during quenching. If the thermal gradients are large (centre to surface or surface to surface), distortion can occur. This means that a uniform temperature is required across the part. This implies that agitation and more importantly, the uniformity of agitation around the part is critical for proper distortion control.


Water is the most common quenchant for all aluminium alloys. It is easy and inexpensive to obtain, and can be carefully disposed of unless severely contaminated. Water is used as a quenchant from ambient temperatures for sheet metal, up to 90°C


Cold water quenching is the most severe. In an early study using cooling curves [2], it showed that quenching into still water caused rapid heat transfer. This study showed that heat transfer at the surface of the part was very turbulent at the metal/water interface. This study also showed that there was a marked difference between hard water and distilled water. Distilled water showed an extensive vapour blanket that extended to very low temperatures.


The cooling rate of water quenching is independent of material properties like thermal conductivity and specific heat. It is primarily dependent on water temperature and agitation [3]


. Water temperature is the


largest primary variable controlling the cooling rate. With increasing water temperature, the cooling rate decreases. The maximum cooling rate also decreases as the water temperature is increased. In addition, the temperature of maximum cooling decreases with increasing water quench temperature. The length of time and stability of the vapour barrier increases, with increasing water temperature. This is shown in Table 1.


Table 1. Effect of water temperature on cooling rates [4]


.


Quenching into water at less than 50-60°C often produces non-uniform quenching. This non uniformity is caused by relatively unstable vapour blanket formation, and manifests itself as spotty hardness, distortion and cracking. Because of this difficulty, it was necessary to develop an alternative to water quenching. Polyalkylene Glycol (PAG) quenchants were developed to provide a quench rate in between that of water and oil. By control of agitation, temperature and concentration quench rates similar to water can be achieved.


Continued on page 12 LUBE MAGAZINE NO.149 FEBRUARY 2019 11


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