jueves, 5 de febrero de 2015

Effect of cryogenic treatment on case-hardening steels. Process alternatives to optimize their final properties.

In the field of material treatment, it is usually considered that cryogenic temperatures are those below 120 K (-153°C). Consequently, conventional subzero treatments, often referred to as shallow cryogenic treatments and usually performed at temperatures around -80°C, cannot be regarded as real cryogenic processes.

Cryogenic temperatures couldn’t be achieved until the late 19th century and, therefore, the emergence of cryogenic treatments in industry is relatively recent. The development of this technology has been based mainly on empirical results. The basic research of the transformations produced in the materials when exposed to cryogenic temperatures is usually conducted with significant delay with regard to development of practical applications.

In general, cryogenic treatments have been considered as separate operations, added to the conventional heat treatments. This is something that has conditioned the development of knowledge in this field, and also the reliability of the results obtained with these processes. Maybe this happens because, very often, this technology is used in tools and finished components, without paying much attention to the previous operations. This approach doesn’t enable a good control over the process results since these depend on the material history before the cryogenic treatment. And, obviously, the previous heat treatments play a crucial role.

In this regard, the consideration of cryogenic treatments as independent operations is a mistake. The right way to contemplate them is not as a supplementary step, but as an integral part of the overall heat treatment process. Only in this way its full potential will be exploited, selecting the route that is most adequate in each case depending on the material considered and the application in which it will be used.

We will try to illustrate it with an example. Let’s consider a case hardening steel like 18NiCrMo5, which is commonly used in applications where high yield strength and good wear resistance are required (shafts, gears, cams, etc.). The heat treatment process of this steel starts with a cementation step in order to increase the carbon content in the surface of the component. The subsequent quenching, followed by a tempering cycle at not more than 200°C, provides a very hard surface while the core remains soft and tough.

When considering the cryogenic treatment of a component made of case hardened steel, two basic strategies could arise. One is to apply it to the already heat treated part, that is, after tempering. The other one is to perform the cryogenic process after quenching but before tempering.

Several investigations focused on studying the effects of cryogenic treatments in this steel grade have been carried out in recent years, but the results seem confusing and sometimes even contradictory. Actually, this happens because in most of these studies only one of the two approaches has been considered, not taking into account that the results that are obtained with each of the treatment strategies are significantly different.

Taking the standard heat treatment as a reference, the results of the studies based on the first route mentioned (cryogenic treatment after tempering) show increased hardness, improved wear resistance and slight increases in tensile strength, but also worse toughness. Moreover, greater fatigue resistance is achieved, accompanied by a marked decrease in the dispersion of the results, which is a very interesting data.

Moreover, the results of the studies conducted on 18NiCrMo5 cryogenically processed between quenching and tempering are somewhat different. Here the cryogenic treatment increases the hardness (as a consequence of a smaller amount of residual austenite), the wear resistance and the dimensional stability. In this case it also increases the toughness but, however, the fatigue resistance worsens significantly.

These considerations concern the cemented layer, since no significant changes were observed in the core, regardless of the approach used for the cryogenic treatment.

In both cases the overall result is clearly positive. Nevertheless, to properly choose the most suitable process route to be used in a specific application, it must be considered the component, the application and, consequently, the most determining failure mode to be faced during operation. If it is foreseen that the material will suffer impacts, it seems that a cryogenic treatment between quenching and tempering is the most appropriate option. However, if fatigue is a determining factor, a cryogenic treatment after tempering appears like the best choice. Moreover, if wear is the main concern, both alternatives are, in principle, valid. In any case, the interest of using cryogenic treatments for increasing the performance of cemented steels seems beyond dispute.

Recent studies conducted in IK4-Azterlan with other steel grades confirm the remarkable influence that the process route has on the results induced by cryogenic treatments. Indeed, cryogenic treatments could even lead to a rethinking of conventional heat treatment schemes for certain materials, thank to innovative and more efficient processes leading to better results.

All this becomes really complex in practice since, for each material, countless combinations of factors such as temperatures, times, number of cryogenic steps, tempering cycles, etc. could be considered. Before subjecting a material to cryogenic treatment, it is convenient to take some time to think about the requirements of the application and how the different process alternatives could satisfy them. Fortunately, very often few simple field tests are enough to assess the results but, sometimes, a more comprehensive study will be required to get more out the huge potential of this novel technology.

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