Cryogenic processing

Paul Stratton and David Groome of Linde Group company BOC describe how it’s possible to improve tools with cold treatment.

Cold treatment has many uses in the manufacture of tools such as forging dies. Many manufacturers use it to transform retained austenite as an alternative to multiple tempering, because it shortens the treatment time and reduces costs. When steels are quenched from the austenitising temperature, martensite begins to form at the martensite start temperature. This temperature depends mainly on the alloy content of the steel – for tool steels it is in the range 200?C-300?C. The transformation continues until the tool cools to the martensite finish temperature. The martensite finish temperature, which also depends on alloy composition, may be below room temperature, so high levels of retained austenite may remain if cooling is done to room temperature only.

The combination of hard martensite and very soft austenite often creates problems. As well as reducing overall hardness, retained austenite affects the machinability of the steel. During grinding, there is a propensity to cracking caused by stress transformation. The combination also creates an inherent dimensional instability due to the 4% volume change as the retained austenite transforms during use particularly where when the temperature varies widely. It is therefore necessary to fully transform retained austenite either by tempering or by cryogenic treatment.

The tempering option entails heating above the martensite start temperature then cooling back to ambient up to six times. In the cryogenic treatment option, quenching is continued to a point below the martensite finish temperature, completely transforming the retained austenite to martensite. Usual treatment temperatures are in the range -70?C for low alloy steels down to -150?C for high alloy steels. The treatment should be carried out within one hour of quenching to avoid possible stabilisation of the austenite, requiring an even lower treatment temperature to transform it. This must be followed by a tempering cycle, as after any cryogenic treatment cycle, to ensure that no brittle, untempered martensite remains when the part is put into service.

Experiments were carried out on D2 using various combinations of cryogenic treatments, tempering and austenitising temperatures. They showed that cryogenic treatment at -90?C followed by a single temper was sufficient to convert all the retained austenite to martensite, saving both time and cost.

Deep cold treatment of tools was introduced in the early 1980s. Today it is used extensively in the USA, but is not so popular in Europe. One reason may be that until recently the mechanism for the great improvement in wear was not fully understood. This led to some inconsistencies in the results obtained. It is now generally acknowledged that the improvement in wear results from the formation of large numbers of nano-scale coherent eta-carbides. When martensite is subjected to deep cold around the boiling point of liquid nitrogen (-196?C) for extended periods, dislocations agglomerate into nucleation sites. When the steel is subsequently tempered the fine eta-carbides precipitate at these sites. It is known that no nucleation sites form in the martensite produced by the cooling itself and it is this, together with the variable processing time, that has caused the inconsistency in results.

Thus as deep cold treatment produces eta-carbides in the martensite only, if the tool still retains a large amount of austenite after quenching, subsequent treatment has less effect. To counter this, the tool must be cooled to transform the retained austenite first, to say -90?C for D2, and then returned to room temperature before the deep cold treatment. Similarly, if the treatment time is too short to agglomerate the dislocations, then there is no effect. The minimum treatment time is reported to be 24 hours with further improvements for up to 48 hours or more. When the processing is carried out correctly, some significant improvements have been reported – up to ten times the life for D2 forging dies.

Equipment for cryogenic processing
Processors can use liquid nitrogen effectively to achieve the temperatures necessary for deep cold treatment and to get quick cool down rates. One of the most common techniques is to use a spray header system with atomising nozzles that convert the liquid nitrogen to very cold gas, as the liquid nitrogen flashes to a vapour and warms up. Only cold gas and not fine droplets of liquid should come in contact with the surface of the part being cooled to avoid ‘spot martensite’ formation. It is possible to control the temperature by controlling the nitrogen flow. Direct cooling is the most efficient way to obtain low cryogenic temperatures for controlled processing.

Cryogenic chambers come in a variety of sizes and configurations. The top-loading Cryogenic Box Freezer, Cryoflex CBF offers an economical solution for tool makers and users. By using liquid nitrogen as the cooling medium, the chamber is suitable for cryogenic treatment and deep cold processing. The chamber can be loaded manually or by a hoist or overhead crane. The interior is made of stainless steel, as are all piping and components that are exposed to the liquid or cold nitrogen gas.

Alternatively Cryoflex-CCF has a chamber with a side-hinged door. The height of the chamber can be aligned with automated part transfer equipment that might be part of a heat treatment processing line. The interior is sized to accommodate a standard heat treatment basket. The unit can also be loaded manually if required.

To summarise, cold treatment can be used to transform retained austenite as an alternative to multiple tempering shortening the treatment time and reducing costs. When performed correctly, further cooling down to -196?C can produce significant improvements in the wear performance of tools. Using liquid nitrogen cooling in Linde’s Cryoflex range of cryogenic treatment chambers can meet all your cold treatment needs.

Dr Paul Stratton is Project Manager with the Linde gases and engineering group; David Groome is a Technical Support Specialist in BOC, a member of The Linde Group.

Click here to find out more