Bad Soden, Germany,

Hardness and dimensional stability thank cooling energy

Nitrogen cooling ensures efficient transformation of retained austenite into martensite

To obtain wear-resistant and dimensionally stable steel components, the retained austenite in the material must be transformed into martensite as completely as possible after hardening. This step has a crucial bearing on the steel quality, for instance when it comes to the ability of tool steel to retain an edge or the dimensional accuracy of very fine contours in casting moulds. The transformation is usually achieved with repeated heating and cooling over a long period of time. Cryogenic treatment offers a time- and energy-saving alternative, which also enables a more comprehensive transformation of the retained austenite.

Hardening of steel is only possible by first allowing the carbon content to dissolve in the austenite at high temperatures followed by quenching of the work piece in liquid oil. This changes the steel’s crystal structure. The dissolved carbon content, hardening temperature and quenching medium determine the nature of the change as well as the properties of the hardened steel at room temperature. In practice, the quenching oils are usually maintained at a constant temperature of 50 to 70 degrees Celsius so that the heat treatment creates reproducible hardness and dimensional stability .. At these temperatures, however, the austenite is not completely transformed into martensite during quenching. In steels with a carbon content of more than 0.5 percent by weight, the formation of martensite during quenching to room temperature may not take place completely.

After hardening, the proportion of retained austenite in the steel can be up to 20 percent by volume. Retained austenite has an adverse effect on dimensional accuracy, wear resistance and hardness, as well as on processing qualities. It is also undesirable from a safety point of view. It is therefore essential to reduce its fraction in the steel. For these reasons, the cryogenic process is nowadays the method of choice, since it combines reproducibly better results with – compared to the conventional procedure – significantly reduced time and energy requirements.

Additional cooling of the hardened parts to temperatures as low as minus 150 degrees Celsius facilitates the transformation of retained austenite into martensite and, for certain alloys and proportions, makes this process possible at all. The hardened components are cooled in cold chambers under controlled conditions, the temperatures (typically between minus 80 and minus 120 degrees) depend on both the alloy involved and any particular requirements . After cooling they are then re-heated. This procedure should be carried out as soon as possible after hardening and repeated several times. The martensite finish temperature (Mf) is a decisive factor here. It indicates the temperature at which the expected rate of transformation will be achieved.

The key element of the cryogenic process is the injection of liquid nitrogen into the refrigeration area of the cold chamber where fans distribute the cryogenic gas, enabling it to act uniformly on the material.

The entire process of cooling and reheating is programmed and centrally controlled using up to date equipment like Messer’s Cryogen® cold chambers, thus ensuring that all parameters for hardening the steel are precisely maintained. These include the speed and duration of cooling, the time for reaching and holding the prescribed temperature,, as well as the speed and duration of heating.. Controlled heating has the advantage that the entire process takes place in a box without air contact, thus preventing the batch from becoming iced over. Another plus is the rapid transition from the holding to the ambient temperature, which saves a great deal of time. Nitrogen and energy are used very efficiently in modern cooling chambers: less than one kilogram of liquid nitrogen is sufficient to treat one kilogram of material. The use of a vacuum insulated pipe for conveying liquid nitrogen from the tank to the cooling facility allows for a further reduction in energy consumption. It is advantageous to use a piping system which is as short as possible; this should therefore be taken into account when planning the system as a whole.

Difference in quality

Not only is the process particularly cost effective, it also improves quality. Comparing the lifetime of cutter blades for wood machining gives a measure of the benefit of cooling. An Austrian company compared tool steels (for industrial blades for removing bark) with and without subsequent cryogenic treatment. The holding temperature used here was minus 150 degrees Celsius. The cryogenic treatment reduced the compressive stresses in the cutter blade and allowed particularly fine needles of martensite to form – a prerequisite for a resilient crystal structure as well as for increased cutting edge retention. The lifetime of tool steels that undergo cryogenic treatment also increases significantly.

The same is true for work pieces in which the accuracy of fine contours plays a crucial role. Moulds for yoghurt pots, for example, must comply with minimum tolerances, since even small variations can cause disruption to the process of preparing, filling and labelling the plastic pots. Cryogenic treatment increases the wear resistance and dimensional stability of the moulds and thus helps to reduce overall costs. Furthermore, cryogenically treated steels are easier to work with.

Typical products and applications

The benefits of cryogenic treatment are exploited for a wide variety of work pieces and applications. Alongside the tool steel and moulds already mentioned there are other typical examples. Cold treatment is used to improve the dimensional stability, among other things, of case hardened injectors, vacuum hardened plain bearing rings and the blades used in electric shavers. The safety aspect is also important in the case of specialised folding bellows,. Durability and safety are at the forefront in applications such as safety chains for the tyres of large vehicles.

Characteristic features of the martensitic transformation

The diffusionless and exothermic transformation begins with the nucleation of martensite at lattice defects in the austenite (dislocations, grain boundaries). The crystal volume increases by three to four percent after the transformation. Depending on the chemical composition of the alloy, a specific content of retained austenite remains. The final extent of transformation is primarily determined by the cooling profile applied.

Effects on the proportion of retained austenite during hardening and tempering

How much retained austenite remains in the material depends on numerous factors. The hardening temperature, the holding time as well as the level of carbon activity in the furnace atmosphere during the heat treatment play a role. During quenching, the type of quenching oil, the oil temperature, the rate of quenching and the oil circulation are factors alongside the work piece parameters (material, geometry, etc.). In the batch removal phase, the immersion time, the draining time, the quenching chamber temperature, and the storage and cooling times are significant. In the cleaning phase, important factors are the temperature of the cleaning solution and steam/spray treatment, as well as the drying and total cycle times. During cold treatment, significant factors are the rate of cooling, the temperature and its distribution, the holding time, heating up and tempering. In order to achieve reproducible quality, treatment specifications that include a work plan as well as monitoring of the process flow and the final quality are essential.

Safe, rapid cooling in every situation

Messer cold chambers enable efficient and reproducible cold treatment in which the customer’s specific requirements can be exactly implemented. They are characterised by high cooling efficiency and operational safety. If the chamber is opened, for instance, the coolant supply automatically shuts off. All of our cold chambers are delivered ready for connection, which speeds up their initial operation at the desired location and thus ensures that all their advantages can be exploited immediately. In addition, the cold chambers can be rapidly and easily redeployed to a different location with minimal installation timel.

The injection of liquid nitrogen permits very efficient and cost-effective operation. It ensures that the cooling temperature is achieved quickly and uniformly. At the same time, a very rapid change in temperature and heating for a quick batch change is possible. Accurate control of the process steps is facilitated through processor controlled temperature regulation. . The high quality construction of the chambers guarantees a long service life. They are also suitable for high loading capacities. Last but not least, the size of the nitrogen tanks can also be specified to precise customer requirements. Experts from Messer consult with customers to determine their product quality needs and seek out appropriate solutions. Analysis of the processes and identification of the potential for improvement are used as a basis for the planning and design of the cold treatment. Once the process has been introduced, long term support and service are an integral component of our business relationship.