Sub-Zero Treatment

Phase Transformation

Evolution of the crystal structure during cooling is driven by the thermodynamic instability of the austenite phase at very low temperatures. As the sub-zero treatment progresses, the remaining austenite, which is soft and non-magnetic, transforms into the much harder and more wear resistant martensite phase. This transformation is a diffusionless process, meaning that the atoms only need to shift their positions slightly to form the new lattice.

The extent of the transformation depends on the temperature reached and the chemical composition of the steel. If the material is not cooled sufficiently, a significant amount of austenite will remain, which can later transform during use and cause the part to change size. This is particularly problematic for precision instruments where even a few micrometers of movement can lead to failure.

By completing the transformation in a controlled environment, the final properties of the part become much more predictable.

Dimensional Stability

Prevention of the spontaneous transformation of austenite over time is the primary reason for incorporating this step into the manufacturing sequence. Sub-zero treatment ensures that the metal does not undergo slow, unwanted changes in volume while it is in service. Retained austenite is unstable at room temperature and can slowly turn into martensite, a process that is accompanied by a small increase in volume.

This expansion can cause internal stresses, warping or the binding of moving parts in precision machinery. In the production of high quality gauge blocks and bearings, this level of stability is a mandatory requirement to ensure the accuracy of the tools. The treatment also reduces the internal stresses within the part, making it less likely to crack during the subsequent tempering or grinding operations.

This contributes to a higher quality finished product with a lower risk of premature failure.

Process Sequence

Integration of the cooling cycle into the heat treatment line requires careful timing and specialized equipment like cryogenic freezers or liquid nitrogen baths. The sub-zero treatment should be applied as soon as the part has cooled to room temperature after quenching to prevent the austenite from becoming stabilized. If there is too much delay, the transformation becomes more difficult to achieve, and the effectiveness of the treatment is reduced.

The parts are typically held at the low temperature for several hours to ensure that the core of the material reaches the target temperature. After the cooling is complete, the part is allowed to warm back to room temperature before it is placed in a tempering furnace. This tempering step is necessary to relieve the stresses created by the new martensite and to achieve the desired balance of hardness and toughness.

The entire sequence is a standard part of the quality plan for high performance steel components.