Changes Resulting From Subzero TreatmentWhen steel is at the hardening temperature it contains a solid solution of carbon and iron known as austenite. When the steel is hardened by sudden cooling, most of the austenite, which is relatively soft, tough, and ductile even at room temperatures, is transformed into martensite, a hard and strong constituent.
If all the austenite were changed to martensite upon reaching room temperature, this process would be an ideal hardening operation, but many steels retain some austenite. In general, the higher the carbon and alloy contents and the higher the hardening temperature, the greater the tendency to retain austenite.
When steel is cooled to subzero temperatures, the stability of the retained austenite is reduced so that it is more readily transformed. To obtain more complete transformation, the subzero treatment may be repeated. The ultimate transformation of austenite to martensite may take place in carbon steel without the aid of subzero treatment, but this natural transformation might require 6 months or longer, whereas by refrigeration this change occurs in a few hours.
The thorough, uniform heating that is always recommended in heat-treating operations should be accompanied by thorough, uniform cooling when the subzero treatment is applied. The cooling cycle will vary from steel to steel and can vary from no soak time to 40 hours depending on the material composition.
The tool or other part is sometimes surrounded by one or more layers of heavy wrapping or insulation to delay the cooling somewhat and ensure uniformity. After the cooling cycle is started, it should continue without interruption.
Subzero treatment may sometimes cause cracking. Normally, the austenite in steel provides a cushioning effect that may prevent cracking or breakage resulting from treatments involving temperature and dimensional changes; but if this cushioning effect is removed, particularly at very low temperatures as in subzero treatments, there may be danger of cracking, especially with tools having large or irregular sections and sharp corners offering relatively low resistance to stresses.
Order of Cooling and Tempering PeriodsThe order or cycle for the cooling and tempering periods has not been standardized. The usual ranges of preheating and hardening temperatures are given; but for a particular steel, the recommended temperatures should be obtained from the manufacturer.
a) Preheat the steel per the manufacturers recommended heat treating schedule.
b) Heat to the hardening temperature.
(Note: Tests indicate that the effect of subzero treatment on high-speed steel may be influenced decidedly by the hardening temperature. If this temperature is near the lower part of the range, the results are unsatisfactory.
c) Quench in oil, salt, or air, down to a workpiece temperature of 150–200 degrees F and then air cool.
d) For high temperature Cryo Cool in refrigerating unit to temperature of −100 to −120 degrees F right after quenching. Dry ice will achieve the needed temperatures, if a low temperature cryo is needed then liquid nitrogen will get the material down to -312F.
(Note: Tests have shown that a delay of one hour has a detrimental effect, and in ten hours the efficiency of the subzero treatment is reduced 50 per cent. This is because the austenite becomes more and more stabilized when the subzero treatment is delayed; consequently, the austenite is more difficult to transform into martensite.)
e)Remove the tool from the refrigerating unit and allow it to return to room temperature.
f) Temper to required hardness for a period of two hours or as recommended on material data sheet.
Note: The time for the second tempering operation is sometimes reduced to about one-half the time required for the first tempering.
For crack sensitive materials or when recommended by supplier the order of operations will be changes so that the cryogenic cooling cycle is between the two tempering cycles.
Benefits to the cryogenic process are dependent on material but typically include reduced wear and increased hardness by 1-2 points on the Rockwell C scale.