Cryogenic treatment of steel
Cryogenic treatment (tempering) is a processing of the material at a temperature below 80K (-315°F / -193°C) resulting in modification of its microstructure and improvement of its properties.
Effects of cryogenic treatment on steel microstructure
Cryogenic tempering of a steel is carried out as a supplemental process following after the conventional heat treatment procedure (Hardening).
Hardening treatment comprises heating the steel above the phase transformation temperature (upper critical temperature), followed by soaking and then rapid cooling (quenching).
When steel is heated above the upper critical temperature, its structure becomes entirely austenitic, which transforms into martensite (supersaturated solid solution of carbon in α-iron) after quenching.
Austenite-martensite transformation is never complete - a certain percentage of austenite is retained in the resulting microstructure. Austenite has face centered cubic (FCC) structure, which is denser than the body centred tetragonal (BCT) structure of martensite. The densities difference causes internal stresses in heat treated steels.
Austenite is softer than martensite therefore high percentage of retained martensite decreases the steel hardness and wear resistance.
Cryogenic treatment results in the following effects of the steel microstructure:
Hardening treatment comprises heating the steel above the phase transformation temperature (upper critical temperature), followed by soaking and then rapid cooling (quenching).
When steel is heated above the upper critical temperature, its structure becomes entirely austenitic, which transforms into martensite (supersaturated solid solution of carbon in α-iron) after quenching.
Austenite-martensite transformation is never complete - a certain percentage of austenite is retained in the resulting microstructure. Austenite has face centered cubic (FCC) structure, which is denser than the body centred tetragonal (BCT) structure of martensite. The densities difference causes internal stresses in heat treated steels.
Austenite is softer than martensite therefore high percentage of retained martensite decreases the steel hardness and wear resistance.
Cryogenic treatment results in the following effects of the steel microstructure:
- Transformation of retained austenite into martensite;
- Internal stresses relief affecting most mechanical properties;
- Precipitation of fine carbide particles (ETA-carbides) uniformly distributed in the martensite grains.
- More homogeneous microstructure due to reduction of micro-voids (pores, cracks).
Benefits of cryogenic treatment
- Wear resistance (due to higher hardness and the presence of hard ETA-carbides);
- Fatigue strength (due to low residual stresses and homogeneous structure);
- Low coefficient of friction (due to higher hardness and the presence of hard ETA-carbides);
- Machining, grinding and polishing finish (due to no/little soft austenite);
Applications of cryogenic treatment of steels
- Cutting tools for different machining operations: sawing, milling, drilling, broaching, turning, slitting, shearing;
- Metal forming tools: dies, molds, punches.
- High precision parts: gauges, guides, shafts;
- Parts of high performance (sport) car engines and transmissions: crankshafts, connecting rods, piston rings, engine blocks, gear parts, camshafts.
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