Cryogenic treatment of steel

Dr. Dmitri Kopeliovich

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
• Benefits of cryogenic treatment
• Applications of cryogenic treatment of steels

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:

• 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).

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• Wear resistance (due to higher hardness and the presence of hard ETA-carbides);
• Mechanical strength;
• Toughness;
• Fatigue strength (due to low residual stresses and homogeneous structure);
• Creep;
• 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);

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• 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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• Annealing and stress relief
• Normalizing
• Hardening
• Case hardening
• Ion nitriding
• Tempering
• Precipitation hardening
• Salt bath heat treatment
• Furnaces for heat treatment
• Basic principles of heat treatment
• Iron-carbon phase diagram

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