CRYOGENIC HEAT TREATMENT SREEBHASH S KUTTY I PhD, ME10F07

What is cryogenic heat treatment? Cryogenic heat treatment is the ultra low temperature processing of materials to enhance their desired metallurgical and structural properties The temperature is about -196°C or 77°K Ultra cold temperatures are achieved using computer controls, a well-insulated treatment chamber and liquid nitrogen (LN2) They are completely environmentally friendly and actually help reduce waste

The process is capable of treating a wide variety of materials, such as ferrous and non-ferrous metals, metallic alloys, carbides, plastics (including nylon and Teflon) and ceramics The entire process takes between 36 to 74 hours, depending on the weight and type of material being treated Strict computer control and proper processing profiles assure that optimum results will be achieved with no dimensional changes or chance of thermal shock The process is not a surface treatment; it affects the entire mass of the tool or component being treated, making it stronger throughout

The hardness of the material treated is unaffected, while its strength is increased Other benefits include reduced maintenance, repairs and replacement of tools and components, reduced vibrations, rapid and more uniform heat dissipation, and improved conductivity A tight control of the temperature curve is required Each process requires a different curve, some remain at -195C for a number of hours and are slowly brought back to room temperature Some materials require reheating to temper the material after cryogenic hardening

Slowly cooling a tool steel to deep cryogenic temperatures and soaking it at this low temperature for a number of hours changes the material's microstructure In ferrite steels, it is the transformation of austenite, a large soft crystal, into martensite, a smaller, harder, more compact crystal The amount of Martensite formed at quenching is a function of the lowest temperature encountered As the temperature reduces to -185C ɳ-carbides start to grow throughout the structure The net result is that the crystal structure is transformed with the boundary adhesion between the various crystal elements

Almost all of the austenite retained in the steel after heat-treating is transformed into a harder form, martensite, by the deep cryogenic process An additional result of a deep cryogenic "soak" is the formation of fine carbide particles, called binders, to complement the larger carbide particles present before cryogenic treatment Untransformed austenite is very brittle and can cause loss of strength or hardness, dimensional instability, or cracking Most medium carbon steels and low alloy steels undergo transformation to 100 % Martensite at room temperature. High carbon and high alloy steels have retained Austenite at room temperature

The martensite and fine carbide formed by deep cryogenic treatment work together to reduce abrasive wear The fine carbide particles support the martensite matrix, making abrasions and scuffing of the cutting tool less Cryogenic processing also relieves residual stresses in metals and some forms of plastics All metals including copper and aluminum, benefit from the residual stress relief that cryogenic treatment promotes Care should be taken to well control the cooling of the metal to lower temperature to avoid thermal shocks

Benefits of Cryogenics Promotes a more uniform micro-structure Reduces abrasive and adhesive wear Permanently changes the structure of the metal resulting in improved machining properties Improved thermal properties Better electrical properties including less electrical resistance Reduced coefficient of friction Less creep & walk, and improved flatness for critical tolerance parts Easier machining, polishing and grinding for better edges and finishes

Reduce the frequency and cost of tool remanufacture Substantially reduce machine downtime caused by tool replacement Improved surface finishing on material being manufactured with treated tooling. Treated tooling stays sharper and in tolerance longer that untreated Reduces catastrophic tool failures due to stress fracture Stress relieves to reduce inherit/residual stress caused by manufacture Increases the overall durability of the treated product

Cryogenics should not be considered to replace the heat treatment, it is a complimentary treatment that enhances what took place during the heat process Cryogenic Processing is not a substitute for heat- treating if the product is poorly treated cryogenic treatment, overheated during remanufacture or overstressed during use This will result in destroying the temper of the steel which is developed during the heat treatment process rendering the cryogenic process useless Cryogenic treatment is an additional treatment to heat- treating

After cryogenic treatment the metals are taken out of the cryogenic equipment and tempered in a proper tempering oven to stabilize the newly formed martensite The process will not work on all metals to improve wear characteristics If the carbon content is too low, or the proper heat treatment is not done correctly, the results may not show any value at all, or may even show the contrary characteristics But controlled cryogenics processing can act as a stress relief in any circumstances

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

Before processing After processing Comparative microphotographs (1000x) of steel samples show the change in microstructure produced by the controlled deep cryogenic process. Uniform, more completely transformed microstructure and less retained austenite at right, is related to improvements in strength, stability and resistance to wear Before processing After processing