The word quenching means: Rapid cooling from a high temperature by immersion in a liquid bath of oil or water.
About 85% of quenching fluids are mineral oil based as the quenching power of water falls rapidly with increase in temperature, whereas that of oil remains fairly constant up to the range 60°C to 80°C. The rise in temperature of oil produces drop in viscosity, a change which assists cooling and so offsets the reverse influence of rise in temperature itself, whereas water does not possess such compensating effect.
Quenching is the process by which the desired mechanical properties and hardness of a metal is achieved by controlled cooling from an elevated temperature using a quench fluid. Quenching is usually used with metals that are alloyed with small amounts of other metals. At high temperature the alloying metals are dissolved in the base metal. If the material is cooled slowly, the alloy elements have time to precipitate out separately. If the metal is quenched, the alloying metals are trapped within the crystal grains, making them harder.
Optimal quench uniformity is essential if the potential for cracking, distortion, residual stress and spotty hardness is to be minimized. This means that heat transfer must be as uniform as possible throughout the quenching process. Further optimal uniformity of quenchant flow around all surfaces is necessary. One of the most important factors affecting quench uniformity is the design of the quench system. Agitation is one of the most critical areas of system design. Propeller agitation of the quench oil is necessary if uniform heat transfer throughout the quenching operation, producing less cracking and distortion has to be achieved.
The performance of quenching oil can be characterized by its ability to extract heat from the part surface. Previously, aqueous quenching media were employed. The aqueous fluids provided extremely rapid cooling setting up excessive amounts of internal stress in the object. Mineral oil based fluids avoided this difficulty. Further, they possess sufficient durability, cleanliness and consistency. Durability refers to additive lifetime after being exposed to thermal stressing while consistency relates to constant additive performance over time and cleanliness relates to amounts of deposits on the work piece and/or staining thereof.
To meet the demands of modern heat treatment, mineral oils have been modified to give optimum cooling rates for alloys. They are usually classified as under:
Accelerated cold quench is the largest sector; bath temperature is less than 80ºC. These oils are used for large components and alloys where distortion or cracking is not a problem.
Marquenching involves quenching steel in hot oil, holding it in the quenching bath until the temperature throughout the work piece becomes uniform, and then cooled in air at a moderate speed to keep the temperature gradient minimal. The main advantage of this process over normal quenching is that this process reduces internal stresses, distortion, stress cracks and also increases the impact resistance. The oil baths used in marquenching are maintained at a temperature usually in the range of 150°C to 200°C.
The basic criteria for selection of quenching oils are:
The quench oil must be continuously agitated to provide uniform surface microstructure. The quench tanks must be periodically cleaned and the quench oil filtered to remove scale, metal shavings and contaminants. For proper filtration a 50-micron double-bag filter is recommended.
When red-hot steel is plunged into the quenching oil, some oxidation of the oil is inevitable, and in the course of time the resulting changes due to oxidation are bound to cause thickening of the oil and formation of sludge. Oils of inadequate chemical stability will suffer rapid degradation. In some cases the quenching oil may require to be changed completely.
It is therefore recommended that condition monitoring by oil analysis is essential to maintain optimum performance of the quenching oil and get the desired results.
Manufacturers can only suggest how to care and get the best results. The end-user must realize that manufacturers can only institute certain controls on the chemistry of quenching oils. The conditions under which it is used are the most important factor.