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The process of heat-treating metals on the industrial scale primarily serves to improve the overall properties of the metals required for their future use. Such metal properties can include toughness, hardness, ductility, machinability, refined grain structure, removal of residual stresses and an overall improvement in the wear resistance. The specific techniques used to both heat and cool metals are not only specific to the metal of interest, but also the applied industry.
It is estimated that over 80% of all industrial heat treatment processes are performed on steel, however a number of other ferrous and non-ferrous metals are also heated and cooled for similar purposes. Ferrous heat-treated metals often include cast iron, alloys, stainless steel and tool steel, whereas some heat-treated non-ferrous metals include aluminum, copper, brass and titanium.
Hardening and Tempering of Metals
- Neutral Hardening: Typically performed on steel, this heat treatment process initially involves quenching and tempering of the metal to retain a final martensite or bainite microstructure. Neutral hardening of steel can improve the heavy loading potential of the metal while simultaneously improving its strength, toughness and sometimes even its temperature resistance. As compared to other hardening processes, neutral hardening ensures that the natural chemical composition of the steel surface remains unchanged.
- Induction Hardening: This heat treatment process is specifically geared towards improving the mechanical properties of ferrous metals such as carbon steels, alloy steels, stainless steels, cast iron, ductile iron, malleable iron or powder metal. To improve the wear resistance, surface hardness and overall fatigue life of these metals, induction hardening creates a more hardened surface without interfering with the core microstructure of the metal.
Annealing
To soften metals in an effort to improve their overall ductility, annealing is a form of heat treatment that heats metals past their temperature to ultimately change their microstructure. Following such high heat exposure, metals are then slowly cooled to allow for further processing while eliminating any potential for cracking of the metal. While steels are most often annealed, other metals to undergo this process can include copper, aluminum and brass.
Soft annealing aims to heat metals to a point where they become extremely soft, thereby allowing for an easier method to manipulate the overall structure of the metal. High-alloy steels that contain higher carbon contents, as well as copper and brass alloys can undergo soft annealing
Tempering
Metals are often tempered to improve their overall hardness and toughness following other heat treatment processes such as neutral hardening and other hardening processes to achieve a desired hardness/toughness ratio of the metal’s final product. A process that typically takes place following quenching or air cooling operations, tempering can also be utilized to relieve any stresses induced to the metal by the quenching process. Depending upon the type of steel that undergoes tempering, a specific temperature range is often utilized to acquire specific metal properties. These temperature ranges include:
- 160-300°C: Low temperature tempering; often utilized for case hardening components and cold working tool steels.
- 300-500°C: Most often used to temper spring steels or similar applications.
- > 500 °C: High temperature tempering; used to quench and temper steels that will be applied for hot working tool and/or high-speed steels.
Quenching
Quench hardening, or quenching, involves heating metals past their upper critical temperature and rapidly returning that metal’s temperature back to room temperature by placing the hot part in water, oil or another suitable liquid to fully harden the structure. By eliminating a lengthy cooling process, quenching alters the microstructure of the metal to produce a uniform hardening of the final product.
- Press Quenching: This form of quenching is highly favorable when heat-treating metals that will be applied for large round or flat components, such as those found in gears, hearing races, wind power generation, saw blades, etc.
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References:
- “Heat Treatment” – Bodycote
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