1. Gas Carburising Hardening
Gas carburising is a surface-hardening process which is carried out at a high temperature - usually above 925°C. The process is usually conducted in a sealed quench furnace in which a carburising gas is introduced. Atomic carbon is generated by the reaction between the gaseous furnace atmosphere and the steel. The carbon diffuses into the metal surface usually to a depth between 1 and 3mm. After carburising the hardness of the carbon enriched surface is developed by quenching, usually in oil. Hardness values typically in the range 58 - 63 HRC are developed in the carburised surface.
Tempering is a low temperature heat treatment process normally performed after a hardening process in order to reach a desired hardness/toughness ratio.
Tempering is the process of reheating the steel at a relatively low temperature leading to precipitation and spheroidization of the carbides present in the microstructure. The tempering temperature and times are generally controlled to produce the final properties required of the steel. The result is a component with the appropriate combination of hardness, strength and toughness for the intended application. Tempering is also effective in relieving the stresses induced by quenching.
Annealing is a heat treatment process used to reduce hardness, increase ductility and help eliminate internal stresses.
Annealing is used to induce softness, to alter ductility, toughness, electric, magnetic or mechanical properties; or to produce a definite microstructure. Steel is annealed by heating the steel to a temperature above the upper critical temperature, holding it there until the temperature is uniform throughout the part, and then slow cooling the part through the transformation range. The heating and cooling rates depend on the composition, shape and size of the part.
Through normalising the steel can obtain a more fine-grained homogeneous structure with predictable properties and machinability.
Normalizing is used to refine the grain structure and to create a more homogeneous structure when a steel is to be reheated for hardening and quenching or full annealing, or to unify the microstructure of the heat effected zone of weldments, segregation in castings and forgings and provide a more uniform structure, and also to provide moderate hardening.
5. Solution Annealing
Solution annealing (also referred to as solution treating) is a common heat treatment process for many different families of materials. Stainless steels, aluminum alloys, nickel-based superalloys, titanium alloys, and some copper-based alloys all may require solution annealing.
The purpose of solution annealing is to dissolve any precipitates present in the material and transform the material at the solution annealing temperature into a single phase structure. At the end of the solution annealing process, the material is rapidly quenched down to room temperature to avoid any precipitation from occurring during cooling through lower temperature ranges where precipitates may form. The single phase solution annealed material will be in a soft state after treatment.
6. Precipitate Hardening
The precipitation hardening (PH) stainless steels are a family of corrosion resistant alloys some of which can be heat treated to provide tensile strengths of 850MPa to 1700MPa and yield strengths of 520MPA to over 1500MPa - some three or four times that of an austenitic stainless steel such as type 304 or type 316. They are used in the oil and gas, nuclear and aerospace industries where a combination of high strength, corrosion resistance and a generally low but acceptable degree of toughness is required. Precipitation hardening is achieved by the addition of copper, molybdenum, aluminium and titanium either singly or in combination.
7. Stress relieving
Stress relieving is applied to both ferrous and non-ferrous alloys and is intended to remove internal residual stresses generated by prior manufacturing processes such as machining, cold rolling and welding. Without it, subsequent processing may give rise to unacceptable distortion and/or the material can suffer from service problems such as stress corrosion cracking. The treatment is not intended to produce significant changes in material structures or mechanical properties, and is therefore normally restricted to relatively low temperatures.
Stress relieving is done by subjecting the parts to a temperature of about 75 ºC (165 ºF) below the transformation temperature,line A1 on the diagram, which is about 727 ºC (1340 ºF) of steel—thus stress relieving is done at about 650 ºC (1202 ºF) for about one hour or till the whole part reaches the temperature. This removes more than 90% of the internal stresses. Alloy steels are stress relieved at higher temperatures. After removing from the furnace, the parts are air cooled in still air.