Post Weld Heat Treatment
Enkon provides 2 major heat treatment services for our clients.
2-Post Wed Heat Treatment
Heat treatment is a method used to alter the physical, and sometimes chemical, properties of a material. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.
Heat treatment techniques include annealing, case hardening , precipitation strengthening, tempering and quenching. It is noteworthy that while the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot welding.
Preheat, as defined within the AWS Standard Welding Terms and Definition, is “the heat applied to the base metal or substrate to attain and maintain preheat temperature”.
Preheating may be performed by the use of gas burners, oxy-gas flames, electric blankets, induction heating, or by heating in a furnace. For good results, it is essential for the heating to be uniform around the joint area .Intense, non-uniform heating is of little use in retarding cooling and may be detrimental in causing higher residual stresses, distortion, or undesirable metallurgical changes in the base material.
When preheating is specified, the entire weld joint should be heated evenly through the material thickness to the desired minimum temperature. To obtain a uniform temperature through the material thickness, it is desirable to apply the heating sources to one side of the material surface and to measure the material temperature on the opposite side. Whenever the heating and temperature measurement must be conducted from the same surface, the inspector must assure that more than just the surface of the material has been heated. It is important to ensure that the entire material thickness has been heated to a uniform temperature.
2-Post Weld Heat Treatment
A number of different types of post-weld heat treatments are used for different reasons and for
- a) Post-weld heat treatment is most generally used for stress relief. The purpose of stress relieving is to remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary in order to reduce the risk of brittle fracture, to avoid subsequent distortion on machining, or to eradicate the risk of stress corrosion.
b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is generally performed after the weld has cooled, but under certain circumstances, it may be necessary to perform this treatment before it has cooled to prevent cracking.
c) Extremely coarse weld structures in steel, such as those obtained with the electro-slag welding process, may require normalizing after welding. This treatment will refine the coarse grain structure, reduce stresses after welding, and remove any hard zones in the heat-affected zone.
d) The precipitation hardening alloys, such as the heat treatable aluminum alloys, are sometimes required to undergo post-weld heat treatment to regain their original properties. In some cases, only an aging treatment is used, although a full solution heat treat and artificial aging treatment will provide better recovery of properties after welding.