HEAT TREATMENT PROCESSES
WELDING PROCESS
The welding process applied to metals joins two components together by fusion. The surfaces to be joined are raised locally to melting point by a source of heat provided a variety of welding methods based on electric arc, electric resistance or flame. The process energy creates a localised molten pool into which the consumable is fed, fusing with the component surfaces and/or previously deposited weld metal. As the molten pool is moved along the joint axis the components are heated, non-uniformly and subsequently cooled, also non-uniformly. Neighbouring elements of material try to expand and contract by differing amounts in accordance with the sequence of the localised thermal cycle. A series of heat treatment operations are associated with the welding processes, arising from the need to control these changes. These form the basis of the subject of Heat Treatment Engineering.
PREHEAT AND POSTHEAT
Preheating involves raising the temperature of the parent material locally, on both sides of the joint to a value above ambient. The need for preheat in usually determined by the pertinent fabrication code and verified by the weld procedure qualification test. Preheat may be required as an aid to welding for one of four basic reasons. Post Heat, this is the term given to the extension of preheat on completion of the welding at the same or increased temperature. Its purpose is to effect diffusion of hydrogen from the joint and reduce susceptibility to the associated form of cracking. It is usually applied to the higher strength carbon manganese steels and the low alloy steels where the risk of hydrogen cracking is higher. Post heat treatments are often specified by the client who has incorporated the heat treatment procedure / welding procedure specification.
POST WELD HEAT TREATMENT
Post weld heat treatment (PWHT) is a process commonly referred to as stress relief.
PWHT is a temperature-controlled process applied to a metal or alloy that has been welded. The process is most commonly applied to a metal in order to reduce the brittle hardness in the weld and to reduce the residual stresses that have been created in the metal due to the welding process.
During the process, the weld is reheated at a controlled rate to a temperature below its lower critical transformation temperature. It is then held (soaked) at that temperature for a specified amount of time and then cooled down at a controlled rate.
When the structure is heated uniformly, the yield strength of the material around the weld is unable to support the initial deformation caused during the welding process. The stress distributions at the higher temperatures occur by a diffusion mechanism, relaxing the residual stresses even further and becoming more uniform. On cooling, provided it is carried out in a controlled manner, the improved stress distribution is retained. The extent to which residual stresses are relaxed will depend on the temperature for any given material.
Post weld heat treatments permits some tempering or ageing effects to take place. These metallurgical changes are very beneficial in that they reduce the high hardness of the welded structures, improving ductility and reducing the risks of brittle fracture.
Post weld heat treatment has mandatory significance governed by the national standards and codes, as well as being required to offer acceptable component life in onerous environments.