In welding, like in many other manufacturing and fabrication processes, the process is not complete until the necessary post-processing has been carried out. Post-weld operations are usually performed for reasons such as improving mechanical properties, relieving stress, and improving aesthetics. The two most common Post-weld operations are Post-Weld Heat Treatment (PWHT) and Finishing.
Post-Weld Heat Treatment
During welding, the welded materials are exposed to very high temperatures that can cause micro-structural changes in them. Also, residual stresses build up in welded materials when they are allowed to cool naturally. If left unaddressed, these stresses and structural changes can severely compromise the mechanical properties of a material and can lead to failure during use. To prevent this, PWHT is required for welded parts. There are two major types of Post-weld heat treatment and they are as follows.
Post heating is usually carried out to prevent Hydrogen Induced Cracking (HIC). HIC results from the permeation of materials by high levels of ambient hydrogen during welding. In Post heating, the welded material is heated to a specified temperature and held at this temperature for a specified period. This process allows the hydrogen to diffuse out of the welded area, thus preventing HIC. Post heating is carried out immediately after welding before the material cools.
In this process, the welded material is first allowed to cool. After this, it is heated to a specified temperature and held at this temperature for a specified amount of time. The material is then cooled gradually at a specified rate. As its name implies, stress relieving is done to relieve stresses that may have built up during welding and natural cooling.
The heat required in both PWHT processes is usually provided by heating systems. While several heating systems can be used for this purpose, Induction heaters such as the Miller Induction Heating Systems are highly recommended. These systems provide uniform heating, precise temperature control, power efficiency, and many more.
It is important to note that most but not all materials require PWHT. The higher the content of carbon or any other alloying element in a material, the more likely it will need PWHT. Similarly, the thicker the material, the more PWHT is required. The required temperatures and time for different materials are covered by codes and standards.
Welding typically results in a visible weld seam along the areas that have been welded. Even machines as highly precise as the Miller TIG welder series produce visible, albeit very small, weld seams. These affect the appearance of the welded part.
Finishing operations are carried out to improve the aesthetic appearance of welded material. In some cases, they are done to improve the dimensional accuracy of a part.
Finishing involves two basic steps; grinding and polishing.
Grinding involves using an abrasive to level the welded area to be consistent with the surface of the material. This can be done using flap wheels, fibre discs, flap discs, or grinding wheels, depending on the type, shape, and size of the welded material, as well as the quality of finishing required.
Polishing typically takes place after grinding. For this process, gentler abrasives are used together with polishing compounds to produce a very neat surface finish. In some cases, it’s impossible to identify the welded joint after finishing.