Heat treatment is a process where metals such as Inconel, stainless steel and aluminium are heated and cooled in a fully controlled environment, to change their microstructure and to bring these metals closer to their equilibrium state, or bring out more of their physical and mechanical characteristics.
There are different heat treatment processes available, this page only outlines the annealing for stress relief of residual tensile stresses due to welding. In our technical section, other types of heat treatments are discussed.
Why are stresses put in the material?
The residual tensile stress from welding inconel, stainless steel or aluminium is created because the weld consumable is often applied in a liquid state.
The welding process consists of heating up the metal and applying the weld in its hottest, most expanded state. When the much cooler consumable material is bonded to the base material, the weld tends to cool rapidly and will attempt to shrink from the ‘drop-in’ temperature.
Because the base material is usually much stronger and not in a molten state, this cannot shrink, leaving the material remaining in a highly stressed “tensioned” state.
This zone is usually just next to the weld joint, which explains why a crack may appear close to the weld.
Typical heat treatment behaviour
The graph below shows a typical situation of a welded metal component before and after heat treatment.
This graph does not refer to any specific material and is purely for illustration purposes
As can be seen from the graph, the residual stresses in the welded metal are positive, which puts the outer surface layer in a tensional state. When the metal is heat treated in a controlled environment to stress relieve the residual stresses, we can see that the metal reaches an equilibrium state and the residual stresses are close to zero.
A generic heat treatment furnace, image courtesy of Tamworth Heat Treatment
Combining heat treatment with shot peening
Heat treating an exhaust system can enhance its properties. Annealing can soften a part and make it more flexible and less prone to cracking, while other treatments may make it stronger.
Heat treatment is typically carried out overnight although some processes may take significantly longer.
Combined with shot peening, which sometimes follows heat treatment on the same day, the material properties may be enhanced even more, as illustrated in the above graph.
GoodFabs can manage both these processes as part of an exhaust programme for a race team or engine builder.
Inspection is normally required both before and after heat treatment and shot peening, so GoodFabs coordinates both procedures prior to final delivery of the finished and inspected part to the customer.