Heat Treatment Services
Preheat is the term associated with the application of heat to a metal component prior to and during welding. There are a number of reasons that preheat may be required, including reduction of residual stresses after the component has been welded, and minimizing absorption of Hydrogen into the weld during welding. Preheat temperatures are generally relatively low, ranging typically from 50°C up to 250°C.
POST WELD HEAT TREATMENT
Post Weld Heat Treatment (PWHT) is the term used to describe the heating of an entire weld to a temperature high enough to reduce the residual stresses within the weld. PWHT temperatures are typically in the range of 600 – 700°C. There are a variety of application methods for heat to weld geometries in order to achieve the desired temperature, ranging from locally applied electrical heaters, to larger gas fired furnaces. The method used is usually determined by the fabrication geometry, size, access restrictions and site constraints
ANNEALING Full Annealing is the process that involves heating steels above their ‘transformation’ temperature and cooling very slowly, which results in the softening of the steel, to make it more suitable for working with.
Stress relief is a term commonly used to refer to Post Weld Heat Treatment; however, it should always be qualified further, as most heat treatments have the effect of reducing stresses.
Normalizing is a similar process to annealing, except that after heating to the desired temperature (commonly around 900°C for typical carbon steels) the steel is cooled in still air, which results in a much more rapid cooling. This has the benefits of improving the grain structure of the material following welding or cold working and reducing hardness and improving machinability of the material.