In thermal cycle of Post Weld Heat Treatment, the weld parts are heated in furnace often at lower temperature range (where material is not liable to phase change), followed by cooling in furnace. Well it’s not that simple. In PWHT heating and cooling is carried out, in controlled mechanized way with specific heating and cooling rates. For this, a number of thermocouples are employed for the monitoring of cooling and heating process. There are four essential elements that must be fulfilled in order to carry out effective post weld heat treatment.
1. Heating Rate
The weld material is allowed to heat with controlled heating rate because on the detrimental side if the heating rate is not monitored, the material will prone to stresses due to high temperature gradient. In order to prevent any high stress level and consequent cracking, it is a recommended practice to heat steel weld parts (for example Carbon Manganese Steels) with heating rate range from 60 – 200 degree Celsius per hour. Normally the monitoring of heating rate is started above three hundred (300) degree Celsius because below this temperature most steels have adequate strength to avoid distortion which can be caused by intensification of stresses. Moreover it is to be noted that heating rate is primarily adjusted according to the weld thickness.
2. Soak Temperature
It is a temperature range where the heated weld part is held for some time, to effectively reduce the stress level. Soak Temperature is set according to the type of material and normally ranges from six hundred (600) to seven hundred and sixty (760) degree Celsius.
3. Soak Time
It is a specific time during which the material is allowed to heat at sock temperature range, in order to provide uniform heat throughout the weld thickness. Most important question here is how to predict the time frame for soaking, to homogenize the heat throughout the material thickness. Well suitable soak time is essential for the reduction of undesirable stress levels and it is determined depending on the joint thickness. Normally the codes call for soak time, one hour 1h per 20-25mm thickness.
4. Cooling Rate
In like manner, cooling rate must also be monitored carefully, in order to avoid higher stress levels and consequent cracking due to large temperature gradients. After giving particular soak time and temperature the material is cooled down in furnace with the regulation of cooling rate by thermocouples. It gives a complete thermal cycle from heating to cooling. Again the monitoring is required from soak temperature to three hundred (300) degree Celsius. Below three hundred degree Celsius the thermally treated weld material has enough strength to countenance the temperature gradients.
PWHT is most common practice in steel fabrication industries and normally carried out using oil or gas burning furnaces. The fuel must be clean from contaminants that may harm the weld job. Type of steel and service applications are the major considerations for applying post weld heat treatment because there are some exceptional applications and grade of materials where PWHT may induce detrimental effects. However its intended use is, to relieve stresses in steel welding that is why it’s commonly referred to as Stress Relieving.