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.
- Heating Rate
- Soak Temperature
- Soak Time
- Cooling Rate
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.
Last Words
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.