PWHT / Pre Heat Services
GIS has more than 15 sets of PWHT equipment and qualified PWHT technicians who are experienced in providing various Heat Treatment services to the Oil, Gas, Petrochemical, Power, Desalination, Construction, and Manufacturing sectors.
GIS specializes in local post-weld heat treatment of carbon steel and alloy steel piping welds using the electrical resistance method, employing ceramic heater pads and electricity as the heating source for stress relieving weld joints. All heat treatment services are designed to minimize downtime, improve structural integrity, and enhance plant life. Additionally, depending on the mobility of the required equipment, many of our heating processes can be applied on-site or at your facility.
PRE AND POST WELD HEAT TREATMENT (PWHT)
Preheating involves raising the temperature of the parent material locally, on both sides of the joint, to a value above ambient. The need for preheat is usually determined by the pertinent fabrication code and verified by the weld procedure qualification test. Preheat may be required as an aid to welding for one of the following reasons:
- To control the rate of cooling,
- To control the diffusion rate of hydrogen in a welded joint,
- To reduce thermal stresses,
- Compensation for heat loss.
During the welding process, the two metal pieces being joined are subjected to extreme temperatures, causing the crystalline structure of the metal to pass through various metallurgical phases. As a result, hardening (and embrittlement) of the metal can occur to varying degrees, usually dependent on carbon content. Heat treatment is designed to reduce the hardness in the heat-affected zone of the metals and increase ductility in these sections. Various pressure vessel codes specify the procedures for post-weld heat treatment.