The Importance of Post Weld Treatment

Post weld treatment of steel and metal fabrications may be required in many instances to reduce residual stresses caused by welding, improve metallurgical properties of the material and minimize corrosion or fracture risk.

Thermal tempering treatment may be required on alloy steels to achieve an acceptable metallurgical structure.

Stress Reliever

Stress relief is the go-to technique for relieving internal stresses in welded fabrications. This process works by heating steel at slightly elevated temperatures to lower its yield point and allow any residual welding stresses to redistribute elastically.

Reducing internal stress levels is vital in order to avoid distortion in welded structures and to preserve optimal metallurgical properties post welding, so techniques like preheating, keeping interpass temperatures within limits (interpass temperature control), and post weld heat treating are utilized as methods for stress reduction.

PWHT reduces residual stress levels, hardness, and tempering levels in weld metal to improve its ductility, strength and resistance to brittle fracture failure under service conditions. It is required by many pressure vessel and piping codes. Mechanical methods of stress relief like vibration stress relief may be helpful but don’t offer the metallurgical advantages provided by thermal treatment; additionally they may not work for all components or weldments with complex shapes.

Stress Corrosion Cracking (SCC) Prevention

Stress Corrosion Cracking (SCC) is an increasingly frequent form of corrosion damage to pipelines and steel structures. SCC involves transgranular cracking that manifests itself by pitting of surface oxide film oxide layer and eventually leading to fracture[1.

Post weld heat treatment (PWHT) provides significant protection against SCC in chloride-rich environments by reducing hardness of as-welded structures and increasing ductility. To accomplish this effectively, heating the material to specific temperatures for an agreed upon amount of time is often employed; for each steel material this should be done independently with professional advice to determine the right time and temperatures to use during PWHT treatment.

Locally Applied Heat (LAH) is an energy-efficient method of PWHT that’s suitable for welding elongated components like pipes or long pressure vessels, using an induction heating element to apply heat locally at the weld area. LAH avoids distortion that might otherwise occur from one point of intense heating by spreading out its effects evenly over its weld area.

Tensile Strength and Ductility

Tensile strength and ductility are critical elements for keeping structures safe and secure, but corrosion can diminish these characteristics, leaving components more susceptible to stress cracking under dynamic loading conditions than ever.

Post weld heat treatment (PWHT) helps strengthen welded structures by reducing residual stresses and improving their ductility and toughness. This is accomplished by heating material below its transformation temperature for several hours at an elevated temperature before slowly cooling it back down to room temperature.

Ductility of materials refers to their ability to deform plastically before fracture. A material can be considered ductile if it can deform under large strains without cracking and can be distinguished from brittle materials by the shape of its stress-strain curve. Tensile tests offer one way of measuring this property, but for more meaningful measurements it may be beneficial to conduct other types of mechanical tests instead.

Hydrogen Induced Cracking (HIC) Prevention

Maintaining high preheat temperatures during welding and post heat or “bake out” are effective strategies for avoiding HIC. This allows hydrogen in the weld metal and HAZ to diffuse away from areas with higher stresses, and reduces the amount of trapped hydrogen which could eventually crack under pressure.

HIC can typically be detected using various nondestructive testing (NDT) methods, including wet fluorescent magnetic particle testing, shear wave NDT techniques and ultrasonic examination. Regular inspection and HIC prevention measures must be in place in order to maintain steel structures’ integrity.

Excessive time or temperature for stress relieve post weld heat treatments can also contribute to HIC. Excessive temperatures can create thermal gradients between areas being post weld heat treated and areas that aren’t, creating an ideal environment for HIC. To minimize this possibility, carefully arrange components in the furnace while making sure there are enough thermocouples present within each component to prevent flame impingement.