Post Heating After Welding

Post heating following welding helps enhance structural integrity by rapidly raising temperatures before allowing them to drop quickly, relieving residual stresses, avoiding distortion, and encouraging better ductility.

Post-weld heat treatment (PWHT) is an integral component of fabrication processes, helping reduce residual stresses to levels below material yield strength.

Stress Relieving

Welding exposes materials to high temperatures and causes microstructural changes which introduce residual stresses into materials. If left unaddressed, these stresses can become damaging and lead to distortion or structural failure; one way of alleviating them is post heating the weld area, says Smith.

Heating can be performed using a furnace, electric resistance heating elements or induction equipment and should remain above the lower transformation temperature of the material. The heating must be uniform to ensure an ideal result.

HIC, Hydrogen Induced Cracking), can be prevented through postweld heat treatment by keeping hydrogen from diffusing into the weld and diffusing more slowly, thus decreasing shrinkage stresses and providing time for hydrogen dissipation, decreasing under-bead cracking risk. Postweld heat treatment also serves to normalize weld structures to improve strength and toughness in marine environments or weather extremes; postweld heat treatment is especially useful when welding structures will be exposed to harsh environmental conditions like marine environments or extreme weather.

Normalizing

Normalizing restores welded material back to its natural state by eliminating internal stresses and irregular grain structures caused by previous manufacturing processes. This reduces weld brittleness while simultaneously increasing toughness and strength; furthermore, this process improves machinability allowing more workable material.

This process may involve the use of gas burners, oxy-gas flames, electric blankets, induction coils or furnace heating for optimal results. Heating temperatures, soak periods and cooling rates must all be carefully managed as uneven temperatures could result in higher residual stress levels, distortion or undesirable metallurgical changes.

Industry codes, such as ASME pressure vessel and piping codes, require PWHT for welds of certain thickness. PWHT is an effective technique that prevents hydrogen-induced cracking within the weld area to extend its lifetime while simultaneously preventing cold cracking in pipes. Furthermore, PWHT can be used on materials formed via other processes (for example forming).

Tempering

Tempering is often necessary to reduce internal stresses and hardness induced by high temperatures in weld areas, which could otherwise lead to distortions during machining or even pressure failures.

Post weld heat treatment methods typically consist of heating the metal to an elevated temperature and holding it there for a predetermined duration, allowing trapped hydrogen atoms to escape, thus preventing embrittlement and corrosion in its entirety.

Critical applications require advanced PWHT techniques for precise thermal control. These may involve controllable heating systems with thermocouples attached for monitoring purposes or induction heating, using coils of electrical resistance wires enclosing metal being treated, powered by an alternating current that induces magnetic fields that cause eddy currents to form within it – providing much more precision than more traditional gas or oil burners used with traditional PWHT techniques.

Annealing

At Weldcrafters, we use various annealing techniques, such as solution annealing and normalizing, to increase the strength and durability of welds. Tempering is also utilized to help ensure they withstand repeated stresses, vibrations and impacts without cracking under repeated strain.

Controlling preheat, interpass and stress relief temperatures helps mitigate thermal shock by preventing localized heating of materials at various temperature zones. This significantly decreases the possibility of new stresses or distortions being created within heat affected zones while upholding structural integrity throughout a weld.

Proper post-weld heat treatment reduces hydrogen embrittlement, which is responsible for cracking in high strength and thick materials. To achieve this goal, techniques like hydrogen bake-out are utilized, which remove hydrogen atoms from the surface material so they cannot diffuse into the weld and cause cracking or damage. We typically utilize an induction heat source which quickly and precisely heats materials compared with burners or resistance heaters.