The Importance of Weld Treatment

Weld treatment goes beyond simply welding processes – it is an integral component of equipment longevity, relieving internal stresses, refining metal structures and increasing mechanical properties for superior reliability and performance.

Temperature regulation is of critical importance in order to achieve optimal results. Without it, over-tempering may compromise metal strength and hardness while under-tempering could result in higher residual stresses, distortion and unintended metallurgical changes.

Vorheizen

Preheat is the practice of applying controlled heat to metal parts prior to welding, such as open flame, gas torch or electric resistance heating coil. Other sources may also provide this service.

Preheating helps prevent cold cracking by slowing the rate of cooling of weld pools/puddles and surrounding base metal, as well as driving away moisture/hydrogen that could otherwise contribute to cracking, redistributing solidification stresses more evenly, and driving off any moisture/hydrogen that might contribute to cold cracking in welds.

Many welding procedures require setting an initial preheat temperature that fits a particular base metal, welding process and section thickness. Maintaining this temperature during welding is key to producing welds with ductile metallic structures and avoiding hydrogen cracking.

Many alloys, such as precipitation hardening steels, require post-weld heat treatment in order to regain their original properties post-weld. This may involve solution annealing or artificial aging; other materials may only require normalizing treatments after welding; which type is determined by code requirements, material properties and service conditions.

Interpass

Temperature management during interpass welding helps prevent cracking and improve weld quality, as it ensures uniform temperatures throughout the weld zone and near to the preheat temperature. In some instances, this may require using a special torch with an adjustable rosebud tip for optimal flame distribution.

Typical weld procedure specifications will specify minimum interpass temperatures and requirements, which vary according to weldment thickness, which can be monitored using digital pyrometers or temperature-indicating crayons such as Tempilstik(r).

Maintaining a consistent interpass temperature will help to minimize cold cracking by expediting hydrogen diffusion out of the weld bead more quickly, and also allow alloy to return from austenite to ferrite more easily between weld passes, improving weld toughness while potentially decreasing postweld heat treatment (PWHT) requirements for precipitation hardening alloys; however, solid solution and corrosion-resistant alloys often require postweld heat treatment that should be completed under guidance by Haynes International.

Wärmebehandlung nach dem Schweißen

Heat treatment post-weld helps mitigate the risks of brittle fracture and hydrogen-induced cracking that are particularly prevalent in areas like oil pipelines. Furthermore, postweld heat treatments help ensure uniform mechanical properties, reduce distortions and fatigue life in critical loads while improving fatigue life in critical loads.

Welding introduces high levels of hydrogen into a material. If the weld is not heated and cooled in an effective manner, this hydrogen may diffuse out of its confines and cause embrittlement or hydrogen-induced cracking (HIC). Post welding heat treatment allows hydrogen to escape safely, thus decreasing risk for HIC.

PWHT can also be used to enhance the microstructure of weld zones through tempering and normalizing processes, improving toughness and resistance to stress corrosion cracking in stainless steel components, for instance. SEM photographs of laser welded NiTi samples show how heat treatment produces smaller precipitates that increase fatigue life [102]. PWHT must be conducted carefully to avoid distortion and uneven stress relief by gradually heating and cooling off weld zones during treatment [103,104].

Quenching

Quenching is the final thermal treatment before welding. This process rapidly cools metal to temperatures below its martensite start point, which reduces tension strength while improving toughness.

Cooling rates depend on the quenchant chosen; air is a popular cooling medium as it is cheap, non-flammable, and offers fast cooling rates; however it may cause distortion to metal parts and is unsuitable for most alloys.

Quenching is an essential step to reduce high residual stresses within the weld zone (HAZ) of fusion-welded pipes. An increase in residual stress concentration increases the risk of cold cracks and fatigue cracks; to mitigate this issue, cooling must occur at an optimal rate to maximize alloying elements dissolved into solid solution while simultaneously minimising distortion and residual stresses – this requires both good gauge control and an established weld procedure qualification specification specification.