Welding is an essential part of operating and maintaining assets in the petroleum (upstream, midstream, downstream) and chemical processing industries. While it has many useful applications, the welding process can inadvertently weaken equipment by imparting residual stresses into a material, leading to reduced material properties..

In order to ensure the material strength of a part is retained after welding, a process known as Post Weld Heat Treatment (PWHT) is regularly performed. PWHT can be used to reduce residual stresses, as a method of hardness control, or even to enhance material strength.

If PWHT is performed incorrectly, or neglected altogether, residual stresses can combine with load stresses to exceed a material’s design limitations. This can lead to weld failures, higher cracking potential, and increased susceptibility to brittle fracture.

Hydrogen induced cracking (HIC) often occurs when high levels of ambient hydrogen permeate into a material during welding. By heating the material after welding, it is possible to diffuse hydrogen from the welded area, thus preventing HIC. This process is known as post heating and should begin immediately after the weld is completed. Rather than being allowed to cool, the material needs to be heated to a certain temperature depending on the type and thickness of the material. It should be held at this temperature for a number of hours dependent on the thickness of the material.

By the time it’s complete, the welding process can leave a large number of residual stresses in a material, which can lead to an increased potential for stress corrosion and hydrogen induced cracking. PWHT can be used to release these residual stresses and reduce this potential. This process involves heating the material to a specific temperature and then gradually cooling it..

Whether or not a material should undergo PWHT depends on a number of factors, including things like its alloying system or whether it’s been subject to heat treatment previously. Certain materials can actually be damaged by PWHT, while others almost always require it..

In general, the higher the carbon content of a material, the more likely it needs PWHT after welding activities have been conducted. Similarly, the higher the alloy content and cross-sectional thickness, the more likely the material is to need PWHT.

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Post Weld Heat Treatment (PWHT), or stress relief as it is sometimes known, is a method for reducing and redistributing the residual stresses in the material that have been introduced by welding.

Stress Relieving is the treatment of a metal or alloy by heating to a predetermined temperature below its lower transformation temperature followed by cooling in air. The primary purpose is to relieve stresses that have been absorbed by the metal from processes such as forming, straightening, machining or rolling.

Stress the sources are numerous, but fortunately there are a variety of ways to relieve it. No, I'm not talking about the hectic pace of our personal lives, although it certainly can apply, but instead I'm referring to the fabrication of metals.

Residual stress is an internal stress that is not a result of externally applied loads. If stress buildup in the weldment is excessive, the fatigue life of the metal is reduced.

Cold working, hot rolling, grinding, quenching treatments, welding, and thermal cutting all can induce residual stress into metal. The nature of residual stress, its distribution, and prediction of the level within a metal is a complex and not completely understood phenomenon, but you can be sure it is present.

Welding, in particular, because of the rapid thermal expansion and contraction created along a very localized area, is a prime source of residual stress. A very high heat source is applied to a small area relative to the cooler surrounding area. That point where the arc is directed is rapidly heated from ambient temperature to temperatures that can be in excess of 3000°F. The metal expands as it is brought to a molten state. As the molten weld pool solidifies along the joint, there is resistance to its shrinkage by the already solidified weld metal and the unmelted base metal adjacent to the weld. This resistance creates a tensile strain in the longitudinal and transverse directions of the weld. Distortion is often the result, and if the stress is excessive, buckling, stress corrosion cracking, and shortened fatigue life are possible.

All welds will have some residual stress, and it will never be totally reduced to zero strain. But the level of stress can be very high depending on certain conditions. Heat input, base metal thickness, cooling rate, restraint of the weldment, and welding process play roles in the level of residual stress induced into a weldment.

Stress relief heat treatment is used to reduce the stresses that remain locked in a structure as a consequence of manufacturing processes.

There are many sources of residual stresses, and those due to welding are of a magnitude roughly equal to the yield strength of the base material. Uniformly heating a structure to a sufficiently high temperature, but below the lower transformation temperature range, and then uniformly cooling it, can relax these residual stresses. Carbon steels are typically held at 1,100 to 1,250°F (600 to 675°C) for 1 hour per inch (25 mm) of thickness.

For example, when a component with high residual stresses is machined, the material tends to move during the metal removal operation as the stresses are redistributed.

After stress relieving, however, greater dimensional stability is maintained during machining, providing for increased dimensional reliability.

In addition, the potential for stress corrosion cracking is reduced, and the metallurgical structure can be improved through stress relieving.

The steel becomes softer and more ductile through the precipitation of iron carbide at temperatures associated with stress relieving.

Finally, the chances for hydrogen induced cracking (HIC) are reduced, although this benefit should not be the only reason for stress relieving.

At the elevated temperatures associated with stress relieving, hydrogen often will migrate from the weld metal and the heat affected zone. However, as discussed previously, HIC can be minimized by heating at temperatures lower than stress relieving temperatures, resulting in lower PWHT costs.

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