The influence of post-weld heat treatment welding on materials

Update: 21-12-2020

The welding temperature must be high enough to melt the […]

The welding temperature must be high enough to melt the connected materials. Therefore, they introduce thermal cycling, which causes the material close to the weld to be heated to a temperature close to the melting point, while the material far from the weld hardly sees a temperature increase. In other words, the material experiences a thermal gradient from ambient temperature to melting temperature.

The part where the structure of the base metal changes due to welding heat is called the heat-affected zone (HAZ) of the weld. The material whose temperature is not high enough to have any significant effect on the material is referred to as the basic material. The part that melts due to the welding operation is called weld metal.

The structure of the weld metal will depend on the composition of the base metal, the composition of the filler metal and the influence of thermal cycling. Usually, we can choose filler metal to achieve the desired result in the weld metal. However, in terms of composition, we can do nothing about the heat-affected zone. (HAZ) Our only function to control the HAZ structure is to control the thermal cycle.

It is also obvious that the thermal cycling in the heat affected zone will have a significant impact on the heat treatment of the material before the welding operation. For example, some parts of the heat-affected zone of the parent metal of carbon steel will rise to a temperature above which phase transformation will occur in the steel. (This is called the critical temperature. For ordinary carbon steel, it is approximately 720°C.)

Upon subsequent cooling, the phase change will occur again. If the cooling rate is fast enough, we may experience a certain degree of quench hardening in this area, resulting in a hard brittle structure. If the cooling is slow enough, then we will experience a thermal cycle similar to normal heat treatment. If the cooling rate is very slow, the thermal cycle will be similar to the annealing cycle of steel.

This means that by changing the energy used during welding (also called heat input) and preheating and post-heating, different structures can be obtained in the weld metal and the heat affected zone of the weld.

Certain materials, such as low-alloy steel, almost always result in a quench-hardened structure in the heat-affected zone during welding. Then, they need further post-weld heat treatment (PWHT) to obtain the desired result. Take our low-alloy steels as an example. They almost always need to be tempered to achieve a suitable strong and tough microstructure.

Some materials have gained great strength through cold working. Cold working only refers to plastically deforming a metal at a certain temperature, and the crystal grains of the metal deformed below this temperature will recrystallize. This treatment leads to an increase in the strength of the cold-worked material.

When welding cold-worked (also known as work hardening) materials, part of the heat-affected zone will experience high enough temperatures to cause recrystallization and phase transformation. This will eliminate cold work and may significantly reduce the strength of the material in the heat affected zone. Please note that post-weld heat treatment cannot reverse this effect.

This softening usually occurs when welding work-hardened aluminum alloys. The heat-affected zone will always be much weaker than the cold-worked base material. The only way to effectively deal with this effect is to design the part so that the weld is placed in a lower stress area, or the thickness of the base metal is greater than the thickness required around the weld to compensate for the loss of the weld. strength.

Another way that aluminum alloys are usually reinforced is through precipitation hardening. Likewise, welding thermal cycling will introduce unaged areas into the heat-affected zone (the temperature is high enough to return the precipitate to the solution and then quench due to the high cooling rates normally associated with welding), and the area is older than age. (The temperature is higher than the temperature required for optimal aging, but not enough to return the precipitate to the solution.) Therefore, welding precipitation hardening (also called aging) materials will reduce the strength of the heat-affected zone. For small parts, the entire part may undergo precipitation hardening cycles.
Welding is an important part of operating and maintaining assets in the petroleum (upstream, midstream, downstream) and chemical processing industries. Although it has many useful applications, the welding process can unintentionally weaken the performance of the device by applying residual stress to the material, thereby reducing the performance of the material. In order to ensure that the material strength of the part is retained after welding, a process called post-weld heat treatment (PWHT) is usually performed. PWHT can be used to reduce residual stress, as a method to control hardness, and even enhance material strength.

If the implementation of PWHT is incorrect or completely ignored, the residual stress will be combined with the load stress, thus exceeding the design limitations of the material. This can lead to welding failures, a higher probability of cracking and increased susceptibility to brittle fracture. Preheaters cover many different types of potential treatments; the two most common types are heating and stress relief:

Post heating:

When high levels of ambient hydrogen penetrates into the material during welding, hydrogen induced cracking (HIC) often occurs. By heating the material after welding, hydrogen can be diffused from the welding area, thereby preventing HIC. This process is called post-heating and should start immediately after welding is completed. The material is not allowed to cool, but to be allowed to cool requires heating it to a certain temperature according to the type and thickness of the material. The temperature should be held here for several hours, depending on the thickness of the material.

relieve pressure:

When the welding is completed, the welding process may leave a large amount of residual stress in the material, resulting in an increased possibility of stress corrosion and hydrogen-induced cracking. PWHT can be used to relieve these residual stresses and reduce this potential. This process involves heating the material to a certain temperature and then gradually cooling it down. Whether a material should undergo PWHT depends on many factors, including its alloying system or whether it has been heat treated before. Some materials may actually be damaged by PWHT, while other materials almost always require it.

Generally, the higher the carbon content in the material, the more PWHT is needed after the welding activity.
Similarly, the higher the alloy content and cross-sectional thickness, the more PWHT is required for the material.

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