Pre Heating (PH) and Post weld heat treatment (PWHT): Is it right for your welding job?

Pre Heating (PH) and Post weld heat treatment (PWHT): Is it right for your welding job?

Welding is a fundamental process in all manufacturing and engineering industries.
Using extreme heat, materials such as durable metal and thermoplastics are joined together to create and repair moving parts of machinery and equipment.
While welding is a simple solution to many complex problems, the process isn’t always over once the heat subsides. In many cases, just stopping there can cause undue stress on the finished product leading to waste and potentially unsafe equipment.
In these instances, pre heating (PH) a post weld heat treatment (PWHT) may be required to complete the job efficiently and safely.

 

Welding manWhy choose Preheating.

Preheating minimizes the temperature difference between the welding arc and the base material. This benefits the weldment in several ways.
First is to lessen shrinkage stresses that can lead to cracking and distortion. Because hot materials expand and cool ones contract, a large temperature variance between the molten weld pool and the relatively cool base material can result in internal stresses as the weldment tries to normalize those temperature differences. These internal stresses increase the risk of cracking and distortion.
Second is to slow the cooling rate of the finished weld and reduce hardness in the heat-affected zone (HAZ), which creates a weld that is less brittle and more ductile. These characteristics are especially important for materials more susceptible to hardness at elevated temperatures, such as cast iron, medium- and high-carbon steel, or high alloy steel.

Third is to slow the cooling rate which allows hydrogen to escape the weld puddle as it hardens and assists in the elimination of cracking that occurs if hydrogen is present.
Fourth is to introduce the necessary heat into the weld area to ensure proper penetration. This benefits thick materials and those that conduct heat quickly. By preheating, you can use less heat in the welding arc and still achieve optimal penetration, because the base material starts out at an elevated temperature.
Preheating is especially important when welding highly restrained weld joints, thick materials and base materials that tend to be more brittle.
Preheating is also good for materials with high-carbon levels and/or additional alloys can make the material stronger and harder, but also more brittle and less ductile, which can lead to potential cracking issues.

 

Post Weld Heat TreatmentWhy choose PWHT

Post Weld Heat Treatment is a method of altering the physical and sometimes chemical properties of metals and essentially can be defined as a controlled heating and cooling of a metal or metal alloy, in its solid state, to produce certain changes in its properties.
It is generally designed to leave the steel free from harmful, unevenly distributed internal stresses, yet leaving it hard and tough enough to be serviceable.
It removes the external brittle micro structures that follow quenching or fast cooling and restores toughness and ductility, according to the temperatures adopted, dictated by specific codes for both mild and alloy steel material.
The term ‘post weld heat treatment’ to some engineers is a rather vague term that is used to describe any heat treatment that is carried out when welding is completed. To others however, particularly those working in accordance with the pressure vessel codes such as  4458 or ASME VIII, it has a very precise meaning.
Heat treatment following welding may be carried out for one or more of three fundamental reasons:

  • To achieve dimensional stability in order to maintain tolerances during machining operations or during shake-down in service
  • To produce specific metallurgical structures in order to achieve the required mechanical properties
  • To reduce the risk of in-service problems such as stress corrosion or brittle fracture by reducing the residual stress in the welded component

Further to the above there are changes to the microstructure of the metal during the welding that can cause the material to suffer residual stress as the metal cools down and contracts putting undue stresses onto the parent metal within the heat affected zone.

 


The use of an appropriate post weld heat treatment method in line with the code requirements and its heating specification removes the stresses by putting the micro structures back to its original state and renders it to be stress free.

Pre Heating and Post weld heat treatment
Extreme heat caused by welding can also reduce the flexibility of a material but an appropriate PWHT will keep it strong while maintaining durability.
While a PWHT may be necessary in many cases, it’s important to remember that a post weld treatment may also cause irreparable damage if administered incorrectly.

There is a certain amount of confusion surrounding post weld treatments but it is critical for the safety of your consumer and the reputation of your business that you always make the right choice regarding PWHT. Doing so will avoid weakening materials unnecessarily, waste, and potential danger.

Post Weld Heat Treatment is also known as Stress relieving, and after materials have been welded, if they cool too quickly, brittle fracture and corrosion can occur caused by the stress of the extreme heat.

The PWHT/Stress relief process is performed by heating to a temperature as detailed in the   welding code for the appropriate material, and holding at that temperature long enough to achieve the desired reduction in residual stresses. The steel is then cooled at a sufficiently slow rate to avoid formation of excessive thermal stresses.

Stress relieving as the name suggests, is heat treatment designed to reduce the residual stresses produced by weld shrinkage. It relies upon the fact that, as the temperature of the metal is raised, the yield strength decreases, allowing the residual stresses to be redistributed by creep of the weld and parent metal. Cooling from the stress relief temperature is controlled in order that no harmful thermal gradients can occur.

 

Hydrogen bake outWhy choose Hydrogen bake out

Otherwise known as hydrogen-induced cracking (HIC), hydrogen bake out removes any atomic hydrogen from the material before it can seep into the metal structure as can sometimes occur as the material cools down after welding.
Hydrogen can cause cracks to the material and render it unsafe and therefore the hydrogen bake out will force any hydrogen out of the metal before it’s used and by definition is a Post Weld Heat Treatment.
Industry regulations state that hydrogen bake out’s should be administered immediately post weld before the metal has had a chance to cool to avoid all risk of hydrogen damaging the material
Various types of steel equipment, particularly in refinery service, absorb hydrogen during operation. Materials selection and design should ensure that this does not cause any damage, but the presence of hydrogen in the steel also needs to be taken into account if modifications or repairs involving welding are required. Hydrogen in the steel will contribute to hydrogen in the weld, and may increase the risk of fabrication hydrogen cracking, or ‘cold cracking’. The usual approach is to impose a hydrogen removal heat treatment, or ‘hydrogen bake out’ prior to welding, to ensure that there is no significant hydrogen left in the steel, in which case, normal welding precautions can then be taken to avoid cracking.
With the above in mind there maybe confusion between a bake out and an intermediate Post heat application. This is carried out at a lower temperature than the PWHT and removes any atomic hydrogen from the material before it can seep into the metal structure as can sometimes occur as the material cools down after welding.
Intermediate Post heat is normally carried out on high alloy steel and carried out immediately upon completion of the welding operation before the metal has had a chance to cool, and then the weld can be allowed to cool to ambient temperature to enable non destructive testing  (NDT) prior to the final PWHT application.

 


To conclude – Should Pre Heating and or Post Weld Heat Treatment be carried out?

Without knowing the specific purpose and regulations surrounding your welding job, it’s impossible to advise whether a PWHT is required and, if so, which is the best method.

Due to the severe implications of a weld not being completed correctly, we’ll always recommend that you consult with a professional if you’re ever in doubt. The following factors, however, are likely to contribute to residual stress of your material and make the need for a PWHT more likely.

  • The thicker the weld, the higher the thermal gradient which may contribute to higher residual stress on the primary material making a PWHT necessary.
  • Welding two different materials together may change the structure of the original metals, and a PWHT may be required.
  • Controlling cooling temperatures can increase rather than reduce the strength of certain steels. Consider a PWHT to avoid changes in structure caused by cooling too fast.

PWHT to avoid changes in structure caused by cooling too fast.

A PWHT should be considered in all welding cases but may not be the best solution. For professional advice and to find out more information, please do not hesitate in contacting Heat Tech.

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