Power plant and calculations: What is the significance of post weld and Pre-weld heat treatments (PWHTs)?

Post weld Heat Treatment (PWHT):

Post Heating in welding is a process where heat is applied to a welded joint after welding to control the cooling rate and reduce the risk of cracking.

When to carry out post weld heat treatment?

It is commonly used in materials that are prone to hydrogen-induced cracking, such as high-carbon steels, alloy steels, and cast iron.

What is the significance of post weld heat treatment?

Importance of post weld heat treatment

Prevent Hydrogen Cracking: Slows down cooling to allow hydrogen to diffuse out, preventing cracks.

Improve Mechanical Properties: Enhances toughness and ductility.

Reduce Residual Stresses: Helps relieve stresses induced by rapid cooling.

Improve Metallurgical Structure: Minimizes hardness variations and prevents brittle micro structures.

What actually is being done in post weld heating process?

In post weld heat treatment, the welded component is heated to a specified temperature say 450 deg C to 700 deg C. This temperature is maintained for a certain duration up to 6 to 8 hours based on material composition and its thickness, later slow cooling is ensured, often using insulation or controlled cooling methods.

What are the different methods of post weld heat treatment (PWHT)?

Torch/Gas cutter Heating: Using an oxy-fuel or propane torch for localized heating.

Furnace Heating: Placing the component in an oven or furnace for uniform heating.

Electric Resistance Heating: Using heating pads or ceramic heaters.

Induction Heating: Applying electromagnetic induction for controlled heating

What is the criteria for post weld heating or heat treatment (PWHT) or heat treatment of carbon and alloy steels as per IBR regulations?

Actually, arc welded butt joints should be post weld heat treated effectively except in the following cases;

For high pressure alloy steel post weld heat treatment (PWHT ) is not done

1-If 0.5% Molybdenum steel having thickness & Tube OD are < 13 mm and < 127 mm respectively

2- If 1 Cr & 0.5 % Molybdenum steel having thickness & Tube OD are < 13 mm and < 127 mm respectively and it is been pre-heated to 125 deg C

3-In case of 2.25 Cr & 1 Molybdenum steel, PWHT is not necessary in following conditions;

a-Maximum Chromium content is 0.3%

b-Maximum Nominal out side diameter is 102 m

c-Maximum thickness of the alloy steel is 8 mm

d-Maximum specified carbon content 0.15%

e-Minimum pre-heated temperature is 150 deg C

For Carbon steel post weld heat treatment (PWHT ) is not done if;

1-Maximum Carbon percentage is 0.30%

2-Maximum thickness is 9 mm

Pre- weld heat treatment

What is the significance of Pre- weld heat treatment?

Pre-weld heat treatment, often referred to as preheating, is a critical process applied to the base material before welding.

Its main goal is to prepare the metal for welding by ensuring that temperature gradients are minimized, thereby reducing the likelihood of thermal stresses, cracking, and other welding defects.

Importance of Pre-weld heat treatment

To reduce or remove moisture content in the materials: Preheating helps evaporate moisture from the base metal, which is particularly important for materials prone to hydrogen embrittlement.

Reduce Thermal Stress: Preheating helps in minimizing the rapid temperature changes that occur during welding. This gradual change reduces residual stresses in the weld and surrounding base metal.

Prevent Cracking: By slowing down the cooling rate, preheat treatment decreases the risk of hydrogen-induced or cold cracking, especially in materials with high carbon content or high strength.

To get quality weld: A controlled preheat ensures that the weld area is at a uniform temperature, improving the fusion between the base material and the weld metal, leading to a more sound joint.

When to use Pre-weld heat treatment process?

Generally applied for Alloy steels, high carbon steels & Cast irons where there is a possibility of cracking due to rapid cooling.

Pre-heating is also done during rainy seasons or cold climate conditions to remove moisture from the steel.

What is the temperature for Pre-heating?

The required preheat temperature varies with the material type and its carbon or alloy content. For example, mild steels might require preheat temperatures of around 100–150°C, while high-carbon or high-strength steels might need 200–300°C or higher.

What are the different methods of Pre heat weld heat treatment ?

Torch/Gas cutter Heating: Using an oxy-fuel or propane torch for localized heating.

Electric Resistance Heating: Using heating pads or ceramic heaters.

Furnace Heating: Placing the component in an oven or furnace for uniform heating.

Induction Heating: Applying electromagnetic induction for controlled heating

In most of the applications gas heating is done.

Tips for best Pre-weld heat treatments:

1-Uniform heating: Select the weld area to be pre-heated, apply the heat uniformly thought the selected area to avoid the development of localized temperature gradients.

2-Proper controlling of Temperature: Ensure pre-heating temperature is within limit. Make use of calibrated temperature sensors.

3-After completing the welding: Allow the metal to cool slowly. Faster cooling again induces stresses and even form cracks.

For conducting pre & post weld heat treatments skill of welder & welding team is also plays a vital role.

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