Welding Precautions for Metals
Welding of any Materials need some procedural requirements which is also called as welding precautions for metals like cleaning, preheating, post heating PWHT, purging etc. However these requirements are specified in various design standards, of welding requirements.
Before moving ahead with welding requirements, we should know about abbreviation used as PWHT. What is PWHT and why is used?
PWHT is Post Weld Heat Treatment, and as name implies it is used before the material that has been welded. It is controlled heat treatment process, used for material to be welded is reheated to a temperature below its lower critical transformation temperature, and then it is detained at that temperature for a specified amount of time.
Most of the times, any heat treatment is performed after welding; however, within the oil& gas, or petrochemical industries, it has a specific meaning. ASME Pressure Vessel and Piping Codes, often need mandatory requirements for PWHT on certain materials to ensure mechanical and metallurgical properties and having a safe design.
The need for PWHT is generally due to the residual stresses and metallurgical micro-structure changes that happen after welding has been completed. For thicker materials the use of PWHT can help reduce any increased hardness levels, and improve toughness and ductility to acceptable levels with reducing stresses that can reach an unacceptable level and exceed design stresses.
Some of the commonly used Materials I have listed below, with their welding precautions or requirements, to follow during site or shop welding.
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Carbon Steels (CS)
Cleaning Requirements: Wire brush/Grind
Preheating Requirements: 100 Deg C when thickness is more than 20mm, with Interpass Temperature 250 Deg C.
Post Heating and Purging for Carbon Steel is not required. However PWHT is required when thickness is more than 20mm.
Heat Input is 2.5 KJ/mm
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Low Temperature Carbon Steels (LTCS)
Cleaning Requirements: Wire brush/Grind
Preheating Requirements: 100 Deg C when thickness is more than 20mm, with Interpass Temperature 250 Deg C.
Post Heating and Purging for low temperature Carbon Steel is not required. However PWHT is required when thickness is more than 20mm.
Heat Input is 2.5 KJ/mm
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Alloys Steels (Cr-Mo Steels)
Cleaning Requirements: Wire brush/Grind
Preheating Requirements: 150-200 Deg C when thickness is more than 20mm, with Interpass Temperature350 Deg C.
Post Heating required for 300 Deg C for 30 to 60 mins. and Purging for Alloys Steels required when Cr > 5 %. However PWHT is required when thickness is more than 13mm.
Heat Input is 2.5 KJ/mm
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Stainless Steels (SS)
Cleaning Requirements: Cleaning with Acetone both joint and filler wires
Preheating, Postheating and PWHT for Stainless Steel is not required, however Purging is required with oxygen content shall not be more than .5%.
Interpass Temperature150 Deg C.
Heat Input is 2.0 KJ/mm
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Duplex Stainless Steels (DSS)
Cleaning Requirements: Cleaning with Acetone both joint and filler wires
Preheating, Postheating and PWHT for Duplex Stainless Steel is not required, however Purging is required with oxygen content shall not be more than 50 ppm
Interpass Temperature 150 Deg C.
Heat Input is 2.0 KJ/mm
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Super Duplex Stainless Steels (SDSS)
Cleaning Requirements: Cleaning with Acetone both joint and filler wires
Preheating, Postheating and PWHT for Super Duplex Stainless Steel is not required, however Purging is required with oxygen content shall not be more than 50 ppm
Interpass Temperature 100 Deg C.
Heat Input is 1.5 KJ/mm
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Inconel Alloys (Inconel)
Cleaning Requirements: Cleaning with Acetone both joint and filler wires
Preheating, Postheating and PWHT for Inconel Alloys is not required, however Purging is required with oxygen content shall not be more than 50 ppm
Interpass Temperature 100 Deg C.
Heat Input is 1.5 KJ/mm