Heat treatment

[Shahil Saini]

Dear all,

Can we Exempt PWHT of Weld metal as per NACE MR0175/ISO 15156-2 for carbon steel? Is there any metallurgical changes that will affect the material??

Regards,

Shahil Saini 

[venkateswara reddy Desireddy]

For PWHT no metalrigical change will occur. During normalising only metalrigical change will occur.

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[pradip kumar Sil]

NACE MR 0175 refers only to the hardness value, not PWHT. Its requirement of client/ designer

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[Amol Betkar]

Dear Shahil,

Exemption of PWHT for Carbon Steel as per NACE MR0175/ISO 15156-2

NACE MR0175/ISO 15156-2 provides guidelines for materials used in environments containing hydrogen sulfide (H2S) in oil and gas production, which are often referred to as sour service conditions. Post Weld Heat Treatment (PWHT) is an important consideration in these environments due to its impact on reducing residual stresses and improving toughness, especially in carbon steels.

PWHT Exemption Criteria

NACE MR0175/ISO 15156-2 specifies conditions under which PWHT may or may not be required for carbon steels:

1. General Requirements:

   • PWHT is typically required to reduce hardness in the heat-affected zone (HAZ) and weld metal to below 250 HB (Brinell Hardness), which helps prevent sulfide stress cracking (SSC).
   • There are certain conditions and steel compositions that may allow for exemption, but these are specific and must meet the guidelines set by the standard.

2. Exemption Conditions:

   • Hardness Control: If the hardness of the weld metal and HAZ can be controlled to acceptable limits (below 250 HB) through welding procedure control and without PWHT, exemption might be considered.
   • Material Composition: Low carbon steels with very low alloy content and controlled welding processes that do not induce high levels of residual stresses might qualify for exemption.
   • Welding Procedure: The welding procedure itself must ensure that the mechanical properties and resistance to SSC are maintained without the need for PWHT.

Metallurgical Changes without PWHT

If PWHT is exempted, there are several metallurgical changes and potential risks to be considered:

1. Residual Stresses:

   • Without PWHT, residual stresses from welding remain in the weld and HAZ. These stresses can contribute to cracking, particularly under corrosive or hydrogen sulfide conditions.

2. Hardness and Brittleness:

   • Welding can increase the hardness of the weld metal and HAZ. High hardness levels (above 250 HB) are susceptible to SSC in sour environments. Controlling hardness without PWHT can be challenging and may require specific welding techniques and filler materials.

3. Microstructural Changes:

   • PWHT helps in tempering the martensitic structures that may form during welding. Without PWHT, these structures can remain brittle and more prone to cracking.

4. Hydrogen Embrittlement:

   • High residual stresses and hardness levels increase the risk of hydrogen embrittlement, where hydrogen atoms diffuse into the steel and cause it to become brittle and crack.

Conclusion

Exempting PWHT for carbon steels as per NACE MR0175/ISO 15156-2 is possible under specific conditions, primarily if the weld hardness can be controlled to below 250 HB and residual stresses are managed. However, it is important to note that:

• Careful consideration of welding procedures and filler materials is necessary.
• Metallurgical risks include increased residual stresses, higher hardness and brittleness, and susceptibility to hydrogen embrittlement.

Therefore, exemption from PWHT should be approached with caution and should strictly adhere to the conditions set forth in NACE MR0175/ISO 15156-2 to ensure the integrity and safety of the welded components in sour service environments.

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Best regards,

Amol K.B

B.E(Mech), CSWIP 3.2.2, NACE CIP 1 & ASNT LII(4M)