Post-weld heat treatment (PWHT) of pipelines usually refers to heat treatment below the metal’s phase transition point, which is also commonly referred to as stress relief. The main functions of post-weld heat treatment are to release residual stress, reduce diffusible hydrogen, and improve microstructure and properties. In addition, post-weld heat treatment (PWHT) has other functions to improve weld joints:
1. Improving the Fracture Toughness of Welded Joints
Applying appropriate PWHT to welded structures, such as low-carbon steel and low-alloy high-strength steel, can improve the fracture toughness of welded joints.
2. Improving the creep properties of weld metal.
For example, PWHT can improve the creep properties of V-added modified steel (such as 2.25Cr-1Mo-0.25V).
3. Improving the Stress Corrosion Resistance of Welded Joints
Given the appropriate material and structural conditions, PWHT can effectively reduce residual stress and hardness in welded joints, thereby improving their stress corrosion resistance.
4. Increasing the Fatigue Strength of Welded Joints
In areas such as branch pipes and open tees, residual stresses overlap with stress concentrations caused by the shape, resulting in a complex triaxial stress state. PWHT can prevent fatigue cracking in these areas.
The following are some common questions regarding post-weld heat treatment (PWHT) of pipelines:
1. Carbon Steel Heat Treatment Requirements for Stress Corrosion
When stress corrosion resistance is required in specific media environments, regardless of the wall thickness of carbon steel pipelines, welded joints should undergo post-weld heat treatment (PWHT) after welding to reduce residual stress in the weld joints, lower the hardness of the weld and heat-affected zone, and improve corrosion resistance.
Standards and specifications do not specify heat treatment requirements for such pipelines. For example, SH/T3501-2021 simply states that for pipelines exposed to specific corrosive media, all thicknesses should be heat-treated according to design requirements. The recommended hold time for carbon steel heat treatment is 600-650°C, with a minimum hold time of 0.5 hours.
8.4 Post-Weld Heat Treatment
8.4.1 Post-weld heat treatment should follow the requirements of the welding procedure specification and be verified through welding procedure qualification. Unless otherwise specified in the design documents, the heat treatment temperature for commonly used steel welded joints should be determined based on the requirements in Table 8.4.1. Austenitic stainless steel welded joints should undergo stabilization heat treatment if required by the design documents. When performing heat treatment using a resistance heating method, relevant specifications must be followed. Post-weld heat treatment of low-temperature steel should comply with the requirements of standard SH/T 3525.
Table 8.4.1 Basic Requirements for Heat Treatment of Common Steel Welded Joints
| Base Material Category | Nominal Thickness δ (mm) | Carbon Content (%) | Heat Treatment Temp (°C) | Equivalent Welded Thickness for Heat Treatment | Minimum Holding Time (h) | Brinell Hardness (HBW) |
| Carbon Steel | >20 | All | 600–650 | — | — | Not requiredᵇ |
| Cr ≤ 0.5% | ≤16 | All | 600–650 | — | — | ≤225 |
| >16 | All | 600–650 | — | — | ≤225 | |
| 0.5% < Cr ≤ 2% | ≤13 | ≤0.15 | 650–700 | — | — | ≤225 |
| >13 | ≤0.15 | 650–700 | δ/25 (minimum 0.5) | 2 + (δ−50)/100 | ≤241 | |
| 2.25% < Cr ≤ 3% | All | ≤0.15 | 700–760 | — | — | ≤241 |
| 3% < Cr ≤ 10% | All | — | 700–760 | — | — | — |
| 1.5% ~ 2.5% Ni | ≤13 | ≤0.15 | 600–650 | — | — | — |
| >13 | ≤0.15 | 600–650 | — | — | — | |
| 3.5% Ni | >16 | — | 600–650 | — | — | — |
The same requirements are also in SH/T3554-2013 and GB50235-2010. The recommended constant temperature time for carbon steel heat treatment is 600-650℃, and the minimum insulation time is 1h.
1. Requirements for Heat Treatment in Branch Pipe Connections
When performing branch pipe connections, the heat treatment thickness should be based on the thickness of the main pipe or the branch pipe and should not include the thickness of reinforcement parts (including full or partial reinforcement sleeves). If the weld seam thickness at the branch connection exceeds twice the thickness listed in Table 6.0.10-1, or if the thickness of any component in the welded joint is less than the minimum thickness specified in Table 6.0.10-1, heat treatment is still required.
The weld thickness for branch pipe connections should be calculated according to the specifications in Table 6.0.10-2.
Table 6.0.10-1 Basic Requirements for Pipe Heat Treatment
| Base Material Category | Nominal Thickness (mm) | Minimum Specified Tensile Strength (MPa) | Heat Treatment Temperature (°C) | Holding Time (min/mm) | Minimum Holding Time (h) |
| Carbon Steel (C), Carbon-Manganese Steel (C-Mn) | ≤19 | All | Not required | — | — |
| >19 | All | 600–650 | 2.4 | 1 |
Notes:
- a) If the deformation exceeds 50% of the minimum elongation specified by the material standard, heat treatment must be carried out. However, if it can be proven that the selected pipe bending or forming method can still retain at least 10% elongation after cold bending and forming, then heat treatment may not be necessary.
- b) For materials that require low-temperature impact testing at 0°C, heat treatment is required if the bending or forming strain exceeds 5%.
- c) Follow the design documentation if it includes specific provisions.
In other words, the PWHT requirements in the pre-standard are based on an assessment of weldability and mechanical properties (strength and brittle fracture), without considering the effects of the design and operating environment (media, temperature), such as alkali embrittlement, hydrogen embrittlement, stress corrosion cracking, and hydrogen attack.
However, NACE SP0472-2020 (Methods and Measures for Preventing On-Line Cracking of Carbon Steel Welds in Corrosive Petroleum Production Environments) stipulates that the minimum PWHT heat treatment temperature for preventing hydrogen-induced cracking (HSC and SSC) is 620°C, and a holding time of at least 1 hour should be specified regardless of the thickness of the base metal. For carbon steel pipelines exposed to corrosive media such as amines or alkali solutions, the post-weld heat treatment temperature for welded joints should be controlled at 635±15°C for a minimum holding time of 1 hour to prevent alkaline stress corrosion cracking. For alkaline carbonate-acid aqueous media, the post-weld heat treatment temperature for welded joints should be no less than 649°C for a minimum holding time of 1 hour.
Table 3
Recommended Post-Weld Heat Treatment (PWHT) Process Parameters for Controlling Stress Corrosion Cracking in P No.1 Carbon Steel Welded Joints Under Stress Cracking Usage Environment
| Material Type | Working Condition Environment Standard (or Document) Reference | PWHT Holding Temperature (°C) |
| P No.1 Carbon Steel | NACE MR0103 Petroleum Refining Industry – Materials for H₂S Stress Corrosion Cracking-Resistant Applications | 620–650 |
| P No.1 Carbon Steel | API RP 941 High-Temperature Hydrogen Attack | 620–650 |
| P No.1 Carbon Steel | NACE SP0403 Corrosion Resistant Materials for Refinery Equipment – Hydrogen-Induced Cracking and Sulfide Stress Cracking | 620–650 |
| P No.1 Carbon Steel | NACE 34108-07 Carbon Steel Crack Prevention under Sour Service – Hydrogen Sulfide Environments | 620–650 |
| P No.1 Carbon Steel | NACE 0472 Prevention of Sulfide Stress Cracking (Sulfur Compounds, Moisture, pH, Carbonate, etc.) NACE TG347 Cracking Control in Sour Environments | 620–650 |
| P No.1 Carbon Steel | NACE 5A171 Ammonia and Ammonium Nitrate Environments | 620–650 |
| P No.1 Carbon Steel | NACE SP0500-2007 Hydrogen Embrittlement Prevention | 620–650 |
| P No.1 Carbon Steel | API 939D, API 939E Hydrogen Sulfide Stress Cracking Prevention | 620–650 |
The appendix states: “It has been demonstrated that a higher PWHT and a hold time of at least one hour at this temperature are required to significantly reduce residual stress.”
Therefore, for post-weld heat treatment of full-wall carbon steel subjected to stress corrosion, attention should be paid to the heat treatment temperature and duration. An appropriate welding procedure qualification and welding procedure specification should be selected based on the design and operating environment requirements, rather than blindly copying standard requirements.
II. Post-weld Dehydrogenation Treatment
- For P91 heat-resistant alloy steel, Table 7 of GB/T 20801-2020 requires:
Unless otherwise specified in the design, the filler metal Ni+Mn content should not exceed 1.2%, and the maximum heat treatment temperature should be ≤ 800°C. If the heat treatment temperature exceeds 800°C, or if the heat treatment temperature is not higher than 800°C but exceeds the filler metal’s Ac1 (lower transition temperature or critical temperature), the weld metal and heat-affected zone should be removed and re-welded, and post-weld heat treatment should be performed again. The minimum heat treatment temperature for 9Cr-1Mo-V steel with a wall thickness of ≤13mm may be 675°C.
The requirement of a Ni+Mn content of no more than 1.2% is because excessive Ni and Mn concentrations lower the austenite to martensite transformation temperature, Ms, and the Mf transformation temperature, respectively. Consequently, the weld joint must be cooled to a lower temperature after welding to complete the austenite transformation, which is detrimental to preventing cold cracks in the weld joint.
2. Regarding the timely heat treatment of chromium-molybdenum alloy steel, various standards address this. For example, SH/T3501-2021, Section 8.4.5, requires: “Pipeline welded joints of chromium-molybdenum alloy steel and steel with a minimum tensile strength equal to or greater than 540 MPa shall be heat treated immediately after welding. Otherwise, they shall be uniformly heated to 300°C to 350°C immediately after welding, then held and slowly cooled. The heating and holding range shall be the same as for post-weld heat treatment, and the holding time shall be no less than 0.5 h.”
The “immediately” mentioned in this provision refers to the weld metal temperature not falling below the preheating temperature. Post-weld heat treatment reduces the cooling rate of the weld joint, thereby reducing the hardened structure and diffusible hydrogen content. It also provides ample time for preparation for the formal heat treatment.
Conclusion
Post-weld heat treatment (PWHT) plays a critical role in enhancing the structural integrity, toughness, and corrosion resistance of welded joints in pipelines. It helps relieve residual stresses, reduce hardness, prevent cold cracking, and improve resistance to stress corrosion and fatigue. Standards like NACE, SH/T3501, and GB/T20801 provide specific guidelines for temperature, duration, and application based on material and service conditions. However, the choice of PWHT parameters should be driven by actual design and operational environments rather than relying solely on general standards. Properly implemented PWHT ensures safe, long-term performance of pipeline systems.
