Code of Practice for Post-Weld Heat Treatment

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1. Subject matter and scope of application This code stipulates the preparation of process documents for post-weld heat treatment. 2. General Provisions Pressure vessel products have their own separate specifications and manufacturing standards; these guidelines should be used in conjunction with the product drawings, standards, and relevant process documents. 3. Process details and technical requirements: In the manufacturing of pressure vessels, the provisions regarding post-weld heat treatment outlined in standard GB150 must be strictly followed. Post-weld heat treatment shall be performed for vessels and their pressure-bearing components that meet one of the following conditions. 3.1 The nominal thickness δs of the steel at Class A and B weld joints shall meet the following conditions: 3.1.1.1 For carbon steel, 15MnNbR, and 07MnCrMoVR, δs > 32 mm (if preheating before welding is carried out at over 100°C, δs > 38 mm). 3.1.1.2 For Q345R and 16Mn, δs > 30 mm (if preheating before welding is carried out at over 100°C, δs > 34 mm). 3.1.1.3 For 15MnVR and 15MnV, δs > 28 mm (if preheating before welding is carried out at over 100°C, δs > 32 mm). 3.1.1.4 For 18MnMoNbR, 13MnNiMoNbR, 15CrMoR, 14Cr1MoR, 12Cr2Mo1R, 20MnMo, 20MnMoNb, 15CrMo, 12Cr1MoV, 12Cr2Mo1, and 1Cr5Mo steels of any thickness ； 3.1.1.5 For welded joints with different steel thicknesses δs, the aforementioned thickness shall be considered as the thinner one; for welded joints formed by joining dissimilar steels, it shall be determined based on the material that requires more severe heat treatment, provided that the temperature does not exceed the lower critical point Ac1 of either steel grade. 3.1.1.6 Unless otherwise specified in the drawing, welded joints of austenitic stainless steel may not require heat treatment. 3.1.2 Containers marked in the drawings as being susceptible to stress corrosion, such as those used for storing liquefied petroleum gas, liquid ammonia, etc. 3.1.3 Containers marked in the drawings as holding toxic, hazardous, or highly dangerous substances. 3.1.4 Containers that require hydrogen removal treatment after welding; however, if post-weld heat treatment is carried out immediately, hydrogen removal treatment may be omitted. 4. Post-weld heat treatment methods 4.1 Furnace heating should be given priority for post-weld heat treatment, and the operating procedures are as follows: 4.1.1 When the welded parts are placed in the furnace, the temperature inside the furnace should be ≤400°C. 4.1.2 After the welded parts are heated to 400°C using power, the heating rate in the heating zone should be ≤5000/δs °C/h, with a maximum of 200°C/h and a minimum of 50°C/h. 4.1.3 During heating, the temperature difference over any 500 mm length within the heating zone shall be ≤ 120°C. 4.1.4 Holding time and temperature are specified in the attached table.
Attached Table: Post-weld heat treatment specifications for common steel grades
Steel grade | Post-weld heat treatment temperature (°C) | Minimum holding time (h)
Arc welding | Electroslag welding
Q235-A, 10, 20 | Q235-B, Q245R | Q235-C | 600–640 | (1) When the thickness for post-weld heat treatment δPWHT ≤ 50 mm, it is [image] h; however, the minimum time shall not be less than 1/4 h.
(2) When the thickness for post-weld heat treatment δPWHT > 50 mm, it is (2 + 1/4 × [image]) h.
09MnD | 580–620 | –
Q345R | 600–640 | After normalizing at 900–930°C, temper at 600–640°C.
16Mn, 16MnD, 16MnDR | –
20MnMo, 20MnMoD | 580–620 | –
18MnMoNbR, 13MnNiMoNbR | 600–640 | After normalizing at 950–980°C, temper at 600–640°C.
20MnMoNb | –
07MnCrMoVR, 07MnNiCrMoVDR, 08MnNiCrMoVD | 550–590 | –
09MnNiD, 09MnNiDR, 15MnNiDR | 540–580 | –
12CrMo, 12CrMoG | ≥600 | –
(1) When the thickness for post-weld heat treatment δPWHT ≤ 125 mm, it is [image] h; however, the minimum time shall not be less than 1/4 h.
(2) When the thickness for post-weld heat treatment δPWHT > 125 mm, it is (5 + 1/4 × [image]) h.
15CrMo, 15CrMoG | ≥600 | –
15CrMoR | After normalizing at 890–950°C, temper at ≥600°C.
12CrlMoV, 12CrlMoVG, 14CrlMoR, 14CrlMo | ≥640 | –
12Cr2Mo, 12Cr2Mol, 12Cr2MolR, 12Cr2MolG | ≥660 | –
1Cr5Mo | ≥660 | –
4.1.5 During the holding period, the temperature difference within any 500 mm length in the heating zone must be ≤ 65°C. 4.1.6 During heating and holding, the atmosphere in the heating area should be controlled to prevent excessive oxidation of the weld surface. 4.1.7 When the temperature of the welded part exceeds 400°C, during cooling, the cooling rate in the heated area must not exceed 6500/δs °C/h; the maximum cooling rate is 260°C/h, while the minimum can be 50°C/h. 4.1.8 When the welded part is taken out of the furnace, the furnace temperature shall be ≤400°C, and it should be cooled in still air after removal from the furnace. 4.2 For long weldments, post-weld heat treatment may be carried out in segments within a furnace. When performing segmented heat treatment, the length of each reheating section shall be no less than 1500 mm. Insulation measures must be taken for the portions outside the furnace to ensure that the temperature gradient does not affect the microstructure and properties of the material. 5. For Class A weld joints where the spherical head of a B.C.D welded joint is connected to a cylinder, as well as for areas that require defect repair, local heat treatment methods may be employed. During local heat treatment, the heating width on each side of the weld should be no less than 2 times the thickness δs of the steel. When joining a pipe to a shell, the heating width must be no less than 6 times the thickness δs of the steel. Insulation measures should be taken in the areas near the heating zone to prevent temperature gradients from affecting the structure and properties of the material. 6. Heat treatment operators must strictly follow the “Safety Operating Regulations for Heating Furnaces”. 7. The quality assurance department shall keep records of all heat treatment time-temperature relationship curves, with a retention period of not less than 7 years. 8. The interpretation of these guidelines lies with the Process Department.