GB/T 37558-2019 in English

This standard specifies the technical requirements for processes such as preventive white spot annealing, grain refinement treatment and stress relief annealing for the post-forging preparatory heat treatment of large steel forgings (effective thickness ≥300mm) for heavy equipment in the energy, metallurgy, transportation and other industries. The main body of the standard contains 10 chapters of technical clauses and 5 informative appendices, which systematically standardize the key points of the whole process control from equipment selection to quality inspection.

1. Annealing process to prevent white spot

When the hydrogen content of the forging exceeds the critical value (1.8-3.6ppm, depending on the type of steel), it is necessary to execute:

• Temperature selection: 650℃±20℃ (refer to Appendix A)
• Hot holding time: calculated according to the formula τ=K·R²/D, where R is the radius of the forging and D is the hydrogen diffusion coefficient (see Table C.2)
• Cooling control: ≤60℃/h above 400℃, ≤30℃/h below 400℃

2. Grain refinement normalizing process

For high alloy steel with serious organizational inheritance phenomenon:

• Temperature gradient: Ac3+(100-150)℃, subsequent normalizing gradually reduces 20-30℃
• Heating segmentation: Elastic-plastic transition zone (600-700℃) requires limited heating (20-80℃/h)
• Cooling requirements: Final cooling temperature should be lower than Ms point but higher than 250℃

1. Specifications of heat treatment equipment

• Furnace temperature uniformity: normalizing ±15℃, annealing ±20℃, tempering ±10℃ (Table 2)
• Instrument system: at least Class D accuracy, 0.5 level recorder (GB/T 30825)
• Effective heating zone inspection cycle: 6 months (extended to 1 year after 3 consecutive qualified tests)

2. Quality inspection system

• Hardness test: According to GB/T 231.1 Brinell hardness or GB/T 17394.1 Leeb hardness
• Grain size assessment: GB/T 6394 specified method, usually required to be ≤ Grade 5
• Hydrogen content verification: Confirm the elimination of white spots through ultrasonic flaw detection (JB/T 5000.15)

1. Key points of process design

• For medium alloy steels such as 34CrMo1A, it is recommended to adopt a two-time normalizing + tempering process (first normalizing at 920℃, second normalizing at 880℃)
• Forgings with a thickness of ＞1000mm need to install a load thermocouple at the thickest part of the section to monitor the actual temperature
• Dehydrogenation annealing is recommended to be carried out simultaneously with high-temperature tempering after normalizing (630-660℃)

2. Dealing with common problems

• White spot defects: Immediately perform dehydrogenation annealing according to the process in Appendix B, and extend the holding time by 30%
• Coarse grains: Use the cyclic normalizing process in Appendix E, combined with isothermal treatment near the Ac1 point
• Shape correction and deformation: After shape correction, stress relief annealing at a temperature 20℃ lower than the previous tempering is required