SUS630 (martensitic precipitation-hardening stainless steel) uses a two-step heat treatment process: solution treatment + aging treatment. This process adjusts strength and hardness, and post-weld parts require re-aging to restore performance. Detailed steps are as follows:
1. Solution treatment (Pre-treatment / Pre-machining Preparation)
* Purpose: Dissolve carbides, copper precipitates, and other phases to homogenize composition, form a supersaturated solid solution, reduce hardness, and improve machinability, laying the foundation for subsequent aging strengthening.
* Process parameters: Heat to 1040–1060℃; hold time depends on workpiece thickness. For standard thickness (≤50mm), hold 30–60 minutes. Add 15 minutes for every additional 25mm of thickness.
* Cooling method: Quench rapidly with water or oil to prevent premature precipitation, which could affect subsequent aging.
* Post-treatment condition: Hardness 28–32 HRB, good plasticity, suitable for cutting, stamping, and other machining.

2. Aging treatment (Core strengthening step)
* Purpose: Low-temperature heating promotes fine copper-based precipitates from the supersaturated solid solution, hindering dislocation motion and increasing strength and hardness. Temperature selection depends on desired mechanical properties.
* Requirements: Perform only after solution treatment. The workpiece must cool to room temperature before entering the aging furnace. Industry standard: hold 4 hours followed by air cooling.
* Effect by temperature:
* 480℃: Hardness 39–45 HRC, tensile strength 1310–1400 MPa; suitable for high-strength and high-hardness applications.
* 550℃: Hardness 36–42 HRC, tensile strength 1210–1275 MPa; balanced strength and toughness, widely used in industry.
* 590℃: Hardness 33–38 HRC, tensile strength 1100–1170 MPa; higher toughness, suitable for medium-strength, high-toughness requirements.
* 620℃: Hardness 30–32 HRC, tensile strength 1000–1030 MPa; good toughness, suitable for low-temperature, impact-resistant applications.

3. Post-weld re-aging treatment
* Purpose: Welding destroys the original aging strengthening effect, reducing strength and hardness by 30–40%; re-aging is required.
* Key Steps: Preheat workpiece to 80–150℃ before welding. After welding, cool to room temperature and perform aging treatment according to the target performance (temperature and hold time) to restore over 90% of original properties.
Critical process considerations:
* Avoid prolonged heating in the 400–450℃ range, which can cause “475℃ embrittlement” and reduce impact toughness.
* For large or complex workpieces, control cooling rate after solution treatment to prevent distortion or cracking due to internal stress.
* Ensure uniform furnace temperature during aging to avoid local deviations that could affect workpiece performance consistency.