The core process for SUS630 (17-4PH) revolves around machining and heat treatment, with the main goal of achieving high strength through “solution + aging” treatment. Machining parameters need to match the material’s condition.
1. Heat treatment (Determines strength and hardness)
Solution treatment (Pre-treatment / Pre-machining preparation)
* Purpose: Homogenize alloy composition, dissolve precipitates, form a supersaturated solid solution, and improve machinability.
* Parameters: Heat to 1040–1060℃, hold for 30–60 minutes (increase 15 minutes per additional 25 mm of workpiece thickness).
* Cooling: Water or oil quench to avoid premature precipitation and ensure effective aging.
Aging treatment (Core strengthening step)
* Purpose: Promote the formation of copper-based precipitates to significantly increase strength and hardness. Temperature is selected based on performance requirements.
* Typical aging parameters (4 hours hold + air cooling):
* 480℃: High strength, tensile ~1400 MPa, hardness 39–45 HRC.
* 550℃: Balanced strength and toughness, tensile ~1275 MPa, hardness 36–42 HRC.
* 590℃: Higher toughness, tensile ~1170 MPa, hardness 33–38 HRC.
* 620℃: Good toughness, tensile ~1030 MPa, hardness 30–32 HRC (suitable for low-temperature applications).
Welding
* Methods: Arc welding, TIG, etc. Preheat workpiece to 80–150℃ before welding.
* Post-weld: Must perform aging treatment to restore 30–40% of lost strength; same parameters as conventional aging.
* Precautions: Avoid excessive heat during welding to prevent coarse grains and reduced toughness.

2. Machining (Adapted to material condition)
Cutting
* Best performed in solution-treated state (hardness 28–32 HRB, good plasticity, low cutting resistance).
* Tooling: High-speed steel or carbide; cutting speed 20–30% lower than for 304 stainless steel.
* Parameters: Feed 0.1–0.2 mm/rev, depth of cut 2–5 mm, avoid overheating tools.
Cold working (Stamping, Bending, Stretching)
* Applicable only in solution-treated state; aged material is harder and less ductile, prone to cracking.
* Guidelines: Limit single-pass deformation ≤15%, complex shapes require multiple steps, intermediate solution treatment can restore plasticity.
Surface Treatment
* Polishing: Mechanical or electrochemical polishing for improved surface finish, suitable for medical or decorative applications.
* Passivation: Acid treatment to form a protective film, enhancing corrosion resistance, especially in humid or mildly corrosive environments.

3. Process precautions
* Avoid prolonged heating in the 400–450℃ range to prevent “475℃ embrittlement” and reduced impact toughness.
* For large or complex parts, control cooling rate after solution treatment to prevent distortion or cracking.
* Ensure parts are fully cooled to room temperature before aging to achieve uniform precipitate distribution.