III. Main Forging Process Routes

1. Hot Forging (Mainstream)

The vast majority of stainless steel forgings are produced via hot forging.

• Heating Temperature: Depends on grade. Austenitic: 1150-1250°C; Ferritic: lower.
• Key Steps:
  • Heating: Step heating or controlled ramp rates to prevent thermal stress cracking (especially in large cross-sections).
  • Preforming: Upsetting, drawing out, piercing, etc.
  • Forming: Die forging, open die forging, or ring rolling.
  • Post-Forging Treatment: Air cooling, slow cooling, or water quenching based on steel type.

Typical Case: A 15.6m diameter austenitic stainless steel support ring for a Generation IV nuclear reactor, formed using “metal constructive technology + ring rolling.” A single piece, weld-free, supporting a 7000-ton load.

2. Warm Forging (Near-Net Shape)

Applicable to specific grades (e.g., TWZ series):

• Process Flow: Forging completion (900-1000°C) → In-line quench in a solution treatment tank (25-30°C water, 25-90s) → Cooling to 550-650°C → Warm forging in this temperature range (≥3 min, 10-20mm reduction in diameter).
• Advantages: Reduces heating cycles, saves energy, product yield can exceed 90%.

3. Radial Forging (Precision Long Shafts/Tubes)

Radial forging machines are ideal for tube billets, bars, and stepped shafts.

• Typical Parameters (Based on TISCO Duplex Stainless Steel Radial Forged Tube Billet Patent):
  • Billet Heating: 1250±10°C, soaking 2.0-8.0h.
  • Start Forging Temperature: 1050-1150°C.
  • Three-Stage Deformation Ratio: 5-8% → 10-15% → 20-30% (progressively increasing).
  • Post-Forging Annealing: 1050±20°C, 6-12h, water quench.
• Austenitic Stainless Steel Radial Forging: Use ”small deformation, high-frequency forging” for breakdown. This creates a fine-grained surface layer, reducing cracking risk. Follow with large deformation, low-frequency forging to achieve full cross-section dynamic recrystallization.

IV. Core Technical Elements

1. Heating and Temperature Control

• Equipment: Box furnaces, car-bottom furnaces, induction heating. Protective atmospheres or coatings are recommended for high surface quality requirements.
• Temperature Window: Strictly monitor initial and final forging temperatures. For austenitic grades: too low final temperature causes work hardening and cracking; too high causes coarse grains, difficult to refine during solution treatment.
• Soaking Time: Determined by billet cross-section. Excessive time leads to grain growth and increased chromium-depleted layer depth.

2. Deformation Amount and Forging Ratio

• Ingot Breakdown: Requires a sufficient forging ratio (typically ≥3) to break down as-cast dendrites and heal internal porosity.
• Final Heat Deformation: Directly impacts final grain size. Research indicates that increasing reduction per pass and reducing the number of heating cycles helps refine grains and improve yield strength.
• Deformation Distribution: Duplex stainless steel radial forging uses an incremental deformation strategy to avoid cracking from a single, large reduction.

3. Post-Forging Cooling and Heat Treatment

• Austenitic Stainless Steel: Typically air cooled. No phase transformation impact, but solution annealing is required to eliminate work hardening and restore corrosion resistance.
• Martensitic Stainless Steel: Must be slow cooled (pit, sand, or furnace cooling) post-forge, followed immediately by annealing to prevent air-hardening cracks.
• Duplex Stainless Steel: Requires rapid cooling (water quench) post-forge to inhibit σ-phase precipitation.
• Precipitation Hardening Stainless Steel: Can be directly solution treated after forging (or combined with forging heating), followed by aging.

4. Tooling and Lubrication

• Dies designed for carbon steel forgings can be reused. However, die materials must withstand higher temperatures and thermal fatigue.
• Lubrication: Water-based graphite, glass-based protective lubricants. Reduces friction, lowers forging load, and improves metal flow.