Different Base Materials of Forgings Lead to Varied Forging Requirements

 Different Base Materials of Forgings Lead to Varied Forging Requirements

Stainless steel is a high-alloy steel with low carbon content (mass fraction of carbon generally ≤ 0.4%) and multiple alloying elements (mass fraction of alloying elements > 13%). Based on its matrix structure, stainless steel can be classified into ferritic, austenitic, and martensitic stainless steels. Stainless steel is characterized by high deformation resistance, low thermal conductivity, sensitivity to overheating, and poor ductility.

The forging requirements vary significantly depending on the base material of stainless steel forgings.

1. Forging Ferritic Stainless Steel

Ferritic stainless steel forgings (e.g., 20Cr13, 10Cr17) contain low carbon (mass fraction ≤ 0.2%) and high chromium (mass fraction: 16%–30%). These steels undergo no structural transformation during heating or cooling, meaning heat treatment cannot refine grains or enhance strength. Only forging can refine their grains. Key points for forging ferritic stainless steel:

· Heating Process:

o Grain growth begins at 600°C. Avoid excessive heating temperatures and prolonged holding.

o Initial forging temperature: 1100–1150°C; final heating temperature ≤ 1000°C.

o Slowly heat to 760°C first, then rapidly raise to the forging temperature to minimize high-temperature exposure.

· Forging Requirements:

o Fully deform the material to refine grains (final forging pass deformation ≥ 12%–20%).

o Final forging temperature: ≤800°C (to prevent grain coarsening) but ≥750°C (to avoid work hardening).

· Post-Forging Treatment:

o Short-term annealing at 700–800°C restores non-brittle properties.

2. Forging Austenitic Stainless Steel

Austenitic stainless steel (e.g., 12Cr18Ni9, 17Cr18Ni9) has low carbon (mass fraction < 0.25%), chromium (17%–19%), and nickel (8%–18%). Its grains grow easily at high temperatures but less aggressively than ferritic types.

Key Forging Guidelines:

· Heating Process:

o Initial forging temperature: 1150–1180°C; final forging temperature ≥ 850°C (to avoid carbide precipitation and cracking).

o Use a weakly oxidizing atmosphere to prevent carburization and intergranular corrosion.

· Forging Requirements:

o For ingots: Begin with light forging; apply heavy forging only after ≥30% deformation.

o Forging ratio: 4–6 for ingots, 2–4 for billets (depending on grain size).

o Ensure uniform deformation via methods like smooth dies, preheated tools (150–450°C), or stacked forging.

o Account for high shrinkage (1.5%–1.7%) during cooling.

· Post-Forging Treatment:

o Air-cool, pit-cool, or sand-cool.

o Perform solution treatment (1020–1050°C + water quenching) to dissolve carbides and refine grains.

3. Forging Martensitic Stainless Steel

Martensitic stainless steel (e.g., 20Cr13, 30Cr13, 40Cr13) has higher carbon (0.1%–4%) and chromium (12%–14%). It transforms from austenite at high temperatures to martensite upon cooling, enabling heat treatment for enhanced mechanical properties.
Key Forging Guidelines:

· Heating Process:

o Initial forging temperature: 1100–1150°C.

o Heat slowly below 850°C to avoid cracking, then rapidly raise to forging temperature.

· Forging Requirements:

o Avoid heavy forging in the 900–950°C range to prevent embrittlement.

o Final forging temperature: ~900°C.

· Post-Forging Treatment:

o Cool slowly in hot sand or a furnace to mitigate martensite-induced stresses.

o Anneal promptly to relieve internal stresses and reduce hardness for machining.

Summary

· Ferritic: Focus on grain refinement via controlled heating/deformation.

· Austenitic: Prioritize uniform deformation and post-forging heat treatment.

· Martensitic: Balance heating rates and post-forging cooling to manage phase transformation stresses.

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