Forging of Stainless Steels: Ferritic, Austenitic & Martensitic Varieties

In our daily lives, stainless steel products are everywhere, from the gleaming cutlery in the kitchen to the sterilized medical instruments in the hospital, and to the sturdy and durable machine parts in the factory. Stainless steel, with its excellent corrosion resistance and high strength, quietly guards our lives and industrial production. But have you ever wondered how these seemingly ordinary stainless steel products are transformed from rough steel ingots into the indispensable "steel guardians" in our lives through a series of complex forging processes? Today, let's delve into the world of stainless steel forging together, lift the veil of mystery, and see the unique processes and precautions for the forging of ferritic, austenitic, and martensitic stainless steels.

Overview of Stainless Steel

Stainless steel is a high-alloy steel with a low carbon content (typically not exceeding 0.4% by mass) and containing various alloying elements (with a mass fraction greater than 13%). Based on its microstructure, stainless steel is primarily categorized into ferritic, austenitic, and martensitic stainless steels. Each type of stainless steel has its own specific requirements in the forging process due to its unique composition and structure.

1. Ferritic Stainless Steel

Ferritic stainless steel contains a low amount of carbon, with a chromium mass fraction ranging from 16% to 30%. This type of steel does not undergo a microstructural transformation during heating and cooling, so it cannot be strengthened or have its grain refined through heat treatment. Instead, forging methods must be employed. Ferritic stainless steel a has low recrystallization temperature and a rapid recrystallization rate. The grains tend to grow easily during heating, resulting in poor forgeability.

2. Austenitic Stainless Steel

Austenitic stainless steel has a carbon mass fraction less than 0.25%, a chromium mass fraction of 17% to 19%, and a nickel mass fraction of 8% to 18%. This type of steel also does not undergo a microstructural transformation during cooling, and like ferritic stainless steel, it cannot be strengthened or have its grain refined through heat treatment. Instead, hot forging deformation and recrystallization are used. Austenitic stainless steel is prone to grain growth at high temperatures, but the tendency is not as strong as that of ferritic stainless steel.

3. Martensitic Stainless Steel

Martensitic stainless steel has a carbon mass fraction ranging from 0.1% to 4% and a chromium mass fraction of approximately 12% to 14%. This type of steel exhibits an austenitic microstructure at high temperatures, which transforms into a martensitic microstructure when cooled to room temperature. Martensitic stainless steel has a high hardness and can have its grain refined and mechanical properties enhanced through heat treatment.

Key Points of Forging Process for Ferritic Stainless Steel

The forging process for ferritic stainless steel requires particular attention to the control of heating temperature and soaking time to prevent grain coarsening. The specific process points are as follows:

1. Heating Temperature Control

Ferritic stainless steel begins to experience grain growth at 600°C, so the heating temperature should not be too high, and the soaking time should not be too long. The commonly used starting forging temperature is 1100°C to 1150°C, and the heating temperature for the final forging pass should not exceed 1000°C. To minimize the dwell time of the billet at high temperatures, after slow heating to 760°C, the billet should be quickly heated to the starting forging temperature.

2. Forging Process Control

Since ferritic stainless steel cannot have its grain refined through heat treatment, it is essential to forge it sufficiently to refine the grains and ensure adequate deformation and uniformity. The deformation for the final pass should be greater than 12% to 20%. The final forging temperature should be below 800°C to prevent the refined grains from re-agglomerating and coarsening. However, to avoid excessive work hardening due to too low a final forging temperature, the final forging temperature should not be below 750°C.

3. Cooling and Subsequent Treatment

After forging, the workpiece should be air-cooled in a dispersed manner to quickly pass through the embrittlement zone at 475°C. A short-time annealing process above 550°C (typically 700°C to 800°C) can restore the embrittled stainless steel to its original non-embrittled state.

Key Points of Forging Process for Austenitic Stainless Steel

The forging process for austenitic stainless steel requires special attention to atmosphere control during heating to prevent the formation of chromium carbides, thereby reducing intergranular corrosion sensitivity. The specific process points are as follows:

1. Heating and Atmosphere Control

During heating, it is crucial to strictly avoid carburization, as carbon readily forms chromium carbide compounds at grain boundaries, leading to chromium depletion in the adjacent matrix and increasing the steel's intergranular corrosion sensitivity. Heating should be conducted in a weakly oxidizing atmosphere. The starting forging temperature is generally 1150°C to 1180°C, and the final forging temperature should not be below 850°C. Otherwise, the precipitation of carbides in the microstructure will increase deformation resistance, making forging more prone to cracking.

2. Forging Process Control

When forging ingots, initial light pressing is recommended. Once the ingot has undergone a deformation of 30%, heavier pressing can be applied. During forging, the workpiece should be fed in one direction to avoid repeated hammering in the same spot, which can cause center cross cracking. The forging ratio for ingots is typically 4 to 6, while for billets it is 2 to 4, depending on the original grain size of the material. To achieve a fine-grained microstructure, the final forging pass should have a sufficiently large forging ratio, with a deformation greater than the critical recrystallization deformation level. Uniform deformation is required throughout the process to obtain a uniform grain structure.

3. Cooling and Subsequent Treatment

Austenitic stainless steel has a particularly high contraction rate during cooling. When forging the final shape, a larger shrinkage allowance (1.5% to 1.7%) should be considered to prevent the forging from being undersized and becoming scrap after cooling. After forging, air cooling, pit cooling, or sand cooling can be employed. To dissolve the carbides that precipitate during forging and air cooling back into the austenite, resulting in a uniform single-phase austenitic microstructure at room temperature, the stainless steel should undergo solution treatment. This involves heating and holding the steel at 1020°C to 1050°C, followed by water quenching. The temperature should not be too high, and the holding time should not be too long to prevent grain growth.

Key Points of Forging Process for Martensitic Stainless Steel

The forging process for martensitic stainless steel requires particular attention to the control of heating temperature to prevent the formation of delta ferrite in the microstructure, which can reduce the steel's ductility. The specific process points are as follows:

1. Heating Temperature Control

The heating temperature for martensitic stainless steel should not be too high, as excessive temperature can lead to the formation of delta ferrite, reducing the steel's ductility. The starting forging temperature is generally 1100°C to 1150°C. Due to the poor thermal conductivity of this type of steel, rapid heating can easily cause cracking. Therefore, heating should be slow up to 850°C to enhance ductility before quickly heating to the starting forging temperature.

2. Forging Process Control

Martensitic stainless steel has a single-phase austenitic microstructure at high temperatures, and there are no special difficulties during forging. However, heavy striking should be avoided in the temperature range of 900°C to 950°C to prevent shattering. There are no special requirements for the deformation of the final pass, and the final forging temperature is typically around 900°C.

3. Cooling and Subsequent Treatment

If martensitic stainless steel forgings are air-cooled after forging, they will immediately transform into a martensitic microstructure. This can result in significant thermal stresses, forging residual stresses, and transformation stresses within the forging, leading to surface cracking. Therefore, after forging, the workpiece should be slowly cooled in hot sand or a furnace, followed by timely annealing to relieve internal stresses and reduce hardness, facilitating subsequent machining operations.

Conclusion

The forging processes of stainless steel vary according to the type of steel, each with its own characteristics and requirements. Ferritic stainless steel requires strict control of heating temperature and soaking time to prevent grain coarsening. Austenitic stainless steel demands special attention to atmosphere control during heating to reduce intergranular corrosion sensitivity. Martensitic stainless steel necessitates of control heating temperature to avoid the formation of delta ferrite. By precisely controlling the heating, forging, and cooling processes, the quality and performance of stainless steel forgings can be effectively enhanced to meet the needs of various industrial applications.

In actual production, the optimization of forging processes needs to be combined with specific material properties, equipment conditions, and production requirements. Only through continuous practice and technological innovation can we ensure product quality while improving production efficiency and reducing costs.