Stainless Steel Forging Guide: Top 3 Types Explained

Stainless steel is a type of high-alloy steel with low carbon content (generally not exceeding 0.4% by mass) and containing various alloying elements (with a mass fraction of alloy elements greater than 13%).

Stainless steel is divided into ferritic stainless steel, austenitic stainless steel, and martensitic stainless steel based on its matrix structure. Stainless steel has high deformation resistance, low thermal conductivity, strong overheating sensitivity, and poor forgeability.

Forging of ferritic stainless steel

Ferritic stainless steel contains a small amount of carbon, with a chromium mass fraction of 16% to 30% (such as 20Cr13, 10Cr17, etc.). This type of steel does not undergo structural transformation during heating and cooling, and cannot use heat treatment methods to improve strength or refine grains, only forging methods can refine grains. Its recrystallization temperature is low, recrystallization speed is fast, grains tend to grow during heating, and forgeability is poor. The key points of the ferritic stainless steel forging process are as follows:

1) The grain of ferritic stainless steel begins to grow at 600°C. To avoid grain coarsening during the heating process, the heating temperature should not be too high, and the holding time should not be too long. The commonly used initial forging temperature is 1100 to 1150°C, and the heating temperature of the last fire should not exceed 1000°C. To reduce the residence time of the billet at high temperatures, it should be rapidly heated to the initial forging temperature after slowly heating to 760°C.

2) Since ferritic stainless steel cannot use heat treatment methods to refine grains, it must be fully forged during forging to refine the grains and ensure sufficient deformation and uniform deformation. The deformation of the last fire should be greater than 12% to 20%. The final forging temperature should be below 800°C to prevent the refined grains from re-aggregating. However, to avoid work hardening caused by too low a final forging temperature, the final forging temperature should not be lower than 750°C.

3) After forging, it should be air-cooled to disperse quickly through the 475°C embrittlement zone. A short-time annealing above 550°C (generally 700 to 800°C) can restore the embrittled stainless steel to its original non-brittle state.

Forging of austenitic stainless steel

The carbon mass fraction of austenitic stainless steel is less than 0.25%, the chromium mass fraction is 17% to 19%, and the nickel mass fraction is 8% to 18%, such as 12Cr18Ni9, 17Cr18Ni9, etc. Austenitic stainless steel does not undergo structural transformation when cooled, nor can it use heat treatment methods to increase strength and refine grains, only thermal forging deformation and recrystallization can be performed. Austenitic stainless steel grains tend to grow at high temperatures, but the tendency to grow is not as strong as that of ferritic stainless steel.

The heating specifications for austenitic stainless steel are similar to those for ferritic steel, with the initial forging temperature generally being 1150 to 1180°C, and the final forging temperature should not be lower than 850°C; otherwise, the precipitation of carbides in the structure increases the deformation resistance, making forging prone to cracking.

 Key points of the austenitic stainless steel forging process are as follows:

1) Strictly avoid carburization during heating. Carbon and chromium easily form chromium carbide compounds at grain boundaries, which depletes the chromium content in the matrix near the grain boundaries and increases the steel’s sensitivity to intergranular corrosion. Heating should be done in a weakly oxidizing atmosphere.

2) When forging steel ingots, start with light pressing. Only when the deformation of the steel ingot reaches 30% can heavy pressing be applied. During forging, the ingot should be fed in one direction to avoid repeated hammering at one spot, which prevents the occurrence of central cross cracks.

3) The forging ratio for steel ingots is set between 4 to 6, and for billets between 2 to 4, depending on the grain size of the raw material. The grain size of austenitic stainless steel greatly affects the corrosion resistance of the steel. To obtain a fine grain structure, ensure that the last heat has a sufficient forging ratio, with a deformation amount greater than the critical deformation degree for recrystallization.

4) Uniform deformation is required during the deformation process to achieve a more uniform grain structure. The following measures can be considered for round disk forgings:

• Use a smooth platform and anvil surface, lubricate if necessary;
• Preheat the platform and anvil surface to 150 to 450°C;
• Add low carbon steel pads to both ends of the disk blank;
• Use stacking forging;
• Apply intermittent compression during deformation;
• Use a casing for upsetting.

5) Austenitic stainless steel has a particularly large shrinkage rate. When forging is in its final shape, a larger shrinkage rate (1.5% to 1.7%) should be considered to avoid the forged piece being scrapped due to insufficient dimensions after cooling.

6) Air cooling after forging. Austenitic stainless steel can be air cooled, pit cooled, or sand cooled after forging.

7) In order to re-dissolve the carbides precipitated during the forging and air cooling process into the austenite, obtaining a uniform and single austenitic structure at room temperature, stainless steel should undergo solution treatment, i.e., heating and holding at 1020 to 1050°C, then water cooling. The temperature should not be too high, and the holding time should not be too long to prevent grain growth.

3. Forging of martensitic stainless steel

Martensitic stainless steel has a carbon mass fraction of 0.1% to 4%, and a chromium mass fraction of about 12% to 14%, such as 20Cr13, 30Cr13, 40Cr13, etc. This type of steel is austenitic at high temperatures and transforms into martensitic structure when cooled to room temperature. It has higher hardness than ferritic and austenitic stainless steels, and its grain size can be refined and mechanical properties improved by heat treatment.

The heating temperature for martensitic stainless steel should not be too high, as too high a temperature can lead to the formation of δ-ferrite, reducing the steel’s plasticity. The initial forging temperature is generally 1100 to 1150°C. This type of steel has poor thermal conductivity, and rapid heating can easily cause cracking. Therefore, it must be heated slowly before reaching 850°C, and only after the plasticity is improved can it be rapidly heated to the initial forging temperature.

This type of steel is a single-phase austenitic structure at high temperatures, and there are no special difficulties in forging, but heavy strikes should be avoided in the 900 to 950°C range to prevent shattering. The deformation amount of the last fire also has no special requirements, and the final forging temperature is generally around 900°C.

If this type of steel is cooled in air after forging, it will immediately transform into a martensitic structure. There are significant thermal stresses, forging residual stresses, and structural stresses within the forged piece, which can easily lead to surface cracking. Therefore, after forging, it should be slowly cooled in hot sand or in a furnace, and timely annealing should be performed to eliminate internal stresses and reduce hardness, facilitating machining.