Improvement of the forgability of 17-4 precipitation hardening stainless steel by ausforming

Abstract

The ausforming process for 17-4 precipitation hardening (17-4PH) stainless steel is compared with the conventional warm-forging process, from the point of both the forgeability and the properties of the forged material. The forgeability is evaluated by upsetting, forward rod extrusion and backward can extrusion. The forging force required in ausforming is about half that required in conventional warm-forging: it especially keeps quite a low level in the temperature range of 200–400°C. Ausforming brings out the excellent upsettability of the material at the warm-working temperature, whilst conventional warm-forging leads to a drastic decrease of workability at 400°C. Forward extrusion and backward extrusion with a reduction of area of 30–70% are also achieved without cracks and seizures. The properties of the forged material are examined with the hardness test, tensile test, impact test, delayed cracking test and the corrosion resistance test. The mechanical properties of the ausformed material are slightly superior compared with those of the solution treated and aged (ST-AG) material. The Charpy impact value of the ausformed and directly aged (AU-AG) material is superior to that of the ST-AG material, keeping nearly the same level from room temperature to −100°C. The corrosion resistance of materials forged by the two processes are almost the same. The surface quality of the ausformed product is good compared with that of the cold-forged product. The ausforming process is applied successfully to the production of various 17-4PH stainless steel parts using a multi-stage forging machine.