Surface hardening of precipitation-hardening nonmagnetic steels

1. Precipitation-hardening nonmagnetic steels of the Fe−Mn−Ni system can be surface hardened by nitriding and also by work hardening of the surface. Surface layers with an elevated hardness to a depth of 0.3 mm can be obtained by nitriding of high-strength nonmagnetic steel 40G14N9Kh3YuF2. The results of comparative wear resistance tests showed that nitrided steel 40G14N9Kh3YuF2 is not inferior to steel 38KhMYuA, which has a fairly high wear resistance after nitriding. 2. Studies of the effect of preliminary heat treatment and the conditions and medium of nitriding on the hardness, depth, and condition of the nitride case showed that the best nitride case on steel 40G14N9Kh3YuF2 is obtained after nitriding at 560 and 600° in ammonia diluted with nitrogen when the steel has a fine-grained structure (after forging or after quenching from 1100° and aging), with an austenite grain size not lower than grade 6. 3. Surface hardening of steel 40G14N9F2 by roller burnishing and by shot peening increases the fatigue strength of smooth samples by 25–30% and notched samples by 90–140%. The increase in the fatigue strength is due to the high surface hardness (H 550) and compressive stresses (σcom~50–70 kg/mm2) in the surface layer.