An investigation into the effect of various surface treatments on fatigue life of a tool steel

The effects of nitriding, nitrocarburizing and shot peening on fatigue behavior of AISI D3 cold work tool steel were investigated. Five batches of fatigue specimens were manufactured according to the ASTM E466-96 standard. All specimens were quenched and tempered to a hardness of 50 HRC. One group of specimens was shot peened with an Almen intensity of 16 A, one group was nitrocarburized, one group was gas nitrided by single stage and another group was also nitrided by double stages. Rotating–bending fatigue tests were carried out. Our results showed that nitriding and nitrocarburizing treatments did not have a beneficial effect on the fatigue resistance of D3 tool steel. Fatigue test results revealed that shot peening increased the fatigue life of specimens by about 14%, while double stage nitriding decreased it by about 8%. Nitrocarburizing and single stage nitriding decreased the fatigue life of specimens by about 29% and 50%, respectively. Nitriding and nitrocarburizing treatments may improve abrasive-wear resistance of this material by increasing surface hardness but may promote the easy nucleation of a fatigue crack in the brittle surface layer and its unimpeded growth into the weaker substrate material. However, double stage nitriding is less damaging to the fatigue behavior of this steel. SEM and metallographic investigations indicated that a compound layer with porous structure was formed during nitriding and nitrocarburizing. So shot peening is the best treatment to improve the fatigue life of AISI D3 tool steel.