Microstructure Enhancement for Fatigue Improvement
EURO PM2003, in Valencia, Spain. Author: Anders Bergmark PM steels with excellent fatigue properties are characterized by high performance microstructures and high density. Much effort has been put on reaching high densities during the last years. High density is important to get high rolling contact fatigue properties. It is well known that a martensitic surface is of outmost importance to get a material that is resistant to high Herzian pressure in combination with sliding. When bending fatigue is considered, however, it must be pointed out that high density alone has a relatively low impact on the fatigue properties. The two microstructure extremes from mechanical performance point of view are Ferrite and Martensite. A survey of data presented in the literature indicates a fatigue limit of 380 MPa in rotating bending for 100% martensite . The alloying system and processing route to get through hardening seems not to be important . The bending fatigue limit at density 7.1 g/cm³ is about 120 MPa for 100% ferrite and 210 MPa for Copper-Carbon PM steel with 2% Cu and 0.8%C. These values indicate that the fatigue performance of martensite is almost 3 times superior to ferrite and almost 2 times superior to Fe2Cu0.8C. The influence of density is twofold. Increased density normally implies smaller pores that are favorable, as higher – critical - strains are needed to start crack initiation. Increased density also means a higher Young’s Modulus, which in turn demands higher stress to obtain the critical strain . An evaluation from data in the literature  reveals that the influence of density in the density interval 7.0 – 7.4 g/cm³ on the fatigue performance is about 5 – 10% per 0.1 g/cm³ . This indicates that density only has an impact of about plus 15 – 30% when the density is increased from 7.1 to 7.4 g/cm³. Microstructure is obviously much more important than density from the bending fatigue performance point of view. This paper presents an overview of the influence of microstructure on the fatigue performance of PM steel. The reference material is Astaloy Mo, 1.5%Mo pre-alloyed material with 0.6% sintered carbon and 100% upper bainitic microstructure. The influence on the fatigue performance is evaluated for systematic variations of the microstructure.