INFLUENCE OF INCLUSION CONTENTS ON FATIGUE PROPERTIES OF SAE-52100 BEARING STEELS

The fatigue behavior of smooth specimens of bearing steels was studied in axial fully reversed loading. Fatigue life distributions were determined at a preselected stress amplitude. Five SAE 52100 variants were investigated with different inclusion contents corresponding to different levels of oxygen, titanium, and sulphur. The fatigue life distributions could be represented by dual sets of Weibull parameters except for the cleanest steels, which could be described with a single set of parameters. Inclusion size distributions were determined on metallographic sections through the steels. Distributions were recorded for sulphides, titanium nitrides, duplex inclusions, and oxides. The causes of fatigue failures were identified on the fracture surfaces. Aluminium- and silicon-containing oxides were the most frequent causes of failures. Most of these inclusions had diameters below 30-mu-m on the fracture surfaces. A model based on fracture mechanics was developed to describe the probability of fatigue failure. It was assumed that fatigue life was controlled by propagation or permanent arrest of small cracks around inclusions. The statistical nature of fatigue life in bearing steels was assumed to originate from the statistical distribution of inclusions of different sizes. The effect of different inclusion types on fatigue life was considered.