Study on transverse-longitudinal fatigue properties and their effective-inclusion-size mechanism of hot rolled bearing steel with rare earth addition

Abstract This study mainly aims to clarify the effect of rare earth (RE) addition on the fatigue properties of hot rolled high-carbon chromium bearing steel, especially crack initiation and propagation behavior induced by long strip type inclusions in the very high cycle fatigue (VHCF) regime. For this purpose, ultrasonic tension-compression fatigue tests were carried out on transverse and longitudinal specimens extracted from hot rolled bearing steels with and without RE addition. As a result, complex inclusions, usually containing inner oxide and peripheral oxide-sulfide duplex inclusion as well as (Ca, Mn)S inclusion, dominate the fatigue property of high clean bearing steel though MnS inclusions may lead to early fatigue failure of transverse specimens and contribute to fatigue anisotropy. RE addition can better improve longitudinal fatigue property of hot rolled bearing steel than transverse fatigue property by modifying common inclusions into complex RE inclusions, which deform easily during hot rolling to reduce the effective inclusion size (EIS) at fatigue source. In contrast with fatigue failure caused by inclusion particle indicating almost synchronous crack propagation in all directions from inclusion particle, the width of instant crack plays an dominating role in crack initiation and propagation process for fatigue failure induced by long strip type inclusion. Fine granular area (FGA) only exists on both sides of the wider part of inclusion strip, and the length-width ratio of instant crack may continuously decrease with crack propagation. Effective inclusion area and corresponding EIS can be determined from the perspective of crack initiation and propagation in the VHCF regime. For VHCF fracture surface with FGA, the effective inclusion area corresponds exactly to the inclusion area included in FGA.