Contact fatigue and crack propagation studies of sintered PM steel

Abstract: Gears made of PM steel are of interest for the automotive industry because from the press/sinter route, complex geometries can be produced to near net shape with only a few processing steps while keeping the material and energy usage at a minimum. However, the PM steels of today have significant performance limitations caused by the inherent porosity which impairs the mechanical properties by acting as stress raisers in the material. The teeth of automotive gears are subjected to pure rolling and sliding as well as a mixture between the two, leading to different mechanisms of wear at different parts of the gear surface. Pure rolling occurs only at the pitch line and the sliding to rolling ratio increases with distance from it. At the vicinity of the pitch line, the force acting on the gear teeth is at its maximum and the repeated cyclic loading leads to fatigue wear. For PM steels, which due to the porosity are highly sensitive to cyclic loads, this results in crack initiation at pores underneath the surface, eventually causing failure of the gear teeth as these cracks propagate to the surface. The SSF project Nanotechnology Enhanced Sintering Steel Processing aims to reduce the gap in performance between PM steels and wrought steels by using nanopowders as a compliment to the standard microsized powder to increase the performance of the material. This thesis aims to increase the knowledge of how different manufacturing parameters, such as sintering temperature, initial powder particles size distribution and addition of nanopowder affect the fatigue properties of the materials. This was done by developing new test equipment for rolling contact fatigue which simulates the contact at the pitch line in a spur gear contact. Utilizing extensive image analysis enables coupling of the different input parameters and their respective pore and microstructure to the fatigue properties. Furthermore, I have investigated how cracks propagate in PM steels using a novel setup which can propagate short cracks in the material. Extensive EBSD studies were performed on the materials after sintering, after case hardening and on cracks after testing. From the rolling contact fatigue tests and image analysis, it was found that materials manufactured using a finer powder fraction together with a higher sintering temperature results in a material well suited for handling cyclic fatigue despite the inherent porosity. Also, by adding a relatively small amount of nanopowder to the standard fraction the fatigue properties were improved considerably. The improved fatigue properties are probably attributed to the reduction of pores with a size in the range of 200 – 400 µm2. These pores are probably the cause of early crack initiation as they are large enough in size as well as in numbers. Smaller pores, although many in numbers do not appear to promote early crack initiation and larger pores, although critical in terms of size, are too few to statistically affect the outcome of the fatigue tests. From the crack propagation studies, it was found that cracks appear to propagate along prior austenite grain boundaries, which for sintered steels are highly related to the particle boundaries after sintering. A finding which helps to explain the deflective pattern of crack propagation found in the rolling contact fatigue studies.