Role of Microstructure Heterogeneity on Fatigue Crack Propagation of Low-Alloyed PM Steels in the As-Sintered Condition

Abstract

Due to their lower production costs, powder metallurgy (PM) steels are increasingly being considered for replacing wrought counterparts. Nevertheless, the presence of a non-negligible volume fraction of porosity in typical PM steels makes their use difficult, especially in applications where cyclic loading is involved. On the other hand, PM offers the possibility of obtaining steel microstructures that cannot be found in wrought. Indeed, by adequately using alloying strategies based on admixing, pre-alloying, diffusion bonding or combinations of those, it is possible to tailor the final microstructure to obtain a distribution of phases that could possibly increase the fatigue resistance of PM steel components. Therefore, a detailed study of the effect of different microstructural phases on fatigue crack propagation in PM steels was performed using admixed nickel PM steels (FN0208) as well as pre-alloyed PM steels (FL5208). Specimens were pressed and sintered to a density of 7.3 g/cm 3 in order to specifically investigate the effect of matrix microstructure on fatigue properties. Fatigue crack growth rates were measured at four different R-ratios, 0.1, 0.3, 0.5 and 0.7 for both PM steels. The negative effect of increasing the R-ratio on fatigue properties was observed for both alloys. The crack propagation path was characterized using quantitative image analysis of fracture surfaces. Measurements of roughness profile and volume fractions of each phase along the crack path were made to determine the preferred crack path. Weak Ni-rich ferritic rings in the FN0208 series (heterogeneous microstructure) caused a larger crack deflection compared to the more homogeneous microstructure of the FL5208 series. It was determined that, contrary to results reported in literature, crack propagation does not pass through retained austenite areas even though fatigue cracks propagated predominantly along prior particle boundaries, i.e., intergranular fracture.

Effect of Short Attritor-Milling of Magnesium Alloy Powder Prior to Spark Plasma Sintering

“…At the same time, the formation of 3D net-like structure of the brittle MgO phase represents an additional weak point for crack propagation. A similar problem was recently revealed also in the case of low-alloyed steel prepared by powder metallurgy, where crack tended to propagate along with the Ni-rich ferrite present as an envelope around more ductile austenite particles [ 30 ]. As shown above, mechanical milling significantly disrupted the MgO shell forming the powder particles’ surface, leading to enhanced diffusion and more effective consolidation during the sintering.…”

Fatigue Crack Growth Characteristics of 34CrMo4 Steel for Gas Cylinders by Cold Flow Forming after Hot Drawing

“…Silva [12] revealed that the cyclic plasticity was the main controlling mechanism of fatigue cracks under the conditions of negative stress ratios. Mousavinasab et al [13] investigated the influence of different microstructures on the FCG rate of PM steel, the conclusion was drawn that reducing the formation of nickel-rich ferrite regions could make PM steel have better fatigue resistance.…”

Effects of Temperature on Wear Properties and Mechanisms of HVOF Sprayed CoCrAlYTa-10%Al2O3 Coatings and H13 Steel

“…Improving the life of the hot extrusion dies and wear-resistant mechanical parts has crucial significance in the industrial application [1][2][3][4][5][6][7]. Reducing friction and wear is an important way to prolong the life of the hot extrusion dies and wear-resistant parts [8][9][10].…”