Influence of notches on fatigue behaviour of PM steels

Powder Metallurgy (PM) is an economical way to produce components in large series. To broaden the range of components that can be successfully produced in PM steel, improved design criteria are needed. Notched fatigue is one domain where the present knowledge is insufficient for the design of highly stressed components.

This work is an investigation of the notched fatigue performance of five PM steels from Höganäs AB. Four point bending fatigue tests (R=-1 and 0) of notched and un-notched specimens were performed to determine the notch sensitivity index, q. SEM fractography was used to reveal the different mechanisms of crack initiation and propagation.

The investigated materials were: Distaloy AE, Distaloy DC, Distaloy HP, Fe +2,0% Cu + 0,8% C, Astaloy Mo, and Astaloy CrL. All materials were pressed to a green density of 7,05-7,10 g/cm3 and sintered 30 min at 1120 deg C. CrL was also sintered at 1250 deg C.

The results showed that the heterogeneous materials (Distaloys and FeCuC) showed low notch sensitivity, q=0,6-0,8 and that the homogeneous materials (Astaloys) showed notch sensitivities in the range q= 0,6-1. Thus, no general rule that covers all PM steels can be stated. High temperature sintering of Astaloy CrL increased both the fatigue limit and the notch sensitivity.

SEM fractography showed that the materials with low notch sensitivity displayed a more trans-particle crack initiation and the materials with high notch sensitivitiy displayed more sinter-neck crack nucleation.

Author: Ericsson, Hans

Source: Luleå University of Technology

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