Influence of carbide and inclusion contents on the fatigue properties of high speed steels and tool steels

In this study the influence of carbide distribution, inclusion contents and the surface machining process on the fatigue properties of tool steels and high speed steels were investigated. Four different steels intended for cold work applications were included, of which three were powder metallurgically processed and one was conventionally ingot cast. One of the powder metallurgy steels was studied in four surface conditions namely hard turned, ground, polished and shot peened. Fatigue testing was performed on hour glass shaped specimens with a load ratio of R=0.05. The staircase method was used to determine the fatigue strength corresponding to a life of two million cycles. The causes of fatigue failures were determined from fracture surfaces. For the polished specimens of all steel grades it was found that internal inclusions and carbides caused the failures. Crack initiating internal carbides were found more frequently for one highly alloyed powder metallurgy steel and for the conventionally cast steel. For the other polished series, internal inclusions were controlling the fatigue failures. Surface crack initiation was encountered on the ground specimens as well as on the specimens that were hard-turned and subsequently shot peened as a final machining process. Fracture mechanics was used to describe the relation between inclusion and carbide sizes observed on fracture surfaces and the fatigue strength. A model was developed to predict the fatigue strength of the specimens. It was shown that the model managed to determine the fatigue life limiting factors, i.e. internal inclusions in “PM23” and ASP2014 and internal inclusions and internal carbides in VANADIS10 and M2. However, the predicted fatigue limits overall were lower for all steels than what was observed in practical experiments, hence the model was conservative.