Lifetime Controlling Defects in Tool Steels

Powder metallurgy (PM) tool steels offer improved mechanical properties compared to their conventional counterparts obtained by ingot metallurgy (IM), in part due to higher alloy element contents being possible but also due to their finer and isotropic microstructure. Traditionally, the mechanical strength of PM tool steels has been controlled by micropores and slag inclusions which in practice result in unpredictable failure. Due to improved processing conditions however these defects have now largely been eliminated, and larger primary carbides as well as carbide clusters now may act as crack initiating sites, such sites being particularly detrimental in the case of fatigue loading and resulting in internal crack initiation. Within this project, the relationship between microstructure and fatigue properties of both PM and ingot metallurgy tool steels is to be studied, with particular focus on the role of microstructural singularities acting as internal defects which however are difficult to detect by conventional techniques. Therefore, ultrasonic high cycle fatigue testing will be used since this technique is particularly well suited to reveal crack-inducing singularities. The crack initiation processes thus occurring will be studied, and the respective effects of both size and nature of the defects will be described esp. with regard to the shape of the S-N curve and the existence of a true fatigue limit. A ranking of the various types of singularities regarding their effect on fatigue behaviour will be established. From the empirical results, defect-fatigue limit relationships will be derived, based on cyclic fracture mechanics, and will be compared to such between the fatigue endurance strength and artificial microdefects introduced into the surface through Focus Ion Beam Technique. Finally, powder metallurgy cold work and high speed tool steels with extremely fine primary carbides will be manufactured through a special low-temperature hot isostatic pressing technique with subsequent cold working and optimized heat treatment to retain the fine carbides and will be used as a model material for assessing the potential of microstructural refinement for improving the mechanical properties of tool steels as well as a promising basis for very fine tools as needed e.g. in micromachining.

This research project, carried out in close cooperation between Vienna University of Tehcnology (Institute of Chemical Technolgies and Analytics) and University of Vienna (Faculty of Physics), was focused on the fatigue behavior of tool steels during cyclic mechanical loading up to very high loading cycle numbers (`gigacycle regime`). Tool steels are used in tools for cutting applications or forming processes, but also in engine components e.g. for diesel direct injection, in which they are exposed to very high numbers of loading cycles during the life of an engine. These materials are high alloy steels, which after heat treatment can reach a service hardness of 60 up to 70 Rockwell C hardness combined with high strength and toughness (bending strength >3000 MPa). The lifetime of the tools made of tool steel grades is limited mostly by wear at the tool surface - e.g. blunting of cutting edges - or by fatigue processes within the tool, i.e. by fracture. In the latter case it is of decisive importance to identify the crack-initiating features within the microstructure. In this project, an ultrasonic frequency resonance testing method was employed operating at 20 kHz. This enabled fatigue testing up to very high loading cycle numbers in reasonable time (10 10 cycles in 5- 6 days, compared to about 8 years in standard fatigue testing). Wöhler curves (lifetime vs. applied loading stress amplitude) were determined for various grades of cold work tool steels and high speed steels; all of them showed a steadily decreasing slope. Thus the existence of a true fatigue limit, which is still frequently claimed to hold for steels, could be clearly disproved at least for the tool steels studied here. Correct fatigue data are however only obtained if residual stresses in the specimens are avoided; it was found that compressive stresses, which are easily introduced into the specimen surface during preparation, strongly affect the fatigue stress levels. Furthermore, it could be shown that fatigue testing up to very high loading cycle numbers (up to 1010) is well suited for identifying crack nucleating micro-constituents in the material, whether they are singularities such as nonmetallic inclusions or large carbide clusters, or standard microstructural features such as primary carbides. It could be shown that in conventional wrought tool steels, produced by ingot metallurgy, large primary carbides and carbide clusters, which are regular constituents of the material and are responsible for the high hardness and excellent wear resistance of the material, act as crack nucleation sites and are thus responsible for fatigue failure. In powder metallurgy tool steels, in contrast, relatively rare nonmetallic, mainly oxidic, inclusions caused fatigue failure, since due to the special production route the primary carbides are too small (<5m) to initiate fatigue cracks. Generally the fatigue endurance strength for powder metallurgy tool steels was found to be at a level almost double that of equivalent steels produced by ingot metallurgy. This indicates that PM tool steel grades are particularly well suited for applications where fatigue loading dominates.