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Abstract: Crack nucleation mechanisms in two types of cold work tool steels were evaluated under monotonic and cyclic loading conditions. The effect of the microstructural constituents: the primary alloy carbides and the tempered martensite matrix, and their interaction, was identified through fractographic analysis and determination of mechanical properties such as the bending strength, , the fatigue limit, , and the fracture strength of the primary carbides under static tensile stressing, . The response under monotonic loading was found to be governed by the fracture of primary carbides. The cracks nucleated when the applied stresses were higher than the carbides fracture strength, accordingly depending on their properties, morphology and arrangement. Under cyclic loading, despite failure origins were located at the primary carbides and cracks emanating from them and propagating through the metallic matrix were evidenced, crack nucleation phenomena could not be explained as in monotonic loading since the applied stresses in fatigue were lower than the determined . Carbides fracture was then probably caused by damage observed in the metallic matrix. Primary carbides acted as stress concentrators and strain localization was more likely occurring in the matrix around them. Thus, fatigue failure was attributed to the destabilization of the tempered martensite of the matrix, induced by the strain localization processes around carbides, which produced their breakage and gave rise to fatigue propagating cracks. [Copyright &y& Elsevier]

The automotive manufacturers currently use advanced high-strength steels (AHSS) to produce lightweight vehicles with increased safety performance. While showing excellent strength-to-weight ratios, AHSS have several limitations due to the high loads required in cold forming and cutting processes, which lead to accelerated wear and premature fracture of tools. Thus, new tool materials with improved mechanical behavior ought to be developed with regard to the tool failure mechanisms. The aim of this work is to shed light on the fracture mechanisms acting in tools (i.e. crack nucleation and propagation) applying the acoustic emission (AE) technique. Bending tests using two different tool steels were monitored in order to establish a relationship between AE signals and fracture events. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the hardness and fracture toughness of different primary carbides present in tool steels have been evaluated by nanoindentation. Toughness was evaluated by the indentation method using Laugier’s equation, after experimentally assessing its applicability. The results obtained allow different types of carbides to be discerned, in terms of hardness and toughness, which ranges from 2.2 to 3.7MPam1/2. It has been found that M7C3 carbides exhibit a marked fracture resistance anisotropy, which is lower in the direction parallel to its larger edge. This anisotropy is related to the crystalline texture. The relationship between hardness and toughness of the analysed carbides can be used as a guideline for the selection of the most appropriate type of carbide that the microstructure should contain in order to optimize the wear behaviour and the fracture resistance of a tool steel. [Copyright &y& Elsevier]

The paper presents the microstructural investigation of a friction-welded joint made of 316L stainless steel with an ultrafine-grained structure obtained by hydrostatic extrusion (HE). Such a plastically deformed material is characterized by a metastable state of energy equilibrium, increasing, among others, its sensitivity to high temperatures. This feature makes it difficult to weld ultra-fine-grained metals without losing their high mechanical properties. The use of high-speed friction welding and a friction time of <1 s reduced the scale of the weakening of the friction joint in relation to result obtained in conventional rotary friction welding. The study of changes in the microstructure of individual zones of the friction joint was carried out on an optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM) and electron backscattered diffraction (EBSD) analysis system. The correlation between the microstructure and hardness of the friction joint is also presented. The heat released during the high-speed friction welding initiated the process of dynamic recrystallization (DRX) of single grains in the heat-affected zone (HAZ). The additional occurrence of strong plastic deformations (in HAZ) during flash formation and internal friction (in the friction weld and high-temperature HAZ) contributed to the formation of a highly deformed microstructure with numerous sub-grains. The zones with a microstructure other than the base material were characterized by lower hardness. Due to the complexity of the microstructure and its multifactorial impact on the properties of the friction-welded joint, strength should be the criterion for assessing the properties of the joint. [ABSTRACT FROM AUTHOR]