Analysis of micro-mechanical damage in tool steels coupling fracture mechanics and acoustic emission

Fracture tests and Acoustic Emission (AE), a technique providing wave-like information, were coupled in this study in order to obtain in-situ data characterization of damaging mechanisms. Characteristic AE signals, i.e. waves with different energy, frequency, amplitude, etc., were analyzed and related to micro-mechanical and damaging mechanisms taking place in the microstructure. The occurrence of these signals varied depending on the considered steel in terms, for instance, of the quantity of registered signal or the stress at which they started to be recorded. The results of this investigation permitted to set the stresses at which crack nucleation and propagation processes started to occur in two tool steels with very different microstructural properties, and they provided very helpful information to understand the failure mechanisms acting in these steels. Introduction One of the outcomes of the increasing environmental and passenger safety regulations in cars, together with the global concern of exhausting fossil fuel resources, is the construction of lightweight vehicles with increased crash resistance. New and advanced materials are currently under development to satisfy the underlying demands on vehicles. However, implementing with success these materials in body-in-white and chassis components requires optimization of the routes by which they are to be manufactured. Among the factors interfering with the viability of producing such components, the premature failure of tools is one of the most important -directly reflectingthe final price of manufactured parts. Thus, understanding the fracture events of tools is crucial to foresee tool lifetime and to further develop tool steels with improved mechanical performance. The interaction between the two main constituents of tool steel microstructures: the primary carbides and the metallic matrix, determines their mechanical properties and hereby, the performance of tools working in industrial applications. A proper comprehension of the micro-mechanical mechanisms leading to damage in the microstructure prior to failure comprises the identification, localization and quantification of the phenomena being involved in the process when a certain load is applied. In order to reach this goal, an innovative field-based approach was undertaken in this work, combining fracture tests with Acoustic Emission (AE) monitoring and wave signal analysis. Despite AE is a well known technique for non-destructive inspection, only scarce data exist in the literature concerning its application for the analysis of micro-damage of tool steels. Fukaura et al. [1] and Yokoi et al. [2] are amongst the few authors who employed this technique to test two tool steels in order to determine the progression of internal damage. The steels used were JIS SKD11 (an equivalent steel type to DIN 1.2379 or AISI D2) and a modified SDK11 with reduced Cr and C content and increased Mo and V. AE signals from carbide cracking could successfully be detected by these authors; the signals started at a certain applied load and the event rates continually increased until reaching the fracture stress. These authors stated that no continuous AE signals existed, but that numerous burst emissions at close intervals were recorded instead. Martinez-Gonzalez et al. [3] showed that three different zones could be distinguished during a bending test in a tool steel sample with regard to the AE events. In the first zone, almost no signals were detected and it neither did any damage at the microstructure. In the second zone, AE signals started to be recorded and they increased gradually in intensity and abruptly in number. During this stage, several broken carbides were discerned at the surface by means of optical microscopy due to the increase of applied stress during the test. Finally at the third zone, the amount of AE signal as well as the cumulative energy increased considerably. At this stage, many cracks were observed to propagate at the microstructure. A better understanding of the correlation between AE signals and micro-damage in monotonic conditions was given by Yamada and Wakayama [4]. Although these authors used AE monitoring to clarify the flexural fracture of cermets, they observed a rapid increase in cumulative AE energy prior to the final fracture and attributed this phenomenon to the main crack formation. They also distinguished two types of AE signal: one was a burst-type signal with high frequency and the other was a low frequency and continuous-type signal. The former was considered to be emitted from micro-cracking while the latter was due to plastic deformation of the binder phase. From this standpoint, this work deals with the coupling of AE techniques to mechanical tests, namely a three point bending test, in case of tool steels with different microstructural features in terms of size, geometry and distribution of primary carbides in the metallic matrix. The main focus here is the study of damage events under monotonic loading and obtaining net waves associated with each one of them. Experimental Procedure Two different cold work tool steels were considered in this study. The first type is a conventional ledeburitic high-carbon, high-chromium tool steel DIN 1.2379 (AISI D2). The second is a special grade of cold work tool steel developed by ROVALMA S.A., named HWS, which in comparison to the aforementioned 1.2379 has lower carbon and chromium content but higher of vanadium. 1.2379 is obtained by ingot metallurgy routes while HWS is produced by powder metallurgy. The main alloying elements found in their chemical composition are shown in Table 1. Table 1. Main alloying elements in the chemical composition of the studied steels (in wt %)