DURING the last decades, induction heat treatments for quenching and tempering as a through-hardening technique have become more and more important. The advantages of induction heat treatments are numerous. The shorter heat treatment cycles lead to a high potential for cost savings. Furthermore, the process provides less deviation of the mechanical properties and less decarburization.[1,2] However, the mechanical properties after induction heat treatments are not always equal to conventionally heat-treated steels. Dengel studied the short time tempering of different heat treatable steels. Even though the tempering mechanisms proceed during the heating as by conventional heat treatments, the temperature regions of the four wellknown tempering stages are shifted to higher temperatures with higher heating rates. Furthermore, the dislocation densities in induction tempered steels are higher because the dwell time at elevated temperatures is shorter in case of rapid heating. Due to the higher heating rates, recovery processes are retarded and more defects, such as dislocations and vacancies, stay in the material when compared to a conventional tempered one. Consequently, this leads to a finer dispersion of cementite during tempering, which in consequence again retards recovery.[3,4] The frequently applied tempering parameter links tempering temperature and time. For a given tempering parameter, a certain hardness should be reached for a given steel. Even if the tempering parameter is taken into account, the hardness values of a heat treatable steel are lower after the induction heat treatment, while for a given tensile strength the reduction of area increases. Other researchers investigated the hardness-toughness relationship of three different heat treatable steels. Slightly lower toughness values were found after a short time heat treatment compared to a conventional heat treatment for a certain hardness level. Also an effect of the induction heat treatment on the mechanical properties of high speed steels is present. Leitner et al. observed a shift of the maximum of the secondary induction hardening curve of a high speed steel. Furthermore, a drop in the maximum hardness was found, which is related to a decrease of alloying elements available in solution due to the shorter heating and holding times. In the last years, especially electron back scatter diffraction (EBSD) has become a powerful tool to characterize the microstructure of martensitic steels in more detail. The microstructure of lath martensite consists of laths, blocks, packets, and the prior austenite grain. Laths are defined as single crystals of martensite with a high density of lattice defects. Blocks consist of several laths, which all exhibit the same crystallographic orientation. Several blocks which share the identical habit plane in the austenite are combined to a packet. To analyze the strength-structural relationships of lath martensitic steels, the block as well as the packet size are important features. The difference between the conventional and the induction heat treatment has never been studied by using a comprehensive set of experiments including high-resolution methods such as transmission electron microscopy (TEM) and EBSD. Furthermore, the individual stages during the induction heat treatment, for example as-quenched or tempered to different stages, have never been analyzed, only fully heat-treated conditions. Therefore, in the present investigation, we study the differences between conventionally and induction- hardened 42CrMo4 steel after quenching. The austenitization temperatures and times as well as the heating and quenching rates for this study are based on two industrial processes. The mechanical as well as microstructural features are investigated by means of light optical microscopy (LOM), scanning electron microscopy (SEM), TEM, EBSD, electron probe micro analysis (EPMA), and mechanical testing, such as hardness and tensile testing. Our results on the induction tempering of this class of steel will be the subject of a subsequent paper.