Influence of structural state on the effects of laser treatment of steel with different chemical compositions. Part III: Tool steels

摘要

This article is the third in a series of studies concerning laser treatment technologies applied to steel with different chemical compositions. The effects of laser treatment of carbon steels [1] and constructional alloy steels [2] are discussed in the first two articles. In the first one, it is demonstrated that increasing carbon content results in clear growth of the hardness of laser-hardened steel, affects the shape of the layer and the character of discontinuities forming in the melted zone. In the second, the significance of carbon share was confirmed and the influence of alloying elements on temperability of steel was evaluated. Moreover, it was demonstrated that laser treatment destroys the diffusive, nitrided layer. The test results found here are the fruit of collaboration between three research centers: Fraunhofer Institute for Production Technology IPT - Aachen, Institute of Industrial Machinery and Transportation of Poznan University of Technology and the Metal Forming Institute in Poznan. This article contains results of laser treatment tests performed on NCI 1LV steel in its initial (annealed) state and hardened at different temperatures, including in layers situated at varying distances from one another. Furthermore, the article provides an evaluation of the quality of alloying by means of powders with varying contents of alloying ingredients of the steel intended for forging tools. The initial structure of NC11LV steel was martensitic with a small amount of residual austenite after hardening at 1020°C, and containing austenite with a small amount of carbides after hardening at 1150°C. It was determined that the hardness of steel in the melted zone (approx. 500HV0.1) is similar to the hardness of NCI 1LV steel hardened at 1150°C, and in the transitional zone, the dispersion of hardness measurement results is irregular; more so in the initial hardening zone at 1020 °C. The influence of laser beam travel speed on the state of the specimens near-surface layer is similar as in articles [1-2]; increasing travel speed reduces the layer's "parameters". Tests show that, regardless of the initial state (hardening temperature), laser treatment does not result in beneficial changes of the surface layer of NCI 1LV steel. However, positive results were obtained in tests of alloying technology for forging tools made of WCL steel. In specimens alloyed using Mo/VC powder, tempering temperature of the alloyed layers was determined (600°C), higher than the tempering temperature of high-speed tool steels. Fifteen variants were evaluated in tests on the selection of technology, in which three groups of powders with different chemical compositions were applied: WC/Co, WC/Co/Cr and Mo/VC, in the appropriate proportion of ingredients and doses. Structural tests of the layers obtained were carried out. Based on evaluation of properties obtained during hardness measurements, tests of mechanical properties (bending and fatigue tests) as well as abrasion resistance tests, the conditions of the best technology were selected. Nitriding applied in the tests caused the quality of the nitrided layer to deteriorate - similarly as before.