Effects of hot forging on the structural condition in HS 6-5-2 high-speed steel

Abstract

Microstructure analysis was performed on rolled bars of high-speed steel after two and three forging cycles, each cycle comprising one upsetting and one drawing out operation. High-speed steels belong to difficult-to-form materials with a narrow forging temperature interval. Forging above the maximum forging temperature may lead to grain coarsening. Below the minimum forging temperature, deformation resistance of the material increases, and the workpiece may fail. Using numerical modelling, special forging dies were designed and effective strain distribution was calculated for an axial cross-section plane in specimens after two and three forging cycles. The purpose of the analysis was to identify the relationship between the amount of effective strain and the shape and size of austenite grain and the volume fraction and density of carbides after forging. The size of prior austenite grains was measured using the linear intercept method which is based on the Snyder-Graff method. Grain shapes were characterized in terms of circularity, which is the difference between the shape in question and a circle. With increasing amount of strain, the grains in the material became finer, as undissolved carbides impeded grain growth. In as-received rolled condition, the austenite grain size was G9. After three forging cycles, it was smaller, G11 (the higher the number, the smaller the grains). Circularity characterizes the complexity of a grain shape. Micrographs of carbide particles were taken using a scanning electron microscope and examined with NIS Elements image analysis software. The majority of carbides were sized between 0.2 and 2 μm. The carbides which are less than 1 μm in size do not shrink in response to increasing strain and their quantity does not change appreciably. Carbides with a size of 1-2 μm show a different behaviour. In the central region of specimens, where strain is the largest, their amounts are much larger than in less-worked regions.