Characterization of industrial high speed steel roll material and effect of annealing prior to heat treatment on the structure and properties
Hot strip mills (HSM) are used to roll cast slabs down from their initial size to plate or sheet, which is then used to manufacture a wide variety of goods for many industrial markets. The working rolls that contact and deform the strip are arguably the primary element of the hot strip mills. The work rolls must have sufficient strength to apply the mechanical force to the strip, while still resisting the wear and thermal stresses inherent to the process. The history of the development of these work rolls spans more than a hundred years. The more recent advancement in work roll technology was the development of High Speed Steel (HSS) work rolls during the 1990's. The HSS rolls utilise a variety of carbide forming elements to improve the wear resistance. The South African Roll Company (SARCO) is an established roll producer and the only one in South Africa with a well-established international client base. SARCO is currently developing a range of high quality HSS rolls and has developed an initial HSS grade, which has shown competitive performance and durability in service compared to established HSS alloys from other roll manufacturers. Although the initial trials show great promise, significant potential for development and associated improvements are available. It has been documented that an additional annealing treatment prior to the regular heat treatment will refine the microstructure of HSS rolls, which should improve mechanical properties and performance. However, although pre-annealing has been documented, the degree of mechanical property improvement associated with the pre-annealing has not been studied and the annealing step does not appear to be commonly applied by roll producers. The first aim of this research project was to investigate the properties of the HSS material currently produced by SARCO to elucidate the source of the combination of high wear resistance and durability. The roll material identified for characterisation consisted of four sets of samples. These were subjected to intensive characterisation, which included chemical analysis, ferrite scope measurements, metallography, hardness testing, Scanning Electron Microscopy (SEM) analysis, X-Ray Diffraction (XRD) analysis and Simultaneous Thermal Analysis (STA). The good combination of performance and durability appears to be the product of the Ni additions, W:Mo ratio, high carbon content, high austenitising temperatures and balanced carbide-former additions. The second objective was to investigate the effect of annealing prior to the typical solution, "soft" quench and tempering heat treatment, on the structure and mechanical properties of the HSS material used for the outer shell of mill rolls. As-cast material was used for the analysis, which was subjected to similar intensive characterisation. High Chromium (HC) used in roll material appears to be more sensitive to both temperature and time variations in heat treatment than HSS. No grain refinement of the HSS material was achieved by the pre-annealing. Higher peak hardness and more gradual reduction in hardness beyond the peak was not found under simulated conditions, indicating that a desired increase in hardness will not be achieved in practice. It was evident that the industrial heat treatment condition cannot be sufficiently simulated by shorter time laboratory tests. The effects of pre-annealing should be researched on lower alloy bainitic HSS material if feasible and the improved wear resistance and toughness benefits of pre-annealing could be quantified by performing industrial trials and fracture toughness testing respectively.
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