Effect of slag properties and nitrogen addition on behaviour of alloying elements during ESR of AISI M41 tool steel A. Ahmed* and A. Fathy High speed tool steels are high alloyed high carbon steels. The processing of these grades is affected to a large extent, by the behaviour of the alloying elements. In this work, the effect of nitrogen alloying, physical properties and chemical composition of fluxes used in electroslag refining (ESR) on hardness, microstructure and alloying elements recovery and homogeneity of two grades, conventional and nitrogen alloyed AISI M41 high speed tool steel, were studied. One melt of each selected grade was melted in an induction furnace. The ingots were then used as consumable electrodes in ESR, where each steel grade was remelted under three different prefused CaF 2 based fluxes. The alloying element behaviour was investigated. The highest recovery was obtained using ESR flux of high viscosity and low metal/slag interfacial tension. Increasing the nitrogen content of the steel results in an increase in alloying element recovery. Furthermore, the ESR process improved the shape, size and distribution of precipitates in the ingots. Also, hardness was improved by both ESR and nitrogen alloying. Keywords: M41, Electroslag remelting, Slag properties, Alloying element, Microstructure and hardness Introduction High speed tool steels are high alloyed high carbon steels with a complex pattern of carbides. 1,2 AISI M41 high speed tool steel is called ‘super hard tool steel’ due to its high hardness level (65–70 HRC) and is often used for making tools, balls and roller bearings, measuring gauges, many types of springs, elements of fuel supply systems, some parts of machine tools and various machines. Nitrogen produces some beneficial effects in steels and therefore it can be regarded as a significant alloying element. In addition to its role with aluminum as a grain refining agent, nitrogen produces marked solid solution hardening and precipitation strengthening reactions which form the basis of many high strength grades. 3,4 Nitrogen additions are also particularly beneficial to the constitution and pitting resistance of austenitic stainless steel grades. 5,6 Nitrogen alloying up to 0?18% of low cobalt contain- ing steels (Co about 2 wt-%) leads to an increase in secondary hardness and heat resistance to the level of DIN S 6-5-2-5 steel with 5 wt-% Co (AISI M41). The magnitude of secondary hardness and heat resistance of DIN S-6-5-2-5 and DIN S6-5-2-10 high cobalt steels cannot be substantially increased by nitrogen alloying; however, it was concluded that by nitrogen alloying, it is possible to reach a high level of the basic properties at lower cobalt content. 7 Electroslag refining (ESR) is an advanced technology for production of high quility steels. The improvement in the quality of steel ingots produced by the ESR process arises from obtaining sound ingots with complete absence of pipe and porosity, clean smooth surfaces, and high product yield. The structure of ESR ingots may be improved through the uniformity, elimination of banding and zone segregation, control over the direction and rate of solidification, control of grain size and control of carbides size. 8–10 Additionally, ESR has a pronounced effect on the as cast structure as a result of the positive influence of decreasing the local solidification time of the metal which results in the formation of fine carbides rich in alloying elements and carbon. 11,12 During ESR, the molten metal produced in the form of droplets or a continuous stream passes through the molten slag. Therefore, by suitable choice of slag composition and properties, chemical reactions can be encouraged or inhibited, i.e slag may be selected to inhibit removal of elements that are to be retained, like strategic expensive alloying elements. This work aims at developing a new grade of steel comparable to AISI M41 high speed steel alloyed with nitrogen, and studying the effect of the ESR flux on various physical and chemical factors. Steel Technology Department, Central Metallurgical Research & Development Institute, PO box 87 Helwan, 11421 Cairo, Egypt *Corresponding author, email azzaazza40@yahoo.com 458 ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 23 October 2007; accepted 4 April 2008 DOI 10.1179/174328108X318356 Ironmaking and Steelmaking 2008 VOL 35 NO 6