Tool steels are utilized on a wide variety of application including forming, shearing, cutting and molding (manufacturing the tools, dies, and molds) where (due to their remarkable properties) high wear resistance, hardness, strength, toughness, heat resistance and other properties are preferred for optimum performance. In addition to alloying, tools steels are considered special because they are very difficult to manufacture and require careful manufacturing in every processing step. The very high alloy content and special microstructure that make them desirable for severe applications also make them difficult to manufacture [ , ].
High-carbon, high-chromium (AISI D series) cold-work tool steels are of the main groups of tool steels which are identified with their high wear resistance and exceptional nondeforming properties. The excellent wear resistance of D-type cold-work tool steels is the result of their high chromium (~12wt.%) and high carbon (1.5 to 2.35wt.%) contents.
Generally, the high-carbon, high-alloy tool steels are particularly difficult to process by the conventional ingot metallurgy route. The main challenge is that the relatively slow cooling of the conventional static cast ingot allows the formation of coarse eutectic carbide structures, which are difficult to break down during hot working [1, ]. For overcoming these problems some new processing methods such as powder metallurgy (PM) and spray forming have been developed for production of the most highly alloyed tool steels, such as high-carbon, high-chromium (AISI D series) and high-speed (AISI T and M series) [1, , , ]. These techniques offer the possibility to produce steels with higher homogeneity, lack of segregations, finer microstructure and uniform distribution of carbides, in comparison to conventional ingot-cast materials, as a result of rapid solidification during atomization. Due to the optimized microstructure, the PM and spray formed high alloyed tool steels are expected to show improved performance in respective applications, where a high hardness, fine carbide size, high toughness, etc are indispensable [ ].
In spite of excellent metallurgical performance of these routes, the bulk of tool steels market is still supplied by products made by the conventional ingot metallurgy approach, due to its low production cost.
Therefore, compared with these techniques, modifying the ingot metallurgy process seems a more practical strategy to obtain fine, more rounded carbides. From several techniques which can be used for achievement of finer, more rounded and isolated carbides in high alloyed tool steels during ingot metallurgy, the addition of modifying elements which is called chemical modification is the simpler, commercially available and less expensive method [ ].
It was reported that by adding small amounts of modifier to high-carbon, high-alloy the eutectic solidification progress of austenite/MxCy can be modified, and therefore, the grain size, shape and...