Solidification and sulfide precipitation in Fe-Cr-Ni alloys were investigated. Five samples containing 0.3 wt% S and 1.5 wt% Mn were investigated and each sample chemistry was adjusted to solidify as primary austenite, primary ferrite or eutectic by tailoring the Cr and Ni contents. 355© 2009 ISIJ or g, depending on the balance between Cr and Ni contents. In the 70wt%Fe-Cr-Ni system, if Ni content is less than 10-12 wt%, the alloy will encounter the d-liquidus first and begin to form d phase when it is cooled down from the liquid state, while if Ni content exceeds 10-12 wt% it will solidify as g phase first after passing the g-liquidus during cooling. 8,10,16) At this stage of solidification, the partition of Fe, Cr and Ni between the liquid and the primary solid phase is dependent on the slope of liquidus surface; i.e., the primary solid phase is rich in Fe and the remained liquid is rich in Cr and Ni. 6,8,15) In the region where Ni content lies between 10-12 wt%, the remaining liquid undergoes a eutectic reaction at the latter stage of solidification where the three phases coexist. When the liquid composition reaches a eutectic valley, partitions of solute elements in d and g phases will occur through eutectic reaction; i.e., d phase is rich in Cr and g phase rich in Ni. In summary, as a result of a specific solidification path along which an alloy undergoes, significant partitioning of alloying elements take place. 6,8,9,11,[13][14][15]17,18) Numerous papers have been published on morphology and characterization of sulfides in steel. Among sulfides, manganese sulfide (MnS) is commonly observed and utilized in steel, especially in free machining grades, and extensive studies have been devoted to understanding the formation of MnS.19-25) When S content is small (e.g., less than 1 wt%), MnS will form either in a monotectic or a eutectic manner during solidification, depending on the amounts of other alloying elements such as C, Si and Al. 20,22) Although these elements are not constituents of sulfides, they can switch the formation process of MnS from monotectic to eutectic reaction by lowering melting temperature of the metal matrix or supplying nucleation sites for MnS, which results in the change in morphology of sulfides. 20,22) Besides this change in sulfide morphology potentially caused by these elements, solidification structures, d, g, or the mixture of d and g, can influence the sulfide morphology as well as distribution and composition because partitions of S and sulfide forming elements vary from phase to phase which has a specific solubility limit of sulfide. Since sulfide morphology, distribution and composition have a critical effect on hot crack susceptibility, it is necessary to have a basic understanding of how sulfides precipitate in different solidification modes.Several researchers carried out in-situ observation of sulfide formation using a confocal scanning laser microscope (CSLM). 26,27) In Si-killed and Al-killed steels, MnS was seen to form rapidly as a film on the surface of the interd...

Fe 36massNi alloy is well known as invar alloy for its low thermal expansion (LTE) near room temperature. Since its strength is relatively low and thus its application is limited, strengthening of LTE alloy has been a matter of concern for industrial use. It has been reported that the combined additions of V and C are effective for enhancing age hardness of Fe 36massNi alloy without remarkable increase in thermal expansion. However, since worldwide depletion of rare metals such as V has been making it difficult to purchase and pushing their prices up, saving these elements is an urgent issue for steel industry. In addition, more strengthened LTE alloy is still in demand. From these aspects, the effect of substitution of V by Ti and Cr on strength of Fe 0.2massC 36massNi 0.8massV alloy is investigated in this paper. As a result, age hardness reached its maximum when a quarter of V was replaced by Ti. Substitution by Cr by about up to a half of V had little effect on age hardness. Based on these results, an LTE alloy, Fe 0.3massC 36massNi 0.6massV 0.2massTi 0.6massCr, was prepared and its properties were measured. Consequently, the alloy had 70 MPa higher tensile strength than the previously reported for Fe 0.2massC 36massNi 0.8massV alloy. In the viewpoint of saving rare metals, influence of reducing nickel on thermal expansion is also discussed.

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