Influence of isothermal treatment on MnS and hot ductility in low carbon, low Mn steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 x 10-4 s -1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The highercarbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility

In situ study of dynamic recrystallization and hot deformation behavior of a multiphase titanium aluminide alloy

Hot-compression tests were conducted in a high-energy synchrotron x-ray beam to study in situ and in real time microstructural changes in the bulk of a β-solidifying titanium aluminide alloy. The occupancy and spottiness of the diffraction rings have been evaluated in order to access grain growth and refinement, orientation relationships, subgrain formation, dynamic recovery, and dynamic recrystallization, as well as phase transformations. This method has been applied to an alloy consisting of two coexisting phases at high temperature and it was found that the bcc β-phase recrystallizes dynamically, much faster than the hcp α-phase, which deforms predominantly through crystallographic slip underpinned by a dynamic recovery process with only a small component of dynamic recrystallization. The two phases deform to a very large extent independently from each other. The rapid recrystallization dynamics of the β-phase combined with the easy and isotropic slip characteristics of the bcc structure explain the excellent deformation behavior of the material, while the presence of two phases effectively suppresses grain growth

Research support for the regional steel industry

Building on a long history of collaboration, the University of Wollongong jointly with BHPSteel, has developed a strategy for strengthening the competitive position of the local steel industry. This strategy is based on the premise that a sustainable alliance between industry and university would help ensure that opportunities are created for the development of innovative and creative solutions to problems facing the steel business and its customers. Focussed research groups were founded in the Institute for Steel Processing and Products, University of Wollongong. These dedicated teams have built a specialised equipment infrastructure that is shared by university and industry researchers. This infrastructure is unique in Australia and is specifically tailored to the needs of the steel industry. As this infrastructure base has grown, BHPSteel\u27s in house research and metallurgical organizations and the University have progressively started to eliminate costly duplication of equipment by sharing facilities. The Institute\u27s major contribution to BHPSteel has been the enhancement of the business through multi-disciplinary collaborative research between BHPSteel and the University to gain a better understanding of the key technical issues facing the company. In addition, the Institute provides advanced education and training to BHPSteel engineers. Through these collaborative efforts, the Institute has contributed to the strengthening of the competitive position ofthe local steel industry in the world market. This model of interaction need not be confined to interaction within Australia, but can be extended to enhance the competitive position of the regional steel industry

Delta-ferrite recovery structures in low-carbon steels

The development of delta-ferrite recovery substructures in low-carbon steels has been observed in-situ utilizing laser scanning confocal microscopy (LSCM). Well-developed sub-boundaries with interfacial energies much smaller than that of delta-ferrite grain boundaries formed following transformation from austenite to delta-ferrite on heating. It is proposed that transformation stresses associated with the austenite to delta-ferrite phase transformation generate dislocations that subsequently recover into sub-boundaries by a process of polygonization. Experimental evidence in support of this proposal was found in a ferritic stainless steel. Thermal cycling through the high-temperature delta-ferrite/austenite/delta-ferrite phase transformation leads to the development of a well-defined recovery substructure, which, in turn, modifies the low-temperature austenite decomposition product from Widmanstätten to polygonal ferrite, with a commensurate change in hardness

Delta-Ferrite Recovery Structures in Low Carbon Steels

The development of delta-ferrite recovery sub-structures in low carbon steels has been observed in-situ utlhsing laser scanniJlg confocal microscopy (LSCM). Well developed subboundaries with interfaCial\u27 energies much smaller than that of delta-fen\u27ite grain boundaries fOfmed following transformation from austenite to delta-fen\u27ite on heating. Transfonnatioll stresses associated with the austenite to delta-fenite phase transt\u27onnation generate dislocations that subsequently recover into sub-boundaries by a process of polygonisation. Experimental evidence in suppol1 of this proposal was found in a ferritic stainless steel. Thermal cycling through the high temperature delta-ferrite/austenite/delta-fel1\u27ite phase b\u27ansformation leads to the development of a strong recovery substructure, wh.ich in turn. modifies the low temperature austenite decomposition product from WidmansUilten/bainite to polygonal fen\u27ite, with a commensurate chauge in hardness

Strain-induced phase transformation during thermo-mechanical processing of titanium alloys

Strain-induced phase transformation studies have been conducted in a Ti-6Al-2Sn-4Zr-6Mo alloy. This alloy was subjected to b sub-transus deformation upon slow cooling from the b-phase field and the effect of strain on the extent and morphology of the phase transformation during hot-deformation was determined. In addition, the transformation kinetics and the morphology of the newly formed a-phase were studied following deformation under different cooling conditions. By applying strain, the rate of the b-to-a phase transformation increased significantly during deformation as well as during slow cooling following deformation. This increase in the kinetics of the b-to-a transformation can be ascribed to straininduced phase transformation. Also, the extent of deformation of the b-phase had a marked effect on the resulting morphology and size of the newly formed a-phase. Transformation of un-deformed b-phase rendered a-phase of acicular morphology only. However, following deformation of the b-phase, acicular as well as globular a-phase morphologies have been observed upon cooling