Titanium-alloyed ferritic chromium steels are a competitive option to classical austenitic stainless steels owing to their similar corrosion resistance. The addition of titanium significantly ...influences their final steel cleanliness.
The present contribution focuses on the detailed metallographic characterization of titanium nitrides, titanium carbides and titanium carbonitrides with regard to their size, morphology and composition. The methods used are manual and automated Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy as well as optical microscopy. Additional thermodynamic calculations are performed to explain the precipitation procedure of the analyzed titanium nitrides.
The analyses showed that homogeneous nucleation is decisive at an early process stage after the addition of titanium. Heterogeneous nucleation gets crucial with ongoing process time and essentially influences the final inclusion size of titanium nitrides. A detailed investigation of the nuclei for heterogeneous nucleation with automated Scanning Electron Microscopy proved to be difficult due to their small size. Manual Scanning Electron Microscopy and optical microscopy have to be applied. Furthermore, it was found that during solidification an additional layer around an existing titanium nitride can be formed which changes the final inclusion morphology significantly. These layers are also characterized in detail.
Based on these different inclusion morphologies, in combination with thermodynamic results, tendencies regarding the formation and modification time of titanium containing inclusions in ferritic chromium steels are derived.
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•The formation and modification of TiN in the steel 1.4520 was examined.•Heterogeneous nucleation essentially influences the final steel cleanliness.•In most cases heterogeneous nuclei in TiN inclusions are magnesium based.•Particle morphology provides important information on inclusion formation.
The microstructure and mechanical properties of conventionally heat treated or thermomechanically treated spring steel Fe–0.56C–1.50Si–0.67Cr–0.59Mn (mass%) are presented after tempering. At ...tempering temperatures between 623 and 673
K continuous cementite films form at the prior austenite grain boundaries when the austenite is quenched without any preceding deformation. Two austenite modifications, a recrystallized variant and an elongated pan-cake grain structure were produced by deformation prior to quenching. The first variant resulted in refined grain boundary carbides after tempering, while for the latter no grain boundary carbides were observed. Both modifications of austenite produced by thermomechanical treatment improve the ductility significantly.
Steels with higher strength, ductility and improved fatigue behavior are required for light-weight structures in the transportation industry. It is shown that for martensitic steels the combination ...of microalloying and an optimized thermomechanical treatment (TMT) results in increased strength and improved ductility. Proper conditioning of the austenite by deformation either refines the austenitic grains or generates a dislocation substructure that is inherited to the martensite structure. In contrast to simply quenched and tempered martensite with no prior deformation, the thermomechanically processed martensite exhibits a more refined structure with refined blocks and is free of grain boundary carbides. Addition of vanadium is beneficial in controlling the austenite grain size during austenitization and for the stabilization of the austenite defect structures that are produced by deformation. It enables to use higher deformation temperatures for TMT, i.e. lower rolling forces can be applied in an industrial process. It is possible to increase the strength and ductility of conventionally heat treated Si–Cr steel by addition of vanadium or by TMT, but the highest improvement is achieved through the combination of both. In this study, an increase of more than 600
MPa in the ultimate tensile strength and an improvement of 40% in the reduction area are reported.
Compressor valves are made of hardened and tempered martensitic stainless steels. They were first produced from ingot cast material. Later continuous casting technology largely replaced this process, ...which led to improved, more uniform, properties and higher yields in the production of hot strip. A further development was made around the turn of the millennium, when strip casting was first used for the production of stainless steel strip, directly from the melt. In this paper the steel grade Zapp 1.4028MO produced via both production routes: a) Continuous casting - hot rolling - cold rolling and b) Strip casting - cold rolling, are compared with respect to their microstructure evolution and static/dynamic mechanical properties. It is shown that strip cast material can not only substitute that of the traditional continuous casting route, but moreover deliver performance advantages with thin stainless steel valves used within air conditioning units and household refrigerators.
The limits of strength and ductility of a medium‐carbon silicon chromium spring steel are investigated for the case of conventional heat treatment including austenitization, quenching and tempering. ...The effect of phosphorus and austenite deformation prior to quenching was studied by measuring mechanical properties after quenching and tempering and by microstructural investigation. Strong influence of phosphorus on the ductility is observed for the quenched and tempered martensite without prior austenite deformation. The minimum in ductility found after tempering at 350°C is explained by the formation of cementite and grain boundary segregation of phosphorus. Two thermomechanical treatments were tested involving different austenite conditions produced by variation of the deformation temperature. The deformed conditions, recrystallized or work‐hardened, exhibit higher ductility at all tempering temperatures tested. A combined thermomechanical treatment is proposed that provides the highest ductility after tempering at 300°C independent of the phosphorus content. All thermomechanical treatments described in this study refine or eliminate carbide films at prior austenite grain boundaries. It was found possible to increase the tensile strength and the fatigue limit by deformation of austenite prior to quenching while maintaining or increasing the ductility level.
The austenite grain coarsening behaviour in a medium carbon Si‐Cr spring steel with and without vanadium was investigated by the thermal etching method. This method is efficient when the ...austenitization temperature is not lower than 900 °C. The average grain sizes on the surfaces of the samples determined by thermal etching vary little from those in the bulk as revealed by chemical etching. The austenite grain coarsening behaviour of the steel with vanadium can be classified in three temperature regimes. Below a critical temperature the vanadium addition is effective to impede grain growth and fine grains are observed. The austenite grain size was significantly smaller than in the steel without vanadium. In a medium temperature regime the steel with vanadium exhibits a bimodal grain size distribution. Above the dissolution temperature of vanadium carbides normal grain growth is observed. An equation was set up to predict the grain coarsening behaviour in the steel without vanadium. The results show how the austenite grain size, which is very important for the toughness and ductility of the spring steels, can be controlled by microalloying with vanadium.
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