Materials Science Forum Vols. 638-642

Abstract: Gaseous ferritic nitrocarburization is one of the major surface hardening methods to improve the fatigue strength of machinery parts made of medium carbon steels. The fatigue strength of nitrocarburized steel parts depends on the hardness profile below the surface; however, the mechanism of its evolution during nitrocarburization has not been fully understood. Recently, as-forged steels, in which thermal refining process like normalizing is omitted from the viewpoints of environmental considerations, energy savings and manufacturing cost reductions, have been widely used in the machinery parts industry. Therefore, it is important to understand the mechanism of hardness increase below the surface of the nitrocarburized steels with respect to the effect of prior refining heat treatment. In the present study, the hardness increase at the subsurface region of nitrocarburized steels containing 0.4mass%C was characterized, and the influence of prior normalizing treatment was investigated. Microstructure of both the as-forged and the normalized specimens was the ferrite/perlite mixture, while the ferrite volume fraction in the as-forged steel was smaller than the latter. These as-forged and normalized steels were gaseous nitrocarburized at 853K for 2 hours under the atmosphere of RX gas and NH3 gas mixture, and then they were oil-quenched to 373K. Overall hardness below the surface after nitrocarburization was higher for the specimen without prior normalizing treatment, although both specimens had the similar nitrogen concentration profiles and precipitation behaviors of the nitrides. However, it was found that the individual ferrite grains in the as-forged steel were more hardened than those in the normalized steel. These indicate that the most likely cause of the hardness increase near the surface after nitrocarburization is the solid solution hardening by dissolved nitrogen and that the ferrite grains of the as-forged steel were likely to soak up more nitrogen and were hardened to the higher degree since the similar amount of nitrogen were incorporated mainly within ferrite grains. Thus, the prior heat treatment strongly affects the amount of hardening through the ferrite fraction.

Abstract: The development of global microstructure characteristics has been compared to the local distribution and extent of Σ3 and Σ3n (1 n 3) grain boundary clusters as a function of thermo-mechanical processing in Type 304 stainless steel. A cold reduction of 5% produced GBE modified microstructures on annealing at 1050°C, containing almost one order of magnitude longer maximum cluster lengths than the corresponding annealing treatments for a reduction of 15%. Differences in the development of the distributions of cluster length scales were observed between the thermo-mechanical treatments. A re-conversion of the longest cluster obtained after GBE processing was observed with long annealing times. The local distribution of Σ3n boundary clusters was assessed, and regions with a low density of clusters are indicative of the onset of GBE conversion of microstructure.

Abstract: In IF steel without interstitials, the Hall-Petch coefficient (ky) is low as about 100 MPa･μm1/2 but it is enhnced by adding some alloying elements. Author et al. has already reported that a small amount of carbon increases ky greatly while nitrogen does not give influence so much. On the other hand, it is already known that phosphorus has a tendency to segregate at grain boundary of polycrystalline ferritic iron and that phosphorus also gives some influence to ky. However, the effect of carbon is not considered on the evaluation of ky in steels containing phosphorus. In this study, effect of phosphorus on the ky of ferritic iron was investigated by varying the grain size in Ti bearing IF-steel with different amount of phosphorus. Besides, the concentration of phosphorus and carbon at grain boundary was estimated by the grain boundary segregation model proposed by Seah et al. and the interaction between phosphorus and carbon was discussed in connection with the change of ky.

Abstract: Precipitation-strengthening is widely applied to high strength steel sheet for automotive use since several strength grades are easily achieved by controlling amount of microalloyed component. Recently, finer carbide dispersion has been required to obtain higher strength by smaller addition of carbide formers like titanium and niobium. Here, interface precipitation, one of the carbide formation phenomena during γ→α transformation, can be the efficient method to promote very fine carbides by lowering precipitation temperature. This study deals with relationship between transformation temperatures and hardness of ferritic steel strengthened by carbides generated by the interface precipitation. Two kinds of 0.04%C steels containing Ti and Nb of the same amount as carbon content in atomic were hot-rolled, followed by the soaking at various temperatures for 600s. The rapid-cooled samples before the soaking for 600s exhibited higher hardness than slow-cooled samples. Large carbides generated by interface precipitation were observed in slow-cooled Ti-bearing steel with a transmission electron microscope. In slow-cooled Nb-bearing steel, large NbC precipitated in austenite before γ→α transformation. The results are suggesting that lowering transformation temperature and suppressing carbides precipitation in austenite are important to obtain high strength by interface precipitation.

Abstract: The aim of the paper is to determine the influence of hot deformation conditions on σ-ε curves and microstructure evolution of new-developed high-manganese C-Mn-Si-Al-Nb austenitic steel. The force-energetic parameters of hot-working were determined in continuous and multi-stage compression tests performed in a temperature range of 850 to 1100°C by the use of the Gleeble 3800 thermomechanical simulator. Evaluation of processes controlling work-hardening were identified by microstructure observations of the specimens water-quenched after various conditions of plastic deformation. Multi-stage compression tests with true strain of 0.29 permit to use the dynamic and metadynamic recrystallization for forming the fine-grained, austenite microstructure of steel in the whole range of deformation temperature.

Abstract: Lifetime of hot-work tool steels is often limited by the development of crack nets as a consequence of thermal or mechanical fatigue loading. In this paper, the isothermal and thermal fatigue behaviour of tool steel AISI H11 (German grade X38CrMoV 5-1) in the temperature range between room temperature and 650°C is investigated. In addition to stress controlled isothermal tension-compression fatigue tests, out-of-phase thermal fatigue tests were carried out using a servohydraulic testing system, keeping the macroscopic total strain of the gauge length constant, while triangular time-temperature cycles were applied. The temperature range was chosen in such a way that numbers of cycles to fracture between 1,000 and 10,000 were achieved. The development of stress amplitudes, plastic strain amplitudes as well as of mean stresses was analysed during the tests. Results of isothermal and thermal fatigue tests are compared and discussed.

Abstract: In this study the effect of thermo-mechanical controlled rolling and continuous cooling of different grades of steel wire rod (e.g. high-carbon for cold drawing applications, medium-carbon micro-alloyed for cold forming) has been analysed through the application of a set of integrated mathematical models simulating hot rolling and continuous cooling, and a laboratory work involving hot rolling simulation on a pilot plant and heat treatments on a laboratory scale. The samples have been characterised by means of instrumented tensile tests, metallographic analyses including determination of pearlite interlamellar spacing, and controlled compression tests. The results show that: - The mechanical strength of high-carbon steel is essentially related to interlamellar pearlite spacing, and can be enhanced through the control of continuous cooling. Improvements in cold drawability can be obtained by means of prior austenitic grain size (PAGS) reduction, through the application of thermo-mechanically controlled hot deformation processes. - In the case of medium-carbon micro-alloyed steels for cold forming, the reduced PAGS achieved by means of thermo-mechanically controlled process reflects on a closer control of as-rolled mechanical properties, avoiding hardness hot spots asking for annealing treatments before cold forming. Moreover, the finer ferrite grain size could affect the forces needed during forming at the same deformation level.

Abstract: To improve the toughness and weldability, the carbon content of the steels has to be deduced, and more and more attention has been attracted to the low carbon and ultra-low carbon steels. To strengthen the microstructure Cu and Nb-bearing steels are developed. However, the knowledge on influence of combined addition of Cu and Nb is still in lack. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The microstructure and mechanical properties are studied in the 6-mm thick as-rolled and tempered ultra-low carbon steel plates with varied copper and niobium content. The experimental results show that if niobium is added without copper, the increase of niobium addition does not have a significant influence on the phase transformation and mechanical properties before tempering. The strength and toughness of those copper-free niobium steels do not vary significantly after tempered at different temperatures, while the strength of niobium steels with 1.8% copper added increases after tempered in the range of 450-650°C and reaches a peak at 500-550°C. If combined with 1.8% copper, the increase of niobium addition from 0.08% to 0.16% improves the hardenabililty and strength significantly, and the strength peak after tempering moves to a lower temperature. The strength of air-cooled niobium steels with 1.8% copper added is usually higher than those water-cooled, while after tempered at a proper temperature, the strength of the latter becomes higher than the former.

Abstract: Medium carbon steels have low yield ratio (YR). In order to obtain high yield strength (YS) for them, different microstructures including ferrite(F)/cementite(C) and ferrite(F)/pearlite(P) are prepared and the microstructure for high YR as well as its mechanism is clarified. The purpose of this study is, therefore, to elucidate the effect of ferrite grain size on YS in the relationship between F/P and F/C structures having the same tensile strength (TS) level. In case of F/C, YS can be significantly improved through grain refinement with a slight decrease in ductile properties. This is particularly clear by comparison of F/C with a ferrite grain size of 0.6m and F/P, where both structures show the same TS level while the YS level is significantly different; F/P representing much smaller YS than that of F/C. It is also to be noted that F/P microstructure shows low YS compared to its high TS with a generation of dislocations at the interface between ferrite and cementite in its pearlite phase. In conclusion it is necessary to consider the difference in the YS-TS relationship between F/C and F/P in order to make the most of its forging processing.

Abstract: Advance high strength steels (AHSS), like dual phase (DP) and transformation induced plasticity (TRIP) steels, offer high strength and toughness combined with excellent uniform elongation. However, the higher alloying content of these steels limit their weldability and the thermal cycle of welding processes destroys the carefully designed microstructure. This will result in inferior mechanical properties of the joint. Therefore, joining processes with a low heat input, like brazing, are recommendable. Data regarding mechanical properties of joints in DP and TRIP steel is limited, especially for brazed joints. Results with respect to the fatigue lifetime of laser brazed butt joints are presented. In DP and TRIP steel, crack initiation takes place at the braze toe. In DP steel the crack propagates through the base metal. In TRIP steel, however, the crack may either follow the interface or may continue through the steel depending on the maximum stress level. The different failure mechanisms are explained on the basis of process conditions, the microstructure and the stress state.