Materials Science Forum Vols. 539-543

Abstract: High silicon steel (up to 6.5 wt.-%Si) is important for the electrical industry because of its magnetic properties. However, its production in low thickness by cold rolling is difficult due to extreme brittleness, mainly caused by ordering processes, making dislocation motion more complex. Nevertheless, these materials appear to be deformable at higher temperatures. The cooling rate after hot deformation, the temperature from which it is cooled and the time delay prior to cold deformation are important elements for the understanding of their workability. Hot torsion tests were performed on Fe-Si steel (4.2 and 5.6 wt.-%Si) under continuous cooling to study the influence of strain and interpass time on ordering and non-recrystallization temperatures. Compression tests at a constant strain rate were used to study the effect of continuous cooling to RT and the delay time between deformations for series of silicon alloys (from 3.3 to 6.3 wt.-% Si) with different thermomechanical treatments. An aging phenomenon due to an ordering reaction at RT was observed. Finally, extrapolating the hot torsion and compression tests parameters to the rolling mill a suitable schedule for hot rolling was found guaranteeing good conditions for further cold rolling.

Abstract: The effect of annealing temperature on grain growth, texture development and magnetic properties of Al-free and Al-1% added non-oriented electrical steel were investigated. Normal grain growth occurred in Al-free steel. On the other hand, abnormal grain growth occurred in Al-added steel which was annealed at 800°C for 24h. Precipitates in these two steels were different. TiN precipitated in Alfree steel, but in the case of Al-added steel, AlN and TiC precipitated. The TiC in Al-added steel was so fine that it inhibited the normal grain growth and finally caused the abnormal grain growth. Main textures of both steels were near {111}<112>, but the intensity of near {111}<112> in the abnormal grain growth was higher than that in the normal grain growth. Magnetic flux density (B50/Bs) was decreased by the grain growth. Especially B50/Bs in the abnormal grain growth was lower than that in normal grain growth. B50/Bs in these steels can be estimated by their three-dimensional textures in vector method.

Abstract: The recently developed small punch (SP) creep test was applied to four different heatresistant ferritic steels, namely, two kinds of conventional ferritic steels which had been actually used in the high-temperature components for long periods and two advanced high chromium ferritic steels for fusion reactor materials to investigate the applicability of the SP creep test. The ratio of the load of SP creep test to the stress of standard uniaxial creep test was calculated so that both the creep rupture curves (load/stress versus Larson-Miller parameter curves) were overlapped to convert the results of SP creep test into those of standard test. As a result, the ratio was determined to be 2.4, irrespective of the kind of ferritic steel. This result indicates that the creep rupture strength of heat-resistance ferritic steels can be estimated using a miniaturized plate-type specimen and this conversion coefficient 2.4 independent of the kind of ferritic steel.

Abstract: Many steels tubes used in the Oil Industry are constantly exposed to hydrogen sulphide (H2S) which can lead to the diffusion of hydrogen into the steel, potentially provoking hydrogen embrittlement and/or stress corrosion cracking. Additionally the critical region in pipelines is usually considered to be the welded joints. In this work SAW welded joints of two API steels, grades X80 and X70, were evaluated using laboratory tests according to the NACE TM0177/96 METHOD A Standard and the slow strain rate test (SSRT) using a sodium thiosulphate solution. The results indicate that both steel grades can be susceptible to HE and SCC, since loss of ductility when submitted to SSRT and failure under NACE TM0177/96 METHOD A were observed.

Abstract: Original position statistic distribution analysis (OPA) is a new analytical method to determine the quantitative distribution of different chemical compositions and their states of the materials. A novel quantitative metal original position analyzer has been invented by combining the technology of Single Discharge Analysis (SDA) and continuous scanning excitement spark spectrum without pre-excitement. This analytical method can be used to analyze the original position statistic distribution of metals. In this study, the following examples were provided: Quantitative statistic distribution of contents of various elements on the deferent locations in materials; Quantitative statistic distribution of weight in deferent contents of the various elements; Accurate calculation of the segregation of various elements in materials; Quantitative description of the homogeneity of materials; Quantitative statistic distribution of weight in different contents of the elements in total excitement signals of materials; Quantitative description of the porosity in materials; Quantitative statistic distribution of inclusion contents on different locations in materials and distribution of inclusion sizes in materials. The concept of original position statistic distribution analysis along with the widely utilization of the metal original position analyzer will provide researchers and scientists a new method to study the internal characteristics and predict the properties of the materials

Abstract: The effects of quench-tempering treatment and composition modification on SAE 8620 steels have been investigated. Experimental results show that the 400°C tempering of SAE 8620 steels can exhibit an excellent overall performance, including the mechanical strength, impact toughness and corrosion resistance. By slightly adding alloy elements into SAE 8620 steels, the modified HSLA steels exhibit excellent hardenability, high mechanical strength and elongation, and ductile fracture behavior. These excellent performances are comparable to those of HSLA-100 steels and can satisfy well the requirement of Grade 100 lifting components.

Abstract: Double-twist torsion tests were used to determine static softening in the hot working range of three tool steels – W1, a carbon steel (1.03% C - 0.8% other elements), A2 and D2, a medium and a high alloy steel, containing 8.45% and 14.82% alloying elements. The carbon steel, that was single-phase austenite in the hot-working range, experienced rapid static recrystallization due to increased diffusion rate caused by C in hot austenite, very little alloying solute and no carbides. Carbides in alloy tool steels, which exist throughout the hot-working range, have a retarding effect on the progress of recrystallization but are responsible for enhancing initiation due to formation of nuclei at the strain concentration near the particle/matrix interface. Static recrystallization (SRX) of the alloy tool steels was compared with austenitic stainless steels, with similar strengths but much greater alloying content, and with microalloyed steels, as well as with the dynamic recrystallization kinetics.

Abstract: SAE 8620 steel is typically used in the carburized condition for powertrain applications in the automotive industry, e.g. gears, roller bearings, camshafts. Such steels always contain retained austenite to varying degrees in the as-hardened and also in the tempered microstructures. As well as retained austenite, heat treatment can produce residual stresses, which lead to distortion (size and shape). The intent of this study was to investigate the effect of heat treatment parameters on the amount of retained austenite, residual stress and distortion in carburized SAE 8620 steel. A specially designed specimen, the Navy C-ring, was used for this study. The steel was first normalized prior to machining the Navy C-ring specimens. The specimens were then heat treated by carburizing at 927°C or 954°C (1700°F or 1750°F) at four levels of carbon potential (0.9, 1.0, 1.1, 1.2) followed by oil quenching and tempering at either 149°C or 177°C (300°F or 350°F). The distortion of the C-ring was evaluated by dimensional measurements of the inner diameter, outer diameter, gap width and thickness for size distortion, as well as flatness, cylindricity and roundness for shape distortion. X-ray diffraction (XRD) techniques were used to determine the residual stress and the amount of retained austenite. The amount of retained austenite was also measured by optical metallography. The amount of retained austenite and the residual stress increased with increasing carburizing temperature and carbon potential and decreased upon tempering. There was not a significant further reduction in the amount of retained austenite and residual stress when the tempering temperature was increased from 149°C to 177°C. Distortion was influenced by both the amount of retained austenite and the magnitude of the residual stress. With increasing retained austenite/residual stress, the distortion became more serious. Based on the distortion data for 3 parameters (OD, gap width and flatness) for the quenched and tempered specimens, the amount of retained austenite for minimum distortion was approximately 25%.

Abstract: High strength steels containing significant fractions of retained austenite have been developed in recent years, and are the subject of growing commercial interest when associated with the TRIP phenomenon during deformation. A new process concept “quenching and partitioning” (Q&P) has been proposed by CSM/USA, and the results show the potential to create a new kind of steel microstructure with controlled amounts of retained austenite, enriched by carbon partitioning. Four steels containing C, Si, Mn, Ni, Cr and Mo, were designed with variation in the Ni and C content, aiming to decrease Bs temperature and to suppress carbide formation during the partitioning treatment. Several heat-treatment procedures were performed in specimens previously machined for tensile testing, while x-ray diffraction was used to determine the fraction of retained austenite. The tensile test results showed that except for the high C high Ni alloy, most of the processing conditions resulted in strengths superior to those of advanced high strength steels (AHSS), although it is importantly recognized that higher alloy additions were used in this study, in comparison with conventional AHSS grades.. A variety of strength and ductility combinations were observed, confirming the potential of the Q&P process and illustrating the strong influence of the final microstructure on the mechanical properties. Experimental results for samples partitioned at 400 °C indicate that higher ultimate tensile strength is associated with higher fraction of retained austenite for multiple heat treatments of each alloy investigated. The amount of retained austenite obtained was generally lower than that predicted by the model. Further studies are in progress to understand the influence of alloying and processing parameters (time/temperature) on the partitioning of carbon and precipitation of transition carbides.