Materials Science Forum Vols. 539-543

Abstract: Three main hypotheses of hydrogen embrittlement (HE) of austenitic steels are discussed based on the studies of the interatomic interactions, hydrogen-induced phase transformations and dislocations properties. Measurements of electron spin resonance and ab initio calculations of the electron structure witness that the concentration of conduction electrons increases due to hydrogen, which enhances the metallic character of interatomic bonds. The hypothesis of brittle hydrogen-induced phases is disproved by the studies of the silicon-alloyed steels: the silicon-caused increase in the fraction of the εH martensite is accompanied by the decrease of HE. Studies of strain-dependent internal friction have shown the hydrogen-caused decrease in the start stress of microplasticity and increase in the velocity of dislocations in accordance with HELP hypothesis. A mechanism of HELP is proposed based on the hydrogencaused enhancement of the metallic character of interatomic bonds, which results in the local decrease of the shear modulus within the hydrogen atmospheres round the dislocations. As consequence, the line tension of the dislocations followed by the hydrogen atoms decreases, which finds its expression in the early start of dislocation sources, decreased distance between dislocations in the pile-ups and increased velocity of dislocations. A mechanism of localization of plastic deformation is proposed based on the observations of the hydrogen-enhanced concentration of equilibrium vacancies.

Abstract: It is well known that microstructural changes occur in a steel bearing, when the bearing is operated under conditions involving high cyclic stresses. When combined with relatively high temperatures, such microstructural changes result in the flaking of the bearing raceway. In this paper, microstructural changes that occurred during rolling contact fatigue were investigated, and the relationship between these changes and fatigue life are discussed in association with the recrystallization behavior of martensite. Conventional bearing steel SUJ2 (SAE52100) was subjected to partial solution treatment at 1133K for 2.4ks followed by oil quenching. The quenched material with a martensitic structure was tempered at 443K for 7.2ks, and then subjected to rolling contact fatigue testing. The testing was performed at temperatures ranging from 373K to 443K and surface pressures of 4.6GPa or 5.5GPa. During testing at 373K, flaking occurred from the surface of the raceway due to non-metallic inclusion and without any marked microstructural changes. On the other hand, in the case of testing at 403K or more, flaking occurs after obvious microstructural changes. Firstly, dark etching constituent (DEC) formed around the area of maximum shear stress, which was followed by the formation of white etching constituent (WEC) within the DEC at 80 and 30 degrees to the rolling direction. TEM observations showed the change from martensite lath to dislocation cell structure within the DEC, and also the existence of fine ferrite grains of 20nm through 100nm within the WEC. Arrhenius plots for the fatigue life indicated that the activation energy of the fatigue process corresponded to that of carbon diffusion in bcc ferrite. These results suggest that rolling contact fatigue originated from the WEC is controlled by the diffusion of carbon in the ferrite matrix.

Abstract: The use of Niobium as a microalloying element is particularly beneficial in increasing strength and toughness through its ability to control austenite grain size during reheating, and via grain refinement and precipitation hardening after austenite transformation. Steels with lower carbon contents can thus be utilised, further enhancing toughness and weldability.

Abstract: A new method for the determination of inclusions of boron and titanium in medium and low alloy steel has been reported in this paper. The technique of original position statistic distribution analysis based on statistic analysis of more than ten thousands primary optical signals is applied in the present method. It is found that the abnormal high intensities in emission lines of boron and titanium occurred when a discharge volatilizes the material containing inclusions of boron and titanium. Some statistic algorithms are developed to distinguish the signals related to nonmetallic inclusions from the signals generated from soluble boron and titanium. Furthermore, mathematical models based on the relationship between the frequency of abnormal sparks and content of inclusions is put forward in this paper to count the contents of inclusions of boron and titanium. The results calculated by the present model have good coincidence with the value obtained by chemical methods. The present method has been applied to the determination of the contents of inclusions of boron and titanium in some certified reference materials and some real medium and low alloy steel samples.

Abstract: The potentialities of using the magnetic Barkhausen noise measurement in characterization of metallurgical transformations have been highlighted in multi-phase High Strength (HS) steels. This kind of steels are composed of different metallurgical constituents, such as ferrite, bainite, martensite or residual austenite. Recently, we found that it was possible to assess the proportion of phases in ferrite-martensite steels and in industrial Dual-Phase steels too. In this work, we show that the Barkhausen noise measurements can be also suitable to follow bainitic transformation in a TRIP steel.

Abstract: The Thixoforming of metals is a new forming technology, which combines the advantages of casting and forging. This technology focuses on structural components with complex shapes, which can not be manufactured using conventional processing methods. Different components made of aluminum and magnesium are already industrially produced. Apart from this, the use of steels in the Thixoforming processes is still being researched and developed. During the last years it became obvious that the Thixoforming of steels may not be a complete substitution for highly automated die-casting or forging processes, but rather a new process route to produce special components with high quality and geometrical demands. Special interest lies in the development and production of filigree components, parts with voids or undercuts, integrated elements such as tubes or tube-systems and joining pieces or products made of different materials. Beside the design of suitable tool materials and tool concepts, the development of homogeneous heating strategies and the development of thixoformable steels is at the center of current research. Therefore, this paper focuses on material requirements which are needed for the Thixoforming of steels and on the production of components and occurring problems. By use of a failure catalogue for Thixoforming of steels, reasons for characteristic failures and suggestions are given to avoid them, so that the quality of thixoformed parts can be improved.