Materials Science Forum Vols. 539-543

Abstract: High pressure gas quenching became a modern way of quenching finally machined engineering components,having many advantages compared to quenching in liquid quenchants.The main shortcoming of this technology is the problem of achieving adequate hardness in the core of bigger workpieces,because of inadequate quenching intensity.Due to the possibility to change gas pressure and its flow velocity,combined with transient spraying of liquid nitrogen during the quenching process ,the intensity of cooling can be instantly increased during selected time intervals.In this way the heat extraction dynamics can be automatically controlled,and a predetermined path of the heat transfer coefficient can be followed.Preliminary experiments show that using the controllable heat extraction(CHE) technology, the mentioned shortcoming can be eliminated.Theoretical background of the CHE technology is described,with particular attention to the depth of hardening,and to residual stresses.Possibilities and prerequisite conditions for application of the CHE technology in vacuum furnaces,and for automatic heat extraction control,are discussed.

Abstract: The influence of high energy density sources on morphological changes in steels was studied by a physical simulation. Strips of carbon steels were subjected to heat cycles including continuous rapid heating to temperatures between 400 and 1200°C and immediate water quenching. The heat cycles were carried out by passing a high intensity electrical current through trapezoidal specimens in a special device allowing to obtain heating rates up to 10000°/s with an excellent temperature control. Real-time temperature recordings were drawn so as to define some characteristic temperatures of continuous austenitization. The evolution of the morphology, initially composed of ferrite and martensite, was examined by means of light and electron (SEM and TEM) microscopy using standard techniques. The results of the examination were related to microhardness measurements. Three distinct stages of the morphological evolution were finally analyzed: - T > AC1 - Beginning of austenite formation in zones of as-tempered martensite, mostly at former austenite grain boundaries. - AC1 << T < AC3 - Development of irregular lath-shaped interface between the growing (austenite) and shrinking (ferrite) phases. - T > AC3 - Final massive transformation of supersaturated ferrite areas.

Abstract: Thermal simulation and arc welding were carried out to test the weldability of atmospheric corrosion resistant steel 07MnCuPTiNb. In thermal simulation experiment, welding thermal cycles with different peak temperature and different cooling rate were adopted and the microstructure and impact toughness were analyzed. In arc welding experiment, different heat input was used and the microstructure, impact toughness, hardness distribution, tensile strength and bending ductility of welded joints were examined. When small heat input is used for welding, the impact toughness of HAZ remains to be good. The broken position of tensile test specimen is located in base metal zone. The joint also has good ductility.

Abstract: An overview of the mechanically driven phase transformations taking place in nanocrystalline pearlitic steels in conditions of the severe plastic deformation (SPD), i.e. combination of high pressure and strong shear strains will be given. Conditions of the discussed experiments (room temperature and moderate strain rates) exclude any thermal origin of the observed transformations. One of them is strain induced cementite decomposition, which is a well-documented phenomenon taking place at cold plastic deformation of pearlitic steels. We explain this process taking into account friction forces at the interface between the hard cementite and ferrite. Under the high pressures and stresses higher than the ferrite matrix yield stress, the later one behaves like a viscoelastic fluid. The friction at the precipitate/matrix interface leads to two effects. One is to induce high strains on the precipitates. This leads to shift of thermodynamic equilibrium towards dissolution of the cementite. The second is wear of the cementite phase due to friction at the ferrite/cementite interface and mechanically induced drag of carbon atoms by the ferrite. This had been recently confirmed in 3D AP experiments, which demonstrated that the process of cementite decomposition starts with depleting of carbides with carbon and formation of non-stoichiometric cementite. The existing theories of atom drag by moving dislocations (ballistic models) can be regarded as one of the many possible mechanism of wear discussed by the wear theory. In that respect the process can be called athermal, as temperature indirectly influences wear processes but is not their main cause. We observed also another strain driven transformation in nanocrystalline pearlitic steel during room temperature high pressure torsion. This is a stress induced α→γ transformation, which has never been observed at conventional deformation of coarse grained iron and carbon steels. This was concluded to have occurred due to a reverse martensitic transformation.

Abstract: A method of obtaining nanocrystallized bulk steels through phase transformation was investigated. Firstly, the austenite grain size of a microalloyed steel was refined to 1~3μm through repeatly heating and quenching; secondly, the samples with ultrafined austenite grains were heavily deformed at different temperature, and the uniform microstructures with some 0.1~0.3μm equiaxed ferrites were obtained.

Abstract: Equal Channel Angular Pressing (ECAP) in a fully pearlitic structured steel 65Mn was successfully carried out at 923 K via route C in this study. The severe shear deformation of ECAP was accommodated by periodical bending, periodical shearing and shearing fracture etc in the pearlitic lamellae. The cementite in the pearlite has higher plastic deformation capability. Excessive imperfections may be introduced into the cementite, which supplies additional energy driving for the following spheroidization of cementite in subsequent processing. After five ECAP passes, the fully pearlitic lamellae evolved into a microstructure of ultrafine-grained ferrite matrix uniformly dispersed with finer cementite particles. The ferrite matrix is homogeneous with an average grain size of ~0.3 micrometers. Two possible mechanism for the spheroidization of cementite were proposed：heterogeneous growth of the fractured cementite fragments, and the precipitation of new fine spherical cementite particles through nucleation and growth.

Abstract: Ultrafine-grained steel sheets with the chemical composition of 0.15%C-0.74%Mn- 0.01%Si have been prepared using a laboratory rolling mill by Super Short Interval Multi-pass Rolling (SSMR) process, in which the inter-pass time is extremely shortened to enhance the cumulative strain. The SSMR process with a finish rolling around Ae3 leads to ultrafine equiaxed ferrite structure with 1μm in average grain size. In order to clarify the grain refinement mechanism in the SSMR process, the deformation substructure in deformed austenite was simulated using 70%Ni-30%Fe, which was a fcc alloy with equivalent stacking fault energy to C-Mn steels. TEM observations have shown that the dislocation substructure in the Ni-Fe alloy hot-rolled by SSMR process mainly consists of dislocation cells, of which size are refined to less than 1μm with shortening inter-pass time. It is concluded that the SSMR process can accumulate the deformation strain in austenite enough to densely nucleate ferrite inside austenite grains.

Abstract: Ferrite grain refinement by hot rolling mostly above Ae3 being followed by an ultra-fast cooling has been investigated. An emphasis has been paid on the interval, Δt, between the finish of rolling and the start of water spray cooling of which cooling rate is more than 1000 °C·s-1. When Δt was nearly equal to zero, ultra-fine ferrite of about 1 or 2 μm in grain diameter was obtained for 1.2 to 1.3 mm thick 0.1%C-1%Mn steel sheets near sheet surfaces or in thickness center regions respectively, although the grain size at Δt of 0.5 s was about 3 μm in both regions. The ferrite grains were almost equiaxed and surrounded by high angle boundaries. This grain refinement is likely to be caused by an increased number of nucleation sites for the transformation from austenite to ferrite due to the ultra-fast cooling. Such a grain refinement mechanism is discussed in terms of prior-austenite structures deduced by the misorientation distribution function analysis.