Band 37, Heft 6 -

To study the mechanism of rare earth (RE) elements on the mechanical properties of axle steels, trace RE were added to LZ50 axle steel, which was melted in vacuum induction furnace. By calculating the thermodynamic and kinetic of RE inclusions, the formation mechanism of inclusions was discussed. And the effects of RE elements on the microstructures and mechanical properties were investigated. The results show that RE and O elements in the molten steels diffused into the interface and increased the thickness of the liquid film. The inclusions transform into 1–3 μm spherical RE compound inclusions instead of 5 μm Al 2 O 3 -SiO 2 with sharp edges. The grain sizes of the steels containing 0.0010 %～0.0026 % RE were decreased, impact toughness and fatigue resistance were improved greatly, compared with the steel without RE. The impact toughness and fatigue resistance of the LZ50 steel with the addition of 0.0010 % RE were increased by 1.65 and 2 times, respectively.

The formation mechanism of inclusions in Si-killed 304 stainless steel was studied by industrial experiments during GOR refining process and thermodynamic calculations. A lager number of CaO-SiO 2 -MgO-Al 2 O 3 -MnO-CrO x liquid spherical inclusions with different size (from 1 μm to 22 μm) had been found after deoxidization of FeSi alloy in 10 minutes. The calculation result of FactSage 6.3 software assisted in confirming that the inclusions in size of less than 5 μm that had less than 30 % CaO mainly came from the deoxidation of FeSi alloy with Ca and Al. The inclusions in size of more than 5 μm that had more than 30 % CaO mainly came from the modification of involved slag droplets through the oxidation of Si and Al and the collision with deoxidation-type inclusions, and the degree of change was bigger for smaller inclusions. The MgO in slag and refractory was reduced by Si and Al in steel, which leaded to the unceasingly increase of Mg content in steel. Subsequently, SiO 2 , MnO and CrO x in inclusion were reduced by Mg, which resulted in the increase of MgO content in inclusion and the degree of increase of MgO content was greater for the larger size of inclusions.

A series of experiments were designed in order to directly heat the billet of 42CrMo4 to the forming temperature in the dies prior to forming and continue to heat the billet during the forming process. Processing parameters during heating and forming were investigated by experimental method and thermo-electro-mechanical coupling FEM. The experimental results show that prior to forming the billet could be rapidly heated to forming temperature under relatively low initial contact pressure, and the heating temperature was proportional to the square of the current intensity. When the heating current remained constant, the heating temperature could not increase with heating time. During the forming process, the billet cooling rate slowed down and the forming time was extended due to the continuous resistance heating during forming. Finally, an incrementally coupled thermo-electro-mechanical model has been developed to analyze the hot-forging process by direct resistance heating. To obtain the transient temperature field prior to forming, a simple model of contact resistance was used in the thermal-electrical simulation, in which the electrical conductance of the contact resistance was proportional to the heating temperature. Contrasted the experimental results and the simulation results, it was found that they coincided well.

A new kind of martensitic ZG1Cr10MoWVNbN heat-resistant steel has been attracted more attentions in recent years, which is mainly applied in ultra-supercritical steam turbines. The ageing property for ZG1Cr10MoWVNbN heat-resistant steel is very important because it often serves for long-time at high-temperature environment. Herein, a long-term ageing heat treatment was conducted on ZG1Cr10MoWVNbN steel at 600 °C heat for 17,000 hours. The microstructure evolution and property variation of the ZG1Cr10MoWVNbN steel were analysed before and after ageing, and also the effect of the precipitates on the mechanical properties was studied. The result showed that strength, the plastic index and impact power of the ZG1Cr10MoWVNbN steel were gradually decreased after long-term and high-temperature ageing at 600 °C due to the changes of martensite morphology and the coarsening of M23C6 carbide precipitation phase. Furthermore, fine precipitation of matrix MX carbide can also attribute to the change of mechanical properties at high temperature.

An experimental investigation of plasma-assisted nitriding of austenitic stainless steel AISI 321 in a low-pressure (7 Pa), low-frequency (50–100 kHz) nitrogen inductively coupled plasma enhanced with ferromagnetic cores has been performed at the temperatures of 470–625 °C, sample biases of ‒500–‒750 V, current densities on the sample surface of 1.2–3.3 mA/cm 2 and nitriding times of 20 and 60 min. It is found that even the short (20 min) ion-plasma treatment results in the formation of nitrided layers with the thickness of up to 40 μm and microhardness of up to 9 GPa.The high speed of nitriding can be explained as a result of the joint action of high ion flux density and high ion energy on the sample surface.

A series of compression tests were performed on Ti-6Al-4V-0.1Ru titanium alloy in nine temperatures between 750 and 1150 °C and a strain rate range of 0.01 to 10s −1 . The hot deformation behaviors of Ti-6Al-4V-0.1Ru showed highly non-linear intrinsic relationships with temperature, strain and strain rate. The flow curves exhibited different softening mechanisms, dynamic recrystallization (DRX) and dynamic recovery (DRV). In this study, the rheological behaviors of Ti-6Al-4V-0.1Ru were modeled using a special hybrid prediction model, where genetic algorithm (GA) was implemented to do a back-propagation neural network (BPNN) weights optimization, namely GA-BPNN. Subsequently, the predicted results were compared with experimental values and GA-BPNN model showed the ability to predict the flow behaviors of Ti-6Al-4V-0.1Ru with superior accuracy. Then a 3-D continuous interaction space was constructed to visually reveal the successive relationships among processing parameters. Finally, the predicted data were applied to process simulation and accuracy results were achieved.

To survey the probability of the re-utilization of nickel containing slag corroded magnesia and alumina-graphite refractories, different amounts of nickel oxide (NiO) were used as one of raw materials to synthesize nickel containing magnesium aluminum oxynitride (MgAlON) composites, and the sintering and oxidation behavior have been explored in this work. The results reveal that with the increasing NiO additive, submicron metallic Ni grains are segregated from the (Ni,Mg)AlON spinel, and these grains are beneficial to improve thermal shock resistance of MgAlON. Though the oxidation reaction of (Ni,Mg)AlON starts at lower temperature than MgAlON, the metallic Ni grains in the plate can improve the oxidation resistance. Hence, the metallic nickel containing (Ni,Mg)AlON composite has not only superior thermal shock resistance but also excellent oxidation resistance, and it is of great potential to utilize in plant. So, the fabrication of metallic nickel containing (Ni,Mg)AlON composite should be a feasible way to reuse the nickel slag corroded magnesia and alumina graphite refractories.

Multi-directional forging (MDF) is one of the most promising severe plastic deformation (SPD) methods used in fabricating large-scale bulk metal materials with ultra-fine grains (UFG). A finite element model for MDF is developed to investigate the strain evolution of Ti-6Al-4V alloy subjected to MDF. Results show that the billet subjected to MDF can be divided into four individual strain zones in terms of the equivalence of effective strain evolution, and that the strain increment in each individual strain zone varies from pass to pass. The deviation between the maximum and the minimum strain increases with the increase of passes and friction coefficient. The effective strain linearly decreases from the core to the exterior of the billet in all three directions after the MDF process. With the increase of the passes and friction coefficient, the gradient of the effective strain in the billet increases in all three directions due to the difference of deformability in different individual strain zones. For the definite friction coefficient, the average and maximum effective strains are in proportion to the accumulative compression strain.

To make use of chromium slag waste, this study examined the technology of the preparation of MgAl 2 O 4 spinel refractory raw material by using chromium slag. The results showed that (1) using the traditional sintering process of synthetic spinel, the highest volume density was 3.14 g/cm 3 and the lowest porosity rate was 9.16 %. With the increase of the proportion of MgO powder mixing, bulk density and porosity fluctuations, and which cannot meet the requirements of refractory raw materials. (2) For MgAl 2 O 4 spinel prepared by the electric-melting process, the highest volume density was 3.27 g/cm 3 and the lowest porosity rate was 8.78 %. Volume density and the porosity remained stable, and they are affected by MgO powder thus the blending ratio is very small, which can satisfy the requirements of refractory raw materials. (3) MgAl 2 O 4 spinel prepared by the electric-melting method is superior to traditional MgAl 2 O 4 spinel in terms of slag resistance and thermal shock resistance.

DP980 steels were joined using fiber laser welding. The welded joint was characterized in terms of hardness distribution and tensile behavior at room temperature, 150 ℃, and 300 ℃, respectively. The fine-grained martensite in supercritical heat affected zone (HAZ) resulted in the highest hardness (428 Hv), while the tempered martensite contributed to the hardness decreasing (‒31 Hv). Both the ultimate tensile strength and yield strength of the base metal and welded joint decreased at 150 ℃, and then increased at 300 ℃ due to dynamic strain aging (DSA). The welded joint exhibited slightly higher yield strength and lower elongation at all the test temperatures compared to base metal due to the hardened fusion zone. The energy absorption reduced slightly with increasing temperature both for base metal and welded joint, and the weld posed a minor effect on the energy absorption. Deformation was one of the requirements for DSA effect. DSA enhanced the hardness of base metal (+78 Hv) and softened zone (+53 Hv). HAZ was not softened enough to become the weakest position during tensile test.

Si 3 N 4 ceramics and 304 stainless steel were joined by the Cu/Ag-Cu/Ti laminated filler metal. Interfacial microstructure of brazed joint and effect of brazing temperature and thickness of Cu foil on mechanical properties were studied in this paper. Research results showed that the interfacial microstructure of the brazed joint might be 304 stainless steel/TiFe 2 /Ag-Cu eutectic+Cu(s,s)/Cu(s,s)/Cu(s,s)+Ag-Cu eutectic/Cu 3 Ti+TiN/Si 3 N 4 ceramics. With the increasing of the brazing temperature, four-point bending strength of the brazed joint initially increased, then decreased. The bending strength reached the maximum value of 53 MPa at 1153 K when the thickness of Cu foil was 500 μm. The bending strength reached the maximum value of 57 MPa with 1 mm thickness Cu interlayer under the brazing temperature of 1153 K.