Volume 34, Issue 5 -

The hot ductility of 20CrMnTi steel with different levels of Cu equivalent was investigated. The results show that Sn and Cu in 20CrMnTi steel are detrimental to its hot ductility. Sn was found to segregate to the boundaries tested by EPMA, moreover, Cu was not found to segregate to boundaries, however, the fracture morphology was examined with SEM and showed many small and shallow dimples on the fracture of steels with large Cu equivalent (>0.15) and fine copper sulfide was found from carbon extraction replicas using TEM. The adverse effect of large Cu equivalent (>0.15) on the hot ductility was due to Sn segregation and fine copper sulfide in the steel as well as their retarding the occurrence of dynamic recrystallization (DRX). The proeutectoid ferrite film precipitating along the austenite grain boundary causes the ductility trough of the five examined steels. Moreover, in this case, the level of Cu equivalent should be controlled below 0.15, which would not deteriorate the hot ductility significantly.

In order to investigate the tensile deformation behavior of ultra-high-strength-steel BR1500HS, a series of isothermal tensile experiments were carried out in a temperature range of 1023˜1123 K and a strain rate range of 0.01˜10 s −1 on a Gleeble-3500 thermo-mechanical simulator. The results indicate that the flow stress initially increases to a peak value and then decreases gradually to a steady state. Based on the analysis of the effect of strain, temperature and strain rate on flow stress, dynamic recrystallization (DRX) type softening characteristics of the stress-strain curve with single peak were identified. The flow stress level decreases with increasing temperature and decreasing strain rate. By regression analysis for Arrhenius type equation, the hot deformation activation energy Q and material constants ( n , α , β and A ) were calculated. Further, the Arrhenius-type constitutive equation of flow stress was developed by considering the effect of strain on the variable coefficients (including activation energy Q , material constants n , α , and A ). The validity of the developed constitutive equation incorporating the influence of strain was evaluated through comparing the experimental and predicted data. Furthermore, the predictability of the developed model was also evaluated using two standard statistical parameters, correlation coefficient ( R ) and average absolute relative error (AARE). And R-value and AARE-value for the model are 0.997 and 4.06% respectively, which indicates that the developed model can precisely estimate the flow behavior for BR1500HS alloy throughout the entire temperature and strain rate range.

Influences of the reduction temperature, dew point and hydrogen content on the morphologies of produced Mo powders by reducing MoO 2 with hydrogen were investigated. It was found that at low temperatures (blow 1173 K), the reduction proceeded through pseudomorphic transformation mechanism, and the product of molybdenum kept the same platelet shape as the initial MoO 2 . However, at a high temperature of 1393 K, the produced molybdenum was formed as perfect spherical-shaped crystals, which indicated that the reduction obeyed the chemical vapor transport (CVT) mechanism. The influence of dew point in the atmosphere on the morphological changes of Mo powders was also important. It was beneficial for the nucleation and growth of Mo crystals following CVT mechanism with increasing the dew point of hydrogen. However, the influence of hydrogen content (25% H 2 –100% H 2 ) on the morphological changes could be neglected at low temperature or high temperature when one reduction mechanism was dominated.

The effect of aging treatment on the microstructures and mechanical properties of new groove T92/Super 304H dissimilar steel joints is studied in this paper. The experimental results show that the heat-affected zone (HAZ) of T92 is mainly composed of coarse-grained and fine-grained martensitic, whereas the microstructure of Super 304H HAZ and weld seam exhibit an austenitic structure. Aging treatment increases the nucleation and growth of second phase particles of the weld joints, especially at T92 side. The weld joints have a low tensile strength (<700 Mpa) and a high tensile strength (>700 Mpa) when the tensile fractures are located at weld seam and T92 base metal, respectively. With increasing aging time, the hardness and tensile strength of the weld joints initially decrease, then increase, and finally stabilize. Moreover, the weld joints have a maximum hardness value at the T92 heat-affected zone. At room temperature condition, the impact absorbed energy reaches the minimum value, which is related to the coarse grain containing equiaxed dendrites in the weld seam.

Prior low cycle fatigue (LCF) deformation in a 316LN austenitic stainless steel reduced the remnant high cycle fatigue (HCF) life as a function of the amount of LCF exposure and the applied strain amplitude. A critical LCF pre-damage was found necessary for an effective LCF-HCF interaction to take place.

Due to its special nature of microwave heating, the reaction processes of high dielectric loss materials under microwave field draw more and more attention. This work attempts to figure out microwave oxidation process of high titanium slag. Based on a through analysis, it can be concluded that high titanium slag phase transformation process under microwave field can be divided into four stages: (i) crystallization of amorphous Fe, Mg, and Ti oxides, (ii) decomposition of Fe 0.5 Mg 0.5 Ti 2 O 5 , (iii) oxidation of (Mg 0.15 Ti 1.85 )O 5 and FeTi 2 O 5 , and (iv) transformation of A-TiO 2 , which is totally from the reaction process (M 3 O 5  → A-TiO 2  → R-TiO 2 ) under conventional resistance heating condition. In addition, when amorphous Fe, Mg, and Ti oxides crystallize, the Raman active vibration modes first was assigned to A-TiO 2 lattice, then dominated by R-TiO 2 lattice. And it is important to mention that with phase transformation, there will grow tumour form on the surface of sample, which eventually develops into squamous layers.

In this paper, a calculation model for decarburization kinetic in solid metal melting process is studied. Firstly the macro-kinetic model is established, and the melting and decarburization process is divided into five steps, which are adsorption and dissolution of O 2 , liquid phase mass transfer of [C], interfacial reaction of [C] and [O], diffusion of CO gas, and melting and dropping of metal. Secondly based on the assumptions that the total decarburization process is steady at the reaction temperature of 1800 K, the interface reaction is in equilibrium state, the composition in the liquid metal is homogenous, all the activity coefficients are 1 and so on, the calculation model is simplified into equations set with six formulas, and is calculated under 10584 conditions with the help of VB software. Results show that the C or O 2 deliverability is the restricted link under different conditions. Finally, a decarburization experiment of melting pig iron at the rate of 0.56 kg/(s·m 2 ) is achieved, and it proves that the simplified calculation model is in uniform with the actual data at a low melting rate.

The interaction between nitrogen and titanium carbonitride (Ti(C,N))-based cermets by microwave heating was applied to develop functional gradient cermets. Ti(C,N)-based cermets was densification and nitrided simultaneously above the eutectic temperature, which greatly shortens the sintering time. Experimental results show that a face centered cubic (fcc) rich surface layer was formed at the surface of the cermets. Meanwhile, a new three layers core/rim structure was found in the cermets and its formation mechanism was proposed in this study. The functional gradient material exhibits excellent comprehensive properties.

In the present paper the theoretical basis and experimental verification of a method, enabling the calculation of defect concentration and their mobility in transition metal oxides and sulphides have been described. The idea of proposed method consists in determination of both these parameters in indirect way, i.e. in studying the influence of aliovalent metallic additions on the oxidation kinetics of a given metal (doping effect). It has been shown that from the results of oxidation kinetics of binary alloys, the enthalpy and entropy of defect formation and their migration can be calculated. These data, in turn, can be used for the calculation of defect concentration and defect mobility in pure, undoped oxides. Such a possibility has been illustrated on the example of nonstoichiometric nickel oxide, Ni 1-y O.

Steel solidification process control, especially in the solidification process of high-alloy steel, and improving the solidification structure have been increasingly gaining interest among metallurgists, particularly the electroslag workers. To further develop the electroslag remelting (ESR) process and to improve the ingot solidification structure, the effects of mould rotation on chemical element distribution and the compact density were investigated in this study. The experimental results showed that chemical element distribution would become more uniform when the mould keeps the reasonable rotation rate. However, the excessive rotation rate would deteriorate the solidification structure of steel. When mould rotation rate was between 0 and 28 r/min, maximum segregation of carbon could decrease from 3.19 to 1.084, and statistical segregation decreased from 0.2636 to 0.0554. Maximum segregation of chromium could decrease from 1.316 to 1.131, and statistical segregation decreased from 0.2753 to 0.0657. The compact density increased from 0.7693 to 0.94. But element segregation would become bigger and compact density would become smaller if rotation rate further increased. The improvement in the solidification structure could be attributed to reasonable mould rotation rate which could initiate movement in the slag pool and further increase the uniformity of the temperature in the slag pool. At the same time, the movement in the slag pool could also affect the metal molten droplet, scattering the liquid drop randomly in the metal pool. But the excessive rotation rate made the slag pool violent motion, so as to drive the molten metal pool to rotate which would carry off enriched steel surrounding the dendrites in the mushy zone and reduce the solute content in the region. As a result, the element segregation would occur.

In the Fe-Fe 2 Nb eutectic system, halo formations were observed at both the hypo- and hypereutectic compositions. This observation cannot be well explained by the existing halo theories based on competitive growth or competitive nucleating alone. In the present paper, a new explanation is proposed to describe the formation of halos in the binary Fe-Fe 2 Nb off-eutectic alloys. By theoretical calculations, the α-Fe and Fe 2 Nb are predicted to be the primary growing phase in the hypo- and hypereutectic compositions, respectively. Based on the experimental observations, the Fe 2 Nb Laves phase was identified as a good nueleant for α-Fe phase, but not vice versa. By analysing of the competitive growth among the involved phases and the growth undercoolings of the two individual eutectic phases, the formation of halos in the Fe-Fe 2 Nb alloys is explained successfully.

In this study, microstructural, mechanical and wear properties of TiB 2 reinforced Al 2014-based composites produced by vacuum infiltration were investigated under different production temperatures. It was observed that the microstructures of the composites consisted of Al matrix and TiB 2 particles. TiB 2 particles exhibit various shapes such as cubic, triangle and spherical. It was observed that the hardness and porosity of the composites increased with increasing TiB 2 content. It was also observed that the production temperature has an important effect on the porosity and hardness of the produced composites. The highest wear resistance was obtained from Al 2014-8% TiB 2 composite among the materials tested. It was observed that the production temperature and hardness are effective parameters on wear of produced composites. Evaluation of microstructural and mechanical test results suggest that it would be beneficial to keep the production temperature around 800 °C and Al 2014-8% TiB 2 composite can be recommended for engineering applications where the hardness and wear resistance are considered to be significant factors.

The generalized Darken method allows a quantitative description of diffusion mass transport in multi-phase materials. The method characterizes the diffusion zone by phase volume fractions. The results of the calculations are compared with experimental concentration’s profiles of nickel, zirconium and aluminum in zirconium doped aluminide coatings deposited on pure nickel by the PVD and CVD methods.