Yingche Ma

Effects of Heat Treatment and Nitrogen on Microstructure and Mechanical Properties of 1Cr12NiMo Martensitic Stainless Steel

A series of heat treatments using the orthogonal experiment method were performed to study the microstructure and mechanical properties of 1Cr12NiMo martensitic stainless steel containing various nitrogen content addition. The results indicate that the optimal heat treatment is annealing at 830 °C for 1 h, austenitizing at 985 °C for 1 h followed by oil quenching, and tempering at 630 °C for 4 h followed by air cooling. Nitrogen addition to 1Cr12NiMo steel can effectively hinder the austenite grain growth, refine the martensite lath, and increase the strength and hardness. The impact toughness of this steel only shows a minor decrease as the nitrogen content increases.

Micro-segregation and Precipitation of Alloy 690 during Isothermal Solidification: the Role of Nitrogen Content

The segregation and precipitation behavior of Alloy 690 containing 0.001-0.11 wt% nitrogen during isothermal solidification at 1370 and 1355 degrees C have been investigated using optical microscopy (OM), electron probe microanalysis (EPMA) and transmission electron microscopy (TEM). The results indicate that the volume fraction of TiN-type nitride formed during isothermal solidification increases with the nitrogen content of Alloy 690. Segregation of Ti and Cr exists in samples solidified at 1370 and 1355 degrees C. The Ti content in the residual liquid markedly decreases and the concentration of Cr increases when the nitrogen content of Alloy 690 increases. Furthermore, N and S also show segregation to some extent in the residual liquids at 1355 degrees C. Accompanying by the segregation of Cr, Ti, C, N and S, sulfides and chromium nitrides form. In a low nitrogen content Alloy 690, sulfur segregates and precipitates in the form of Ti4C2S2 and (Cr, Ti)S, but in the form of (Cr, Ti)S or CrS in a high nitrogen content Alloy 690. (Cr, Ti)N-type nitrides with an fcc crystal structure have been identified in a sample with 0.11 wt% nitrogen.

Effects of Oxygen on the Microstructure of Ti47Al0.7B Alloys

The effects of oxygen on the microstructure of Ti-47Al-0.7B (at. pct) alloy for as-cast automotive valves were investigated. Six alloys with oxygen content from 0.4 to 1.4 at. pct were prepared by induction melting and centrifugal casting in CaO crucible under protective atmosphere. The microstructures were observed by optical microscopy (OM) and scanning electron microscope (SEM). The results show that the increase of oxygen content led to grain refinement and enhanced the microhardness as well as the alpha(2) volume fraction in the TiAl-based alloys.

Changes of Oxygen Content in Molten TiAl Alloys as a Function of Superheat during Vacuum Induction Melting

Experimental studies on the melting process of titanium aluminum alloy have been pursued from the viewpoint of contamination TiAl alloys were prepared with vacuum induction melting (VIM) in calcia crucibles at 1873 K and 1923 K in order to determine the behavior of the oxygen content as a function of temperature time and frequency of power The experiment results showed that alloys were uncontaminated except for the increasing of oxygen content which was introduced from the reaction CaO(s)=Ca(in TiAl)+O(in TiAl) and the standard Gibbs energy of the reaction was determined to be Delta G(0)=274000-102 8T (J/mol) Oxygen content increased slowly with the melting time by about 50 x 10(-6) wt pct/min and decreased with induction melting frequency Lower superheat and higher melting frequency can be used to reduce oxygen content increasing rate

Modeling of filling and solidification process for TiAl exhaust valves during suction casting

Investment and suction casting (ISC) represents an economic and promising process route to fabricate automotive exhaust valves of γ-TiAl based alloys, but information available on the metal flow and the temperature changes during mould filling and solidification process for the ISC process is meager. A sequentially coupled mathematical flow-thermal model, based on the commercial finite-volume/finite-difference code FLOW-3D and the finite-element code PROCAST, has been developed to investigate the ISC process. In term of calculating the flow and temperature fields during the filling and solidification stages, potential defects including the gas bubbles and the surface air entrainment occurred in the mould filling process and the shrinkage porosities formed in the solidification process are predicted and the reasons for the formation of these defects are also analyzed. The effects of filling pressure difference control methods and moulds on gas bubble and surface air entrainment behavior are presented. It is found that by changing the filling pressure difference control methods from general suction casting to “air leakage” suction casting and reducing air leakage flow rates, the gas bubbles are eliminated effectively, and the surface air entrainment attenuate dramatically. With resort to a mould with a tetragonal runner, the surface air entrainment decrease to the lowest level. Finally, the water analogue and suction casting experiments of exhaust valves are implemented for further validation of the simulation results.

Heat-affected zone crack healing in IN939 repaired joints using hot isostatic pressing

Simulation defects in IN 939 superalloy were repaired by gas tungsten arc welding (GTAW) using Nimonic C263 as filler metal. The microstructures and chemical compositions of fusion zone under the condition of as-welded and post-weld heat treatment (PWHT) were investigated by optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe X-ray microanalysis (EPMA). Moreover, the liquation cracking and strain age cracking in heat-affected zone (HAZ) were characterized as well. Post-weld hot isostatic pressing (HIP) was also applied to heal the HAZ cracks. The results show that elements segregation at as-welded joints could be significantly reduced by PWHT. HAZ cracks consisted of liquation cracking and strain age cracking. The liquation cracking was shorter than the strain age cracking. HIP is an effective way to heal the cracks in the repaired joints especially for liquation cracking. Though the strain age cracking can be healed by HIP, the healed trace concentrated with Al, Ti, B, and Zr elements can still be detected. Post-weld HIP is a necessary process to obtain crack-free joints.

Evaluating the Microstructures and Mechanical Properties of Dissimilar Metal Joints Between a New Cast Superalloy K4750 and Hastelloy X Alloy by Using Different Filler Materials

Two kinds of filler materials were used to join dissimilar alloys between a new cast superalloy K4750 and Hastelloy X by tungsten gas arc welding (GTAW). The segregation behavior, interfacial microstructure and mechanical properties of the dissimilar joints were evaluated. The results show that both filler materials can be used to obtain sound dissimilar joints successfully. Microstructural observation show that no obvious cracking is observed in the joints achieved by both filler materials. The segregation extent of various elements in Hastelloy X weld metal is more severe than that in the K4750 weld metal. No unmixed zones were observed at the interfaces. Transition areas with the chemical compositions various between the K4750 alloy and the Hastelloy X alloy were found at the joint interfaces. The maximum width of the transition area between the K4750 weld metal and Hastelloy X base metal is smaller than that between the Hastelloy X weld metal and K4750 base metal. The ultimate tensile strength and yield strength of the joints with Hastelloy X filler material are slightly higher than those with K4750 filler material, however, the K4750 filler material results in a higher total elongation and fusion zone microhardness than those with Hastelloy X filler material. Both dissimilar joints fractured with a ductile feature which exhibits tearing edges and dimples. Hastelloy X filler material is suggested to be more suitable for joining of K4750 superalloy and Hastelloy X dissimilar metals in terms of obtaining superior comprehensive mechanical properties.

σ-Phase Precipitation Mechanism of 15Cr-15Ni Titanium-Modified Austenitic Stainless Steel During Long-Term Thermal Exposure

The σ-phase precipitation in 20% cold-worked 15Cr–15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment. During the isothermal aging stage, the amount of σ-phase was significantly increased at beginning and gradually became constant. The aging time only slightly affected the size and morphology of the σ-phase. Conversely, during the isochronal aging stage, the amount of σ-phase grew rapidly with the increase in the aging temperature. The σ-phase with a large amount of stacking faults was prone to nucleate around the (Ti, Mo)C particles or at the grain boundaries. The (Ti, Mo)C particles can act as effective nucleation sites, where the σ-phase directly precipitates from the austenitic matrix. From this work, the growth of σ-phase is found to be influenced by the aging temperature, and a new mechanism of σ-phase precipitation from austenite has been proposed.

Microstructure and Solidification Characteristics and Segregation Behavior of Superalloy K4169

The solidification process and segregation feature of K4169 alloy is studied by DSC, Thermo-Calc simulation and isothermal solidification analyses. The segregation and precipitation behavior of superalloy K4169 during isothermal solidification at 1360 and 1120 °C have been observed by the means of optical microscopy (OM), electron probe microanalysis (EPMA), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results stated that the solidification began with the formation of primary γ dendrites, and accompanied by the formation of MC carbides. During the solidification, Nb, Mo and Ti were enriched in the residual liquid phase, which eventually lid to the formation of MC carbides and Laves phases. The liquidus of K4169 was 1354 °C, and the solidus was 1240 °C. Its solidification sequence can be identified as follows: L → γ at 1354 °C; L → γ + MC at 1290 °C; L → γ + Laves at 1120 °C.

Influence of Orthogonal Heat Treatments on Mechanical Properties of HT-9 Ferritic/Martensitic Steel

A series of heat treatments by orthogonal experimental method were performed to study the mechanical properties of HT-9 ferritic/martensitic steel. The results show that the tempering temperature is the most important factor affecting the yield strength (Rp0.2) and elongation (EL%) of HT-9 steel. With the increments of tempering temperature, EL% increases and Rp0.2 decreases gradually. Both normalizing temperature and tempering temperature show influence on DBTT of HT-9 steel. Considering the tensile strength and impact toughness properties with no abrupt reduction of tensile strength, the optimal heat treatment regime is selected as follows: normalizing at 1000 ℃ for 0.5 h followed by oil cooling, and tempering at 760 ℃ for 1.5 h followed by air cooling.