Papers by Author: Jian Xin Dong

Abstract: With the development of aircraft engine, higher requirement was put forward on turbine disk materials. In the present work, new thoughts on improving high temperature properties of superalloys have been proposed and a newly developed candidate turbine disk material for 850°C-900°C application with a composition of Ni-Co-Cr-W superalloy has been investigated. The results show that W is beneficial for mechanical properties. Microstructural characteristics and hot deformation of this new alloy were studied by optical microscope (OM), field emission scanning electric microscope (FESEM) and energy dispersive X-ray spectrometer (EDX) and differential scanning calorimetry (DSC). The results show that the main precipitates in the as-cast condition are γ’ phase, primary MC carbides and eutectic phase. The incipient melting temperature, γ’ solvus and MC solvus are 1312°C, 1220°C and 1356°C respectively. Cracks are observed in the tested samples after hot deformed at 1160°C to 1220°C with 30% strain. They initiated at the surface of the samples and propagated along the grain boundaries and also initiated at the interface of carbides and matrix.

Abstract: High temperature compression tests at a deformation temperature range of 1273K~1473K with various strain rates of 0.01s^-1~0.1s^-1 on as-cast GH625 alloy were carried out, aiming at the current research status that the deformation process of cogging and the recrystallization behavior of ingot are still in the study. The results indicated that the recrystallization nuclei of ingot formed not only along the original grain boundaries, but also in the interdendrite. Dynamic recrystallization volume percent increased with the increase of temperature and the decrease of strain rate. When the temperature was high and strain rate was low, the dynamic recrystallization behavior of as-cast GH625 alloy was dominated by discontinuous recrystallization. However, when the temperature was low and strain rate was high, continuous recrystallization also existed. These results can provide some reliable experimental support for the cogging process design.

Abstract: Microstructure and mechanical property of tungsten inert gas (TIG) welding joint of 740H pipe were studied by optical microscope, scanning electron microscope and HV tester in this paper. The principal segregated elements in fusion zone of 740H weldment were Nb and Ti. The discontinuous MC-type phase with irregular shape and γ′ phase were distributed in the interdendritic region. The γ′, M₂₃C₆ and MC phases on grain boundary in heat affected zone (HAZ) dissolved into matrix. Hardness of weld metal and HAZ were apparantly lower than that of base metal. The previous weld bead was the heat affected zone of the subsequent weld metal, thus hardness was different in different area of the fusion zone (FZ). After the postweld heat treatment of 800°C/4h followed by air cooling, the volume fraction of γ′ in the interdendritic region was larger than that in the dendrite arm. The hardness of HAZ and FZ were approximately equal to that of base metal (BM).

Abstract: K419 superalloy turbine wheel was more susceptible to hot tearing than K418 one when they were used for auto turbocharger turbine wheel. The fracture and microstructure characteristics in the K418 and K419 turbine wheel blades were analyzed. The segregation of alloying elements was analyzed by EDS. The probable equilibrium phases in the two kinds of superalloys, the effects of aluminum, titanium and niobium contents on the precipitation of γ and γ/γ eutectic phase and the segregation of alloying elements were studied by Thermo-Calc software. The results show that the hot tearing in the K418 and K419 superalloy turbine wheel blades is caused by the fracture of dendrites structures, while the amount of γ/γ eutectic in K419 is more than that in K418, resulting in K419 being more susceptible to hot tearing. Titanium and niobium, the strong positive segregation elements promote the formation of γ/γ eutectic, which lead to severe hot tearing susceptibility of the superalloy.

Abstract: The Nb-contained austenitic heat-resistant steel 18Cr10NiNb(TP347H) has been widely used as super-heater and re-heater tube material for modern ultra-super-critical (USC) power plants in the world. High temperature structure stability is considered to be one of the most important factors for long-term service. Long-term aging at 650 °C for this steel was conducted till 10,000 hours. Effect of aging time on microstructure was studied by means of SEM and TEM. Tensile tests were carried out after aging at 650 °C for different times. Experimental results show that MX phase and M₂₃C₆ carbides are major strengthening precipitates in this steel. With increasing of aging time, fine nano-size MX particles precipitate in grains and its size keeps about 50 nm till 10,000 h at 650 °C. Carbide M₂₃C₆ mainly precipitates at grain boundaries but coarsens quickly. Investigation results show that MX phase plays the most important strengthening effect in grains. The amount of MX phase increases with increasing of Nb and C contents. The effects of Nb and C contents on mole fractions of MX phase in 18Cr10NiNb steel have been calculated by using Thermo-Calc software.

Abstract: The second phase precipitation strengthening effect of , +, + phases in INCONEL718 type and newly developed INCONEL740® Ni-base superalloys and their structure stability have been studied. Experimental results show that careful control of Nb, Ti and Al for alloy modification can not only raise their strengths but also improve structure stability at high temperatures. These modified newly developed alloys are suggested to be used at higher temperatures.

Abstract: A recent developed Ni-Cr-Co-Mo-Nb-Ti-Al type nickel-base superalloy, INCONEL 740, has been selected for the application of USC boilers at the temperature above 750°C. This paper focuses on the structure stability improvement of this alloy. Phase computation by Thermo-Calc has been adopted to study main influencing factors on precipitating phases of the alloy and the results show that the ratio of Al/Ti plays an important role. Four new modified alloys in adjustment of Al and Ti contents and in control of Si level were designed and melted for experimental study. The results indicate that the modified alloys exhibit more stable structure stability at 750, 800 and 850°C long time exposure. The newly developed alloy can be adopted for engineering production and application for USC power plants at temperature above 750°C.

Abstract: High performance alloy spring steel D701 for accelerated railway wagons is improved 60Si2CrVA steel. Its properties have been better than 60Si2CrVA steel.The difference between of them is using controlled rolling process for D701 steel.The affect of hot treatment process was studied by orthogonal test on D701 steel property in the article. The mechanical property σs is 1800MPa; σb is 1925MPa; δ is 9.5%, ψ is 36.5%.

Abstract: High temperature structure stability of 2 important Nb-containing Ni-base superalloys Inconel 718 (Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) and Inconel 740 (Ni-25Cr-20Co-0.5Mo-2Nb -1.8Ti-0.8Al) have been studied on the relationship of microstructure to mechanical properties via SEM, TEM, SAD, XRD and quantitative determination of precipitated phases by micro-chemical analysis. The longest exposure times are 50,000hrs at 650°C for Alloy 718 and 4,000hrs at 704°C, 725°C and 760°C for Alloy 740. The structure instabilities for these 2 Alloys are: 1) strengthening phases γ″/γ′ coarsening; 2) meta-stable strengthening phases change to stable phase, such as γ″→ δ-Ni3Nb for Alloy 718 and γ′→ η-Ni3Ti for Alloy 740; 3) embrittling phases formation, such as α-Cr and σ-phase formation in Alloy 718 and high Si-containing G-phase formation in Alloy 740. On the base of understanding phase changes at high temperature exposures the structure stability of Inconel 718 and Inconel 740 has been improved both by minor adjustment of alloys chemistry. The modifications of Alloy 718 and Alloy 740 are to be developed in the near future.

Abstract: Inconel 718(Ni-19Cr-18Fe-3Mo-5Nb-1Ti-0.5Al) nickel-base superalloy strengthened mainly by Ni3Nb type γ″ and partially by Ni3Al type γ′ precipitation is today’s most widely used superalloy in the world. China has paid special attention on Inconel 718 research and development. Systematic long-term research project has been conducted in close cooperation among our university, research institutes and factories. The goal of this long-term project is in 2 steps. First step is to improve Alloy 718 to get high quality and the second step is for improving the alloy temperature capability from 650oC to 680-700oC. The basic idea for alloy improvement is still to keep the chemical composition in the range of specification by small adjustment or control of minor elements, such as S, P, N, Si and Mg. The main achievements are segregation control by adjustment of S, P, Si, control of N for cleanliness, micro-alloying of Mg for grain boundary strengthening and control of low S and high P for stress rupture life improvement. Modification of Alloy 718 is based on structure stability study and its improvement in adjustment of main second phase strengthening alloy elements Nb, Ti and Al in total amount and the ratios among them. The goal is to achieve more stable second phase strengthening by control of alloy elements. Experimental results show that the future of modified 718 alloys is very attractive to raise the temperature capability improvement from 650oC to 680-700oC.