Papers by Author: Xi Shan Xie

Abstract: The Cu-containing austenitic heat-resistant steel 18Cr-9Ni-3CuNbN has been widely used as superheater and reheater tube material for modern ultra-super-critical (USC) power plants in the world. High temperature structure stability is considered to be an important factor for long-term service. Long-term aging at 650°C for this steel was conducted from 100 to 10,000hours. Effect of aging time on microstructure was studied by means of SEM, TEM and 3DAP (three dimensional atom probe). Micro-hardness tests were carried out after aging at 650°C for different times to be considered as a representative of strength. Experimental results show that Cu-rich phase, MX and M23C6 are major strengthening precipitates in this steel. With on increasing of aging time, fine nano-size Cu-rich phase particles precipitate in the grains and its size is in the range of several nanometers to 35nm till 10,000h at 650°C. The fraction of MX also increases with aging time and its average size is about 100nm till 10,000h. Carbide M23C6 mainly precipitates at grain boundaries and coarsens quickly. Investigation results show that the most important strengthening effect for 18Cr-9Ni-3CuNbN steel is contributed by Cu-rich phase and MX in the grains and M23C6 carbide at the grain boundaries.

Abstract: Super304H, a Cu-containing 18Cr-9Ni-3CuNbN heat-resisting steel is wildly used as an superheater/reheater tube material for ultra-super-critical (USC) power plants all over the world. It is recognized that the Cu-rich phase is an important strengthening phase for Super304H. However, the detail precipitation behaviour and its strengthening effect are still not very clear. Investigated material was taken from routine production and was aged at 650°C for different times. The precipitation of Cu-rich phase in Super304H was studied by three dimensional atom probe (3DAP) and TEM. Experimental results show that Cu-rich clusters have been formed at very early stage of 650°C aging. The Cu-rich particle images have been clearly caught just after 650°C aging for 5h. The Cu atoms gradually concentrate to Cu-rich particles and the other elements (such as Cr, Ni etc) diffuse away from Cu-rich particles to γ-matrix with the increasing of aging time. The Cu-rich particle size and its density have been determined as a function of aging time. Cu-rich particles still keep nano-size and distribute homogenously in grains even after long time (1,000h) aging, which is one of the most important reasons for keeping good strength of Super304H heat-resistant steel at high temperatures.

Abstract: Inclusions are un-avoidable even in super-clean engineering alloy steels because of the necessary melting process. These inclusions (such as TiN, AlN etc) are considered as harmful phases especially for ultra-high strength alloy steels. The unique experiments (in-situ tension and in-situ fatigue tests) have been conducted in a loading chamber of scanning electron microscope. TiN often characterizes with large blocky cubic morphology. Cracks easily initiate at the sharp corners of TiN cubic particles or sometimes directly initiate in TiN particles because of its brittleness. These cracks propagate to the matrix and to introduce early failure. AlN small particles (in several microns) often distribute as inclusion chains in steels. At tensile and fatigue tests cracks very often initiate at the inclusion chains among AlN small particles and line up to develop voids, which rapidly propagate to the matrix till early failure. These important results reveal the harmful effect of inclusions in micro-scale and can be connected with tensile and fatigue loading processes for understanding the early failure mechanisms.

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: Specially designed SEM in-situ tensile and fatigue tests have been conducted to trace the entire process of crack initiation and propagation till fracture in an ultra-high strength steel MA250. TiN is a typical inclusion and its average size is in the range of 8~10μm in MA250 steel. The micro-mechanism of the effect of TiN inclusion on crack initiation and propagation at tensile and fatigue tests both have been studied in detail. Experimental results show the harmful effect of TiN on tensile and fatigue properties both. This work is helpful to establish the practical life prediction model for the characteristic inclusion parameters in ultra-high strength steel components. It also enlightens us to eliminate TiN in the further development of ultra-high strength steels.

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.

Abstract: The microstructure of directionally solidified (DS) NiAl-28Cr-5Mo-1Hf eutectic alloy was investigated by SEM and TEM. The eutectic alloy is composed of NiAl matrix , Cr (Mo) phase and a small amount of Heusler phase. Tensile testing of this alloy was performed from room temperature to 1373K. The brittle-to-ductile transition temperature (BDTT) is dependent on strain rate, with two orders of increase on strain rate resulting in a100K increase in transition temperature. The observation of fracture surface showed that at temperature above BDTT, the fractograph changes from NiAl cleavage and debonding along NiAl/Cr(Mo) interface boundary to ductile in nature.