Papers by Keyword: Heat Resistant Steel

Abstract: Although welding results in premature failure by type IV fracture under high temperature creep conditions, the alloy design of light elements such as boron addition and nitrogen reduction enhances the creep lifetime of 9Cr heat resistant steel. In particular, the simulated heat affected zone (SHAZ) sample of new 9Cr steel (called TA steel) shows about 10 times longer creep lifetime than that of the standard Gr. 91 steel. The welded TA steel is thus expected to exhibit good creep properties because its SHAZ sample has coarser grains and suppresses type IV fracture. The preservation of base metal’s microstructure after welding results from the precipitate morphology, such as high grain boundary coverage by precipitates and low amount of MX being nucleation sites of ferrite grains during the a-g phase transformation. In addition, the increase of stability of M₂₃C₆ affects high pinning pressure toward grain boundary migration upon rapid heating during welding. First-principles calculations confirm the increased stability when boron is absorbed by M₂₃C₆. Moreover, the calculations reveals that boron decreases the coherency between matrix and M₂₃C₆, suppressing grain coarsening during creep tests in TA steel. It is concluded that the increased microstructural stability during welding and long high temperature exposure generates the elongated creep lifetime in welded TA steel including about 0.01 wt% boron and less than 0.01 wt% nitrogen.

Abstract: The current paper proposes a so-called Zc method to evaluate creep strain and rupture life of heat resistant steels by considering the scattering distribution of experimental data, where the Zc value indicates the deviation of the fitted σ-P curves at different creep strains to the main curve. It is shown that the value of Zc increased rapidly at first and then became stable with increasing creep strain, meanwhile, the degree of deviation between experiment data and the corresponding fitted curves came to decrease. The current method provides a statistic method for predicting creep stain and rupture life and get good agreement with experiment result.

Abstract: This paper mainly introduces the properties of welded joints of supercritical boilers, combined with the actual construction of a power plant, of these new type heat-resistant steel we Ultra supercritical unit; heat resistant steel; weldability al exploration.

Abstract: Based on heat resistant steel ZG40Cr24, test alloys were cast by intermediate frequency induction furnace with non-oxidation method by alloying of aluminium and silicon. The oxidation resistance at 1100°C for 500 hours of test alloys was carried out according to oxidation weight gain method. The thermal diffusion were tested by Laser Heat Conductivity. The thickness of oxide scale was detected by Coating Thickness Meter. Experimental results showed that the thermal diffusion of oxide scale affected its oxidation resistance exactly, the lower thermal diffusion coefficient matched the higher oxidation resistance. The oxide scale thermal diffusion coefficient of ZG40Cr24+2%wt.Si+4%wt.Al was only 0.00092cm².s^-1, endowing itself 0.0633g.m^-2.h^-1 oxidation weight gain rate, reaching the complete oxidation resistance. The mechanism of the effect of thermal diffusion on oxidation resistance lay in that the lower thermal diffusion represented the inert inner particles of materials, the few quantity of diffusion particles, and lower transporting and moving rate. So the oxidation rate slowed down, realizing higher oxidation resistant property for oxide scales.

Abstract: Synchrotron X-ray microtomography(SR-μCT) scans have been carried out on sample coupons extracted from the fracture specimens of a 10.86% Cr heat resistant steel exposed to crep deformation at 873K over stresses of 120, 150, and 180 MPa. The 3D cavitation characteristics in terms of void volume fraction, numbwer density and size distribution as a function of the applied stress has been determined by quantitative analysis of the reconstructed tomograohy slice datasets. The relationship between heterogenous spatial distribution of creep voids and variation in rupture life has been exploited in terms of microstructural sites during the onset of creep embrittlement.

Abstract: Effect of carbide precipitation on pinning force and migration mechanism of boundaries of martensite laths was considered in a 3%Co modified P911. The dimensions of second phase precipitations, martensite laths and dislocation densities were measured by means of transmission electron microscopy. The pinning forces retarding the motion of the lath boundaries, that arise from M(C,N) nanoscale precipitations and M₂₃C₆ particles were evaluated by using different models. The pinning pressure evaluated by taking into account a non-uniform distribution of M₂₃C₆ particles was high enough to stabilize the lath martensite structure during tempering and long term ageing. On the other hand, significant coarsening of martensite laths occurred in neck portions of samples subjected to long-term creep tests. Additional effects from dislocation density and applied stress on the motion of lath boundaries are considered in some details.

Abstract: Microstructure evolution in a P911 heat resistant steel was examined under conditions of aging and creep at a temperature of 600°C and an applied stress of 200 MPa. The tempered martensite lath structure (TMLS) evolved after heat treatment consisted of prior austenite grains (PAG), packets, blocks and laths. The mean transverse lath size and the interior dislocation density were about 345 nm and 3.5 × 10¹⁴ m^-2, respectively. Various second phase particles precipitated upon tempering. Fine MX carbonitrides were homogeneously distributed throughout the tempered martensite laths, while relatively coarse M₂₃C₆ carbide particles were located on high-and low-angle boundaries. Upon creep test, precipitation of Laves phases was found. The stability of TMLS during creep is discussed in detail.

Abstract: Influences of aging on the creep rupture properties of super-clean 9%CrMoV steel and 1%CrMoV steel, the heat resistant steels for steam turbine rotors of thermal power plants, are investigated. Using the as-received and the aging-treated materials of the two steels, creep rupture tests are carried out at 566°C. Creep rupture lives, creep fracture modes as well as the microstructural changes of the specimens are examined. It is made clear that the creep strength and the microstructural stability of super-clean 9%CrMoV steel are superior to those of 1%CrMoV steel in long-term services.

Abstract: During superplastic forming, dies are subjected to high temperatures and severe environmental conditions. Optimum material grade choice and die design have to take into account all these combined parameters. Microstructure evolution and high temperature mechanical properties are investigated and reported for various Heat Resistant Cast Steels. New die concepts are suggested for energy and cost savings.

Abstract: In this work, two heats of 9Cr ferritic/martensitic heat resistant steels with different carbon and nitrogen contents were prepared. The steels were designed to have much lower carbon content than conventional 9-12Cr heat resistant steels for obtaining dense nano-sized MX precipitates. Microstructure of the two steels in different heat treatment states was analyzed by electron backscatter diffraction (EBSD) method. The results show that grain boundary character is greatly affected by carbon and nitrogen contents. Martensite in the steel with 0.02wt.% carbon and ultra low nitrogen is easier to recrystallize than that in the steel with ultra low carbon and 0.03wt.% nitrogen during tempering treatment. The effect of grain boundary character on stress rupture properties is also discussed.