Cr Ferritic Heat-resistant Steel Research Articles

Grain Size Dependence of Secondary Creep Rate in Pure Metals and Single-phase Alloys

Grain size dependence of secondary creep rate in pure metals and single-phase alloys and the mechanisms/models to interpret the dependence have been reviewed. Two types of the grain size dependence were reported, both of which show a negative dependence approximately below 100 μm. The model proposed by Garofalo in 1960s assumes that the density of dislocations generated at grain boundaries and sub-boundaries determines the secondary creep rate, which is not experimentally supported. Mclean’s model considers preferential subgrain growth near grain boundaries, which might be important in practical steels and alloys with sub-grains such as high Cr ferritic heat resistant steels. Grain boundary sliding (GBS) and its accommodation process in grain is considered as a source of the negative grain size dependence. A finite element modeling performed by Crossman and Ashby, which simulated a deformation process where GBS is accommodated by the power law creep process in the grain interior, indicates that the negative grain size dependence cannot be interpreted by the accommodation process. Based on substructure observation and internal stress measurements, Terada established a “core and mantle” model where the mantle region near grain boundary has no internal stress. This model reasonably interprets the negative grain size dependence.

Correlation of deformation with damage progression behavior around a notch tip under creep and fatigue conditions for W-added 9Cr steel including weld joint

Research concerning heat-resistant steels for the application in fossil-fired power plants has progressed remarkably during the past 60 years. This has resulted in improvements in the electrical efficiency of fossil-fired power plants. Currently, there are plans and programs to develop ultra-supercritical plants designed to operate at steam temperature and pressure conditions of 600/650 °C and 32 MPa. The W-added 9%Cr ferritic heat-resistant steel, that is, ASME grade P92, has been developed as a boiler material for this ultra-supercritical plant. Boiler materials, whose performance is critical for ultra-supercritical plant, are required to possess high creep resistant properties. In addition, these materials are exposed to fatigue induced by thermal stresses, that is, they are operated under creep-fatigue interacting conditions. In this study, mechanical tests under the condition of high temperature creep-fatigue interaction were conducted for P92 steel under stress-controlled and various load frequency conditions using the in-situ observational creep-fatigue testing machine to observe the damage formation behavior around a notch tip composed of voids in mesoscale. On the basis of these results, the effects of damage formation behavior on crack growth life were clarified. Furthermore, for the case of creep deformation, the numerical analyses of vacancy diffusion and concentration around a notch tip were conducted using our proposed numerical method of local stress-induced vacancy diffusion behavior, which is a nanoscale phenomenon to relate these behaviors to the damage formation behavior in mesoscale (μm scale).

Effect of Cr Content on Softening during Aging in High Cr Ferritic Heat Resistant Steel

Effect of Cr Content on Softening during Aging in High Cr Ferritic Heat Resistant Steel

Development of the modified high Cr ferritic heat-resistant steel

High Cr ferritic heat-resistant steels have been the important structural materials for elevated temperature application in the advanced power plants. Owing to the severe difficulties encountered by energy shortage and environment pollution, it is imperative to improve the serving temperature up to 650° which would largely elevate the thermal efficiency of generating station. In this paper, the phase transformation behaviours, strengthening mechanism and the formation the precipitates of high Cr ferritic steels were summarised. Based on these, the modified high Cr ferritic steel for 650 °C condition was developed by our group. The tensile and creep tests indicate that the modified steel has the outstanding mechanical properties, suggesting it may be the candidate materials in advanced power plants for 650 °C grade. Furthermore, developments of the modified high Cr ferritic steels will be presented and discussed in views of microstructural control, alloy design, phase transformation behaviours and high-temperature performance.

Effect of interstitial solutes on precipitation behavior of 9–12% chromium ferritic steels

Effects of nitrogen and carbon content on precipitation behavior in two 9–12% Cr ferritic heat-resistant steels during isothermal transformation were investigated. Isothermal aging treatments at 700 °C after solution annealing were carried out for the different periods up to over 220 h. Microstructure characterization was performed by many characterization methods, such as physicochemical phase analysis, X-ray diffraction and scanning electron microscope etc. It was found that Cr-rich M2N nitride was formed as the main precipitate in the alloy A with content of 0.15% N and 0.03% C in wt%, together with some Cr-rich M23C6 carbide and Nb-rich MN nitride. Comparatively, the aged alloy B with 0.05% N and 0.11% C, predominantly contains Cr-rich M23C6 carbides. Besides, both alloys showed the similar microstructure evolution process: The precipitates were found to be formed initially along prior austenite grain boundaries, then grew toward interior of grain in the form of cell with the increasing aging time. Meanwhile, the cell growth feature was also discussed based on the experimental observation in well-controlled specimens.

Formation and evolution of precipitates in high Cr ferritic heat-resistant steels

For high Cr ferritic heat-resistant steels, dispersion hardening has played a vital role in improving creep strength at high temperatures. In this paper, the various types of precipitates in high Cr ferritic steels are reviewed, concerning M3C, M23C6, MX, M2X, Laves phase and Z phase. The evolution of these precipitates is a complicated process, since the dissolution of one type of precipitate may be overlapped with the precipitation of another type or other types. The composition of some precipitations varies towards the equilibrium composition under certain conditions, which is relevant to the solubility and diffusivity of the alloying element, while the composition of some other precipitates changes slightly. For the precipitates in high Cr ferritic steels, the preferred nucleation sites involve prior austenite grain boundaries, lath boundaries, dislocations and the interfaces between the precipitates and the matrix.

Characterisation of crack growth behaviour for W-added 12%Cr ferritic heat resistant steel under the creep-fatigue interaction conditions

The W-added 12%Cr ferritic heat resistant steel has been developed as a turbine rotor material which is used under high temperature creep-fatigue conditions. In this study, crack growth tests using C(T) specimen under the creep-fatigue interaction conditions were conducted under various conditions of stress holding time, applied stress and temperatures. As a result, the effect of load frequency on crack growth behaviour and its life for this steel was characterised by the three-dimensional curved surface based on the concept of non-equilibrium science. Furthermore, a predictive law of crack growth life under the creep-fatigue interaction conditions was derived based on the concept of Q* with the transition function of crack growth life from fatigue to creep.

169 クリープ・疲労相互作用条件下でのW添加12Cr鋼のき裂成長寿命則

169 クリープ・疲労相互作用条件下でのW添加12Cr鋼のき裂成長寿命則

Carbonitride Dissolution and Austenite Grain Growth in a High Cr Ferritic Heat-resistant Steel

Carbonitride Dissolution and Austenite Grain Growth in a High Cr Ferritic Heat-resistant Steel

Role of Boundary Strengthening on Prevention of Type IV Failure in High Cr Ferritic Heat-Resistant Steels

Microstructure evolution of newly developed 9Cr-3W-3Co-V, Nb steel with boron addition (B steel) has been analyzed during HAZ thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ) and post-weld heat treatment (PWHT) to elucidate the prevention mechanism of type IV failure by boron addition. It was found that enhancement of the boundary strengthening by precipitates is the main reason for prevention of type IV failure by boron addition. In B steel HAZ, original austenite is reconstituted through martensitic α to γ reverse transformation during the heating and original martensite is reconstituted through martensitic transformation during cooling of the Ac3 HAZ thermal cycle. This process allows M23C6 carbides to precipitate at the prior austenite grain (PAG) and block boundaries during PWHT even if the chemical segregation of carbide forming elements exists. The effect of boundary strengthening on the creep property has also been investigated. Microstructure evolution during creep was compared among Gr.92 with different Ac3 HAZ microstructures prepared by three kinds of heat treatments and B steel. The results revealed that both the boundary length and kernel average misorientation value decreased in all samples during creep. However, this process occurred very rapidly in Ac3 HAZ simulated Gr.92, whereas it was significantly retarded in the other samples with sufficient boundary strengthening by precipitates. This result confirms that the precipitates formed at PAG and block boundaries play the most important role to stabilize the microstructure of Ac3 HAZ simulated samples during creep and prolong the creep life.

Martensite–austenite transformation kinetics of high Cr ferritic heat-resistant steel

Abstract The isochronal transformation behavior from martensite (α′) to austenite (γ) of modified high Cr ferritic heat-resistant steel was analyzed using the kinetic information extracted from differential thermal analysis results. The thus obtained α′→γ transformation kinetic process has been described by a JMAK-like model including three overlapping processes: site saturation nucleation, diffusion-controlled growth, and impingement correction for random distribution of nuclei. It is found that both the diffusion of alloying components and the inhibition of carbide precipitates strongly affect the α′→γ transformation kinetic processes. The α′→γ transformation (at relatively low heating rates) is of diffusion-controlled nature, and the obstruction of carbides during the migration of the α′/γ interface leads to a decrease in the pre-exponential factor of diffusion coefficient.

Precipitation kinetics of M23C6 in T/P92 heat-resistant steel by applying soft-impingement correction

Abstract

Influence of austenitization temperature on phase transformation features of modified high Cr ferritic heat-resistant steel

The phase transformation features of high Cr ferritic heat-resistant steel have been investigated using Differential Thermal Analysis (DTA) in accordance with different austenitization temperatures as 950°C, 1050°C, 1150°C, 1200°C and predicted by Thermo-calc calculation. The results based on DTA curves on heating showed that the magnetic transition and the Ac1 temperature occurred at 744.9°C and 850.9°C, respectively. The measured Ac3 temperature was around 919.5°C, the obtained austenitization temperature range was about 69°C. Two new phenomena were found: firstly, the onset temperature of martensite transformation increases strongly and then not obviously with the increase of austenitization temperature; secondly, the δ-ferrite abnormally forms in the specimen austenitized at 950°C due to the existence of un-dissolved M23C6 particles.

2C6 水素昇温脱離分析法による高Crフェライト系耐熱鋼のクリープ余寿命評価(<OS3>構造材料の環境強度と劣化・損傷2)

2C6 水素昇温脱離分析法による高Crフェライト系耐熱鋼のクリープ余寿命評価(<OS3>構造材料の環境強度と劣化・損傷2)

Effect of groove configuration on Type-IV creep damages in Gr.122 steel welds

Creep strength of high Cr ferritic heat-resistant steel welds decreases more rapidly than base metals due to Type-IV creep damage formed in the heat-affected zone (HAZ) at high temperatures. This paper aims to elucidate the effect of groove configuration of welds on the formation and evolution of Type-IV creep damage. Creep tests using base metal, simulated HAZ and two-types of weld joint; gas tungsten arc and electron beam welds, of the ASME Gr.122 steel were conducted. Decreasing the groove angle and width of HAZ was effective in prolonging the creep life; however, it could not prevent Type-IV failure. Type-IV creep voids were mostly observed in the area about 1.0–3.0 mm below the specimen surfaces, whereas their detailed distributions were dependent on the groove configurations. Both the distribution of the multiaxial stress state and creep strain in HAZ can be considered to influence the formation of Type-IV creep damage. Damage mechanics analysis taking the multiaxial stress state into account can predict fairly well the distribution of Type-IV creep voids and failure times of the weld joints without regard to the groove configurations.

Martensite transformation in the modified high Cr ferritic heat-resistant steel during continuous cooling

Abstract

Effect of Austenitization Temperature on Phase Transition Features of High Cr Ferritic Heat-Resistant Steel

The phase transformation of high Cr ferritic heat-resistant steel has been investigated by using differential scanning calorimeter and predicted by Thermo-calc calculation. The steel specimens were hot rolled and followed by air cooling, and then heated from room temperature up to different austenitization temperature as 800°C, 900°C, 1000°C and 1100°C. The DSC curves during heating process showed that the magnetic transition temperature and the Ac1 temperature are 744.9°C and 850.9°C, respectively. The austenitization range was about 58°C. The onset and offset temperature of martensite transformation both increase with the increase of austenitization temperature. The experimental results and the Thermo-calc calculated results both displayed that M23C6 carbides precipitated at around 950°C, and δ-ferrite started to form at about 1020°C.

Athermal martensite transformation of modified high Cr ferritic heat resistant steel undergoing different quenching temperatures

The thermal dilation behaviours of modified high Cr ferritic heat resistant steel quenched at different temperatures were employed to investigate the kinetics of martensite transformation. The martensite fraction and formation rate were obtained as a function of temperature. A model considering ‘spread’ martensites was introduced to explore the influence of quenching temperatures on martensite transformation. Both onset and offset temperatures of martensite formation decrease with the increase in quenching temperatures, and the reaction rate increases rapidly at the beginning of transformation and reaches a peak when a small quantity of martensite forms. The fitted data based on the proposed phase transformation model indicated that the width/length ratio of martensite laths decreases with the increase in quenching temperatures.

Effect of Chromium, Aluminum and Nickel on Microstructure and Reverse-S Type Creep Rupture Strength of High Cr Ferritic Heat Resistant Steels

Effect of Chromium, Aluminum and Nickel on Microstructure and Reverse-S Type Creep Rupture Strength of High Cr Ferritic Heat Resistant Steels

D124 実用12Crフェライト系耐熱鋼のクリープボイド生成挙動と微視組織変化(OS6 保全・設備診断技術)

D124 実用12Crフェライト系耐熱鋼のクリープボイド生成挙動と微視組織変化(OS6 保全・設備診断技術)