Heat-to-heat variation in long-term creep strength of some ferritic steels

Heat-to-heat variation in creep life has been investigated for some ferritic steels, mainly 12Cr steels, using long-term creep data in the NIMS Creep Data Sheets. The tempered martensitic plain 12Cr and 12Cr–1Mo–1W–0.3V steels exhibit large heat-to-heat variation in creep life, as shown by about one order of magnitude difference in time to rupture or more between the strongest and weakest heats. On the other hand, low-Cr steels of tempered bainitic 1Cr–1Mo–0.25V, ferritic–pearlitic 2.25Cr–1Mo and ferritic 9Cr–1Mo steels exhibit small heat-to-heat variation in creep life. The heat-to-heat variation in long-term creep strength is correlated with the degradation behaviour at long times, which depends on initial strength and concentrations of Al, nitrogen and Cr. The present results suggest that taking the mechanisms responsible for the heat-to-heat variation in creep life into account, quality of heat resistant steels as well as reliability of remaining life estimation can be further improved.     

Improved approach for predicting weld creep strength factors of ferritic steels

Abstract

The cross-weld (CW) creep strength of ferritic steels is typically lower than that for parent metal (PM), and in the past the ratio of CW to PM creep strength (weld strength factor – WSF) was assumed to be limited to ～80%. For newer Cr steels WSF can be significantly lower for a typical design life of 100 000 h or more. The possibility of low WSF is also accommodated in the current design codes such as EN 13445, but no suggested WSF values are given for guidance. Assuming a too high WSF for such welds obviously results in an unsafe (too long) predicted creep life. Unfortunately, as a further complication the WSF of the newer Cr steels can decrease when the operating temperatures are increased for improved efficiency of future power plants. It is hence important that reliable and sufficiently high values of WSF can be guaranteed. However, there is often much less extensive data on the creep strength of welds than on parent steel, and also the extrapolation to long term values of WSF can add more relative uncertainty than what is expected in extrapolating the long term creep strength of parent steel. Here an improved approach is proposed to predict WSF using the Wilshire creep model to obtain the relationship between the CW creep strength and the corresponding parent material (PM) strength. The Wilshire model directly provides the WSF value for each CW data point, when the expected normalised stress is based on the CW time to rupture at stress and temperature. The corresponding master curve parameters are those for PM, when the PM hot tensile strength is also known. The WSF data points for each CW test can then be fitted for temperature and stress dependence. This approach avoids fitting distortion in WSF, unlike the traditional assessment where a master curve is first obtained for the CW creep strength. As an example, WSF of welded P91 steel at 100 000 h is here predicted in the temperature range of 550–650°C.     

Cr concentration dependence of overestimation of long term creep life in strength enhanced high Cr ferritic steels

Creep rupture data and microstructural degradation during aging of high Cr ferritic boiler steels with enhanced creep strength have been studied with special attention to prediction of long term creep rupture life. Tempered lath martensite structure in the high Cr ferritic steels remains unchanged during short term aging, whereas static recovery of the lath martensite structure proceeds when diffusion distance during aging becomes sufficiently long as is the case in long term creep. The static recovery brings about premature failure in long term creep and decreases in apparent activation energy for creep life. The decrease in activation energy is responsible for overestimation of rupture life reported in strength enhanced high Cr ferritic steels. The boundary from a short term region with high activation energy Q_H to a long term region with low activation energy Q_L moves towards longer time with decreasing Cr concentration. The difference in activation energy (Q_H − Q_L) primarily determines the extent of overestimation of rupture life predicted from short term data. In general, the extent of overestimation is less serious at 9%Cr as compared to 12%Cr.     

Development of high strength ferritic steel for interconnect application in SOFCs

High-Cr ferritic model steels containing various additions of the refractory elements Nb and/or W were studied with respect to oxidation behaviour (hot) tensile properties, creep behaviour and high-temperature electrical conductivity of the surface oxide scales. Whereas W additions of around 2 wt.% had hardly any effect on the oxidation rates at 800 and 900 °C, Nb additions of 1% led to a substantially enhanced growth rate of the protective surface oxide scale. It was found that this adverse effect can be alleviated by suitable Si additions. This is related to the incorporation of Si and Nb into Laves phase precipitates which also contribute to increased creep and hot tensile strength. The dispersion of Laves phase precipitates was greatly refined by combined additions of Nb and W. The high-temperature electrical conductivity of the surface oxide scales was similar to that of the Nb/W-free alloys. Thus the combined additions of Nb, W and Si resulted in an alloy with oxidation resistance, ASR contribution and thermal expansion comparable to the commercial alloy Crofer 22 APU, but with creep strength far greater than that of Crofer 22 APU.     

Effect of strain rate on low cycle fatigue with hold time in 9Cr rotor steel

Abstract

One of the main mechanisms of turbine rotor damage is cyclic thermal stress produced during transition conditions, such as warm-up and shutdown processes. In this paper, the cyclic damage with ramp rate is studied in terms of material properties. To evaluate thermal stress damage during the cyclic operation, experiments were carried out on low cycle fatigue with hold time and a range of strain rates for COST FB2, 9Cr ferritic steel. Initially, the experimental fatigue life was evaluated using the time fraction and ductility exhaustion methods. The two analysis methods are compared with real life cases of different hold times and strain rates, and the accuracy is discussed. Next, to predict the plastic deformation curve for low cycle fatigue with hold time more accurately, an attempt was made to simulate cyclic hysteresis loop behaviour numerically using a time dependent viscoplastic constitutive model, and this was compared with the experimental hysteresis loop curve for each cycle number.     

关于1Cr18Ni9扁钢压制开裂的裂纹分析

对某公司供货的2种规格的1Cr18Ni9扁钢进行了试验,对来料的化学成分、力学性能和金相组织进行了测定。确定厂家固溶后的过酸洗是导致省煤器撑架压制开裂的主要原因。     

Oxidation resistance of novel ferritic stainless steels alloyed with titanium for SOFC interconnect applications

Chromia (Cr₂O₃) forming ferritic stainless steels are being developed for interconnect application in Solid Oxide Fuel Cells (SOFC). A problem with these alloys is that in the SOFC environment chrome in the surface oxide can evaporate and deposit on the electrochemically active sites within the fuel cell. This poisons and degrades the performance of the fuel cell. The development of steels that can form conductive outer protective oxide layers other than Cr₂O₃ or (CrMn)₃O₄ such as TiO₂ may be attractive for SOFC application. This study was undertaken to assess the oxidation behavior of ferritic stainless steel containing 1 weight percent (wt.%) Ti, in an effort to develop alloys that form protective outer TiO₂ scales. The effect of Cr content (6–22 wt.%) and the application of a Ce-based surface treatment on the oxidation behavior (at 800 °C in air + 3% H₂O) of the alloys was investigated. The alloys themselves failed to form an outer TiO₂ scale even though the large negative ΔG of this compound favors its formation over other species. It was found that in conjunction with the Ce-surface treatment, a continuous outer TiO₂ oxide layer could be formed on the alloys, and in fact the alloy with 12 wt.% Cr behaved in an identical manner as the alloy with 22 wt.% Cr.     

Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.     

High-temperature tensile and creep properties of a ferritic stainless steel for interconnect in solid oxide fuel cell

The purpose of this study is to investigate the high-temperature mechanical properties of a ferritic stainless steel (Crofer 22 APU) for use as an interconnect material in planar solid oxide fuel cells (pSOFCs). Tensile properties of the Crofer 22 APU steel are evaluated at temperatures of 25-800 °C. Creep properties are evaluated by constant-load tests at 650-800 °C. Several creep lifetime models are applied to correlate the creep rupture time with applied stress or minimum creep rate. Experimental results show the variation of yield strength with temperature can be described by a sigmoidal curve for different deformation mechanisms. The creep stress exponent, n, has a value of 5 or 6, indicating a power-law creep mechanism involving dislocation motion. The apparent activation energy for such a power-law creep mechanism is estimated as 393 kJ mol⁻¹ through some thermally activated relations. Creep rupture time of the Crofer 22 APU steel can be described by a Monkman-Grant relation with a time exponent, m = 1.11. The relation between creep rupture time and normalized stress is well fitted by a universal simple power law for all of the given testing temperatures. Larson-Miller relationship is also applied and shows good results in correlating the creep rupture time with applied stress and temperature for the Crofer 22 APU steel. Fractographic and microstructural observations indicate most of the creep cavities are nucleated along grain boundaries and a greater amount of cavities are formed under high stresses.     

New ECCC assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91)

A first assessment of creep rupture strength for steel grade X10CrMoVNb9-1 (Grade 91) was performed by ECCC in 1995. The results were included in the European standard EN 10216. Due to a significant increase of test data and test duration it was decided in 2005 to make a re-assessment of the extended database. Different procedures have been used independently by different assessors. The method with the best overall fit of the data set has found to be the ISO CRD method. This is characterized by a two steps procedure: in the first step the mean isotherms are evaluated from the test data, afterwards the evaluated isotherms are used for averaging by a Manson–Haferd master-curve. The results have been chosen as the basis to specify long term creep rupture strength values in a new ECCC data sheet for X10CrMoVNb9-1 (Grade 91).     

Alloy design and creep strength of advanced 9%Cr USC boiler steels containing high concentration of boron

Abstract

The research and development project of National Institute for Materials Science (NIMS) for advanced ferritic heat resistant steels for 650°C ultra super critical (USC) plants, revealed that the addition of >0·01 mass% boron to a 0·08C–9Cr–3W–3Co–V–Nb–,0·003N steel remarkably improves the long term creep strength. Boron enriched in M₂₃C₆ carbides near prior austenite grain boundaries suppresses the coarsening of carbides during creep deformation, leading to excellent microstructural stability and creep strength. If creep strength was further improved by the addition of nitrogen, it was found to enhance precipitation of fine MX. Addition of excess nitrogen to the high boron containing steel was found to reduce creep rupture lives and ductility. This results from a decrease in the amount of effective boron, which is dissolved in M₂₃C₆ and suppresses its coarsening, resulting from the formation of coarse BN at normalising temperature. The highest creep strength is obtained with steel of the following composition: 0·08C–9Cr–3W–3Co–0·2V–0·05Nb–0·008N–0·014B (mass%), which has an improved creep strength compared to P92. The 10⁵ h extrapolated creep rupture strength at 650°C is ～100 MPa. This steel also shows good creep ductility even in the long term. In conclusion, high boron bearing 9Cr–3W–3Co–V–Nb steel combined with the addition of 0·008 mass% nitrogen is a promising candidate for thick section components in the 650°C USC plants as it shows superior creep strength without impaired creep ductility.     

ZG12Cr9Mo1Co1NiVNbNB耐热钢材料开发应用研究

随着火电超超临界机组蒸汽参数提高到28 MPa~30 MPa/600℃/620℃,对耐热钢的性能要求进一步提高,在目前耐热钢铸件材料无法满足该参数长期运行的条件下,开发了一种含Co和B的新型耐热钢材料。利用先进的JMatPro计算软件对ZG12Cr9Mo1Co1NiVNbNB进行成分配比和组织及相的模拟,并按计算确定的成分进行试制,通过对进口试块的常规力学性能、高温瞬时拉伸、断裂力学、高温持久-蠕变性能、组织稳定性及相分析和多回火试验后常规力学等使用性能和工艺性能进行了测试、分析,研究表明ZG12Cr9Mo1Co1NiVNbNB材料可适用于620℃等级超超临界机组阀门、汽缸等高温高压部件。     

热处理对ZG1Cr10MoVNbN组织和力学性能的影响

超临界汽轮机高压内缸材料的力学性能的提高是超临界汽轮机技术进步的一个重要组成部分。针对这方面的要求,进行超临界汽轮机高中压汽缸材料的力学性能的研究,为设计制造高性能的超临界汽轮机提供可靠的数据依据。     

Quantitative evaluation of hydrogen embrittlement susceptibility in various steels for energy use using an in-situ small punch test

Recently, as hydrogen has been increasingly applied in the field of new energy, it has become necessary to evaluate the mechanical characteristics of hydrogen embrittlement (HE) when materials are used to reduce costs as well as ensure safety in hydrogen facilities. However, to obtain a large amount of data in a short period of time and ensure reliability when selecting materials used in hydrogen energy applications, a simple test method for screening the HE susceptibility of materials under high-pressure hydrogen environments should be established and applied. In this study, the HE behaviors of three structural steels to be used in the hydrogen energy field were examined at room temperature and low temperatures under high-pressure hydrogen environments using the newly established in-situ small punch test method. The effects of test temperature and punch velocity on the HE susceptibility of each steel were quantitatively evaluated using the characterizing factor, known as the relative reduction of thickness.