Experimental study of remaining creep life of SA-304L stainless steel using small punch creep test

In this paper, the remaining creep life of SA-304L stainless steel at elevated temperatures is studied. The small punch creep tests (SPCT) were performed on SA-304L virgin and aged materials at constant loads. Times to fracture and the minimum deflection rate in SPCTs were recorded, and the time-temperature parametric analysis was performed, based on experimental results. The constants of the Larson–Miller parameter and the Monkman–Grant relationship were obtained for different consumed creep life ratios, and eventually the equations were established to explain the variation of constants of the Larson–Miller parameter and the Monkman-Grant relationship with respect to the consumed creep life ratio.     

The effect of prior fatigue on the creep behaviour of type 321 stainless steel

An investigation was made of the influence of prior elevated-temperature fatigue (at 400°C, 650°C and 750°C) on the creep behaviour of type 321 stainless steel stressed at 200 MPa at 650°C. Prior fatigue was found to accelerate the creep rate, increase the creep ductility, and decrease the time to failure. The acceleration in creep rate was ^∝e^1.8t^0.68, where e is the strain amplitude and t is the duration of the fatigue test. Transmission electron microscopy showed that the dislocation structures produced by prior fatigue tended to persist throughout the subsequent creep test. A stress-aided recovery mechanism is suggested to account for these effects.     

The multiaxial creep ductility of austenitic stainless steels

Calculations of creep damage under conditions of strain control are often carried out using either a time fraction approach or a ductility exhaustion approach. In practice, calculations of creep damage are further complicated by the presence of multiaxial states of stress. In the case of the time fraction approach, there are a number of models that can be used to predict the effect of state of stress on creep rupture strength. In particular, Huddleston developed a model from data on stainless steels. The R5 procedure uses a ductility exhaustion approach to calculate creep damage and includes a model for use under triaxial states of stress. The aim of this paper is to describe the development of this model, which is based on considerations of cavity nucleation and growth and was developed from multiaxial creep data on Type 304 and 316 steels.     

高能球磨法制备不锈钢-TiC纳米复合粉末的研究

在真空条件下采用高能球磨法以不锈钢粉末、钛粉和碳粉为原料制备了不锈钢-TiC超细复合粉末.X射线、扫描电子显微镜、比表面积评价等分析技术被用来对球磨过程中粉末的微观状态进行了分析.结果表明,随着球磨时间的增加,不锈钢复合粉末逐渐细化,用X射线衍射方法计算的最终晶粒可达到20nm.同时,钛粉和碳粉在球磨过程中发生反应形成TiC,可获得纳米级的不锈钢-TiC复合粉末.     

Effect of grain refinement on strain hardening and fracture in austenitic stainless steel

The present study aims to investigate the effect of grain refinement on strain hardening behaviour and fracture surface characteristics in 316LN austenitic stainless steel (ASS). The ASSs with varying grain sizes were obtained through 90% cold rolled reduction and subsequently phase reversion annealing treatment. The results showed that the grain refinement from coarse-grained (CG) structure to ultrafine-grained (UFG) structure increased the yield strength whilst maintaining a reasonable ductility. The strain hardening curves in all the samples were divided into three stages. The fractures in all the samples were ductile fracture with dimples. The subtle differences in the strain hardening behaviour and fracture surface characteristics among the samples with various grain sizes from CG structure to UFG structure were influenced by the deformation mechanisms of austenite.     

Effect of thermal ageing on creep and oxidation behaviour of Type 316H stainless steel

The UK has unique experience in operating high temperature civil nuclear power systems, known as advanced gas cooled reactors (AGRs). One of the primary challenges for extending the lifetime of the AGR power stations is to understand the interaction that occurs between the AGR CO₂ environment and creep-fatigue cracking behaviour. This is one of the life limiting degradation mechanisms for steel components within the reactor pressure vessel. This paper addresses the effect of thermal aging on material internal state that controls both the creep deformation and oxidation behaviour of Type 316H stainless steels when they are exposed at a simulated AGR environment. Experimental results from creep tests are discussed with respect to a multi-scale self-consistent model, while experimental results from oxidation tests are considered with respect to the application of measured short term data to predict the long term oxidation behaviour. Finally, the interaction between oxidation and creep and its impact on high temperature structural integrity of AGR nuclear systems are discussed.     

TiC/Cr18Ni8不锈钢钢结硬质合金原位反应合成研究

以钛铁粉、铬铁粉、铁粉、镍粉和胶体石墨等为原料,原位反应合成了TiC/不锈钢钢结硬质合金,并用扫描电镜(SEM)、X射线衍射等对烧结试样进行了组织结构分析.结果表明:钢结硬质合金主要相组成为TiC和Fe-Cr-Ni固熔体,TiC颗粒细小,形状规则,最大约3μm,大部分在1μm以下;随烧结温度的升高,钢结硬质合金的孔隙度减小,密度和硬度升高,但TiC颗粒略有长大.在相同烧结条件下,C/Ti原子比为0.9的钢结硬质合金的密度和硬度比C/Ti原子比为1.0的钢结硬质合金高,所合成的TiC颗粒更细小,分布更均匀.     

Mechanical properties characterisation of welded joint of austenitic stainless steel using instrumented indentation technique

The instrumented indentation test is a promising nondestructive technique for evaluating mechanical properties of metallic materials. In this study, the localised mechanical properties of welded joint of 304 austenitic stainless steel were characterised with the instrumented indentation test. The single V-groove welded joint was produced using the electric arc welding. A series of instrumented indentation tests were carried out at different regions, including base material, weld zone and heat-affected zone (HAZ). A soft zone regarding strength properties was found in the coarse-grain HAZ. The results show that the HAZ has the lowest yield strength and tensile strength (263.6 MPa, 652.5 MPa) compared with the base material (307.4 MPa, 807.9 MPa) and the weld zone (285.6 MPa, 702.1 MPa). In addition, characterisations of microstructure, microhardness and conventional tensile tests have been performed for comparison. The results reveal that the localised mechanical properties of welded joint of austenitic stainless steel can be represented effectively with the instrumented indentation technique.     

Effect of Ni content on the tensile properties and strain-induced martensite transformation for 304 stainless steel

The effect of Ni content (8.3-12 wt.%) on the tensile properties and strain hardening behavior was studied on type 304 stainless steels (STS) used for the membrane of LNG storage tanks. The tensile test temperature was varied from 25 °C to −196 °C. At room temperature, the hardening and ductility indices (tensile strength, strain hardening exponent and elongation) increased with decreasing Ni content. For the 8.3-9.0 wt.% Ni STS, a lower yield point was observed at temperatures below −60 °C. It was due to the dynamic strain softening and/or transformation-induced plasticity (TRIP) that accompanied the rapid increase in the amount of strain-induced martensite (α′) at low strains. Neither dynamic strain softening nor TRIP was observed for the 12 wt.% Ni STS because only the ?-martensite transformation was produced at the low strains.     

A new high temperature life correlation model for austenitic and ferritic stainless steels

Low-cycle fatigue tests on 429EM ferritic stainless steel and 316L austenitic stainless steel were carried out in a wide range of temperatures from room temperature to 750 °C. The Tomkins fatigue life model was applied to correlate the fatigue life with crack propagation rate and this model matched well with the fatigue life of 429EM stainless steel but not for the 316L stainless steel. A new life prediction model was developed to consider the temperature effect on fatigue life. The predictions show good agreement with experimental results for both materials. The predicted lives were within a±2X scatter band at all test temperatures.     

Effect of lanthanum on the precipitation of NbC in bcc Fe

The effect of lanthanum on the precipitation of NbC in bcc Fe has been investigated by the first-principles calculations in combination with the experiments. The interactions of La–C are repulsive in the first-, second- and third-nearest-neighbour configurations, and the binding energies of La with Nb were predicted to be positive, and converge to a weak value. The addition of La decreases the solubility of C (above 763?K) and Nb, and increases the chemical potential of these two solutes, thus promoting the precipitation of NbC in bcc Fe. The experimental results show that the La-contained Nb-bearing steel has a relatively faster precipitation rate of NbC than the La-free steel, during isothermal treatment in ferritic region.     

Ageing and cracking of simulated heat affected zones in 321 stainless steel

The heat affected zone (HAZ) of stabilised austenitic stainless steel welds may exhibit a serious form of intercrystalline cracking during service at high temperature. This type of embrittlement, well known as stress relief cracking, is related to thermal ageing: a fine and abundant intragranular Ti(C,N) precipitation appears near the fusion line during service at high temperature and modifies the mechanical behaviour of the HAZ. To analyse this embrittlement micro mechanism and to assess the lifetime of real components, different HAZ were simulated by various solution heat treatments, cold rolling and ageing conditions. The mechanical behaviour of these resulting materials was investigated using creep and tensile tests on smooth bars. Then, creep tests were carried out and simulated on notched bars by finite element calculations. A damage model was identified from intergranular damage measurements made on notched specimens and compared with calculated mechanical fields. Further tests on fatigue precracked specimens showed that crack propagation occurred under stress relaxation conditions in simulated HAZ material.     

Grain-boundary diffusion and precipitate trapping of hydrogen in ultrafine-grained austenitic stainless steels processed by high-pressure torsion

This study was conducted to clarify the effects of grain boundaries and precipitates on room-temperature hydrogen transport in two types of austenitic stainless steels with ultrafine-grained structures produced by high-pressure torsion (HPT) and subsequent annealing. The grains in the Fe-25Ni-15Cr (in mass%) alloy containing Ti and the Fe-25Cr-20Ni alloy were refined by the HPT-processing to ∼150 and ∼85 nm, respectively. The high-temperature annealing after the HPT processing led to the precipitation of η-Ni₃Ti for the former and σ-FeCr for the latter. In the HPT-processed specimens, hydrogen diffusivity was enhanced through short-circuit diffusion because of the increased population of grain boundaries in comparison with the increased opportunity of hydrogen trapping on dislocations. As for the post-HPT-annealed specimens having the precipitates, the hydrogen diffusion was hindered by the hydrogen trapping on η-Ni₃Ti precipitates, but was not affected by σ-FeCr precipitation. This depends on the affinity between hydrogen and constituting elements.     

Evaluation of tensile properties of austenitic stainless steel 316L with linear hardening by modified indentation method

Abstract

The instrumented indentation test has been widely used for the non-destructive evaluation of the tensile properties of metal materials. The true stress–strain curve, yield strength and tensile strength can be obtained by this method. In the present study, a new modified indentation algorithm was used to determine the tensile properties of austenitic stainless steel 316L by taking into account its linear hardening characteristic. As received 316L was solution treated at four different temperatures in the range of 1223–1473K for 2 h followed by water quenching. The effect of solution treatment temperature on the tensile properties of 316L was investigated by the instrumented indentation test using the modified indentation algorithm. Results reveal that the new modified indentation algorithm can be used to estimate the tensile properties of austenitic stainless steel with linear hardening.     

The effect of pit size and density on the fatigue behaviour of a pre‐corroded martensitic stainless steel

UNS S17400 steel is used in turbines for the aerospace and utility industries. While it is generally corrosion resistant, it is susceptible to pitting when exposed to aqueous chloride environments. Effects of pitting characteristics, such as depth, width, and local density on fatigue life, have been studied in this work to better inform criteria for component replacement or repair. While pit depth correlates well with cracking, the deepest pit never initiated the crack that ultimately led to failure. The clustering of pits, or local pitting density, also correlated well with crack initiation location; however, the densest region of pitting was not always the location where cracking occurred. There is likely no single metric that directly correlates pitting with fatigue cracking, rather there is a combination of pitting characteristics that ultimately lead to cracking. The results from this work suggest that pit depth and local pitting density are among the more important metrics.     

Austenitic stainless steel bearing Nb compositional and plastic deformation effects

Austenitic stainless steel has excellent ductility. Consequently, it has capability for heavily cold deformation, despite its high strength and high work hardening ability. Austenitic stainless steel predominantly contains high levels of chromium and nickel. Additional elements may be added to enhance performance. The target of this paper is to melt and cast several austenitic stainless steel alloys with different Nb contents. Furthermore the effects of the chemical composition on strength as well as the effect of cold rolling on the creation of induced martensite phase are also studied. The microstructural investigation shows that grain coarsening was observed on the as-cast structure accompanying with thick grain boundary carbides along with carbide agglomerations at the triple points. Hot deformation diminishes the grains as well as the carbide films surrounding the grains. Solution treatment creates austenitic grains free of grain boundary carbides. Cold deformation creates highly elongated grains associated with wavy pancaked structure. Numerical modeling extensively used to detect the proof strength at high temperatures (up to 600 °C). The detected proof strength decreases drastically by raising the deformation temperatures. Nb was found to increase the proof strength even at high temperatures. The measured mechanical properties of the alloys under investigation are higher than that of detected ones by Kimura model, where the model did not pay attention to the Nb effect. Elliason model for the flow curve of different alloys has been extensively studied and applied. The detected results have been verified by the microstructural changes during deformation.     

Tensile elongation of lean-alloy austenitic stainless steels: transformation-induced plasticity versus planar glide

An Fe–13Cr–3.4Mn–0.47C lean-alloy stainless steel was made austenitic by solution annealing at 1250°C. Tensile tests between 20 and 200°C indicated enhancement of ductility at higher temperatures. At 200°C where planar glide, manifested as deformation twinning, was the dominant deformation mechanism, a uniform tensile elongation of 102% was achieved. At 20°C where deformation-induced α′-martensitic transformation replaced deformation twinning as the dominant deformation mechanism, tensile elongation was significantly impaired. The tensile elongation contribution by the planar glide was estimated to be at least four times that of the α′-TRIP (transformation-induced plasticity) mechanism. The results indicate that inexpensive lean-alloy austenitic stainless steels exhibiting pronounced α′-formation at room temperature could become highly formable at higher temperatures.     

Effect of grain size on the hydrogen-assisted cracking in duplex stainless steels

The embrittlement behavior of 2205 duplex stainless steels with two different grain sizes in 26 wt% NaCl (pH 2) under cathodic potential were investigated by slow strain rate testing. The electrochemical permeation technique was used to characterize the permeation rate and effective diffusivity of hydrogen. The results indicated that both the effective diffusivity and the susceptibility of hydrogen embrittlement were lower for the finer grain size specimen. Ultimate tensile strength (UTS) and uniform elongation (UEL) decrease linearly with decreasing logarithm of strain rate. The dependence of UTS and UEL on the logarithm of strain rate was higher for the finer grain specimen. The microstructural examination revealed that internal cracks resulted from hydrogen embrittlement of the ferrite phase under cathodic charging conditions were arrested by austenite in duplex stainless steels.     

On the stress corrosion cracking mechanisms of austenitic stainless steels

In this paper, experimental results on stress corrosion cracking in austenitic stainless steels are described. Crack growth data in sodium chloride solution for AISI 304 steel obtained for different metallurgical conditions, acoustic emission data recorded during crack growth and fractographic observations have been discussed with a view to identifying the operating mechanism. Some of the experimental observations such as crack propagation occurring in discontinuous jumps of the order of a few microns, lowering of the threshold stress intensity andJ-integral values on sensitization and cold working, typical transgranular fractographic features, transition in mode of fracture from transgranular to intergranular in sensitized conditions and activation energies of the order of 50 to 65 kJ/mol can all be accounted by hydrogen embrittlement mechanism. Hydrogen generated at the crack tip by corrosion reaction diffuses ahead of the crack tip under hydrostatic stress and influences the deformation process at the crack tip and also leads to the brittle component of the crack advance in jumps.