Study on microstructural changes in thermally-aged stainless steel weld-overlay cladding of nuclear reactor pressure vessels by atom probe tomography

The effect of thermal aging on microstructural changes was investigated in stainless steel weld-overlay cladding composed of 90% austenite and 10% δ-ferrite phases using atom probe tomography (APT). In as-received materials subjected to cooling process after post-welding heat treatments (PWHT), a slight fluctuation of the Cr concentration was already observed due to spinodal decomposition in the ferrite phase but not in the austenitic phase. Thermal aging at 400 °C for 10,000 h caused not only an increase in the amplitude of spinodal decomposition but also the precipitation of G phases with composition ratios of Ni:Si:Mn = 16:7:6 in the ferrite phase. The chemical compositions of M₂₃C₆ type carbides seemed to be formed at the austenite/ferrite interface were analyzed. The analyses of the magnitude of the spinodal decomposition and the hardness implied that the spinodal decomposition was the main cause of the hardening.     

Thermal aging effect on Z3CN20.09M Cast Duplex Stainless Steel

Instrumented impact tests were performed on Z3CN20.09M Cast Duplex Stainless Steel (CDSS) aged at 400 °C for up to 3000 h. The material was cut from the primary pipe of a PWR to investigate the accelerated thermal aging embrittlement effect on the dynamic fracture properties. The load–deflection curves from the instrumented impact tests described both the loading and the fracture stages, from which the dynamic ultimate loading level and the dynamic ultimate strengths were calculated as a function of increased aging time. The increase in the dynamic ultimate strengths with aging time is much higher than those for the static ultimate strengths. With the increment of the aging time, both the crack initiation energy and the crack propagation energy decrease, which are separated form the total impact energy taking the maximum loading level as the crack initiation point. It is shown that the reduction tendency of the fracture propagation energy is the predominant reason for the reduction of the impact energy. With the increment of the aging time, the unloading slopes reflecting the crack propagation speed increase and the fracture surface patterns change from ductile fracture with shallow dimples to cleavage brittle fracture caused by dislocations piling up in the ferrite matrix. Mechanical properties of inner wall with fine equi-axed grains are much better than those of the outer wall with coarse columnar structures.     

Numerical simulations of the phase separation properties for the thermal aged CDSS with Phase Field Model

Experiments and numerical simulations with Phase Field Model and Finite Element Analysis were carried out to investigate the phase separation dynamic properties and the corresponding thermal aging degradation mechanism. Experimental results from transmission electron microscopy and atomic force microscopy show that thermal aging causes the Cr-rich phase precipitate and form clusters. A phase field dynamic model was developed with constitutive relations and empirical potential functions to investigate the phase separation dynamics in the ferrite phase. Numerical results integrated with cell dynamical system method show clearly the micro structure morphology and the phase separation coarsening with aging time. The evolution process of the phase separation was quantitatively illustrated and reproduced macroscopically. The scattering pattern becomes clearer and the corresponding radius becomes smaller along with the increasing aging time. The average characteristic length increases firstly then decreases and enters a more stable stage. With the increment of the local Cr concentration, the evolution of the phase morphology was quite different. Finite Element Analysis simulation results with the Gurson-Tvergaard-Needleman void model show that the damage initiated more easily in the ferrite matrix for the Cr atoms forming clusters with increasing aging time. The phenomenological simulations with Phase Field Model and Finite Element Analysis were in remarkably good agreement with experimental results and analytical considerations.     

Basic investigation for life assessment technology of modified 9Cr–1Mo steel

In order to develop life assessment techniques for aged components made of modified 9Cr–1Mo steel, specimens were artificially deteriorated by aging, creep and fatigue tests at elevated temperatures, and associated changes in the microstructure and mechanical properties were examined. It was observed that aging resulted in formation of Laves phase causing a decrease in toughness. The creep damage in base metal could be correlated with decrease in hardness, while creep damage in weldments could be correlated with the area fraction and density of creep voids. Creep rupture in weldments occurred in the fine-grained heat affected zone by the formation and growth of creep voids. The fatigue damage in base metal correlated to the maximum length of a crack among micro-cracks initiated during fatigue cycles.     

显微硬度法分析核电站主管道热老化趋势

使用显微维氏硬度计和冲击试验机研究了核电站主管道材料Z3CN20.09M在400 ℃加速热老化10 000 h前后的力学性能变化。结果表明,热老化导致试验材料的冲击吸收能下降;构成试验材料的铁素体相的显微维氏硬度上升,奥氏体相的显微维氏硬度基本保持不变。通过研究材料组织特征,剖析显微硬度与冲击韧性的关系,探索将显微硬度测试方法作为核电站主管道材料热老化趋势预测方法的可能性。     

Fracture mechanics evaluation of cast duplex stainless steel after thermal aging

For the primary coolant piping of PWRs in Japan, cast duplex stainless steel, which is excellent in terms of strength, corrosion resistance and weldability, has conventionally been used. Cast duplex stainless steel contains the ferrite phase in the austenite matrix, and thermal aging after long-term service is known to decrease fracture toughness. Therefore, we evaluated the integrity of the primary coolant piping for an initial PWR plant in Japan by means of elastic plastic fracture mechanics. The evaluation results show that the crack will not grow into an unstable fracture and the integrity of the piping will be secure, even when such through-wall crack length is assumed to be as large as the fatigue crack length grown for a service period of up to 60 years.     

Mechanical property and microstructural change by thermal aging of SCS14A cast duplex stainless steel

The aging behavior, especially saturation, of JIS SCS14A cast duplex stainless steels was investigated on the basis of the mechanical properties and microstructural changes during accelerated aging at 350 °C and 400 °C. The aging behavior of the materials mainly proceeds via two stages. During the first stage, the generation and concentration of the iron-rich and chromium-enriched phase in ferrite occurs by phase decomposition. The first stage corresponds to aging times of up to 3000 h at 400 °C. During the first stage, the ferrite hardness achieved is approximately 600 VHN, and the Charpy impact energy is almost saturated. During the second stage, the precipitated chromium-enriched phase aggregates and coarsens, and the G phase precipitation also occurs. The second stage corresponds to the aging times range of 3000-30 000 h at 400 °C. During the second stage, the ferrite hardness achieved is about 800 VHN; however, further hardening exceeding 600 VHN does not influence the Charpy impact energy.     

沉淀硬化马氏体不锈钢热老化脆化的热电势检测研究

研究了17-4PH沉淀硬化马氏体不锈钢在400℃下不同时效时间力学性能和热电势变化规律,提出了热电势和冲击韧性的经验公式,并通过在核电厂服役13 a的主蒸汽隔离阀阀杆进行验证。验证结果显示,随着时效时间的延长,材料的冲击韧性下降,屈服强度、抗拉强度和硬度升高,断面收缩率和断裂伸长率下降。材料的热电势变化与冲击韧性呈现指数相关性,材料的屈服强度、抗拉强度、硬度和热电势呈现出较好的线性关系。通过热电势检测评估的冲击韧性和实测值显示出较好的符合性。      

Effect of thermal aging on fracture toughness of RPV steel

The effect of thermal aging on mechanical properties and fracture toughness was investigated on pressure vessel steel of light water reactors. Submerged are welded plates of ASME SA508 C1.3 steel were isothermally aged at 350°C, 400°C and 450°C for up to 10,000 hrs. Tensile, Charpy impact and fracture toughness testings were conducted on the base metal and the weld heat affected zone (HAZ) material to evaluate whether thermal aging induced by the plant operation is critical for the integrity of the pressure vessel or not. Tensile properties of the base metal was not changed by thermal aging as far as the thermal aging conditions were concerned. Relatively distinct degradation was observed in fracture toughness J_IC and J-resistance properties of both the base metal and the weld HAZ material, while only slight changes were observed in Charpy impact properties for both of them. However, it was concluded that the effect of thermal aging estimated by 40–80 years of plant operation on fracture toughness of both materials is small.     

A study on NDE method of thermal aging of cast duplex stainless steels

To maintain the integrity of applications of the duplex stainless steels currently in service, a study was conducted to develop a method to nondestructively estimate their Charpy-impact energy at room temperature. It was found that hardness of the ferrite phase is a reliable indicator of the process of embrittlement during long-term heating of duplex stainless steels. However, further information on the ferrite phase and the austenite phase is required for the estimation of Charpy-impact energy. An equation composed of the hardness values of ferrite and austenite phases, the ferrite content and the average spacing of ferrite phase islands was presented as a method applicable to the nondestructive estimation of Charpy-impact energy at room temperature.     

Effect of aging at 700 °C on precipitation and toughness of AISI 321 and AISI 347 austenitic stainless steel welds

A detailed knowledge of changes in microstructures and mechanical behaviour that occur in austenitic stainless steels with or without Nb/Ti-stabilized weld during heat treatment is of great interest, since the ductility and toughness of the material may change drastically after long aging times. Two kinds of materials, i.e. AISI 321 base and without Ti-stabilized weld steel and AISI 347 base with Nb-stabilized weld steel, were compared during aging at 700 °C up to 6000 h. Both materials present increased amount of precipitate and decreased impact energy as the aging time increases. The decreased extent of impact energy with aging is almost the same for both base materials. However, it presents differences for 347 and 321 weld samples. The latter shows a more drastic decrease of impact energy than the former due to the different amount of precipitates. 321 weld sample precipitates more numerously than 347 weld sample due to the absence of stabilized Ti/Nb on the former. Large amount of carbides is formed on 321 weld sample immediately after welding. The carbides are transformed to sigma phase, which is mainly responsible for the much more sigma phase precipitation compared with other samples, after high-temperature aging. The fractographs showed, in general, brittle fracture mode in 321 weld impact-fractured specimens after aging at 700 °C for 6000 h. However, other samples show ductile fracture mode in general. Several approaches should be employed to control sigma phase precipitation in weld material. These approaches include: decreasing content of ferrite and M₂₃C₆ carbide in weld and selecting Nb added weld wire during welding.     

Microstructural changes and fracture behavior of CF8M duplex stainless steels after long-term aging

Microstructural changes and fracture behavior in cast CF8M duplex stainless steel after aging at 300–450°C for 300–10?000 h have been investigated. Both, optical microscopical and transmission electron microscopical analyses, hardness and ferrite content measurements have been carried out in this study. Strengthening and aging phenomena of the ferrite phase have been identified by hardness measurements. Spinodal decomposition and heterogeneous precipitation of G-phase were found to be responsible for strengthening of the ferrite phase after aging with a temper parameter (see Appendix A) in the range ca. 1.8–4.5. Three different fracture modes, dimples, cleavage and α/γ grain boundary separation, have been observed for Charpy V-notch and CT test specimens fractured at 20°C.     

CF-8M铸造不锈钢热老化的无损检测和评估方法研究

核电站主管道铸造不锈钢在280~325 ℃下长期运行服役过程中存在热老化脆化问题,韧性会大幅下降,为检测和评估主管道材料的热老化程度,通过对核级CF 8M静态铸造不锈钢主管道材料在400 ℃下热老化10 000 h的样品进行了热电势测量,研究了不同热老化阶段的力学性能（冲击能）和热电势与老化时间的关系,获得了热老化影响因素归一化后的参数与力学性能（冲击能）和热电势的对应关系式。结果表明,在热老化初始阶段冲击能下降较快,达到8 000 h后冲击能下降趋势已趋于平缓。在试验周期内,随着对数老化时间的增加,热电势呈线性增加;随着热电势的增加,冲击能开始下降较快,后期下降趋势变缓,逐渐趋于饱和,冲击能随热电势变化的形式和冲击能随热老化时间变化的形式相似。对热老化影响因素进行归一化后的参数值与热电势呈线性关系,基于该关系式可利用热电势检测技术评估服役部件热老化后的性能下降程度。     

Microstructural and mechanical property changes with aging of Mo-41Re and Mo-47.5Re alloys

The changes in microstructure and mechanical properties of Mo-41Re and Mo-47.5Re alloys were investigated following 1100 h thermal aging at 1098, 1248 and 1398 K. The electrical resistivity, hardness and tensile properties of the alloys were measured both before and after aging, along with the alloy microstructures though investigation by optical and electron microscopy techniques. The Mo-41Re alloy retained a single-phase solid solution microstructure following 1100 h aging at all temperatures, exhibiting no signs of precipitation, despite measurable changes in resistivity and hardness in the 1098 K aged material. Annealing Mo-47.5Re for 1 h at 1773 K resulted in a two-phase αMo + σ structure, with subsequent aging at 1398 K producing a further precipitation of the σ phase along the grain boundaries. This resulted in increases in resistivity, hardness and tensile strength with a corresponding reduction in ductility. Aging Mo-47.5Re at 1098 and 1248 K led to the development of the χ phase along grain boundaries, resulting in decreased resistivity and increased hardness and tensile strength while showing no loss in ductility relative to the as-annealed material.     

热老化温度对高Si含量F/M钢中Laves相析出行为和冲击性能的影响

铁素体马氏体钢(F/M钢)是铅冷快堆堆芯的主要候选材料之一,提高材料中的Si含量可提高其抗腐蚀性能,但同时会促进Laves相的析出从而影响材料韧塑性。针对一种Si含量为0.98%的F/M钢,开展了3种温度(500、550、600℃)下5000 h的热老化实验,研究了温度对Laves相析出行为和冲击性能的影响。结果表明,热老化温度升高能够促进Laves相的形核和粗化,且温度从550℃提高至600℃,Laves相的粗化速率从3.7 nm/h^1/3提高至9.0 nm/h^1/3。另一方面,热老化温度升高将加速冲击性能的退化,在550℃和600℃下热老化500 h,冲击功(AKV)值分别下降至热老化前的51%和39%,而在500℃下热老化2500 h,A_KV值仍保持热老化前的75%。Laves相的析出与冲击性能退化有强烈的对应关系,是冲击性能退化的主要原因。     

Mechanisms of stress relief cracking in titanium stabilised austenitic stainless steel

The heat affected zone (HAZ) of AISI 321 welds may exhibit a serious form of cracking during service at high temperature. This form of damage, called ‘stress relief cracking’, is known to be due to work hardening but also to aging due to Ti(C,N) precipitation on dislocations which modifies the mechanical behaviour of the HAZ. The present study aims to analyse the latter embrittlement mechanism in one specific heat of 321 stainless steel. To this end, different HAZs are simulated using an annealing heat-treatment, followed by various cold rolling and aging conditions. Then, we study the effects of work hardening and aging on Ti(C,N) precipitation, on the mechanical (hardness, tensile and creep) behaviour of the simulated HAZs and on their sensitivity to intergranular crack propagation through stress relaxation tests performed on pre-cracked CT type specimens tested at 600 °C. It is shown that work hardening is the main parameter of the involved mechanism but that aging does not promote crack initiation although it leads to titanium carbide precipitation. Therefore, the role of Ti(C,N) precipitation on stress relief cracking mechanisms is discussed. An attempt is made to show that solute drag effects are mainly responsible for this form of intergranular damage, rather than Ti(C,N) precipitation.     

Toughness investigation on simulated weld HAZs of SQV-2A pressure vessel steel

In order to get detailed information about weld HAZs toughness of SQV-2A steel and determine the optimum welding and heat treatment parameters, the toughness of simulated CGHAZs (coarse grained heat affected zone) and CGHAZs (intercritically reheated CGHAZ) were systematically investigated. The influence of tempering thermal cycles on weld ICCGHAZs toughness was clarified. The effect of post weld heat treatments (PWHT) on weld CGHAZs toughness was also determined. The results showed that high toughness (absorbed energy >200 J) of weld HAZs could be achieved by selecting the optimum welding and PWHT parameters (cooling time Δt_8/5: 6–40 s, PWHT: 893 K, 3.6–7.2 ks). Tempering thermal cycles with peak temperature of above 573 K could remarkably improve the toughness of deteriorated ICCGHAZs and reduce the hardness, when cooling time Δt_8/5(2) of the reheating thermal cycle was 6 s, which implies that welding of SQV-2A without PWHT is possible, provided that low heat input welding is adopted and welding procedure is correctly arranged. Metallography and fractography revealed that M–A constituents in weld HAZs played an important role in controlling weld HAZ toughness.     

Effect of hydride orientation on fracture toughness of Zircaloy-4 cladding

Hydrogen embrittlement is one of the major degradation mechanisms for high burnup fuel cladding during reactor service and spent fuel dry storage, which is related to the hydrogen concentration, morphology and orientation of zirconium hydrides. In this work, the J-integral values for X-specimens with different hydride orientations are measured to evaluate the fracture toughness of Zircaloy-4 (Zry-4) cladding. The toughness values for Zry-4 cladding with various percentages of radial hydrides are much smaller than those with circumferential hydrides only in the same hydrogen content level at 25 °C. The fractograghic features reveal that the crack path is influenced by the orientation of zirconium hydride. Moreover, the fracture toughness measurements for X-specimens at 300 °C are not sensitive to a variation in hydride orientation but to hydrogen concentration.     

Fatigue crack growth behavior of weld heat-affected zone of type 304 stainless steel in high temperature water

The fatigue crack growth behavior of the weld heat-affected zone (HAZ) of type 304 stainless steel in high temperature water which simulates the boiling-water reactor environment was investigated to clarify the effects of welding residual stress, cyclic frequency f and thermal aging on crack growth rate. A lower crack growth rate of the HAZ than of the base metal was observed in both the high temperature water and the ambient air caused by the compressive residual stress. The crack closure point was measured in the high temperature water. The effect of the welding residual stress on the crack growth rate of the HAZ can be evaluated separately from the environmental effect through the crack closure behavior. The high temperature water increased the crack growth rate at a cyclic frequency of 0.0167 Hz but did not affect it much at 3 and 5 Hz. The crack growth behavior of the thermally aged HAZ at 400 °C for 1800 h was almost the same as that of the unaged material tested at 0.0167 and 5 Hz in the high temperature water.     

Microstructural stability of modified 9Cr–1Mo steel during long term exposures at elevated temperatures

This paper describes the stability of microstructure in normalized and tempered modified 9Cr–1Mo steel exposed to service temperatures in the range of 773–873 K for different time durations. A detailed microstructural and microchemical analysis of the secondary phases was carried out using optical and electron microscopy techniques. The microstructural observations, supported by hardness measurements showed that the lath morphology of the tempered martensite was retained even after 10 000 h of aging. The coarsening of M₂₃C₆ carbide was observed until 5000 h, when the Laves phase started appearing. The microstructural features observed are discussed in conjunction with the embrittlement observed in this steel on high temperature aging exceeding 5000 h.