Study on fracture toughness evaluation method of SA738Gr.B steel heat affected zone in ductile-brittle transition region

The Master Curve (MC) method is widely used to evaluate the fracture toughness of ferritic steels for reactor pressure vessels in the ductile-brittle transition zone, but this method is only applicable to macroscopically uniform ferritic steels. Based on the measured fracture toughness data of China-produced SA738Gr.B steel Heat-Affected Zone (HAZ) in the as-welded and heat-treated state, the problems of the standard MC method for characterizing the fracture toughness of the HAZ are discussed. Then the Bi-modal master curve method (BMC), SINTAP method, and Single point estimation method (SPE) were used to characterize the fracture toughness of the HAZ of SA738Gr.B steel in the ductile-to-brittle transition zone. The comparative analysis found that the BMC method is better in terms of the accuracy of the estimation and the over-dispersion envelope was caused by the heterogeneity. The estimation results of the SINTAP and SPE models are conservative but can be used for the safety evaluation of the actual structure. The study provided technical support for SA738Gr.B steel to exempt post-weld heat treatment issue.     

焊后热处理对SA517 Gr.F焊接接头力学性能与组织的影响

为更好地掌握焊后热处理(PWHT)对SA517Gr.F调质钢焊接接头性能的影响,对SA517Gr.F调质钢手工电弧焊(SMAW)焊接接头进行了焊态(AW)与PWHT试样力学性能和组织分布特征的对比分析研究。分析结果表明:PWHT与AW相比,焊接接头室温拉伸和360℃高温拉伸的抗拉强度均有所下降;焊缝和热影响区(HAZ)峰值硬度降低。PWHT降低了焊接接头的残余应力,但未能改善焊缝和HAZ的冲击性能。建议对于SA517 Gr.F调质钢若满足ASME规范案例N-71-18条件,可免除焊后热处理。     

Impact of structural steel flexibility and restraints gaps on the dynamic behaviour of piping

The effect of gaps present in the seismic supports of nuclear piping systems and of the flexibility of the steel structure to which intermediate supports are attached, is studied in this paper. An actual piping system is used to investigate the impact of structural steel and mechanical snubber gaps on the dynamic behaviour of piping. An evaluation is thus performed of the finite element modeling techniques employed by the designers in the dynamic analysis of piping systems.     

AP1000核岛主设备用SA508-3钢锻件的监造

ASME SA508-3钢具有优越的可焊性、较好的抗中子辐照脆化性能和非常好的断裂韧性以及冲击韧性,因此被广泛应用于压水堆核电站核岛压力容器的制造中。AP1000三代核电机组的一些主设备,如反应堆压力容器、蒸汽发生器、稳压器的全部大锻件及一些重要部件均采用了这一钢种。通过对SA508-3钢锻件制造过程中的技术要点的分析,指出了该钢种的锻件在制造过程中的质量关注重点,提出了对该钢种锻件实施监造过程中的监督方法和监督重点。     

Mechanism of dynamic strain aging and characterization of its effect on the low-cycle fatigue behavior in type 316L stainless steel

Low-cycle fatigue tests were carried out in air in a wide temperature range from 20 to 650 °C with strain rates of 3.2 × 10⁻⁵–1 × 10⁻² s⁻¹ for type 316L stainless steel to investigate dynamic strain aging (DSA) effect on the fatigue resistance. The regime of DSA was evaluated using the anomalies associated with DSA and was in the temperature range of 250–550 °C at a strain rate of 1 × 10⁻⁴ s⁻¹, in 250–600 °C at 1 × 10⁻³ s⁻¹, and in 250–650 °C at 1 × 10⁻² s⁻¹. The activation energies for each type of serration were about 0.57–0.74 times those for lattice diffusion indicating that a mechanism other than lattice diffusion is involved. It seems to be reasonable to infer that DSA is caused by the pipe diffusion of solute atoms through the dislocation core. Dynamic strain aging reduced the crack initiation and propagation life by way of multiple crack initiation, which comes from the DSA-induced inhomogeneity of deformation, and rapid crack propagation due to the DSA-induced hardening, respectively.     

Influences of the dynamic strain aging on the J–R fracture characteristics of the ferritic steels for reactor coolant piping system

The leak-before-break (LBB) design of the piping system for nuclear power plants has been based on the premise that the leakage due to the through-wall crack can be detected by using leak detection systems before a catastrophic break. The piping materials are required to have excellent J–R fracture characteristics. However, where ferritic steels for reactor coolant piping systems operate at the temperatures where dynamic strain aging (DSA) could occur, the fracture resistance could be reduced with the influence of DSA under dynamic loading. Therefore, in order to apply the LBB design concept to the piping system under seismic loading, both static and dynamic J–R characteristics must be evaluated.Materials used in this study are SA516 Gr.70 for the elbow pipe and SA508 Cl.1a for the main pipe and their welding joints. The crack extension during the dynamic and the static J–R tests was measured by the direct current potential drop (DCPD) and the compliance method, respectively. This paper describes the influences of the dynamic strain aging on the J–R fracture characteristics with the loading rate of the pipe materials and their welding joints.     

Experimental and theoretical investigations of KWU on leak-before-break behaviour and its application on excluding of breaks in nuclear piping of KWU-plants

The fracture of pipes with longitudinal and circumferential cracks was investigated by experiments and theoretical approaches (flow stress criteria and limit load analyses).The experiments show that the critical crack dimensions can conservatively be determined by fracture mechanics.The tests and calculations are applied to the primary coolant piping with hypothetical longitudinal and circumferential defects. Reactor systems, design, fabrication, stress analysis, material, non-destructive testing, quality control and inservice inspection are considered referring to the leak-before-break behaviour. On the basis of the extreme toughness of the materials, the known loads, the high level of non-destructive examinations, the leakage monitoring system and the high quality of manufacture and processing it is shown that a spontaneous failure need not to be postulated.     

高温高压动水回路中A106Gr.B和A672Gr.B60耐冲蚀作用的研究

碳钢和低合金钢由于Cr含量较低,易发生流动加速腐蚀,对安全产生严重影响。现阶段最先进的第三代核电技术AP1000使用Cr含量较高的A672Gr.B60代替A106Gr.B作为供水管的材料来减小流动加速腐蚀(FAC)的影响。试验采用腐蚀失重、X射线光电子能谱(XPS)、扫描电镜(SEM)和X射线能量色散谱(EDS)等方法来分析流速、碱化剂以及物质组成对A672Gr.B60和A106Gr.B的影响。经336h的实验,结果显示A672Gr.B60较A106Gr.B具有更好的耐冲蚀性能,氧化膜更致密,不易脱落。使用乙醇胺(ETA)作为碱化剂相比于氨水能更好地降低FAC,也能使氧化膜更加致密。流速的变化会影响氧化物颗粒的分布、大小、形状,也会影响氧化膜的厚度。实验生成的氧化膜主要成分为Fe3O4。     

Application of the leak-before-break concept in nuclear power plant safety analysis

NIKIÉT. Translated from Atomnaya Énergiya, Vol. 75, No. 6, pp. 426-430, December, 1993.     

Influence of gap size on the dynamic behaviour of piping systems

This paper addresses the effects of dynamic events induced by support motion on piping systems with snubbers having variable gap sizes. The investigation consists of 3 parts: (i) Mathematical examination of a linear I DOF mass-spring-snubber system with gap size zero or infinity. (ii) Numerical analysis of a piecewise linear I DOF mass-spring-snubber system with varying gap sizes, by means of a simple computer code. (iii) Numerical analysis of a realistic three-dimensional piping system with 3 snubbers, each having 4 different gap sizes, with the aid of a non-linear F.E. code.     

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.     

Observations on the effect of post-weld heat treatment on J-resistance curves of sa-106b seamless piping welds

Ontario Hydro has developed a leak-before-break (LBB) methodology for application to large diameter piping (21, 22 and 24 inch) Schedule 100 SA106B heat transport (HT) piping as a design alternative to pipe whip restraints and in recognition of the questionable benefits of providing such devices. Ontario Hydro's LBB approach uses elastic-plastic fracture mechanics (EPFM).In order to assess the stability of HT piping in the presence of hypothetical flaws, the value of the material J-integral associated with crack extension (J— R curve) must be known. In a material test program J-resistance curves were determined from various pipe heats and four different welding procedures that were developed by Ontario Hydro for nuclear Class 1 piping. The test program was designed to investigate and quantify the effect of various factors such as test temperature, crack plane orientation and welding effects which have an influence on fracture properties. An acceptable lower bound J-resistance curve for the piping steels and welds were obtained by machining maximum thickness specimens from the pipes and weldments and by testing side-grooved compact tension specimens. This paper addresses the effect of test temperature and post-weld heat treatment on the J-resistance curves from the welds.The fracture toughness of all the welds at 250°C was lower than that at 20°C. Welds that were post-weld heat treated showed high crack initiation toughness, J_lc, rising J-resistance curves and stable and ductible crack extension. Non post-weld heat treated welds, while remaining tough and ductile, showed comparatively lower J_Ic, and J-resistance curves at 250°C. This drop in toughness is possibly due to a dynamic strain aging mechanism evidenced by serrated load-displacement curves. The fracture toughness of non post-weld heat treated welds increased significantly after a comparable post-weld heat treatment.The test procedure was validated by comparing three test results against independent tests conducted by Materials Engineering Associates (MEA) of Lanham, Maryland. The J_Ic and J-resistance curves obtained by Ontario Hydro and MEA were comparable.     

Dynamic strain aging in stress controlled creep-fatigue tests of 316L stainless steel under different loading conditions

Stress controlled fatigue-creep tests were carried out for 316L stainless steel under different loading conditions, i.e. different loading levels at the fixed temperature (loading condition 1, LC1) and different temperatures at the fixed loading level (loading condition 2, LC2). Cyclic deformation behaviors were investigated with respect to the evolutions of strain amplitude and mean strain. Abrupt mean strain jumps were found during cyclic deformation, which was in response to the dynamic strain aging effect. Moreover, as to LC1, when the minimum stress is negative at 550 °C, abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While the minimum stress is positive, mean strain only jumps once at the end of deformation. Similar results were also found in LC2, when the loading level is fixed at −100 to 385 MPa, at higher temperatures (560, 575 °C), abrupt mean strain jumps occur at the early stage of cyclic deformation and there are many jumps during the whole process. While at lower temperature (540 °C), mean strain only jumps once at the end of deformation.     

Effect of thermal aging on the room temperature tensile properties of AISI type 316LN stainless steel

Round tensile specimens of AISI type 316LN stainless steel, thermally aged at 1123 K for 0, 2, 10, 25, 100, 500 and 1000 h, were tested for tensile properties at room temperature at a strain rate of 7.7 × 10⁻³ s⁻¹. The changes in tensile properties were correlated to the transmission electron microscopic studies. The various stages of nitrogen repartitioning including Cr–N cluster formation, intragranular and subsequent cellular precipitation of Cr₂N were found to have a strong influence on the yield strength (YS) and ductility of the material. However, the changes in ultimate tensile strength (UTS) with aging were negligible. The results of electrochemical extraction of secondary phases clearly indicated a two-slope behavior. X-ray diffraction analysis of electrochemically extracted residue suggested that the initial smaller sloped line corresponded to the precipitation of the Cr₂N phase while the line with larger slope at longer aging time corresponded to the domination of chi phase precipitation.     

Influence of dynamic strain aging on the ductile tearing of C–Mn steels: modelling by a local approach method

C–Mn steels and associated welds can be susceptible to dynamic strain aging (DSA). In this case, fracture toughness passes through a minimum when the temperature increases from 20 to 300 °C. Since Charpy V-Notch data are not affected by the DSA phenomenon, the method for predicting the evolution of J_0.2 and dJ/da in the temperature domain where DSA occurs is still an open question. The purpose of the present study is the assessment of this decrease in fracture toughness using a local approach method. The results of J–R tearing resistance tests, characterized by crack initiation resistance J_0.2 and tearing modulus dJ/da relative to a base metal (A48 French standard steel) and manual metal arc deposited metals are presented and discussed. The local approach determination of J_0.2 was performed using the Rice and Tracey model. Comparisons between experimental results and the results of local approach modelling indicate that both J_0.2 and dJ/da decreases with DSA can be correctly predicted.     

焊后热处理对SA517 Gr.F焊接接头力学性能与组织的影响

In order to better understand the effect of PWHT (post weld heat treatment) on SA517 Gr.F quenched and tempered steel welded joint, the mechanical properties and microstructure distribution characteristics of SA517 Gr.F welded joint by SMAW (shielding metal arc welding) before and after PWHT were compared and analyzed. The analysis results show that, compared with that of as-weld welded joint, both the room temperature and 360℃ high temperature tensile properties of the welded joint decrease, and the peak micro-hardness of weld metal and HAZ also decrease after PWHT. The residual stress of the welded joint is decreased by PWHT, however, the impact properties of the weld and HAZ are not improved. It is suggested that PWHT can be exempted for SA517 Gr.F quenched and tempered steel if the requirements of ASME Code Case N-71-18 can be satisfied.     

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.     

Numerical fluid-hammer analysis by the method of characteristics in complex piping networks

This paper describes the method of characteristics as used to calculate fluid-hammer problems in complex piping networks. The formulation is based on the one-dimensional Navier-Stokes equation that contains the viscous term expressed as wall friction. A stepwise solution procedure is constructed from compatibility relations along characteristics and appropriate boundary conditions describing various types of pipe joints.The close agreement between the numerical result and the acoustic solution validates the boundary conditions for sudden area change based on steady-state flow conditions. A non-reflecting far-end boundary condition is devised that enables certain portions of a complex system to be analyzed. A dummy-junction boundary condition proved useful in treating systems containing long pipe sections in comparison to other pipes of the system. The numerical results obtained for the sodium-loop piping arrangement of the experimental breeder reactor II with the sodium-water reaction as the pressure disturbance are presented and discussed.