Estimation of the local mechanical properties of pipeline steels and welded joints by use of the microshear test method

In the present work, a microshear test method has been used to investigate the local mechanical properties distribution of API X60 grade pipeline steels and their welded joints, including microshear strength, microshear plasticity and microshear toughness. The results obtained from the microshear test have been compared with those from conventional tests, such as the tensile test and Vickers' hardness test, and shown good coincidence. The experimental results also prove that the microshear test can be used successfully to estimate accurately the local mechanical properties distribution of practical welded joints with great convenience and low test cost.     

Local uniform heat treatment and its effects on the toughness of welded C-Mn pressure vessel steel

The present paper gives the results of a research programme on the influence of a local postweld heat treatment on the toughness of welded joints in a C-Mn pressure vessel steel. The welded test plates, 50 mm thick, were locally heat treated using electrical resistance heating over a width of about 200 mm. The influence of the local heat treatment on the toughness of the welded joints was evaluated using wide plate test specimens. The results indicate that an important improvement in toughness is obtained by the local heat treatment, resulting in toughness levels which are comparable to those obtained by a furnace stress relief heat treatment.     

Development of a statistical lower bound fracture toughness curve

A collection of mechanical property data has been used to develop reference lower bound fracture toughness curves. Many heats of nuclear pressure vessel steel (A533B-1, A508-1, A508-2, A540, A302B, A537-1 and A537-2), heat affected zone material, manual arc and submerged arc weld material were used in the study. Several referencing techniques were used to remove heat to heat variation for a particular material type. The currently employed technique, using RT_NDT, was found to inflate variance in some situations instead of reducing it. Alternative and more promising techniques were developed, the best one using the precracked instrumented Charpy impact test. First, mean curves relating the reference test to fracture toughness were developed. Second, the distribution of actual data about the curves was characterised in terms of variance and the form of distribution. Both variance and form of distribution changed with temperature. The results were used to generate a lower bound reference curve (a local tolerance bound), using the function:

$\text{f = A + B tanh}\text{t ? T}{}_0\text{C}$

($\text{f}$ is normalised fracture toughness and $\text{t}$ is normalised temperature).     

The fracture mechanical significance of cracks formed during stress-relief annealing of a submerged arc weldment in pressure vessel steel of type A508 Class 2

In large weldments of type A508 Cl2 cracks can form in the heat-affected zone during stress-relief annealing. The significance of such cracks with respect to catastrophic fracture is of interest from the point of view of safety, in particular for nuclear pressure vessels. In this investigation the size of reheat cracks, as formed and after fatigue growth, has been compared with the critical size for fast fracture. The latter was assessed by determination of the fracture toughness of the heat-affected zones. The fracture toughness of the heat-affected zones did not differ much from that of the parent material. The presence of microcracks reduced the fracture toughness (of a special type of simulated specimen) at 20°C by about 20%. The fracture mechanical evaluation indicates that the cracks formed during stress-relief annealing should not impair the safety of the vessel under normal conditions, except for particular geometries and when the cracks may rapidly link together during fatigue.     

Effects of a stress relief heat treatment on the toughness of pressure vessel quality steels—Influence of the plate thickness

This paper gives the results of the third phase of a research programme on the influence of post weld heat treatments on the toughness of welded joints in pressure vessel quality steels. Special attention is paid to the minimum plate thickness for which a post weld heat treatment should be recommended. In addition to the qualification tests, several large-scale tests were carried out on test plates of various thicknesses. The results indicate that, although the present code requirements are satisfactory for a C-Mn steel, they should be completely altered for an Nb-microalloyed steel, welded with a high heat input.     

Effects of geometry and flow properties on the fracture toughness of a C–Mn reactor pressure vessel steel in the lower shelf region

To estimate the fracture toughness of thick section nuclear reactor pressure vessel (RPV) steel in the irradiated condition, it is necessary to apply a size effect correction to the test results obtained on small-scale surveillance specimens. This correction is usually derived using toughness data obtained on different sizes of fracture mechanics testpieces made of non-irradiated material, for which the flow properties are quite different from those of material in the irradiated state. This paper describes the results of a fracture toughness test programme carried out on a C–Mn steel plate for two different specimen geometries (10 mm thickness precracked Charpy and 25 mm thickness compact tension) in the lower shelf region of the temperature/fracture–toughness curve. A comparison of the fracture behaviour and failure micromechanisms has been made for the material in the ‘start of life’ condition and after the application of cold prestraining, which was used to simulate the effects of neutron irradiation on flow properties during service. Although the Master Curve methodology predicts no size effects on the lower shelf, size effects were observed.     

The fracture toughness behaviour of austenitic steels and weld metal including the effects of thermal ageing and irradiation

Stainless steel components used in nuclear power plant must be capable of maintaining reasonable mechanical properties after thermal ageing and irradiation damage has accumulated over the lifetime of the system. This study examines the fracture toughness behaviour of wrought and welded Type 316 material in long-term thermally aged and irradiated conditions. The results indicate that whilst some potentially detrimental microstructural changes have occurred during ageing, the degradation in mechanical properties is not large. In wrought material some comparisons are made between the toughness of Type 316 grade and recent results obtained on modified Type 316LN grade materials. The effects of welding processes on oxygen and inclusion content have been quantified in MMA and TIG welds, and the results have been used to explain the higher toughness of TIG-welded material. Comparisons of fracture toughness after irradiation have also been made between arc welds in Type 316 and other grades.     

The reasonable design of Charpy-size specimen for fracture toughness test in nuclear surveillance

Use of a precracked and side-grooved Charpy-size specimen is an economical and convenient single-specimen method of evaluating the elastic-plastic fracture toughness of nuclear pressure vessel steels. This paper has studied the influence of side-groove depth on fracture toughness and stable increment of crack on several pressure vessel steels in detail. Test results are compared with those of large-size specimens which the National Standard of China (i.e. GB 2038) requires. The research results indicate that using a precracked Charpy-size specimen with side-groove depth 30% and adopting the energy before the maximum load of a three-point bending test curve, the elastic-plastic fracture toughness of materials can be evaluated conservatively when the crack begins to propagate, and the reasonable design of a Charpy-size specimen for fracture toughness tests in nuclear surveillance has been established. At the same time, the constraint effect and thickening action of the side-groove on a Charpy-size specimen have also been discussed, and theoretically explained.     

Experimental investigation of fatigue behaviour for welded joint with mechanical heterogeneity

Because of material and mechanical heterogeneity in welded joints, experimental studies of low-cycle fatigue (LCF) life and fatigue crack initiation behaviour were performed on weld metal, heat-affected zone (HAZ) and base metal of 16 MnR pressure vessel steel. A fatigue damage test was carried out by introducing a new damage variable. The fatigue damage evolution laws in each zone of a welded joint were obtained. A circular-notched compact tension specimen was designed and the whole process of fatigue damage, crack initiation and crack growth in each zone was investigated. The effect of mechanical heterogeneity in welded joints on the fatigue damage behaviour and fatigue crack initiation life was also investigated. The experimental and theoretical results indicate that the fatigue behaviour is markedly different in three zones of welded joints; there is a trend toward reduction in the fatigue life of HAZ as compared with the base metal. The weld metal does not have the same fatigue behaviour as confirmed by LCF life and crack growth rate. This study leads to the conclusion that fairly accurate estimates of fatigue life for welded joints should include the whole process of fatigue damage evolution and fatigue crack initiation as well as crack growth. Furthermore, a local experimental method should be carried out for weld metal, HAZ and base metal, respectively. Finally, the local fatigue failure criterion based on the weakest chain model has been presented.     

Measurement of crack propagation velocity in nuclear pressure vessel steel (SA516 gr. 70)

An attempt has been made to develop a simple, reliable and cost-effective device for measuring the dynamic crack propagation velocity in a nuclear pressure vessel steel (SA516 gr. 70). The experimental method is described and a simple digital approach is proposed. The experimentally determined dynamic crack velocity has been utilized to obtain elastic dynamic stress intensity factors by INSAMCR (a two-dimensional dynamic finite element code which is a modified version of SAMCR developed by Dr Schwartz at the University of Maryland). A relationship between instantaneous crack tip velocities and dynamic stress intensity factors for pressure vessel steels is estimated using dynamic crack propagation velocities determined by a proposed measuring device. The relationship between the dynamic stress intensity factor and time history and the dynamic arrest toughness for each test are obtained using the generation mode dynamic finite element analysis. A function ƒ(å) = 1·356 − 2·672å + 6·494å² − 4·539å³ + 1·461å⁴ is suggested which may be useful to predict the relationship between the dynamic fracture toughness (K(å)) and the dynamic crack arrest toughness (K_Ia) for SA516 gr. 70 steel (say K(å) = K_Ia ƒ(å) where å is the dynamic crack propagation velocity).     

Effect of temperature gradient on crack initiation

Compact tension (CT) specimens of pressure vessel steel A533B have been subjected to linear distributions of temperature along the crack line. The temperature dependence of fracture toughness for the A533B steel creates a fracture toughness gradient in such a specimen. The specimens simulated an irradiated pressure vessel wall and a functionally gradient material. The CT specimens subjected to linear distributions of temperature were tested to evaluate the fracture toughness for crack initiation. In all tests, the temperature at the crack tip was kept at −10°C or −55°C. If the value of the temperature gradient ahead of the crack tip exceeded a critical value, the fracture toughness deviated from the toughness obtained under a uniform temperature of −10°C or −55°C.     

Creep crack growth behavior in the HAZ of weldments of W containing high Cr steel

The creep and creep crack growth properties of W strengthened 11Cr–0.4Mo–2W steel welded joints have been investigated at 923 K. The joints were prepared using gas tungsten arc (GTA) welding and electron beam (EB) welding. Most of the joint specimens were ruptured in their heat affected zone (HAZ), and inevitably resulted in shorter creep lives than those of the base metals. The investigation of creep properties of simulated HAZ specimens showed that fine grains produced by heating around A_c3 were obviously responsible for the degradation of creep strength in welded joints. The creep lives of smooth specimens for EBW joints were about twice longer than those for GTAW joints, however brittle type IV fracture occurred even in the EBW joints with narrower HAZ width for long-term creep test. The FEM analysis used creep data from simulated HAZ specimens and so the experimental results for creep properties of welded joints could be explained. The creep crack growth properties in the HAZ of weldments were investigated using CT specimens. In the pre-cracked CT specimens, the crack initiation time was affected by mechanical constraint, whereas the difference of the crack growth rate between welded joints and base metal was negligible for the present high-strengthened steel.     

Fracture toughness of 15X2HMFA steel and its welds

The generalized experimental fracture toughness results of industrial semi-products, made from Cr---Ni---Mo---V steel and intended for VVER-1000 reactor pressure vessel manufacture, are presented. On the base of test results of nine steel heats having a common purity (sulfur to 0.018%; phosphorus to 0.015%; and copper to 0.20%) and of the same number of heats having a higher purity (sulfur to 0.012%; phosphorus to 0.012%; and copper to 0.06%) the fracture toughness temperature dependences were constructed. These differ appreciably from the reference curves for this material according to PNAE G-7-002-86. The temperature dependences K_1c for 15X2HMFA steel welds were also obtained. By this, for welds produced using manual arc welding and the types RT-45A and RT-45B electrodes as well as electroslag welding with Sv-16X2HMFTA wire and OF-6 flux, these dependences were constructed on the limited number of welded samples, as these welding methods are, practically, not used in the manufacture of such reactors. The fracture toughness assessment for welds produced by submerged arc welding was carried out for two variants of production processing (Sv-10XGNMAA wire and AH-17M flux; Sv-08XGHMTA wire and NF-18M flux), which were applied in the fabrication of circumferential welds for a VVER-1000 reactor.     

The effect of a long post weld heat treatment on the integrity of a welded joint in a pressure vessel steel

The effect of a long post weld heat treatment on the microstructure and mechanical properties of a welded joint in a 0·2%C-1·4%Mn-0·5%Mo pressure vessel steel was studied. Multipass submerged-arc welds were made at a heat input of 1·2 and 4·3 kJ mm⁻¹. Individual microstructural regions observed in the heat-affected zone of the actual weld were simulated. These regions were brittle in the as-simulated condition. Post weld heat treatment for periods of up to 40 h at 620°C resulted in a significant improvement in the Charpy impact toughness. At the same time, a loss of the heat-affected zone and weld metal hardness and transverse weld strenghth occurred. A fracture toughness (J_Ic) of 134 kJ m⁻² was measured in the heat-affected zone of the 4·3 kJ mm⁻¹ welds after prolonged post weld heat treatment. The improvement in weldment toughness with post weld heat treatment was primarily attributed to softening of the structure.     

三代非能动核电项目SA-508国产大锻件性能不符合项探讨

三代非能动核电项目RPV和SG用SA-508 Gr.3 C．1和SA-508 Gr.3 Cl.2锻件设计要求严格,制造难度高,在国产化制造过程中出现了诸多的性能不符合项。定量比较了三代非能动核电项目RPV和SG用SA-508锻件的设计要求与ASME规范要求的差异,并统计分析了RPV和SG用SA-508锻件的力学性能实测值。结果表明,RPV用SA-508 Gr.3 C．1锻件的不符合项的主要原因是低温韧性不足,SG用SA-508 Gr.3 C．2锻件的不符合项的主要原因是低温韧性、室温和高温抗拉强度。最后研究了RPV和SG用SA-508锻件化学成分和热处理工艺特点,分析了造成国产化大锻件性能不稳定的原因。为克服SA-508大锻件的制造现状与设计要求的矛盾,亟需对大锻件生产工艺进行优化或采用新型的制造工艺进一步提升大型锻件的性能。     

Analysis of biaxial loading effect on fracture toughness of reactor pressure vessel steels

The local stress–strain state (SSS) near the crack tip and its connection with the crack tip opening displacement and J-integral under biaxial loading have been studied by finite element methods in elastic–plastic finite strain statement. Numerical investigations have been performed for various crack lengths and two types of biaxial loading (tension and bending) under conditions of small- and large-scale yielding. To predict the biaxial loading effect on cleavage fracture toughness, the procedure has been elaborated, this being based on the revealed regularities for SSS near the crack tip under biaxial loading and brittle fracture criterion proposed earlier. Prediction of the biaxial loading effect on cleavage fracture toughness has been performed as applied to reactor pressure vessel steel. The calculated results have been compared with available experimental data. Alternative approaches for prediction of the biaxial loading effect on fracture toughness have been discussed.     

Characterization of high strength and high toughness Ni–Mo–Cr low alloy steels for nuclear application

The reactor pressure vessels of PWRs have mostly been made of SA508 Grade 3 (Class 1) low alloy steels which have revealed moderate mechanical properties and a moderate radiation resistance for a 40 or 60 year operation. The specified minimum yield strength of the material is 345 MPa with a ductile–brittle transition temperature of about 0 °C. While other materials, most of which are non-ferrous alloys or high alloyed steels for a higher temperature application, are being developed for the Generation-4 reactors, low alloy steels with a higher strength and toughness can help to increase the safety and economy of the advanced PWR systems which will be launched in the near future. The ASME specification for SA508 Grade 4N provides a way to increase both the strength and toughness by a chemistry modification, especially by increasing the Ni and Cr contents. However, a higher strength steel has a deficiency due to a lack of operating data for nuclear power plants. In this study, experimental heats of SA508 Grade 4N steels with different chemical compositions were characterized mechanically. The preliminary results for an irradiation embrittlement and the HAZ properties are discussed in addition to their superior baseline properties.     

国产F92耐热钢焊接接头性能试验与研究

对国产F92钢焊接热影响区的粗晶区进行热模拟试验.不同热处理条件后试样的硬度试验,冲击韧性试验和组织分析表明,焊后热处理对改善焊接接头的塑韧性具有显著作用.焊接工艺试验表明,国产F92钢存在一定的冷裂倾向,实际生产宜采用多层多道焊,层间温度的控制也很重要.     

Radiation embrittlement modelling for reactor pressure vessel steels: I. Brittle fracture toughness prediction

Modelling for the irradiation effect on brittle fracture toughness of reactor pressure vessel (RPV) steel is performed on the basis of the probabilistic model for fracture toughness prediction proposed by the authors earlier. The irradiation effect on parameters controlling plastic deformation and brittle fracture of RPV steels is analyzed. The physical mechanisms are considered which control the cleavage microcrack nucleation for RPV steels in the unirradiated and irradiated states and also in state after post-irradiation annealing. Prediction of the temperature dependence of brittle fracture toughness is performed as applied to irradiated 2.5Cr–Mo–V reactor pressure vessel steel. Modelling of the fluence effect and the phosphorus and copper content effect on brittle fracture toughness is carried out. It is shown that the probabilistic model based on a new formulation for brittle fracture criterion allows the adequate modelling for the irradiation effect on fracture toughness for RPV steel. Application of alternative models is discussed for fracture toughness prediction for irradiated RPV steels.     

Effects of a stress relief heat treatment on the toughness of pressure vessel quality steels welded with high heat input processes

The research described in this paper is an extension of an earlier research programme on the influence of a post weld heat treatment on the toughness of welded joints in pressure vessel quality steels.¹ In this part of the work special attention was paid to the heat input of the welding process involved and the influence of the subsequent heat treatment. In addition to the qualification tests, several wide plate tests were carried out.The results indicate that, while for some steels the influence of a stress relief heat treatment on the toughness of the joint depends considerably on the heat input of the welding process, it is independent of the heat input of the welding process for other types of steel.