Stress-Corrosion Cracking of Stainless Steel Superheater Tubing

Stainless steel has been widely used in challenging environments typical to nuclear power plant structures, due its excellent corrosion resistance. Nickel filler metals containing high chromium concentration, including Alloy 82/182, are used for joining stainless steel to carbon steel components to achieve similar high resistance to stress corrosion cracking. However, the joint usually experience weld metal stress corrosion cracking (SCC), which affects the safety and structural integrity of light water nuclear reactor systems. A primary driving force for SCC is the high tensile residual stress in these welds. Due to large dimension of pressure vessel and limitations in the field, non-destructive residual stress measurement is difficult. As a result, finite element modeling has been the de facto method to evaluate the weld residual stresses. Recent studies on this subject from researchers worldwide report different residual stress value in the weldments [5]. The discrepancy is due to the fact that most of investigations ignore or underestimate the thermal recovery in the heat-affect zone or reheated region in the weld. In the current study, the effect of heat treatment on thermal recovery and microhardness is investigated for materials used in dissimilar metal joint. It is found that high equivalent plastic strains are predominately accumulated in the buttering layer, the root pass, and the heat affected zone, which experience multiple welding thermal cycles. The final cap passes, experiencing only one or two welding thermal cycles, exhibit less plastic strain accumulation. Moreover, the experimental residual plastic strains are compared with those predicted using an existing weld thermo-mechanical model with two different strain hardening rules. The importance of considering the dynamic strain hardening recovery due to high temperature exposure in welding is discussed for the accurate simulation of weld residual stresses and plastic strains. Finally, the experimental result reveals that the typical post-buttering heat treatment for residual stress relief may not be adequate to completely eliminate the residual plastic strains in the buttering layer.

Now-a-days, electron beam welding is extensively used in piping and pressure vessel of nuclear industry because of its advantages with respect to no use of filler material, lower distortion and residual stresses. Therefore, the paper aims to evaluate weld pool size, deformation and residual stresses in 13.5 mm thick pipe weld of outer diameter 170mm made of austenitic stainless steel (SA312 Type 304L) in a butt joint configuration prepared by Electron Beam Welding (EBW). Pipe weld with back plate has been prepared by EBW with heat input 825 and 700 J/mm and qualified as per requirements of ASME Sections III. Post-welding, axial distortion, depth of penetration, weld width, and weld area have been measured. A comprehensive three-dimensional coupled thermo-mechanical finite element analysis of pipe weld joint has been performed to predict the weld pool size (weld depth penetration and weld width) and axial distortions. Experimental and predicted results with respect to weld pool size and distortions compares well which confirms the validity of the numerical analysis procedure. After validating the numerical analysis procedure, effect of beam power (heat inputs) on weld residual stresses on pipe weld joint have been evaluated by carrying out numerical analyses. The results showed that lower beam power input resulted in reduced weld pool size, residual stresses and HAZ. The residual stresses in pipe weld joint have been compared with available studies in literature for different welding processes such as multi-pass arc welding, gas tungsten arc welding (GTAW) for approximately same pipe diameter to thickness ratio. It has been found that the residual stresses, distortion, weld pool size and HAZ in the pipe weld joint of EBW are lower compared to other welding processes. It has also been brought out that beam input power of 600J/mm is sufficient for manufacturing pipe weld joint of thickness 13.5 mm. Details of the studies are elaborated in this paper.

Residual stresses that arise as a result of welding can cause distortion, and also have significant implications for structural integrity. Martensitic filler metals with low transformation temperatures can efficiently reduce the residual stresses generated during welding, because the strains associated with the transformation compensate for thermal contraction strains during cooling. However, it is vital that a low weld transformation temperature is not obtained at the expense of other important material properties. This article outlines the alloy design process used to develop appropriate low transformation temperature filler materials for the mitigation of residual stresses in both low alloy ferritic and austenitic stainless steel welds. Residual stresses in single pass, 6 mm bead in groove welds, on 12 mm thick plates, have been measured and compared against those obtained with commercially available conventional austenitic and ferritic filler materials. The filler metals developed here exceeded requirements in terms of weld mechanical properties, while significantly reducing the maximum residual stress in the weld and heat affected zone.

Risk based treatment of degradation and fracture in nuclear power plants has emerged as an important topic in recent years. One degradation mechanism of concern is stress corrosion cracking. Stress corrosion cracking is strongly driven by the weld residual stresses (WRS) which develop in nozzles and piping from the welding process. The weld residual stresses can have a large uncertainty associated with them. This uncertainty is caused by many sources including material property variations of base and welds metal, weld sequencing, weld repairs, weld process method, and heat inputs. Moreover, often mitigation procedures are used to correct a problem in an existing plant, which also leads to uncertainty in the WRS fields. The WRS fields are often input to probabilistic codes from weld modeling analyses. Thus another source of uncertainty is represented by the accuracy of the predictions compared with a limited set of measurements. Within the framework of a probabilistic degradation and fracture mechanics code these uncertainties must all be accounted for properly. Here we summarize several possibilities for properly accounting for the uncertainty inherent in the WRS fields. Several examples are shown which illustrate ranges where these treatments work well and ranges where improvement is needed. In addition, we propose a new method for consideration. This method consists of including the uncertainty sources within the WRS fields and tabulating them within tables which are then sampled during the probabilistic realization. Several variations of this process are also discussed. Several examples illustrating the procedures are presented.

The part-through-wall crack perpendicular to the circumferential weld on the outside surface of a spent nuclear fuel (SNF) multiple-purpose canister (MPC) can be shown to be the most limiting fracture configuration driven by the welding residual stress (WRS). A series of semi-elliptical cracks of various sizes is chosen to calculate the stress intensity factors (K) under a bounding residual stress (i.e., the stress distribution that bounds all WRS in a canister). The threshold stress intensity factor (KISCC) of the canister material in the storage environment is used to determine a critical flaw size, below which the stress corrosion cracking would be unlikely to take place. This result can be considered as the flaw disposition criterion should a surface flaw be detected during the inservice inspection as required by the aging management program (AMP), and can be proposed to American Society of Mechanical Engineers (ASME) Section XI Code Case N-860, “Examination Requirements and Acceptance Standards for Spent Nuclear Fuel Storage and Transportation Containment Systems.”

Some cases of Stress Corrosion Cracking (SCC) failures in the proximity of welded zone of core internals and pipes in Primary Loop Recirculation, which are made of austenitic stainless steel, were reported in existing boiling water reactors, from the late 1970s to 1980s. As a countermeasure against SCC, low-carbon stainless steel was developed in order to reduce susceptibility to SCC, and used as standard material at that time. However, SCC failures were still observed in the core internals and pipes made of low-carbon stainless steel in recent years. It is well understood that residual tensile stress due to welding largely affects occurrence and growth of SCC in low-carbon stainless steel. Based on these observations, stress reduction methods for the pipe welded zone have already been developed such as Induction Heating Stress Improvement (IHSI) and Heat Sink Welding (HSW). However, these stress reduction methods are applied to only large-bore, thick-wall pipes, because it is difficult to apply these established countermeasures to the small-bore, thin-wall pipes which diameter is 50A and below. Thus stress reduction method for small-bore pipe has not been established. In this study, residual tensile stress reduction method that is applicable to the welded zone of small-bore pipe has been developed. The stress reduction method uses rapid quenching of inner surface of the pipe by cooling water after heating outer surface of the pipe by a heating device. Just after starting to cool the inside of the pipe, the temperature of the inner surface is low and tensile stress is generated. On the other hand, since temperature of outer surface is high, large temperature difference between inside and outside surface of the pipe develop and the high through-wall thermal stress is generated. When the temperature difference between the inner and outer surfaces of the pipe is large, thermal stress exceeds the yield stress on the pipe inner surface, and plastic deformation occurs on the inner surface of the pipe. The residual stress of the inner surface becomes compressive after the heat treatment due to the residual layer of plastic strain. In this study, the effectiveness of this method is shown by comparing the residual stress on the inner surface of the pipe before and after the application of this method by mock-up tests.

Residual stresses in a CrMoV-2CrMo pipe weld: Part 1—The as-welded condition

T-joint welding is the most common type used in heavy equipment industry. Localized heating and subsequent cooling induce residual stress in the weld process. Service loading overlapped with residual stress influences the strength of the welded structure and leads to premature failure of structure. In this study, finite element analysis is used to study the influence of welding groove on residual stress and distortion in T-joint welding. The simulation consists of sequentially coupled thermal and elastic-plastic analysis using an element birth and death method to model the addition of weld metal to the workpiece. The results of numerical simulation are verified by comparing with experiment from published literature. The results show that welding groove has a significant influence on welding residual stress.

The welding process, incorporating rapid heating and cooling, generates distortion and residual stress in weldments. Welding distortion and residual stress in welded structures can result in problems such as dimensional inaccuracies during assembly and raise concerns regarding safety during service. Therefore, accurate prediction and reduction of residual stress are critical in improving the quality of a weldment. In the present paper a new method of analysis is proposed to predict welding residual stress by considering solid phase transformations during the welding process. This method is applied to two cases, involving medium carbon steel and low carbon steel respectively. The analysis of medium carbon steel revealed the presence of compressive residual stress due to martensite formation when phase transformation was considered. However, in low carbon steel the residual stress obtained considering the effect of phase transformation did not differ significantly from that obtained when phase transformation was ignored.

Unstable fractures generally occur in brittle materials under low-temperature service conditions. Toughness and welding residual stress are the main factors that should be evaluated when defining a brittle crack propagation path. In this study, a rainbow welding technique was proposed and confirmed as being significantly useful in preventing unstable fractures in weld joints. The residual compressive stress in the crack front was particularly useful for decreasing the possibility of brittle fracture. The objective was to examine the effect of high welding consumable toughness welding residual stress, especially for avoiding brittle fracture through welding residual compressive stress.

The fabrication of structural member using dissimilar steels renders steel structures lighter and more economical. However, it always involves welding process and produces different residual stresses in welds as compared with welding of similar steels. This paper presents the characteristics of residual stresses in welds of similar and dissimilar steel weldments by carrying out three-dimensional (3-D) thermal elastic-plastic finite element (FE) analysis. The materials used in this investigation were SM400, SM490, SM520 and SM570, widely used structural steels in welded structure. Results show that the maximum longitudinal residual stresses in welds of the similar steel weldments increase with increasing yield stress of the steel welded (SM400 < SM490 < SM520 < SM570). In case of the dissimilar steel weldments, the difference between the longitudinal residual stresses in welds increases with increasing yield stress of the steel welded together with SM400 (SM490 < SM520 < SM570).

Weld residual stress can contribute to the reduction of structure lifetime and accelerate the formation of fatigue cracks, brittle fractures, or stress corrosion cracking. Distortion can have a significant impact on the dimensional ac-curacy of assembly, structure strength, and fabrication cost. In the past two decades, there have been many significant and exciting developments in the prediction and mitigation of weld residual stress and distortion. This paper reviews the recent advances in mitigation techniques that have been applied in the structure design, manufacturing, and postweld stages. The techniques used in the structure design stage include selecting the type of weld joint and weld groove, using balanced welding, determining appropriate plate thickness and stiffener spacing, and considering distortion compensation. Mitigation techniques used in the manufacturing stage include welding sequence optimization, reducing welding heating input, selecting low-transformation-temperature filler metals, prebending, precambering, constraints, trailing and stationary cooling, in-processing rolling, transient thermal tensioning, and additional heat sources. Postweld mitigation techniques include postweld heating and mechanical treatment. Finally, the remaining challenges and new development needs were discussed to guide future development in the field of mitigating weld residual stress and distortion.

Using FEM to determine the thermo-mechanical stress in tube to tube–sheet joint for the SCC failure analysis

Failure analysis of cracking in S30408 weld joint between cylinder and flange of pure steam sterilization pot

To investigate the distribution pattern and magnitude of residual stress at the weld toe of pipe-sphere connections in welded hollow sphere joints, the blind-hole method was employed to measure welding residual stress at both the spherical weld toe and pipe weld toe, yielding the magnitude and distribution characteristics of the residual stress. Additionally, the VISUAL-Environment finite element simulation software was utilized to obtain the temperature field, residual stress field, and residual stress distribution curves at the weld seam. The results indicate that welding residual stress in the entire welded hollow sphere joint exhibits both periodic and stochastic characteristics, with these two patterns showing good consistency. Meanwhile, finite element analysis results reveal the presence of both residual tensile and compressive stresses on the spherical surface and the connected pipe, with the welding residual stress maintaining a self-equilibrated state across the entire specimen. This study provides a foundational basis for subsequent research on the fatigue performance of grid structures with welded hollow sphere joints.