Control of welding residual stress for ensuring integrity against fatigue and stress–corrosion cracking

https://doi.org/10.1016/j.nucengdes.2006.05.006Get rights and content

Abstract

The availability of several techniques for residual stress control is discussed in this paper. The effectiveness of these techniques in protecting from fatigue and stress–corrosion cracking is verified by numerical analysis and actual experiment. In-process control during welding for residual stress reduction is easier to apply than using post-weld treatment. As an example, control of the welding pass sequence for multi-pass welding is applied to cruciform joints and butt-joints with an X-shaped groove. However, residual stress improvement is confirmed for post-weld processes. Water jet peening is useful for obtaining a compressive residual stress on the surface, and the tolerance against both fatigue and stress–corrosion cracking is verified. Because cladding with a corrosion-resistant material is also effective for preventing stress–corrosion cracking from a metallurgical perspective, the residual stress at the interface of the base metal is carefully considered. The residual stress of the base metal near the clad edge is confirmed to be within the tolerance of crack generation. Controlling methods both during and after welding processes are found to be effective for ensuring the integrity of welded components.

Introduction

It is essential to ensure the integrity of a welded joint against fatigue or corrosion during their long use in welded structures such as power plants, industrial machines, or transportation vehicles. The factors that affect fatigue strength are residual stress, stress concentration, the mechanical properties of the material, and the macro- and micro-structure. Residual stress is one of the most important factors, and it is well known that residual stress is more of a concern for high-cycle fatigue than the other factors. Stress–corrosion cracking usually occurs when the following three factors superpose at the same time: material corrosivity, the environment of the components, and tensile stress, including residual stress, thus residual stress becomes very critical for stress–corrosion cracking when it is difficult to improve the material corrosivity of the components and their environment during operating conditions. Because residual stress must be considered in order to maintain the integrity of welded components, the reduction of residual stress is effective in preventing high-cycle fatigue and stress–corrosion cracking. Therefore, it is important to control and reduce residual stress during the welding process or after the welding process for the integrity of the welded structures. Effective methods of residual stress control need to be developed and applied to the welded components.

There have been many reports on reducing residual stress. Residual stress control during the welding process can easily protect welded components without any additional procedure after welding. Controlling welding conditions such as heat input or constraints affects the residual stress distribution (Boulton and Lance Martin, 1936, Kihara and Masubuchi, 1954, Watanabe and Satoh, 1961, Nagaraja Rao et al., 1964, Alpsten and Tall, 1970, Satoh and Terasaki, 1976). The material properties of the base metal and weld metal have an influence on residual stress (Bru et al., 1997, Ohta et al., 2000) when a consumable with a low-temperature martensitic phase transformation is applied to a high-tensile steel structure. Pre-strain (Kumose et al., 1954) and pre-heating (Benson and Watson, 1955, Burak et al., 1979) are effective in reducing residual stress. In addition, controlling constraint conditions (Guan et al., 1994a), cooling near a weld pool (Guan et al., 1994b), or combining vibration with welding (Aoki et al., 1995) have been developed as in-process methods of residual stress reduction.

The procedure of residual stress reduction after welding is also important. Post-weld heat treatment (Greene and Holzbaur, 1946, Burdekin, 1963, Fidler, 1982, Smith and Garwood, 1982) and peening (Kawai and Koibuchi, 1975, Al-Obaid, 1986, Ohta et al., 1996) are typical methods for reducing residual stress. A procedure after welding is often used for components already in operation. An induction heating method for stress improvement after the welding process has been developed to reduce residual stress in a welded pipe joint (Ueda et al., 1986). The methods of residual stress reduction by plastic deformation from a mechanical force applied after welding (Matsuoka and Naoi, 1983, Porowski et al., 1990, Nayama and Akitomo, 1994), cooling by liquid nitrogen after welding (Watanabe et al., 1983), or compulsive vibration after welding (Watanabe and Satoh, 1955) have also been developed.

The effectiveness of residual stress control both during and after the welding process is discussed in this paper. To prove the effectiveness of residual stress control, the precise prediction of residual stress is needed for an accurate evaluation of the lifetime of welded structures. The techniques of numerical analysis and measurement, which we have already developed, are applied to the precise evaluation of the residual stress.

Optimizing the sequence of welding passes during the welding process reduces residual stress. The effects of residual stress on fatigue strength at a weld toe in a multi-pass fillet-welded joint were evaluated. It was confirmed that the fatigue strength was nearly the same at a high-stress amplitude; however, at a low-stress amplitude, the fatigue strength of the specimen with controlled welding passes was higher than that of the conventional specimen. Next, the residual stresses in butt-welded joints with an X-shaped groove were evaluated. The effect of the residual stress distribution on fatigue crack propagation was verified in a plate with a butt-joint. Because the distribution of residual stress depends on the sequence of welding passes, an optimum welding sequence was determined from the residual stress distribution which was the most reliable means of preventing stress–corrosion cracking with consideration of the operating conditions of actual welded pipe joints.

The procedure after welding was also studied, particularly for applying preventive maintenance. Water jet peening has been developed as a preventive maintenance technique for the internal components of the reactor vessels of nuclear light-water power plants. This technique can be performed in water without an adhesive; therefore, performing this procedure during plant operation is easily possible. The compressive residual stress on the material surface was improved by water jet peening, and the tolerance against fatigue and stress–corrosion cracking was verified. The tungsten inert gas (TIG) cladding method has also been developed as a preventive maintenance technique for small-diameter pipes that penetrate pressure vessels. A corrosive-resistant material is clad on the inner surface of the pipe, and the residual stress due to cladding was confirmed not to affect the corrosion-resistance both for the clad metal and the base metal.

The residual stress evaluation in this study was mainly done by numerical analysis. Measuring methods such as X-ray diffraction and stress relief with a strain gage were used to verify the analytical result. There are two numerical methods for calculating residual stress: inherent strain analysis and thermal elastic–plastic analysis. The inherent strain method can calculate the residual stress by elastic analysis once an inherent strain database is constructed (Ueda and Fukuda, 1989, Mochizuki et al., 1995, Mochizuki et al., 1997, Mochizuki and Toyoda, 2001). Thermal elastic–plastic analysis is effective when the material properties at all temperatures have been gathered. This technique has usually been done in two-dimensional modeling because of its timeconsuming computer time (Ueda and Yamakawa, 1971, Friedman, 1975, Rybicki and Stonesifer, 1979, Argyris and Doltsinis, 1981, Leblond et al., 1986, Mochizuki et al., 2000a), but the three-dimensional calculation has recently become popular due to the development of high-speed computers (Karlsson and Josefson, 1990, Brown and Song, 1992, Janosch and Clergè, 1997, Karlsson et al., 1997, Michaleris et al., 1997, Dong et al., 1998, Mochizuki and Toyoda, in press). After considering the characteristics of each analytical technique, both the inherent strain method and the thermal elastic–plastic analysis were used in this study (Mochizuki et al., 2000a). ADINA Ver. 6.1 was used for the thermal elastic–plastic analysis and an own FE-code was used for the inherent strain analysis. In the thermal elastic–plastic analysis, phase transformation effect for ferritic steels is considered by changing material properties properly within the transition temperature region.

Section snippets

Fillet-welded joint

It is possible to reduce the residual stress by changing the welding pass sequence in a multi-pass fillet-welded joint. Other factors that affect fatigue strength, such as-welded joint configuration or mechanical properties (Peterson, 1959, Almen and Black, 1963), can be kept the same as in conventional welding procedures. This method does not need any additional treatment after welding to reduce residual stress. The effects of residual stress on fatigue strength at a weld toe in a multi-pass

Water jet peening

Water jet peening has been developed to improve residual stress on a weld surface (Enomoto et al., 1994). This technique is particularly useful for preventive maintenance of the internal components of pressure vessels in nuclear light-water power plants because it can be processed in water without an adhesive during plant maintenance (Sagawa et al., 2000). Numerical analysis was performed to clarify the fundamental characteristics of the residual stress improvement on the material surface.

Summary

Methods of residual stress reduction were discussed for both in-process control during welding and post-weld control. The sequence optimization of welding pass deposition for multi-pass welding was applied to cruciform fillet-welded joints and butt-welded joints with an X-shaped groove. The effectiveness in preventing fatigue and stress–corrosion cracking in the in-process control method was verified by numerical analysis and actual experiment. Water jet peening and TIG cladding were

Acknowledgments

The author especially appreciates Profs. Hideo Kobayashi of Tokyo Institute of Technology and Masaki Shiratori of Yokohama National University for their valuable and significant comments about this study. The author also thanks also thank Messrs. Katsumasa Miyazaki, Katsuhiko Hirano and Munetoshi Zen of the Mechanical Engineering Research Laboratory, Hitachi, Ltd., for their kind support in the experiment and numerical analysis, and Messrs. Hideyo Saito, Sadato Shimizu, and Osamu Oyamada of

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