Welding Deformation and Residual Stress Prevention
By Ninshu Ma, Hidekazu Murakawa and Yukio Ueda
()
About this ebook
- The basic theories including "theory of elastic-plastic analysis" and "inherent strain theory" , and analysis procedures are described using a simple three-bar model
- Online simulation software to perform basic analysis on welding mechanics
- Examples of strategic methods and procedures are illustrated to have solved various welding-related problems encountered in the process of construction
- Appendices present data bases for welding residual stresses, temperature dependent material properties, etc.
Ninshu Ma
Ninshu Ma received his doctoral degree in Engineering from Osaka University in 1994 and then worked for 21 years as a professional consultant in the field of computer-aided engineering at Japan Research Institute. He’s currently a professor at Joining and Welding Research Institute, Osaka University. His research focuses on the development of computing methods and their FEM software for analysis of multi-physical phenomena in joining and forming processes. Recent work has centered on thermal-mechanical coupling analysis on various joining processes of dissimilar materials as well as additive manufacturing processes and the assessment of structural components.
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Welding Deformation and Residual Stress Prevention - Ninshu Ma
Authors
Chapter 1
Introduction to Welding Mechanics
1.1 Basic Concepts of Welding and Welding Mechanics
Fusion welding always produces thermal stress, deformation, and residual stress in products. The mechanism of production of stress and deformation and their influences upon the performance of welded structures are matters of keen concern for engineers. The theory of welding mechanics [1, 2] presents the theoretical framework for understanding these consequences. In classic welding mechanics, theories are based on an empirical formula and using simplified models. However, recently, thermal elastic-plastic analysis [3–6] by the finite element method (FEM) has become popular and has been applied to simulate the mechanical behavior of metal during welding.
In practice, general structures are fabricated by a series of production processes such as cutting, bending, joining, straightening, residual stress relieving, etc. (as shown in Table 1.1). These processes may be classified into the following three groups: utilization of thermal process, mechanical device, and chemical reaction.
Table 1.1. Manufacturing Processes for Welded Structures
Welding belongs to the first group, so-called fusion joining (welding), that is, to join components by melting a small portion of the metal.
The energy sources for melting metal are gas, arc, plasma, electron beam, laser, etc. These are selected according to the type of structure and the efficiency of the manufacturing. For example, since a heavy pressure vessel is composed of many thick plates, a highly efficient heat source such as electron beam welding, which provides a concentrated large amount of heat input, is often adopted. For large structures such as ships, bridges, etc., arc welding is widely used because its machine is cheap and can be handled easily.
Generally, fusion welding is conducted by moving a heat source along the weld line. The joining process by any type of fusion welding is almost always the same and includes the heating process, melting process, and solidification process no matter what kind of welding heat source is employed. Therefore, arc welding is selected as an example, and the basic concepts of computational welding mechanics are presented here.
Many different types of welded joints, such as butt joints, fillet joints, etc., as illustrated in Fig. 1.1, are used according to the shape characteristics of the structural components. The groove type also differs according to plate thickness to obtain sufficient penetration, as shown in Fig. 1.2 [7].
Fig. 1.1 Types of welded joints.
Fig. 1.2 Types of groove shapes.
In arc welding, the surface of the groove of the base metal is heated and molten by arc heating. Simultaneously, the electrode is also molten and fills into the groove. After cooling down, the joint is formed. Figure 1.3 is a schematic illustration of the electric power source and the welding arc. The type of welding power source, such as direct current, alternating current, and pulse current, must be selected to fit the type of welded joint.
Fig. 1.3 Schematic illustration of electric power and welding arc.
In arc welding, the heat input per unit weld length is expressed by
(1.1.1)
In this equation, the current of the direct-current source is denoted by I, the voltage between the electrode and the base plate by U, and the traveling speed of the electrode by v. η is the efficiency of the heat input, which takes into account loss of electric energy by radiation, convection, etc. For example, in submerged arc welding, a high efficiency such as η = 0.9–0.99 is expected, and η = 0.3~0.6 for low efficiency of TIG (tungsten inert-gas) welding [5].
When the heat source of welding travels along the weld line, the temperature distribution in the base plate changes as a function of time elapsed. For example, when a point heat source moves on the surface of a thick plate, the temperature distributions are calculated as indicated in Fig. 1.4 [8]. Beneath the source, the temperature is the highest and gradually decreases from the heat source toward the starting point of welding. The temperature distribution in the vertical plane against the weld line indicates a concentric circle. As a result, the portion where the temperature is above the melting point is regarded as the weld metal, and the surrounding area, in the case of steel, which is heated above the A c1 transformation temperature (723°C), forms the heat-affected zone (HAZ), as illustrated in Fig. 1.5. The HAZ exhibits different mechanical properties in hardness and ductility, which influence the strength and reliability of the welded