Welded Joint

a segment of a structure or article where parts are joined by welding; the joined parts may be of the same material or of different materials.

Classification of joints and welds. The relative positions of the joined components are the basis for the various categories of joints: butt joints, T-joints, lap joints, and corner joints. Each type of joint has specific characteristics that depend on the welding method chosen, whether arc welding (Figure 1), electroslag welding (Figure 2), resistance welding (Figure 3), or some other method. The segment of the welded joint directly connecting the components being welded is called the weld. All types of welds can be classified according to the metal-deposition technique used as single-pass welds, welds formed from the center toward the ends, and welds formed by backstep welding. Welds can be also distinguished by the spatial positioning during welding as vertical, horizontal, downhand, and overhead welds. Welds can also be classified according to the method by which their cross sections are formed as single-layer and multilayer welds. State and industry standards specify the basic types of welded joints, the structural components of edges and welds, and the tolerances and feasible ranges for the thicknesses of the components beingjoined. These data apply to all types of welds.

Characteristics of welded joints. A welded joint contains a number of zones formed in the material of the welded components. These zones differ from the base material and among themselves in chemical composition, structure, physical and mechanical properties, microstressing, and macrostressing. A welded joint made by fusion welding (Figure 4,a) exhibits a zone containing the weld metal, a fusion zone, a heat-affected zone, and a zone where the properties and structure of the base metal remain unchanged. A welded joint formed by pressure welding (Figure 4,b) does not have a zone containing weld metal or a fusion zone. It consists of a joint zone, in which interatomic links have been formed between the joined components, a mechanically affected zone, and a zone of the adjacent base metal.

Figure 2. Types of welded joints and welds used in electroslag welding: (a) butt joint, (b) T-joint, (c) corner joint; (1) butt weld, (2) fillet weld, (3) weld for corner joint

The material in the weld may consist of an alloy formed by the melted base material, the electrode material, and/or filler material, or it may consist solely of the melted base metals. In the heat-affected zone, the base metal does not undergo melting; however, in individual sections of this zone the effects of heating and cooling can change the properties and structure of the base metal in various ways. In the general case of fusion welding of low-carbon steel, the heat-affected zone of a welded joint consists of the sections shown in Figure 5. The overheated section (I) directly adjoins the fusion zone. The material in this section has been heated to a temperature above 1100°C and has acquired a coarse-grained structure; consequently, the strength of the material has been lowered. In the grain-refined, or normalized, section (II), the material has been heated to temperatures in the range of 900°-1100°C; this heating causes a reduction in grain size and an increase in strength. In the partially grain-refined section (III), the metal has been heated to temperatures in the range of 700°-900°C. The metal in this section is characterized by a nonhomogeneous structure or partial grain refining. In the recrystallized section (IV), the metal has been heated from 500°C to a temperature corresponding to the critical point A₁, which causes a reduction in strength and, sometimes, plasticity. In the aging section (V), the metal has been heated to temperatures of 100° to 500°C; it does not exhibit any visible structural changes but differs from the original base metal by its lower strength, which is most strongly pronounced in the temperature range of 100°-300°C. The width of the heat-affected zone during welding depends on the welding method, the technological process, the thermal conditions, and the thermal and physical properties of the base metal.

Figure 3. Types of welded joints and welds used in resistance welding: (a) butt joint formed by resistance welding, (b) butt joint formed by fusion welding, (c) lap joint formed by a single row of spot welds, (d) lap joint formed by multiple rows of spot welds, (e) lap joint formed by a single seam weld

Properties of welded joints. The quality of a welded joint is determined by the joint’s operational condition and the resistance to failure caused by brittleness or fatigue. The operational condition of a welded joint is characterized by a complex combination of properties in alternating zones—layers that differ from the base metal and from one another in their strength properties. The layers with higher strength properties are conventionally called hard, and the adjacent layers with lower strength properties are called soft. Many factors determine which layers

Figure 4. Welded joints formed by (a) fusion welding and (b) pressure welding; (1) weld metal, (2) fusion zone, (3) joint zone, (4) heat-affected zone, (5) adjacent base metal

qualify as soft layers. They include the properties of the base metal and welding materials, the methods and conditions for welding and heat treating, and the temperature and rate at which loads are applied. The weld itself, the fusion zone, the weakened sections of the heat-affected zone, and the interspersed layers of other nonbase metals may be soft layers. Deformations are localized in the soft layers. If the layers are extremely thin, the deformations do not lower the load-carrying capacity of the welded joint. However, if soft layers are relatively thick, the load-carrying capacity of the welded joint is limited by the properties of the soft layers.

In designing and fabricating welded structures, it is necessary to take into account the influence of the stress-strain state on the operational condition of welded joints, the accuracy of joint dimensions and shapes, and the stability of such properties during actual use. In a stress-strain analysis, a distinction is made between the zone of plastic deformations, the zone of elastic deformations, and intrinsic residual stresses, both tensile and compressive. Diagrams of temporary and residual longitudinal deformations and stresses in the butt joint of a plate made of carbon steel are shown in Figure 6.

The resistance of welded joints to failures caused by brittleness or fatigue depends on the properties of the metal and on the presence of stress concentrators and deformation concentrators in the metal. Some concentrators are part of the design of a joint, for instance, sections where the cross section of a joint changes abruptly, as in a transition from the weld metal to the base metal in a T-joint or lap joint. Others are formed by the production process, for example, abrupt transitions with reentering angles where the weld is reinforced, poor penetration, incomplete coalescence, and cuts. Still other concentrators may have a physical or chemical origin, such as pores, slag inclusions, and cracks in the welds or the heat-affected zones.

The formation of a welded joint is accompanied by a thermoplastic process of deformation of the base metal. This process is most clearly exhibited in welded steel structures; it causes brittleness in some areas of the heat-affected zone. The greatest degree of brittleness is caused by aging that occurs during the deformation process of the metal at temperatures of 150°-300°C. In the aging sections, the welded joints exhibit a limited resistance to failure caused by brittleness.

Figure 5. Diagram of the heat-affected zone: (I) overheated section, (II) grain-refined (normalized) section, (III) partially grain-refined section, (IV) recrystallized section, (V) aging section; (1) weld metal, (2) fusion zone

During the formation of a welded joint, the dimensions of the components being joined are reduced, both in the longitudinal direction and the transverse direction. This shrinkage is taken into account in designing and fabricating welded objects.

Design principles for welded joints. Two methods for calculating the strength of welded joints under static loads are used in the USSR: the limit-state method, used in civil engineering, and the method of permissible stresses, used in machine building.

Figure 6. Temporary and residual deformations and stresses in the butt joint of a plate made of carbon steel: (a) plate, (b) diagram of temporary deformations for ∊_max< ∊_t, (c) diagram of temporary deformations for ∊_max > ∊_t, (d) diagram of residual deformations ∊_res, (e) diagram of residual stresses σ_t; (1) zone of plastic deformations from compression, (2) zone of elastic deformations, (3) and (4) tensile and compressive stresses and deformations

Industrial standards and design norms for structures specify the design values of welded joints made of steel of various strengths. The values include tensile strength , compressive strength , shear strength in butt welds , and shear strength in fillet welds , as well as the permissible tensile and compressive stresses σ^w and shear stress τ^W. The calculation of fatigue effects on welded joints in the metal structures of machines is made according to generally accepted methods of fatigue calculation for machine parts. The effects of low temperatures on the operational condition of welded joints can be taken into account in the design and fabrication stages by the choice of appropriate base metals and welding materials, by correct design and selection of adequate processes and by material quality-control methods.

In calculating the strength of welded joints under static loads, the effects of stress concentrators and of temperature are usually negligible for commonly used carbon steels and low-alloy steels. In calculating the fatigue strength of welded joints, the effects of stress concentrators and residual stresses are taken into account by specifying the values of permissible stresses. The limit-state method is used to calculate the fatigue resistance of welded joints in bridge spans and in industrial steel structures.