Chapter 27 Welding Welding Processes • Welding Specifications • Welding Metallurgy • Weldability • Welding Discontinuities and Defects • Repair Welding • Adhesive Bonding

Arc welding encompasses several manual and automatic processes. Arc welding is a welding process in which the heat required to melt the filler metal or fuse the joint is generated by an arc struck between an electrode and the workpiece. The joint area is shielded from the atmosphere until it is cool enough to prevent the absorption of harmful impurities from the atmosphere. Arc welding is the most commonly used method of joining metals. It is also used for surfacing. Surfacing (weld overlay) is used to rebuild worn surfaces to their original dimensions and to restore corrosion resistance to corroded surfaces. Arc welding processes include shielded metal arc welding (SMAW), gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), flux cored arc welding (FCAW), and submerged arc welding (SAW). See Figure 27-1.

Oxyfuel welding uses the burning of various types of fuel gases to produce the heat needed to fuse metal. Oxyfuel welding (OFW) is a welding process in which the heat is generated by the burning of a fuel gas with oxygen. The most widely used fuel gas is acetylene, forming an oxyacetylene combination. See Figure 27-2.

Resistance welding generates the heat required to fuse metal by passing high-amperage current through mating workpieces. Resistance welding (RW) is a welding process in which heat is generated by passing a high current through mated workpieces. See Figure 27-3. Resistance welding is used to make localized (spot) or continuous (seam) joints.

Specialty welding processes require specialized equipment. Specialty welding processes require specialized equipment. Specialty welding processes include laser beam welding (LBW), plasma arc welding (PAW), electron beam welding (EBW), and electroslag welding (ESW). See Figure 27-4.

Solid-state welding uses various forms of energy to join metals below their melting points. Solid-state welding (SSW) is a welding process in which metals are joined at temperatures below their melting points. Solid-state welding processes include friction welding, ultrasonic welding, and explosion welding. See Figure 27-5.

The distance (clearance) between the surfaces being joined has a significant effect on the strength of a brazed or soldered joint. Brazing and soldering are joining processes that use brazing filler metals and solders that melt below the melting temperatures of the metals being joined. The joint clearance is extremely narrow, so that the filler metal flows into the joint area by capillary action. See Figure 27-6. The difference between brazing and soldering is the melting temperatures of the brazing filler metal and the solder. The brazing filler metal has a melting temperature above 450°C (840°F), and solders have a melting temperature below 450°C (840°F). Braze welding is a process in which a brazing filler metal is used to make joints that have large joint clearances that do not fill by capillary action. Braze welding is most often used to join cast iron.

American Welding Society specifications cover every type of standardized welding filler metal, brazing filler metal, and solder composition. Welding rods and electrodes are classified by the AWS in a series of specifications that cover specific alloy families. For example, A5.1 covers carbon steel covered arc welding electrodes. See Figure 27-7. The American Society of Mechanical Engineers (ASME) issues similar specifications for boilers and pressure vessels, but uses the prefix uppercase letters SF in their designation system. For example, SF-5.1 covers carbon steel arc welding electrodes.

Welding procedures provide all the basic information required to produce a sound joint between two metals. A welding procedure is a set of specific requirements for welding that is described in the various welding codes and broken down into the welding procedure specification (WPS), the procedure qualification record (PQR), and the welder performance qualification (WPQ). See Figure 27-8. The first part of qualifying a weld procedure is to prepare a detailed WPS. All information pertaining to joint design, base metal, weld metal, preheating and post-weld heat treatment, shielding gas, purge gas, electrical characteristics, and welding technique are listed. Then, a sample weld is made using the proposed WPS. Next, the actual parameters used to weld the sample are recorded in the PQR. The sample is then cut up, and tensile and bend tests are performed on the weld (plus impact tests, if required). For weld overlay qualifications, a chemical analysis of the overlay is required, as well as bend tests. If the samples tested are deemed acceptable, the WPS is considered qualified.

A welded joint comprises three metallurgically distinct regions, which are the weld bead, the heat-affected zone, and the base metal. As a result of the heat input of welding, a welded joint consists of three metallurgically distinct regions. The regions include the weld bead, the heat-affected zone, and the base metal. See Figure 27-9.

Characteristic discontinuities in welds include cracking, hydrogen cracking, incomplete penetration, incomplete fusion, porosity, slag inclusions, and undercutting. Discontinuities are invariably caused by improper use of the process employed. Characteristic weld discontinuities include cracking, hydrogen cracking, incomplete penetration, incomplete fusion, porosity, slag inclusions, and undercutting. See Figure 27-10.

When making a structural repair weld, the weld must not be made in the area of the highest stress. Structural weld repair is restoration of a load-bearing structure by welding to meet performance requirements. Examples of structural weld repairs are restoring a broken rotating shaft or rebuilding a storage tank wall that has corroded down to an unacceptable wall thickness. See Figure 27-11.

Surfacing welds are used to build up worn surfaces. Surfacing is a repair welding technique that applies one or more layers of weld metal to restore corroded, worn, or cavitated components in order to extend their useful life. See Figure 27-12. Surfacing weld repair can be used for many applications.

A weld repair plan details all required steps to successfully complete a repair weld. A weld repair plan specifies all the steps required for a certain repair weld. When developing a weld repair plan, many factors must be considered, including determining the necessity of the repair, any applicable repair codes, identifying the base metal, controlling distortion, and the repair welding procedures. See Figure 27-13. Depending on the situation, not every factor may be applicable, but all should be considered. An improper welding procedure or incomplete weld repair plan can increase the chance of weld failure.

The types of forces exerted on adhesive bonded parts affect the bond’s mode of failure. Adhesive bonded parts normally have a high resistance to shear stresses because the entire surface area of the joint contributes to the strength of the bond. See Figure 27-14. However, adhesive bonded parts exhibit relatively low resistance to peeling and cleavage (tearing). Thus, if the load is concentrated at the end of the bond, the joint may start to fail from the loaded end, leading to incremental separation in the body of the joint (“unzipping”).

Adhesives should be carefully chosen based on the materials to be bonded, the application, the environment, and the performance requirements. Adhesives are selected based on the application and service requirements of the bonded part. See Figure 27-15. For example, epoxy phenolic adhesives form strong bonds and have good moisture retention, making them suitable for joining some metals, glass, and phenolic resins. Polyacrylate esters are not suitable for structural joints but may be used as pressure-sensitive tape.