Welding stresses

Cautions. Precise determination of the cause of weld-root cracks can be difficult. Determining correct repair procedures and preventive measures may require metallographic examinations and analyses. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

Although not commonly listed as a weld defect, high-welding stress nevertheless provides an essential ingredient to stress-corrosion cracking and other failures. It differs in an important respect from other types of weld defects in that stresses cannot be visually identified or revealed by conventional nondestructive testing techniques. 

Stress-corrosion cracking can result from high-welding stresses in or immediately adjacent to the weld (Figs. 15.18 and 15.19). 

The cracks illustrated in Fig. 15.18A resulted from weld metal-shrinkage along the long axis of the weld. The cracks in Fig. 15.18B were caused by shrinkage along the short axis of the weld. The cracking illustrated in Fig. 15.18D may be caused by lack of penetration. 

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Overhaul pump replace casing or repair by welding. Stress-relieve after welding as needed. 

The ASME Codes recommend that hydrostatic tests be run at a temperature that is usually above the nil-ductility temperature of the material. This is, in effect, a pressure-temperature treatment of the vessel. When tested in the relatively ductile condition above the nil-ductihty temperature, the material will yield at the tips of cracks and flaws and at points of high residual weld stress. This procedure will... 

Figure 15.19 shows various crack orientations that can occur in connection and attachment welds. Applied stresses from external loading of these components can add to the residual weld stresses, producing still higher stress loads. This can increase the susceptibility to stress-corrosion cracking and can affect orientation and location of crack paths. 

Welding stresses are not visually observable. The common nondestructive technique available for determining residual stresses is x-ray diffraction. 

Dissmilar metal weld stress rupture Stress-corrosion cracking Quality control... 

Equipment found to suffer from cracking included tanks, absorbers, carbon treater drums, skimming drums, and piping. All welds of deaerator vessels in carbon steel should be post-weld stress relieved to minimize cracking and pitting. (Bond)5... 

After the preliminary testing and necessary repairs are completed, it may be necessary to heat-treat the equipment to remove forming and welding stresses, restore corrosion-resistance properties to heat-affected materials, and prevent... 

Residual stresses are due to the manufacturing processes that leave stresses in a material. Welding leaves residual stresses in the metals welded. Stresses associated with welding are further discussed later in this module. 

The bainite that has been formed occupies more space than the original austenite lattice. This elongation of the material causes residual compressive and tensile stresses in the material. Welding stresses can be minimized by using heat sink welding, which results in lower metal temperatures, and by annealing. 

Lightly Loaded Attachments A lightly loaded attachment is one where the weld stress due to mechanical loads is not over 25% of the allowable stress for fillet welds and the temperature difference between the shell and attached member is not more than 14° C (25° F). 

Although the core shroud is under low stress during normal operation from applied loads, the core shroud is highly susceptible to SCC due to tensile residual weld stresses, cold work during fabrication, and high fluence for the middle part of core shroud located in the core region. IGSCC of Type 304 stainless steel core shrouds have been reported in many... 

IGSCC of the shroud support has been reported in several plants and has initiated in Alloy 182 welds due to tensile residual weld stress as discussed below. 

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