Welding of stainless steels

The main problems encountered in welding stainless steels are the same as those seen in heat treatment. The heat of welding (portions of the base metal adjacent to the weld may be heated to 430 to 870 °C, (or 800 to 1600 °F) can cause sensitization and formation of intermetalhc phases, thus 

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Another potential risk of the welding of stainless steel, and presumably the inhalation of hexavalent chromium has a geno-toxic effect on germ cells. The adjusted relative risk for spontaneous abortion for pregnancies associated with exposure to paternal stainless-steel welding was 3.5 (Hjollund et al. 2000)... 

Table la. 11 lists electrodes and welding rods suitable as filler metals for the arc welding of stainless steels. 

Table la.ll Filler metals for arc welding of stainless steels (Ref. 11, 15)... 

Arc Welding of Stainless Steels, in ASM Metals Handbook, Vol. 6 Welding Brazing, Soldering. 

If zinc-rich paints wiU be used on structural steel components, shop priming is preferred. Field application of zinc-rich paints should be done after all welding of stainless steel components has been completed and after insulation has been applied. Otherwise, stainless steel components should be temporarily covered with plastic sheathing to prevent deposition of overspray and splatter. 

Intergranular corrosion at the interface weld of stainless steel (Reclaru et a ., 2001). 

Intergranular corrosion is an especially severe problem in the welding of stainless steels, when it is often termed weld decay. Figure 17.19 shows this type of intergranular corrosion. 

Special attention was given to the welding of stainless steel butt joints, of which there are approximately 2000 in the entire reactor. The inert-gas, nonconsumable-electrode method was u.sed almost entirely. Welds were inspected to considerably higher standards than required by the ASME code. In addition to being subjected to dye-penetrant inspection, every... 

The oxide formed on stainless steel is electrically conductive. Stainless steel has a poor thermal conductivity and should not be used in applications requiring good thermal conductivity. Welding of stainless steel can affect the corrosion resistance in the heat-affected zone (HAZ). This can be controlled by limiting the amount of carbon in the material to minimize formation of chromium carbide and by using special passivation procedures. 

It is used in arc-welding rods for stabilized grades of stainless steel. Thousands of pounds of niobium have been used in advanced air frame systems such as were used in the Gemini space program. The element has superconductive properties superconductive magnets have been... 

Many instances of intergranular stress corrosion cracking (IGSCC) of stainless steel and nickel-based alloys have occurred in the reactor water systems of BWRs. IGSCC, first observed in the recirculation piping systems (21) and later in reactor vessel internal components, has been observed primarily in the weld heat-affected zone of Type 304 stainless steel. 

Liquid-Metal Corrosion Liquid metals can also cause corrosion failures. The most damaging are liqmd metals which penetrate the metal along grain boundaries to cause catastrophic failure. Examples include mercury attack on aluminum alloys and attack of stainless steels by molten zinc or aluminum. A fairly common problem occurs when galvanized-structural-steel attachments are welded to stainless piping or eqmpment. In such cases it is mandatoty to remove the galvanizing completely from the area which will be heated above 260°C (500°F). 

The initi MIC examples studied in the 1970s were weld failures of stainless steel piping that saw only potable drinking water. The numbers of water-exposed systems that have been verified as being affected by MIC are legion. 

Weld overlays of stainless steel or cobalt-based wear-resistant and hard-facing alloys such as Stellite may salvage damaged equipment. In addition, weld overlays incorporated into susceptible zones of new equipment may provide cost-effective resistance to cavitation damage. 

Dents in tubing can induce erosion failures, especially in soft metals such as copper and brass. Welding and improper heat treatment of stainless steel can lead to localized corrosion or cracking through a change in the microstructure, such as sensitization. Another form of defect is the inadvertent substitution of an improper material. 

Most defects can be detected using one or more appropriate nondestructive testing techniques. However, in the absence of routine nondestructive testing inspections, identification of defects in installed equipment is generally limited to those that can be observed visually. Defects such as high residual stresses, microstructural defects such as sensitized welds in stainless steel, and laminations will normally remain undetected. Defects that can be detected visually have the following features ... 

Cautions. Certain types of stainless steel welds are metallurgically designed to form two compositionaUy distinct phases to reduce the... 

Figure 15.9 Cross section of stainless steel weld showing crevice corrosion along a site of incomplete fusion. (Magnification 15x.)...

Note that low carbon or stabilized grades of stainless steel do not possess intrinsically greater corrosion resistance than their unadjusted counterparts. Their sole value in typical cooling water systems results from their resistance to sensitization and potential weld decay that can result when the metals are welded. It is therefore not economically justifiable to specify low carbon or stabilized grades of stainless steel for typical cooling water system components that are not to be welded. 

Cihal, V. and Jezek, J., Corrosion of Stainless Steel in the Immediate Vicinity of the Weld Metal , Brit. Corr. J., 7, 76 (1972)... 

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