Heat-affected zone formation

Welding carbon and low-alloy steels can create residual stress and cause undesirable microstructure changes, e.g., formation of martensite, both of which make steel more vulnerable to hydrogen embrittlement. - - Both the fusion zone and heat-affected zone regions of the weld can have microstructures that vary from the base metal. 

As discussed below, the hydrodynamic regime is responsible for the increase in the machining over-cut and for the formation of heat affected zones around the microhole. This is an undesired effect and one should try to avoid machining in this situation. Strategies to reduce this regime are presented in Chapter 7. 

Optimal surface quality is achieved by maximising the chemical etching and at the same time minimising the local heating in order to avoid the formation of heat affected zones. This implies in particular an optimal supply of electrolyte to the machining zone. 

Welds of dissimilar metals result in (1) Warping, buckling, and/or excessive residual stresses caused by different thermal expansion coefQcients. (2) Hard spots in heat-affected zones and fusion zones are caused by formation of hard intennetallic compounds such as carbides. The heating/cooling cycle can harden HAZs and can cause a sensitized HAZ in nonstabilized alloys. 

The composition of type 439L will be foimd in Table 7.5. This alloy is nonhardenable through heat treatment and has excellent ductility and weldability. It resists intergranular attack and formation of martensite in the as-welded, heat-affected zone, but is subject to 885°F (475°C) embrittlement. 

Alloy B-2 has improved resistance to knifeline and heat-affected zone attack. It also resists formation of grain-boimdary precipitates in weld-heat-affected zone. 

Alloy C-22 resists the formation of grain boundary precipitates in the weld-heat-affected zone. Consequently, it is suitable for most chemical process applications in the as-welded condition. 

Although columbium (niobium) stabilized alloy G from formation of chromium-rich carbides in the heat-affected zones of the welds, secondary carbide precipitation still occurred when the primary columbium carbides dissolved at high temperatures, and the increased carbon in the matrix increases the tendency of the alloy to precipitate intermetallic phases. Alloy G-3 has lower carbon (0.015% maximum vs. 0.05% maximum for alloy G) and lower columbium (0.3% maximum vs. 2% for alloy G). The alloy also possesses slightly higher molybdenum (7% vs. 5% for alloy G). 

HASTELLOY alloy B-2—An improved wrought version of HASTELLOY alloy B. Alloy B-2 has the same excellent corrosion resistance as alloy B, but with improved resistance to knife-line and heat-affected zone attack This alloy resists the formation of grain-boundary carbide precipitates in the weld heat-affected zone, thus making it suitable for most chemical process applications in the as-welded condition. Alloy B-2 also has excellent resistance to pitting and stress-corrosion cracking it is particularly well suited for equipment handling hydrogen chloride gas. and hydrochloric, sulfuric, acetic, and phosphoric acids. 

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. 

---