The main concerns in austenitic stainless steel weld­ing are solidification, liquation, and PIC. Addition­ally, avoiding grain boundary chromium depletion via carbide precipitation (i. e., sensitization or ‘knife­line’ attack) is critical to maintaining in-service corrosion resistance (Chapter 2.09, Properties of Austenitic Steels for Nuclear Reactor Applica­tions). A key factor in solidification cracking resis­tance is proper control of the weld chemistry to form delta ferrite in the initial solid.98-100 Delta ferrite formation breaks up long, linear solidification bound­aries and acts to scavenge tramp elements from the liquid and prevent their concentration at interden­dritic boundaries during terminal solidification.101,102

Delta ferrite levels are typically controlled to ~5-10% by volume to impart solidification cracking resistance but retain the mechanical properties of a face-centered cubic alloy. Specifically, the body — centered cubic ferrite is susceptible to cleavage at low temperatures and to spinodal decomposition of the ferrite into iron-rich (a) and chromium-rich (a0) phases at intermediate temperatures. Control of delta ferrite levels for different alloys is given in handbooks and can be predicted via the experimen­tally based Schaeffler or Delong diagrams, or com­putationally, by multicomponent phase diagrams, as shown in Figure 21.103-107 As a practical example, it is often difficult to weld over fully austenitic metals (e. g., nickel-based alloys) with austenitic filler metals (e. g., 308), as the increased nickel (from dilution) promotes the primary austenite solidification mode.

As with most fusion welds, liquation cracking can be controlled by minimizing solidification segrega­tion (e. g., faster cooling rates and finer, more equiaxed structures) and by using lower heat input.

As discussed earlier, stainless steels can be susceptible to PIC via partially coherent M23C6 carbide precipita­tion. One distinction relative to nickel-based alloys is that cracking may more likely occur on heating or with longer in-service exposures, as carbon may be in solution as-welded, and precipitation kinetics are usually slower than for high-chromium nickel-based alloys.10