Sensitization characteristics of AISI 304 stainless steel using flux coated TIG welding
Austenitic stainless steels are commonly adopted in petrochemical and nuclear industries because of their good weldability, high corrosion resistance and mechanical strength. One of the disadvantages that limit the application of these stainless steels is their susceptibility to intergranular corrosion because of sensitization when they come across to the high temperature range of 500℃- 900℃ during heat treatment or welding. A lot of research has been carried out to understand the mechanism of sensitization and its effect on properties of stainless steel.The investigation from the literature survey show that the carbon atoms rapidly diffuse to grain boundaries within high temperature range (500℃- 900℃) and unite with chromium (Cr) to form chromium carbides (CrC). The carbon diffuses at a very fast rate to the grain boundaries during the precipitation while the diffusion of Cr occurs a much slower rate, results in Cr depleted zone. The regions adjoining to grain boundary became Cr deficient and get preferentially affected by corrosive media, thus results in IGC. This phenomenon is known as sensitization, leading to this the component gets premature failure.
Various researchers have used many approaches to stop the IGC after welding. Most of their results revealed that it can be only delayed but cannot be prevented. The researchers recommended to use stainless steel grade with low carbon content because CrC precipitation occurs when C precipitates with Cr to form CrC. The stainless steel grades 304Land316L having extra low carbon maximum upto0.03% are developed for this purpose. The addition of stabilizing elements elevates resistance to IGC in austenitic SS. In this treatment, addition of elements is involved, these elements can be titanium (Ti), boron (Bo), niobium (Nb), zirconium (Zr) etc. These elements are involved due to their higher affinity to form carbides than chromium. These elements are called stabilizers because they stabilize the sensitization