Hot cracking in ferritic stainless steels is not as common as with austenitic stainless steels due to the lower coefficient of thermal expansion of the ferritic stainless steels and the greater solubility of sulphur and phosphorusin ferrite. However, excessive amounts of stabilising elements such as Nb and Ti can encourage hot cracking and also reduce hot ductility. Other elements exhibiting a detrimental influence on hot cracking susceptibility are the non-metallics C, N, S, P and Mn. S is three times as detrimental as C and 48 times as detrimental as Mn. It is suggested that the S and P should be maintained at levels similar to those in austenitic stainless steels, to avoid the risk of hot cracking.
Therefore, ferritic stainless steels stabilised with Nb and Ti are more susceptible to hot cracking compared to non stabilised grades. Steels containing Nb exhibit higher propensity to hot cracking than those containing Ti, while steels containing both elements exhibit highest susceptibility. The risk of hot cracking in stabilised grades of ferritic stainless steels is reduced when the total interstitial element content (C + N) is less than 0.02%wt and Nb is lower that 0.2%wt. Titanium content should also be maintained below 0.65%wt, approximately.
The effect of other alloying elements on hot cracking, such as silicon, is more significant in fully austenitic weld metals than ferritic ones. For example, the silicon solubility in austenite at 1300°C is 10 times lower than that in ferrite and therefore, silicon-bearing low melting phases that promote hot cracking do not form during welding of ferritic stainless steels. Ferritic stainless steels with 1%wt Si are not susceptible to hot cracking during welding.