In this bulletin, the individual effects of residual P and S on the behavior of fusion welds in austenitic stainless steel have been differentiated. Knowing how carefully each individual element must be controlled in the final composition has profound impact on the economics of producing the alloy in the first place (e.g., the selection of starting stock, the use of recycled scrap, and subsequent melt chemistry adjustment). This work will serve both the economics and the technology of welding austenitic stainless steels.
For the most widely used austenitic stainless steels solidification hot cracking attributed to the formation of low-melting constituents is, far and away, the predominant weldability problem; with problems from Si, P, and S (whether present as intentionally-added minor elements to enhance machinability or weld penetration, or as residuals from the steel-making process) being the most prevalent. To overcome or offset problems in the fusion zone, the composition of the filler metal (e.g., consumable covered electrode or flux-cored or solid wire) is typically and easily adjusted to result in 5 to 10 volume percent of ferrite (i.e., a Ferrite Number, FN, of 5 to 15) in the solidified weld. The ferrite scavenges the Si, P, and S, tying them up in solid solution so that their detrimental effects are eliminated in the deposit. Dealing with these elements in the heat-affected zone is much more problematic. The options are: 1) controlling the base metal chemistry to keep unnecessary residual elements to an absolute minimum; or 2) developing welding procedures (such as controlled deposition, substructure morphology control, etc.) that cause otherwise troublesome low-melting constituents to be distributed in a more benign way. The former is far more attractive. It precludes problems in field welding or other circumstances where either welder skills or supervision are not certain. On the other hand, driving P and S residuals to extremely low levels is neither technologically simple nor necessary, carries an economic penalty, and can, in at least some cases, lead to unwanted side effects. Good materials science and sound business practice demands that the individual effects of these elements be identified and understood before melting practice is changed.
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| Number of Pages : | 36 |
| Published : | 2003 |
