June 17, 2014
Q: We are a small company that fabricates and welds various parts and products for numerous companies. One of our customers is supplying us with parts for a particular weldment. All of the individual parts are made from A36 steel except for one, which is made from 12L14 steel. We are using GMAW with an 0.045-in.-dia. E70S-6 electrode with 90 percent argon/10 percent CO2 shielding gas in spray transfer mode. We have been experiencing a high rate of centerline cracking in the welds. Can you recommend a better filler metal or weld process to eliminate these cracking issues?
A: As long as you are not having any weld quality issues with the A36-to-A36 welds, you are using a very robust process and filler metal. However, for the A36-to -2L14 welds, there are several areas of concern.
The 12L14 steel is what is referred to as a free-machining steel. During machining the metal tends to come off in long, curled strands that can actually slow down the machining process because of to the need to remove these strands. The strands tend to be razor-sharp, which is also a safety hazard to the machinist. Therefore, free-machining steels have alloying elements added to them to create weak spots in the metal that allow these strands to break frequently or produce small chips during cutting operations.
These alloying elements typically have higher levels of phosphorus and sulfur or lead, which is not generally added to steel. This particular grade of free-machining steel has lead added to it. This is troublesome for two main reasons.
First, any process that involves melting this grade of steel, such as welding or cutting, should be done with operator safety in mind. When lead melts it can emit lead vapors, and if inhaled, can cause lead poisoning. Operators should wear appropriate ventilated breathing masks. Contact your safety department for recommendations.
Second, during the welding process a certain amount of alloy is recovered from the base metal that ultimately becomes part of the final weld metal in the joint. Based upon the welding information you provided, there is a fair amount of alloy recovery from the base metal, which happens to include some lead. Since lead has the lowest melting point of any of the alloying elements in the weld puddle, it is the last to solidify in the joint, which just so happens to occur in the center of the weld. Lead does not have the mechanical properties necessary to withstand the internal stresses from weld solidification and shrinkage, which is a big reason that you are experiencing cracking.
Typically, this type of free-machining steel is not recommended for applications involving welding. One of the other free-machining steels containing only phosphorus or sulfur may be specified instead, which may have some success in welded applications (although not recommended). In these cases, the recommendation would be a FCAW process with an E70T-5 electrode or a low-hydrogen, basic slag system. A basic slag is less likely to produce a buildup of phosphorus or sulfur in the weld metal, which can help minimize cracking.
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