Diagnose failures of stainless steel flux core welds



Why does a single pass stainless steel weld with FCAW always fail inspection? David Meyer and Rob Koltz take a closer look at what could be behind these failures. Getty Images

Q: We repair welding steel scrapers inside a drying system that is in a humid environment. Our welds fail inspection due to porosity, undercut and cracking of the weld. We weld A514 to A36 with 0.045 ” diameter, all positions, 309L cored with 75% argon / 25% carbon dioxide gas for better wear resistance.

We tried using a carbon steel electrode, but the welds wear out too quickly and we found that the stainless steel performs better. All welds are made flat and measure 3/8 in. Due to extreme time constraints, all welds are done in one pass. What could cause our welds to fail?

A: Based on the information you provided, we see some areas of concern.

Undercut usually occurs due to weld parameters that are far out of specification, improper weld technique, or both. We cannot comment on the welding parameters because we do not know them. The undercut that occurs in the 1F position usually results from excessive handling of the weld pool or too fast or too slow travel speed.

Since welders try to drop a 3/8-in. One pass fillet weld with small diameter flux-cored wire, the possibility of over-handling of the welding gun may be partly to blame. However, this appears to be a use case of the wrong tool for the job rather than a technical glitch, and here’s why.

Porosity is caused by impurities in the weld seam, loss or excess of shielding gas, or excessive moisture absorption in the cored wire. You mentioned that this is repair work inside a dryer for a wet substrate, so if the solder joints are not thoroughly cleaned, this can be the main cause of porosity.

The filler metal you use is all position flux cored wire, and these types of wires have a quick freeze slag system. This is necessary to support the weld pool when vertical welding up or above. The downside of quick-freezing slag is that it will solidify before the puddle of molten solder below. If gases are still being released, they will often be trapped and later appear as porosity or as traces of surface worms. This is magnified when flat welding with a small diameter wire and trying to deposit a large weld in one pass, as in your application.

Weld cracking at the start and stop of your welds can be caused by several factors. It is likely that you will encounter a lack of fusion (LOF) at the root of the weld since you are using a small diameter wire to lay a large weld bead. With high residual weld stresses and LOF at the root, weld cracking is a common occurrence.

For this diameter of wire, you should use two or three passes to complete a 3/8-in. fillet weld, not one. You may find that it is faster to make three passes without defects than to do a single pass weld with defects and then have to make repairs.

However, the other issue that can play a bigger role in weld cracking is the incorrect level of ferrite in the weld, which is often one of the main causes of cracking. 309L wire was developed for welding stainless steel to carbon steel, not carbon steel. The specific solder chemistry of this product also takes into consideration some base metal dilution of the two base materials. Thus, in an application of stainless steel to carbon steel, some alloy pickup from the stainless steel helps to balance the chemistry and produces an acceptable ferrite number. Using a filler metal containing about 50% ferrite, such as 312 or 2209, will eliminate the possibility of cracking due to low ferrite.

The best method to provide superior wear resistance is to weld the joint with a standard carbon or stainless steel electrode, then add a layer of hardfacing electrode. However, you mentioned that you are under a very tight time constraint and that not every multi-pass welding scenario is possible.

Try converting to a larger diameter wire such as 1/16 inch or larger. Staying with a gas shielded flux-cored wire is ideal as it provides better solder cleaning action and better draft protection than an unfrozen wire. However, instead of an all position wire, a wire intended only for the flat, horizontal position would minimize porosity or worm tracking. You also need to change your filler metal from 309L to 312 or 2209.



Leave A Reply