Q: I am making a replacement fire place grate of 0.75-inch thick mild steel. My problem-is that the grate keeps burning up. I am trying to determine which stainless steel to try next. A friend suggested using either a 310 or 347 stainless steel, or an Inconel material. What do you recommend?

A: Materials can differ in their resistance to oxidation at elevated temperatures. Typical mild steel has a relatively low oxidation resistance at elevated temperatures compared to general Austenitic stainless steels like 304L. Of stainless steels generally available, those with higher chromium contents exhibit better oxidation resistance at elevated temperatures. Materials like 310 stainless have better oxidation resistance than 309 while 309 is better than 304L.

A 347 stainless has comparable resistance to a 304L stainless so there is no reason to use it. The main reason welders chose a 347 stainless is for creep strength — but that won't solve your problem.

Inconel materials yield the best resistance to oxidation at elevated temperatures, but they are expensive.

The least costly solution to consider is a 304L stainless steel first as it is readily available and easily welded with a 308L filler metal. A 309 stainless would be the next logical choice since it also is readily available and easily welded with a 309L filler metal. A 310 would be best of all from a service life perspective, but it has some tendency for hot cracking during welding.

Q: My company manufactures structural components made from 6061-T6 aluminum.We are currently using 5356 wire and are experiencing crater cracking. After grinding out the cracks, we would like to fill in the craters using 4043 wire. Is this an acceptable practice?
A: It is never advisable to mix 4043 and 5356 filler metals. The high silicon content of 4043 filler metal and the high magnesium content of 5356 filler metal, forms a brittle intermetallic compound, Mg2Si. This intermetallic compound segregates at the grain boundaries, reducing weld metal ductility and toughness. It will not cause solidification cracking, but may affect the serviceability of the weldment. The best course of action is to fix whatever caused the crater to crack in the first place, and then make the repair using 5356. Usually cracking occurs because there is not enough filler metal into the weld. Also, check to ensure that the weld itself is slightly convex, never concave.

Q: We are quoting some cast aluminum deck chocks in A357.0- T6 aluminum for use on an all-aluminum ship made of 6082 T6 aluminum plate. Can the chocks be successfully welded to the ship's deck? If so, what filler metal should we use? Can the strength of the chocks and the deck in the weld area be restored to original levels? If so, how?
A: A357-T6 castings can be easily welded to 6083-T6 sheet. For a marine application use 5356 filler. Unfortunately, the strength of the weld and heat affected zone will only be about 25 ksi. It is possible, but impractical, to heat treat after welding to restore the T6 properties. To accomplish this, the component would have to be solution treated at 1,050 degrees F, water quenched, then aged at 400 degrees F. The component also would distort badly during the quenching.