Erosion of stainless steel

Have we any metallurgists out there?

I used a scrap of stainless steel (most probably 304 grade) to make a vaporiser tube for a pilot light. For various reasons the vaporiser sits low in the flame where it gets hot but not too hot (about 650C from the brown/red colour).

After about 40-50 hours use pits appeared in the surface of the tube - looking like the holes that are found after grinding down a bad mig weld. There is no overall scaling or any other obvious form of corrosion.

I have replaced the tube with a new one made from traceable grade 316 and will see what that does but can any one explain what happened to the first one?

One school of thought is that 304 is subject to attack from sulphur in a reducing atmosphere (the pilot burns petrol so sulphur is not unexpected). Some references suggest sulphur can burn the nickel out of some grades of stainless (is this the problem that BMW had with their nicasil cylinder liners?).

The position of the vaporiser in the flame is in the area which is usually described as reducing, also hot unburned gases must be flowing around the underside. Pits are both top and bottom.

Last question - as Stanley did not have stainless steel what did they use for vaporisers?

Mike





Edited 1 times. Last edit at 06/07/05 12:38PM by Mike Clark.

Hi MIke
I understand that 400C is considered the upper limit for vapourising units using oil derivatives as the oil will break down into its constituent parts and carbonize. therefore you are running too darn'd hot
Hope this is helpfull
regards Les

Les,

The vaporiser is a straight line job not a loop so the fuel goes through without hitting the hot spots which seeem unavaoidable with loops. The hottest bit of the tube can get dull red on the outside but I suspect the fuel which has a tremendous cooling effect on the vaporiser does not get anything like as hot. The tube is 8mm o/d x 5mm bore. Proof of the pudding is as King Alf found in the eating for it does not carbon up in practice. Since my last post I have been trying a mild steel tube - made from seamless hydraulic pipe and this works just as well although with the increased thermal conductivity it needs to be a bit shorter than the stainless version. I turn up the jet (needle control) so the vaporiser doesn't overheat.

Mike