We are currently considering upping our material thickness to please the crowds. Although it offers no advantage in terms of crack prevention.
Cracks only form at stress raisers or at 'as manufactured' defects. If a weld is imperfection free and designed correctly it will not crack. For a weld to just break the manifold must be loaded above its design ability. The combination of thermal loading and gas pressure on a manifold/stainless steel is not enough to do this. Working at a where I do we have stainless pipework exposed to unbelievable service conditions. Several hundred degrees, containing water flows of around 0.5Tonnes/second, and pressures of 150bar, thats the ability of stainless!
By using a thicker pipe the weld depth will have to increase. Going from 1mm to 3mm pipe for example gives triple the weld depth. Assuming that the aspect ratio of the weld (width to height) remains constant (probalby 1.5ish) then the width of weld will increase with increasing pipe thickness. This increase in weld width, together with increased heat application will give a larger HAZ (Heat affected Zone). This HAZ is the area in which the grain structures of the material has been changed, and it is beneficial to minimise this area. The smaller the weld the better, thats why laser welding is so popular typical aspect ratios can be around 10-12, that means for a 1mm thick weld the width is 1/10mm!
By using a thin material we would use a Tig welded root bead. The root bead is the first weld in an weld procedure, and it would be very uncommon (almost never) that a weld be made up of only one run/bead. The root bead would be autogenous i.e. no filler rod would be used, and only the parent metal would be used to fuse together and form the weld pool. This method has three main advantages.
1. In order to obtain an imperfection free high quality weld the filler rods have to be meticulousely clean. When welding the root bead if using a filler rod, surface impurities on the rod that are dragged into the weld may cause an imperfection. By using only the parent metal for the root bead you are insured that the weld is of high quality as the two mating weld preps fuse together to form the root bead.
2. You can get no better filler material than the parent. By using the actual manifold material to weld you are insuring that the root bead is perfectly matched mettalurgically. This alleviates any potential residual stresses in the root bead that may be present if using incorrectly matched homogenous welding techniques.
3. The inner surface of the weld will be much higher quality using thin walled than thicker walled. As the weld has a much smaller width and depth the inside of the weld will be smoother and much tighter packed. This is difficult to expalin without drawing aids.
The process would be completed by running a final bead of filler rod over the top. This further adds to the strength of the weld by stitching the two ends of pipe together and gives a more aesthetically pleasing weld finish.
All welds will also be argon purged. This further inhances the inside finish of the pipe and insures proper penetration of the weld and zero oxidation in the weld pool. It is just as importnat to keep out gaseous impurities as solid ones.
I'm afraid thats my best stab at explaining it. I'm not the welder. I have CC'd him on this e-mail so I am sure he will correct me where I've slipped up. Once the website is up and running I will give you the address and you can let the GTiR lads know.
Another point worth adding. The welder has previousely worked at PPR at silverstone, manufacturing exhausts for cars ranging from Porsche, NSX, to Formula 3, all using 16swg stainless with no cracks or returns.
We may also consider using mild steel as a secondary range for heavily tuned cars as these are much easier to make and extremely durable in thermal fatigue situations.
any questions please ask, and il pass them on for you
I see this on my own manifold.
