Joe,
You get what you pay for 4 sure...........in a nut shell, I would ask them:
Were are they made
What company supplies the raw tubing
How are they welded.............QA
As RIF........here is some tech info & my tweaking:
18/8 stainless 304 is used for many exhaust system components. For those who don't know 304 can be made in 18/0 upwards the 18 refers to the % of chromium and the 8 to the % of nicol.
The quality/Supplier of that tubing plays a big part despite the % figure above
If you use a magnet, it will be stronger on the lower nickel content 304 as opposed to the higher and hence the lower will degrade quicker.
For the most demanding "cost is no object" applications, Inconel tubing is commonly used. The term has become something of a generic reference to a family of austenitic nickel-chromium-based superalloys which have good strength at extreme temperatures and are resistant to oxidization and corrosion. Because of the excellent high-temperature properties, Inconel can offer increased reliability in header systems, and in certain applications, it is the only material which will do. The high-temperature strength properties can enable weight-reducing designs, since, for a given reliability requirement, Inconel allows the use of much thinner-wall tubing than could be used with other materials. The catch, as usual, is that Inconel tubing is quite expensive.
Certain Inconel alloys retain very high strength at elevated temperatures. One of the favorites for header applications is Inconel-625, a solid-solution alloy containing 58% Nickel, 22% Chromium, 9% Molybdenum, 5% Iron, 3.5% Niobium, 1% Cobalt. It has good weldability using inert-gas-shielded-arc processes, and good formability in the annealed condition, and has a lower thermal expansion rate than the stainless alloys commonly used in exhaust systems. Weldability and formability are both important because of the somewhat limited availability of Inconel tubing sizes, which often makes it necessary to form tubing sections from sheet. As with many metals, the high-temperature strength diminishes as the amount of time the parts are exposed to extreme temperatures increases.
Inconel tubing is nearly essential in high-output turbocharged applications such as Formula-One cars and a few Cup teams. No $$$ spared allows them to be able to use Inconel for their headers, both for reliability and for weight savings.
Some teams are routinely using headers made from (0.020 inch) wall Inconel tubing. To compare, in view of the immense heat load imposed by the exhaust gasses of contemporary Formula One engines, many doubt that a set of stainless headers, even in (0.065 inch), would survive. A BMW F-1 engine at full power, graphically illustrates this demanding environment.
There are several austenitic stainless alloys which are commonly used in exhaust systems. In order of reducing temperature capabilities, they are 347, 321, 316 and 304. In addition, special variations in the basic alloy chemistry (carbon, nickel, titanium and niobium) are available to enhance the high temperature strength of these alloys.
Regarding the use of stainless, In NASCAR Cup racing, the 304 and 321 stainless alloys were used more often than Inconel, depending on the preferences of the various teams. In view of the facts that thinwall Inconel headers are (a) very fragile and readily damaged by inadvertent mishandling, (b) "grotesquely" expensive, and (c) provide almost immeasurable gains on a 3600 pound vehicle, his opinion is that the use of Inconel headers is not prudent stewardship of his resources. For a peek at the magnitude of the costs involved, one fabricator told that a single 1-D "U" bend of 2-inch diameter, 0.032-inch wall Inconel tubing would cost somewhere in the neighborhood of $200, whereas the same bend in 321-stainless would be in the $65 range.
Although titanium has been made to work quite well in exhaust valve applications, the practical temperature limits for titanium alloys suitable for tubing is quoted at about (575 °F), which makes that material suitable for lightweight tailpipes in various applications and in certain motorcycle applications as well.
One might ponder why the same materials used for titanium exhaust valves are not used for exhaust tubing. Apparently, the simple reason is cost vs. benefit, since the estimated cost of thin sheets of Ti-6242 were estimated at over $150 per pound in large-quantity purchases. Add to that the fact that this material lacks the ductility to be readily formed into tubes, plus the fact that there would be problems welding the seams of a rolled tube, and yet more problems forming the welded straight tubes into bends, and it becomes evident that there are more suitable materials for exhaust tubing use.
Burns Stainless LLC - Technical Articles
321 stainless steel is used in the aviation industry ( Aircraft exhaust,Boiler Shells,Cabin heaters,Fire walls, Pressure vessels)for it's higher quality alloys which withstand high heat applications.
Most other manufacturers use lower quality 304 stainless. 304 is not as durable as 321 in handling the constant heat cycling exposure produced during the lifetime of an automobile. 304 is also not as resilient as 321 in withstanding the acids given off by an internal combustion engine. If you are looking for the best a lifetime product can offer, 321 stainless steel is the best choice.............I have seen many Vettes owners have the AIR fittings fail.
Some due to a poor grade of 304...............some due to poor welding/prep..............some due to both!