Stainless steel in exhaust service is usually discussed as a single choice, but the grades a fabricator reaches for behave differently from one another in ways that matter over years rather than months. 304 is the familiar austenitic baseline. 321 and 347 are austenitic grades that have been stabilised to tolerate heat better. 439 is a ferritic grade that contains no nickel at all. Setting them side by side explains why two systems that both look like stainless can end up in very different condition.
The family name hides a real split. Austenitic grades are non-magnetic in the annealed condition, work harden readily when formed, and depend on nickel and chromium to hold their structure. Ferritic grades have a different crystal structure, contain no nickel, and behave more like a plain steel that happens to resist rust well. Both can carry the word stainless on the certificate while behaving nothing alike on the bench.
Exhaust service is a demanding test for any of them. The metal sees repeated heating and cooling, condensate that forms on cool-down, vibration transmitted from the engine and the road, and in many markets salt thrown up from the surface. A grade that survives all of that in one application can disappoint in another, and the reason is usually metallurgical rather than a matter of thickness.
304 is the grade most exhaust work starts from, and it earns that position. It forms and bends predictably, it welds cleanly with sound technique, and it resists the ordinary corrosion of a road car well enough to last a long service life. Tubing, muffler bodies, midpipes and tips are all built from it in quantity, and a fabricator who knows 304 knows the reference point against which the others are judged.
Its weakness appears when it is held in the heat range where chromium carbides can form along the grain boundaries. In that condition the material can become sensitised: the boundaries lose the chromium that was protecting them, and corrosion then attacks the metal from the inside out, along paths that are invisible from the outside until the part is well into failure. The effect is time and heat dependent, which is exactly why it is easy to ignore.
Our own T304 work includes the twin-tube cat-back built for the BMW S58, and the Z-style and equal-length midpipes for the S55 and S58 engines. Those parts show what 304 does well: repeatable forming, reliable welding and a consistent surface that takes a finish without drama.
321 and 347 take the 304 idea and add an element that ties up the carbon before it can cause trouble. In 321 that role is played by titanium; in 347 by niobium. Because the carbon is locked into stable particles rather than left free to migrate to the grain boundaries, the metal keeps its corrosion resistance through the heat cycles that would sensitise a plain austenitic grade.
The practical result is a grade that holds up better in the hottest parts of an exhaust and in applications that see long, steady heat rather than short trips. That benefit is bought at the counter: these grades cost more and are less common in stock, so they tend to be specified where the duty genuinely justifies them rather than as a blanket choice.
A stabilised grade is not a licence to abuse the material. Forming, welding and cleanliness still decide the outcome, and a badly shielded weld in 321 will fail before a well-made joint in 304 ever does. The grade widens the margin; it does not remove the requirement for care.
439 is the grade that carries no nickel. Its resistance to rust comes from its chromium content and from the way it is processed, not from an austenitic structure. Formability is good, and because it contains no nickel it is a sensible choice where cost pressure is real and the duty is moderate.
What changes on the bench is the way it behaves when welded and the way it behaves under mechanical load. Ferritic grades are generally less tolerant of being worked hard, and they have their own rules about heat input and about distortion. A fabricator moving from 304 to 439 for the first time will notice the difference in the first bend and the first joint.
The single most useful thing to understand about stainless in exhaust service is that the corrosion problem is usually not the outside surface. Condensate forms on the inside of a system when it cools, and those deposits sit in the low points, in the crevices behind hangers, and under clamps that never quite dry out. Given chlorides from road salt or from the environment, that is where pitting and crevice attack begin.
Welding adds a second front. Where a joint has been made without adequate shielding, the back side of the weld can come out oxidised and rough. That rough, discoloured surface has lost much of its protective oxide and gives corrosion somewhere to start, right at the point where stress also concentrates.
The third front is mechanical. Corrosion and fatigue work together: a pit becomes a stress raiser, the stress raiser becomes a crack, and the crack grows with each heat cycle. This is why two identical-looking systems, one welded with good gas coverage and one not, can age so differently.
All of these grades are TIG welded in our shop, and the grade decides how much attention the joint demands. 304 is willing and predictable. 439 moves differently and distorts more readily, so the sequence and the clamping change. The stabilised grades behave much like 304 in the arc but are specified precisely because the finished joint must keep working in a hot location.
The constant across all of them is gas coverage. On an exhaust tube the inside of the weld matters as much as the outside, and back-purge practice is not optional where the part will live in a wet, hot environment. The grade sets the expectation; the technique delivers it.
A mixed build is normal and sensible. The hot end of a system, the sections closest to the engine that see the highest and most sustained heat, is the natural home for a stabilised grade. The cooler sections further back can be made in 304 or in a ferritic grade without giving up much.
The point of specifying per section is to spend the material budget where the duty is worst. Specifying a stabilised grade along a tailpipe that never gets properly warm is cost without benefit, and specifying a plain grade in the hottest location is a compromise the customer may discover years later.
The questions below cover the ground that actually changes the answer. Asking them early avoids a system being built in the wrong grade for its duty.
Once those are settled, the grade list for the job is usually obvious, and the conversation moves on to geometry.
If the system is a general road exhaust and the budget is ordinary, 304 will do the work and last well. If the hot end will see sustained heat, a stabilised grade is the safer answer. If the part is a cost-sensitive section that never runs very hot, 439 deserves consideration. The choice is not a matter of prestige; it is a matter of matching the metal to the conditions.
You can see the stainless work we already produce on our stainless steel exhaust system pages, and the titanium alternative under the titanium exhaust system range. Tube stock sits under exhaust pipe, fabricated parts under titanium fab components, and muffler work under titanium exhaust muffler. The full material list is on the shop by material page.
Whichever grade is chosen, the material certificates travel with the job, and the shop works from them in the same way it works from a drawing. If a question about grade arises on a specific part, it can be put to us directly at tigindustry@fupower.com.
