SS 321 stainless steel for heat and welded hot service

Carbon in austenitic stainless steel wants to combine with something when the steel is heated between roughly 425 °C and 850 °C. Given only chromium, it forms chromium carbides at grain boundaries and robs the surrounding metal of corrosion resistance. Titanium has a much stronger pull on carbon than chromium does. With enough titanium present, carbon forms titanium carbides instead, scattered through the grains, and chromium is left alone.

ASTM A240 sets the titanium at no less than five times the carbon plus nitrogen content, with an upper limit of 0.70%. That ratio is what you should check on a 321 test certificate. Titanium that is too low relative to carbon leaves the steel only partly stabilised.

Low carbon alone prevents sensitisation during the short heating of a weld. It does not reliably prevent it over months or years at 500 °C to 800 °C. Stabilised 321 does. It also keeps better strength at temperature than the low-carbon grades, with useful creep and stress-rupture properties.

In air, 321 resists scaling to around 900 °C in intermittent service and about 925 °C continuously, similar to 304. Its advantage is that the material stays sound and resistant to intergranular attack after long periods hot, especially if it later sees moist or acidic conditions during shutdowns.

321 is an excellent grade, but it is a specialist one. A few practical points come up again and again.

• It does not polish well. Titanium carbonitride particles drag out during polishing and leave a streaky finish, so 321 is not used for decorative or mirror-finish work.
• Titanium does not cross the welding arc well. 321 is often welded with 347 filler, which is stabilised with niobium (columbium) instead.
• Its general corrosion resistance at room temperature is about the same as 304, not better. Stabilisation is about grain boundaries, not overall rust resistance.
• It is non-magnetic when annealed, like other 300-series grades.

347 does the same job with niobium in place of titanium and is often preferred for welding consumables and some heavy sections. 304H relies on controlled higher carbon for creep strength and accepts some sensitisation. Pressure equipment designers in power and petrochemical plants choose between them case by case. For sustained high-temperature pressure parts, a 321H variant with controlled carbon is also made. Our power and IBR boiler page discusses the wider material choices for steam service.

The most common 321 parts we see enquiries for are thin-walled. Metallic expansion bellows for hot process and exhaust lines are a classic example: they are made from thin 321 sheet, welded, and then run hot for years while flexing. Exhaust pipes and silencers for diesel generator sets and engines are another. Both see repeated heating and cooling through the sensitising range, plus condensate that can turn acidic when the system cools.

Larger users include refineries and petrochemical plants for hot process lines and heater parts, power plants for auxiliary hot piping, and furnace builders for muffles, baskets and fixtures that run below the range where heat-resisting ferritic grades like SS 446 become necessary. Aircraft exhaust systems were one of the original uses of the grade.

321 welds easily with standard TIG, MIG and stick processes. As mentioned above, 347 filler is common because niobium transfers across the arc better than titanium. After welding, normal cleaning and pickling apply. For components that will run in the sensitising range under corrosive conditions, some specifications call for a stabilising heat treatment, which encourages the titanium to tie up any remaining carbon. Your fabricator or the design code will say whether this is needed.

We supply 321 as seamless pipe and ERW tube, sheet and strip for bellows and exhaust work, round bar, wire, flanges and fittings. Sizes and product standards (for example ASTM A312 TP321 for pipe, A240 for sheet) are confirmed at enquiry, with a test certificate for every lot.