TP321 Stainless Pipe

TP321 is a titanium-stabilized 18-10 austenitic stainless pipe. The chromium and nickel levels match 304's neighborhood; the entire difference is titanium, added to grab carbon before chromium can. Left unstabilized, chromium carbides precipitate at grain boundaries in the 425–850 °C window - the sensitization that turns weld HAZs and hot service into intergranular corrosion problems. Titanium binds carbon as titanium carbide first, so the pipe can hold that temperature window for years without sensitizing.

Product Forms

Bar & Rod

Plate & Sheet

Coil & Strip

Pipe & Tube

Fitting: Flange, Tee, Elbow, Reducer etc.

Forging: Ring, Shaft, Circle, Block etc.

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Table 1 - ASTM A312 chemical composition, TP321 (UNS S32100) and TP321H (UNS S32109)

The titanium ratio, worked out - the line every SERP page quotes and none explains. For a heat with 0.05% C and 0.02% N, TP321 requires Ti ≥ 5 × (0.07) = 0.35% - with the 0.70% ceiling giving the melt shop a real working window. Order a heat pinned at the carbon maximum, 0.08% C + 0.03% N, and the floor rises to 5 × 0.11 = 0.55%. The ceiling matters as much as the floor: titanium above ~0.7–0.8% stops dissolving cleanly, forming titanium nitride stringers that show up as surface defects in polished tube and as hard spots at the weld line. A mill certificate sitting comfortably at 0.4–0.6% Ti is exactly what a healthy TP321 heat looks like.

Table 2 - ASTM A312 mechanical requirements, TP321 / TP321H

TP321 vs TP321H - what the H actually buys. The H grade shifts carbon into a 0.04–0.10% band and requires a coarser grain structure (ASTM No. 7 or coarser), because coarse grains and retained carbon raise creep-rupture strength at temperature. In practical terms: below roughly 538 °C, TP321 and TP321H are interchangeable in most design work; when ASME Section I / B31.1 allowable stresses at boiler-tube temperatures govern the wall thickness, TP321H is the grade that carries the higher numbers. If your spec says "321" on a 500 °C line and the wall thickness calculation keeps failing, the missing word is H.

Table 3 - TP321 pipe supply program (typical stock range)

Table 4 - Weight quick math: kg/m = (OD − WT) × WT × 0.02466 (OD, WT in mm)

Weight is the fastest audit of any stainless quote - density is fixed, the formula is public, and a declared weight more than a percent or two off Table 4 says the thickness, not the price, needs a second look.

Where this pipe earns its keep - and its boundaries

TP321 pipe is the standard choice where a plain 304 line would sensitize: furnace and superheater tubing, boiler and heat-recovery lines, exhaust and thermal-oxidizer service, and post-weld equipment that lives in the 425–850 °C window. For buyers piping refinery and utility steam circuits in the Saudi Gulf coast corridor, the deciding table is short: sustained metal temperature above ~425 °C, or welds that will sit there - specify TP321/TP321H; below it, 304L is genuinely enough and we will say so.

The boundaries are equally plain. Chloride-rich duty is not this grade's fight - pitting resistance is 304-class, and hot seawater or brine lines belong to 316L at minimum, duplex or 6Mo beyond. And where creep governs above 538 °C for pressure design, the H grade - or an Incoloy 800H line once temperatures leave the stainless envelope entirely - is the correct product, not thicker 321.

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