UNS S32100 Forgings Type 321 seamless rolled rings, flanges, shafts, bars, discs and tube sheets to ASTM A182 Grade F321

Hold an unstabilised austenitic stainless steel such as 304 in the band from about 427 to 816 °C, or cool it slowly through that band, and carbon migrates to the grain boundaries and combines with chromium. The zones beside those carbides end up short of chromium, lose their passive film and become a path for intergranular attack. This is sensitisation, and it shows up in anything welded, stress relieved or run hot.

Type 321 deals with the problem through chemistry rather than by cutting carbon alone. Titanium has a far stronger affinity for carbon than chromium does, so a titanium addition of at least five times the carbon content ties carbon up as stable titanium carbide during heat treatment. Chromium stays in solution and remains available for corrosion protection. That is the whole design intent, and it is why 321 displaces 304 for service in and above the precipitation range.

What this means in a heavy forging

For forgings the benefit applies twice. Heavy open-die sections and large rolled rings cool slowly from forging temperature and spend real time in the sensitisation band before they ever reach service, so stabilisation makes the grade much more forgiving in thick sections than 304 or 304L.

Type 321 hot works readily and takes ring rolling well. Titanium carbides and carbonitrides are stable at forging temperature, so they do not dissolve and re-precipitate the way chromium carbides do, and grain boundary chemistry stays predictable through the forging sequence. The constraints are the usual austenitic ones: a fairly narrow working window, higher flow stress than carbon steel, and a need to finish above the temperature where the structure stops recrystallising so that the final solution anneal returns a uniform grain size right through the section.

Equivalents are nominal cross-references for enquiry purposes. Chemistry limits, testing and acceptance criteria differ between standards, so the specification written on the purchase order governs what we produce and certify. Name the specification you have to satisfy and we will confirm compliance before accepting the order.

Two notes on the titanium requirement

The 0.70 percent ceiling matters as much as the minimum. Excess titanium forms coarse inclusions that spoil surface finish and machinability, which is why the element is capped rather than simply specified as a minimum.

Some editions and readings of A182 set the F321 titanium minimum against carbon plus nitrogen, as 5 × (C + N), rather than against carbon alone. If your design margin turns on that distinction, state the basis on the enquiry and we will aim the heat chemistry to it. For F321H the titanium minimum is commonly 4 × C, again to 0.70 percent maximum.

Watch the silicon limit when comparing documents: A182 forgings allow 1.00 percent maximum while A240 plate is usually held to 0.75 percent maximum. Judging a forging certificate against a plate data sheet is a routine source of false rejections.

These are acceptance minimums, not expected results. A properly solution-annealed 321 forging normally tests well clear of them at room temperature.

Typical strength against temperature

These are published typical values given for orientation. They are short-term tensile figures and say nothing about creep or stress rupture. For sustained load above roughly 540 °C, design to the creep and rupture allowables in the governing code, and consider whether 321H, with its controlled 0.04 to 0.10 percent carbon range, is the correct specification instead. Where the duty is genuinely 25Cr-20Ni oxidation and creep service, the right grade is 310H and not 321.

Published physical values vary between sources and with temperature. Use them for preliminary sizing and heat transfer estimates, and take governing values from your design code or from the certificate for the heat supplied.

Most forging suppliers in China buy billet. This works starts a step earlier, at the furnace. Steel is melted and refined on site, forged or ring rolled, heat treated, machined and tested without leaving the plant, so chemistry can be aimed at the forging it will become rather than accepted as delivered.

Rapid cooling from solution temperature is the control point that decides whether this grade performs. A sluggish quench on a heavy section defeats the reason for buying a stabilised steel, so quench arrangements are matched to section thickness on heavy rings and discs. If the application calls for a stabilising anneal in addition to solution annealing, which is usual for severe polythionic acid or hot caustic service, state it on the enquiry so the thermal cycle is planned and recorded from the start rather than argued about afterwards.

Every item is forged from a single piece of steel melted on site, with no welding and no purchased billet of unknown origin. All work is to customer drawing, size list or sketch.

In practice the 321 against 347 decision usually comes down to fabrication preference and to whatever the end user specification already names, since both deliver stabilised behaviour. The 321 against 321H decision comes down to whether the code case needs creep allowables. The 321 against 310H question should not arise: if the duty is sustained load at high temperature with heavy oxidation, 321 is the wrong grade.

• A182 F321
• A182 F347
• AISI 347H
• AISI 330
• A182 F44
• A182 F51
• A182 F53
• A182 F55
• 904L
• Sanicro 28
• Nitronic 50
• 17-4 PH
• A286
• Incoloy 825

Grade data compiled and reviewed by the metallurgical department, Jiangyin Jiangnan Metal Co., Ltd. Composition and mechanical minimums are quoted from ASTM A182 / A182M Grade F321. Elevated-temperature and physical values are published typical data and are given for guidance only. The specification named on the purchase order and the certificate issued for the heat supplied take precedence over anything on this page. Last reviewed 7 October 2026. A plain-text version of this page is available at UNS-S32100.md.