Type 321 stainless steel

Identity and stabilization

Type 321, UNS S32100, is an austenitic stainless steel stabilized with titanium to resist chromium-carbide sensitization. ATI describes its principal advantage as resistance to intergranular corrosion after exposure in the 427–816°C carbide-precipitation range. This is a particular corrosion mechanism, not universal immunity to corrosion or damage from heat. ATI stabilized stainless technical sheet

ATI's product page lists chromium 17–19%, nickel 9–12%, carbon ≤0.08%, nitrogen ≤0.10%, and titanium from 5 × (C + N) to 0.70%, by mass. It lists plate, sheet and strip against ASTM A240, ASME SA-240 and AMS 5510. Those are producer summaries without identified standard editions; use the ordered specification for full acceptance requirements. ATI 321 product data

As an original chemistry-check example, C = 0.050% and N = 0.040% give 5 × (C + N) = 0.450% Ti. This arithmetic checks only the stated ratio: it does not demonstrate complete chemistry compliance, processing condition or corrosion performance.

321 and 321H are distinct orders

ATI identifies the higher-carbon variant as S32109, with carbon 0.04–0.10%, and explicitly notes that the H-grade minimum-stabilizer formula differs. Do not apply the ordinary 321 titanium formula unchanged to a 321H acceptance decision. ATI composition table and footnotes

Outokumpu's current Therma range identifies its 321H/4878 as EN 1.4878 / UNS S32109, with furnace equipment and case-hardening boxes among applications. That producer's high-temperature plate offering does not establish that generic 321 sheet has the same condition or creep performance. Outokumpu Therma 321H

Specify exact grade, product standard and edition, form, thickness, condition, surface finish, test orientation and required inspection documents. Keep producer names separate from generic grade identity. Require explicit dual certification where needed rather than inferring it from overlapping carbon limits. Grain size, heat treatment and other product requirements still need confirmation.

Mechanical properties: preserve the basis

ATI's 321 plate entry gives:

Confirm dimensions, condition and test requirements against the purchase specification before using these values for acceptance. ATI plate specifications

Rolled Alloys separately publishes average elevated-temperature tensile data: at 68°F, tensile strength 93.3 ksi and 0.2% yield strength 36.5 ksi; at 1200°F, 52.3 ksi and 24.5 ksi. An average supplier dataset and a minimum product requirement have different purposes. Rolled Alloys tensile table

Neither set is automatically an allowable design stress. For hot service, determine whether short-term strength, creep strain, rupture life, fatigue or corrosion loss controls the design. Specify the required duration and permissible distortion. A component can remain unbroken yet fail its function through time-dependent sagging or loss of alignment.

Service temperature and corrosion boundaries

Rolled Alloys describes common 321 service in the 1000–1600°F range and discusses a stabilizing treatment for optimum intergranular-corrosion and polythionic-acid stress-corrosion resistance. It does not thereby qualify every component for every atmosphere in that range. Rolled Alloys service guidance

Outokumpu's Therma datasheet gives 850°C for 321H/4878 in Table 2, footnoted as guidance for air under EN 10095, while Table 16 in the same edition gives 800°C. Confirm the applicable guidance with the producer rather than silently resolving this discrepancy into one rating. Neither entry establishes a generic 321 assembly temperature or pressure limit. Outokumpu Therma range datasheet

ATI describes general corrosion resistance as broadly similar to 304 and cautions that prolonged exposure in the carbide-precipitation range can affect resistance in severe media. Stabilization does not remove the need for an exposure assessment. ATI corrosion discussion

Record actual metal temperature, load history, atmosphere, deposits, contaminants and shutdown conditions. Include wet cleaning and condensates where they occur. Choose evidence for the actual medium and fabrication condition; an intergranular-corrosion test does not establish pitting, crevice-corrosion or chloride stress-corrosion performance in every environment.

Welding and finishing

Rolled Alloys recommends AWS ER347 bare wire and E347 covered electrodes for its 321 product. The filler designation therefore need not repeat the base-metal number. Rolled Alloys welding guidance

ATI explains that titanium can be lost during welding and warns against carbon pickup from oils and nitrogen pickup from air. That makes cleanliness and shielding part of maintaining the intended weld metallurgy. ATI welding discussion

Select the procedure for the real joint, consumable, dilution and service requirement. Include attachment and repair welds in the process review. Do not interpret a supplier filler recommendation as qualification of any dissimilar-metal connection. Establish mechanical tests, inspection and any corrosion evidence required by the application.

Define final surface condition and cleanup after thermal processing. Separate a solution anneal, stabilizing treatment and stress relief by their intended purpose; do not prescribe one universal heat cycle for all thicknesses and assemblies. Verify fit and distortion at the final inspection stage, and retain traceability through forming, machining and finishing.

Bounded thermal-expansion example

Rolled Alloys lists a mean expansion coefficient of 10.3 × 10⁻⁶/°F from 70°F to 1000°F. For a freely expanding 1,000 mm member across that interval, original arithmetic gives:

ΔL = 1,000 × 10.3 × 10⁻⁶ × (1000 − 70) = 9.579 mm.

The corresponding coefficient is 18.54 × 10⁻⁶/K and temperature rise 516.67 K, yielding the same result. This is a preliminary free-expansion estimate; it excludes gradients, restraints, joint motion and property scatter. It is not a restrained thermal-stress calculation or a guarantee of final dimensions. Rolled Alloys physical properties and interval footnote

Source editions and related reference

ATI's technical sheet identifies Version 1, 18 February 2014. Rolled Alloys' footer gives ©2011, Bulletin No. 106Use 09/16. The Outokumpu Therma datasheet footer identifies 1565EN:5, November 2022. The producer webpages show no separately verified technical revision date here. Source editions and this draft's review date do not replace current order-specific standards or construction-code requirements.

Grades, tempers and conditions