Stainless Steels — Family Index

Tier-3 taxonomy of stainless steel grades. Five metallurgical families, ~50 common grades, with composition, properties, equivalents, and selection rules. Companion to materials-steel (carbon and alloy steels) and steel-grades.

1. At a glance

Stainless steel is iron-base alloy with Cr ≥ 10.5% (IUPAC / EN 10088 threshold). Stainlessness comes from a self-healing passive film of Cr₂O₃ that forms in the presence of oxygen. Below ~10.5% Cr the film is discontinuous and the alloy rusts.

Family	Crystal	Magnetic?	Hardenable by HT?	Typical Cr	Typical Ni	Key trait
Austenitic (200/300)	FCC	No (mostly)	No (cold-work only)	16–26%	6–22%	Toughness, formability, weldability, corrosion
Ferritic (400)	BCC	Yes	No	11–30%	0–1%	Low cost, no Ni, SCC-resistant, magnetic
Martensitic (400)	BCT	Yes	Yes (quench + temper)	11.5–18%	0–2.5%	Hardness, edge retention, wear
Duplex	FCC + BCC ~50/50	Yes	No	19–27%	4–7%	High strength + Cl-SCC resistance
Precipitation-hardening (PH)	Mixed, age-hardened	Yes	Yes (aging 480–620 °C)	12–17%	4–9%	Hi-strength + corrosion, aerospace

Density 7.7–8.0 g/cm³ (austenitic high end due to Ni). Thermal conductivity 15–25 W/m·K (vs. ~50 for carbon steel — stainless is a poor thermal conductor). CTE 10–18 ×10⁻⁶/K (austenitic highest; ferritic lowest, close to carbon steel — relevant for clad/bimetal joining).

2. Designation primer

Multiple overlapping naming systems — agents pick correct equivalent based on context.

• AISI / SAE 3-digit — legacy US, e.g. 304, 316, 410. Still dominant in spec sheets and shop floor talk.
• UNS (Unified Numbering System) — six-character, prefix indicates family. Used in ASTM specs.
  • S = stainless (e.g. S30400 = 304, S31600 = 316, S32205 = duplex 2205)
  • N = Ni-base (e.g. N06625 = Inconel 625, N08825 = Incoloy 825) — beyond stainless, see Tier 3 Ni-superalloys note
  • K = misc. Fe-base (K91560 = HSLA, not stainless)
• EN 10088 (European, replaces DIN 17440) — number 1.4xxx for stainless plus a name code. e.g. 1.4301 = 304, 1.4401 = 316, 1.4404 = 316L, 1.4462 = 2205, 1.4571 = 316Ti, 1.4542 = 17-4PH.
• JIS (Japanese) — SUS prefix mirrors AISI: SUS304, SUS316, SUS410.
• GB (China) — 0Cr18Ni9 (= 304), 00Cr17Ni14Mo2 (= 316L). Older system; newer GB/T 20878 uses S-prefix similar to UNS.

ASTM specs reference UNS: A240 (plate/sheet), A276 (bars), A479 (bars for pressure), A312 (welded pipe), A182 (forged flanges), A351 (castings). EN equivalent: EN 10088-1 (general), -2 (sheet), -3 (bar).

3. Austenitic stainless (200 and 300 series)

FCC structure, stabilized by Ni (and N, Mn, C). The largest family by tonnage (~70% of all stainless produced). Non-magnetic in solution-annealed condition (cold work can induce martensite and weak ferromagnetism). Excellent toughness even at cryogenic temperatures. Not hardenable by heat treatment — strengthened only by cold work or N alloying.

3.1 The grade table

Grade	UNS	EN	Cr	Ni	Mo	C max	YS (MPa)	UTS (MPa)	Elong %	Notes / use
201	S20100	1.4372	16–18	3.5–5.5	—	0.15	310	655	40	Mn-N substituted for Ni (budget). Cookware, transit
202	S20200	1.4373	17–19	4–6	—	0.15	310	620	40	Like 201, slightly higher Ni
301	S30100	1.4310	16–18	6–8	—	0.15	275	760	60	Cold-work strengthens dramatically. Springs, structural
302	S30200	1.4319	17–19	8–10	—	0.15	275	620	55	Older grade, superseded by 304
303	S30300	1.4305	17–19	8–10	—	0.15	240	620	50	Free-machining (+0.15–0.35% S). Screw-machine parts. Reduced corrosion
304	S30400	1.4301	18–20	8–10.5	—	0.08	215	515	55	The workhorse. Sinks, cookware, food eqpt, hardware
304L	S30403	1.4307	18–20	8–12	—	0.030	170	485	55	Low-C for welded service, avoids sensitization
304H	S30409	1.4948	18–20	8–10.5	—	0.04–0.10	205	515	40	Higher C for elevated-T creep (boilers)
305	S30500	1.4303	17–19	10.5–13	—	0.12	205	485	50	High Ni — minimal work hardening. Deep-drawn, spun parts
309 / 309S	S30900 / S30908	1.4828	22–24	12–15	—	0.20 / 0.08	205	515	40	High-T furnace parts to ~1090 °C
310 / 310S	S31000 / S31008	1.4845	24–26	19–22	—	0.25 / 0.08	205	515	40	High-T to ~1150 °C. Heat treat fixtures
316	S31600	1.4401	16–18	10–14	2–3	0.08	205	515	55	Mo for pitting. Marine, chemical, pharma
316L	S31603	1.4404	16–18	10–14	2–3	0.030	170	485	55	Low-C 316. Welded chemical service
316Ti	S31635	1.4571	16–18	10–14	2–3	0.08	205	515	50	Ti-stabilized, EU-favored over 316L for ≥ 425 °C
316H	S31609	1.4401	16–18	10–14	2–3	0.04–0.10	205	515	40	Elevated-T 316 (creep)
316LN	S31653	1.4429	16–18	10–14	2–3	0.030	295	580	40	N-strengthened 316L. Nuclear, cryo
317L	S31703	1.4438	18–20	11–15	3–4	0.030	205	515	40	More Mo than 316. Pulp & paper, FGD scrubbers
317LMN	S31726	1.4439	17–20	13.5–17.5	4–5	0.030	240	550	40	Even higher Mo + N
321	S32100	1.4541	17–19	9–12	—	0.08	205	515	40	Ti-stabilized (Ti ≥ 5×C). Exhaust manifolds, expansion joints
347	S34700	1.4550	17–19	9–13	—	0.08	205	515	40	Nb-stabilized (Nb ≥ 10×C). Aerospace, welded high-T
904L	N08904	1.4539	19–23	23–28	4–5	0.020	220	490	35	Super-austenitic, 1.5% Cu. Sulfuric acid service
254 SMO	S31254	1.4547	19.5–20.5	17.5–18.5	6–6.5	0.020	300	650	35	6Mo super-austenitic + 0.2% N. Seawater, bleach
AL-6XN	N08367	1.4529	20–22	23.5–25.5	6–7	0.030	310	690	30	6Mo + Cu, similar role to 254 SMO
654 SMO	S32654	1.4652	24–25	21–23	7–8	0.020	430	750	40	7Mo super-austenitic. Hottest brines

3.2 Key facts

• L grades carry ≤ 0.030% C to prevent Cr₂₃C₆ precipitation in the 425–815 °C sensitization range during welding HAZ exposure.
• H grades carry slightly higher C (and minimum solution-anneal temperature requirements) for higher creep strength above ~525 °C.
• N alloying (LN grades, super-austenitic) is the modern strengthener — replaces some Ni, raises yield, improves pitting (N appears in PREN with weight 16×).
• Austenitic stainless cannot be hardened by quenching — there is no austenite-to-martensite transformation on cooling for stable grades. Cold work raises strength substantially: 301 quarter-hard → YS 515 MPa; full-hard → YS 965 MPa.

4. Ferritic stainless (400 series, low-Ni)

BCC structure, no austenitizing on heating (some grades have small γ-loop), so not hardenable by quenching. Cheaper than austenitic — no/low Ni — but less formable, with lower toughness and a ductile-to-brittle transition. Magnetic. Better thermal conductivity than 300s (good for heat exchangers). Excellent resistance to chloride stress-corrosion cracking — a major advantage over 304/316.

Grade	UNS	EN	Cr	Ni	Other	YS (MPa)	UTS (MPa)	Elong %	Use
405	S40500	1.4002	11.5–14.5	—	0.1–0.3 Al	170	415	20	Resists γ formation — weldable thick sections
409	S40900	1.4512	10.5–11.75	—	6×C–0.5 Ti	205	380	20	Auto exhaust (huge tonnage). Low-cost, OK to ~700 °C
410S	S41008	1.4000	11.5–13.5	0.6	—	205	415	22	410 with C limited to 0.08 — non-hardenable variant
429	S42900	—	14–16	—	—	205	450	22	Stabilized version, Ni-cat substrate
430	S43000	1.4016	16–18	0.75	—	205	450	22	The everyday ferritic 18Cr — appliances, trim, sink rims
430F	S43020	1.4104	16–18	—	0.15 S	205	480	25	Free-machining variant
434	S43400	1.4113	16–18	—	0.75–1.25 Mo	240	450	20	Auto trim where chloride exposure (de-icing salt)
436	S43600	1.4526	16–18	—	0.75–1.25 Mo, Nb	240	450	22	Better than 434 for corrosion + formability
439	S43035	1.4510	17–19	0.5	Ti-stab	205	415	22	”Ferritic 304 replacement” — replacing 304 in domestic hot water tanks, cookware
441	S44100	1.4509	17.5–19.5	—	Nb + Ti dual-stab	240	430	25	High-T exhaust (manifolds, downpipes), 950 °C
444	S44400	1.4521	17.5–19.5	1	1.75–2.5 Mo, Ti+Nb	275	415	20	”Ferritic 316” — boilers, brackish water
446	S44600	1.4762	23–27	—	—	275	515	20	High-Cr ferritic for ~1100 °C — burner parts, muffles
26-1 (E-Brite)	S44627	1.4575	25–27	—	1 Mo, ELI	275	450	22	Super-ferritic for chloride service
29-4-2	S44800	—	28–30	2–2.5	3.5–4.2 Mo	415	550	20	Super-ferritic for tubing, chlorinated environments

Ferritic limitations: at thicknesses > ~3 mm the heat-affected zone of welds suffers grain coarsening and embrittlement, so heavy-section welded fabrication is rare in standard ferritics. Stabilized variants (439, 441, 444) and ELI (extra-low interstitials — C+N) variants mitigate this.

475 °C embrittlement — high-Cr ferritics (≥ ~17% Cr) lose toughness on long exposure between 400 and 540 °C due to α’ phase decomposition. Avoid using 446 in this range; OK at room T and above ~600 °C.

5. Martensitic stainless

The 400-series grades with enough C to austenitize on heating, then transform to martensite on quenching. Hardenable to HRC 50+ for the high-C grades. Magnetic always. Corrosion resistance lower than 300s (less Cr in solid solution after carbide formation), but acceptable for mild atmospheric / fresh-water service.

Grade	UNS	EN	Cr	C %	YS (MPa, tempered)	UTS (MPa)	Hardness (max)	Use
403	S40300	1.4000	11.5–13	0.15 max	620	760	HRC 38	Turbine blades
410	S41000	1.4006	11.5–13.5	0.15 max	620	760	HRC 41	Valves, pump shafts, fasteners, cutlery handles
414	S41400	—	11.5–13.5	0.15 max	795	825	HRC 45	410 + 1.5–2.5 Ni for toughness
416	S41600	1.4005	12–14	0.15 max	275	515	HRC 35	Free-machining 410 (+S). Bushings, gears
420	S42000	1.4021	12–14	0.15+	1480 (hardened)	1720	HRC 50	Cutlery, surgical, plastic-mold cavities
420F	S42020	1.4029	12–14	0.15+	950	1080	HRC 40	Free-machining 420
420HC	S42000	1.4034	12–14	0.45	—	—	HRC 56	High-C 420 — knife blades (lower-end)
422	S42200	—	11–12.5	0.20–0.25	760	965	HRC 36	Steam-turbine blades, valves to 540 °C
431	S43100	1.4057	15–17	0.20 max	825	965	HRC 38	Fasteners + bearings + valve stems
440A	S44002	1.4109	16–18	0.60–0.75	—	—	HRC 56	Mid-tier knife steel, kitchen cutlery
440B	S44003	1.4112	16–18	0.75–0.95	—	—	HRC 58	Better edge retention than 440A
440C	S44004	1.4125	16–18	0.95–1.20	—	—	HRC 60	Premium 440 — bearings, knives, valve seats
440F	S44020	—	16–18	0.95–1.20	—	—	HRC 58	Free-machining 440C
13-8PH bdry	—	—	—	—	—	—	—	See PH section

Heat treatment — austenitize at 980–1060 °C, oil or air quench, temper. Tempering response forms a “double-tempering” curve due to retained-austenite decomposition. Low-temperature temper (150–370 °C) maximizes hardness; high-temperature temper (565–650 °C) trades hardness for toughness. Never temper martensitic stainless in the 400–550 °C window — temper embrittlement reduces toughness severely. 440C is left as-quenched + low-temper (typically RT → HRC 58–60).

Corrosion of martensitic: Cr-carbide precipitates lock up Cr, reducing matrix Cr below the passive threshold locally. 440C in chloride service is poor. Used for mechanical-property reasons, not corrosion.

6. Precipitation-hardening (PH) stainless

A hybrid family — martensitic, semi-austenitic, or austenitic matrix, strengthened by precipitation of fine intermetallics (Cu-rich, Ni-Al, Ni-Ti) during a controlled aging treatment at 480–620 °C. Combines high strength (UTS up to 1400 MPa) with corrosion resistance better than martensitic, and machinability in the solution-annealed condition before aging. Aerospace bread-and-butter.

Grade	UNS	EN	Type	Cr	Ni	Other	Condition	YS (MPa)	UTS (MPa)	Elong %	Use
17-4PH	S17400	1.4542	Martensitic-PH	15–17.5	3–5	3–5 Cu, Nb	H900	1170	1310	10	The most common PH. Valves, pump shafts, aerospace fittings
17-4PH	S17400	1.4542	Martensitic-PH	15–17.5	3–5	3–5 Cu, Nb	H1025	1000	1070	12	Best toughness/strength balance
17-4PH	S17400	1.4542	Martensitic-PH	15–17.5	3–5	3–5 Cu, Nb	H1150	725	930	16	Lower strength, max toughness, lower SCC risk
15-5PH	S15500	1.4545	Martensitic-PH	14–15.5	3.5–5.5	2.5–4.5 Cu	H900	1170	1310	10	Modified 17-4 — lower δ-ferrite, better transverse properties
13-8PH (Mo)	S13800	1.4534	Martensitic-PH	12.25–13.25	7.5–8.5	2–2.5 Mo, Al	H950	1410	1520	10	Aerospace fasteners, landing gear
17-7PH	S17700	1.4568	Semi-austenitic-PH	16–18	6.5–7.75	0.75–1.5 Al	TH1050 / RH950	1380	1450	6	Springs, diaphragms, aircraft skins
PH 15-7 Mo	S15700	1.4532	Semi-austenitic-PH	14–16	6.5–7.75	2–3 Mo, Al	RH950	1450	1550	6	Stronger 17-7, used in retainers
Custom 450	S45000	1.4507	Martensitic-PH	14–16	5–7	1.25–1.75 Cu, Nb, Mo	H900	1140	1240	10	Higher general corrosion than 17-4
Custom 455	S45500	—	Martensitic-PH	11–12.5	7.5–9.5	1.5–2.5 Cu, 0.8–1.4 Ti, Nb	H900	1620	1720	10	Ultra-high-strength PH
A286	S66286	1.4980	Austenitic-PH (Fe-Ni-Cr)	13.5–16	24–27	1–1.5 Mo, 1.9–2.35 Ti, Al, V	Aged	670	970	25	Hot fasteners to 700 °C — turbine bolts
19-9 DL	S63198	—	Austenitic	18–21	8–11	Mo, W, Ti, Nb	—	540	800	20	Aircraft exhaust, jet engine high-T

Treatment sequence (typical 17-4PH):

1. Solution-anneal at 1040 °C → quench. Material now soft (~ HRC 33, YS ~ 760 MPa). Machinable.
2. Age at chosen H-temperature: H900 = 900 °F (482 °C, 1 h), H1025 = 1025 °F (552 °C, 4 h), H1150 = 1150 °F (621 °C, 4 h), H1150-M (double-temper 1400+1150 °F) for max SCC resistance.
3. Cool in air. Done — no further machining needed except finish grinding.

SCC caution — 17-4 H900 has high strength but is susceptible to chloride and hydrogen SCC. Use H1025 or H1150 for chloride service or stressed marine applications. Avoid in H₂S service (NACE MR0175).

7. Duplex stainless

Roughly equal volume fractions of austenite and ferrite, achieved by chemistry balance (high Cr, lean Ni, +N). Combines the strength of ferritic with the corrosion resistance of austenitic. Magnetic. Yield strength roughly double that of 316 → thinner/lighter components. Excellent chloride SCC resistance (failures common in 316). Tonnages have grown 5× over the past 20 years for oil & gas, chemical, and desalination.

Class	Grade	UNS	EN	Cr	Ni	Mo	N	PREN	YS (MPa)	UTS (MPa)
Lean duplex	2101 (LDX)	S32101	1.4162	21–22	1.35–1.7	0.1–0.8	0.20–0.25	~26	480	680
Lean duplex	2202	S32202	1.4062	22–24	1–2.8	0.45 max	0.18–0.26	~26	450	650
Lean duplex	2304	S32304	1.4362	21.5–24.5	3–5.5	0.05–0.6	0.05–0.2	~26	450	600
Std duplex	2205	S31803 / S32205	1.4462	22–23	4.5–6.5	3–3.5	0.14–0.22	~35	450	655
Std duplex	25Cr	S31200	—	24–26	5.5–7.5	1.2–2.0	0.14–0.2	~33	450	690
Super duplex	2507	S32750	1.4410	24–26	6–8	3–5	0.24–0.32	~43	550	800
Super duplex	DP3W	S39274	—	24–26	6.8–8	2.5–3.5 W 1.5–2.5	0.24–0.32	~42	550	800
Super duplex	Z100 (Z3CN)	S32760	1.4501	24–26	6–8	3–4	0.20–0.30	~41	550	750
Hyper duplex	S32707	S32707	—	26–29	5.5–9.5	4–5	0.30–0.50	~48	700	920
Hyper duplex	S33207	S33207	—	29–33	6–9	3–5	0.40–0.60	~50	750	930

PREN — Pitting Resistance Equivalent Number:

PREN = %Cr + 3.3 × %Mo + 16 × %N

(W-containing grades use 3.3×(%Mo + 0.5×%W) instead.) PREN > 40 is the empirical threshold for chloride seawater service. 2205 at PREN ≈ 35 is OK for brackish but marginal for warm seawater; 2507 at 43 is widely used in offshore service; super-austenitics like 254 SMO (PREN ≈ 43) play the same role.

Sigma-phase embrittlement — duplex grades held between ~600 and 900 °C precipitate the brittle Fe-Cr σ phase. Even short exposures (minutes at 850 °C) drop toughness. Must control welding heat input (typically 0.5–2.5 kJ/mm for 2205), interpass temperature (< 150 °C), and avoid PWHT in this range. Solution-anneal at 1040–1100 °C and water-quench restores duplex structure.

Max service temperature — duplex de-rated above ~280 °C (475 °C embrittlement of the ferrite phase, like high-Cr ferritics) and not used above ~320 °C in ASME B31.3 service.

8. Pitting resistance — PREN ranking

Rank	Grade	Family	Typical PREN
1	S33207 (hyper duplex)	Duplex	~50
2	S32707	Duplex	~48
3	654 SMO	Super-austenitic	~56
4	2507 / S32750	Super duplex	~43
5	254 SMO / S31254	Super-austenitic	~43
6	AL-6XN / N08367	Super-austenitic	~45
7	Z100 / S32760	Super duplex	~41
8	904L / N08904	Super-austenitic	~34
9	317LMN / S31726	Austenitic	~33
10	2205 / S32205	Std duplex	~35
11	317L / S31703	Austenitic	~29
12	316 / S31600	Austenitic	~24
13	316L / S31603	Austenitic	~24
14	444 / S44400	Ferritic	~24
15	2304 / S32304	Lean duplex	~26
16	2101 / S32101	Lean duplex	~26
17	321 / S32100	Austenitic	~18
18	347 / S34700	Austenitic	~18
19	304 / S30400	Austenitic	~18
20	430 / S43000	Ferritic	~17
21	410 / S41000	Martensitic	~12

Selection rule of thumb:

• Fresh water, indoor atmosphere → PREN > 17 (304, 430)
• Mild marine atmosphere → PREN > 24 (316)
• Splash zone / brackish → PREN > 32 (904L, 2205)
• Warm seawater (< 30 °C, oxygenated) → PREN > 40 (2507, 254 SMO)
• Hot chloride brines / chlorinated water → PREN > 45 (hyper duplex, 6/7Mo super-austenitic)

PREN does not capture crevice corrosion susceptibility well — for crevice service add ~5 to the threshold. CPT (Critical Pitting Temperature, ASTM G48 Method E) is a better experimental ranking.

9. Cross-reference / equivalence table

AISI	UNS	EN (number)	EN (name)	JIS	Family
201	S20100	1.4372	X12CrMnNiN17-7-5	SUS201	Austenitic
301	S30100	1.4310	X10CrNi18-8	SUS301	Austenitic
304	S30400	1.4301	X5CrNi18-10	SUS304	Austenitic
304L	S30403	1.4307	X2CrNi18-9	SUS304L	Austenitic
304H	S30409	1.4948	X6CrNi18-10	SUS304H	Austenitic
309S	S30908	1.4833	X6CrNi22-13	SUS309S	Austenitic
310S	S31008	1.4845	X8CrNi25-21	SUS310S	Austenitic
316	S31600	1.4401	X5CrNiMo17-12-2	SUS316	Austenitic
316L	S31603	1.4404	X2CrNiMo17-12-2	SUS316L	Austenitic
316Ti	S31635	1.4571	X6CrNiMoTi17-12-2	SUS316Ti	Austenitic
321	S32100	1.4541	X6CrNiTi18-10	SUS321	Austenitic
347	S34700	1.4550	X6CrNiNb18-10	SUS347	Austenitic
904L	N08904	1.4539	X1NiCrMoCu25-20-5	SUS890L	Super-austenitic
—	S31254	1.4547	X1CrNiMoCuN20-18-7	SUS312L	Super-austenitic
409	S40900	1.4512	X2CrTi12	SUH409	Ferritic
430	S43000	1.4016	X6Cr17	SUS430	Ferritic
439	S43035	1.4510	X3CrTi17	SUS430LX	Ferritic
444	S44400	1.4521	X2CrMoTi18-2	SUS444	Ferritic
446	S44600	1.4762	X10CrAlSi25	SUH446	Ferritic
410	S41000	1.4006	X12Cr13	SUS410	Martensitic
420	S42000	1.4021	X20Cr13	SUS420J1	Martensitic
440C	S44004	1.4125	X105CrMo17	SUS440C	Martensitic
17-4PH	S17400	1.4542	X5CrNiCuNb16-4	SUS630	PH
15-5PH	S15500	1.4545	X5CrNiCu15-5	—	PH
17-7PH	S17700	1.4568	X7CrNiAl17-7	SUS631	Semi-aust. PH
13-8PH	S13800	1.4534	X3CrNiMoAl13-8-2	—	PH
A286	S66286	1.4980	X6NiCrTiMoVB25-15-2	SUH660	Austenitic PH
2205	S32205	1.4462	X2CrNiMoN22-5-3	SUS329J3L	Duplex
2507	S32750	1.4410	X2CrNiMoN25-7-4	SUS327L1	Super duplex
2304	S32304	1.4362	X2CrNiN23-4	SUS323L	Lean duplex

10. Selection heuristics

Quick map from application to grade. Refine with PREN, mechanical, and cost criteria.

• Kitchen sink, domestic cookware, household hardware → 304 (1.4301). Adequate for non-marine atmosphere, food-safe, cheap.
• Marine fittings, boat hardware, coastal architecture → 316/316L for moderate exposure; duplex 2205 for higher strength + better SCC; super-duplex 2507 for splash zone or seawater immersion.
• Chemical plant heat exchanger (chloride-bearing) → 904L, 254 SMO, or duplex 2205/2507 depending on Cl⁻ concentration and temperature.
• Cutlery (knives): kitchen → 420HC or 440A (cheap, easy to sharpen); premium → 440C or proprietary high-Cr-V tool stainless (e.g. 154CM, CPM-S30V — outside AISI codes).
• Aerospace fasteners → A286 for hot zones to ~700 °C, 17-4PH H1025 or H1150-M for moderate-T airframe, Custom 455 for ultra-high-strength but limited corrosion.
• Aerospace landing gear → 13-8PH or maraging steels (outside stainless family).
• Automotive exhaust manifold → 409 (cold end, cheap) or 441 (hot end, > 800 °C). Catalytic converter shell typically 409 or 439.
• Food processing equipment, sanitary tubing (3-A) → 304 for low-Cl; 316L electropolished for pharma / pure-water / WFI lines.
• Pharmaceutical bioreactor, ASME BPE → 316L with Ra < 0.5 µm (mechanical polish) or < 0.4 µm electropolished, with full traceability of mill heat.
• High-Cl pitting environment (FGD, bleach, hot brine) → 254 SMO, AL-6XN, or super-duplex 2507.
• High-temperature furnace muffles, hangers, radiant tubes → 309S (to 1090 °C), 310S (to 1150 °C), 446 (to ~1100 °C). For higher T, move to Ni-base (Inconel 600/601 — out of stainless family).
• Cryogenic service (LNG, LIN, LHe) → 304L, 316L, or 304LN preserve toughness to 4 K (no DBTT in FCC austenitic). Ferritic, martensitic, and duplex grades are unsuitable below room temperature in structural roles.
• Oil & gas downhole (sour service per NACE MR0175) → duplex 2205 or 2507 (with HV ≤ 32 HRC max); martensitic and 17-4PH H900 prohibited or restricted by partial-pressure of H₂S.
• Pump shafts, valve stems, mild corrosion + medium strength → 17-4PH H1025.
• Springs, washers, retainers, diaphragms → 301 cold-rolled hard, 17-7PH RH950, or 302 spring temper.
• Free-machining bushings, screw-machine parts → 303 (austenitic), 416 (martensitic), 430F (ferritic). Note reduced corrosion vs. base grade.

11. Failure modes

Five failure modes dominate stainless service issues. Each maps to specific families.

11.1 Chloride stress-corrosion cracking (Cl-SCC)

300-series austenitic grades crack transgranularly in chloride solutions above ~60 °C under tensile stress. Classic site: insulation on outside of austenitic vessels trapping moisture and chlorides — “CUI” (corrosion under insulation). 316 is slightly worse than 304 for Cl-SCC in the moderate-stress regime due to its higher Mo content shifting the alloy toward less Ni — counterintuitive, since 316 is better for pitting. Ferritic and duplex grades are far more SCC-resistant; this is the main motivation for duplex in chemical-plant piping and heat exchangers.

Mitigation: use duplex (2205, 2507) or ferritic (444, 26-1); avoid chloride-bearing insulation; design to keep external surface dry; PWHT to relieve weld residual stress.

11.2 Sensitization and intergranular corrosion

When austenitic stainless is held in 425–815 °C (most dangerous around 670 °C), Cr₂₃C₆ precipitates at grain boundaries, depleting adjacent matrix of Cr below the passive threshold. The depleted boundaries corrode preferentially when exposed to acidic chloride or oxidizing media. The classic “weld decay” — corrosion bands a few mm from the weld where the HAZ saw sensitization temperatures.

Mitigation:

• L grades (304L, 316L) — keep C ≤ 0.030% so there isn’t enough C to form continuous boundary carbides.
• Stabilized grades (321 = Ti, 347 = Nb, 316Ti) — Ti/Nb tie up C as TiC/NbC at higher T, leaving Cr free.
• Solution-anneal post-weld — heat to 1040 °C, water-quench. Dissolves carbides. Not practical for large fabrications.

ASTM A262 (Practice E “oxalic-acid etch”, or Practice A “Streicher”) tests for sensitization.

11.3 Pitting corrosion

Local breakdown of the passive film in halide environments (chloride mostly). Pits initiate at inclusions (MnS in particular), grow autocatalytically, and can perforate a wall while the bulk surface looks pristine. Worse with temperature, Cl⁻ concentration, and oxidizing species (FeCl₃, ClO⁻).

Mitigation: pick higher PREN; specify low-S grades (≤ 0.005% S); good surface finish (Ra < 0.8 µm); passivate after fabrication (HNO₃ or citric per ASTM A967).

11.4 Crevice corrosion

A geometric variant of pitting — initiates in shielded geometries (gasket faces, lap joints, deposits) where local chemistry becomes acidic and oxygen-depleted. CCT (Critical Crevice Temperature, ASTM G48 Method F) ranks alloys. For seawater service the rule of thumb is CCT ≥ ambient + 5 °C; 316 (CCT ~ -3 °C) fails crevice corrosion in seawater; 2507 (CCT ~ 35–45 °C) is safe.

Mitigation: eliminate crevices by design (full-penetration welds, smooth radii, no lap joints); use higher-PREN alloy; specify Mo-rich gaskets or graphite-foil; cathodic protection.

11.5 Sigma-phase embrittlement

In duplex and high-Cr austenitic grades (≥ 22% Cr), the brittle σ phase (Fe-Cr intermetallic, ~ FeCr stoichiometry) precipitates between 600 and 900 °C. Drops Charpy toughness from 200 J to < 30 J in hours. Critical for welding heat-affected zones — duplex requires strict heat-input control. Solution-anneal at 1050 °C + rapid quench dissolves σ.

Other phases to be wary of: χ phase (similar T range as σ, in Mo-rich grades), α’ (alpha-prime) in high-Cr ferrite at 400–540 °C (“475 °C embrittlement”), and carbonitrides in N-bearing duplex.

12. Cross-references

• materials-steel — parent steel taxonomy (carbon, low-alloy, tool, stainless overview).
• steel-grades — sister Tier-3 covering carbon, HSLA, alloy, and tool steels.
• fasteners-taxonomy — fastener grades and head styles; many call out 18-8, A2/A4 (= 304/316), or 17-4.
• joining-welding — weldability table, filler-metal matching (308/316/2209/2509), preheat/interpass for stainless.
• surface-treatments — solution anneal, age hardening, austenitize-quench-temper schedules.
• surface-treatments — galvanic series, passivation, environmental corrosion modes.

13. Citations

• ASM Handbook Vol. 1 (10th ed.): Properties and Selection: Irons, Steels, and High-Performance Alloys, ASM International, 1990.
• SSINA, Designer Handbook: Stainless Steel for Design Engineers, Specialty Steel Industry of North America.
• Outokumpu, Handbook of Stainless Steel, 2013 edition (free PDF; comprehensive grade and welding data).
• ASTM A240 — sheet/plate; A276 — bars; A479 — bars for pressure-vessel service; A312 — welded austenitic pipe; A182 — forged flanges; A351 — castings; A967 — passivation; A262 — sensitization tests; G48 — pitting and crevice tests.
• ISO 15510 — chemical compositions of stainless steels.
• EN 10088-1/-2/-3 — chemical composition and product forms for stainless steels.
• A. J. Sedriks, Corrosion of Stainless Steels, 2nd ed., Wiley, 1996 — the standard reference for SCC, pitting, crevice, sensitization.
• NACE MR0175 / ISO 15156 — materials for sour-service oil and gas; restricts hardness and grade selection.
• BSSA (British Stainless Steel Association) — https://www.bssa.org.uk/ — quick reference and case studies.
• ASTM — https://www.astm.org/ — current specifications and test methods.