ASTM A193 Grade B8 / B8M austenitic stainless bolting (304, 316, A2-70, A4-70)

ASTM A193 Grades B8 and B8M are austenitic stainless steel bolting made from AISI 304 (B8) and AISI 316 (B8M). Class 1 is solution treated at 75 ksi tensile, and Class 2 is strain hardened for higher strength that falls as diameter increases.

Names by standard

What it is called in each region

Region	System	Designation	Norma	Match
USA	ASTM A193/A193M	Grade B8 (Class 1, 1A, 2)304-type austenitic stainless bolting. Class 1A is solution treated in the finished condition; Class 2 is strain hardened.	ASTM A193/A193M	Same material
	ASTM A193/A193M	Grade B8M (Class 1, 1A, 2)316-type stainless bolting with 2.00–3.00% molybdenum.	ASTM A193/A193M	Same material
	AISI/SAE (steel type)	304 (UNS S30400) for B8; 316 (UNS S31600) for B8MA193 lists the steel types by AISI and UNS number. Steel types are owned by the 304 and 316 pages.	ASTM A193/A193M	Equivalent
UK	BS EN ISO 3506-1	A2-70 / A4-70BSI adopts ISO 3506. A2 (304 type) and A4 (316 type) class 70 have lower strength requirements than B8 Class 2 and are a different specification.	BS EN ISO 3506-1	Closest equivalent
EU	EN 10269 name	X5CrNi18-10 (1.4301) for B8; X5CrNiMo17-12-2 (1.4401) for B8MEN 10269 gives solution-annealed and cold-worked (+C700) conditions with elevated and low-temperature properties. Not the same ordering specification as A193.	EN 10269	Closest equivalent
	EN ISO 3506-1	A2-70 / A4-70ISO property classes for stainless bolts, screws and studs: 700 MPa tensile and 450 MPa proof stress for class 70.	EN ISO 3506-1	Closest equivalent
India	IS 1367 (Part 14)	Part 14 (A2 / A4 property classes)Part 14 of the Indian fastener standard is identical with ISO 3506 (the nuts section was read). Indian projects often name B8 or B8M by the ASTM designation.	IS 1367 (Part 14):2023	Closest equivalent
Japan	JIS B 1054-1	A2-70 / A4-70Japanese stainless fastener properties standard, listed against ISO 3506-1.	JIS B 1054-1:2013	Closest equivalent
	JIS G 4303	SUS304 / SUS316Stainless bar steels from which stainless bolts are made. The bolt-property requirements come from another standard.	JIS G 4303 (bar)	Closest equivalent

On equivalence

B8 and B8M are ordering specifications for bolting made from 304 and 316 type steels. The steels themselves are owned by the 304 and 316 pages, so the A193 names are listed as "same" here and the steel names as "equivalent".
ISO 3506 A2-70 and A4-70 are the common metric stainless classes. They correspond to 304-type and 316-type steels but set their own strength requirement: 700 MPa tensile and 450 MPa proof stress. They are close in strength to B8 Class 2 at 1 to 1¼ in, not to Class 1 (515 MPa). Do not use one for the other without checking.
A2 and A4 do not match B8 and B8M exactly. A2 is a 304-type steel and A4 a 316-type steel, but sources differ on which steel types sit inside A2 and A4. Check the steel grade on the certificate.
EN 10269 X5CrNi18-10 and X5CrNiMo17-12-2 are 1.4301 and 1.4401 as bar for fasteners. Their solution-annealed yield (190 and 200 MPa minimum) is slightly below A193 Class 1 (205 MPa).
India: IS 1367 (Part 14) follows ISO 3506. We read the nuts section, which BIS records as identical with ISO 3506-2:2020. BIS records the bolts section (Sec 1) as identical with ISO 3506-1:2009 in its 2018 edition, since superseded by a 2023 edition; we did not read its text.
Japan: JIS B 1054-1:2013 is the Japanese counterpart of ISO 3506-1. SUS304 and SUS316 (JIS G 4303) are the bar steels.

Chemical composition

Composition limits, mass %

Element	B8 (304)USA · ASTM A193/A193M, Grade B8 (heat analysis)	B8M (316)USA · ASTM A193/A193M, Grade B8M (heat analysis)	X5CrNi18-10 (1.4301)EU · EN 10269 (cast analysis)	X5CrNiMo17-12-2 (1.4401)EU · EN 10269 (cast analysis)
C	≤0.08	≤0.08	≤0.07	≤0.07
Si	≤1.00	≤1.00	≤1.00	≤1.00
Mn	≤2.00	≤2.00	≤2.00	≤2.00
P	≤0.045	≤0.045	≤0.045	≤0.045
S	≤0.030	≤0.030	≤0.015	≤0.015
Cr	18.0–20.0	16.0–18.0	17.0–19.5	16.5–18.5
Ni	8.0–11.0	10.0–14.0	8.0–10.5	10.0–13.0
Mo	—	2.00–3.00	—	2.00–2.50
N	—	—	≤0.10	≤0.10

Mechanical properties

Strength and hardness

Condition	Tensile strength	Yield strength	Elongation	Hardness
Class 1 and 1D (carbide solution treated), B8 and B8MSpecified minimums, all diameters. Elongation 30%, reduction of area 50%.ASTM A193/A193M, via Torqbolt	≥515 MPa≥75 ksi	≥205 MPa≥30 ksi	≥30 %	≤223 HBW / ≤96 HRB
Class 1A (solution treated in the finished condition), B8A and B8MASpecified minimums, all diameters. Elongation 30%, reduction of area 50%.ASTM A193/A193M, via Torqbolt	≥515 MPa≥75 ksi	≥205 MPa≥30 ksi	≥30 %	≤192 HBW / ≤90 HRB
B8 Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter up to ¾ in (up to M20). Elongation 12%, reduction of area 35%.ASTM A193/A193M, via Torqbolt	≥860 MPa≥125 ksi	≥690 MPa≥100 ksi	≥12 %	≤321 HBW / ≤35 HRC
B8 Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over ¾ to 1 in (over M20 to M24). Elongation 15%, reduction of area 35%.ASTM A193/A193M, via Torqbolt	≥795 MPa≥115 ksi	≥550 MPa≥80 ksi	≥15 %	≤321 HBW / ≤35 HRC
B8 Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over 1 to 1¼ in (over M24 to M30). Elongation 20%, reduction of area 35%.ASTM A193/A193M, via Torqbolt	≥725 MPa≥105 ksi	≥450 MPa≥65 ksi	≥20 %	≤321 HBW / ≤35 HRC
B8 Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over 1¼ to 1½ in (over M30 to M36). Elongation 28%, reduction of area 45%.ASTM A193/A193M, via Torqbolt	≥690 MPa≥100 ksi	≥345 MPa≥50 ksi	≥28 %	≤321 HBW / ≤35 HRC
B8M Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter up to ¾ in (up to M20). Elongation 15%, reduction of area 45%.ASTM A193/A193M, via Torqbolt	≥760 MPa≥110 ksi	≥655 MPa≥95 ksi	≥15 %	≤321 HBW / ≤35 HRC
B8M Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over ¾ to 1 in (over M20 to M24). Elongation 20%, reduction of area 45%.ASTM A193/A193M, via Torqbolt	≥690 MPa≥100 ksi	≥550 MPa≥80 ksi	≥20 %	≤321 HBW / ≤35 HRC
B8M Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over 1 to 1¼ in (over M24 to M30). Elongation 25%, reduction of area 45%.ASTM A193/A193M, via Torqbolt	≥655 MPa≥95 ksi	≥450 MPa≥65 ksi	≥25 %	≤321 HBW / ≤35 HRC
B8M Class 2 (carbide solution treated and strain hardened)Specified minimums, diameter over 1¼ to 1½ in (over M30 to M36). Elongation 30%, reduction of area 45%.ASTM A193/A193M, via Torqbolt	≥620 MPa≥90 ksi	≥345 MPa≥50 ksi	≥30 %	≤321 HBW / ≤35 HRC
EN 10269 X5CrNi18-10, solution annealed (+AT)Specified, diameter up to 160 mm; yield and elongation are minimums. Impact KV2 at least 100 J at 20 °C. Comparison only.EN 10269, via Torqbolt	500–700 MPa73–102 ksi	≥190 MPa≥28 ksi	≥45 %	—
EN 10269 X5CrNi18-10, cold worked (+C700)Specified, diameter up to 35 mm; yield and elongation are minimums. Impact KV2 at least 80 J at 20 °C. Comparison only.EN 10269, via Torqbolt	700–850 MPa102–123 ksi	≥350 MPa≥51 ksi	≥20 %	—
EN 10269 X5CrNiMo17-12-2, solution annealed (+AT)Specified, diameter up to 160 mm; yield and elongation are minimums. Impact KV2 at least 100 J at 20 °C. Comparison only.EN 10269, via Torqbolt	500–700 MPa73–102 ksi	≥200 MPa≥29 ksi	≥40 %	—
ISO 3506 property class 70 (A2-70, A4-70)Specified minimums for bolts, screws and studs. Class 80 is 800 MPa tensile and 600 MPa proof stress. Comparison only.BS EN ISO 3506, via SCI Advisory Desk AD 342	≥700 MPa≥102 ksi	≥450 MPa≥65 ksi	—	—

In practice

Behaviour, heat treatment and uses

Characteristics

B8 is a 304-type steel (18–20% chromium, 8–11% nickel). B8M is a 316-type steel with 2.00–3.00% molybdenum, which improves resistance to chlorides and pitting. See the 304 and 316 pages for the steels.
Class 1 bolts are carbide solution treated, which keeps chromium carbides dissolved to maintain corrosion resistance. Strength is low at 75 ksi tensile and 30 ksi yield, and it does not depend on diameter.
Class 2 bolts are solution treated and then strain hardened, usually by cold drawing the bar. Strength is higher but depends on diameter: B8 Class 2 is 125 ksi tensile up to ¾ in and 100 ksi at 1¼ to 1½ in, because cold work reaches less of the larger bar.
Class 2 sizes are limited in A193 to 1½ in. A Class 2 stud is marked with SH after the grade symbol (B8SH, B8MSH) to distinguish it from Class 1. Older practice, still written into A320, was a line under the grade mark.
Austenitic bolting keeps ductility at low temperature. ASTM A320 includes B8 and B8M for low-temperature service without an impact test down to about −325 °F (−198 °C); below that A320 requires impact testing for them too. Check the design code for allowable stress at temperature.
Stainless bolts gall on stainless nuts. Lubricant and a different nut grade (A194 Grade 8 for B8, 8M for B8M) are normally specified.
ASTM A193 warns purchasers to read its strain-hardening appendix carefully when Class 2 is ordered, because the strength depends on cold work and diameter.

Heat treatment

Class 1 and 1D: carbide solution treated. Class 1A: carbide solution treated in the finished condition, so threads and finished shape are treated together.
Class 2: carbide solution treated and then strain hardened. Cold drawing gives the strength, and the smaller the diameter, the larger the effect.

Usos habituales

Flanges, valves and bolted joints exposed to corrosion in chemical, food and marine plant (B8M where chlorides are present)
Cryogenic and low-temperature joints in stainless piping
Class 2 bolts where more strength is needed than Class 1 without changing the steel
Paired with A194 Grade 8 (B8) or Grade 8M (B8M) nuts

Bolting steels

Similar grades compared

Grado	C %	Mn %	Cr %	Ni %	Tensile, first listed condition
A193 B8 / B8M	≤0.08	≤2.00	18.0–20.0	8.0–11.0	≥515 MPa
A193 B7	0.37–0.49	0.65–1.10	0.75–1.20	—	≥860 MPa
A193 B16	0.36–0.47	0.45–0.70	0.80–1.15	—	≥860 MPa
A320 L7	0.38–0.48	0.75–1.00	0.80–1.10	—	≥860 MPa
A194 2H	≥0.40	≤1.00	0.80–1.10	≤0.40	500–650 MPa

Composition from the US standard where one exists, otherwise the first standard listed on each grade's page. Strength conditions differ between grades; open a grade for its full table.

Preguntas resueltas

Common questions

What is the difference between A193 B8 and B8M?

B8 is made from 304 stainless steel and B8M from 316, which adds 2.00–3.00% molybdenum. B8M resists chlorides and pitting better, so it is the usual choice in marine and chemical service. Both come in Class 1 (solution treated) and Class 2 (strain hardened).

What is the difference between Class 1 and Class 2 for A193 B8?

Class 1 is carbide solution treated only, with 75 ksi minimum tensile and 30 ksi yield at all sizes. Class 2 is strain hardened as well, reaching 125 ksi tensile up to ¾ in for B8, but strength falls as diameter increases and the class stops at 1½ in. Class 2 is marked with SH after the grade symbol (for example B8SH); older practice was a line under the symbol.

Is A193 B8M the same as A4-70?

No. Both are 316-type stainless bolting, but A4-70 is an ISO 3506 property class (700 MPa tensile, 450 MPa proof stress) and B8M is an ASTM ordering grade. B8M Class 1 is 515 MPa tensile, and B8M Class 2 depends on diameter. They are close in use but not interchangeable on a drawing.

What nut should be used with A193 B8M?

A194 Grade 8M nuts are the usual match for B8M, and Grade 8 for B8. Stainless on stainless threads can gall, so lubricant is normally used.

Can B8 be used at low temperature?

Yes. Austenitic stainless steel keeps its toughness at low temperature, which is why ASTM A320 includes B8 and B8M for cryogenic service and does not require an impact test for them down to about −325 °F (−198 °C); below that A320 requires impact testing for them too. Class 2 values still follow the diameter rules.

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References

Sources

ASTM International, A193/A193M scope and abstract (alloy-steel and stainless steel bolting for high temperature or high pressure service) — Scope, grade families, nuts and A320 cross-reference; the full tables are paywalled.
Torqbolt, ASTM A193/A193M grades B7, B7M, B16, B8, B8M — Composition, strength by diameter band, minimum tempering temperature and supplementary requirements, transcribed from A193.
Hex Technology, The Stud Guide: B7s, B16s and other common bolt materials — Typical service ranges, nut pairings and grade-selection notes from a fastener maker.
Torqbolt, EN 10269 bolting steels overview — Grades covered by EN 10269 and the compositions and properties of X5CrNi18-10, X5CrNiMo17-12-2, C35E and 21CrMoV5-7.
SCI Advisory Desk AD 342, Stainless steel fasteners — BS EN ISO 3506 grades A2 and A4, property classes 50/70/80 and their strengths.
Projectmaterials, What is ASTM A320 — L7, L7M and L43 summary, impact energy, nut grades and typical LNG and ethylene uses.
Bureau of Indian Standards, IS 1367 (Part 14/Sec 2):2023, identical with ISO 3506-2:2020 — Indian stainless fastener standard follows ISO 3506; this record is the nuts section.
DIN Media, JIS B 1054-1:2013 Mechanical properties of corrosion-resistant stainless steel fasteners, Part 1 — Title, year and ISO 3506-1 counterpart of the Japanese stainless fastener standard.
Standards.ie (NSAI shop), EN 10269:2013 Steels and nickel alloys for fasteners with specified elevated and/or low temperature properties — Title, scope and status of the CEN standard.
ASTM A193/A193M-10a, hosted copy of the full standard — B8 and B8M composition, Class 1, 1A and 2 strength by diameter in inch and metric, Table 5 marking (Class 2 = SH suffix). 2010 edition, read by the independent checker.
Allfasteners (Australia), B8 and B8M stud bolt kits to A193/A194 — Class 2 studs identified by the B8SH and B8MSH head marking.
Bureau of Indian Standards, IS 1367 (Part 14/Sec 1):2018, identical with ISO 3506-1:2009 (superseded by the 2023 edition) — BIS record for the bolts, screws and studs section of the Indian stainless fastener standard.

Values summarize published limits and typical data. The governing standard and the current edition control; check them before specifying.