Socket Cap Screws Manufacturer in India | Stainless A2 / A4 Cap Screws

Overview

Stainless A2 / A4 Cap Screws to ISO 4762 / DIN 912: Manufacturer & Stockist

Socket cap screws put a strong, compact fastener in places a spanner cannot reach. In stainless steel they add corrosion resistance and a clean finish for food machines, marine fittings, medical and laboratory equipment, and architecture.

Exotic Alloy manufactures stainless socket cap screws in Mumbai, India, and stocks A2 and A4 sizes for machine builders, OEMs and distributors across India. We also supply Germany, Europe, the Gulf and Africa. Heads are cold formed with the socket and optional knurl. Threads are rolled, and every lot is passivated and PMI-checked.

Stainless is not 12.9

A2-70 is about 57 % as strong as a black 12.9 cap screw, and stainless threads can gall if fitted dry. Choose the grade for the environment, check the load, and lubricate. For high-strength alloy steel, see 12.9 Allen bolts and socket head bolts.

ISO 4762 / DIN 912 A2-70 / A4-70 / A4-80 Knurled heads ASTM F837 inch PMI verified

Stainless cap screw at a glance

Steel: A2 (304), A4 (316); A1, A3, A5 to order
Class: 70 to M24, 50 above (ISO 4762)
Strength: A2-70: 700 MPa tensile, 450 yield
Head: Plain or knurled, hex socket
Assembly: Lubricate or use anti-seize
Related: Hollow Allen, hex cap screws, button head screws

ISO 4762 M8 × 25, A2-70, knurled head, drawn to scale (top view at 83 %). Steel group and property class are marked on the head with the maker’s mark, as in the photo.

Specification

Socket Cap Screw Specification

The range we manufacture and supply.

StandardsISO 4762, DIN 912; ISO 3506-1 (properties); ASME B18.3 and ASTM F837 (inch)

Size rangeM3 to M36 (M1.6 to M64 to order); No. 0 to 1½″ inch; lengths per ISO 4762 / B18.3

ThreadMetric coarse 6g, fine pitch to order; UNC / UNF 3A; rolled threads

Property classA2-70, A4-70 (to M24); A2-50, A4-50 (above M24); A4-80 on request

SteelA2: 1.4301 / 1.4307 (304 / 304L); A4: 1.4401 / 1.4404 (316 / 316L); A1, A3, A5 and duplex to order

FinishPassivated (ASTM A967); plain; anti-galling coating or wax on request

HeadKnurled or plain cylindrical head; low head DIN 7984 and hollow screws also available

MarkingSteel group and class (A2-70, A4-80) and maker’s mark

CertificationMill certificate; EN 10204 3.1 with PMI and mechanical tests

PackingBoxes of 100 / 200 / 500, bulk cartons, labelled kits

Steel grades

Stainless Steel Groups & Property Classes (ISO 3506-1)

The marking has two parts: the letter and number give the steel group, and the figures after the dash give the strength class.

Steel group	Type	Typical grades	Corrosion resistance	Use
A1	Austenitic, free-machining	303 (1.4305)	Lower: sulphur added for machining	Machined parts, not for wet service
A2	Austenitic Cr-Ni	304 / 18-8 (1.4301, 1.4307)	Good in air, water and mild chemicals	General stainless: machinery, food plants, architecture
A3	Austenitic, stabilized	321 (1.4541)	As A2, better after welding	Heat-exposed parts
A4	Austenitic Cr-Ni-Mo	316 / 316L (1.4401, 1.4404)	Better: molybdenum resists chlorides	Marine, coastal and chemical plants
A5	Austenitic Cr-Ni-Mo, stabilized	316Ti (1.4571)	As A4, better after welding	Heat-exposed, chloride service

Property classes for austenitic steels

Property class	Tensile R_m min, MPa	Yield R_p0.2 min, MPa	Elongation min, after fracture	How it is made	In ISO 4762
50	500	210	0.6 d	Annealed or lightly worked	Above M24
70	700	450	0.4 d	Cold headed, work hardened	Up to M24
80	800	600	0.3 d	Higher cold work	On agreement
100	1000	800	0.2 d	Heavy cold work (ISO 3506-1:2020)	On agreement

ISO 3506-1. Strength comes from cold working, not heat treatment, which is why stainless screws cannot be hardened like 12.9. For indoor swimming pool atmospheres, A4 is not recommended for load-bearing parts; use high corrosion resistant steel such as 1.4529.

Strength

How Strong Are Stainless Socket Cap Screws?

Loads = stress area × minimum strength (our calculation). Classes 80 and 100 are made on request; ISO 4762 itself lists class 70 up to M24 and class 50 above.

Minimum loads by size

Size	A_s mm²	A2 / A4-70 yield load kN	A2 / A4-70 min. tensile kN	A4-80 min. tensile kN	12.9 min. tensile kN, for comparison	Hex key mm
M3	5.03	2.3	3.5	4.0	6.1	2.5
M4	8.78	4.0	6.1	7.0	10.7	3
M5	14.2	6.4	9.9	11.4	17.3	4
M6	20.1	9.0	14.1	16.1	24.5	5
M8	36.6	16.5	25.6	29.3	44.7	6
M10	58	26.1	40.6	46.4	70.8	8
M12	84.3	37.9	59.0	67.4	102.8	10
M16	157	70.7	109.9	125.6	191.5	14
M20	245	110.2	171.5	196.0	298.9	17
M24	353	158.8	247.1	282.4	430.7	19
M30	561	117.8 (class 50)	280.5 (class 50)	448.8	684.4	22
M36	817	171.6 (class 50)	408.5 (class 50)	653.6	996.7	27

Our calculation, A_s × stress: class 70 = 450 / 700 MPa, class 50 = 210 / 500 MPa, A4-80 = 800 MPa, 12.9 = 1220 MPa.

Dimensions

Socket Cap Screw Dimensions: ISO 4762 & ASME B18.3

Size	Pitch P	d_k max plain / knurled	k max head height	s hex key	t min key engagement	w min wall under socket	b thread length, ref.
M1.6	0.35	3 / 3.14	1.6	1.5	0.7	0.55	15
M2	0.4	3.8 / 3.98	2	1.5	1	0.55	16
M2.5	0.45	4.5 / 4.68	2.5	2	1.1	0.85	17
M3	0.5	5.5 / 5.68	3	2.5	1.3	1.15	18
M4	0.7	7 / 7.22	4	3	2	1.4	20
M5	0.8	8.5 / 8.72	5	4	2.5	1.9	22
M6	1	10 / 10.22	6	5	3	2.3	24
M8	1.25	13 / 13.27	8	6	4	3.3	28
M10	1.5	16 / 16.27	10	8	5	4	32
M12	1.75	18 / 18.27	12	10	6	4.8	36
M14	2	21 / 21.33	14	12	7	5.8	40
M16	2	24 / 24.33	16	14	8	6.8	44
M20	2.5	30 / 30.33	20	17	10	8.6	52
M24	3	36 / 36.39	24	19	12	10.4	60
M30	3.5	45 / 45.39	30	22	15.5	13.1	72
M36	4	54 / 54.46	36	27	19	15.3	84
M42	4.5	63 / 63.46	42	32	24	16.3	96
M48	5	72 / 72.46	48	36	28	17.5	108
M56	5.5	84 / 84.54	56	41	34	19	124
M64	6	96 / 96.54	64	46	38	22	140

DIN EN ISO 4762, mm. The knurled head d_k max is slightly larger than the plain head; allow for it in counterbores.

Size	Threads / in UNC / UNF	A head dia. max / min, in	H head height max / min, in	J hex key in	T key engagement min, in	G wall min, in
No. 0	— / 80	0.096 / 0.091	0.060 / 0.057	0.050	0.025	0.020
No. 1	64 / 72	0.118 / 0.112	0.073 / 0.070	1/16	0.031	0.025
No. 2	56 / 64	0.140 / 0.134	0.086 / 0.083	5/64	0.038	0.029
No. 3	48 / 56	0.161 / 0.154	0.099 / 0.095	5/64	0.044	0.034
No. 4	40 / 48	0.183 / 0.176	0.112 / 0.108	3/32	0.051	0.038
No. 5	40 / 44	0.205 / 0.198	0.125 / 0.121	3/32	0.057	0.043
No. 6	32 / 40	0.226 / 0.218	0.138 / 0.134	7/64	0.064	0.047
No. 8	32 / 36	0.270 / 0.262	0.164 / 0.159	9/64	0.077	0.056
No. 10	24 / 32	0.312 / 0.303	0.190 / 0.185	5/32	0.090	0.065
1/4″	20 / 28	0.375 / 0.365	0.250 / 0.244	3/16	0.120	0.095
5/16″	18 / 24	0.469 / 0.457	0.312 / 0.306	1/4	0.151	0.119
3/8″	16 / 24	0.562 / 0.550	0.375 / 0.368	5/16	0.182	0.143
7/16″	14 / 20	0.656 / 0.642	0.438 / 0.430	3/8	0.213	0.166
1/2″	13 / 20	0.750 / 0.735	0.500 / 0.492	3/8	0.245	0.190
5/8″	11 / 18	0.938 / 0.921	0.625 / 0.616	1/2	0.307	0.238
3/4″	10 / 16	1.125 / 1.107	0.750 / 0.740	5/8	0.370	0.285
7/8″	9 / 14	1.312 / 1.293	0.875 / 0.864	3/4	0.432	0.333
1″	8 / 12	1.500 / 1.479	1.000 / 0.988	3/4	0.495	0.380
1-1/4″	7 / 12	1.875 / 1.852	1.250 / 1.236	7/8	0.620	0.475
1-1/2″	6 / 12	2.250 / 2.224	1.500 / 1.485	1	0.745	0.570

ASME B18.3 socket head cap screws, inches; stainless screws use the same dimensions.

Group / condition	Designation	Diameter in	Full-size tensile ksi	Yield ksi	Elongation % min	Hardness
Group 1 or 2, AF (annealed)	F837A (Gr 1) / F837D (Gr 2)	0.060–1.500	85 max	55 max	40	95 HRB max
Group 1 or 2, CW (cold worked)	F837B (Gr 1) / F837E (Gr 2)	0.060–0.750	80 min	40 min	25	50 HRA min
Group 1 or 2, CW1 (cold worked)	F837C (Gr 1) / F837F (Gr 2)	0.060–0.750	102 min	65 min	20	59 HRA min
Group 5, HT (martensitic)	F837G	0.060–0.190	180 min	—	—	36–43 HRC
Group 5, HT (martensitic)	F837H	0.250–1.500	180 min	120 min	12	36–43 HRC

ASTM F837 mechanical properties of stainless steel socket head cap screws. Group 1 covers 304-type steels, Group 2 316-type and Group 5 the hardenable martensitic grade.

Galling & torque

Galling, Lubrication & Tightening Torque

Thread flanks, enlarged (schematic). Galling is the most common assembly problem with stainless screws, and once it starts the screw usually cannot be saved.

Do

Use an anti-seize or lubricant on the thread and under the head
Tighten slowly and steadily, by hand or with a slow tool
Keep threads clean: dirt and damage start galling
Use torque values for the lubricant you actually use

Don't

Fit dry stainless screws into stainless threads with an impact driver
Use 12.9 torque tables for stainless screws
Keep forcing a screw that tightens unevenly: back it off and clean it
Reuse a galled screw or nut

Tightening torque at μ = 0.14 (lubricated)

Size	A2 / A4-70: F_M kN	M_A N·m	A4-80: F_M kN	M_A N·m	Hex key mm
M3	1.6	1.0	2.1	1.3	2.5
M4	2.7	2.2	3.7	2.9	3
M5	4.5	4.4	6.0	5.9	4
M6	6.3	7.4	8.4	9.9	5
M8	11.6	18	15.5	24	6
M10	18.5	36	24.7	47	8
M12	27.0	61	36.0	81	10
M16	51.0	151	68.0	202	14
M20	79.7	296	106.3	395	17
M24	114.8	510	153.1	680	19

Our calculation by the VDI 2230 method: 90 % of the minimum yield strength used, friction coefficient 0.14 in the thread and under the head, ISO 273 medium clearance hole. Other friction coefficients: use the calculator.

Torque calculator

Stainless Socket Cap Screw Torque Calculator

Torque and preload for ISO 4762 stainless screws at 90 % of yield (VDI 2230 method), for any property class and friction coefficient.

Your screw

Size

Property class

Friction coefficient

Use the friction coefficient your lubricant maker publishes. Dry stainless threads can be 0.3 or higher and scatter widely. Values are guidance; confirm critical joints by test.

Result

18.1

N·m torque

6

mm hex key

Preload F_M:11.6 kN

Yield load (A_s × R_p0.2):16.5 kN

Preload / yield load:71 %

Corrosion

Choosing A2 or A4, and Fitting Stainless to Other Metals

The galvanic area rule (schematic). A stainless screw is the more noble metal in most joints with aluminium, zinc-coated or carbon steel parts.

Environment	Choose	Why
Indoor, dry or occasionally wet	A2	Chromium oxide film is enough
Outdoor, urban or rural	A2, or A4 for long life	Rain and pollution, low chlorides
Coastal, de-icing salt, swimming pool surrounds	A4	Molybdenum resists pitting by chlorides
Food plants with salt, brine or chlorinated cleaners	A4	Chlorides in washdown water
Indoor pool ceilings, road tunnels	HCR (1.4529)	Risk of stress corrosion cracking for A4

A2

304 / 304L

General-purpose stainless.

304 304L

A4

316 / 316L

Chloride and chemical resistance.

316 316L

A1 / A3 / A5

303 · 321 · 316Ti

Free-machining and stabilized grades.

303 321 316Ti

Beyond A4

Duplex · HCR

Offshore, tunnels and pools.

2205 1.4529

Manufacturing

How Our Stainless Socket Cap Screws Are Made

1

Wire

Cold-heading quality 304 / 316 wire, PMI-checked on receipt.

2

Heading

Head, socket and knurl formed; the cold work gives class 70 strength.

3

Thread rolling

Rolled threads, smoother flanks and less galling than cut threads.

4

Cleaning

Degreased to remove heading oil and iron pick-up.

5

Passivation

Nitric or citric passivation to ASTM A967 restores the oxide film.

6

Inspection

PMI, socket gauging, dimensions and tensile tests per lot.

Applications

Where Our Stainless Socket Cap Screws Are Used

Food & beverage machines

Washdown-resistant A2 and A4 screws on frames, guards and fillers.

Pharma & medical

Clean-room and laboratory equipment.

Marine fittings

A4 cap screws on deck hardware, pumps and boat equipment.

Chemical & water treatment

Valves, dosing pumps and instrument housings.

Automation & packaging

Aluminium profiles, linear modules and covers.

Architecture

Visible fixings on balustrades, glazing and signage.

Quality & testing

Socket Cap Screw Inspection & Certification

Check	Method	Applied to
Grade verification	PMI (XRF spectrometer)	Every lot
Hex socket	Go / no-go gauges	Every lot
Head & thread	Callipers, ring gauges	Every lot
Tensile & yield	ISO 3506-1 / ASTM F837	Every lot
Passivation	Water immersion or copper sulphate test (ASTM A967)	Passivated lots
Magnetic permeability	Permeability meter	On request

Every shipment includes

Mill certificate with heat number
PMI and dimensional report
Lot number on every box

On request

EN 10204 3.1 certificate
Salt spray or intergranular corrosion test
Low-magnetic screws with permeability report

FAQ

Socket Cap Screw Questions, Answered

A socket cap screw is a screw with a cylindrical head and a hexagon socket, turned with an Allen key. Metric socket cap screws are made to ISO 4762 or DIN 912 and inch ones to ASME B18.3. In stainless steel they are usually A2-70 (304) or A4-70 (316), often with a knurled head for grip.

A2 is chromium-nickel stainless steel of the 304 type; A4 adds molybdenum, as in 316, which gives much better resistance to chlorides from sea water, de-icing salt and many chemicals. Both are made in the same property classes, so an A2-70 and an A4-70 screw have the same strength. Use A4 near the sea, in chemical plants and in food processing with salt or cleaning agents.

A2 is the ISO 3506-1 steel group, an austenitic chromium-nickel stainless steel such as 304. The 70 is the property class: a minimum tensile strength of 700 MPa and a minimum 0.2 % yield strength of 450 MPa. Class 70 is reached by cold working the steel during heading.

No. A2-70 and A4-70 have a minimum tensile strength of 700 MPa, about 57 percent of the 1220 MPa of class 12.9. An M8 A2-70 screw has a minimum tensile load of about 25.6 kN against 44.7 kN for 12.9. A4-80 reaches 800 MPa. Choose stainless for corrosion resistance and 12.9 alloy steel where strength is critical.

Stainless steel threads can gall: under pressure and sliding, the thin oxide film on the flanks breaks and the metals cold weld and tear, locking the screw. It is most likely with dry threads, fast power tools, fine threads and the same grade on both parts. Prevent it with an anti-seize or lubricant, slower tightening and clean, undamaged threads.

Austenitic stainless steel is non-magnetic when annealed, but the cold heading and thread rolling used to make A2-70 screws convert part of the structure and make them slightly magnetic. A4 grades are usually less magnetic than A2. A slight attraction to a magnet does not mean the screw is not stainless; a PMI test confirms the grade.

For an ISO 4762 M8 A2-70 or A4-70 screw with a lubricated or anti-seize friction coefficient of 0.14, tightening to 90 % of yield gives about 18.1 N·m and 11.6 kN preload (VDI 2230 method). Dry stainless can have a friction coefficient of 0.3 or more: at 0.30 the same 90 % utilisation is reached at about 25.9 N·m but gives only 8.4 kN preload, and the scatter is large, so lubricate and confirm by test.

Yes, usually. A small stainless screw in a large aluminium part is generally acceptable because the large aluminium area shares the galvanic attack. In wet or salty service, isolate the metals with nylon or EPDM washers and sleeves, or seal the joint, and avoid the reverse case of small aluminium or zinc-coated fasteners in large stainless parts.