ASTM A574 — Alloy Steel Socket Head Cap Screws
The standard for alloy steel socket head cap screws — heat treated to 170–180 ksi tensile, stronger than Grade 8.
ASTM A574 covers quenched and tempered alloy steel socket head cap screws, #0 (0.060″) through 4″, where high strength is required. Head and socket dimensions come from ASME B18.3; A574 sets the material and strength. At 170–180 ksi tensile they are stronger than an SAE Grade 8 bolt (150 ksi).
Covers
Alloy steel socket head cap screws, #0–4″
Dimensions
ASME B18.3
Threads
Class 3A through 1″
Strength by size
| Size | Proof | Tensile | Hardness |
|---|---|---|---|
| Up to ½″ | 140 | 180 | HRC 39 min |
| Over ½″ | 135 | 170 | HRC 37 min |
Minimum loads by size
Stress area × the strength above, as A574 lists them. TPI = threads per inch. Proof load is what the screw holds without stretching for good; tensile load is the least it breaks at.
Coarse thread (UNC)
| Size | TPI | Proof (lb) | Tensile (lb) | Area (sq in) |
|---|---|---|---|---|
| #2 | 56 | 518 | 666 | 0.00370 |
| #4 | 40 | 846 | 1,090 | 0.00604 |
| #6 | 32 | 1,270 | 1,640 | 0.00909 |
| #8 | 32 | 1,960 | 2,520 | 0.0140 |
| #10 | 24 | 2,450 | 3,150 | 0.0175 |
| #12 | 24 | 3,380 | 4,350 | 0.0242 |
| ¼″ | 20 | 4,450 | 5,730 | 0.0318 |
| 5/16″ | 18 | 7,340 | 9,440 | 0.0524 |
| 3/8″ | 16 | 10,800 | 13,900 | 0.0775 |
| 7/16″ | 14 | 14,900 | 19,100 | 0.1063 |
| ½″ | 13 | 19,900 | 25,500 | 0.1419 |
| 5/8″ | 11 | 30,500 | 38,400 | 0.226 |
| ¾″ | 10 | 45,100 | 56,800 | 0.334 |
| 7/8″ | 9 | 62,400 | 78,500 | 0.462 |
| 1″ | 8 | 81,800 | 103,000 | 0.606 |
| 1-1/8″ | 7 | 103,000 | 129,000 | 0.763 |
| 1-1/4″ | 7 | 131,000 | 165,000 | 0.969 |
| 1-3/8″ | 6 | 156,000 | 196,000 | 1.155 |
| 1-1/2″ | 6 | 190,000 | 239,000 | 1.405 |
| 1-3/4″ | 5 | 256,000 | 323,000 | 1.90 |
| 2″ | 4½ | 338,000 | 425,000 | 2.50 |
Fine thread (UNF)
| Size | TPI | Proof (lb) | Tensile (lb) | Area (sq in) |
|---|---|---|---|---|
| #2 | 64 | 552 | 709 | 0.00394 |
| #4 | 48 | 925 | 1,190 | 0.00661 |
| #6 | 40 | 1,420 | 1,830 | 0.01015 |
| #8 | 36 | 2,060 | 2,650 | 0.01474 |
| #10 | 32 | 2,800 | 3,600 | 0.0200 |
| #12 | 28 | 3,600 | 4,600 | 0.0258 |
| ¼″ | 28 | 5,100 | 6,500 | 0.0364 |
| 5/16″ | 24 | 8,120 | 10,400 | 0.0580 |
| 3/8″ | 24 | 12,300 | 15,800 | 0.0878 |
| 7/16″ | 20 | 16,600 | 21,400 | 0.1187 |
| ½″ | 20 | 22,400 | 28,800 | 0.1599 |
| 5/8″ | 18 | 34,600 | 43,500 | 0.256 |
| ¾″ | 16 | 50,400 | 63,400 | 0.373 |
| 7/8″ | 14 | 68,700 | 86,500 | 0.509 |
| 1-1/4″ | 12 | 145,000 | 182,000 | 1.073 |
| 1-3/8″ | 12 | 178,000 | 224,000 | 1.315 |
| 1-1/2″ | 12 | 213,000 | 269,000 | 1.581 |
How they’re made
A574 screws are alloy steel made to a fine-grain practice, then quenched and tempered — that heat treatment is where the strength comes from. Threads are Class 3A through 1″ diameter.
How to spot it
The head is round with a hex socket, sized to ASME B18.3. Head markings vary by maker — a maker’s name or logo, knurls on the head, the size, or a lot code. When the spec matters, go by the paperwork.
Stainless socket heads
A574 is alloy steel only. Stainless socket head cap screws are not A574 and are not heat treated to these numbers — give us a call at 405-670-1234 if you need strength for stainless.
We stock socket head cap screws
Alloy steel and stainless socket head cap screws, coarse and fine thread. Give us the size, length and finish.
Quick answers
- What is the tensile strength of a socket head cap screw?
- Alloy steel socket head cap screws (ASTM A574) are 180 ksi minimum through 1/2″ and 170 ksi over 1/2″.
- Are socket head cap screws stronger than Grade 8?
- Yes. At 170–180 ksi tensile they are stronger than an SAE Grade 8 bolt (150 ksi).
- How much load can a 1/2" socket head cap screw hold?
- A 1/2″-13 alloy socket head cap screw holds at least 19,900 lb before it stretches for good (proof load) and breaks at no less than 25,500 lb.
- What is the proof strength of ASTM A574?
- 140 ksi through 1/2″ and 135 ksi over 1/2″.
- Are stainless socket head cap screws ASTM A574?
- No. A574 is alloy steel only; stainless socket head cap screws are not heat treated to those numbers.