ASTM F879 – Stainless Steel Socket Cap Screws
ASTM F879 specifies the chemical and mechanical requirements for stainless steel socket button and flat countersunk head cap screws intended for applications requiring general corrosion resistance. The specification covers austenitic stainless steel alloys in different material conditions, including annealed and cold-worked conditions, with requirements for chemical composition, tensile properties, yield strength, elongation and hardness.
The specification covers stainless steel alloys such as 304, 304L, 305, 384, 18-9LW, 302HQ, 316 and 316L, with the applicable alloy and material condition determining the required mechanical properties. The chemical composition and mechanical requirements are specified separately in the standard and provide the basis for material selection and inspection of finished fasteners.
Material Chemical Composition:
ASTM F879 specifies chemical composition requirements for the stainless steel used in socket cap screws. The specification covers several austenitic stainless steel alloys, with each alloy having defined limits for carbon, manganese, phosphorus, sulfur, silicon, chromium, nickel and other applicable elements. These limits distinguish the permitted material grades and provide a consistent basis for the mechanical and corrosion-related characteristics of the fasteners.
The chemical composition requirements for the applicable stainless steel alloys are summarized in the table below. The specified limits should be considered together with the corresponding material condition and mechanical property requirements of ASTM F879
| UNS Designation | Alloy | Carbon(max) | Manganese(max) | Phosphorus(max) | Sulfur(max) | Silicon(max) | Chromium | Nickel | Copper | Molybdenum |
|---|---|---|---|---|---|---|---|---|---|---|
| S30400 | 304 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0 to 20.0 | 8.0 to 10.5 | 1.00 | … |
| S30403 | 304L | 0.030 | 2.00 | 0.045 | 0.030 | 1.00 | 18.0 to 20.0 | 8.0 to 12.0 | 1.00 | … |
| S30500 | 305 | 0.12 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0 to 19.0 | 10.5 to 13.0 | 1.00 | … |
| S38400 | 384 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 15.0 to 17.0 | 17.0 to 19.0 | … | … |
| S30430 | 18–9LW | 0.10 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0 to 19.0 | 8.0 to 10.0 | 3.0 to 4.0 | … |
| S30433 | 302HQ | 0.03 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0 to 19.0 | 8.0 to 10.0 | 3.0 to 4.0 | … |
| … | 304J3ᴬ | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 17.0 to 19.0 | 8.0 to 10.5 | 1.00 to 3.00 | … |
| S31600 | 316 | 0.08 | 2.00 | 0.045 | 0.030 | 1.00 | 16.0 to 18.0 | 10.0 to 14.0 | … | 2.00 to 3.0 |
| S31603 | 316L | 0.03 | 2.00 | 0.045 | 0.030 | 1.00 | 16.0 to 18.0 | 10.0 to 14.0 | … | 2.00 to 3.0 |
Mechanical Property:
ASTM F879 specifies mechanical property requirements for stainless steel socket cap screws according to the applicable alloy and material condition. The requirements include tensile strength, yield strength, elongation and hardness, with different minimum values specified for the respective property classes.
The required mechanical properties are related to the material condition of the stainless steel, including annealed and cold-worked conditions. The applicable requirements are summarized in the table below, providing the specified minimum strength and ductility values for each ASTM F879 material class.
| Alloy Group | Alloy Condition | Full Size Product Tests - Tensile Strength ksi | Full Size Product Tests - Extension min | Machined Specimen Tests - Tensile Strength ksi | Machined Specimen Tests - Yield Strength ksi | Machined Specimen Tests - Elongation % min | Core Hardness - Vickers | Core Hardness - Rockwell |
|---|---|---|---|---|---|---|---|---|
| All | AF | 85 max | 0.6D | 85 max | 55 max | 40 | 210 max | 95 HRB max |
| All | CW | 80 min | 0.4D | 80 min | 40 min | 25 | 150 min | 50 HRA min |
| All | CW1 | 102 min | 0.4D | 87 min | 65 min | 20 | 220 min | 59 HRA min |
Breaking Strength for Full Size Fasteners：
The breaking strength of full-size fasteners varies with the nominal diameter and thread size, as the available tensile stress area increases with fastener size. The table below provides the tensile stress area and corresponding breaking strength in pounds-force (lbf) for the applicable ASTM F879 fastener sizes. These values provide a reference for the tensile load at which a full-size fastener is expected to reach failure under axial loading.
| Nominal Size | Thread Size | Tensile Stress Area, in² | AF lb. min | CW lb. min | CW1 lb. min |
|---|---|---|---|---|---|
| 0 | 0.060–80 | 0.00180 | 153 | 115 | 147 |
| 1 | 0.073–64 | 0.00263 | 223 | 168 | 215 |
| 2 | 0.086–56 | 0.00370 | 314 | 237 | 303 |
| 3 | 0.099–48 | 0.00487 | 414 | 312 | 399 |
| 4 | 0.112–40 | 0.00604 | 513 | 386 | 495 |
| 5 | 0.125–40 | 0.00796 | 676 | 509 | 652 |
| 6 | 0.138–32 | 0.00909 | 772 | 581 | 745 |
| 8 | 0.164–32 | 0.0140 | 1191 | 897 | 1149 |
| 10 | 0.190–24 | 0.0175 | 1490 | 1122 | 1438 |
| 1/4 | 0.250–20 | 0.0318 | 2705 | 2037 | 2609 |
| 5/16 | 0.312–18 | 0.0524 | 4457 | 3356 | 4299 |
| 3/8 | 0.375–16 | 0.0775 | 6587 | 4959 | 6354 |
| 1/2 | 0.500–13 | 0.1419 | 12061 | 9082 | 11636 |
| 5/8 | 0.625–11 | 0.226 | 19210 | 14464 | 18532 |