ASTM F879 – Mechanical & Material Properties

ASTM F879 is the standard specification governing the chemical and mechanical requirements for stainless steel inch hexagon socket button (SBHCS) and flat countersunk (SFHCS) head cap screws. Intended for applications requiring general corrosion resistance, this specification covers fasteners with nominal thread diameters ranging from 0.060 through 0.625 inches. To apply this specification effectively, users must distinguish between the two primary alloy conditions:

  • Condition AF: Austenitic alloy in an annealed condition (85 ksi maximum).
  • Condition CW: Austenitic alloy that has been annealed and then cold-worked to develop higher strength (95 ksi minimum).
When ordering under ASTM F879, it is technically critical to specify the quantity, dimensions (thread pitch and length), alloy group, and condition to ensure the fasteners meet the intended performance environment.

Mechanical Properties

The mechanical integrity of F879 fasteners is verified through two primary methods: full-size product tests and machined specimen tests. Because the geometry of button and flat countersunk heads can affect load distribution, the specification provides specific breaking strength values that are adjusted for these configurations.

  • Ductility: Condition AF offers higher elongation (40% min) compared to Condition CW (20% min), making it more suitable for applications where material "give" is preferred over raw strength.
  • Strength: Condition CW fasteners are processed to achieve significantly higher tensile and yield strengths through cold working.
  • Testing Requirement: Screws with a length equal to or greater than three diameters must be tested full-size to meet the breaking strength requirements specified in the tables below.

Mechanical Requirements
Alloy Condition Full Size Product Machined Specimen Tests Core Hardness
Tensile Strength, ksi Extension, (min) Tensile Strength, ksi Yield Strength, ksi Elongation, (min) Vickers Rockwell
AF 85 max 0.6D 85 max 55 max 40% 210 max 95 HRB max
CW 80 min 0.4D 80 min 40 min 25% 150 min 50 HRA min
CW1 102 min 0.4D 87 min 65 min 20% 220 min 59 HRA min
Breaking Strength Value For Full Size Fasteners
Screw Size - Threads per Inch Tensile Stress Area, (in^2) Alloy Condition, lb, (min)
AF CW CW1
#0-80 0.0018 153 115 147
#1-64 0.00263 223 168 215
#2-56 0.0037 314 237 303
#3-48 0.00487 414 312 399
#4-40 0.00604 513 386 495
#5-40 0.00796 676 509 652
#6-32 0.00909 772 581 745
#8-32 0.014 1191 897 1149
#10-24 0.0175 1490 1122 1438
1/4-20 0.0318 2705 2037 2609
5/16-18 0.0524 4457 3356 4299
3/8-16 0.0775 6587 4959 6354
1/2-13 0.1419 12,061 9082 11,636
5/8-11 0.226 19,210 14,464 18,532

Material Properties

ASTM F879 fasteners are manufactured from Group 1 Austenitic Alloys, including common grades such as 304, 304L, 316, and 316L. The choice of alloy determines the fastener's performance in corrosive environments and its magnetic permeability.

  • Corrosion Resistance: To ensure maximum resistance, Condition AF screws must be annealed by heating to 1900 ± 50°F and cooled rapidly to prevent carbide precipitation.
  • Chemical Precision: The composition limits (Carbon, Manganese, Phosphorus, Sulfur, Silicon, Chromium, Nickel, Copper, and Molybdenum) are strictly controlled. For example, 316/316L grades include Molybdenum to provide enhanced protection against pitting in chloride-rich environments.
  • Supplementary Requirements: For specialized applications, users can specify S3 (Permeability) to ensure Condition AF screws do not exceed a magnetic permeability of 1.05 at 100 Oe.

Austenitic Alloys
Composition, (%)
UNS Designation Alloy Carbon Manganese Phosphorus Sulfur Silicon Chromium Nickel Copper Molybdenum
S30400 304 0.08 2 0.045 0.03 1 18.0 to 20.0 8.0 to 10.5 1 -
S30403 304L 0.03 2 0.045 0.03 1 18.0 to 20.0 8.0 to 12.0 1 -
S30500 305 0.12 2 0.045 0.03 1 17.0 to 19.0 10.5 to 13.0 1 -
S38400 384 0.08 2 0.045 0.03 1 15.0 to 17.0 17.0 to 19.0 - -
S30430 18-9LW 0.10 2 0.045 0.03 1 17.0 to 19.0 8.0 to 10.0 3.0 to 4.0 -
S30433 302HQ 0.03 2 0.045 0.03 1 17.0 to 19.0 8.0 to 10.0 3.0 to 4.0 -
S31600 316 0.08 2 0.045 0.03 1 16.0 to 18.0 10.0 to 14.0 - 2.00 to 3.0
S31603 316L 0.03 2 0.045 0.03 1 16.0 to 18.0 10.0 to 14.0 - 2.00 to 3.0