ASTM A574 – Mechanical, Material & Chemical Properties

The ASTM A574-04 specification establishes the technical requirements for quenched and tempered alloy steel hexagon socket-head cap screws, ranging in size from 0.060 to 4 inches in diameter. This standard is critical for applications requiring high-strength fasteners where specific mechanical, chemical, and metallurgical properties must be strictly controlled. It defines essential manufacturing protocols, including the requirement for alloy steel made to fine-grain practice and heat treatment by oil quenching and tempering at a minimum of 650°F (343°C). To ensure quality, the specification outlines rigorous testing methods for hardness, tensile strength, and proof loads, while also providing clear limits for surface discontinuities and decarburization to prevent structural failures.

Mechanical Properties

The mechanical integrity of ASTM A574 fasteners is verified through specific hardness and tensile testing protocols based on screw diameter.

  • Hardness Requirements: Finished screws must maintain a hardness of 39 to 45 HRC for sizes 0.500 in. and smaller, and 37 to 45 HRC for sizes 0.625 in. and larger.
  • Tensile and Proof Loads: Fasteners must meet the minimum tensile loads and proof loads specified in the tables below for coarse threads and fine threads.
  • Yield Strength: For screws larger than 1.500 in. where machined specimens are used, the minimum yield strength is 153 ksi at 0.2% offset.
  • Wedge Testing: Screws are subjected to tension testing using a specific wedge angle under the head to ensure failure occurs in the body or thread section rather than the head-to-shank junction.

Mechanical Requirements
Thread Nominal Diameter Full-size Screw Machined Test Specimen Hardness
Tensile Strength, (min) ksi Proof Load, (nom) ksi Yield Strength at 0.2% Offset, (min) ksi Tensile Strength, (min) ksi Elongation (min) 4D Reduction Area (min) Rockwell (HRC)
< 0.5 in 180 140 - - - - 39 – 44
< 1 in 170 135 - - - - 37 – 44
> 1 in 170 135 153 170 10% 35% 37 – 44
Tensile Requirements For Coarse Thread Screws
Screw Size - Threads per Inch Tensile Load min (lbf) Stress Area (in²) Proof Load min (lbf)
#1-64 473 0.00263 368
#2-56 666 0.00370 518
#3-48 877 0.00487 682
#4-40 1090 0.00604 846
#5-40 1430 0.00796 1110
#6-32 1640 0.00909 1270
#8-32 2520 0.01400 1960
#10-24 3150 0.01750 2450
1/4-20 5730 0.03180 4450
5/16-18 9440 0.05240 7340
3/8-16 13900 0.07750 10800
7/16-14 19100 0.10630 14900
1/2-13 25500 0.14190 19900
5/8-11 38400 0.22600 30500
3/4-10 56800 0.33400 45100
7/8-9 78500 0.46200 62400
1-8 103000 0.60600 81800
1-1/8-7 129000 0.76300 103000
1-1/4-7 165000 0.96900 131000
1-3/8-6 196000 1.15500 156000
1-1/2-6 239000 1.40500 190000
1-3/4-5 323000 1.90000 256000
2-4.5 425000 2.50000 338000
2-1/4-4.5 552000 3.25000 439000
2-1/2-4 680000 4.00000 540000
2-3/4-4 838000 4.93000 666000
3-4 1010000 5.97000 806000
3-1/4-4 1210000 7.10000 958000
3-1/2-4 1420000 8.33000 1120000
3-3/4-4 1640000 9.66000 1300000
4-4 1880000 11.08000 1500000
Tensile Requirements For Fine Thread Screws
Screw Size - Threads per Inch Tensile Load min (lbf) Stress Area (in²) Proof Load min (lbf)
#0-80 324 0.00180 252
#1-72 500 0.00278 389
#2-64 709 0.00394 552
#3-56 941 0.00523 732
#4-48 1190 0.00661 925
#5-44 1490 0.00830 1160
#6-40 1830 0.01015 1420
#8-36 2650 0.01474 2060
#10-32 3600 0.02000 2800
1/4-28 6500 0.03640 5100
5/16-24 10400 0.05800 8120
3/8-24 15800 0.08780 12300
7/16-20 21400 0.11870 16600
1/2-20 28800 0.15990 22400
5/8-18 43500 0.25600 34600
3/4-16 63400 0.37300 50400
7/8-14 86500 0.50900 68700
1-12 113000 0.66300 89500
1-1/8-12 146000 0.85600 116000
1-1/4-12 182000 1.07300 145000
1-3/8-12 224000 1.31500 178000
1-1/2-12 269000 1.58100 213000

Material Properties

The chemical composition is tightly regulated to ensure that the alloying elements provide sufficient hardenability for high-strength requirements.

  • Core Chemistry: Standard alloy steel must meet specific minimums for Carbon (0.33%) and maximum limits for Phosphorus (0.035%) and Sulfur (0.040%) based on heat analysis.
  • Alloying Strategy: Elements such as chromium, nickel, molybdenum, or vanadium must be present in sufficient quantities to ensure that specified strength properties are met after oil quenching and tempering.
  • Prohibited Elements: The intentional addition of bismuth, selenium, tellurium, or lead to the heats of steel is strictly prohibited.

Chemical Requirements
Carbon (min) Phosphorus (max) Sulfur (max)
Heat Analysis Product Analysis Heat Analysis Product Analysis Heat Analysis Product Analysis
0.33% 0.31% 0.035% 0.040% 0.040% 0.045%
Chemical Composition, %
Grade Designation 4037 4042 4137 4140 4142 4145 4340 8740 5137M 51B37M
UNS Number G40370 G40420 G41370 G41400 G41420 G41450 G43400 G87400 - -
Carbon Heat Analysis 0.35-0.40 0.40-0.45 0.35-0.40 0.38-0.43 0.40-0.45 0.43-0.48 0.38-0.43 0.38-0.43 0.35-0.40 0.33-0.40
Product Analysis 0.33-0.42 0.38-0.47 0.33-0.42 0.36-0.45 0.38-0.47 0.41-0.50 0.36-0.45 0.36-0.45 0.33-0.42 0.31-0.42
Manganese Heat Analysis 0.70-0.90 0.70-0.90 0.70-0.90 0.75-1.00 0.75-1.00 0.75-1.00 0.60-0.80 0.75-1.00 0.30-0.50 0.30-0.50
Product Analysis 0.67-0.93 0.67-0.93 0.67-0.93 0.71-1.04 0.71-1.04 0.71-1.04 0.57-0.83 0.71-1.04 0.27-0.53 0.27-0.53
Phosphorus (max) Heat Analysis 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.035 0.035
Product Analysis 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040
Sulfur (max) Heat Analysis 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040 0.040
Product Analysis 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045 0.045
Silicon Heat Analysis 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35 0.15-0.35
Product Analysis 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37 0.13-0.37
Nickel Heat Analysis - - - - - - 1.65-2.00 0.40-0.70 - -
Product Analysis - - - - - - 1.65-2.05 0.37-0.73 - -
Chromium Heat Analysis - - 0.80-1.10 0.80-1.10 0.80-1.10 0.80-1.10 0.70-0.90 0.40-0.60 0.90-1.20 0.95-1.25
Product Analysis - - 0.75-1.15 0.75-1.15 0.75-1.15 0.75-1.15 0.67-0.93 0.37-0.63 0.85-1.25 0.90-1.30
Molybdenum Heat Analysis 0.20-0.30 0.20-0.30 0.15-0.25 0.15-0.25 0.15-0.25 0.15-0.25 0.20-0.30 0.20-0.30 - -
Product Analysis 0.18-0.32 0.18-0.32 0.13-0.27 0.13-0.27 0.13-0.27 0.13-0.27 0.18-0.32 0.18-0.32 - -
Boron Heat Analysis - - - - - - - - - 0.0005-0.003
Product Analysis - - - - - - - - - B

Metallurgical Properties

To maintain surface integrity and prevent brittleness or premature fatigue failure, the specification sets strict metallurgical boundaries.

  • Carburization/Decarburization: There shall be no evidence of total decarburization or carburization on the surfaces of the heat-treated screws when measured in accordance with the specification.
  • Partial Decarburization Limits: The depth of partial decarburization is strictly limited to the values defined in Table 5, based on the number of threads per inch.
  • Surface Discontinuities: The standard restricts the depth and location of cracks, seams, and laps; for instance, no laps are permitted at the thread root or on the flanks below the pitch line.
  • Measurement Protocols: Decarburization and carburization are evaluated using either optical or microhardness measurement methods on a longitudinal section of the thread area.

Decarburization Limits
Threads/in. Thread Height, hs 0.75 hs from Root to Crest, min 0.1 hs; Root, max
48 0.013 0.010 0.001
44 0.014 0.011 0.001
40 0.015 0.011 0.002
36 0.017 0.013 0.002
32 0.019 0.014 0.002
28 0.022 0.017 0.002
24 0.026 0.020 0.003
20 0.031 0.023 0.003
18 0.034 0.026 0.003
16 0.038 0.029 0.004
14 0.044 0.033 0.004
13 0.047 0.035 0.005
12 0.051 0.038 0.005
11 0.056 0.042 0.006
10 0.061 0.046 0.006
9 0.068 0.051 0.007
8 0.077 0.058 0.008
7 0.088 0.066 0.009
6 0.102 0.077 0.010
5 0.123 0.092 0.012
4.5 0.136 0.102 0.014
4 0.153 0.115 0.015