ASME SF-568, specification for carbon and alloy steel externally threaded metric fasteners covers chemical and mechanical requirements for nine property classes of carbon and alloy steel externally threaded metric fasteners in nominal thread diameters M 1.6 through M 100 suited for use in general engineering applications. Some detail from the standard is given in this article to show areas of its usefulness. For more information please refer the specification.
Property Classes
There are nine property classes as shown below.
| Property Class |
Nominal Product Diameter, mm |
Material and Treatment | Tempering Temperature, ºC Minimum |
|---|---|---|---|
| 4.6 | M5 – M100 | Low or medium carbon steel | _ |
| 4.8 | M1.6 – M16 | Low or medium carbon steel, partially or fully annealed as required | _ |
| 5.8 | M5 – M24 | Low or medium carbon steel, cold worked | _ |
| 8.8 | M16 – M72 | Medium carbon steel, product is quenched and tempered | 425 |
| 8.8 | M16 – M36 | Low carbon martensite steel, product is quenched and tempered | 425 |
| 8.8.3 | M16 – M36 | Atmospheric corrosion resistant steel, product is quenched and tempered | 425 |
| 9.8 | M1.6 – M16 | Medium carbon steel, product is quenched and tempered | 425 |
| 9.8 | M1.6 – M16 | Low carbon martensite steel, product is quenched and tempered | 425 |
| 10.9 | M5 – M20 | Medium carbon steel, product is quenched and tempered | 425 |
| 10.9 | M5 – M100 | Medium carbon alloy steel, product is quenched and tempered | 425 |
| 10.9 | M5 – M36 | Low carbon martensite steel, product is quenched and tempered | 340 |
| 10.9.3 | M16 – M36 | Atmospheric corrosion resistant steel, product is quenched and tempered | 425 |
| 12.9 | M16 – M100 | Alloy steel, product is quenched and tempered | 380 |
Classes 8.8.3 and10.9.3 have atmospheric corrosion resistance and weathering characteristics comparable to those of the steels covered in ASME Specification A588.
The ASME standard SF-568 also gives chemical composition requirements, mechanical requirements, mechanical testing requirements, proof loads, tensile strength values and property class identification symbols for various property classes.
The standard also contains information on the following.
1.   Ordering information.
2.   Materials and manufacture – heading practice, threading practice, heat treatment and zinc coating.
3.   Workmanship.
4.   Number of tests, retests, test methods, inspection and responsibility.
5.   Product marking, packaging and package marking.
Significance of Property Class Designations
Property classes are designated by numbers where increasing numbers generally represent increasing tensile strengths. The designation symbol has the following significance.
The one or two numerals preceding the first decimal point approximates 1/100 of the minimum tensile strength in N/mm2.
The numeral following the first decimal point approximates 1/10 of the ratio, expressed as a percentage, between minimum yield stress and minimum tensile strength.
The number 3, following the second decimal point, is an indicator that the material has atmospheric corrosion resistance and weathering characteristics comparable to those of the steels covered in ASME Specification A588.
Selection of Material
Designers and purchasers may use following information (conversion guidance) for selecting material for a property class.
| Property Class | Material specification approximately meeting required mechanical properties. |
|---|---|
| 4.6 | ASME A 307, Grade A |
| 8.8 | ASME A 449 and ASME A 325, Types 1 and 2 |
| 8.8.3 | ASME A 325, Type 3 |
| 9.8 | Mechanical properties of Class 9.8 are approximately 9 % higher than that of ASME A 449 |
| 10.9 | ASME A 354, Grade BD and ASME A 490, Types 1 and 2 |
| 10.9.3 | ASME A 490 |
| 12.9 | ASME A 574 |
Class 8.8 is applicable to fasteners larger than M16, except for specification ASTM A 325M bolts where M16 and larger bolt diameters are Class 8.8.
Comparison of Different Strength Grades
Comparison of Strength Grades under different standards is as under.
| SAE | B.S. | IS ISO DIN |
ULTIMATE TENSILE STRENGTH | HARDNESS | |||||
|---|---|---|---|---|---|---|---|---|---|
| Tonnes/in2 (Kgf/mm2) |
Newtons/mm2 Minimum (Kgf/mm2) |
Pounds/in2 Minimum (Kgf/mm2) |
BHN | HRB | HRC | ||||
|  |  | 4.6 |  | 400 (40.8) |  | 114 209 | 67 95 |  | |
| GRADE 1 |  |  |  |  | 60,000 (42.3) | (121) (241) | 70 100 |  | |
|  |  | 4.8 |  | 420 (42.8) |  | 124 209 | 71 95 |  | |
|  |  | 5.6 |  | 500 (51.0) |  | 147 209 | 79 95 |  | |
| GRADE 2 |  |  |  |  | 74,000 (52.1) | (154) (241) | 80 100 |  | |
|  |  | 5.8 |  | 520 (53.0) |  | 152 209 | 82 95 |  | |
|  | P |  | 35/45 (55.2/71.0) |  |  | 152 207 |  |  | |
|  |  | 6.8 |  | 600 (61.2) |  | 181 238 | 89 99 |  | |
|  | R |  | 45/55 (71.0/86.8) |  |  | 201 285 |  |  | |
|  | S |  | 50/60 (78.9/94.7) |  |  |  |  | (20/33) | |
|  |  | 8.8 |  | 800≤M16 830>M16 (81.6) (84.6) |  | 219/285 242/319 |  | 20/30 23/34 | |
| GRADE 5 |  |  |  |  | 1,20,000 (84.6) | (266) (318) |  | 25/34 | |
|  | T |  | 55/65 (86.8/102.6) |  |  | 248 335 |  | (24/36) | |
|  | U |  | 60/70 (94.7/110.5) |  |  | 269 331 |  | (28/36) | |
|  | V |  | 65/75 (102.6/118.4) |  |  | 293 370 |  | (31/40) | |
| GRADE 8 |  |  |  |  | 150000 (105.7) | (311) (362) |  | 33/39 | |
|  |  | 10.9 |  | 1,040 (106.0) |  | 295 362 |  | 31/39 | |
|  | W |  | 70/80 (110.5/126.3 |  |  | 311 375 |  | (33/41) | |
|  | X |  | 75/85 (118.4/134.2) |  |  | 341 410 |  | (37/44) | |
|  |  | 12.9 |  | 1220 (124.4) |  | 353 412 |  | 38/44 | |
|  | Y |  | 80/90 (126.3/142.0) |  |  | 363 429 |  | (39/46) | |
|  | Z |  | 100 Min (157.8) |  |  | 444 |  | (47) | |
|
|||||||||
