Property Classes 4.8–12.9: What the Numbers Mean

How to Read a Property Class Designation

A designation such as 8.8 or 10.9 is two whole numbers separated by a point, and the two parts are read separately rather than as one value. GB/T 3098.1-2010 §5 defines them. The number before the point is one hundredth of the nominal tensile strength Rm,nom in MPa. The number after the point is ten times the ratio of the nominal yield strength to the nominal tensile strength.

Reading 8.8 back through that rule: the first number gives a nominal tensile strength of 800 MPa, and the second gives a yield-to-tensile ratio of 0.8, so the nominal yield strength is 800 × 0.8 = 640 MPa. GB/T 3098.1-2010 Table 1 (PDF p.9) fixes the ratio for each second number: 0.6 for .6, 0.8 for .8 and 0.9 for .9.

That clause also records two variations. A fastener that keeps a class’s material properties but has a reduced actual load-carrying capacity is marked with a leading zero, as in 08.8. The designation system may also be used above 39 mm in diameter when the limits of GB/T 3098.1-2010 Table 2 and Table 3 are met.

The First Digit: Nominal Tensile Strength

GB/T 3098.1-2010 Table 3 (PDF p.10-11) lists a nominal and a minimum tensile strength for each property class: the nominal is the value the first digit encodes, and the minimum is the value a test must reach.

Property class	Rm,nom (MPa)	Rm,min (MPa)
4.8	400	420
8.8, d <= 16 mm	800	800
8.8, d > 16 mm	800	830
10.9	1000	1040
12.9	1200	1220

GB/T 3098.1-2010 Table 3 splits class 8.8 by diameter: the minimum tensile strength is 800 MPa at d <= 16 mm and 830 MPa above that. The scope note in §7 of GB/T 3098.1-2010 states that the d <= 16 mm column does not apply to bolted structures and that bolted structures begin at d >= M12, and that class 9.8 is specified only for d <= 16 mm.

The Second Digit: Yield Ratio and Yield Strength

The second number fixes the yield side of the class, and the nominal value recomputes from the class number. GB/T 3098.1-2010 Table 3 (PDF p.10-11) gives 800 × 0.8 = 640 MPa for 8.8, 1000 × 0.9 = 900 MPa for 10.9, and 1200 × 0.9 = 1080 MPa for 12.9.

Property class	Yield measure	Nominal (MPa)	Minimum (MPa)
4.6	ReL	240	240
4.8	RPf at 0.0048d	320	340
8.8, d <= 16 mm	RP0.2	640	640
8.8, d > 16 mm	RP0.2	640	660
10.9	RP0.2	900	940
12.9	RP0.2	1080	1100

Which measure applies depends on the class. GB/T 3098.1-2010 Table 3 pairs class 4.6 with a lower yield strength ReL of 240 MPa, and pairs classes 4.8, 5.8 and 6.8 with a proof stress RPf measured at 0.0048d, because a lower yield strength is not defined for them: 320 MPa nominal and 340 MPa minimum for 4.8, 400 and 420 MPa for 5.8, and 480 MPa nominal and minimum for 6.8. The notes to Table 3 record that RP0.2 is permitted when ReL cannot be measured, and that the RPf minimum for 4.8, 5.8 and 6.8 is calculated from the proof-stress ratio rather than measured.

The nominal proof stress SP is a separate row of GB/T 3098.1-2010 Table 3: 580 MPa for class 8.8 at d <= 16 mm, 600 MPa for 8.8 above 16 mm, 830 MPa for 10.9 and 970 MPa for 12.9.

Designation Reading Matrix

Element of the designation	What it encodes	Defined in
First number, as the 8 in 8.8	Rm,nom / 100, in MPa	GB/T 3098.1-2010 §5
Second number, as the 8 in 8.8	ReL,nom / Rm,nom, or RP0.2,nom / Rm,nom, times 10	GB/T 3098.1-2010 §5 and Table 1
Leading zero, as 08.8	Class material properties, reduced actual load-carrying capacity	GB/T 3098.1-2010 §5
Point	Separator between the two numbers	GB/T 3098.1-2010 §5

What Changes as the Class Number Rises

Higher classes carry smaller specified minimum ductility. GB/T 3098.1-2010 Table 3 (PDF p.10-11) sets the minimum elongation after fracture A for a machined test piece at 12 % for 8.8, 10 % for 9.8, 9 % for 10.9 and 8 % for 12.9, and the minimum reduction of area Z at 52 % for 8.8, 48 % for 9.8 and 10.9, and 44 % for 12.9.

Property class	A min (%)	Z min (%)	KV min at -20 C, d >= 16 mm (J)
8.8	12	52	27
9.8	10	48	27
10.9	9	48	27
12.9	8	44	under study

The impact-energy row follows GB/T 3098.1-2010 Table 3: classes 8.8, 9.8 and 10.9 carry a minimum absorbed energy of 27 J at -20 C for d >= 16 mm, and the value for 12.9 is recorded as under study. Hardness is bounded from both sides by GB/T 3098.1-2010 Table 3, which sets a Vickers minimum of 250 HV for 8.8 at d <= 16 mm, 255 HV above 16 mm, 290 HV for 9.8, 320 HV for 10.9 and 385 HV for 12.9, with maxima of 320, 335, 360, 380 and 435 HV respectively.

Two heat-treatment limits come from GB/T 3098.1-2010 Table 2 (PDF p.9-10). The minimum tempering temperature is 425 C for classes 8.8, 9.8 and 10.9 and for 12.9 made from alloy steel, and 380 C for the 12.9 row made from carbon steel with added elements. The notes to Table 2 state that a 12.9 surface shall not show a metallographically measurable layer of white phosphorus, and that selection of class 12.9 requires care: the manufacturer’s capability, the service conditions and the tightening method all enter, and the service environment may cause stress corrosion cracking.

Nut Classes Use a Different Numbering

A nut is marked with a different scheme. GB/T 3098.2-2015 §4 (PDF p.7) defines a nut’s property class as a single number equal to the first digit of the highest bolt property class it may be paired with, so a class 8 nut pairs with an 8.8 bolt. The pairing rule itself is GB/T 3098.2-2015 Table 2 (PDF p.7):

Nut property class	Highest bolt property class it may be paired with
5	5.8
6	6.8
8	8.8
10	10.9
12	12.9

Thin nuts use a two-number form. GB/T 3098.2-2015 §4 records that the leading 0 marks a lower load-carrying capacity than 1-type and 2-type nuts, and that the second number is one hundredth of the nominal proof stress tested with a hardened test mandrel, so class 04 has a nominal proof stress of about 400 MPa and class 05 about 500 MPa. The bolt class and the nut class are therefore two different numbering systems read side by side, and the pairing in GB/T 3098.2-2015 Table 2 states which nuts may go with which bolts.

Adoption Relationship Recorded as MOD

GB/T 3098.1-2010 records its relationship to the international standard as MOD ISO 898-1:2009, meaning it was prepared by modification of that standard. GB/T 3098.2-2015 records a MOD relationship to ISO 898-2:2012. A MOD relationship names the international standard that the national standard was derived from and modified. Nothing in either fact layer states that a class number in one document may be used in place of a class number in the other.

How the Values Are Measured: Test Methods

The tensile and yield values behind the class numbers are measured under separate test-method documents. MIL-STD-1312-8A (1984) §1.1 places the scope on testing structural fasteners in tension at room temperature, and states that it defines the tensile strength of the fastener itself rather than the strength of the fastener in a sheet combination.

MIL-STD-1312-8A (1984) §4.2.2 requires the nut used to test an externally threaded fastener to be strong enough to ensure bolt failure, and states that a test ending in nut failure is not a satisfactory test of the bolt. §5.4 reads the load-deflection curve so that the first peak is the structural failure load and the second peak is the ultimate load. §5.5 determines the yield load by Johnson’s two-thirds approximate method: a line is drawn at two-thirds the slope of the straight portion of the curve, a second line is drawn parallel to it and tangent to the curve, and the point of tangency is taken as the yield load.

MIL-STD-1312-8A (1984) §5.6 requires a minimum of five specimens for each fastener diameter and test condition. Minimum tensile and yield stresses by class and diameter are collected on the site’s bolt grade strength chart. Where the question is the torque needed to reach a target preload, the site’s bolt torque chart is organised by thread size and property class.

References

• GB/T 3098.1-2010 - Mechanical properties of fasteners - Bolts, screws and studs
• GB/T 3098.2-2015 - Mechanical properties of fasteners - Nuts
• MIL-STD-1312-8A (1984) - Fastener Test Methods, Method 8: Tensile Strength
• ISO 898-1:2009 - named by GB/T 3098.1-2010 as the standard it was prepared by modification (MOD) of; not consulted directly in this entry.