B8 Class 2

B8 Class 2

ASTM A193/A193M-11 specifies the mechanical properties of bolts, screws and nuts made of austenitic (amongst others) stainless steels for high temperature or high pressure service and other special purpose applications.

In ASTM A193/A193M-11, austenitic steels are divided into 6 classes (1, 1A, 1B and 2, 2B and 2C) and 27 grades (B8, B8A, BBC, BBCA, B8M, B8MA, B8M2, B8M3, BBP, B8PA, B8N, B8NA, B8MN, B8MNA, B8MNLCuN, B8MLCuNA, B8T, B8TA, B8R, B8RA, B8S, B8SA, B8LN, B8LBNA, B8MLN, and B8LNA).

Midland Bright Steel supplies B8 Class 2 and B8 Class 2B material (both 304 stainless, cold drawn to increase tensile properties) that enable fastener manufacturers and others to produce products that meet or exceed the properties required by the above standard.

We stock round bar from 10 mm to 75 mm diameter, and hexagons from 10 mm to 60 mm AF.

We can also produce flats and squares not covered by the standard, on request, to reduce machining costs (up to 101.6 mm (4”) square and flats to 100 mm x 50 mm (4” x 2”]). Please enquire detailing the properties required.

Background

Grade 304 is the standard “18/8” stainless; it is the most versatile and most widely used stainless steel. It has excellent forming and welding characteristics. The balanced austenitic structure of Grade 304 enables it to be severely deep drawn without intermediate annealing, Grade 304 also has outstanding welding characteristics.

Grade 304L, the low carbon version of 304, does not require post-weld annealing and so is extensively used in heavy gauge components (over about 6mm). Grade 304H with its higher carbon content finds application at elevated temperatures. The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Good oxidation resistance in temperatures up to 870ºC. Continuous use of 304 in the 425-860ºC range is not recommended if subsequent aqueous corrosion resistance is important. Grade 304L is more resistant to carbide precipitation and can be heated into the above temperature range. 304H will become sensitized in the temperature range of 425-860ºC; this is not a problem for high temperature applications, but will result in reduced aqueous corrosion resistance.

Grade 304 is excellent in a wide range of atmospheric environments, and many corrosive media, although it is subject to pitting and crevice corrosion in warm chloride environments, and to stress corrosion cracking about about 60ºC. Grade 304 is considered resistant to potable water with up to about 200mg/L chlorides at ambient temperatures, reducing to about 150mg/L at 60ºC.

The austenitic structure also gives these grades excellent toughness, even down to cryogenic temperatures.

Heat treatment is by solution annealing – heated to 1010-1120ºC and cooled rapidly. These grades cannot be hardened by thermal treatment. Cold working (normally drawing) produces enhanced levels of tensile strength. All austenitic stainless steel fasteners are normally non-magnetic in the annealed condition.

It is common for 304 grades to be stocked in “Dual Certified” form. These items have chemical and mechanical properties complying with both 304 and 304L specifications.

Chemical Composition

See table below for B8 Class 2 material chemical composition (standard chemical compositions for a range of 304 stainless steels is also given for information).

Grade		C	Mn	P	S	Si	Cr	Ni
B8	Min	–	–	–	–	–	–	–
	Max	0.08	2.0	0.045	0.03	1.0	20.0	11.0
304	Min	–	–	–	–	–	18.0	8.0
	Max	0.08	2.0	0.045	0.03	0.75	20.0	10.5
304L	Min	–	–	–	–	–	18.0	8.0
	Max	0.03	2.0	0.045	0.03	0.75	20.0	12.0
304H	Min	0.04	–	–	–	–	18.0	8.0
	Max	0.10	2.0	0.045	0.03	0.75	20.0	10.5

Mechanical Properties

The mechanical properties of material supplied for conversion into bolts, screws and studs conforms to the requirement of the standard below. For comparison, the “normal” mechanical properties of 304 stainless are given for information

Grade	Size Dia (“)	Size Dia (mm)	Tensile Strength (MPa) min	Yield Strength 0.2% Proof (MPa) min	Elong (% in 4D min)	Red of Area (% min)	Hardness (HRC max)
B8 CL2	<= 3/4	<=19.05	860	690	12	35	35
B8 CL2	> 3/4 <=1	>19.05 <=25.4	795	550	15	35	35
B8 CL2	> 1 <= 1.1/4	>25.4 <= 31.75	725	450	20	35	35
B8 CL2	> 1.1/4 <= 1./12	>31.75 <=38.1	690	345	28	45	35
B8 CL2B	> 1.1/2 <=2	>38.1 <=50.8	655	515	25	40	35
B8 CL2B	>2 <=2.1/2	>50.8 <=63.5	620	450	30	40	35
B8 CL2 B	>2.1/2 <= 3	>63.5 <=76.2	550	380	30	40	35
304			515	205	40	55	15
304L			485	170	40	55	15
304H			515	205	40	55	15

Note: the standard allows that, for diameters of 1.1/2” (38.1mm), centre (core) properties may be lower than indicated by test results which are based on values determined at ½ radius.

Physical Properties

Typical physical properties of 304 grade stainless steel in the annealed condition.

Grade	Density (kg/m³)	Elastic Modulus (GPa)	Mean Co-eff of Thermal Expansion (μm/m/°C)	Thermal Conductivity (W/m.K)	Specific Heat 0-100 °C (J/kg.K)	Elec Resistivity (nΩ.m)
			0-100 °C 0-315 °C 0-538 °C	At 100 °C At 500 °C
304/L/H	8000	193	17.2 17.8 18.4	16.2 21.5	500	720