B8M Class 2

B8M 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, B8MNA, B8MNLCuN, B8MLCuNA, B8T, B8TA, B8R, B8RA, B8S, B8SA, B8LN, B8LBNA, B8MLN, and B8MLN, and B8LNA).

Midland Bright Steels supplies B8M Class 2 and B8M Class 2B material (both 316 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 with details of properties required.

Background

Grade 316 is the standard molybdenum-bearing grade. The molybdenum gives 316 better overall corrosion resistant properties and has resistance to pitting and crevice corrosion in a range of atmospheric environments and chloride and other corrosive media. Stress corrosion cracking starts above about 60ºC. 316 is considered resistant to potable water with up to about 1000mg/L chlorides at ambient temperatures, reducing to about 500mg/L at 60ºC. It has good oxidation resistance up to 870ºC. Continuous use of 316 in the 425-860ºC range is not recommended if subsequent aqueous corrosion resistance is important. Grade 316L is more resistant to carbide precipitation and can be used in the above temperature range. Grade 316H has higher strength at elevated temperatures and is sometimes used for structural and pressure-containing applications at temperatures above about 500ºC.

It has excellent forming and welding characteristics. Grade 316L, the low carbon version of 316 and is immune from sensitisation (grain boundary carbide precipitation). Grade 316H, with its higher carbon content has application at elevated temperatures, as does stabilized grade 316Ti.

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

Heat Treatment is by solution annealing – Heat to 1010-1120ºC and cool 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 316 and 316L to be stocked in “Dual Certified” form. These items have chemical and mechanical properties complying with both 316 and 316L specifications.

Chemical Composition

See table below for B8M Class 2 material chemical composition (standard chemical compositions for a range of 316 stainless steels is also given for information).

Grade		C	Mn	P	S	Si	Cr	Ni	Mo
B8M	Min	–	–	–	–	–	16.0	10.0	2.0
	Max	0.08	2.00	0.045	0.03	1.00	18.0	14.0	3.0
316	Min	–	–	0	–	–	16.0	10.0	2.00
	Max	0.08	2.0	0.045	0.03	0.75	18.0	14.0	3.00
316L	Min	–	–	–	–	–	16.0	10.0	2.00
	Max	0.03	2.0	0.045	0.03	0.75	18.0	14.0	3.00
316H	Min	0.04	0.04	–	–	0	16.0	10.0	2.00
	Max	0.10	0.10	0.045	0.03	0.75	18.0	14.0	3.00

Mechanical Properties

The mechanical properties of material supplied for conversion into bolts, screws and studs conforms to the requirements of the standard below. For comparison, the “normal” mechanical properties of 316 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	758	655	15	45	35
B8 CL2	> 3/4 <=1	>19.05 <=25.4	690	550	20	45	35
B8 CL2	> 1 <= 1.1/4	>25.4 <= 31.75	655	450	25	45	35
B8 CL2	> 1.1/4 <= 1./12	>31.75 <=38.1	620	345	30	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	20
304L			485	170	40	55	20
304H			515	205	40	55	20

NOTE: the standard allows that, for diameters over 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 for 316 grade stainless steels:

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
316/L/H	8000	193	15.9 16.2 17.5	16.3 21.5	500	740