Stainless Steel Fasteners

 

Introduction

According to BS EN ISO 3506, stainless steel fasteners are specified. Bolts, screws, and studs are covered in Part 1. Part 2 deals with nuts. These requirements took the place of BS 6105. The most recent edition, which was published in 2020, now covers Duplex and 6% Mo Austenitic stainless steels as well as a new "100" property class for these steels (as well as for the previous "A4" and "A5" fastener classes).

Chemical Composition of Stainless Steel Fasteners

Table 1 displays the chemical makeups of the various fastener grades. For comparison, the roughly corresponding grades in other systems are displayed. Compared to the standards for the feedstock product, typically bar, this standard's composition variations are far larger.

Grade	Chemical Composition1(%)										Notes	Alternative names
	C	Si	Mn	P	S	Cr	Mo	Ni	Cu	N
A1	0.12	1.00	6.5	0.020	0.15/0.35	16.0-19.0	0.70	5.0-10.0	1.75-2.25	–	2 3 4	303S31, 303S42, 1.4305
A2	0.10	1.00	2.00	0.050	0.030	15.0-20.0	–5	8.0-19.0	4.0	–	6 7	304, 394S17 (BS 3111), 1.4301, 1.4567
A3	0.08	1.00	2.00	0.045	0.030	17.0-19.0	–5	9.0-12.0	1.00	–	8	321, 1.4541, 347, 1.4550
A4	0.08	1.00	2.00	0.045	0.030	16.0-18.5	2.00-3.00	10.0-15.0	4.0	–	7 9	316, 1.4401, 1.4578
A5	0.08	1.00	2.00	0.045	0.030	16.0-18.5	2.00-3.00	10.5-14.00	1.00	–	8 9	316Ti, 1.4571, 316Cb, 1.4580
A8	0.030	1.00	2.00	0.040	0.030	19.0-22.0	6.0-7.0	17.5-26.0	1.50	–	–	6% Mo, 1.4547, 1.4529
C1	0.09-0.15	1.00	1.00	0.050	0.030	11.5-14.0	–	1.00	–	–	9	410, 1.4006
C3	0.17-0.25	1.00	1.00	0.040	0.030	16.0-18.0	–	1.50-2.50	–	–		431, 1.4057
C4	0.08-0.15	1.00	1.50	0.060	0.15-0.35	12.0-14.0	0.60	1.00	–	–	2 9	416, 1.4005
F1	0.08	1.00	1.00	0.040	0.030	15.0-18.0	–10	1.00	–	–	11 12	430, 1.4016, 430Ti, 1.4520 430Cb, 1.4511
D2	0.040	1.00	6.00	0.040	0.030	19.0-24.0	0.10-1.00	1.50-5.5	3.00	0.05-0.20	13	Lean Duplex, DX 2202 1.4062, 1.4162 LDX 2101
D4	0.040	1.00	6.00	0.040	0.030	21.0-25.0	0.10-2.00	1.00-5.5	3.00	0.05-0.30	14	2304 1.4362
D6	0.030	1.00	2.00	0.040	0.015	21.0-23.0	2.50-3.5	4.5-6.5	–	0.08-0.35	–	2205 1.4462
D8	0.030	1.00	2.00	0.035	0.015	24.0-26.0	3.00-4.5	6.0-8.0	2.50	0.20-0.35	15	Zeron 100 1.4501, Ferrinox 255 1.4507

Mechanical Properties of Stainless Steel Fasteners

The standard currently recognises four "property classes" that are arbitrarily given to the various austenitic stainless steel fastener grades. For each of these property classes, Table 2 displays the 0.2% proof stress, tensile strength, and elongation values.

Steel Group	Steel Grade	Property Class	Tensile Strength Rm1 min MPa	0.2% Proof Stress      Rp0.21 min MPa	Elongation A2 min mm
Austenitic Fasteners	A1, A2, A3	50	500	210	0.6d
		70	700	450	0.4d
		80	800	600	0.3d
	A4,A5,
		70	700	450	0.4d
		80	800	600	0.3d
		100	1000	800	0.2d
	70	700	450	0.4d
	80	800	600	0.3d
	100	1000	800	0.2d

The limitations on the size of fastener (M24) for which the characteristics in table 2 can be guaranteed have been eliminated in this most recent iteration. The standard is only applicable up to 39 mm diameter, according to a general provision. If the features can be reached above that, the standard may still be implemented, provided that all parties are in agreement. These modifications result in an increase from 24 to 39 mm in the maximum diameter on which the class 70 and 80 attributes can be stated without debate. This is a reflection of the former 24 mm limit's conservatism. It is true that characteristics from classes 70 and 80 are accessible above 39 mm, but this must always be negotiated with the provider.

The steel is shown as being in the annealed condition by property class 50.

Property class 70, which specifies a "cold drawn" for the bar stock from which the fasteners are manufactured, is the most typical and widely accessible supply condition.

A very hard cold drawn bar serves as the foundation for property class 80.

The official standard now includes the property class 100 condition, which is defined as having a minimum tensile strength of 1000 MPa.

The nominal tensile stress region of the thread is used to compute and report all tensile stress values.

Instead of using a prepared test piece, the length of the bolt or screw itself is used to establish the elongation measurement. Instead of being given as a percentage elongation, it is expressed in millimetres (mm) of extension, thus A = (L2 – L1)

(where L1 = original length and L2 = length after fracture)

d = nominal diameter of bolt, screw or stud

The three "property classes" that are now part of the standard are given to the three grades of Duplex stainless steel fasteners. For each of these property classes, Table 3 lists the 0.2% proof stress, tensile strength, and elongation values.

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