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Chemical composition and mechanical properties of stainless steels to BS 6744

Introduction

The current version of BS 6744 ‘Stainless steel bars – Reinforcement of concrete – Requirements and test methods’ was published in 2016. The new version removes plain round bars from its scope and mainly applies to just ribbed stainless reinforcing steel bars in strength grade 500. Thus grades 200 and 650 have been removed, although 500 MPa. plain bar and 650 MPa. ribbed bar are now in an informative annex, (Annex I). The standard now includes fatigue property requirements, see below.

The standard lists the composition limits for nine stainless steel grades in three categories, i.e. ‘Alloys used for low magnetic permeability’, ‘Alloys used for corrosion resistance to chloride ingress in concrete’ and ‘Alloys used for special applications, e.g. where the steel may be exposed’.   See below.

The grade designations are harmonised with the European steel number system, but currently there is no European standard for these products, although a European standard BS EN 104031, ‘Steel for the reinforcement of concrete – Stainless steel’, is in preparation. The symbols employed in the standard have also been further harmonised with those employed in other current European standards.

The steels included in BS 6744 can also be found in BS EN 10088-3. The specified composition ranges are the same, but with some differences in the number of decimal places for the percentages of some elements shown. The ranges are however shown in this article. The tensile properties are also included in this article, but these are not expressed in the same way as in BS EN 10088-3, the general standard for stainless steel bars. The strength levels for bars to BS 6744 are expressed as the ‘Grade’ and indicate the proof strength of the steel. Guidance on steel number selection in a range of service environments is given in Annex B of the standard.

The information from the previous version of the standard, (2009), is summarised in the BSSA Special Report ‘The use of stainless steel reinforcement in bridges‘. This report however was produced some years before the current standard was issued and has not been updated, so you should always consult the full version of the current standard in order to check for any changes.

As always the data given is not intended to replace that shown in individual standards to which reference should always be made.

BS 6744 chemical composition tableA

Chemical composition (% by mass – max unless stated)
Steel designation number C Si Mn S Cr Ni Mo Cu P N
Alloys used for low magnetic permeability
1.4311 0.03 1.0 2.0 0.03 17.5/19.5 8.5/11.5 0.045 0.12/0.22
1.4436 0.05 1.0 2.0 0.03 16.5/18.5 10.5/13.0 2.5/3.0 0.045 0.11
Alloys used for corrosion resistance to chloride ingress in concreteB
1.4311 0.03 1.0 2.0 0.03 17.5/19.5 8.5/11.5 0.045 0.11
1.4162 0.04 1.0 4.0/6.0 0.015 21.0/22.0 1.35/1.70 0.1/0.8 0.1/0.8 0.045 0.12/0.22
1.4362 0.03 1.0 2.0 0.015 22.0/24.5 3.5/5.5 0.1/0.6 0.1/0.6 0.035 0.05/0.20
1.4462 0.03 1.0 2.0 0.015 21.0/23.0 4.5/6.5 2.5/3.5 0.035 0.10/0.22
1.4404 0.03 1.0 2.0 0.03 16.5/18.5 10.0/13.0 2.0/2.5 0.045 0.11
1.4429 0.05 1.0 2.0 0.03 16.5/18.5 10.5/13.0 2.5/3.0 0.045 0.12/0.22
Alloys used for special applications, e.g. where the steel may be exposedC
1.4501 0.03 1.0 1.0 0.015 24.0/26.0 6.0/8.0 3.0/4.0 0.035 0.20/0.30
1.4529 0.02 0.50 1.0 0.010 19.0/21/0 24.0/26.0 6.0/7.0 0.50/1.50 0.030 0.20/0.30

A. This does not provide an exhaustive list of designations

B. These are alloys for which the most academic research on chloride resistance is available and which have the most historic use for this application. Refer to manufacturer’s certificate for CCTL value.

C. Specialist corrosion advice might be required to specify.

BS 6744 tensile property table

Grade
Tensile Properties (min unless stated)
Characteristic Yield Strength, fyk A Stress Ratio, Rm/Rp0.2 Elongation at fracture, A5 Total elongation at maximum force, Agt
B500 500 1.08 14 5

A. Consult the full standard for the definition of this term, but it is based on  prescribed probability limits of  ‘one-sided’ statistical analyses of 0.2% Proof Strengths
Proof strength and stress ratio units are MPa (N/mm2)
Elongation values are percentages (%) (Elongation is A5, total is Agt)

 

Minimum Fatigue Property Requirements

Fatigue stress range (MPa) for N> =5 x 106 cycles with a stress ratio, smin /smax = 0.2 for different bar sizes
Nominal bar size dia. (mm) <=16 >16 <=20 >20 <=25 >25 <=32 >32 <=50
Stress Range, MPa 200 185 170 160 150

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