Steels_ Metallurgy and Applications, Third Edition

Engineering steels 229

Table 3.6

Grade C%

420S29 0.14-0.2

420S37 0.2-0.28

431S29 0.12-0.2

304S 15 0.06 max.

310S31 0.15 max.

Cr% Ni%

....

11.5-13.5

12-14

15-18 2-3

17.5-19 8-11

24-26 19-22

austenitic and martensitic/ferritic grades respectively. The interventing letter S
also signifies a stainless steel but the last two digits, chosen arbitrarily in the
range 11 to 99, indicate variants within the main type. Examples are shown in
Table 3.6.
Quenched and tempered grades are no longer included in BS 970, following
the publication of the following European standards:

BS EN 10083-1: 1991 Quenched and tempered steels (special steels)

BS EN 10083-2:1991 Quenched and tempered steels (unalloyed

quality steels)

In the former, a useful comparison is provided of grades in the new European
standard and equivalent grades in previous national standards. This information
is shown in Table 3.7.
For many years, BS 970 adopted a system whereby a single letter was used
to specify a particular range of tensile strength in the quenched and tempered
condition. This system is still widely used in the UK and the full range of letters
and the associated tensile ranges are shown in Table 3.8.
Former versions of BS 970: Part 1 contained an appendix which provided a
guide to the selection of Category 1 through hardening steels, based on tensile
strength and limiting ruling section. This guide was prepared for applications
where the most important criterion was the level of mechanical properties required
in the finished part and with the aim of assisting in the selection of the most
cost-effective grade for a given tensile strength and section size. Although many
of the steel designations are now obsolete with the introduction of European
standards, it was considered worthwhile to maintain this information in the
present text and this is shown in Table 3.9. The first column in this table (Heat
treatment condition) lists the various ranges of tensile strength according to the
lettering system P-Z defined above. Along each row of the table, the ruling
section increases and the various grades of steel that will satisfy the required
combination of tensile strength and section size are identified. Thus a simple
C-Mn steel (080M30) is recommended for an application calling for a tensile
strength of 625-775 N/mm 2 (Q condition) in small section sizes. On the other
hand, a 2.5% Ni-Cr-Mo steel (826M40) is recommended when a tensile strength
of 1075-1225 N/mm 2 (W condition) is required in section sizes greater than
150 ram.