Steels_ Metallurgy and Applications, Third Edition

152 Steels: Metallurgy and Applications

2. To achieve the strength levels of controlled-rolled materials in steels of lower
   alloy content.

The latter approach is particularly attractive in that it utilizes steels of lower
carbon equivalent and therefore provides improved weldability. This effect is
shown schematically in Figure 2.9.
An extension of normal accelerated cooling after rolling is that employing
the faster cooling rates of direct quenching. Whereas the former is concerned
with refinement of the ferrite grains, the latter is concerned with the formation
of lower temperature transformation products such as bainite and martensite.
Direct quenching avoids the reheating costs associated with conventional off-line
hardening treatments but still requires a subsequent tempering treatment.

Standard specifications

For many years, the relevant UK standard for structural steels was BS 4360 Weld-
able structural steels and provided the comprehensive yield strength-toughness
matrix shown in Figure 2.10. Thus there were four levels of strength (40, 43,
50 and 55), the numbers referring to the minimum tensile strengths in kgffmm 2.
Within each strength grade, there were various sub-grades which represented
increasing levels of toughness, designated by the following nomenclature:

Minimum Charpy V value of 27J at:

A no requirement DD -30"C

B 20~ E -40~

C 0~ EE -50~

D -20~ F -60*C

O4

E

E

Z

500

400

>" 300

.~

E i

200

55F

9 9 55EE 9 55C

No

impact strength

requirement

50F 50DD

9 9 50EE 9 9 50D 9 50C 9 50B 9 50A

9 43EE 9 43D 9 43C 9 43B 9 43A

9 40EE

9 40D 9 40C 9 40B 9 40A

I I I I I^1 ~.

-60 -40 -20 0 20 v

Temperature (~ for 27J Charpy V value

Figure 2.10 Yield strength-impact strength requirements in BS 4360:1986 Weldable
structural steels (product forms-plates, strip and wide flats)