Steels_ Metallurgy and Applications, Third Edition

232 Steels: Metallurgy and Applications

Table 3.8

Reference Tensile strength

symbol (N/mm 2)

P 550-700

Q 625-775

R 700-850

S 775-925

T 850-1000

U 925-1075

V 1000-1150

W 1075 - 1225

X 1150-1300

Y 1225-1375

Z 1550 min.

Steel prices

Because of the very large number of steel grades covered by BS 970, one can only
be very selective in attempting to provide an introduction to the cost structure of
engineering steels. Purely on an arbitrary basis, the selection was narrowed down
to some of the grades in the 19-29 mm ruling selection column in the Category
1 steels tabulation (Table 3.9). However, this selection was augmented by one
further grade, namely 826M40, which is recommended for the attainment of the
highest strength (1075-1225 N/mm 2) in the largest ruling section (150-250 mm)
in this table.
The prices of the six steels concerned, in the form of large billets, are shown
in Figure 3.23. This indicates a steady increase in price which reflects the
progressive addition of alloying elements such as chromium, molybdenum and
nickel.

Machinable steels

Machining is an important stage in the production of most engineering
components and, in many automotive transmission parts, machining can account
for up to 60% of the total production costs. It is not surprising therefore that
the engineering industries have called for steels with improved and consistent
levels of machinability, whilst still maintaining the other properties that ensure
good service performance. The common machining processes include turning,
milling, grinding, drilling and broaching and several of these operations might
be carried out on an automatic lathe in the production of a single component.
Each of these processes differs in terms of the metal cutting action and involves
different conditions of temperature, strain rate and chip formation. Therefore
the machining performance of a steel cannot be defined by means of a single
parameter.