Steels_ Metallurgy and Applications, Third Edition

Engineering steels 287

rod has a tensile strength which is about 100 N/mm 2 lower than that obtained on
patenting, s~
As indicated in the previous section, the strength of pearlitic steels is influenced
very markedly by the carbon content, which controls the amount of pearlite in
the structure, and also by the pearlite interlamellar spacing (So). However, for a
given carbon content, the volume fraction of pearlite can be increased and the
interlamellar spacing refined by depressing the austenite to pearlite transformation
temperature. This can be achieved by:

1. Increasing the prior austenite grain size.


2. Increasing the rate of cooling from rod rolling.


3. Adding alloying elements such as chromium and manganese.

Jaiswal and Mclvor 5~ developed the following equation to illustrate the effects

of cooling rate and composition on the tensile strength of controlled cooled, plain

carbon steel rod:

Tensile strength (N/mm 2) = [267(logCR)- 293] + 1029(% C)

+ 152(% Si)4-210(% Mn)

1

+ 442(% P)[ + 5244(% Nf)

This equation relates to steels with a prior austenite grain size of ASTM 7 and
CR is the cooling rate in *C/s at 700"C, i.e. before the start of transformation.
Although nitrogen has the largest strengthening coefficient, carbon provides a
much bigger contribution to the strength of these materials because it is present
in very much higher concentrations.

Wire drawing

Prior to wire drawing, the hot-rolled rods must be cleaned in order to remove

scale. This may be carried out mechanically by grit blasting or by subjecting

the rods to a series of bending and twisting operations. However, for the more

critical applications, acid pickling is employed followed by a neutralizing wash.

The cleaned rod is then coated with lime or zinc phosphate for dry drawing or

with a layer of copper or brass for wet drawing.

As illustrated in Figure 3.45, pearlitic steels work harden very rapidly during

wire drawing to develop tensile strengths well in excess of 2000 N/mm 2.

Micro.alloy, high-carbon rod

Jaiswal and Mclvor 5~ have described the use of micro-alloy additions in high-

carbon rod in order to compensate for the lower tensile strengths that are achieved

in controlled cooled rod compared with patented material. These authors show

that additional strength can be achieved by the following mechanisms:

1. Refinement of the pearlite intedamellar spacing with the addition of chromium.


2. Solid solution strengthening using higher silicon contents.


3. Precipitation strengthening by the addition of vanadium.