Steels_ Metallurgy and Applications, Third Edition

236 Steels: Metallurgy and Applications

Following a brief description of machinability testing, the role of free cutting

additives will be discussed. Attention will then be turned to the composition and

machining characteristics of:

1. Low-carbon free cutting steels.


2. Medium-carbon steels.


3. Low-alloy steels.


4. Stainless steels.

Machinability testing

Many organizations have developed their own individual test methods for evalu-
ating machinability and these can range from tests involving tool failure in only a
few minutes of metal cutting to those simulating commercial practices and lasting
several hours. However, reproducibility is of absolute importance and, regardless
of the type of test employed, the cutting conditions such as speed (e.g. peripheral
bar speed in m/rain), feed (rate of travel of cutting tool in mm/rev) and depth of
cut (metal removed per cut in ram) must be carefully controlled.
A widely adopted laboratory test for machinability is the Taylor Tool Life test
in which the life of the tool is determined at various cutting speeds. As illustrated
in Figure 3.24, plots of tool life (T) against cutting speed (V) provide a straight
line giving the equation:

VT n ---C

where C and n are constants.
One particular parameter of tool life which features prominently in machining
evaluations is V20, the cutting speed that will provide a tool life of 20 minutes.
However, such short-term tests may not necessarily provide an accurate guide to
the performance in longer term industrial machining operations.
Although simple turning tests such as those described above are useful in steel
development or for quality control purposes, steel users often call for longer
term tests involving multi-machining operations. Thus up to two tonnes of bright
drawn bar stock might be consumed over a period of about seven hours in
tests involving the production of the type of test piece shown in Figure 3.25.
This involves turning, drilling, plunge cutting and parting operations and the
cutting rates are adjusted so as to maintain a specific dimensional tolerance in
the component over a simulated shift period. The machinability rating of a given
steel sample can then be expressed in terms of components per hour or production
time per component.
In each of the laboratory tests for machinability, rigorous attention must be
given to maintaining standard characteristics in the cutting tools in terms of grade,
hardness and tool geometry.

Role of free cutting additives

Various elements are added to steel in order to improve the machining perfor-

mance. The main free cutting additives and their perceived action in improving

machinability are as follows.