HEAT TREATING TOOL STEEL

We will now cover as much as possible the heat-treatment of the tools and cutters that you will be
making. To prevent the spoilage of tools during heat-treatment, you must understand the changes that
take place in the steel when it is heated, quenched and tempered.

That part of this chapter which deals with the heating cycles for typical tool steels, the quenching or
cooling procedure, and the effects of single tempering operations on the structure and properties of the
steel.

In most cases, the life of tools if proportional to their hardness can be developed in plain carbon steel by
heat-treatment if the steel contains over about 0.50 % carbon, provided the section is not very large.

When tools are made in large sections, plain carbon steel cannot be hardened adequately, and it is
necessary to add allowing elements in order to increase harden-ability.

The elements usually used for this purpose are chromium, manganese, and molybdenum.

Steel becomes hard during heat treatment because of the formation of a micro structure called
martensite. For some tool applications, the wear resistance provided by the martensite structure alone
is not sufficient, and therefore, tungsten and vanadium as well as chromium and molybdenum, are
introduced into tool steel.

These elements combine with some of the carbon in the steel to form very hard particles of carbides.
These gives the heat-treated steel much better abrasion resistance than can be developed without the
presence of alloy carbides.

When the tool operates at high speed or under high pressure or is in contact with hot metal, as in
forging, etc., special tool steels must be used that are resistant to the high temperatures encountered.

Vanadium, cobalt, and chromium combined with tungsten or molybdenum give the steel the necessary
resistance to softening at high temperatures.

Finally, in some applications, the life of the tool is more dependent on toughness than on hardness.

TYPICAL TOOL STEELS

The compositions of tool steels that we use for tool making are as follows.

PLAIN CARBON TOOL STEELS, Carbon 1.14, Manganese 0.22, Silicon 0.16 per cent.