HEAT TREATMENT OF TOOL STEEL and HEAT TREATMENT OF HIGH SPEED STEEL

  HEAT TREATMENT OF TOOL STEEL


First of a1l, the purpose
or functional requirements of the tool to be used should
be understood clearly. Accordingly the heat-treatment
will be carried on the tool to obtain the desired qua1ities in the tool steel
to meet the needed objective. For example a cutting tool requires particularly
sufficient strength, high hardness and high wear-resistance. Therefore, it is
initially shaped by forging operation which should be carried out at
temperature 850°- 950°C with the help of hammers. Next normalizing is to be
performed on it to relieve the stresses and strains developed during forging
and to have a uniform grain structure. Next the tool steel is hardened by
heating followed by sudden or rapid cooling depending upon the
carbon percentage in tool steel.
The various heating
ranges of different tool steels are as follows

C%  in tool steel        Heating Range

 0.7- 0.8%                    780-850°C

0.8- 0.95%                   765-790°C

0.95-1.10%                     750-775°C

Above 1.10 %              740-760°C


The tool steel is heated to this
temperature range and kept at that temperature
for sufficient time to achieve uniform
structure inside the metal. Then it is quenched by immersing
it into a bath of fresh water (rapid cooling). For securing more homogenous
cooling
and reduce danger of
cracking, brine solution (10% brine or caustic soda solution) may be used.
After hardening, tempering is performed to remove extra hardness and
brittleness induced during hardening. This is performed by reheating the
hardened tool steel up to 150-300°C once again and same is cooled in oils to
reduced internal
stresses. Tempering is carried over  to tool steel for the purpose of increasing
its usefulness and to provide good results in
its
performance. The important point is to note that drills and milling cutters
should be hardened through out. But certain tools
like screw taps, screw dies, lathe, planer and shaping tools
are not hardened throughout the surface of the cutting
tools.


    HEAT TREATMENT OF HIGH SPEED STEEL

First
the HSS tool is heated to about 850°C and kept at this temperature for 4 to 5 hours. This is done to dissolve all the
carbides or homogenization of WC, VC
and Cr
4C3. After
it, tool is heated to 1200°C for 1-4 minutes. The purpose of heating to high
temperature is that more the substance is cooled from high temperature to lower
temperature difference, the more will be the hardness.
Tool 
is not kept at such temperature for sufficient longer time. After this,     it is quenched
in salt bath to 650°C and kept at this temperature for 10-20 minutes. Direct quenching to room temperature is
dangerous. Then the tool is oil quenched. For increasing the life of HSS tool,
surface treatment processes are also done like, liquid cyaniding, gas cyaniding and solid or dry cyaniding.

8 thoughts on “HEAT TREATMENT OF TOOL STEEL and HEAT TREATMENT OF HIGH SPEED STEEL”

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