Hardenability and Heat Treatment of Tool Steels

Kallol Mondal

Department of Materials Science and Engineering

Indian Institute of Technology, Kanpur

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How can a steel be hardened? Why?

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Hardenability is the ability of Fe-C alloy to be hardened

due to formation of martensite.

For every different steel alloy there is a specific

relationship between the martensite formation and the

cooling rate.

Hardenability is not hardness. It is a qualitative measure of

the rate at which hardness decreases with distance from

the surface because of decreased martensite content.

High hardenability means the ability of the alloy to

produce a high martensite content throughout the volume

of specimen.

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Jominy test (Hardenability test)

Hardness profile

Jominy test set up

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Relation between Jominy test, cooling rate and CCT diagram (Eutectoid steel)

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Hardenability curves for five different steel alloys, eachcontaining 0.4 wt% C.

Approximate alloy compositions (wt%) are as follows: 4340–1.85 Ni, 0.80 Cr, and 0.25

Mo; 4140–1.0 Cr and 0.20 Mo; 8640–0.55 Ni, 0.50 Cr, and 0.20 Mo; 5140–0.85 Cr; and

1040 is an unalloyed steel.

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Design of wear resistant Gear

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Steel Composition

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A hardness of HRC 40 in a 9310 steel corresponds to a Jominy distance of

10/16 in. (10oC/s). At the same Jominy distance, the other steels

shown have the following hardness at the critical location:

1050 HRC 28

1080 HRC 36

4320 HRC 31

8640 HRC 52

4340 HRC 60

In Table, we find that the 86xx steels contain less alloying elements than the

43xx steels; thus the 8640 steel is probably less expensive than the 4340

steel and might be our best choice. We must also consider other factors

such as durability.

Solution

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Heat treatment of Tool Steel

High speed (HS) tools steels are used for high cutting speeds such as drills,

mill cutters, taps.

Hot-worked tool steels are intended to withstand combinations of heat,

pressure, and abrasion associated with shearing, punching or forming of

metals during manufacturing at high temperatures.

Cold-work tool steels are those intended for applications that do not

Require prolonged or repeated heating in the range above 205-260oC.

Example: Punches, thread rolling dies, etc.

Shock resisting tool steels are those intended for applications requiring

toughness and resistance to shock-loading such as hammers, chisels,

punches, driver bits and others.

Water hardening tool steels are shallow hardened and have relatively low

resistance to softening. They are suitable for woodworking tools, hand

metal cutting tools such as taps and reamers and cutlery.

Tool Steels and their usage

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Isothermal Transformation Diagram for Water Quenched Carbon Tool Steel

These steels must be cooled very rapidly

past 500 C to avoid pearlite.

Additional Mn – 0.5% - is added

for hardening saw blades by oil

quenching.

Note the %Martensite as a function of

temperature

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Isothermal Transformation Diagram for Oil Quenched

Carbon-Chromium Tool Steels

The slower cooling rate of oil results in

small thermal gradients and

more uniform transformations

through the cross sections – eg.,

a ball bearing steel.

Proeutectoid cementite curve

g pearlite

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Steps for heat treatment of Tool Steels

Austenitization

Quenching

Tempering

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Lets discuss and self-learn

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