Changing the Changing the Properties of SteelsProperties of Steels

SteelsSteels

Can be heat treated to alter properties Can be heat treated to alter properties by either:by either:

Heating and rapid cooling Heating and rapid cooling (quenching)(quenching)

Heating and slow coolingHeating and slow cooling

QuenchingQuenching

The steels shown in blue on the The steels shown in blue on the following slide can be heat treated to following slide can be heat treated to harden them by quenching.harden them by quenching.

Metals

Ferrous metals Non-ferrous metals

Steels Cast Irons

Plain carbon steels

Low alloy steels

High alloy steelsStainless & Tool steels

Grey Iron

White Iron

Malleable & Ductile Irons

Low carbon steels

Medium carbon steels

High carbon steels

Hardening TemperaturesHardening Temperatures

The temperatures for hardening depend The temperatures for hardening depend on the carbon content. on the carbon content.

Plain carbon steels below 0.4% will not Plain carbon steels below 0.4% will not harden by heat treatment. harden by heat treatment.

The temperature decreases from approx The temperature decreases from approx 820 deg C as carbon content increases 820 deg C as carbon content increases from 0.4% up to 0.8%, where temperature from 0.4% up to 0.8%, where temperature is approx 780 deg C. is approx 780 deg C.

Above 0.8% the temperature remains Above 0.8% the temperature remains constant at 780 deg C.constant at 780 deg C.

AusteniteAustenite

This is the structure of irons and This is the structure of irons and steels at high temperatures (over steels at high temperatures (over 800 deg C). 800 deg C).

For quench hardening all the For quench hardening all the material must start as Austenite.material must start as Austenite.

Quenching causes the Austenite to Quenching causes the Austenite to be partially or totally transformed to be partially or totally transformed to Martensite.Martensite.

MartensiteMartensite

Only formed by very rapid cooling Only formed by very rapid cooling from the austenitic structure. from the austenitic structure.

Needs to be above the Needs to be above the Critical Critical Cooling Rate.Cooling Rate.

The needle-like The needle-like structure of structure of martensite, the martensite, the white areas are white areas are retained austenite.retained austenite.

HardenabilityHardenability

This is dependent upon the chemical This is dependent upon the chemical composition of the steel alloy.composition of the steel alloy.

The addition of Nickel, Chromium The addition of Nickel, Chromium and Molybdenum will slow the and Molybdenum will slow the transformation to other phases and transformation to other phases and allow more martensite to form.allow more martensite to form.

Most heat treatable steels are alloys Most heat treatable steels are alloys rather than plain carbon steels.rather than plain carbon steels.

Quenching MediaQuenching Media

Four commonly used quenching media:Four commonly used quenching media: Brine – the fastest cooling rateBrine – the fastest cooling rate Water – moderate cooling rateWater – moderate cooling rate Oil – slowest cooling rateOil – slowest cooling rate Gas – used in automatic furnaces, Gas – used in automatic furnaces,

usually liquid nitrogen, can be very fast usually liquid nitrogen, can be very fast cooling.cooling.

Too rapid cooling can cause cracking in Too rapid cooling can cause cracking in complex and heavy sections.complex and heavy sections.

Depth of HardeningDepth of Hardening

Due to the Due to the mass effectmass effect, not all the , not all the section of a large component may be section of a large component may be hardened due to too slow a cooling hardened due to too slow a cooling rate. rate.

This may leave a soft core, or in This may leave a soft core, or in extreme cases prevent hardening extreme cases prevent hardening altogether.altogether.

The Heat Treatment ProcessThe Heat Treatment Process

How you heat treat makes all the difference to the steel you get

“Martensite” (brittle)

“Pearlite” (ductile)

BCC + Fe3C with different microstructures

TemperingTemperingThe brittleness of martensite makes The brittleness of martensite makes hardened steels unsuitable for most hardened steels unsuitable for most applications.applications.

This requires the steel to be tempered This requires the steel to be tempered by re-heating to a lower temperature by re-heating to a lower temperature to reduce the hardness and improve to reduce the hardness and improve the toughness. This treatment the toughness. This treatment converts some of the martensite to converts some of the martensite to another structure called bainite. another structure called bainite.

Tempering Temperatures

Slow Cooling Rate ProcessesSlow Cooling Rate Processes

NormalisingNormalising AnnealingAnnealing SpheroidisingSpheroidising Stress-relief annealingStress-relief annealing

NormalisingNormalising

1.1. Heat to Upper Critical Temperature, Heat to Upper Critical Temperature, at which point the structure is all at which point the structure is all AusteniteAustenite

2.2. Cool slowly in air.Cool slowly in air.

3.3. Structure will now be fine equi-axed Structure will now be fine equi-axed pearlite.pearlite.

4.4. Used to restore the ductility of cold Used to restore the ductility of cold or hot worked materials whilst or hot worked materials whilst retaining other properties.retaining other properties.

AnnealingAnnealing

1.1. Heat to above Upper Critical Heat to above Upper Critical Temperature, at which point the Temperature, at which point the structure is all Austenitestructure is all Austenite

2.2. Cool very slowly in the furnace.Cool very slowly in the furnace.

3.3. Structure will now be large-grained Structure will now be large-grained pearlite.pearlite.

4.4. Used to improve the properties of Used to improve the properties of cast and forged steels prior to cast and forged steels prior to machining.machining.

Grain GrowthGrain Growth

A glass vial containing a liquid that foams. Shaking results in a fine foam, which slowly coarsens with time. The coarsening process is somewhat analogous to grain growth in solids.

The same vial, after allowing some time for the foam to coarsen. The process occurs in order to reduce the surface per unit volume.

SpheroidisingSpheroidising

1.1. Heat to just below Lower Critical Heat to just below Lower Critical Temperature. (about 650-700 deg C)Temperature. (about 650-700 deg C)

2.2. Cool very slowly in the furnace.Cool very slowly in the furnace.

3.3. Structure will now be spheroidite, in Structure will now be spheroidite, in which the Iron Carbide has ‘balled which the Iron Carbide has ‘balled up’.up’.

4.4. Used to improve the properties of Used to improve the properties of medium and high carbon steels prior medium and high carbon steels prior to machining or cold working.to machining or cold working.

Process (stress-relief) Process (stress-relief) AnnealingAnnealing

1.1. Heat to below Upper Critical Heat to below Upper Critical Temperature to cause Temperature to cause recrystallisationrecrystallisation

2.2. Cool very slowly in the furnace.Cool very slowly in the furnace.

3.3. Structure will now be equi-axed Structure will now be equi-axed pearlite.pearlite.

4.4. Used to maximise the ductility of low Used to maximise the ductility of low carbon steels and other materials carbon steels and other materials after cold working.after cold working.

Cold WorkingCold Working

Cold roll to “pancake” grains

Increases hardness and strength at the expense of ductility.