1. Describes in basic terms, the following heat-treatment processes:- tempering- annealing- normalizing- hardening

2. Explains, indicates why the processes in the above objectives are used

3. States how low-carbon steels can be given a hard case4. States why low-carbon steels are sometimes given a hard case5. Describes in basic terms a suitable heat-treatment process for

common steels, given the properties required

Various types of heat treatment processes are used to change the following properties or conditions of the steel:

•   Improve the toughness• Increase the hardness• Increase the ductility• Improve the machinability• Refine the grain structure • Remove the residual stresses• Improve the wear resistance

The Hardening ProcessesSteels can be heat treated to high hardness and strength levels. The reasons for doing this are obvious. Structural components subjected to high operating stress need the high strength of a hardened structure. Similarly, tools such as dies, knives, cutting devices, and forming devices need a hardened structure to resist wear and deformation.

QuenchingIn this process steels which contain sufficient carbon, and perhaps other alloying elements, are cooled sufficiently rapidly from above the transformation temperature to produce Martensite.

There is a range of quenching media of varying severity, water or brine being the most severe, through oil and synthetic products to air which is the least severe.

TemperingAfter quenching the steel is hard, brittle and internally stressed. Before use, it is usually necessary to reduce these stresses and increase toughness by 'tempering'. There will also be a reduction in hardness and the selection of tempering temperature dictates the final properties.

As a rule of thumb, within the tempering range for a particular steel, the higher the tempering temperature the lower the final hardness but the greater the toughness.

Note:In general terms, if steel is heated until it glows red and is quenched in clean water immediately, it becomes very hard but also brittle. This means it is likely to break or snap if put under great pressure. On the other hand, if the red hot steel is allowed to cool slowly, the resulting steel will be easier to cut, shape and file as it will be relatively soft.

Main objectives of

1. Hardening

> To increase the hardness of the metal, so that it can resist wear

> To enable it to cut other metals, ie to make it suitable for cutting tools

2. Tempering

> To reduce brittleness of the hardened steels and thus to increase its ductility

> To remove the internal stresses caused by rapid cooling of steel

> To make steel sufficiently tough to resist shock and fatigue

Type of work Tempering Temperature,oC

Tempering colour

Scrapers & lathe tools for brass

220 Light straws or pale yellow

Hammer faces, planers for steel, screwing dies for brass

230 Straw or dark straw

Moulding, and planing cutters for haedwood, punches and dies

255 Yellowish brown or brown

Gold chisels and sets for steel, cold chisels for cast iron, chisels for wood

280 Purple

Cold chisels for wrought iron 290 Violet

Screw drivers 295 Blue

Springs 300 Dark blue

Example 1: Hardening and Tempering ProcessIf a mild steel or silver steel screw driver blade has been manufactured at some point it will have to be ‘’hardened’ to prevent it wearing down when used.

1. The screw driver blade is heated, slowly at first, warming up the whole blade. Then the heat is concentrated on the area at the end of the blade. This gradually becomes ‘red’ hot.

2. The screw driver blade is removed quickly from the brazing heart, with blacksmiths tongs and plunged into clean, cold water. Steam boils off from the water as the steel cools rapidly. At this stage the blade is very hard but brittle and will break easily.

3. The screw driver blade is cleaned with emery cloth and heated again on the brazing hearth. Heat is concentrated at the end of the steel blade. The steel must be watched very carefully as it changes colour quite quickly. A blue line of heat will appear near the end of the blade and it travels towards the tip as the temperature rises along the blade. When the line of blue reaches the tip the brazing torch is turned off. The blue indicates the correct temperature of ‘tempering’.

4. The screw driver blade is placed on a steel surface, such as an anvil face. This conducts the heat away and allows slow cooling of the screw driver blade. When cold, the blade should be tough and hard wearing and unlikely to break or snap. This is due to the tempering process.

Example 2: Case Hardening of Mild Steel

Case hardening is a simple method of hardening steel. It is less complex than hardening and tempering.

This techniques is used for steels with a low carbon content. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm.

One advantage of this method of hardening steel is that the inner core is left untouched and so still processes properties such as flexibility and is still relatively soft.

1. The steel is heated to red heat. It may only be necessary to harden one part of the steel and so heat can be concentrated in this area.

2. The steel is removed from the brazing hearth with blacksmiths tongs and plunged into case hardening compound and allowed to cool a little. The case hardening compound is high in carbon.

3. The steel is heated again to a red colour, removed from the brazing hearth and plunged into cold, clean water.

The steel rod should now have a hardened outer surface and a flexible, soft interior. The process can be repeated to increase the depth of the hardened surface.

The Softening ProcessesHardening is reversible. If a hardened tool needs to be remachined, it may be softened by heat treatment to return it to its machinable condition. Most steels weld better in their soft state than in their hardened state; softening may be used to aid weldability.

Annealing is a heat process whereby a metal is heated to a specific temperature /colour and then allowed to cool slowly. This softens the metal which means it can be cut and shaped more easily. Mild steel, is heated to a red heat and allowed to cool slowly. However, metals such as aluminium will melt if heated for too long.

Aluminium can be annealed but care must be taken whilst heating. The flame should be held at a distance to the aluminium so that it gives a generalised heating to the metal.

A ‘trick of the trade’ is to rub soap on to the surface of the aluminium and then heat it on the brazing hearth. It takes only a short time for the soap to turn black. The brazing torch should be turned off immediately and the aluminium allowed to cool slowly. It is now annealed and should be very soft and malleable.

You anneal metals to relieve internal stresses, soften them, make them more ductile, and refine their grain structures.

Normalizing is a type of heat treatment applicable to ferrous metals only. It differs from annealing in that the metal is heated to a higher temperature and then removed from the furnace for air cooling. The purpose of normalizing is to remove the internal stresses induced by heat treating, welding, casting, forging, forming, or machining. Stress, if not controlled, leads to metal failure; therefore, before hardening steel, you should normalize it first to ensure the maximum desired results. Normalized steels are harder and stronger than annealed steels.

The animation above shows that metals that have not been annealed are very difficult to deform.

The animation above shows that an annealed metal is usually softer and can be deformed more easily than metals that are not annealed.

Annealed metals are relatively soft and can be cut and shaped more easily. They bend easily when pressure is applied. As a rule they are heated and allowed to cool slowly.

Hardened metals are difficult to cut and shape. They are very difficult if not impossible to bend. As a rule they are heated and cooled very quickly by quenching in clean, cold water.

It should be noted that not all steels will respond to all heat treatment processes, the following table summaries the response, or otherwise, to the different processes.

  Anneal

Normalise Harden

Temper

Low Carbon <0.3% yes yes no no

Medium Carbon 0.3-0.5% yes yes yes yes

High Carbon >0.5% yes yes yes yes

Low Alloy yes yes yes yes

Medium Alloy yes yes yes yes

High Alloy yes maybe yes yes

Tool Steels yes no yes yes

Stainless Steel (Austenitic eg 304, 306)

yes no no no

Stainless Steels (Ferritic eg 405, 430 442)

yes no no no

Stainless Steels (Martensitic eg 410, 440)

yes no yes yes

Approximate Soaking Periods for Hardening, Annealing, and Normalizing Steel

Conclusion

• The process of heat treating is the method by which metals are heated and cooled in a series of specific operations that never allow the metal to reach the molten state.

• The purpose of heat treating is to make a metal more useful by changing or restoring its mechanical properties.

• Through heat treating, we can make a metal harder, stronger, and more resistant to impact. Also, heat treating can make a metal softer and more ductile.

• The one disadvantage is that no heat-treating procedure can produce all of these characteristics in one operation. Some properties are improved at the expense of others; for example, hardening a metal may make it brittle.

STAGES OF HEAT TREATMENT

Heat treating is accomplished in three major stages:

Stage l Heating the metal slowly to ensure a uniform temperature

Stage 2Soaking (holding) the metal at a given temperature for a given time and cooling the metal to room temperature

Stage 3Cooling the metal to room temperature