Annealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.) 5. Hardening (introduction...) 5.1. Material-related Preconditions for the Hardening by Means of Heat Treatment 5.2. Principle of Hardening by Means of Heat Treatment 5.3. Kinds of Hardening by Means of Heat Treatment 5.4. Hardness-related Shaping of the Workpieces 5.5. Hardening Defects 5.6. Specific Working Hints for Practical Execution of Work 5.7. Hardness Measurement

Annealing, Hardening, Tempering - Course: Working techniques of heat treatment of steel. Trainees' handbook of lessons (Institut für Berufliche Entwicklung, 27 p.)

5. Hardening

By hammering, bending and cold rolling of iron materials and nonferrous metals a strengthening of the metal structure is already reached, this is called "cold straining". Another kind is the "age-hardening" of light metals which is carried out after the finished heat treatment by storing the materials of several days' duration.

The most important kind is the "hardening by means of heat treatment" by which the structure of steels is changed systematically. This kind of heat treatment is used for making the steels hard and wear-resistant for certain purposes.

5.1. Material-related Preconditions for the Hardening by Means of Heat Treatment

Unalloyed steel is an iron material which can be formed in hot or cold state without special after treatment Unalloyed steel contains, apart from other chemical elements, the element "carbon" in portions of 0.02 to 2.1 per cent, which especially influences the properties "hardness" and "strength".

High carbon content:	- great hardness and strength
	- low toughness and elasticity
Low carbon content:	- low hardness and strength
	- great toughness and elasticity

When the steel is heated, the properties of the steel are changed in dependence on the carbon content

That is why the carbon content of the steel must be known prior to the heat treatment!

Conditions for hardenability:

Carbon content:	above 0.35 per cent - hardenable
	below 0.35 per cent - not hardenable

Steels of a carbon content below 0.35 per cent can be hardened at their surfaces when carbon is added to the steel from outside by means of a special process (casehardening).

What is the purpose of hardening by means of heat treatment?

What minimum carbon content must a steel have for being hardenable?

What influence on the mechanical properties does a high carbon content have?

5.2. Principle of Hardening by Means of Heat Treatment

Three working steps are required for hardening:

- Heating of the workpiece up to hardening temperature (over 723°C) in dependence on the carbon content of the steel;

- Holding the temperature according to the grade of steel and the size of the workpiece;

- Sudden cooling down (quenching) of the workpiece still being at hardening temperature.

In order to reach the correct hardness for the respective steel, the required hardening temperature must be met

Selection of hardening temperatures for unalloyed steels:

Carbon content in percent	0.5	0.6	0.7	0.8	1.0	1.5
Hardening temperature in degree centigrade	830	815	800	780	770	770

The lower the carbon content, the higher must be the hardening temperature!

The holding time up to hardening temperature is dependent on the grade of steel and the size of the workpiece. Small and difficult-to-form parts only require short holding times of a few minutes duration. With increasing size of the parts and a high carbon content, a longer holding time is required.

5.3. Kinds of Hardening by Means of Heat Treatment

Quenching:

Steels which can be hardened without special preparations are hardened by this process. In this case, the steel is heated to hardening temperature and quickly cooled down once a time. As a result, the material is very hard and brittle and it can show serious internal stresses; in case of unfavourable conditions, the workpiece can distort or break.

Figure 21 - Temperature/time diagram of quenching - 1 heating, 2 keeping constant. 3 quick cooling down - I Temperature, II Time

Interrupted hardening:

By this process, steels are treated which are especially sensitive to break and distortion. The material is quenched only for a short time in a powerful quenching medium (water) until hissing is finished, after having been heated up to hardening temperature; subsequently, it is kept in a mild quenching medium (heated oil) until temperature balance. Only then it is further cooled down in air. A favourable variant is therefore the method where the material powerfully quenched is suspended into a hot bath of 200°C until temperature balance; by this, stresses occuring during the cooling-down process and the danger of break formation are effectively avoided.

Figure 22 - Temperature/time diagram of interrupted hardening - 1 heating, 2 keeping constant, 3 quick cooling down, 4 slow cooling down - I Temperature, II Time

Hot quenching:

By this process, workpieces of complicated shapes are treated. After having been heated to hardening temperature, the workpiece is cooled down in a hot bath at temperatures between 180°C and 500°C until temperature balance according to the grade of steel. Then it is cooled down (for any length of time) to ambient temperature by means of which the workpiece subsequently shows minor internal stresses only. Salt melting baths are preferably used as hot baths, the temperature of the bath must be derived from the grade of steel.

Figure 23 - Temperature/time diagram of hot quenching - 1 heating, 2 keeping constant, 3 retarded cooling down, 4 residual cooling down - I Temperature, II Time

Within this mild hardening process the holding time for the workpiece in the melting bath has to be considered for the cooling-down process. The following rule is valid:

For every 10 mm in diameter or thickness, the workpiece must be held in the melting bath for 60 seconds!

Example:

A heated shaft of 75 mm diameter has to be cooled down in a melting bath. What holding time has to be met in the melting bath?

The shaft has to be kept in the melting bath for 450 seconds or 7.5 minutes, respectively.

Which working steps are required for the hardening process?
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What feature determines the interrupted hardening?
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5.4. Hardness-related Shaping of the Workpieces

As to small workpieces, hardening is effected in the entire structure; as to greater workpieces, the degree of hardness decreases internally.

Unfavourable shapes of workpieces or working imperfections can cause crack formation after hardening.

The following problematic points can cause defects:

- irregular breakthroughs
- sharp edges
- boreholes directly made into outer surfaces
- abrupt transitions between thick and thin parts

Measures:

Sharp edges have always to be rounded; in case of abrupt transitions, a separated individual-part-hardening with subsequent joining of the individual parts is more favourable.

Figure 24 - Workpieces compared to hardness - I Favourable shape of workpiece, II Unfavourable shape of workpiece - 1 shaping of opening, 2 shaping of internal edges, 3 boreholes, 4 transition points in cross-sektion

Hints:

Quenching cracks can be determined when a dilute colour paint is coated over the hardened surfaces - existing cracks ink and become visible!

Particularities for hardening in melting baths:

- When suspending into melting baths it has to considered that the workpieces are correctly emerged according to their shape.

- The workpieces must evenly be wetted; air cushions must not be produced! Air cushions slow down the quenching process and cause soft spots in the hardness layer!

Figure 25 - Correct dipping of various workpieces into a melting bath

- Before being suspended into the melting bath, the workpieces must be predried. Minor amounts of moisture cause, at those great temperature differences, explosive evaporation of water. The vapour throws off the heated bath fluid from the melting tank.

- Vapours produced in the melting baths must be sucked off the hardening room by means of effective suction devices.

5.5. Hardening Defects

Feature	Causes of defect by
	heating	quenching	tempering
Steel to soft	hardening temperature too low, too little heated, workpiece too much cooled prior to quenching	quenching bath too hot, quenching bath too small, wrong quenching medium, quenching speed too slow, quenching time too short	tempering temperature too high, wrong temper colour, when tempering from inside, too slowly cooled down
Steel irregularly hard	irregular heating, sulphur taking in from fuel gas, scaled workpiece, sticking melting bath when using melting baths	too big tongs' bit, unclean quenching bath, pieces to be hardened lie too crowded, unsuitable covering, wrong move in the bath (vapour bulbs), annealing skin and scale	irregularly heated
Steel too hard	too high hardening temperature	quenching medium too coarse	tempering temperature too low
Distorted work-pieces	due to great cross-section differences heated wrong, unfavourable position in annealing furnace, heated too quickly and unevenly, workpiece partly superheated, covered inadequately or even not, too long kept onto hardening temperature	cooled down too coarsely, emerged wrong	-
Workpieces with cracks	irregularly and too much heated, sharp screwings not covered, not preheated	irregularly quenched - quenched too coarsely - suspended wrong

5.6. Specific Working Hints for Practical Execution of Work

- Finished parts which have to be protected from scaling or carbon loss are favourably heated in a salt bath or hardened in a packing.

Packing material can be, for low hardening temperature, peashaped-screened and roasted charcoal; in case of higher hardening temperatures, it can be burnt coke grit. It is practicable to wrap the parts into paper, additionally.

- As to the heating of workpieces of great cross-section differences or sensitive points, protective covers made of sheet metal, asbestos or clay must be laid onto the weaker or more sensitive points in order to protect them from being overheated.

- Prior to hardening, it is practicable to preheat the workpieces to about 500°C and to hold them at that temperature. At the beginning of hardening, the workpieces are quickly heated to hardening temperature and then, they are further machined.

Why is it favourable to harden the workpieces in a carbon containing packing?
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5.7. Hardness Measurement

Hardness is the resistance a body puts up against the forcible entry of a harder body.

The easiest possibility to check this hardness is the use of a used fine file by means of which a file test is carried out If the file slips on the edge and does not penetrate, the material is harder than the file.

Figure 26 - File test

Frequently, the exact hardness is however required for the application of a workpiece. In those cases, the hardness must be determined by means of special testing devices.

According to the kind of power action onto the piece to be tested, we distinguish test procedures with statical or dynamical power action.

Principle of any hardness measurements:

A testing piece penetrates into the material to be tested - a value for the hardness of the material is derived from the deformation produced therefrom.

The produced hardness values are specified without measuring unit, only with a symbol according to the hardening process. Example for a hardness specification:

Figure

How can hardness be checked in a simple way?
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