Hardening of steel is obtained by a suitable quench from within or above the critical range. The temperatures are the same as those given for full annealing. The soaking time in air furnaces should be 1,2 min for each mm of cross-section or 0,6 min in salt or lead baths. Uneven heating, overheating and excessive scaling should be avoided.

The quenching is necessary to suppress the normal breakdown of austenite into ferrite and cementite, and to cause a partial decomposition at such a low temperature to produce martensite. To obtain this, steel requires a critical cooling velocity, which is greatly reduced by the presence of alloying elements, which therefore cause hardening with mild quenching (e.g. oil and hardening steels).

Steels with less than 0,3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0,7 % due to an increased tendency to retain austenite in high carbon steels Fig. 1.

Figure 1. Variation of hardness of martensite and bainite with carbon content

Water is one of the most efficient quenching media where maximum hardness is required, but it is liable to cause distortion and cracking of the article. Where hardness can be sacrificed, whale, cotton seed and mineral oils are used. These tend to oxidise and form sludge with consequent lowering of efficiency.

The quenching velocity of oil is much less than water. Ferrite and troostite are formed even in small sections. Intermediate rates between water and oil can be obtained with water containing 10-30 % Ucon, a substance with an inverse solubility which therefore deposits on the object to slow rate of cooling. To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. To prevent steam bubbles forming soft spots, a water quenching bath should be agitated.

Fully hardened and tempered steels develop the best combination of strength and notch-ductility.
Tempering and toughening

The martensite of quenched tool steel is exceedingly brittle and highly stressed. Consequently cracking and distortion of the object are liable to occur after quenching. Retained austenite is unstable and as it changes dimensions may alter, e.g. dies may alter 0,012 mm.

It is necessary, therefore, to warm the steel below the critical range in order to relieve stresses and to allow the arrested reaction of cementite precipitation to take place. This is known as tempering.

*
150-250°C. The object is heated in an oil bath, immediately after quenching, to prevent related cracking, to relieve internal stress and to decompose austenite without much softening.
*
200-450°C. Used to toughen the steel at the expense of hardness. Brinell hardness is 350-450.
*
450-700°C. The precipitated cementite coalesces into larger masses and the steel becomes softer. The structure is known as sorbite, which at the higher temperatures becomes coarsely spheroidised. It etches more slowly than troostite and has a Brinell hardness of 220-350. Sorbite is commonly found in heat-treated constructional steels, such as axles, shafts and crankshafts subjected to dynamic stresses. A treatment of quenching and tempering in this temperature range is frequently referred to as toughening, and it produces an increase in the ratio of the elastic limit to the ultimate tensile strength.

The reactions in tempering occur slowly. Reaction time as well as temperature of heating is important. Tempering is carried out to an increasing extent under pyrometric control in oil, salt (e.g. equal parts sodium and potassium nitrates for 200-600°C) or lead baths and also in furnaces in which the air is circulated by fans. After the tempering, the objects may be cooled either rapidly or slowly, except for steels susceptible to temper brittleness.

Temper colours formed on a cleaned surface are still used occasionally as a guide to temperature. They exist due to the interference effects of thin films of oxide formed during tempering, and they act similarly to oil films on water. Alloys such as stainless steel form thinner films than do carbon steels for a given temperature and hence produce a colour lower in the series. For example, pale straw corresponds to 300°C, instead of 230°C (Table 1).

Table 1.

Temper Colour	Temperature °C	Objects
Pale straw	230	Planing and slotting tools
Dark straw	240	Milling cutters, drills
Brown	250	Taps, shear blades for metals
Brownish-purple	260	Punches, cups, snaps, twist drills, reamers
Purple	270	Press tools, axes
Dark purple	280	Cold chisels, setts for steel
Blue	300	Saws for wood, springs
Blue	450-650	Toughening for constructional steels

For turning, planing, shaping tools and chisels, only the cutting parts need hardening. This is frequently carried out in engineering works by heating the tool to 730°C, followed by quenching the cutting end vertically. When cutting end gets cold, it is cleaned with the stone and the heat from the shank of the tool is allowed to temper the cutting edge to the correct colour. Then the whole tool is quenched. Oxidation can be reduced by coating the tool with charcoal and oil.