Quench hardening of steel

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.

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.

Changes during tempering

The principles underlying the tempering of quenched steels have a close similarity to those of precipitation hardening. The overlapping changes, which occur when high carbon martensite is tempered, are shown in Fig. 2 and as follows:

• Stage 1. 50-200°C. Martensite breaks down to a transition precipitate known as c-carbide (Fe2,4C) across twins and a low carbon martensite which results in slight dispersion hardening, decrease in volume and electrical resistance.
• Stage 2. 205-305°C. Decomposition of retained austenite to bainite and decrease in hardness.
• Stage 3. 250-500°C. Conversion of the aggregate of low carbon martensite and c-carbide into ferrite and cementite precipitated along twins, which gradually coarsens to give visible particles and rapid softening, Fig. 3.
• Stage 4. Carbide changes in alloy steel at 400-700°C. In steels containing one alloying addition, cementite forms first and the alloy diffuses to it. When sufficiently enriched the Fe3C transforms to an alloy carbide. After further enrichment this carbide may be superseded by another and this formation of transition carbides may be repeated several times before the equilibrium carbide forms. In chromium steel, changes are: Fe3C®Cr7C3®Cr23C6. In steels containing several carbide-forming elements the reactions are often more complex, and the carbides which decompose are not necessarily followed by carbides based on the same alloy elements. The transformation can also occur in situ by gradual exchange of atoms without any appreciable hardening; or by resolution of existing iron carbides and fresh nucleation of coherent carbide with considerable hardening that counteracts the normal softening that occurs during tempering. In some alloy steels, therefore, the hardness is maintained constant up to about 500°C or in some cases it rises to a peak followed by a gradual drop due to breakdown of coherence and coalescence of the carbide particles. This age-hardening process is known as secondary hardening and it enhances high temperature creep properties of steel (e.g. steel E in Fig. 2). Chromium, for an example, seems to stabilise the size of the cementite particles over a range 200-500°C. Vanadium and molybdenum form a fine dispersion of coherent precipitates (V₄C₃Mo₂C) in a ferrite matrix with considerable hardening. When over-ageing starts the V₄C₃ grows in the grain boundaries and also forms a Widmanstätten pattern of plates within the grain.

Figure 2.Tempering curves for 0,35 % C steel and die steel

Figure 3. Low carbon lath martensites have a high Ms temperature and some tempering often occurs on cooling, i.e. autotempering.

Tempering at 300°C causes precipitation of carbides within the laths in Widmanstätten form (Fig. 3). Tempering at 500°C promotes the recovery of the dislocation tangle into cells within the laths with carbides precipitated along boundaries. Tempering at 600°C gives rise to recrystallisation into equioxed ferrite with carbides re-nucleated at the boundaries.

Quench cracks

The volume changes, which occur when austenite is cooled, are: a) expansion when gamma iron transforms to ferrite; b) contraction when cementite is precipitated; c) normal thermal contraction.

When steel is quenched these volume changes occur very rapidly and unevenly throughout the specimen. The outside cools most quickly, and is mainly martensitic, in which contraction (b) has not occurred. The centre may be troostitic and contraction (b) started.

Stresses are set up which may cause the metal either to distort or to crack if the ductility is insufficient for plastic flow to occur. Such cracks may occur some time after the quenching or in the early stages of tempering.

Quench cracks are liable to occur:
a) due to presence of non-metallic inclusions, cementite masses, etc.;
b) when austenite is coarse grained due to high quenching temperature;
c) owing to uneven quenching;
d) in pieces of irregular section and when sharp re-entrant angles are present in the design.

The relation of design to heat-treatment is very important. Articles of irregular section need special care. When steel has been chosen which needs a water-quench, then the designer must use generous fillets in the corners and a uniform section should be aimed at. This can sometimes be obtained by boring out metal from bulky parts without materially affecting the design; examples are given in Fig. 4.

A hole drilled from the side to meet a central hole may cause cracking and it should be drilled right through and temporarily stopped up with asbestos wool during heat-treatment. A crack would also form at the junction of the solid gear with the shaft. There is a serious danger of cracks at the roots of the teeth, owing to the great change in size of section. This design could be improved by machining the metal away under the rim to make a cross-section of uniform mass.

Figure 4. The relation of design to heat-treatment

Fundamentals of heat-treatment

Heat-treatment of steel involves the change of austenite, a face-centred cubic iron lattice containing carbon atoms in the interstices, into a body-centred cubic ferrite with a low solubility for carbon.

The carbon atoms segregate into areas to form cementite. This involves mobility or diffusion of the carbon atoms and both time and temperature are important. Atomic movements are rapid at high temperatures but increasingly sluggish as the temperature decreases.

As the rate of cooling of an austenitised steel increases the time allowed for the changes is shortened and the reactions are incomplete at 600-700°C. Residual austenite, therefore, transforms at lower temperatures, with shorter movements of atoms and finer structures. At temperatures below about 250°C diffusion is so slow that another transition structure is formed.

The effect of rapid cooling on the critical points is complex (Fig. 5). Increase in the rate of cooling has the following effects:

1. Arrest temperatures are depressed.
2. Ar3 merges with Ar1 producing a single depressed point known as Ar". Fine laminated troostite is formed.
3. Accelerated cooling causes another arrest to appear at 350-150°C, known as Ar". Troostite and martensite are formed.
4. Rapid quenching causes Ar" to merge into Ar". Martensite is formed.
5. The arrest due to the formation of bainite at 500-250°C does not usually appear with carbon steel, but is present with many alloy steels.

Figure 5. Effect of cooling rate on the transformation of austenite