Tempering of martensite

7.3.5 Tempering of martensite

The presence of martensite in a quenched steel, while greatly increasing its hardness and TS, causes the material to be brittle. Such behavior is hardly surprising, since the formation of martensite is

Mechanical properties II – Strengthening and toughening 421 accompanied by severe matrix distortions. The hardness and strength of martensite increase sharply

with increase in C content. Contributions to the strength arise from the carbon in solution, carbides precipitated during the quench, dislocations introduced during the transformation and the grain size.

Although the martensite structure is thermodynamically unstable, the steel will remain in this condition more or less indefinitely at room temperature because, for a change to take place, bulk diffusion of carbon, with an activation energy Q of approximately 83 kJ mol −1 atom is necessary. However, because there is an exponential variation of the reaction rate with temperature, the steel will be able slowly to approach the equilibrium structure at a slightly elevated temperature, i.e. rate of reaction = Aexp[−Q/kT ]. Thus, by a carefully controlled tempering treatment, the quenching stresses can be relieved and some of the carbon can precipitate from the supersaturated solid solution to form a finely dispersed carbide phase. In this way, the toughness of the steel can be vastly improved with very little detriment to its hardness and tensile properties.

The structural changes which occur on tempering may be considered to take place in three stages. In the primary stage, fine particles of a cph carbide phase (ε-carbide) of composition about Fe 2,4 C, precipitates, with the corresponding formation of low-carbon martensite. This low-carbon martensite grows at the expense of the high-carbon martensite until, at the end of this stage, the structure consists of retained austenite, ε-carbide and martensite of reduced tetragonality. During the second stage any retained austenite in the steel begins to transform isothermally to bainite, while the third stage is marked by the formation of cementite platelets. The precipitation of cementite is accompanied by a dissolution of the ε-carbide phase so that the martensite loses its remaining tetragonality and becomes bcc ferrite. The degree to which these three stages overlap will depend on the temperature of the anneal and the carbon content. In consequence, the final structure produced will be governed by the initial choice of steel and the properties, and hence thermal treatment, required. Alloying elements, with the exception of Cr, affect the tempering of martensite. Plain carbon steels soften above 100 ◦

C owing to the early formation of ε-carbide, whereas in Si-bearing steels the softening is delayed to above 250 ◦

C, since Si stabilizes ε-carbide and delays its transformation to cementite. Alloying additions (see Table 7.2) thus enable the improvement in ductility to be achieved at higher tempering temperatures. When a steel specimen is quenched prior to tempering, quenching cracks often occur. These are caused by the stresses which arise from both the transformation and the differential expansion produced when different parts of the specimen cool at different rates. To minimize such cracking, the desired properties of toughness and strength are often produced in the steel by alternative heat- treatment schedules; examples of these schedules are summarized in Figure 7.28, from which it will become evident that advantage is taken of the shape of the TTT curve to economize on the time the specimen is in the furnace, and also to minimize quenching stresses. During conventional

Table 7.2 Influence of alloying additions on tempering. Element

Retardation in tempering Ratio of retardation of per 1% addition

tempering to depression of M s

C −40

Negative

Co

Cr

Mn

Mo

Ni

Si

V 30 > 1.0

422 Physical Metallurgy and Advanced Materials

Transformation ature

M S to bainite 200

Time (s)

Figure 7.28 Diagrams showing the heat-treatment procedure during isothermal annealing (a), martempering (b) and austempering (c).

annealing, for example, the steel is heated above the upper critical temperature and allowed to cool slowly in the furnace. In isothermal annealing the steel is allowed to transform in the furnace, but when it has completely transformed, the specimen is removed from the furnace and allowed to air-cool, thereby saving furnace time. In martempering, the knee of the TTT curve is avoided by rapid cooling, but the quench is interrupted above M s and the steel allowed to cool relatively slowly through the martensite range. With this treatment the thermal stresses set up by very rapid cooling are reduced. Such a procedure is possible because at the holding temperature there is ample time for the temperature to become equal throughout the sample before the transformation begins, and as a result the transformation occurs much more uniformly. After the transformation is complete, tempering is carried out in the usual way. In austempering, quenching is again arrested above M s and a bainite product, having similar properties to tempered martensite, is allowed to form.

Alloying elements also lower the M s temperatures and, consequently, greater stresses and distortion are introduced during quenching. This can be minimized by austempering and martempering as discussed above, but such treatments are expensive. Alloying elements should therefore be chosen to produce the maximum retardation of tempering for minimum depression of M s ; Table 7.2 shows that (1) C should be as low as possible, (2) Si and Co are particularly effective, and (3) Mo is the preferred element of the Mo, W, V group, since it is easier to take into solution than V and is cheaper than W.

Some elements, particularly Mo and V, produce quite high tempering temperatures. In quantities above about 1% for Mo and 0.5% for V, a precipitation reaction is also introduced, which has its maximum hardening effect at 550 ◦

C. This phenomenon of increased hardness by precipitation at higher temperatures is known as secondary hardening and may be classified as a fourth stage of tempering. 2–2 1 2 Mo addition produces adequate temper resistance and changes the precipitate to Mo 2 C, which is more resistant to overageing than Cr 7 C 3 , which is present in most alloy steels. High

V additions lead to undissolved V 4 C 3 at the quenching temperature, but 0.5V in conjunction with 2Mo does not form a separate carbide during tempering but dissolves in the Mo 2 C. Cr also dissolves in Mo 2 C but lowers the lattice parameters of the carbide and hence lowers the temper resistance by decreasing the matrix/carbide mismatch. However, 1Cr may be tolerated without serious reduction in temper resistance and reduces the tendency to quench crack. Si decreases the lattice parameter of matrix ferrite and hence increases temper resistance. A typical secondary hardening steel usually contains 0.4C, 2Mo, 0.5V, 0.5Si and 1.5Cr, with 1.8 GN m −2 TS and 15% elongation.