H. K. D. H. Bhadeshia

Supersaturated solutions are prominent in this list and the extent of metastability depends both on the excess concentration and on the equilibrium solubility. It can be demonstrated that excess carbon which is forced into solution in martensite is the major contributor to the stored energy of martensite.

The calculations presented in Table 2 show the components of the stored energy of martensite in a typical low--alloy martensitic steel Fe-0.2C-1.5Mn wt%. It is necessary to define a reference state, which is here taken to be an equilibrium mixture of ferrite, graphite and cementite, with a zero stored energy. Graphite does not in fact form because it is too slow to precipitate; the effect of replacing the graphite with cementite is to increase the stored energy by some 70 J mol^-1.

When transformations occur at low temperatures, it is often the case that substitutional elements like manganese and iron cannot diffuse during the time scale of the experiment, whereas carbon is still mobile. The transformation then happens in such a way that the Fe/Mn ratio is maintained constant whilst the carbon redistributes subject to this constrain, until its chemical potential becomes uniform. This is known as paraequilibrium. Unlike the equilibrium state, because the iron and manganese atoms are trapped during transformation, their chemical potentials are no longer uniform. This adds a further 315 J mol^-1 to the stored energy.

When bainite forms, the transformation mechanism is displacive, there is a shape deformation, which leads to an additional 400 J mol^-1 of stored energy. Since there is no diffusion during transformations, but the carbon partitions following growth, the total stored energy is that for the paraequilibrium state added to the strain energy term, giving a net value of 785 J mol^-1.

Martensite is not only a diffusionless transformation, but it frequently occurs at low temperatures where its virgin microstructure is preserved. Even the carbon remains trapped in the product crystal. Furthermore, the strain energy term associated with martensite is greater at about 600 J mol^-1 because the plates tend to have a larger aspect ratio (thickness/length). There may also be twin interfaces within the martensite plates, which cost about 100 J mol^-1. The trapping of carbon inside the martensite adds a further 629 J mol^-1, which makes the total stored energy in excess of 1700 J mol^-1!

Table 2: Stored energies of a variety of microstructures

In high-carbon steels, the precipitation of excess carbon begins with the formation of a transition carbide, such as ε (Fe_2.4C). ε-carbide can grow at temperatures as low as 50^oC. Indeed, most of the iron carbides can precipitate at low temperatures, well below those associated with the motion of substitutional solutes. This is because they grow by a displacive mechanism which does not require the redistribution of substitutional atoms (including iron); carbon naturally has to partition. This corresponds to a process known as paraequilibrium transformation in which the iron to substitutional solute ratio is maintained constant but subject to that constraint, the carbon achieves a uniform chemical potential.

Martensite is said to be supersaturated with carbon when the concentration exceeds its equilibrium solubility with respect to another phase. However, the equilibrium solubility depends on the phase. The solubility will be larger when the martensite is in equilibrium with a metastable phase such as ε carbide. Some 0.25 wt% of carbon is said to remain in solution after the precipitation of ε-carbide is completed.

Although most textbooks will begin a discussion of tempering with this first stage of tempering, involving the redistribution of carbon and precipitation of transition carbides, cementite can precipitate directly. This is particularly the case when the defect density is large. Trapped carbon atoms will not precipitate as transition carbides but cementite is more stable than trapped carbon.

More micrographs of vanadium & molybdenum carbide precipitation in tempered martensite

Severe Tempering

The cobalt plays a key role in retarding the recovery of martensite during tempering, thereby retaining the defect structure on which M₂C needles can precipitate as a fine dispersion. By increasing the stability of body-centred cubic iron, it also reduces the tendency of martensite to revert to austenite during tempering. The carbon concentration is balanced such that all the cementite is replaced by the much finer alloy carbides during secondary hardening.

Impurity concentrations and inclusions are kept to a minimum by vacuum induction melting and vacuum arc refining. Unlike conventional steels, the manganese and silicon concentrations are also kept close to zero because both of these elements reduce the austenite grain boundary cohesion.

The steel is VIM/VAR double-melted and forged or rolled into the final form.

The as-received steel is usually "homogenised" at 1200^oC for 8 hours. This is because the cast and forged alloy contains banding due to chemical segregation. Austenitisation is at about 850^oC for 1 h, followed by quenching in oil to ambient temperature and cryogenic treatment to reduce the amount of retained austenite from some 2% to less than the detection limit. The sample is then tempered in the range 500-600^oC, depending on the properties required. Since the Ae₁ temperature is about 485^oC, thin films of nickel-rich austenite grow during tempering. The films are apparently beneficial to the mechanical properties.

The optimum combination of strength and toughness is obtained by tempering at 470^oC. The as-quenched steel has a martensitic microstructure with a few undissolved MC (5-12 nm) and M₂₃C₆-type carbides (20-100 nm). The high toughness (about 160 MPa m^1/2) in the as-quenched state is believed to be due to the low strength, the cleanliness of the steel and the fact that the undissolved carbides are spherical. It has been suggested that the toughness in this state can be further improved by refining the M₂₃C₆ particle size; since the steel is not used in the as-quenched condition, the significance of this result is in emphasising the need for cleanliness. Any inclusions must clearly be smaller than the M₂₃C₆ particle size-range.

Tempering at 430^oC, 5 h is associated with a minimum in toughness because of the precipitation of relatively coarse cementite platelets in a Widmanstätten array. An increase in the tempering temperature to 470^oC leads to the coherent precipitation of needle--shaped molybdenum--rich zones, and a peak in the strength; the precipitation occurs at the expense of the cementite particles, so the increase in strength is also accompanied by a large increase in toughness. The formation of austenite films may also contribute to the toughness.

Further tempering leads to the precipitation of M₂C carbides, recovery of the dislocation substructure, and a greater quantity of less stable reverted-austenite. The austenite that forms at higher temperatures has a lower nickel concentration and its instability is believed to be responsible for the decrease in toughness beyond about 470^oC tempering, in spite of the decrease in strength.

Case Studies: Creep-Resistant Steels

Creep resistant steels must perform over long periods of time in severe environments. The typical service life is over a period of 30 years, at tempertures of 600°C or more, whilst supporting a design stress of 100 MPa. They are therefore required to resist both creep and oxidation. Their microstructures must clearly be stable in both the wrought and welded states. To resist thermal fatigue, the steel must have a small thermal expansion coefficient and an high thermal conductivity; ferritic steels are much better than austenitic steels with respect to both of these criteria.

The conditions described above correspond to low strain rates and relatively low temperatures. The mechanism of creep then involves the glide of slip dislocations. Diffusion-assisted dislocation climb in necessary for continued deformation when the glide process is obstructed, for example by the presence of precipitates in the glide plane. An applied stress assists the climb process via a force which tends to push the dislocation onto a parallel plane, such that it can by-pass the particle.