The Tempering of Martensite

Martensite is a very strong phase but it is normally very brittle so it is necessary to modify the mechanical properties by heat treatment in the range 150-700°C. This process, which is called tempering, is one of the oldest heat treatments applied to steels although it is only in recent years that a detailed understanding of the phenomena involved has been reached.

Essentially, martensite is a highly supersaturated solid solution of carbon in iron, which, during tempering, rejects carbon in the form of finely divided carbide phases. The end result of tempering is a fine dispersion of carbides in an a-iron matrix, which often bears little structural similarity to the original as-quenched martensite.

It should be noted that, in many steels, the martensite reaction does not go to completion on quenching, resulting in varying amounts of retained austenite which does not remain stable during the tempering process.

Tempering of plain carbon steels
The as-quenched martensite possesses a complex structure. This occurs in the first-formed martensite, i.e. the martensite formed near Ms, which has the opportunity of tempering during the remainder of the quench. This phenomenon, which is referred to as auto-tempering, is clearly more likely to occur in steels with a high Ms.
On reheating as-quenched martensite, the tempering takes place in four distinct but overlapping stages:

up to 250°C, precipitation of a-iron carbide; partial loss of tetragonality in martensite
between 200 and 300°C, decomposition of retained austenite
between 200 and 350°C, replacement of a-iron carbide by cementite; martensite loses tetragonality
above 350°C, cementite coarsens and spheroidizes; recrystallization of ferrite.

Tempering stage 1
Martensite formed in medium and high carbon steels (0.3-1.5% C) is not stable at room temperature because interstitial carbon atoms can diffuse in the tetragonal martensite lattice at this temperature. This instability-increases between room temperature and 2500°C, when iron carbide precipitates in the martensite.

Tempering stage 2
During stage 2, austenite retained during quenching is decomposed, usually in the temperature range 230-300°C. Cohen and coworkers detected this stage by X-ray diffraction measurements as well as dilatometric and specific volume measurements. However, the direct observation of retained austenite in the microstructure has always been rather difficult, particularly if it is present in low concentrations. The little available evidence suggests that in the range 230-300°C, retained austenite decomposes to bainite, ferrite and cementite, but no detailed comparison between this phase and lower bainite has yet been made.

Tempering stage 3
During the Third stage of tempering, cementite first appears in the microstructure as a Widmanstatten distribution of rods, which have a well-defined orientation relationship with the matrix which has now lost its tetragonality and become ferrite.
This reaction commences as low as 100°C, and is fully developed at 300°C, with particles up to 200 nm long and ~15 nm in diameter. Similar structures are often observed in lower carbon steels as quenched, as a result of the formation of Fe3C during the quench. During tempering, the replacement of transition carbides and low-temperature martensite by cementite and ferrite.

During the third stage of tempering the tetragonality of the matrix disappears and it is then, essentially, ferrite, not supersaturated with respect to carbon. Subsequent changes in the morphology of the cementite particles occur by an Ostwald ripening type of process, where the smaller particles dissolve in the matrix providing carbon for the selective growth of the larger particles.

Tempering stage 4
It is useful to define a fourth stage of tempering in which the cementite particles undergo a coarsening process and essentially lose their crystallographic morphology, becoming spheroidized. The coarsening commences between 300 and 400°C, while spheroidization takes place increasingly up to 700°C. At the higher end of this range of temperature the martensite lath boundaries are replaced by more equiaxed ferrite grain boundaries by a process which is best described as recrystallization. The final result is an equiaxed array of ferrite grains with coarse spheroidized particles of Fe3C partly, but not exclusively, in the grain boundaries.
The spheroidization of the Fe3C rods is encouraged by the resulting decrease in surface energy. The particles, which preferentially grow and spheroidize are located mainly at interlath boundaries and prior austenite boundaries, although some particles remain in the matrix. The boundary sites are preferred because of the greater ease of diffusion in these regions. The original martensite lath boundaries remain stable up to about 600°C, but in the range 350-600°C, there is considerable rearrangement of the dislocations within the laths and at those lath boundaries which are essentially low angle boundaries.

Role of carbon content
Carbon has a profound effect on the behavior of steels during tempering. Firstly, the hardness of the as-quenched martensite is largely influenced by the carbon content, as is the morphology of the martensite laths which have a {111} habit plane up to 0.3 % C, changing to {225} at higher carbon contents.
The Ms temperature is reduced as the carbon content increases, and thus the probability of the occurrence of auto-tempering is less. During fast quenching in alloys with less than 0.2 % C, the majority (up to 90%) of the carbon segregates to dislocations and lath boundaries, but with slower quenching some precipitation of cementite occurs.

On subsequent tempering of low carbon steels up to 200°C further segregation of carbon takes place, but no precipitation has been observed. Under normal circumstances it is difficult to detect any tetragonality in the martensite in steels with less than 0.2 % C, a fact which can also be explained by the rapid segregation of carbon during quenching.

Mechanical properties of tempered plain carbon steels
The intrinsic mechanical properties of tempered plain carbon martensitic steels are difficult to measure for several reasons. Firstly, the absence of other alloying elements means that the hardenability of the steels is low, so a fully martensitic structure is only possible in thin sections. However, this may not be a disadvantage where shallow hardened surface layers are all that is required. Secondly, at lower carbon levels, the Ms temperature is rather high, so tempering is likely to take place. Thirdly, at the higher carbon levels the presence of retained austenite will influence the results. Added to these factors, plain carbon steels can exhibit quench cracking which makes it difficult to obtain reliable test results. This is particularly the case at higher carbon levels, i.e. above 0.5% carbon.
Provided care is taken, very good mechanical properties, in particular proof and tensile stresses, can be obtained on tempering in the range 100-300°C. However, the elongation is frequently low and the impact values poor. Plain carbon steels with less than 0.25% C are not normally quenched and tempered, but in the range 0.25-0.55 % C heat treatment is often used to upgrade mechanical properties.

The usual tempering temperature is between 300 and 700°C allowing the development of tensile strengths between 1700 and 800 MPa, the toughness increasing as the tensile strength decreases. This group of steels is very versatile as they can be used for crankshafts and general machine parts as well as hand tools, such as screwdrivers and pliers.

The high carbon steels (0.5-1.0%) are much more difficult to fabricate and are, therefore, particularly used in applications where high hardness and wear resistance are required, e.g. axes, knives, hammers, cutting tools. Another important application is for springs, where often the required mechanical properties are obtained simply by heavy cold work, i.e. hard drawn spring wire. However, carbon steels in the range 0.5-0.75% C are quenched, and then tempered to the required yield stress.