“Martensite is our friend,” so sayeth the heat treater, but what is martensite, really? And why is a tempered martensitic structure the single-minded goal of every heat treater when hardening steel? Let’s learn more.

Martensite Formation

In order to form martensite we need to heat steel into the austenite field (above Ac₃) and quench rapidly enough from the austenite phase to avoid pearlite formation. The rate must be fast enough to avoid the nose of the Time-Temperature-Transformation (TTT) curve – the so-called critical cooling rate for the given steel. The formation of martensite involves the structural rearrangement (by shear displacement) of the atoms from face-centered cubic (FCC) austenite into a body-centered tetragonal (BCT) martensitic structure. This change is accompanied by a large increase in volume and results in a highly stressed condition. This is why martensite has a higher hardness than austenite for the exact same chemistry.

The martensite transformation, while not instantaneous, is significantly faster than diffusion-controlled processes such as ferrite and pearlite formation that have different chemical compositions than the austenite from which they came. Thus, martensite is a meta-stable, strain-induced state that steel finds itself in. The resultant steel hardness is (primarily) a function of its carbon content (Fig. 1).

Martensite Morphology

Morphology is a term used by metallurgists to describe the study of the shape, size, texture and phase distribution of physical objects. Martensite can be observed in the microstructure of steel in two distinctly different forms – lath or plate – depending on the carbon content of the steel (Fig. 2). In general, lath martensite is associated with high toughness and ductility but low strength, while plate martensite structures are much higher in strength but tend to be more brittle and non-ductile.[2]

For alloys containing less than approximately 0.60 wt.% carbon, lath martensite appears as long, thin plates often grouped in packets (Fig. 3). Plate (or lenticular) martensite is found in alloys containing greater than approximately 0.60 wt.% carbon. The microstructure is needle-like or plate-like in appearance across the complete austenite grain (Fig. 4). With carbon contents between 0.60 and 1.00 wt.% carbon, the martensite present is a mixture of lath and plate types.

As the carbon content increases, so-called high-carbon martensite twins begin to replace dislocations within the plates. This transformation is accompanied by the volumetric expansion mentioned earlier, creating (residual) stress in addition to the strains due to interstitial solute atoms. At high carbon levels these stresses can become so severe that the material cracks during transformation when a growing plate impinges on an existing plate.[3] Thus, coarse martensite (Fig. 5) and plate martensite are less desirable structures in most applications.

Ms and Mf Temperatures

The martensite transformation begins at the martensite start (M_s) temperature and ends at the martensite finish (M_f) temperature and is influenced by carbon content. Increasing the carbon content of the austenite depresses the M_s and M_f temperatures, which leads to difficulties in converting all of the austenite to martensite. M_s and M_f temperatures are also important in welding, as they influence the residual stress state.[5] M_s and M_f temperatures can be calculated, and if you need to know them for a particular steel, one source for this data is at www.thomas-sourmail.org/martensite.html, which lists over 1,000 different steel types.

Tempered Martensite

All steels containing martensite should be tempered. As heat treaters, we need to know that martensite in steel produces a hard, brittle microstructure that must be tempered to provide the delicate balance necessary between strength and toughness needed to produce a useful engineering material. When martensite is tempered, it partially decomposes into ferrite and cementite. Tempered martensite is not as hard as just-quenched martensite, but it is much tougher and is finer-grained as well.

Final Thoughts about Martensite

The heat treater might be interested to know that martensite formation is not restricted just to steels because other alloy systems produce crystallographic changes of a similar nature (Table 1). Learning more about martensite is an essential part of what we need to do as heat treaters since it is one of the defining characteristics of our industry. IH