Named in honor of A. Martens, martensite is the hardest microstructure that can be formed in a carbon or alloy steel. In a polished and etched steel specimen, martensite appears as an acicular microconstituent. The level of hardness in a fully martensitic microstructure is commensurate with the carbon content of the steel; almost regardless of the amounts of other alloying elements present. Consequently, a very low-carbon steel, even in the martensitic condition, will not be very hard. It is important to note that with a carbon content of about 0.60%, the maximum hardness that can be achieved in steel is roughly 68 HRC; and a higher carbon content will not achieve any real increase in the maximum obtainable hardness.
A martensitic structure is produced when austenite is continuously cooled at a rate faster than that steel’s critical cooling rate. With low hardenability, austenitized steel must usually be cooled by quenching, in oil or water, to produce a martensitic microstructure.
Some alloy steels with high hardenability will form martensite when the austenitized structure is air cooled. Regions of steel that are austenitized by the localized heat of welding have the potential for forming martensite because cooling rates in welds can be
notably fast, but the final microstructure also depends on the hardenability of the steel.
The higher hardness in the martensitic microstructure is accompanied by lower ductility and toughness that, under many circumstances, can increase susceptibility to cracking. These shortcomings can be relieved by thermal tempering of the martensitic microstructure. Postweld stress relief of steel welds is the most effective procedure to obtain highly satisfactory combinations of strength, hardness, ductility and toughness in steel. Welds in steel with a carbon content of 0.25% or less are not very susceptible to cracking due to martensitic formation in the weld or heat-affected zone. Thus, most structural steels can be used in the as-welded condition without concern for martensitic
cracking.
Tempering of Martensite. Martensite, in the asquenched condition, is generally unsuitable for engineering applications because it can be quite brittle. It requires a tempering heat treatment to effectively increase its ductility and toughness while only moderately reducing its strength. Tempering consists of reheating the steel to an appropriate temperature (always below the austenitizing temperature, A,) and holding at that temperature for a short time. The heat treatment allows the carbon to precipitate in the form of very small carbide particles. The resulting microstructure is tempered martensite. The necessary compromise between hardness and toughness can be obtained by adjustments to the correct tempering temperature and holding time. The higher the temperature, the softer and tougher the steel.