216 Section 3 Ferrous Metallurgy composition. During the martensite transformation, internal stresses may become so great that some austenite is not transformed. Austenite that persists at room temperature is called retained austenite. Properties of Martensite. The yield strength of the martensite sample is much higher than a pearlite sample. A UNS G10800 steel with a fine pearlite structure may have a yield st rength of 54.5 ksi (376 MPa). When the same alloy is quenched to form martensite, the yield strength is well over 142 ksi (979 MPa), almost three times higher. However, most applications also need toughness, which is to say high ductility, elongation, and impact strength. The elongation of as-quenched martensite is virtually nil. Once the yield point of a tensile specimen is reached, the sample fails. It qualifies as “brittle.” The sharp tips and edges of the acicular martensite needles mean that when a crack begins in the metal, the stress concentration at the tip of the needle is extremely high. The metal has no choice but to fail. Applications of Martensitic Steels. Fully martensitic steel, often called fresh martensite, does not have many applications because of the lack of ductility. The extreme hardness m eans any finishing must be done by grinding, not cutting. Fully martensitic parts resist wear very well. The sharp edges of some cutting tools for wood may be heated and quenched to a full martensite microstructure, as discussed in Chapter 13. However, the martensite cannot withstand shock loading. Instead, the martensitic steel must be tempered. 10.5 Additional Thermal Processing to Improve Properties Subjecting a quenched martensitic steel to a tempering thermal cycle gives the metal good ductility without sacrificing te nsile strength. Tempering is the process of reheating metal after quenching in order to increase its ductility and relieve stress. At the other extreme, the ductility of pearlite can be enhanced even more by spheroidizing. Spheroidizing is the process of heating and slowly cooling steel to produce a globular or spheroidal form of iron carbide in the microstructure. 10.5.1 Tempering Martensite When a steel part has been heated and quenched to form martensite and is then reheated to a temperature between 300°F (150°C) and 1200°F (650°C) for on e hour, the body-centered tetragonal (bct) martensite decomposes into body-centered cubic (bcc) ferrite and tiny spherical cementite particles. The sharp needle structure is replaced with very small, rounded particles, reducing the stress-riser effect. The large stresses on the atomic and nanometer scale are relieved. Dislocations can again move through the steel, so some ductility is restored. If a tempered part is struck with a hammer, it will not shatter. The extremely fine cementite particles give tempered martensite a very high yield strength, just as fine pearlite has higher strength than coarse pearlite. The yield strength of quenched UNS G10800 steel tempered at 500°F (260°C) for one hour is near 138 ksi (951 MPa), and the elongation is 13%. Copyright Goodheart-Willcox Co., Inc.