Low-Temperature Bainite: A Thermal Stability Study
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RODUCTION
THE microstructure studied in this work is also referred to as nanostructured bainite, since it consists of carbon-supersaturated plates of bainitic ferrite, only a few tens of nanometers thick, interwoven with thin films of carbon-enriched austenite. Given that it is obtained isothermally at low temperatures, which is between 200 °C and 350 °C, it is also referred to as low-temperature bainite. Cementite precipitation during the bainitic transformation has been proven to be significantly retarded by high enough amounts of silicon[1]; thus, such steels usually contain a Si content higher than 1.5 wt pct. This complex microstructure provides the steel with a high hardness and strength owing to the combined influence of several types of obstacles to dislocation motion, such as interfaces and dislocations, and also to solid-solution strengthening and to the interaction between carbon and defects, which has been well documented.[1–12] Tempering has been historically associated with the heat treatment performed in martensitic steels to enhance the toughness and ductility while maintaining a high degree of strength and hardness. The most important microstructural changes introduced by this treatment are the depletion of carbon in the as-quenched
MIGUEL A. SANTAJUANA, ROSALIA REMENTERIA, JOSE A. JIMENEZ, FRANCISCA G. CABALLERO, and CARLOS GARCIA-MATEO are with the MATERALIA Group, Department of Physical Metallurgy, Spanish National Center for Metallurgical Research (CENIM-CSIC), Avda. Gregorio del Amo 8, 28040 Madrid, Spain. Contact e-mail: [email protected] MATTHIAS KUNTZ is with the Robert-Bosch GmbH, Materials and Process Engineering Metals, Renningen, 70465 Stuttgart, Germany. Manuscript submitted October 19, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
martensite matrix, the formation of carbides, and, when present in the microstructure, the transformation of retained austenite into ferrite and carbides. Although bainitic ferrite has morphological similarities to martensite, the former one is less carbon saturated, which explains why hardness and strength drop rapidly during tempering in martensitic steels, as carbon is no longer in solid solution state. On the other hand, bainitic steels’ strength relies to a lesser extent on carbon in solid solution, and it is the ferrite plate thickness, which is the dominant factor that controls the strength. Thus, major changes in strength do only occur as a result of the plate thickness coarsening.[13] The bainite transformation in high-carbon high-silicon steels leads to high amounts of retained austenite (up to 40 pct) in the form of films between ferrite plates and as blocks between sheaves.[14,15] In this context, the final properties of tempered bainitic steels depend largely on the stability of the retained austenite and the investigation of its decomposition into a mixture of ferrite and cementite becomes of the major importance during tempering.[1] Previous works have established the general basis that unraveled the tempering resistance of nanostructured bain
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