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Licensed Unlicensed Requires Authentication Published by De Gruyter November 21, 2017

Influence of austenization temperature on microstructure and mechanical properties of a new ultra-high strength low alloyed steel

Einfluss der Austenitisierungstemperatur auf die Mikrostruktur und die mechnaischen Eigenschaften eines neuen ultrahochfesten niedriglegierten Stahles
Ya-Ya Feng, Chi Xu, Xiang Su, Yu-Lin Sun, Xi Pan, Yue-De Cao and Guang Chen
From the journal Materials Testing

Abstract

The effects of austenization temperature on the microstructures and mechanical properties of a newly designed ultra-high strength low alloy martensitic steel were systematically studied. The microstructures of the martensitic steels which were quenched from different temperatures between 860 and 980 °C were investigated by transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) and discussed. The results showed that the martensite laths were found to coarsen slowly and the carbide precipitates dissolved gradually with increasing austenization temperature. As the austenization temperature increased from 860 to 980 °C, the volume of retained austenite and the numerical ratio of high angle grain boundaries (HAGBs) were observed to increase while the numerical ratio of low angle grain boundaries (LAGBs) decreased. Rockwell C hardness (HRC), tensile strength and yield strength increased at first and then decreased, while impact toughness was greatly improved with increasing austenization temperature. The fracture mechanism was brittle fracture when austenitized at low temperatures, while it was ductile fracture when austenitized at high temperatures. The mechanical properties were significantly influenced by the formation of retained austenite, the dissolution of carbides, and the numerical ratio of HAGBs and LAGBs.

Kurzfassung

Für den vorliegenden Beitrag wurden die Auswirkungen der Austenitisierungstemperatur auf das Gefüge und die mechanischen Eigenschaften eines neu designten ultrahochfesten niedriglegierten Stahles systematisch untersucht. Die Mikrostrukturen des martensitischen Stahles, der von verschiedenen Temperaturen zwischen 860 und 980 °C abgekühlt wurde, wurden hierzu mittels Rasterelektronenmikroskopie, Transmissionselektronenmikroskopie und Elektronenrückstreudiffraktometrie untersucht und werden im vorliegenden Beitrag diskutiert. Die Ergebnisse zeigen, dass die Größe der ehemaligen Austenitkörner mit der Austenitisierungstemperatur zunimmt. Es stellte sich heraus, dass die Martensitplatten langsam gröber werden und dass die Carbidausscheidungen sich mit zunehmender Austenitisierungstemperatur graduell auflösen. Wenn die Austenitisierungstemperatur sich von 890 bis 980 °C erhöht, nahm der Volumenanteil an Restaustenit zu. Die Ergebnisse der Elektronenrückstreudiffraktometrie deuten darauf hin, dass der zahlenmäßige Anteil von Kleinwinkelkorngrenzen mit zunehmender Austenitisierungstemperatur abnehmen und entsprechend die Zahl der Großwinkelkorngrenzen zunimmt. Die Rockwell-C-Härte, die Zugfestigkeit und die Dehngrenze nahmen zunächst zu und dann ab, während die Kerbschlagzähigkeit entscheidend mit zunehmender Austenitisierungstemperatur verbessert wurde. Die Bruchausbildung war spröde, wenn bei niedrigen Temperaturen austenitisiert wurde, während sie duktil war, wenn bei höheren Temeperaturen austenitisiert wurde. Die erreichte Verbesserung der Kerbschlagzähigkeit kann der Ausbildung von Restaustenit zugeschrieben werden, sowie der Auflösung von Carbiden und der zahlenmäßigen Erhöhung der Großwinkelkorngrenzen.


*Correspondence Address, Prof. Dr. Guang Chen, Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, P. R. China, E-mail:

Ya-Ya Feng, born in 1986, is a PhD student in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on metallic materials and ultra-high strength steel.

Chi Xu, born in 1990, is a PhD student in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on metallic materials and nano-phase high strength steel.

Xiang Su, born in 1990, is a PhD studentin the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on metallic materials and martensitic steels.

Yu-Lin Sun, born in 1992, is a PhD student in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on metallic materials and 3D printing.

Xi Pan, born in 1990, is a PhD student in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on metallic materials and Ti alloys.

Yue-De Cao, born in 1992, is a PhD student in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China with a focus on investment casting.

Professor Dr. Guang Chen, born in 1962, is working in the Engineering Research Center of Materials Behavior and Design, Ministry of Education, Nanjing University of Science and Technology, China in the area of metallic materials.


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Published Online: 2017-11-21
Published in Print: 2017-11-15

© 2017, Carl Hanser Verlag, München