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Development of a heat treatment method to form a duplex microstructure of lower bainite and martensite in AISI 4140 stee
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
2014 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Research on bainite and martensite structures has indicated that lower bainite needles have a refining effect on the lath martensitic structure. Lower bainte needles partitions prior austenite grains and will consequently have a refining effect on the subsequent formed lath martensite. Smaller austenite grains will result in smaller lath martensitic packets and blocks and will result in enhanced mechanical properties.   In order to create a variation of lower bainte structure in a matrix of martensite, two different heat treating methods were tested. The work was focused towards the formation of lower bainite during isothermal heat treating in molten salt, above and below the MS-temperature. Both un-tempered and tempered samples were analyzed .Two different materials were tested, both were AISI 4140 but with a slightly difference in hardenability. The material provided by Ovako Steel is 326C and 326F the later had a higher hardenability. In order to better distinguish the two structures from each other when studied under a microscope, a variation of etching methods were tested.  It was possible to create a variation of lower bainite structures in a matrix of martensite.  326F shows less amount of lower bainite and provides a higher average surface hardness before tempering.

Place, publisher, year, edition, pages
2014. , 66 p.
Keyword [en]
AISI 4140, 326C, 326F, Isothermal heat treatment, Martensite, Bainite, Molten salt, vacuum furnace, gas quenching.
National Category
Metallurgy and Metallic Materials
URN: urn:nbn:se:kth:diva-165084OAI: diva2:807195
Subject / course
Physical Metallurgy
Educational program
Master of Science - Materials Science and Engineering
Available from: 2015-04-23 Created: 2015-04-23 Last updated: 2015-04-23Bibliographically approved

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