THE EFFECT OF TEMPERING ON MICROSTRUCTURE AND HARDNESS OF Fe-1C-2Si (wt%) AND Fe-1C-2Si-2Cu-1Ni (wt%) NANOBAINITIC STEELS

Date

2025-6-20

Author

Öztürk Taş, Zeynep

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Nanostructured bainitic steels have been a focus of interest in recent years, since they exhibit extremely high strength without compromising from toughness. With careful alloy design, excellent combinations of tensile strength and fracture toughness values -for example, 2.5GPa-30MPa.m1/2 and 1.7GPa-130MPa.m1/2- can be obtained through a cheap and simple production route which involves isothermal bainitic transformation at a low temperature (just above Ms temperature). The low temperature transformation leads to a very fine microstructure, which is composed of packets of bainite sheaves each consisting of alternating layers of ferrite platelets (20-50 nm width) and retained austenite films, which is the key to the extraordinary mechanical properties. Owing to their superior properties, nanobainitic steels are now replacing their conventional counterparts in armour applications of defense industry and automotive industry as safety guide rails. Most appreciated benefits of copper in steels are corrosion inhibition, solid solution strengthening, and precipitation hardening. It is well established that the age hardening of copper bearing steels improve their strength. The strengthening is achieved by the precipitation of copper in ferritic matrix. As bainite consists of ferrite platelets, it is possible that copper particles can precipitate within these platelets and further improve the strength of nano-bainitic steels. However, there is very limited study in the literature for copper bearing nano-bainitic steels. In this thesis, the effect of copper as an alloying element on ageing behaviour, microstructure, hardness, thermal stability and transformation kinetics of nanobainitic steels was studied in detail. As a result, Fe-1C-2Si-2Cu-1Ni (wt%) steel showed superior thermal stability in comparison to Fe-1C-2Si (wt%) steel. Moreover, the copper-containing steel exhibited a secondary hardening peak upon tempering either at 450°C or 500°C, while the hardness of the copper-free steel decreased steadily.

Subject Keywords

Nanobainitic transformation, Thermal stability, Precipitation hardening

URI

https://hdl.handle.net/11511/115087

Collections

Graduate School of Natural and Applied Sciences, Thesis