Enhanced thermal stability in nanostructured bainitic steel

Abstract

We report an attempt at increasing the thermal stability of nanocrystalline bainite to tempering heat-treatments, by enhancing the silicon concentration of the alloy. Validation experiments have been conducted using synchrotron X-radiation during tempering heat treatment. It is found that the change in alloying successfully stabilises the austenite at elevated temperatures by retarding cementite formation to temperatures as high as 500 • C. Other changes reflected in the lattice parameters of the major phases have revealed further information about the mechanisms involved.Key words: : nanostructured bainite, thermal stability, synchrotron radiation, silicon, cementite precipitation Nanostructured carbide-free bainitic steel consists of very thin plates of bainitic ferrite in an interconnected network of retained austenite, with a thickness ranging between 20 and 100 nm. These steels exhibit resistance to tempering as measured by the change in hardness [1,2], since retained austenite decomposes to a dispersion of carbides at the plate boundaries, which act to slow coarsening, even at severely high temperatures [3]. However, austenite is regarded as a desirable phase since retained austenite can enhance the ductility and increases the work hardening rate, retarding plastic instability [4][5][6]. The likely explanation is that formation of strain-induced martensite causes a redistribution of stresses and changes the composite characteristics [7][8][9]. Stabilisation of retained austenite would enable surface treatment at elevated temperatures after the nanoscale grain structure is produced. Galvanising treatments or 1 C. Hulme-Smith gratefully acknowledges funding from EPSRC and Rolls-Royce plc

Tempering of Low-Temperature Bainite

“…[12,13] Development of alloys with increased temperature stability is in progress; this will rely on further suppressing carbide precipitation to increase the stability of the nanostructure. [14] So there are a several strong motivations to characterize the tempering behavior of these nanostructured steels.…”

Thermally Stable Nanocrystalline Steel

“…Although previous work [22] has shown that the addition of large amount of cementite-insoluble elements can delay the thermal decomposition, it is unlikely that such an approach can sufficiently suppress cementite precipitation: ultimately, a mixture of ferrite and cementite is required by equilibrium. A novel approach was therefore considered: to minimize the carbon content of retained austenite and thereby reduce the driving force for the precipitation of carbides.…”

“…Two approaches were considered to develop novel alloys: an extension of previous work [22] to introduce as many atoms that are insoluble in cementite as possible and a new concept to minimize the carbon content while still obtaining the desired microstructure.…”

“…[22] The alloy included manganese to suppress M s and B s and for hardenability (Fe-1.037 C-1.97 Mn-3.89 Si-1.43 Al (wt pct)). Manganese is effective in both roles as it reduces the driving force for the transformation of austenite to ferrite, DG c!a .…”

Hydrogen diffusion and the percolation of austenite in nanostructured bainitic steel

“…Previous work on nanostructured bainite has shown that the decomposition occurs during isothermal heat treatment at temperatures of 400 • C or above [27,29,38,39]. However, these experiments involve continuous heating, so dilatometric experiments were done to measure the temperature range over which the substantive decomposition of austenite occurs.…”