On ascertaining hardness‐microstructure relationship in annealed steel for varying carbon content

Abstract

Plain carbon steels of five different carbon contents up to eutectoid composition have been provided with the conventional full annealing treatment. Qualitative and quantitative studies of the microstructure are carried out using optical microscopy and field emission scanning electron microscopy techniques. Even under similar cooling rate pertaining to the full annealing treatment (furnace cooling); microstructural modifications of individual microconstituents are feasible with varying chemistry (carbon content) of plain carbon steel. The refinement of microstructure for both proeutectoid α‐ferrite and pearlite is found to be accentuated with increasing carbon content of annealed steels till eutectoid composition. Hall‐Petch type relationship explains adequately the relationship between hardness and the size of the microconstituent both in terms of the grain size of proeutectoid α‐ferrite and the interlamellar spacing of pearlite. Henceforth an empirical relationship is developed for the first time taking into account the rule of mixture to correlate overall hardness, carbon content, grain size of proeutectoid α‐ferrite and interlamellar spacing of pearlite. The empirical relationship originated thereby is found to closely match the experimental results.

Quantitative hardness‐carbon content‐microstructure correlation in normalized plain carbon steel

“…It is interesting to note that the slope of Hall–Petch type relationship ( K PF ) in the present investigation (involving non‐equilibrium still air cooling pertaining to normalizing treatment) for hardness variation in proeutectoid α‐ferrite region is 392 HV μm −1/2 (as per equation (10)). This value is much higher than the K PF value (238 HV μm −1/2 ) obtained for the annealing treatment involving slow furnace cooling in the earlier investigation by the research group of present corresponding author [10]. Such an augmented hardening effect of the proeutectoid α‐ferrite region in the present work can be eventually correlated to the evolution of more proportion of subgrains within proeutectoid α‐ferrite region under transformation stress exerted during evolution of greater proportion of pearlite region all around for higher gross carbon content under faster rate of cooling (air cooling) of steel as already exemplified with electron back scattered diffraction study, Figure 7.…”

“…The plain carbon steels of hypoeutectoid category (containing 0.05 weight %, 0.192 weight %, 0.35 weight % and 0.48 weight % carbon) and eutectoid category (containing 0.79 weight % carbon) with a detailed chemical composition as enumerated in earlier work has been taken up for normalizing heat treatment process [10]. In this normalizing process, the small pieces of specimens were austenitized at 917 °C, 874 °C, 850 °C, 828 °C, 769 °C for steels containing 0.05 weight %, 0.192 weight %, 0.35 weight %, 0.48 weight % and 0.79 weight % carbon, respectively (corresponding to the fully austenitic domain) for a duration of 1 hour followed by cooling in still air to the room temperature.…”

“…Indeed, the timetemperature-transformation diagram for hypoeutectoid steel is shifted to right with increasing carbon content that eventually lowers down the actual transformation temperature for a particular cooling rate (here that pertaining to still air cooling), either for the evolution of proeutectoid α-ferrite or for the evolution of pearlite [6]. This would eventually increase the respective degree of undercooling to enhance nucleation rate, thereby resulting in structural refinement so as to evolve proeutectoid α-ferrite of smaller grain size and pearlite with smaller interlamellar spacing [10].…”

“…This would eventually enhance the nucleation rate to result in structural refinement [6]. Besides this basic preamble being practiced since the beginning of technological progress on steel, the effect of solute (carbon) content on structural refinement in steel even for similar cooling rate (furnace cooling pertaining to full annealing treatment) was investigated for the first time in a recent work of the present corresponding author's research group [10]. Here the shift of time-temperature-transformation diagram to the right with increasing carbon content of steel was correlated to the greater degree of undercooling for phase transformation so as to conceive structural refinement and finally ending up with development of a mathematical relationship for obtaining hardness straightway from microstructural information.…”