WCN Spring 2015

WCN

applied to the polished surfaces and optical metallography was conducted. Pearlite interlamellar spacing was measured using Field Emission Scanning Electron Microscopy (FESEM) images taken at a magnification of 10,000x, employing a circular intercept method according to ASTM E-112 [9] . Error bars reported for all plots represent the standard error of the data sets. Results and Discussion Hardness values are shown in Figure 1 as a function of cooling rate for both alloys. It is apparent that the 1080V+Nb alloy exhibits greater hardness values than the 1080V alloy. Hardness data is only shown for cooling rates of 2.5 and 5°C/s for the 1080V+Nb steel since non-pearlitic microstructural constituents including martensite and occasionally bainite were observed via FESEM at higher cooling rates in the Nb containing alloy. In addition, more martensitic/bainitic constituents were observed at these cooling rates in the Nb alloyed steel versus the 1080V steel as qualitatively shown in Figure 2 for a cooling rate of 12.5°C/s. Both samples were etched using a 4 pct Picral etchant and the regions that appear light correspond to pearlite whereas the darker regions not attacked by the etchant correspond to martensitic regions. Although no quantitative analysis was conducted, it is apparent that more martensite formed in the 1080V+Nb alloy, which suggests increased hardenability or slower pearlite transformation kinetics.

S S Figure 2 – Low magnification FESEM taken from samples continuously cooled from 1,093°C at 12.5°C/s for a) 1080V and b) 1080V+Nb steels. Samples etched with 4 pct Picral a) b)

construction steels [7]. FESEM micrographs are shown in Figure 3 for the alloys investigated in the present study for constant cooling rates of 2.5 and 5°C/s. A refinement of ILS and pearlite colony size for the 5°C/s cooling rate is qualitatively observed for the 1080V+Nb alloy. Results from ILS

The

obtained

hardness

values

corresponding pearlitic microstructures are also presented in Table 2 and greater hardness values are obtained in the Nb alloyed steel. The hardness values are also observed to increase with cooling rate for both alloys. to fully

800

700

600

S S Figure 3 – FESEMmicrographs of 1080V (a and c) and 1080V+Nb (b and d) alloys cooled from 1,093°C at constant cooling rates of 2.5°C/s (a and b) and 5°C/s (c and d)

HV, 1kgf

500

and pearlite colony size measurements are presented in Table 2 and plotted in Figure 4 as a function of cooling rate. Increased ILS refinement is observed with cooling rate whereas pearlite colony size seems less dependent on cooling rate for the 1080V steel. ILS refinement is observed for both cooling rates in the niobium alloyed steel. A different trend with cooling

As evident from Equation (1), pearlite colony size and ILS contribute to strengthening in pearlitic steels. In addition, microalloying may contribute to increased strength through precipitation strengthening. Effects of niobium additions to eutectoid alloys have received only limited attention; Jansto reports ILS refinement through niobium additions in high carbon

400

0 4 8 12

Cooling Rate, °C/s

S S Figure 1 – Vickers hardness as a function of cooling rate

25

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