Second Stage of Upper Bainite in a 0.3 Mass Pct C Steel
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INTRODUCTION
FERRITE particles of various shapes appear early at the grain boundaries in isothermal transformation of hypoeutectoid austenite at fairly high temperatures. They were examined in a preceding study.[1] Large groups of parallel plates will soon predominate, especially below the eutectoid temperature where pearlite can form and soon will predominate. Groups of ferrite plates will continue to form but to a smaller fraction. Below the ‘‘nose’’ temperature of pearlite they will soon predominate again until interrupted by martensite. It was thus possible to study the effect of temperature on the groups of ferrite plates over the whole range of temperature and it was observed that there is no sharp change of morphology that could indicate a transition of growth mechanism from Widmansta¨tten ferrite to the acicular ferrite of upper bainite. This supports the suggestion that the only difference between Widmansta¨tten ferrite and upper bainite is the presence of cementite in the latter. Initiated by the occurrence of cementite at some time, depending on temperature, the austenite remaining in the interspaces between the ferrite plates will finally transform and the end product is defined as upper bainite. In the present study, the progress of the transformation initiated by cementite was studied with SEM, Scanning Electron Microscopy. The same alloy was employed as in the preceding study but two related JIAQING YIN, MATS HILLERT, and ANNIKA BORGENSTAM are with the Department of Materials Science and Engineering, KTH (Royal Institute of Technology), Brinellva¨gen 23, 100 44 Stockholm, Sweden. Contact e-mail: [email protected] Manuscript submitted June 23, 2016. Article published online January 5, 2017 1444—VOLUME 48A, MARCH 2017
alloys were added. The microstructures of bainite have recently been discussed in two review papers[2,3] but the present study was undertaken to examine what is generally regarded as the second stage of bainite formation. In the preceding study it was essential to examine ferrite plates over their full length which is why that study emphasized grain boundary nucleated plates where one can identify the starting point. Parallel plates of ferrite often occur in large groups, which represent the first stage of bainite formation. Mehl[4] called this final microstructure feathery bainite. In the present alloys feathers were found in the whole temperature range that was examined. Figures 1 and 2 give two examples (see Reference 1 for experimental procedure). Figure 2 from a low temperature gives an indication of how common the feathery microstructure can be. The term feathery describes the appearance in the plane of polish when there are parallel groups of plates on both sides of the grain boundary but Mehl also applied the term to a case where the parallel plates were present only on one side of the grain boundary. Such units could be as useful in the present study. An example is shown in the left-hand part of Figure 2. For bainite there are two conflicting hypotheses which cannot be reconciled, the diff
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