60  Prakt. Met. Sonderband 50 (2016) 
during the Flash bainite rapid heating and cooling cycle !®, the rate of cooling of KAB steels The spec 
towards the Bs temperature is of secondary importance. colors m 
urple st 
However due to local segregations the necessary conditions for the formation of bainite nuclei > Ne ec 
may not be met throughout the whole cross-section, which can lead to the localized formation the surfa 
of martensite as previously observed within welds [. It is therefore of interest how this using ho 
microstructural feature could be characterized and reliably differentiated from the bainite, dependir 
during metallographic observations. The very short transformation times at low temperatures choose tl 
render the diffusion of carbon almost negligible, suggesting a substantial super saturation, grained | 
which consequently results in a very poor contrast when etched in the as quenched condition grained : 
using common attack etchants, like nital and Viella. 500%. A 
The aim of this paper is to assess the suitability of color etching for the quantitative analysis whereby 
of novel experimental KAB steels, where bainite is formed rapidly at very low temperatures. For X-ra 
; undeforr 
Materials and methods interpret 
The alloys studied are 2 experimental KAB steels, which nominal compositions are shown in Result 
Table 1. The main difference between the 2 alloys is their Mn content, which results in 
different Bs and Ms temperatures. The alloy with a low Mn content is termed KAB 1 and the Alloyins 
higher alloyed variant KAB Hi-Mn. The alloys were vacuum cast under Argon in the form of affected 
10kg ingots, homogenized at 1200°C for 2 hours, and forged at 1160°C with a deformation that at a 
ratio of 5. in the cu 
Table 1: Nominal steel compositions (in w %) form, as 
— Da Sl TD ET mm, pi — grew unl 
< Si ; Mn | Mo ; Cr ; Al Ni+Cu+Co+W+V | PS ; Ti+Zr perpend: 
A a lus de den Sn I bist co have a m 
KABHi-Mn | 0.6 12 5° 035] 7 1.7 | 0.6 | 0.01 [0.02] 002 forthe c 
— ; ; — is thoug! 
Samples for microstructural observations were cut using a water cooled abrasive disk and 
prepared using standard metallographic techniques. The procedure consisted of planar 
grinding, to remove about 0.5 mm of material, followed by fine grinding using a stone with 
320-grit, polishing proceeded with 9 pm, 3 pm and 1 pm diamond suspensions and the final 
step was polishing with 0.05 pm colloidal silica, as summarized in Table 2. 
Table 2: Parameters used in specimen preparation 
Surface and Abrasive Load [N] rpm Direction Time 
[minutes] 
Planar grinding on abrasive stone 55 250 Contra 2 
320-grit grinding 20 200 Contra 3 
9-um Diamond on psa-backed silk cloth 20 200 Contra 25 
3-um Diamond on psa-backed synthetic pad 20 200 Contra 2.5 Figur. 
0.05-um colloidal silica on psa-backed ; deterı 
polyurethane pad (25% conc.) 30 150 Comp. ;