grain refinement happening, this means that because of the heating, inside the grain boundaries of the ferrite (white)austenite starts to grow becoming small grains, which after cooling down become ferrite again, therefore the newmaterial contains smaller size ferrite grains than the base material (see Figure 12).In Figure 14 the area close to the fusion line can be seen, which is composed mainly with ferrite (white), and a smallportion of austenite (black small grains), which starts forming due to the heating, however it just starts forming when itcools down, so the amount of austenite is small; this microstructure is called Widmannstatten ferrite, in which ferrite10
MS43010 - NOVEMBER25, 2019Figure 12: Base material of the low alloy steel (20X magnification).Figure 13: Heat Affected Zone (HAZ) (20X magnification).is shown as lines or long grains, the reason of this deformation is a phenomenon called coarse grain; while heatingthe grain boundaries diffuse and join creating larger grains and after the deformation this area starts to cool downwith direction to the center of the welding, which has a concave form, causing the grains to stop growing and stayeddeformed.The weld zone shown in Figure 15 demonstrates the formation of acicular ferrite grains (white), which have the shapeof large grains. Since the weld region is heated to temperatures as high as 2200oC, the liquid steel dissolves oxygen. Asthe liquid weld metal cools from this temperature in the temperature range 2000–1700oC, the dissolved oxygen anddeoxidizing elements in liquid steel react to form complex oxide inclusions. In the temperature range 1700–1600oC,solidification to ferrite (BCC phase) starts and envelops these oxide inclusions; and the ferrite transforms to austenite11
MS43010 - NOVEMBER25, 2019Figure 14: Area near the fusion line (20X magnification).Figure 15: Weld zone (5X magnification).(FCC phase). In the temperature range from 1600 to 800oC, austenite grain growth may occur. In the temperature range800–300oC, the austenite decomposes to different ferrite morphologies. The austenite to ferrite decomposition startswith the formation of allotriomorphic ferrite, which with continued cooling, the Widmannstatten ferrite nucleates at thegrain boundaries and extends into the untransformed austenite–grain interiors, as seen in Figure 14. Further cooling tolow temperatures, the acicular ferrite would nucleate on the inclusion, this promotes a chaotic arrangement of lenticularferrite plates within an austenite grain, and this phenomenon is considered to be one of the mechanisms by which thetoughness is improved in the welds.12
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- Spring '18
- Dr. maarten bakker