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Ebook Welding metallurgy (2nd edition): Part 2

(BQ) Part 2 book "Welding metallurgy" has contents: Post-solidification phase transformations, weld metal chemical inhomogeneities, weld metal chemical inhomogeneities, formation of the partially melted zone, difficulties associated with the partially melted zone,. and other contents. | Welding Metallurgy, Second Edition. Sindo Kou Copyright 2003 John Wiley & Sons, Inc. ISBN: 0-471-43491-4 9 Post-Solidification Phase Transformations Post-solidification phase transformations, when they occur, can change the solidification microstructure and properties of the weld metal. It is, therefore, essential that post-solidification phase transformations be understood in order to understand the weld metal microstructure and properties. In this chapter two major types of post-solidification phase transformations in the weld metal will be discussed. The first involves the ferrite-to-austenite transformation in welds of austenitic stainless steels, and the second involves the austenite-to-ferrite transformation in welds of low-carbon, low-alloy steels. 9.1 FERRITE-TO-AUSTENITE TRANSFORMATION IN AUSTENITIC STAINLESS STEEL WELDS 9.1.1 Primary Solidification Modes The welds of austenitic stainless steels normally have an austenite (fcc) matrix with varying amounts of d-ferrite (bcc) (1–7). A proper amount of d-ferrite in austenitic stainless steel welds is essential—too much d-ferrite (տ10 vol %) tends to reduce the ductility, toughness, and corrosion resistance, while too little d-ferrite (Շ5 vol %) can result in solidification cracking. A. Phase Diagram Figure 9.1 shows the ternary phase diagram of the Fe–Cr–Ni system (8). The heavy curved line in Figure 9.1a represents the trough on the liquidus surface, which is called the line of twofold saturation. The line declines from the binary Fe–Ni peritectic reaction temperature to the ternary eutectic point at 49Cr–43Ni–8Fe. Alloys with a composition on the Cr-rich (upper) side of this line have d-ferrite as the primary solidification phase, that is, the first solid phase to form from the liquid. On the other hand, alloys with a composition on the Ni-rich (lower) side have austenite as the primary solidification phase. The heavy curved 1ines on the solidus surface in Figure 9.1b more or less follow the trend of the liquidus .