Dissolution Kinetics of NbC Particles in the Heat-Affected

Dissolution Kinetics of NbC Particles in the Heat-Affected...

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Dissolution Kinetics of NbC Particles in the Heat-Affected Zone of Type 347 Austenitic Stainless Steel LEIJUN LI and ROBERT W. MESSLER, Jr. Dissolution kinetics of NbC particles in the heat-affected zone (HAZ) of Type 347 austenitic stainless steel were experimentally studied by a rapid cooling method. Coupons with 1.5-mm thickness were water quenched on a GLEEBLE thermomechanical simulator at various instances during the heating portion of a welding thermal cycle. Particle dissolution kinetics data were obtained by statistical analysis of digital images of resulting microstructure. For most of the test alloys, a good correlation exists between the dissolution kinetics and the susceptibility to cracking in the HAZ. The faster the Nb-carbide particles dissolve, the more resistant the alloy is to constitutional liquation cracking. The rate at which particles dissolve seems to be affected by their stoichiometry. If Nb:C ratios in bulk alloys are closer to the ideal stoichiometry of 7.7, NbC particles formed in the alloy are more likely to be stable and, hence, slower to dissolve. I. INTRODUCTION kinetics of NbC particle dissolution during welding. [11] How- ever, there are only a few studies on microstructural revolu- H OT cracking in the weld heat-affected zone (HAZ) tion and particle dissolution during welding of low alloy due to grain boundary liquation is commonly observed in steels. [12–15] There is only one published experimental study austenitic stainless steels, heat-treatable nickel-based super- on the dissolution of second-phase particles in austenitic alloys, heat-treatable aluminum alloys, and some titanium stainless steel under rapid welding thermal cycles. [16] The alloys. There are two types of liquation cracking in welding: lack of experimental data on kinetics, as important as it is one the result of microsegregation of trace solute elements said to be, seems to be attributable to the labor-intensive that promote the formation of low-melting constituents, often experimental procedure for sample preparation and lack of in the form of films at grain boundaries, [1] and the other is efficient, quantitative metallographic analysis methods for the result of an eutectic-like reaction around a dissolving large numbers of samples. Recent advances in digital-image particle during nonequilibrium heating, generally referred analysis technology have made it possible to efficiently to as “constitutional liquation.” [2,3] The major difference obtain kinetic information from tested specimens. between the two types of liquation cracking is that the former Employing a modern GLEEBLE* thermomechanical sim- generally occurs in single-phase alloys and the latter in multi- phase alloys. *GLEEBLE is a trademark of DSI, Inc., Poestenskill, NY.
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Dissolution Kinetics of NbC Particles in the Heat-Affected...

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