Intragranular nucleation of austenite

The mechanism of intragranular nucleation of austenite in a duplex stainless steel (Fe-23.1 Cr-6.1 Ni-3.1 Mo-O.165 N-0.017 C, wt.%) weld metal and heat-affected zone (HAZ) has been examined. In the weld metal the acicular austenite is found to nucleate intragranularly on inclusions and subsequent plates form sympathetically resulting in a fine interlocked microstructure. Austenite plates adopt the Kurdjumov-Sachs orientation relationship with the ferrite matrix and grow with diffusion-controlled mechanism as evident from partitioning of substitutional alloying elements. At least one set of fine intrinsic dislocations on the austenite/ferrite interphase interfaces is observed suggesting that the boundaries are semi-coherent. The high cooling rates involved in the HAZ result in a supersaturated ferrite matrix where precipitation of intragranular austenite occurs as a result of reheating associated with subsequent passes. Austenite particles in the HAZ nucleate preferentially away from gain boundary austenite allotriomorphs indicating that intragranular precipitation is favoured by the supersaturated matrix.

Modelling, simulation and validation of a twelve-pulse autotransformer rectifier for aerospace applications

A microcap SPICE circuit-level model of a 12-pulse autotransformer based rectifier for an aircraft fuel-pump motor drive is described. The importance of including the nonlinear magnetising inductance of the interphase transformers is illustrated. Small supply voltage distortions are seen to result in current imbalance in the interphase transformers, degrading the rectifier input current, and may lead to infringement of the power quality specification. The model has been validated for various operating supply voltages, frequencies and output powers, against measurements from a 3.75 kW unit.