Point counting of intermetallic phase precipitates in ferritic-austenitic stainless steels

Microstructural control is essential to fully exploit the properties of ferritic-austenitic stainless steels. Intermetallic phases are widely regarded as detrimental and their presence is generally considered unacceptable. Whilst it is recognised that most specifications stipulate that no deleterious third phases are permitted, there are occasions when it becomes important to perform a quantitative assessment. When the limitation of such phases to below a critical level is essential, methods are required to enable accurate and reliable quantification. The work investigated the effects of different techniques for the preparation of both metallographic sections and acetate replicas on the results of subsequent point counting. Manual point count estimates of low intermetallic phase volume fractions (i.e. < 5 %) were made in a range of duplex and superduplex stainless steels. The most promising laboratory method was assessed by an interlaboratory round-robin study. Whilst good repeatability of measurements was achieved at one laboratory, significant differences were apparent in the results of the interlaboratory study, indicating that manual point counting of a low phase volume fraction has inherent limitations, which should be recognised when setting acceptance criteria. Measurement of properties relevant to service remains the preferred way to establish the absence of deleterious levels of intermetallic phase. Copyright © 2007, TWI Ltd.