Surface alloy depletion and martensite formation during glass to metal joining of austenitic stainless steels

Abstract

Preoxidised and glass to metal (GtM) sealed austenitic stainless steels displayed a ferritic (bcc) layer near the metal/oxide interface, as determined by electron backscatter diffraction and X-ray diffraction. Through electron probe microanalysis, it was determined that this layer was depleted of alloying elements due to the oxidation and sealing processes. Characterisation of the layer morphology suggested that it formed through the martensite transformation mechanism. Thermochemical modelling with ThermoCalc also supported a martensitic transformation as opposed to diffusional ferrite formation. The composition gradient through the layer was correlated to the Eichelman and Hull empirical relationship for martensite start (M_s) temperatures. Because of Cr, Mn and Si depletion during preoxidation and glass sealing, M_s temperatures near ambient are possible in this surface region. The martensite layer was non-uniform, however, with laths extending deeper into the alloy due to stabilised growth in the material above its M_s temperature. This behaviour was characterised by image analysis techniques and discussed in terms of martensite stability and microstructural effects. Possible negative aspects of bcc phase formation on GtM seal properties are discussed, and analyses of alternative alloys 21-6-9 (tradename Nitronic 40; Armco Holding Corp., West Chester, OH, USA) and 22-13-5 (Nitronic 50) showed reduction or elimination of martensite after GtM joining.