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.

Surface phase transformation in austenitic stainless steel induced by cyclic oxidation in humidified air

“…The more extensive α' martensite formation observed at the specimen surface is considered to be caused by Cr depletion through a combination of carbide formation and oxidation [21,23,24]. Fig.…”

“…The surface properties of AISI 304-type stainless steels are influenced by the formation of α' martensite (BCC) that forms from austenite (FCC) at the surface during cooling from elevated temperature [21,22,23,24]. The α' martensite formation in AISI 304-type stainless steels has previously been attributed…”

“…Mukhopadhyay et al [22] have shown, using acoustic emission, that the formation is martensitic, as opposed to diffusion controlled transformation. The α' martensite formation also occur at other locations than grain boundaries; Susan et al [23] have reported formation of α' martensite at the surface when cooling to room temperature after oxidation at 1000-…”

“…to Cr depletion at grain boundaries through carbide formation [21] and the subsequent increase of the temperature of martensitic transformation (M s ) [21,23,24]. Mukhopadhyay et al [22] have shown, using acoustic emission, that the formation is martensitic, as opposed to diffusion controlled transformation.…”

“…The α' martensite formation at the surface occurs due to depletion of Cr, Mn and Si which increases the M s temperature making the formation of α' martensite above room temperature possible [23]. La Fontaine et al [24] showed that Cr depletion from oxidation during thermal cycling up to 970 • C in air was responsible for α' martensite formation which gave intergranular corrosion causing intergranular cracking as a consequence.…”