Correlation Between Experimental and Calculated Phase Fractions in Aged 20Cr32Ni1Nb Austenitic Stainless Steels Containi
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INTRODUCTION
SINCE 1973, 20Cr-32Ni-1Nb stainless steel (2032Nb SS), has become an extensively used alloy for cast manifolds in steam reformer furnaces.[1] The hydrogen reformer manifold collects the hydrogen and carbon monoxide products from the catalytic reformation of methane and steam.[2] These manifolds are typically exposed to temperatures between 1033 K and 1123 K (760 C and 850 C), and as the reformation reaction is endothermic, axial stresses of between around 2 and 4 MPa are exerted on the pipe leading to longitudinal creep.[3] 20Cr-32Ni-1Nb stainless steel was originally introduced as a more economical alternative with nominally better creep properties than the previously used HK40 and HPNb stainless steels, which contain a larger carbon content (~0.44 wt pct C) than the 2032Nb (~0.10 wt pct C). During long-term aging, the 2032Nb alloy exhibits in-service embrittlement issues that the more costly HK40 and HPNb variants do not. As a result, during repair of aged components, liquation and ductility-dip cracking at the weldments occur because of the transformation of niobium carbides into a low melting temperature silicide, known as G-phase (Ni16Nb6Si7). NbC to G-phase transformation coupled with the low carbon content MATTHEW P. DEWAR, Graduate Student, is with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4, Canada. Contact e-mail: mpdewar@ ualberta.ca ADRIAN P. GERLICH, Associate Professor, is with the University of Waterloo, Waterloo, ON, Canada Manuscript submitted April 3, 2012. Article published online October 10, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A
causes an increased susceptibility to liquation cracking, as well as a drop in ductility upon welding thermal cycles.[4] In HK40 and HPNb steels, G-phase is partially, or completely substituted by g-phase (Cr3Ni2Si(C,N)), which has a lesser susceptibility to liquation cracking. The higher carbon content also promotes the decomposition of G-phase at lower temperatures on heating, and precipitates M23C6 at higher temperatures on cooling. As a result, increasing the carbon content helps in retaining the ductility and the fracture stress of the manifold.[4] G-phase has been characterized in 20Cr-32Ni-1Nb (2032Nb) and 20Cr-25Ni-1Nb (2025Nb) stainless steels by numerous authors including Chen et al.,[5] Hoffman and Magnan,[6] and Powell et al.[7] Embrittlement issues from the deleterious G-phase have previously been amended through a solution-annealed heat treatment, at temperatures from 1422 K to 1505 K (1149 C to 1232 C) for a duration from 1 to 1.5 hours.[4] G-phase generally precipitates in carbon-deficient solutions, after Nb(C,N) becomes unstable from a co-segregation process of silicon and oxygen toward the dendrite boundaries.[8] Nitrogen-containing austenitic stainless steels have been thoroughly studied in alloys such as AISI 347 stainless steel,[9,10] NF704, and 20Cr25Ni alloys,[11,12] but to the author’s knowledge have never been investigated for the 2032Nb variant. Extensive study from
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