Formation of Intergranular M23C6 in Sensitized Type-347 Stainless Steel

Abstract

Formation of intergranular M23C6 carbides and thereafter Cr-depletion zones in commercially available type-347 stainless steels were observed by optical microscopy, conventional transmission electron microscopy and analytical transmission electron microscopy. At the early stage of sensitization, only NbC carbides were observed both intragranularly and intergranularly. At the later stage, formation of intergranular M23C6 carbides and intergranular corrosion were found. A model is proposed to describe the formation kinetics of intergranular precipitation of NbC and of M23C6, as well as Cr-depletion zones, thermodynamically. Evolution mechanisms of intergranular precipitates were found in the order of 1) the formation of intergranular NbC, 2) the coarsening of NbC, 3) the formation of M23C6, then 4) the coarsening of M23C6 and formation of Cr-depletion zones.

Decorated Dislocations with Fine Precipitates Observed by FIB-SEM Slice-sectioning Tomography

“…3(b). 16) In addition, a representative bright-field TEM image of these aligned precipitates showed that fine intragranular NbC precipitates were, in fact, associated with dislocations, as shown in Fig. 3(c).…”

“…16) Moreover, striations across the sample were occasionally found, known as the curtain effect, probably caused by the uneven chemical composition of the sample, typically at grain boundaries.…”

“…13,14) In addition, precipitation hardenings and increase of flow stress are expected by the presences of NbC. 15) The sensitization caused by the formation of M23C6 and simultaneous Cr-depletion region can be suppressed by precipitation of NbC, 16) and the lowering C content in solid solution results the improvements of the formability. 17) The precipitation behavior of nano-sized NbC particles has been the subject of intensive studies, and it has been shown that NbC precipitation occurs in a heterogeneous manner, intragranular preferentially at dislocations and at stacking faults to release a part of the elastic strain.…”

Assessment of the Electrolyte Composition in the Degree of Sensitization in AISI 347H Stainless Steel

“…Typical applications refer to industries as oil extraction, re neries, thermo-electric and nuclear power plants due to its high corrosion resistance at high temperature 2) . The principal characteristics of this alloy are the combination of good mechanical property and corrosion resistance in comparison to conventional ASS due to the presence of Nb which reduces the sensitization phenomenon and increases creep resistance as a result of the formation of NbC 3) . Thermodynamically, the formation of niobium carbides are more favored and takes place in a broader range of temperatures 4) as compared to chromium carbides (450-850 C) 5,6) .…”

“…The precipitation of NbC occurs in grain boundaries, non-coherent twin boundaries, dislocations and stacking faults 5,7,8) . A requisite for nucleation and growth of niobium carbides is met when there is at least a Nb:C mass ratio of 10:1 3) . In this alloy, the highest af nity of C for Nb than Cr enhances the resistance to intergranular corrosion (IGC) by reducing the susceptibility to localized attack in virtue of the minimization of chromium depleted zones 9) .…”