Sensitisation and Evolution of Chromium-depleted Zones in Fe-Cr-Ni-C Systems

Abstract

The development of chromium concentration profiles in austenitic stainless steels, due to the grain boundary precipitation of carbides has been modelled, taking account of multicomponent effects, both in the estimation of the state of equilibrium at the carbide/matrix interface and in diffusion. A comparison against published experimental data shows that the theory accounts for the development of the depleted zone as well as self-healing, unlike recent work where these effects are treated as separate phenomena. At the same time, the present model preserves local equilibrium at the precipitate-matrix interface and provides a natural explanation for the observation of delays in reaching the minimum chromium content.

Migração De Molibdênio Na Interface Matriz/Precipitado Em Metais De Solda De Ligas Ni-Cr-Mo

“…(10,11) Em aços inoxidáveis e superligas de níquel da família 600, a precipitação de carbonetos de cromo causa o fenômeno conhecido como sensitização. (12,13) Nas superligas da classe Ni-Cr-Mo, que têm sido empregadas na deposição de revestimentos resistentes à corrosão sobre a superfície de equipamentos nas indús-trias químicas, petroquímicas, nuclear e de petróleo e gás, verificam-se a formação de fases secundárias do tipo topologicamente compactas (topologically close-packed TCP) ricas em molibdênio, tais como a fase P, a fase µ e a fase σ. (14)(15)(16) Estas fases têm sido reportadas na literatura como prejudiciais às propriedades mecânicas e de resistência à corrosão das ligas de níquel.…”

Model predictions of grain boundary chromium depletion in Inconel 690

“…However, there are difficulties in modelling chromium depleted zones near grain boundaries because all alloys are a multi-component systems and grain boundary precipitation kinetics is complex in such a system. It is inevitable therefore that assumptions are made in various models developed over the years [10][11][12][13][14][15]. A common assumption is that the concentration of carbon is uniform throughout the matrix and grain boundary at any instant.…”

“…Good agreement between the model predictions and experimental measurements on Inconel 690 is achieved. However, as Sourmail et al [15] have pointed out, the two-stage description of the process violates the reasonable assumption that the interface should remain in local equilibrium during diffusioncontrolled carbide growth. Sourmail et al [15] recently proposed another approach where a finite-difference model is linked to thermodynamic calculation software and carbon activity and interface composition can be calculated.…”

“…However, as Sourmail et al [15] have pointed out, the two-stage description of the process violates the reasonable assumption that the interface should remain in local equilibrium during diffusioncontrolled carbide growth. Sourmail et al [15] recently proposed another approach where a finite-difference model is linked to thermodynamic calculation software and carbon activity and interface composition can be calculated. The authors claim that the approach shows a natural transition from sensitization to desensitization and thus explained the observation of delays in reaching the minimum chromium content.…”

“…The proposed model has the advantages of the model proposed by Sourmail et al. [15], i.e. naturally predicting the transition from sensitisation to desensitisation and delays in reaching the minimum chromium concentration at grain boundaries.…”