Role of alloying elements in steels on metal dusting

Abstract

Metal dusting of Fe‐Ni‐Cr alloys has been observed in industrial processes in strongly carburizing atmospheres at temperatures from 450°C to 800°C. At temperatures below 650°C the alloys are generally not able to form dense, well adherent oxide layers in spite of relatively high Cr‐contents, therefore, metal dusting can take place. Already a lot of experimental work has been done to elucidate the mechanism and to compare the resistance against metal dusting for high alloy steels [1]. The intention of this study was to obtain additional information concerning the role of alloying elements and the effects of carbide precipitates in austenitic high alloy steels such as Alloy 800. The susceptibility to metal dusting was determined by measuring the metal loss under metal dusting conditions of Fe‐20%Cr‐32%Ni alloys modified with additions of different carbide formers (W, Mo, Nb) or oxide formers (Si, Al). The samples were exposed at 600°C in a CO‐H2‐H2O‐gas mixture for repeated periods up to 500 – 1500 h. The attack by the oxidizing and carburizing atmosphere leads to the precipitation of internal carbides and metal dusting and more or less to formation of an oxide layer. In comparison to the undoped material, the addition of carbide formers retards the initiation of metal dusting attack. The additions of Si and Al seem to prevent metal dusting under the given laboratory conditions. When carbides are present at the metal surface, they affect the initial oxide growth and have a negative effect on the protectivity of scales. Very striking is the effect of Ce, this rare earth element is generally known to favour Cr‐oxide formation and to improve the adherence of the oxide layer [2], but in the case of metal dusting it clearly enhances metal dusting and metal wastage.

Metal dusting

“…This was observed in in-situ studies [33,40] by Auger electron spectroscopy (AES) and was seen also from AES sputter profiles taken on the Si-and Al-doped 20Cr-32Ni-alloys after 3 hours exposure [9]. These profiles indicate SiO 2 and Al 2 O 3 on top of a Cr-rich oxide scale, obviously formed in the initial "transient state" of oxidation, since in long-term oxidation SiO 2 and Al 2 O 3 should form below Cr 2 O 3 according to thermodynamic rules.…”

“…Clearly beneficial effects of Si and Al additions were observed [9]. The samples with 2.5% Si and 4.5% Al showed no mass loss by metal dusting up to 1500 hours, but small spots of attack were detected.…”

“…Especially the widely used Alloy 800 proved to be very susceptible and many failure cases were reported of this alloy. In a series of exposures with model alloys on the base of Alloy 800, i. e. Fe-20% Cr-32% Ni, the role of several alloying elements was tested, concerning metal dusting resistance [9]. The base alloy showed strong attack and severe mass loss after short time, (Fig.…”

Occurrence of metal dusting – referring to failure cases

“…But such scale may have defects, where carbon ingress and metal dusting can start and will lead to the well-known appearance of pits and holes in high alloy steels. The defects can be caused by heterogeneities in the surface, inclusions and precipitates (TiN, WC, NbC ....), scratches or edges [15]. Defects may also arise from oxide cracking and spalling, due to creep, fatigue and thermal cycling.…”

Development of Materials Resistant to Metal Dusting Degradation-Vol 2

“…As defects develop in the oxide scale, locally, pits start to form due to carbon ingress, and the pits continue to grow at several isolated locations on the oxide surface. [20][21][22][23][24][25][26][27][28][29] The metal dusting process involves two steps: onset of initiation and propagation, the latter being more important since the metallic components in service are expected to last years and the rate of attack plays a large role in establishing the component service life. However, even the not-so-resistant Nibase alloy can have an incubation time of the order of a few thousand hours; therefore, it is desirable to expedite the initiation step for the Ni-base alloys and to conduct propagation tests in the laboratory over long time periods in environments that simulate process conditions.…”