Eutectic precipitates and microstructure of electroslag remelted 15Cr-22Ni austenitic heat-resistant steel with varying Nb contents

The microstructure,eutectic carbides and intermetallic compounds in as-cast ingots with varying Nb contents produced by electroslag remelting were studied.The solidification behavior,microsegregation of alloying elements and its influence on precipitates were analyzed.The increase in Nb content from 0.64 to 1.40 wt.%has no effect on the dendrite morphology and secondary dendrite arm spacing.The total area fraction of eutectic NbC and Laves phase in as-cast ingots increases with the increase in Nb content.The eutectic precipitates in the ingot with 0.64 wt.%Nb are mainly NbC,and the others are Fe_(2)Nb-type Laves phase.Increasing the Nb content of the steel significantly promotes the precipitation of Laves phases and lowers the precipitation temperature of NbC eutectic carbides.Eutectic carbide M_(2)C is precipitated only in the ingot with 1.4 wt.%Nb.The average concentration of Nb in both interdendritic and intragranular regions increases with the increase in Nb content of the steel,leading to a change in the morphology of Fe_(2)Nb-type Laves phase from honeycomb to blocky.Increasing the Nb content exerts little effect on the microsegregation degrees of Si,Mo,Cr and Ni in the steel.

Precipitate Behavior in Fe–20Cr–30Ni–2Nb Austenitic Heat-Resistant Steel

In this study, the precipitation behavior of a new austenitic heat-resistant steel （Fe-20Cr-30Ni-2Nb, in at%） was investigated. The effects of alloying addition of boron （B） and lanthanum （La） on the microstructure of the austenitic steel were scrutinized using SEM, EPMA, TEM, and XRD. The results showed that the addition of B enhanced the precipitation of bar-type Laves phase. A small precipitate with high La concentration was observed at the grain boundary in the alloy without aging; similar precipitates without La also presented in region adjacent to the La single phase. This result indicates that La can exist independently and does not contribute to the formation of new compounds. However, in both B- and La-modified alloy, B appeared in the precipitate free zone. In the alloy containing both B and La, only Fe2Nb Laves- phase precipitates, as indicated by the XRD result.

Water vapor effects on the oxidation resistance of modified potassium silicate coating at high temperature

An inorganic potassium silicate coating with pigments of alumina,aluminum phosphate,NiCrAlY and copper chromite black was prepared on CB2 stainless steel.Oxidation behavior in either ambient air or O_(2)+H_(2) O mixture at 630℃ for 2000 h was comparatively studied,and the coating exhibited excellent resistance under both test conditions.The water vapor considerably accelerated the oxidation of the uncoated CB2 steel,as the hydroxide,the main constituent of the coating,had a negligible evaporation rate at test temperature,while it had a limited effect on the coated sample.Meanwhile,the existence of coating may prolong or eliminate the incubation period in the O_(2)+H_(2) O mixture at 630℃.After oxidation,the coating matrix is in an amorphous state and fillers as alumina and copper chromite black are stable in the coating.Leucite(KAlSi_(2) O_(6))formed by Al from NiCrAlY and potassium silicate in the coatings was detected after tests either in O_(2) or O_(2)+H_(2) O mixture.