J. B. Gnanamoorthy

Influence of grain size on the oxidation resistance of 2 4 1 Cr-1Mo steel

The effect of grain size on the oxidation of a 241Cr-1Mo steel has been studied at 823 K in air. The oxidation rate was found to decrease with increasing grain size (varied form 16 to 59 μm). The decrease in oxidation rate with increasing grain size is attributed to the smaller grain-boundary area which results in a decrease in the short-circuit-diffusion paths. The enhancement in the oxidation rate with decreasing grain size has been confirmed further by acoustic emission (AE) tests which show more AE activity for smaller-grain-size specimens during oxidation as well as during cooling following the oxidation, indicating a faster oxidation rate and thicker scale, leading to more cracking and spailing.

The oxidation behaviour of the weld metal, heat affected zone and base metal in the weldments of 2.25Cr-1Mo steel

Abstract Microstructurally different regions of weld metal, heat affected zone (HAZ) and base metal were identified in the weld pads of 2.25Cr-1Mo steel. The specimens from the weld metal, the HAZ and the base metal were separated out of the weld pads and oxidized for different durations from 2 to 500 h in air at 773 K. All three types of specimens followed parabolic kinetics until the weld metal suffered spallation. While the oxidation rates of the base metal and the weld metal were similar, the HAZ had a much higher rate than those in the other regions. The composite weldment, consisting of the three regions, when oxidized at 773 K, was found to have developed a considerably thicker oxide scale over the HAZ as compared to those of the base metal and the weld metal. When measured by a surface profilometer, the difference in thicknesses that was found to be a fraction of a micrometer higher over the HAZ in 2 h of exposure had increased to more than 14 μm in 500 h. Oxide scales formed over the different regions were characterized by X-ray diffraction and scanning electron microscopy (SEM/EDX).

Effect of thermal aging on the oxidation behaviour of 9wt.%Cr1wt.%Mo steel

The effect of thermal aging at 793 and 873 K of quenched-and-tempered thick-section 9wt.%Crue5f81wt.%Mo steel on its oxidation behaviour has been studied in air at 773 K for a maximum exposure of 2000 h. The oxidation rate was found to increase with increasing aging temperature. This increase has been attributed to the depletion of free chromium in the alloy matrix during aging with a corresponding enrichment of the chromium content of M23C6 at the boundaries of the lath martensite and proeutectoid ferrite present inherently in the microstructure. The enhancement of the oxidation rate of the quenched-and-tempered material upon thermal aging has been confirmed by acoustic emission (AE) tests. These showed more AE activity for an aged specimen during oxidation as well as during subsequent cooling, indicating thicker scale formation leading to more cracking and/or spalling. The influence of chromium depletion, due to thermal aging, on the oxide scale composition has also been confirmed by scanning electron microscopy and energy-dispersive X-ray analysis of the oxide-alloy interfaces.

Oxidation behaviour of 21/4Cr-1Mo steel with prior tempering treatments at 998 K for different durations

Normalized 21/4Cr-1Mo steel has been tempered at 998 K for durations up to 50 h, and then oxidized in air at 773, 873 and 973 K for a maximum duration of 1000 H. The extent of the prior tempering treatment was found to influence the oxidation behaviour of the steel significantly. In general, the oxidation resistance of the steel decreased with increasing duration of prior tempering. However, a pronounced influence has been observed during oxidation at 973 K, when at the end of a 6 h exposure the specimens with prior tempering for 50 h were found to have a weight gain 2.5 times more than the specimen without prior tempering. From the results of the pre- and post-oxidation analyses of the oxide-alloy matrix interfaces by SEM/EDAX, the observed oxidation behaviour could be attributed to the degree of depletion of free (effective) chromium from the alloy by the precipitation of secondary phases of chromium compounds during tempering for different durations. The secondary precipitates in the specimen tempered for 50 h at 998 K can become enriched in chromium by one order of magnitude more than that in the specimen with no prior tempering. Such a drastic depletion of chromium from the matrix causes the formation of a less protective inner oxide layer during oxidation. Acoustic emission tests carried out to assess the mechanical stability of the scale showed that the 50 h tempered specimen suffered cracking after about 4 h oxidation at 973 K, which results in subsequent enhanced oxidation.

Synergistic influence of alloy grain size and Si content on the oxidation behavior of 9Cr-1Mo steel

The synergistic influence of prior-austenite grain size and silicon content of 9Cr−1Mo steel on the resistance to scale spallation has been studied in air at 773 K (for 500 hr) and 973 K (12 hr). Two steels, irrespective of their grain size and Si content, did not show spallation during oxidation at 773 K. Spallation occurred at 973 K, and fine-grain steels exhibited less spallation resistance than coarse-grain ones (in low-as well as high-Si steels). Among the four possible combinations of grain size ans Si content, the steel with low Si and fine grains showed least resistance to spallation, while the steel with high Si and coarse grains showed the best resistance. Spallation was found to initiate in the areas adjoining the oxide ridges formed at the alloy grain boundaries. Oxide scales at the ridges and within the grains were analyzed by scanning electron microscopy (SEM/EDX) and secondary-ion mass spectrometry (SIMS). These analyses suggest depletion of silicon from the areas adjoining grain boundaries, resulting in thicker scaling that triggers spallation in such areas. For similar grain-size materials, the necessary thickness for spallation was attained earlier with low-Si steel rather than in high-Si steel.

Influence of variation in grain size on the oxidation behaviour of low-chromium steelsvis-à-vis that of high-chromium steels

This letter examines the factors responsible for the opposite oxidation trends in the oxidation behaviour of chromium-containing steels

Influence of prior-austenite grain size on the oxidation behavior of 9 wt.% cr-1 wt.% Mo steel

The effect of grain size on the oxidation of 9 wt.% Cr-1 wt.% Mo steel (with prior-austenite grain sizes of 90, 210, and 360 μm) has been studied at 973 K in air. After the initial stages of oxidation (up to 60 min), the fine-grain specimen (90 μm) suffered heavy spallation. A similar spallation took place after 120 min in the case of the specimen with intermediate grain size (210 μm), and after 360 min in a coarse-grain (360 μm) specimen. This increase in the duration for the onset of pronounced spallation with grain size has been attributed to the smaller area of the grain boundaries which are the locations of higher growth stresses leading to spallation. This dependence of spallation characteristics on the alloy grain size has been confirmed by the incidence of higher acoustic-emission activity during AE monitoring. Scanning electron microscopy (SEM) has confirmed the occurrence of pronounced spallation of the oxides formed in the areas adjoining the grain boundaries.

Oxidation behavior of 2.25Cr-1Mo steel with prior tempering at different temperatures

The oxidation behavior of a normalized 2.25Cr-1Mo steel tempered previously for 10 hr at different temperatures between 873 and 1023 K has been studied up to a maximum duration of 1000 hr in air at 773–973 K. The oxidation resistance of the steel was found to decrease significantly with the temperature of tempering. Tempering of this steel is reported to cause microstructural changes involving precipitation of Cr as carbides and a decrease in the effective (free) Cr contents, that could influence the oxidation resistance of the Cr-containing alloys. Relative compositions across the thickness of the oxide scales, as analyzed by SEM/EDX and SIMS, suggest that a less Cr-rich (and less protective) and thicker scale on the steel formed because previous tempering caused extensive depletion of free Cr.

Effect of tempering time on the oxidation behavior of 21/4Cr-1Mo steel

Abstract21/4Cr-1Mo ferritic steel is generally used in the normalized and tempered condition for steam-generator tubes in fast breeder reactors. The microstructure of the alloy changes on tempering, depending on the type of carbides formed, as a function of tempering time. At the very early stage of tempering (up to ∼30 min), most of the cementite (Fe3C) dissolves, causing trapping of Cr and Mo to form their carbides. With an increase in the tempering time more Cr and Mo are used up for carbide formation. This leads to a decrease in the available Cr content, which is generally responsible for the formation of a protective oxide scale. It is thus expected that a decrease in the oxidation resistance will occur with an increase in tempering time.nOxidation tests were carried out on specimens prepared after five tempering treatments; viz. 10 min, 30 min, 60min, 120min, and 180min at 998 K. Oxidation was carried out in air both for long duration (1000 hr at 773 K) and for short duration (6hr at 873, 973, and 1073 K). Results indicate a decrease in the oxidation resistance with increasing tempering time. At 1073 K, however, no significant change in oxidation was noticed. This could be perhaps due to the vanishing of the tempering effect by the dissolution of carbides at this temperature.

Oxidation behavior of microstructurally-different regions in the weldment of 9Cr-1Mo steel

Regions with different microstructures have been identified in the weldments of 9Cr-1 Mo steel. Weldments comprising threthree regions, i.e., weld metal, heat-affected zone (HAZ), and base metal, were oxidized in air at 923 K for different durations up to 500 hr. The crown area of the weld metal was found to form a thicker oxide scale than the other regions of the weldment. When the oxidation kinetics of different regions were compared (by separating out the coupons of the HAZ and the crown and root portions of the weld metal and then oxidizing them), the crown area of the weld was found to oxidize at a much higher rate than the others. Scanning electron microscopy (SEM) was carried out to assess the morphological variations in the different regions of the weldment. The compositional variations in the scales over the different regions have been characterized by the energy-dispersive analyses of X-rays (EDX), and the results thereof have also been corroborated by secondary ion mass spectrometry (SIMS).