Michael M. Shea

Transmission electron microscopy study of austempered nodular iron: Influence of silicon content, austenitizing time and austempering temperature

Abstract The structures produced by austempering nodular iron were evaluated by transmission electron microscopy to explain the influence of austempering temperature, austenitizing time and silicon content on the impact properties. No carbide precipitation was evident in a 2.88 wt. % Si nodular iron austenitized for 15 min at 950 °C and austempered for min at 370 °C. This iron had the highest impact strength. Austempering at 340 and 400 °C resulted in the formation of transition carbides in bainitic ferrite laths and a lower impact strength. In addition, both a lower silicon content and a longer austenitizing time promoted carbide formation in the bainitic ferrite during austempering at 370 °C.

Residual stress and microstructure in quenched and tempered and hot oil quenched carburized gears

The residual stress and microstructure in carburized cases developed by a conventional quench and temper and a hot oil quench were determined for two automotive differential gears. A Ithough the results for the two gears could not be directly compared because of the different geometries, the changes in residual stress produced by hot oil quenching was found to depend on the hardenability of the steels. Hot oil quenched, carburized SAE 4130 drive pinion gears had higher compressive residual stresses than quenched and tempered gears, although the microstructures were similar. The higher compressive stresses are believed caused by 1) the initiation of austenite transformation nearer the core because of the lower thermal gradient, and 2) the elimination of tempering. In contrast, hot oil quenched, carburized SAE 1526 ring gears had lower compressive residual stresses than quenched and tempered gears. The microstructure of the carburized case of quenched and tempered gears was mainly martensitic while bainite was the primary microconstituent in hot oil quenched gears. The lower residual stress in the hot oil quenched ring gears may result from the lower transformation strain associated with the austenite → bainite transformation.

Hardening response of carbonitrided rimmed and aluminum-killed SAE 1010 steels

The critical cooling rate to achieve fully martensitic structures was determined for rimmed and aluminum-killed SAE 1010 steel containing various amounts of carbon and nitrogen. Fine grained aluminum-killed steels required a cooling rate approximately two times higher that rimmed steel for all carbon andnitrogen contents evaluated. Alloying may be required to reduce the critical cooling rate to a rate achievable in oil-quenched carbonitrided components processed from aluminum-killed steel.

Influence of strain rate and temperature on the deformation behavior of a metastable high carbon iron-nickel austenite

Austenitic specimens of Fe-15 wt pct Ni-0.8 wt pct C were tested in tension at strain rates of 10−4 s−1 and 10−1 s−1 over the temperature range −20°C to 60 °C. The influence of strain rate and temperature on the deformation behavior depended on whether stress-assisted or strain-induced martensitic trans-formation occurred during testing. Under conditions of stress-assisted transformation, the ductility was low and independent of strain rate. However, when strain-induced transformation occurred, the duc-tility increased significantly and the higher strain rate resulted in greater ductility and more transfor-mation. Although the ductility increased continuously with temperature, the amount of strain-induced transformation decreased and no martensite was observed above 40 °C. Microstructural examination showed that the martensite was replaced by intense bands and that these bands contained very fine (111) fcc twins. The twinning resulted in enhanced plasticity by providing an additional mode of deformation as slip became more difficult due to dynamic strain aging at the higher temperature. This study confirms that the substructure following deformation will depend on the proximity of the deformation temperature to theMsσ temperature. At temperatures much greater thanMsσ, austenite twinning will occur, while at temperatures close toMsσ, bcc martensite will form.

Amplitude distribution of acoustic emission produced during martensitic transformation

Amplitude distribution analyses were made of acoustic emission signals produced during martensitic transformation in Fe16Ni1C and Fe1.3C (where the compositions are given in weight per cent), which exhibit microcracking, and in Fe30Ni which does not microcrack. For each composition the amplitude distribution followed the power law model proposed by Pollock, n(Vi) = kVi−b. The distribution parameter b for Fe16Ni1C and Fe1.3C was larger than for Fe30Ni. The value of b continuously increased with volume fraction ƒ transformed for Fe30Ni whereas, for Fe16Ni1C, b increased with ƒ up to ƒ = 0.4 and decreased thereafter. These results are interpreted as evidence that some microcracking occurs during the formation of high carbon martensite.

Surface Hardening and Microstructural Changes in 304 Stainless Steel Resulting from Elevated Temperature Ultrasonic Vibration

The influence of 40 kHz ultrasonic vibration at 925 °C on hardness and substructure has been studied for 304 stainless steel. Vibration at 33 MPa and 40 MPa stress amplitude produced an increase in hardness although the degree of hardening was nonuniform throughout the specimens. Hardening was greatest near the surface and decreased with distance from the surface. The amount of hardening and depth of the hardened region increased nonlinearly with the stress amplitude of the vibration. The surface hardening resulted from an increased dislocation density, deformation twinning, and enhanced precipitation of dispersed M23C6 carbide. The incidence of deformation twinning in the surface region was attributed to the inherently low stacking fault energy of the material, high strain rate, and severe strain localization along slip bands. Although deformation was more uniform at the surface, grain boundary regions in the interior of vibrated specimens showed a hardening compared to unvibrated annealed samples. In this case, the strengthening resulted from a dislocation cell structure primarily near grain boundaries combined with increased carbide precipitation.

Twinning in metastable FeNiC austenite during elevated temperature deformation

Abstract Twinning observed in an Fe15Ni0.8C metastable austenite (where the composition is in approximate weight per cent) deformed at temperatures greater than M d (the highest temperature at which martensite forms from strained austenite) provides an alternative mode of deformation, once slip becomes difficult because of dynamic strain aging caused by carbon-dislocation interaction. The twinning process is related to the presence of carbon in the high carbon FeNi alloy since similar alloys without carbon do not undergo twinning at elevated temperatures. A trend towards increased ductility is observed with increasing temperature beyond M d . This is explained as a result of dynamic softening provided by the twinning process which serves to counteract the negative effects of dynamic strain aging on ductility.

Enhanced age hardening of 7075 aluminum alloy after ultrasonic vibration

Abstract The influence of ultrasonic vibration (40 kHz ; ϵ = ± 6.4 × 10 −5 ) at 5 °C on the hardening behavior of commercial grade 7075 aluminum alloy during aging at 120 °C was investigated. Compared with the corresponding values for unvibrated material, vibration increased the hardening kinetics and resulted in higher hardness at the peak-aged condition. The main effect of vibration occurred during the initial stage of aging where Guinier-Preston (GP) zones determine the hardening behavior. The results of this study support the hypothesis that vibration increases GP zone nucleation, resulting in a higher volume fraction of zones early in the aging process. Zone growth during this period was unaffected by vibration. The hypothesis of more zone nucleation is consistent with the observed distinct plateau region in hardness due to the relief of solute supersaturation required for zone formation. The plateau region represents an incubation period for nucleation of η′ during which GP zones become ordered. Although the sequence of precipitation was unaltered by vibration, after aging for 24 h, the zone diameter and η′ size were smaller in specimens which had been vibrated than in normally processed specimens.