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Further Observations of Linear Arrays of Transition-Iron-Carbide Precipitates in Tempered 4340 Steel

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Abstract

4340 steel was quenched to form lath martensite and then tempered at 200 °C for 1 h. Microstructural features of a large lath were examined in detail via transmission electron microscopy, including bright-field imaging, centered-dark-field imaging, and electron diffraction. Linear arrays of transition-iron-carbide precipitate were characterized. Rodlike arrays measured about 100 nm in length and were aligned predominantly along <100> martensite directions. The rodlike arrays consisted of small, closely spaced, nearly equiaxed transition-iron-carbide precipitates of less than 10 nm in diameter. Kinks in the rodlike arrays resulted is deviations from <100> directions by about 20°–30°. A less-common type of linear array measured 500 nm in length. These longer features were aggregates of kinked rodlike arrays that seemed to be “attached” by kinked segments, again at 20°–30° from <100> martensite directions. Transition-iron-carbide precipitates showed little if any association with martensite matrix dislocations that were aligned predominantly along <111> directions. A hypothesis was offered in which precipitates aligned along <100> martensite directions are a remnant of spinodal decomposition that develops planar modulations of high- and low-carbon regions prior to the first stage of tempering.

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Notes

  1. 4340 steel is a Society of Automotive Engineers (SAE) designation. It refers to a nickel–chromium–molybdenum steel with nominally 0.40 wt. pct. carbon.

  2. In high-carbon steels (0.8 wt. pct. C), martensite in the early stages of aging or tempering shows measurable differences in the two lattice parameters, a and c, for this well-known tetragonal phase. In 4340 steel (with 0.4 wt. pct. C), and after tempering at 200 °C, little if any tetragonality remains. Nonetheless, there may be some remnant of the original tetragonality. This issue is addressed in the paper.

  3. In the previous work on this same alloy, these features were rodlike in nature; therefore, the source of the lenslike or lenticular appearance is uncertain. Later in this paper, it will be shown that in fact these lenslike features are aggregates of much smaller precipitates, and the array of dots directly beneath the lens at “D” in Fig. 2b is a reasonably good representation of this aggregate.

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Acknowledgments

Support from the Advanced Steel Processing and Products Research Center is acknowledged.

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  1. Colorado School of Mines - Metallurgical and Materials Engineering, 1500 Illinois Street, Golden, CO, 80401, USA

    Steven Thompson

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Thompson, S. Further Observations of Linear Arrays of Transition-Iron-Carbide Precipitates in Tempered 4340 Steel. Metallogr. Microstruct. Anal. 7, 680–691 (2018). https://doi.org/10.1007/s13632-018-0492-8

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