Further Observations of Linear Arrays of Transition-Iron-Carbide Precipitates in Tempered 4340 Steel
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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.
Keywords4340 steel Transition-iron-carbide precipitates Rodlike precipitate arrays Kinked segments Planar modulations of high- and low-carbon regions
Support from the Advanced Steel Processing and Products Research Center is acknowledged.
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