Prior austenite grain size and tempering effects on the dislocation density of low-C Nb–Ti microalloyed lath martensite

Low carbon lath martensitic microstructures are used in various steel products requiring high strength and toughness. These microstructures are conventionally produced through re-austenitizing and quenching followed by low or high temperature tempering. It is also possible to produce lath martensite through direct quenching immediately following thermomechanical processing. In this study, deformation below the austenite recrystallization temperature before quenching to form martensite was simulated through laboratory scale Gleeble processing of a 0.2 weight percent carbon ASTM A514 steel microalloyed with up to 0.21 weight percent niobium. Thermomechanical processing generally increases the dislocation density of the as-quenched martensite, which is sensitive to the austenite grain size before thermomechanical processing. The hardness of the thermomechanically-processed steels is generally greater than steels austenitized at comparable temperatures without deformation; this hardness difference is attributed to the increase in dislocation density and increased lath misorientation in the thermomechanically-processed conditions. The hardness is generally independent of prior austenite grain size for the thermomechanically processed conditions in contrast to conventionally austenitized and quenched conditions, which have a Hall-Petch correlation with austenite grain size. The strength increase of the thermomechanically processed conditions compared to the conventionally austenitized and quenched conditions is maintained after tempering. However, there is a larger drop in strength for small prior austenite grain sizes for both conventionally austenitized and quenched and thermomechanically processed steels. Overall, the strength of these lath martensitic steels can be directly related to dislocation density through a Taylor hardening model.

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Effect of Prior Austenite Grain Size on Crystallographic Characteristics and Low‐Temperature Toughness of a Quenched Low Carbon Low Alloy Steel

steel research international ◽

10.1002/srin.202100274 ◽

2021 ◽

Author(s):

X.N. Xu ◽

Y. Tian ◽

Q.B. Ye ◽

R.D.K Misra ◽

Z.D. Wang

Keyword(s):

Grain Size ◽

Prior Austenite ◽

Low Carbon ◽

Low Alloy Steel ◽

Austenite Grain Size ◽

Austenite Grain ◽

Crystallographic Characteristics ◽

Prior Austenite Grain Size ◽

Low Temperature Toughness ◽

Prior Austenite Grain

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A New Systematic Approach Based on Dilatometric Analysis to Track Bainite Transformation Kinetics and the Influence of the Prior Austenite Grain Size

Metals ◽

10.3390/met11020324 ◽

2021 ◽

Vol 11 (2) ◽

pp. 324

Author(s):

David San-Martin ◽

Matthias Kuntz ◽

Francisca G. Caballero ◽

Carlos Garcia-Mateo

Keyword(s):

Heat Treatment ◽

Grain Size ◽

Prior Austenite ◽

Bainitic Transformation ◽

Transformation Rate ◽

Transformation Kinetics ◽

Austenite Grain Size ◽

Austenite Grain ◽

Prior Austenite Grain Size ◽

Prior Austenite Grain

This investigation explores the influence of the austenitisation heat treatment and thus, of the prior austenite grain size (PAGS), on the kinetics of the bainitic transformation, using as A case study two high-carbon, high-silicon, bainitic steels isothermally transformed (TIso = 250, 300, 350 °C), after being austenised at different temperatures (γTγ = 925–1125 °C). A methodology, based on the three defining dilatometric parameters extracted from the derivative of the relative change in length, was proposed to analyse the transformation kinetics. These parameters are related to the time to start bainitic transformation, the time lapse for most of the transformation to take place and the transformation rate at the end of the transformation. The results show that increasing the PAGS up to 70 µm leads to an increase in the bainite nucleation rate, this effect being more pronounced for the lowest TIso. However, the overall transformation kinetics seems to be weakly affected by the applied heat treatment (γTγ and TIso). In one of the steels, PAGS > 70 µm (γTγ > 1050 °C), which weakly affects the progress of the transformation, except for TIso = 250 °C, for which the enhancement of the autocatalytic effect could be the reason behind an acceleration of the overall transformation.

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Exploring the Difference in Bainite Transformation with Varying the Prior Austenite Grain Size in Low Carbon Steel

Metals ◽

10.3390/met8120988 ◽

2018 ◽

Vol 8 (12) ◽

pp. 988 ◽

Cited By ~ 4

Author(s):

Liangyun Lan ◽

Zhiyuan Chang ◽

Penghui Fan

Keyword(s):

Grain Size ◽

Prior Austenite ◽

Low Carbon Steel ◽

Low Carbon ◽

Bainite Transformation ◽

Austenite Grain Size ◽

Austenite Grain ◽

Austenite Grains ◽

Prior Austenite Grain Size ◽

Prior Austenite Grain

The simulation welding thermal cycle technique was employed to generate different sizes of prior austenite grains. Dilatometry tests, in situ laser scanning confocal microscopy, and transmission electron microscopy were used to investigate the role of prior austenite grain size on bainite transformation in low carbon steel. The bainite start transformation (Bs) temperature was reduced by fine austenite grains (lowered by about 30 °C under the experimental conditions). Through careful microstructural observation, it can be found that, besides the Hall–Petch strengthening effect, the carbon segregation at the fine austenite grain boundaries is probably another factor that decreases the Bs temperature as a result of the increase in interfacial energy of nucleation. At the early stage of the transformation, the bainite laths nucleate near to the grain boundaries and grow in a “side-by-side” mode in fine austenite grains, whereas in coarse austenite grains, the sympathetic nucleation at the broad side of the pre-existing laths causes the distribution of bainitic ferrite packets to be interlocked.

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