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Influence of cooling rate on microstructure and properties in low-temperature tempered medium-carbon low-alloy martensitic steels
Rupinen, Michael Christopher, Jr.
Rupinen, Michael Christopher, Jr.
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2024
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The use of steels requiring thick section sizes (greater than 12.5 cm) with high strength (>1200 MPa) and impact toughness (>35 J at room temperature) maintained throughout the component has raised new questions regarding the effect of alloying additions and cooling rate on properties. Large section sizes may lead to significant differences in cooling rate throughout the component during processing causing both hardenability and autotempering to be of relevance. The effect of autotempering (tempering of the martensite during quenching) is thought to be of particular importance as these steels are typically tempered at or below 204 °C while the martensite start temperatures may be well over 350 °C. The goal of this study was to investigate the effects of alloying additions and cooling rate on the impact toughness and strength of two sets of medium-carbon low alloy steels following a 204 °C temper.
The first set of three alloys referred to as the “Base” alloys had systematic variations in Mo and V content. The results of the Base alloy study showed that hardenability was the key factor in determining toughness as the least-hardenable steel often had upper bainite present that led to a significant decrease in the upper shelf impact energy. Differences in toughness between fully martensitic conditions of the three alloys were small and were attributed to differences in the prior austenite grain size, carbon content, and strength between the alloys.
The second set of alloys included two AF9628 steels with 0.25 and 1 wt pct Si respectively. These highly hardenable steels were used to determine the effects of cooling rate (i.e. autotempering) and Si on low temperature tempering response and on microstructural evolution during cooling. Additionally, a general Hollomon-Jaffe based tempering parameter model was developed to quantify the degree of autotempering experienced by every incremental fraction of martensite formed during quenching and to directly compare autotempering effects to isothermal tempering effects. The experimental results using the AF96 steels showed agreement between tempering parameter and hardness for autotempered conditions validating this approach.
Specimens of the AF96 alloys were also quenched in various media to produce different degrees of autotempering. Tensile and Charpy testing along with Mossbauer effect spectroscopy, transmission electron microscopy and high-energy X-ray diffraction were used to determine effects of Si and cooling rate. The results showed significant decreases in impact toughness and yield strength with decreased cooling rate in fully martensitic, low temperature tempered conditions for both alloys. The decreased yield strength and impact energy with slower cooling were attributed to the changes in retained austenite. Slower cooling rates led to increased retained austenite fractions in both alloys (leading to decreased YS through yielding at low strains), but the slower cooling rate specimens also showed increased decomposition of retained austenite following tempering at 204 °C. It is therefore believed that the decomposition of retained austenite during tempering (seen only in slower cooled specimens) is the mechanism responsible for the lower impact toughness in slow-cooled specimens.
Significant differences in the fine-scale microstructure were observed between the two AF9628 alloys due to the effects of Si. Increased Si content was shown to inhibit carbide formation during autotempering, increase the fraction of retained austenite during slow cooling, and inhibit carbide formation during low temperature tempering. Comparing the mechanical properties of the two alloys, similar strengths were measured for both, but the low Si variant had significantly reduced toughness which was attributed to the increased carbide fractions and possibly increased cementite precipitation.
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