Fine–scale structural features of intercritically aged HSLA-100 plate steel and their influence on yield strength and low-temperature impact toughness

Journal Article · Thu Feb 15 04:00:00 EST 2018 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2017.12.037· OSTI ID:22804907

Highlights: • The fine-scale microstructure of HSLA-100 steel intercritically aged at 677 °C was characterized. • Continuous austenite films evolved into isolated islands; large islands transformed to brittle martensite upon quenching. • Copper precipitates in martensite were coarse, and some sub-grain coarsening occurred within martensite. • Intercritically aged steel possessed an inferior strength-toughness balance as compared with sub-critically aged steel. - Abstract: Fine-scale structural features of an HSLA-100 steel intercritically aged at 950 K (677 °C) for 4.2 ks (70 min.) were investigated via transmission electron microscopy. The microstructure consisted of approximately 90 vol. pct. of low‑carbon lath martensite. Average lath width was about 0.25 μm, unchanged from the as-quenched condition, owing to a high number density of epsilon‑copper precipitates at these boundaries. About 10 vol. pct. of medium‑carbon austenite existed as coarsened, broken-up islands between laths; martensite lath boundary coverage by austenite was notably reduced compared to the as-quenched condition. Greater continuity between adjacent low‑carbon martensite laths was accompanied by coarse copper precipitates within laths and an order of magnitude lower dislocation density than the as-quenched martensite. Collectively, these features produced a sub-grain coarsening effect. Austenite islands larger than about 0.5 μm were unstable with respect to the formation of fresh, medium‑carbon martensite regions. Copper precipitation within austenite islands (owing to carbon and copper enrichment) might also have contributed to austenite transformation upon quenching. Intercritical aging was associated with a strong-and-undesirable deviation from strength-toughness behavior at sub-critical aging temperatures. Two key factors were identified as being responsible for this negative outcome: (i) a sub-grain coarsening effect associated with “delamellarization” of low‑carbon martensite and austenite and (ii) the formation of brittle martensite upon quenching. Quantitative estimates of the changes in strength and impact toughness are provided for microstructural differences between intercritically aged and subcritically aged material.

• OSTI ID: 22804907
• Journal Information: Materials Characterization, Journal Name: Materials Characterization Vol. 136; ISSN 1044-5803; ISSN MACHEX
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
AUSTENITE
COPPER
DISLOCATIONS
FILMS
MARTENSITE
MICROSTRUCTURE
PRECIPITATION
STEELS
TRANSMISSION ELECTRON MICROSCOPY
YIELD STRENGTH