Effect of B and N Content and Austenitization Temperature on the Tensile and Impact Properties of Modified 9Cr-1Mo Steels

• Published in: Metals (Basel ) Vol. 13; no. 6; p. 1124
• Main Authors: Rejeesh, Ravindran, Barik, Rakesh, Mitra, Rahul, Kostryzhev, Andrii, Das, Chitta, Albert, Shaju, Chakrabarti, Debalay
• Format: Journal Article
• Language: English
• Published: Basel MDPI AG 01.06.2023
• Subjects: auger electron spectroscopy; austenitization temperature; Austenitizing; B and N contents; Charpy impact test; Chromium molybdenum steels; Crack initiation; Ductile-brittle transition; Grain boundaries; Hot rolling; Impact strength; Mechanical properties; microalloyed MX precipitates; Microscopy; Microstructure; modified P91 steels; precipitate stability; Precipitates; Spectrum analysis; Steel; Temperature; Transition temperature
• ISSN: 2075-4701; 2075-4701
• DOI: 10.3390/met13061124

The present study investigates the relative effect of B and N concentrations and the austenitization temperature on the microstructure and mechanical properties (tensile and Charpy impact) of modified 9Cr-1Mo (P91) steels. Initially, a B-free P91 steel (with 500 ppm N) and four different B-containing steels (25–100 ppm) with varying N concentrations (20–108 ppm) were hot-rolled, normalized from different austenitization temperatures (1000–1100 °C/1 h) and finally tempered at 760 °C for 1 h. A Charpy impact test shows that the ductile–brittle transition temperature (DBTT) of all the B-added steels decreases with an increase in the austenitization temperature, where the 100 ppm B steel offers the lowest DBTT (−85 °C). Similarly, the strength increases with the increase in the austenitization temperature (1100 °C), with a slight drop in ductility. The influence of precipitates on the microstructure and mechanical properties is explained considering the B enrichment at the precipitates and the thermodynamic stability of the precipitates. The 100 ppm B steel (containing the maximum B and minimum N), normalized from 1100 °C austenitization, shows the best combination of tensile and Charpy impact properties, owing to the effective dissolution of coarse M23C6 and MX precipitates during the normalization treatment and the formation of fine B-rich (Fe,Cr)23(B,C)6 precipitates during the subsequent tempering.