Hardenability of steels for precision castings

Abstract

1. The standard medium-carbon steel 45L, widely used for manufacturing cast machine parts, which can be melted in high-frequency induction furnaces with an acid lining, has a high hardenability and tendency to grain growth. 2. Steel 50PPL, which can be melted in high-frequency induction furnaces with a basic lining, has a minimal hardenability. The ideal critical diameter of basic 50PPL is 8.6 mm, and does not increase at quenching temperatures of 800–1050°. Acid steel 50PPL has satisfactory hardenability, with an ideal critical diameter of 14.6 mm in quenching from 800° and 17,6 mm in quenching from 950°. 3. It was found that the fine austenite grain size (grade 12) of basic 50PPL hardly changes with quenching from 800–1000°; for acid 50PPL the austenite grain size increases from grade 12 to grade 11 at these quenching temperatures. 4. Raising the silicon content above 0,3% in acid 50PPL increases the hardenability due to the increasing resistance of austenite to decomposition. 5. Preliminary high-temperature heat treatment (quenching from 1250° and tempering at 650°) of acid 50PPL increases the dispersity of carbide and nitride inclusions and thus reduces the tendency to austenite grain growth, reduces the grain size, and lowers the hardenability. Preliminary heat treatment (quenching from 880° and tempering at 650°) has no effect on the kinetics of austenite grain growth or the hardenability of 50PPL. Hot plastic deformation (forging with cooling in air) increases the hardenability.