Properties of Medium Carbon Chromium Steel

A study was made of the mechanical properties and hardenability of medium carbon chromium steel which was developed as corrosion resisting steel under weak acid corrosive atmosphere. It was proved that retained hydrogen had an extremely bad effect especially on the ductility of this steel. Therefore, the process of production was investigated in order to increase the ductility. As the effect of boron on hardenability was different with nitrogen content, the relation between the effect of boron and content of such nitride forming elements as aluminum and titanium was examined. The following results were obtained. (1) In order to guarantee 8 percent of elongation (JIS No 4 tensile specimen) in steel with 140 Kg/mm2 of tensile strength, hydrogen content had to be kept less than 1 ppm. Hydrogen content was decreased less than 1 ppm by slow cooling of the bloom after blooming. (2) The effect of boron on hardenability was remarkable in steels containing nitrogen of less than 40 ppm even if only boron was added to the steels. But in order to make boron more effective for hardenability, aluminum more than 0.01 pet., and aluminum more than 0.04 pet. or titanium more than 0.02 pet. were required for steels containing about 0.006 pet. of nitrogen and for steels containing about 0.01 pet. of nitrogen respectively. (3) The effect of boron on hardenability corresponded with content of acid soluble boron which was decided by solubility products of (B)(N), (Al)(N) and (Ti)(N). But in all cases, the relationship between hardenability and the content of acid soluble boron did not always hold good. In steels containing aluminum, the content of acid soluble boron which was necessary to secure the same hardenability was much smaller than the content in steels without addition of aluminum. (4) It was proved from the investigation on distribution of boron by α-track method that the hardenability was related to the concentration of boron on pre-austenite grain boundaries in steel without addition of aluminum.