An Investigation on Boron-Treated Steels. I : On the Hardenability of Boron-Treated Medium-Carbon Steels, especially the Effect of Nitrogen-Content in Steels

As fundamental-studies on boron-treated steels, the hardenability and the effect of nitrogen in it were studied with medium carbon steel. The maximum hardenability is obtained with the addition of boron by the order of 0.003%, being decreased sharply with further increase of boron. When the nitrogen content is less than about 0.008%, the effect on the improving action of boron on the hardenability is almost nil, but with increasing of its amount over 0.008% the effect of boron decreases markedly, until the nitrogen content increases up to about 0.02% in which the effect of boron almost diminishes. When steel is strongly deoxidized and denitrogenized by adding aluminium or titanium, prior to boron addition, the effect of boron appears markedly. These results may be attributed to fact that boron accerelates to crystallize out a nitrogen compound during quenching or tempering of steels.