RELATION OF C AND MN CONTENT ON HARDENABILITY OF CARBON SPRING STEEL

In these days, one of our troubles in spring production is that spring leaves whose sizes are 13×100×250-1250mm are not hardened enough to the center of the section in usual oil quenching because of low hardenability characteristics of plain carbon spring steels (JIS SUP 3, SUP 4). It is, therefore, desirable to improve hardenability of these steels.For this purpose, authors firstly consider to increase Mn content, and then have experimented and calculated the effect of C and Mn content upon the hardenability of these steels.From these results, authors have determined the minimun limit of Mn content needed for the desired hardenability, and also observed the eftect of C content at constant Mn content.The results obtained are as follows:(1) Hardenability of SUP 3 and SUP 4 is too low to get the desired hardening effect.(2) To obtain the desired hardenability, it is necessary that the ideal critical size D1 practically determined by Jominy End-Quench Test is 2 inches or over. From our experiments, it is found that the ratio of practically determined D1 to calculated D1 from chemical analysis and grain size is in the range from 0.75 to 1.0 and mean value is about 0.9.(3) It is necessary that needed minimum content of Mn for JIS SUP 3 and SUP 4 is at least 0.9-1.0%.(4) Improvement of hardenability is expected by increasing C in 1% Mn series, but not in 0.4% Mn.