For the purpose of finding some low-cost alloys that resist to rupture at high stress at a temperature of 700-900°C, heat-resisting high-manganese steels in which part of nickel was replaced by manganese were investigated. Even if part of 10% Ni in 20% Ni content was replaced by 10% Mn, the creep rupture strength of 10% Mn alloy containing proper amount of carbon, and nitrogen was superior to that of N-155 alloy, and the high-manganese steels showed the least problems that might be posed by forging of high-alloy steels.
The reason was based on the fact that the 10% Mn alloy containing high carbon or high nitrogen could be forged, because the deformation resistance became lower as the manganese content was increased and therefore the alloy with the excellent forgeability could be prepared therewith. The alloy containing about 4% Mn had the longest creep rupture life.
But if the carbon content in 10% Mn alloy was increased to 0·48%, or the nitrogen content in it was increased to 0·38%, the rupture life became very longer and was equal to that of 4% Mn alloy at 700°C, and alloy that was more excellent in creep strength than N-155 alloy was obtained.
In this case, the creep rupture life of 10% Mn alloy at 700°C became longer by addition of vanadium or phosphorus, but the effects of these elements disappeared with the alloy at above 800°C. At 700°C, the rupture life of 10% Mn alloy containing phosphorus became about four times as long as that of the alloy without phosphorus addition, but the rupture elongation became lower. The oxidation-resistance was found to be less worse as the manganese content was increased, but remarkably worse by addition of vanadium or phosphorus.
Therefore, these alloying elements had a harmful effect on forgeability of the alloy, and there was a possibility of generation of cracking when the alloy containing phosphorus was forged. The best chemical composition of a high-manganese alloy obtained from the above facts was as follows:
C 0·21%, Cr 19·96%, Ni 9·52%, Co 20·13%, Mn 9·91%, Mo 3·01%, W 2·79%,
Cb 0·97%, N 0·38%, P 0·012%, Si 0·10%, O2 4·6 p. p. m.