ON THE TEMPER-BRITTLENESS OF STEELS

In this investigation, a statistic survey on the phenomenon of temper-brittleness is made with 164 actual charges of plain carbon and special steels.The results show that the temper-brittleness is primarily related to the constitution of steel. The elements Cr, Mn, Si, Ni and P are confirmed to promote the phenomenon. Closer observation of the phenomena reveals the fact that there are two kinds of temper-brittleness:- the one is observed at the tempering range 450°-525°, and is not affected by the rate of cooling after tempering, while the other occurs above 525° and shows the vast difference in the impact value between rapidly cooled and slowly cooled samples; which has hitherto been known as the temper-brittleness. To distinguish these two kinds of temper-brittleness the present authur has called the former as the "First Temper-Brittleress" and the latter as the "Second Temper-Brittleness" respectively.The so-called carbide theory has been developed. When martensite, a super-saturated solid solution f carbide, is tempered, the precipitation of the carbide occurs at the tempering range 450°-525°, This precipitation is the cause of the first temper-brittleness; at the tempering temperature above 525° the redissolution of the precipitated carbide bigins to take place as the result of the increaing solubility of the carbide in α-iron. This causes the difference of impact value between rapid and slow coeled samples, that is the second temper-brittleness.The carbide, which causes the temper-bittleness, is ascertained to be Fe3C or its solid solid solution; and it is considered that the occurence of the temper-brittleness in steels depends on the correlation of these carbides and α phase.It is possible, in all steels, to observe more or less the first temper-brittleness, which is considered to be the combined effect of Si, Mn, P and eventually Cr present in the steels.It has often been reported that even in the case of samples having an identical composition and subjected to the same heat treatment, there is a great variation in the susceptibility of the temperbrittleness. The present authur believes that this is due to the existing oxides in the steels, and the more the oxides present, the larger the ratio of the susceptibility.The remeding action of Mo and W is confirmed in the temper-brittleness of several steels, and its mechanismis is described.