1990 Volume 76 Issue 8 Pages 1372-1379
21/4 Cr-1 Mo steels which were alloyed with V, Nb, Ta or Ti up to 0.23 at% were studied to evaluate the effect of each carbide forming element on the hydrogen attack. Steels were quenched following the austenitization at 950 or 1 250°C and tempered at 690°C for 19.4 h, and exposed to hydrogen at 600°C and 300 kgf/cm2 for 1000 or 3 000 h.
At the higher austenitizing temperature of 1 250°C, alloying of these elements always decreased the bubble volume density, represented by the bubble volume per unit area of the prior austenite grain boundary. The effect in the reduction of the bubble volume density increased in the order of V, Nb, Ta and Ti, and it correlated well with the standard free energy change for the formation of MC type carbides.
In the case of austenitization at 950°C, a little effect to decrease bubble formations was unexpectedly found out in steels containing the higher content of Nb, Ta or Ti than that at 1 250°C, and the bubble volume density did not decrease in Nb containing steels. These results are considered to be originated from the undissolved carbides at the lower austenitizing temperature of 950°C. In a longer exposure of 3000 h, addition of Nb, Ta or Ti with the range of 0.05 to 0.24 at%, regardless of the content, always increased the bubble volume density.
