抄録
The embrittlement of ultra-high strength steels due to the action of hydrogen introduced by Cd-electroplating has been studied in stress rupture (sustained load) and tensile tests for notched specimens.
4340 steel, SNCM 9, 300 M steel, H-11 Mod. steel and 18%Ni maraging steel, heat-treated to the 200 kg/mm2 tensile strength level, and 5.5W-2.5Cr steel and SKS 4, heat-treated to the 170 kg/mm2 tensile strength level, were examined.
The results of the present work are:
(1) Susceptibility of hydrogen embrittlement estimated by redution of area and notch tensile strength were as follows:
SNCM 9\fallingdotseq4340\gtrsimH-11 Mod.>SKS 4>300 M>18%Ni maraging steel>5.5W-2.5Cr.
(2) Susceptibility of hydrogen embrittlement estimated by delayed failure properties were as follows:
4340\fallingdotseqSNCM 9>300 M>H-11 Mod.\fallingdotseqSKS 4>18%Ni maraging steel\gtrsim5.5-2.5Cr.
(3) 5.5W-2.5Cr steel and 18%Ni maraging steel were noticeably less susceptible to embrittlement by hydrogen than the others.
(4) Of the hydrogen embrittled plain tensile specimens, 5.5W-2.5Cr steel showed cup and cone type fractures similar to the unplated specimen, but others showed brittled fractures whose failure was initiated just below the surface of specimens.
Failure-initiated regions of SNCM 9, 4340 steel, 300 M steel and SKS 4 show the coexistence of intergrannular and transgranular crackings with respect to pre-austenite grain boundaries in away similar to the hydrogen-induced delayed failure, but the interganular cracking was predominant.
