Crack growth behaviour of heat-treated 4140 steel in air and gaseous hydrogen

Abstract

The fatigue crack growth rates (FCGRs) of AISI 4140 steel specimens which were subjected to various heat treatment procedures were performed on compact-tension specimens both in air and gaseous hydrogen. Experimental results indicated that the acceleration of crack growth in hydrogen was more pronounced for QT (quenched and tempered) specimens tempered in the range of 230 to 370 degrees C as compared to those specimens tested in air. Tempering in such a temperature range coincided with the 'one-step temper embrittlement' range of the 4140 steel. For specimens tempered at a higher temperature, e.g. 550 degrees C, the influence of hydrogen on FCGRs became insignificant. In case of specimens austempered (AT specimens) at 230-300 degrees C, the yield strength could reach approximately 1500 MPa with reduced gaseous hydrogen embrittlement (GHE) in fatigue tests as compared to QT specimens of the same strength level. The extent of accelerated FCGRs in hydrogen or the susceptibility to GHE could be related to the fracture surface of specimens covered by brittle features. The change in fracture modes of several heat-treated specimens in hydrogen, relative to the fracture behaviour in air, was also discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.