Fatigue fracture of AISI 4340 DVM steel in gaseous hydrogen & effect of tempering temperature

The effect of dry hydrogen (p = 102 kPa at room temperature) on AISI DVM austenized at 1113 K and tempered at 463, 53 and 743 K has been investigated by means of determinations of fatigue crack growth rate (FCGR) using CT samples (L-R and C-L orientation) precracked in laboratory air up to a value of ΔK of about 11 MPa√m. The fatigue tests were conducted both in air (RH = 45-50%) and hydrogen environment under the following conditions: R = 0.2, Pmax = 2.5 kN, sine wave of 3 and 0.5 Hz. The FCGR were found to be independent on the frequency. The fracture surface of samples fatigated in air, showed brittle striations and, at higher ΔK, some transgranular fracture, while in those obtained in dry hydrogen both an increasing percentage of quasi-cleavage facets and secondary cracks were observed. The fatigue behaviour of the specimens tempered at 463 and 553 K in the two environments considered was much diversified. The fracture surfaces revealed a mixed fracture mode: intergranular along the grains of primary austenite and in some zones, transgranular. The da/dN - ΔK curves showed that the FCGR was controlled by a condition of stress corrosion cracking. A KISCC value of 13.7 MPa√m was estimated. The appearance of a plateau in these curves, at nominal increasing ΔK, is presumably attributable to a slowing of the primary cracking owing to the occurrence of an increasing number of secondary cracks.