Hydrogen embrittlement of 4340 with martensitic and bainitic microstructures for fastener applications

Abstract

The hydrogen embrittlement (HE) resistance of martensitic and bainitic microstructures has been evaluated in a range of hardness levels relevant to large fastener applications. Alloy 4340 was processed via quench and tempering, austempering, and austempering and tempering to achieve martensitic, bainitic, and tempered bainitic microstructures. Bulk hardness, smooth-sided, and notched round bar tensile testing were used to evaluate mechanical properties, while dilatometry, electron microscopy, and x-ray diffraction were utilized to characterize the various microstructures. Hydrogen embrittlement was assessed via incremental step load (ISL) testing of notched tensile specimens in 3.5 pct NaCl with concurrent cathodic polarization with an applied current density of 0.1mA·cm-2. Additional tests utilized the direct current potential drop method, concurrent with the ISL test, to probe crack initiation. At constant hardness, tensile mechanical properties varied by microstructure, with yield point and work hardening behavior being the principle factors contributing to this disparity. Hydrogen embrittlement susceptibility increases with increasing strength or hardness, with some microstructural effects. The onset of dislocation motion, measured through microyield stress, does not correlate with HE resistance, and plastic deformation appears a necessary aspect of the HE processes. Increased measures of flow stress decrease the apparent microstructural effects. The austempered bainitic conditions are more susceptible to hydrogen than the martensitic conditions, for a given yield strength. The highly tempered bainitic condition offers improvement in Kth over a comparable hardness/strength martensitic condition. The most resistant materials fail at high stress intensities, via transgranular cracking, while the greatest strength conditions fail along prior austenite grain boundaries at a similarly low Kth. Intergranular fracture along austenite grain boundaries presents a lower bound limit for Kth. Notch root stress and strain in hydrogen were determined from the equivalent strain energy density method, which indicated that Kth is directly proportional to the square-root of the notch strain. The relationship is independent of microstructure, strength, yield behavior, or fracture mode in hydrogen. The DCPD technique indicates all conditions have limited or no stable crack growth, and the largest difference in stress intensity factor between crack initiation and unstable propagation was 3 pct; the ISL test is therefore correlated with the onset of unstable crack growth.