WEB Hydrogen cracking in a "martensitic" stainless steel obtained from Selective Laser MeltingWednesday (23.09.2020) 11:50 - 12:05 S: Structural Materials 1 Part of:
This study investigates the microstructure and resistance to hydrogen cracking of two 17-4PH steels of the same composition. The two materials were comparatively studied: the first one is a conventional wrought material; the second one was obtained from additive manufacturing, more specifically Selective Laser Melting (SLM). They show very different microstructure: while the conventional wrought material has the expected martensitic microstructure, the SLM-ed steel has a delta-ferritic solidification microstructure with coarse grains. On the other hand, the yield strength of the two steels is not very different, in spite of their different grain microstructure. A detailed analysis of the strengthening mechanisms was conducted, based on one hand on EBSD measurement of delta-ferrite grain size (in the SLM-ed material) and martensite block size (in the wrought one), and on the other hand, on the measurement of dislocation density using neutron diffraction peak broadening. This analysis suggests a stronger contribution of precipitation strengthening in the SLM-ed material. This is consistent with high resolution SEM imaging, as well as low energy EDX mapping, showing copper nanoscale precipitation.
Electrochemical hydrogen permeation showed a slightly faster diffusion of hydrogen in the ferritic SLM-ed steel compared to its martensitic wrought counterpart. This slight difference in hydrogen diffusion in the two materials is discussed in view of their respective microstructure.
Hydrogen cracking was studied using slow strain rate tests under cathodic charging on the wrought and SLM-ed 17-4PH steels after ageing 4h at 580°C. The tensile tests were conducted at rates ranging from 10-5 to 10-7 /s. Significant reduction in elongation was observed for the two materials, although more pronounced for the SLM-ed one. The tensile specimens show no necking at all, although the stress-strain curves demonstrate a very clear load drop before final fracture. Side observations of the tensile specimens show multiple secondary cracks. It is concluded from those observations that failure occurs from initiation/growth/coalescence of subcritical cracks, followed by the final critical fracture. Fractography examination reveals very different fracture surfaces in the two materials. The results are explained by the difference in tortuosity of the possible cracking paths in the two materials.