Thickness and microstructure effect on hydrogen diffusion in creep-resistant 9% Cr P92 steel and P91 weld metal

Abstract

Martensitic 9% Cr steels like P91 and P92 show susceptibility to delayed hydrogen assisted cracking depending on their microstructure. In that connection, effective hydrogen diffusion coefficients are used to assess the possible time-delay. Limited data on room temperature diffusion coefficients reported in literature vary widely by several orders of magnitude (mostly attributed to variation in microstructure). Especially P91 weld metal diffusion coefficients are rare so far. For that reason, electrochemical permeation experiments had been conducted using P92 base metal and P91 weld metal (in as-welded and heat-treated condition) with different thicknesses. From the results obtained, diffusion coefficients were calculated using to different methods, time-lag, and inflection point. Results show that, despite microstructural effects, the sample thickness must be considered as it influences the calculated diffusion coefficients. Finally, the comparison of calculated and measured hydrogen concentrations (determined by carrier gas hot extraction) enables the identification of realistic diffusion coefficients.

Effects of Wall Thickness Variation on Hydrogen Embrittlement Susceptibility of Additively Manufactured 316L Stainless Steel with Lattice Auxetic Structures

“…Whereas the thickest strut, T1.4, displayed fewer surface flaws, the mechanical properties were more affected by H-charging. This can be attributed to reversible trapping sites, such as dislocation cores [ 23 ], which are increased with increasing cross-section, as reported in the literature [ 38 , 39 ]. This is in an agreement with the mechanical properties illustrated in Table 2 .…”

“…Moreover, the deterioration in ductility is directly proportional to the thickness increase, which is attributed to the increase in reversible H-trapping sites as the thickness increases. It has been reported in the literature [ 38 , 39 ] that reversible H-trapping sites are increased with an increase in thickness, and, consequently, more diffusible H exists in the T1.4 specimen. In addition, the depth beneath the edges affected by the existence of H was increased with increasing thickness, which was confirmed by fracture surface fractography, as shown in Figure 6 a2–c2.…”

“…One of the most interesting findings in the literature is the scarcity of studies concerning the additive manufacturing of thin-walled structures. The effect of the wall thickness variation on the HE susceptibility of different steel grades has been reported [ 38 , 39 , 40 ]. Rhode et al [ 38 ] concluded that, as the wall thickness of the H-charged specimens increased, the H-spread through the specimens was delayed, resulting in an increased diffused hydrogen breakthrough time compared with the thinner samples, which was ascribed to the increased length of the H diffusion path.…”

“…The effect of the wall thickness variation on the HE susceptibility of different steel grades has been reported [ 38 , 39 , 40 ]. Rhode et al [ 38 ] concluded that, as the wall thickness of the H-charged specimens increased, the H-spread through the specimens was delayed, resulting in an increased diffused hydrogen breakthrough time compared with the thinner samples, which was ascribed to the increased length of the H diffusion path. However, the number of hydrogen trapping sites increases in the thicker specimens.…”

Influence of PWHT Parameters on the Mechanical Properties and Microstructural Behavior of Multi-Pass GTAW Joints of P92 Steel

“…Weldments of P92 steel offer poor mechanical properties at room temperature due to the formation of brittle fresh martensite in the weld fusion zone (WFZ). Other factors, including heterogeneous microstructure formation along weldments, the evolution of residual stresses along weldments, the formation of soft δ ferrite patches, the presence of diffusible hydrogen and the hardening of coarse-grained HAZ are also considered major causes of failure in P92 weldments [16,[19][20][21][22][23][24][25]. Heterogeneity in the microstructure as a result of high heat input mainly leads to variation in mechanical properties, i.e., hardness and impact toughness.…”