PSI - Issue 68

Adam Ståhlkrantz et al. / Procedia Structural Integrity 68 (2025) 1051–1058 Ståhlkrantz et al./ Structural Integrity Procedia 00 (2025) 000–000

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Table 2: Amount of RA in the different samples and what type of heat treatment used.

Sample

Amount of RA [%]

Type of heat treatment performed Fully austenitized, quenched and tempered Fully austenitized and quenched and partitioned Intercritically austenitized and quenched and partitioned Fully austenitized and austempered Intercritically austenitized and austempered

A B C D E

0.9 8.0

10.2 13.4 14.9

The effect of the different heat treatments is also evident in the mechanical properties of the material, as seen in Figure 5, where the results from the tensile test for the uncharged and charged specimens are presented. It is evident that the fracture behavior changes with increasing RA amount. The RA is transforming into martensite due to the mechanical stress and strain during a tensile test, increasing the global strain hardening rate in the material and thereby delaying the onset of localized plasticity (necking). This phenomenon is commonly referred to as transformation induced plasticity (TRIP). In a tensile test, the TRIP effect results in increased uniform elongation. Plotting the RA fraction of the samples against the uniform elongation shows a linear relation towards the amount of RA Figure 2 A, as expected.

Figure 2: A) RA plotted towards elongation at the maximum load showing a linear relation towards the amount of RA in the samples. B) shows the drop in uniform elongation after charging with respect to RA. When RA no longer transforms at higher strains (either due to the high mechanical stability of the remaining austenite or simply due to all of the austenite having been transformed), the strain hardening rate drops and necking (or localized plasticity) typically follows rapidly. At this stage of the tensile test, the surrounding matrix has to plastically accommodate the very hard fresh martensite islands, whose interfaces with the matrix now provide microcrack and damage nucleation sites. The local ductility of the material is thus reduced compared to a RA free material. This behavior is readily observed in the macrographs taken from the fracture surfaces of the tensile specimens Figure 5. Materials A and B with the lowest RA fractions exhibit the smallest fracture surface areas, which translate to highest degrees of true fracture strain as defined in Hance (2016). The presence of strongly trapped hydrogen changes the ductility behavior of the specimens. When comparing the areas of the fracture surfaces of both uncharged and charged specimens visually in Figure 5, it is evident that charging with hydrogen has reduced the true fracture strain of all specimens, resulting in an increased surface area of the fracture. Comparison of the stress-strain curves indicates that the uniform elongation has also decreased with charging, with the magnitude of the decrease varying from specimen to specimen. The observed changes in fracture and mechanical properties will herein be described in terms of local and global formability adapted from Hance (2016). The global formability is indicated by true strain at necking, ε u (1), where ε Agt is the elongation at maximum load, plotted on the x-axis in Figure 3. The local formability is indicated by true fracture strain, TFS (2), where A 0 is the initial area and A f is the area of the fractured surface. TFS is plotted on the y-axis in Figure 3. The ranking indicated is defined by the formability index, F.I. (3), which gives the lines used for characterization of the quality of the local and global formability in Figure 3 (fair, F.I. = 0.1, good, F.I. = 0.2 and very good, F.I. = 0.3).