Embrittlement Fracture in a 17-4 PH Stainless Steel after Aging at 400°C
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The embrittlement fracture mechanism caused by microstructural evolution of 17-4 PH stainless steel at long term aging was studied using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The solution treated specimen consists largely of lath martensite with a small fraction of elongated δ-ferrite. The spherical particles existed a little in the martensite matrix, while no precipitates were present in the δ-ferrite at the solution treated specimen as non-aging. The precipitation of Fe-Cu in the δ-ferrite causes the aged hardening after long term aging accormpanied by decreases in elongation and charpy V-notch energy absorption. The increased fraction of brittle fracture on the fractured surface by impact and tensile test reveals that the embrittlement of the 17-4 PH alloys during long term aging is mainly caused by the precipitation hardening in the δ-ferrite matrix.Keywords:
Charpy impact test
Embrittlement
Precipitation hardening
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Morphology
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In the present work, we use an advanced EBSD method to analyze the two prominent types of martensite microstructures that are found in the binary Fe-Ni system, lath martensite (27.5 at.% Ni) and plate martensite (29.5 at.% Ni). We modify, document, and apply an analytical EBSD procedure, which was originally proposed by Yardley and Payton, 2014. It analyzes the distributions of the three KSI-angles (ξ1, ξ2, and ξ3, KSI after Kurdjumov and Sachs), which describe small angular deviations between crystal planes in the unit cells of martensite and austenite—which are related through specific orientation relationships. The analysis of the angular distributions can be exploited to obtain high-resolution, color-coded micrographs of martensitic microstructures, which, for example, visualize the difference between lath and plate martensite and appreciate the microstructural features, like midribs in large plate martensite crystals. The differences between the two types of martensite also manifest themselves in different distributions of the KSI-angles (wider for lath and narrower for plate martensite). Finally, our experimental results prove that local distortions result in scatter, which is larger than the differences between the orientation relationships of Kurdjumov/Sachs, Nishiyama/Wassermann, and Greninger/Troiano.
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A new approach is developed to predict the radiation embrittlement of reactor pressure vessel steels. The Charpy transition temperature shift data contained in the Power Reactor Embrittlement Database is used in this study. The results indicate that this new embrittlement predictor achieved about 67.3% and 52.4% reductions respectively, in the uncertainties for General Electric (GE) Boiling Water Reactor plate and weld data compared to Regulatory Guide 1.99, Rev. 2 (RG1.99/R2). The implications of irradiation temperature effects for the development of radiation embrittlement models are then discussed. A new approach for the Charpy trend curve is also developed, which incorporates the chemical compositions into the governing fitting equation. This approach reduces the uncertainty of Charpy data fitting and provides an expedient scheme to link and project the surveillance test results to those for reactor pressure vessel steels.
Embrittlement
Charpy impact test
Boiling water reactor
Light-water reactor
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TEM was used to study the morphology and crystallography of lath martensite in low and medium carbon steels in the as-quenched and 200/sup 0/C tempered conditions. The steels have microduplex structures of dislocated lath martensite and continuous thin films of retained austenite at the lath interfaces. Stacks of laths form the packets which are derived from different (111) variants of the same austenite grain. The residual parent austenite enables microdiffraction experiments with small electron beam spot sizes for the orientation relationships (OR) between austenite and martensite. All three most commonly observed ORs, namely Kurdjumov-Sachs, Nishiyama-Wassermann, and Greninger-Troiano, operate within the same sample.
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The heat treatment technique for 35CrMo steel to get lath martensite microstrure and the contact fatigue of this lath martensite were researched.The results indicate that the contact fatigue life of this lath martensite is four times higher than the condition of quenched and tempered microstructure of this steel.
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A significant number of investigations have been performed on the determination of orientation relationships (OR) in high carbon, plate-martensitic steels. However, very little is known on the exact nature of ORs in technologically important lath martensitic steels. In the present study, in a series of low alloy steels with carbon contents between 0.1-0.4 wt%, the existence of retained austenite as thin films (∼200 Å thick) around the martensite lath boundaries, makes it possible to do direct crystallographic analysis between martensite and austenite by microdiffraction. The most commonly observed orientations for lath martensite-retained γ are <111> α' //<110> γ //<100> α' . Fig. 1 shows an example of a highly symmetric SAD pattern which was interpreted as follows: Considering only one martensite lath at a time, the <lll> α' and <110> γ combination corresponds to Kurdjumov-Sachs (K-S) OR, and <110> α and <100> γ corresponds to Nishiyama-Wassermann (N-W) OR. The coexistance of these two relationships may be taken as evidence that as many variants as necessary occur to provide maximum flexibility for martensite nucleation.
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Martensite in carbon steels forms in different morphologies, often referred to as lath andplate martensite. The alloy composition has a strong effect on the morphology, for instance in car-bon steels there is a morphological change of the martensite microstructure from lath martensite atlow carbon contents to plate martensite at high carbon contents. In the present work a decarburizedhigh-carbon steel, enabling the isolation of carbons' influence alone, has been studied in order to in-vestigate the changes in morphology and hardness. From the results it is concluded that there is acontinuous change of hardness with increased carbon content. The increasing hardness slows down atabout 0.6 wt%C before decreasing at higher carbon contents. This is in accordance with the change inmorphology since it was found that lath martensite dominates below 0.6 wt%C and the first units ofgrain boundary martensite and plate martensite appear above 0.6 wt%C. At high carbon contents thedominating morphology is plate martensite, but retained austenite is also present.
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The toughness of martensitic steels is strongly related to their fine and complex morphologies. To control the toughness of martensitic steels, the effect of carbon content on the morphology of lath martensite must be studied. However, these morphological changes are difficult to clarify using conventional two-dimensional observations because their morphologies are complex and tangled. Previously, we analysed and reported the three-dimensional microstructures of ultra-low-carbon lath martensite. Here we look further at three-dimensional microstructures of low-carbon lath martensite. Lath martensite in both specimens contains a few coarse packets in a prior austenite grain. The coarse packets in low-carbon lath martensite contain plate-like blocks stacked from end to end of the coarse packets and many fine blocks embedded in the coarse packets. The fine and included blocks are much smaller than the plate-like blocks in size. A part of the fine blocks belongs to the crystallographic packets different from the surrounding coarse packet, which should be regarded as 'fine' packets. The fine packets are also observed in ultra-low-carbon martensite. On the other hand, low-carbon martensite contains fine blocks that belong to the same crystallographic packet as the surrounding coarse blocks. These results suggest that the block structure in the coarse packet of low-carbon martensite is more entangled with each other than that of ultra-low-carbon martensite.
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