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Materials Durability, Mechanical Testing, Nondestructive Testing, Characterization
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Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 179-182, September 30–October 3, 2024,
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Ductility dip cracking (DDC) is a detrimental solid-state cracking phenomenon that can occur during welding of copper-nickel (Cu-Ni) alloys used in naval vessels. The presence of these cracks has several deleterious effects, including reduced fatigue life and increased susceptibility to corrosion. The mechanism of DDC remains highly debated and understudied, especially in material systems outside of Ni-Cr-Fe alloys. The predominant mechanisms that have been proposed include: 1. Grain boundary sliding, 2. Precipitate-induced strain, and 3. Impurity element segregation. In the present body of research, thermal-mechanical testing over a wide range of strain rates and temperatures was performed using a Gleeble 3500. Both flow-stress and fracture morphology of wrought 70/30 Cu- Ni are considered. Following fracture, microstructural analyses using both scanning electron microscopy and optical microscopy were conducted to observe and quantify intergranular cracking and fracture surface features. Results show a strong correlation among fracture morphology, ductility, and temperature.
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 208-211, September 30–October 3, 2024,
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Decarburization of steel parts during heat treating results in a lower surface hardness, undesirable residual stress profiles, and poor part performance. Significant effort has been made towards preventing decarburization and determining the impact of annealing time and temperature on decarburization rate. Much of the published research has focused on medium carbon steels, ranging from 0.3wt% C to the eutectoid composition. The goal of the current research is to determine decarburization rates for steels with carbon concentrations above the eutectoid concentration. AISI 52100 steel was heated in air for 12, 24, and 36 hours at three temperature ranges (below A 1 , above A cm , and between A 1 and A cm ). Optical microscopy was used to determine the carbon concentration as a function of depth from the surface. The diffusion coefficients of carbon in austenite and ferrite plus cementite phase assemblages were calculated. These diffusion coefficients can be used in a finite difference simulation to predict decarburization at different temperatures and times.
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 234-238, September 30–October 3, 2024,
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Residual stresses are unavoidable in heat treatment and surface engineering and their presence can be advantageous or disastrous for the performance of components. Residual stresses cannot be measured directly, but are determined from strain measurements, either non-destructively from diffraction-based methods, or destructively from relaxation-based methods. In this presentation, three examples of stress determination from strain measurements showcase some of the possibilities. In the first example lattice strains are determined with energy dispersive analysis with synchrotron radiation in relation to the phase fraction during martensite formation in a soft martensitic stainless steel. The second example shows synchrotron lattice determination with energy dispersive analysis during in-situ tensile loading of super martensitic stainless steel containing reverted austenite. The third example concerns determination of residual stresses in internally oxidized bulk metallic glass with laboratory X-ray diffraction analysis of lattice strains and displacements by stress relaxation during incremental ring-core excavation of micron-scale columns with focused ion beam milling in an SEM.
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 316-320, September 30–October 3, 2024,
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Thermochemical treatments like carburizing and carbonitriding allow to improve the properties in low-alloyed steels, which depend mainly on the distributions of residual stresses and microstructures. As the fatigue properties depend mainly on the latter, a fundamental understanding must be established regarding their formation during the cooling after the enrichment treatment. This study introduces an experimental and simulation analysis of microstructure and internal stresses evolutions and their couplings. Influence of the carbon and nitrogen enrichments is highlighted. An original experimental technique is introduced to follow in situ by High-Energy XRD the phase transformation kinetics and the evolutions of the internal stresses during cooling, inside laboratory scale samples with C/N composition gradients. The usual trends are confirmed regarding the carburizing: the carbon-enriched case is the last to undergo phase transformations. Due to the phase transformation strains, the surface ends up with compression residual stresses, whereas the center is put in tension. Conversely, for carbonitriding, unusual profiles of microstructures and residual stresses are observed. The presence of nitrogen induces a drastic loss of hardenability in the enriched case. This modifies the chronology of the phase transformations and this leads to tensile residual stresses at the surface for the studied cooling conditions. In the nitrogen-enriched case, a fine microstructure is formed during cooling and retained austenite remains, leading to a lower hardness than in the martensite layer beneath. A coupled thermal, mechanical and metallurgical model predicting the phase transformation kinetics and the evolutions of internal stresses is set up. It takes account of the local carbon and nitrogen concentrations in the case. For carburizing, predictions are in good agreement with experiment. Simulations for carbonitriding achieve to predict the tensile stresses in the nitrogen-enriched case, which are due to the loss of hardenability. In both cases, residual stresses come mostly from phase transformation plasticity strains.
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 321-326, September 30–October 3, 2024,
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One of the methods of evaluating the mechanical properties of a material in the case of its limited amount is the use of techniques that employ the miniaturized test specimens. The basic properties used mostly for residual life evaluation are tensile strength, impact notch toughness or impact notch toughness transition curve, fracture toughness, creep and high cycle fatigue. For example, by semi-destructive sampling of operating power equipment, actual material properties can be obtained which are crucial for predicting the residual life of the equipment. Furthermore, the local material properties of the weld joint in individual zones can be determined. In this paper applicability of these test methods is described, specific examples of use are given and reference is made to the existing ISO/ASTM 52909:2022 standard for the use of sub-size samples.
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 327-331, September 30–October 3, 2024,
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Advanced characterization techniques and modeling are used to get new insight on the microstructural evolutions occurring during the tempering of low-alloyed steels with initial martensitic microstructure. Tempering temperatures from 150°C to 600°C, are considered to make vary the metallurgical phenomena activated, form carbon segregation to defects to precipitation of different types of carbides (transition, cementite, alloyed). A large range of carbon compositions, from 0.1 to 0.7 wt.% are investigated, with the same main experimental technique: in situ HEXRD at synchrotron beamlines, with complementary post mortem fine-scale characterizations by TEM and 3D-APT. In the middle of this range (~0.3wt.%), the usual sequence is observed: successive precipitation of transition and cementite carbides. New observations concern the carbon concentrations outside this range. For high carbon concentrations (~0.6wt.%), the same sequence occurs but the martensite/ferrite matrix remains highly supersaturated in carbon compared to equilibrium, for a long time and even after the precipitation of cementite. For low carbon concentrations (~0.1wt.%) most of the carbon starts to segregate at defects (dislocations, lath boundaries). This enters in competition with the transition carbides which are almost fully hindered, whereas cementite precipitates afterwards. Two previous models from literature are combined to predict the concomitant kinetics of carbon segregation and precipitation. Segregation puts the transition carbides at a disadvantage with cementite and for this reason, the latter precipitates earlier than usually reported. The effects of nitrogen enrichment (up to ~0.4 wt.%N, context of carbonitriding thermochemical treatments) in austenite domain of stability (before the martensitic quench) are also investigated. In low-alloyed steel considered (23MnCrMo5), nitrides are formed upon enrichment (CrN, MnSiN 2 ). This has a strong impact on the precipitation sequence, compared to model systems previously investigated (Fe-N, Fe-C-N).
Proceedings Papers
IFHTSE2024, IFHTSE 2024: Proceedings of the 29th International Federation for Heat Treatment and Surface Engineering World Congress, 346-351, September 30–October 3, 2024,
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The automotive industry has searched for alternatives to reduce the weight of vehicles without neglecting the user’s safety by using new materials. Advanced high-strength steels of complex phases are used in structural applications requiring good performance and reducing the weight of vehicles. However, these steels have shown edge cracking, known as fissure, during processing, which has become a challenge for steelmakers and other companies that rely on them to manufacture structural components. Such defects can be associated with the interaction between the different microstructural constituents of the steel, such as various phases and precipitates generated during its processing to achieve the required mechanical properties. The present work presents the studies evaluate the effect that processing and chemical composition exerts on edge cracking in complex phase steels of grade 800 MPa produced by different steelmaking routes.