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Carbonitriding
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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, 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
HT 2021, Heat Treat 2021: Proceedings from the 31st Heat Treating Society Conference and Exposition, 187-195, September 14–16, 2021,
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Low pressure carbonitriding and pressurized gas quenching heat treatments were conducted on four steel alloys. Bending fatigue tests were performed, and the highest endurance limit was attained by 20MnCr5+B, followed by 20MnCr5, SAE 8620+Nb, and SAE 8620. The differences in fatigue endurance limit occurred despite similar case depths and surface hardness between alloys. Low magnitude tensile residual stresses were measured near the surface in all conditions. Additionally, nonmartensitic transformation products (NMTPs) were observed to various extents near the surface. However, there were no differences in retained austenite profiles, and retained austenite was mostly stable against deformation-induced transformation to martensite during fatigue testing, contrasting some studies on carburized steels. The results suggest that the observed difference in fatigue lives is due to differences in chemical composition and prior austenite grain size. Alloys containing B and Nb had refined prior austenite grain sizes compared to their counterparts in each alloy class.
Proceedings Papers
HT 2021, Heat Treat 2021: Extended Abstracts from the 31st Heat Treating Society Conference and Exposition, 9-13, September 14–16, 2021,
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This paper compares and contrasts heat treat processes and equipment typically used to harden gears. It discusses the basic design and operation of vacuum, controlled atmosphere, and hybrid furnaces and process techniques such as carburizing, carbonitriding, nitriding, nitrocarburizing, and neutral hardening. It also includes information on operating and maintenance costs, using batch integral quench furnaces as the base case for comparison. A discussion on when to consider continuous furnace types is included as well.
Proceedings Papers
HT 2019, Heat Treat 2019: Proceedings from the 30th Heat Treating Society Conference and Exposition, 152-159, October 15–17, 2019,
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Low pressure carbonitriding (LPCN) has the potential to improve the impact and fatigue strength of steel components through the enrichment of nitrogen and the effect of carburizing at higher temperatures. The work described in this paper investigates the influence of boron on the LPCN response of 20MnCr5 steel and the effect of niobium on that of 8620. LPCN treatments were developed to achieve a surface hardness of ~700 HV and case depth of 0.65-0.75 mm in four alloys: 20MnCr5, 20MnCr5 + B, 8620, and 8620 + Nb. The hardness and case microstructure of treated and quenched test samples are correlated with bending fatigue measured in Brugger fatigue specimens, which simulate the root of a gear tooth.
Proceedings Papers
HT2017, Heat Treat 2017: Proceedings from the 29th Heat Treating Society Conference and Exposition, 87-90, October 24–26, 2017,
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An empirical model has been developed by studying the rate of case formation for various materials. This model uses the relationship observed between material hardenability and case rate. Material hardenability is based on a modified version of Ideal Diameter by using the nominal steel composition with the exception of holding carbon constant at 0.40% (a rough estimation of the carbon level required for effective case depth). It predicts the level of diffusion needed to meet specification requirements and adjust parameters to optimize process design and resulting product characteristics. Inputs needed for process control are the equivalent diffusion number and a material classification. All times, temperatures, carbon set points and level off parameters are automatically downloaded to the equipment for process execution.
Proceedings Papers
HT2015, Heat Treat 2015: Proceedings from the 28th Heat Treating Society Conference, 308-312, October 20–22, 2015,
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A software simulation tool, CarbonitrideTool, has been developed by Center for Heat Treatment Excellence (CHTE) to predict the Nitrogen and Carbon concentration profiles in selected steels. In this paper, the introduction of the software will be presented. In addition, enhancements have been made to improve the CarbonitrideTool. The diffusion of nitrogen increases the amount of retained Austenite (RA) by changing the Ms temperature. In this paper, the modification has been made to calculate the RA fraction. The empirical prediction of microhardness profile will also be presented. The results of verification experiments will be presented and discussed.
Proceedings Papers
HT2011, Heat Treating 2011: Proceedings from the 26th Heat Treating Society Conference, 9-16, October 31–November 2, 2011,
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Late research projects show that retained austenite, if stabilized by nitrogen, has a positive influence on the fatigue strength of work pieces. The combined diffusion profile of carbon and nitrogen applied in a carbonitriding process plays the major role, besides the process temperature. Yet today, only the carbon potential is somehow controlled and even this is not easy to achieve. This paper will present a new system able to measure and control both, the carbon potential and the nitrogen potential independently. The knowledge of the activities of nitrogen and carbon in iron and the effect of alloying elements on such activities as well as the solubilities offers an easy to use method to apply the potentials on real steels.
Proceedings Papers
Franz T. Hoffmann, Matthias Steinbacher, P.D. Brigitte Clausen, Sebastian Bischoff, Heinrich Klümper-Westkamp ...
HT2011, Heat Treating 2011: Proceedings from the 26th Heat Treating Society Conference, 115-121, October 31–November 2, 2011,
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In the last centuries carbonitriding was mainly used to enhance the hardenability of unalloyed steels. IWT developed gas-carbonitriding and low-pressure-carbonitriding processes to increase fatigue behavior and quality compared to case hardening. For example, modern gas-carbonitriding processes make it possible to extend materials ́ strength, so that the limit of use of a given alloy can be expanded. The paper shows examples for the treatment of ball bearing and case hardening steels. The treatment results in microstructures, which are unusual, compared with conventional heat treated parts. They are characterized by high amounts of retained austenite and carbonitride precipitations. By a controlled process, which has been developed in cooperation with PROCESS-ELECTRONIC, it is possible to adjust surface carbon- and nitrogen content independently. Low pressure carburized parts have the advantage that no internal oxidation occurs. So they have the potential of leading to a higher strength. Nowadays LP-carburizing is used in a wide range, whereas LP-carbonitriding processes are at a starting point. In this paper possibilities and limitations of this process are shown. So, inline controlling of LP-processes in a classical way is not possible, but simulation guided process control. The paper will give examples for LP-carbonitriding processes and the resulting microstructure.
Proceedings Papers
HT2011, Heat Treating 2011: Proceedings from the 26th Heat Treating Society Conference, 285-294, October 31–November 2, 2011,
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Austenitic stainless steels are critical in the modern economies with applications ranging from food processing and cryogenic machinery to medical implants and aerospace instrumentation. Tough, resistant to low-temperature embrittlement and many forms of corrosion, these steels are, nevertheless, prone to scratching and galling in service. Case hardening was found to be effective when combined with corrosive surface treatments or in low-pressure, direct plasma-ion discharges, but inhibited in simple atmospheric-pressure furnaces. This paper presents preliminary evaluation of new, rapid (3-4 hrs) nitriding and carbonitriding treatments at low- (500-565°C) and high- (1100°C) temperature ranges involving injection of high-voltage, electric arc-activated, N2-based, NH3 and hydrocarbon gas mixes to the conventional box furnace. Reported data includes characterization of stainless steel product layers using SEM-EDS, XRD, OM, Leco elemental analysis, and microhardness profiling, as well as laser gas analysis of the residual furnace atmosphere.