Thermo-chemical diffusion processes play an important part in modern manufacturing technologies. They exist in many varieties depending on the type of diffusing elements used and the respective process objectives and procedures. To improve wear and/or corrosion performance of precisely machined steel components, gas nitriding is selected as the most preferred thermo-chemical surface treatment. Conventional gas nitriding of steels is a multi-hour, sometimes multi-day hardening process carried out at ferritic temperatures and including a complete heat treatment cycle: normalizing, austenitizing, martensitic quenching and tempering. An alternative, subcritical-temperature austenitic nitriding process is evaluated with the purpose of accelerating the treatment and optimizing the hardness and toughness of nitrided layers while minimizing the distortion of steel parts treated. The alternative process involves liquefied nitrogen cryogenic quenching as well as aging. This study presents results of experimental work on AISI 4140 steel, examining the interplay between the nitriding and tempering conditions and phase transformations in both ferritic (525oC) and subcritical, nitrogen-austenitic (610oC) processes. Thermodynamic models, used to design processing conditions, are applied also in the microstructural interpretation of nitrided layers. Results are verified using the SEM, EPMA and EDS techniques. Kinetics of interstitial diffusion, isothermal martensite transformation, as well as dimensional control of nitrided parts is also presented. Carburizing is, by far, the most widely adopted method in surface hardening. Problems with intergranular oxidation (IGO), energy efficiency and carbon footprint of conventional endothermic atmosphere (CO-H2-N2) carburizing is forcing heat treating and manufacturing companies to move toward increasingly capital- and operating-cost expensive, low-pressure (vacuum furnace) carburizing methods. In response, a new activated and alternate carburizing method (A2A carburizing) has recently been developed, bridging the endothermic atmosphere and vacuum processes, where a plasma-activated, oxygen-free, non-equilibrium nitrogen-hydrocarbon gas blend is utilized. The optimization of industrial A2A carburizing processes involves improvement of case uniformity of parts at different locations in the charge as well as between different sides on the parts. Connected to the optimization, a computational fluid dynamics (CFD) study is conducted for examination of gas flow field inside the furnace and trays holding steel parts treated. To mitigate soot in the atmosphere and minimize the poorly carburized contact area between parts, effects of different combinations of nitrogen-hydrocarbons mixture on soot formation in atmosphere, deposition on metal surface and graphite growth at carburizing temperature are investigated. N2- 0.4%C3H8-1%CH4 mixture is proven to be able to provide proper carburizing hardened case with less soot in atmosphere, less coke deposition on metal surface, as well as minimized marginally carburized contact zone. A soot formation mechanism for non-equilibrium atmosphere in A2A carburizing is discussed. The carburizing processes have been investigated for decades, yet it still faces challenges concerning performance, reliability and process control. Since carburized parts must meet tolerances and specifications of particular applications, it is necessary to accurately predict carbon concentration profiles as a function of processing conditions. Proper carbon distribution is critical for satisfactory and reliable service life of carburized parts. Based on experimental work and theoretical developments, a software CarbTool© has been created for atmosphere and low pressure carburizing methods which consider the thermodynamics, mass transfer kinetics and carbon diffusion aspects of the carburizing process and the gas-steel interface condition. The models are capable now to accurately predict the surface carbon concentration and the carbon concentration profile in the steel, i.e. the most important outcomes of the process.

"Case hardening, a process which includes a wide variety of techniques, is used to improve the wear resistance, by diffusing carbon (carburization), nitrogen (nitriding) and/or boron (boriding) into the outer layer of the steel at high temperature, and then heat treating the surface layer to the desired hardness without affecting the softer, tough interior of the part. In this research, a nitrogen-hydrocarbon gas mixture was used as the process atmosphere for carburizing steels. It can offer a cost and part quality alternative to the conventional endothermic atmosphere and vacuum processes. It can hold the promise for matching the quality of work parts processed in vacuum furnace, i.e. eliminating the intergranular oxidation which normally occurs in the endogas atmosphere. The process control of nitrogen-hydrocarbon atmosphere is also investigated in the research. Modified shim stock method is used to measure the carbon pickup and constant carbon flux modeling tool is used afterwards to predict the carbon profile. With minimum modification, commercially available equipment or sensors can be used to monitor non-equilibrium process atmosphere. Gas nitriding was also studied. For nitriding, the kinetics of the nitriding process with hydrocarbon gases addition and electric arc discharge activation of the nitrogen diluted ammonia atmosphere were investigated. Prior to and during the nitriding, hydrocarbon gases were reacted with metal surface and removed oxidation layers, which can accelerate nitriding process. Overall, nitriding with this unique gas mixture provides an alternative to a long-hour pure ammonia nitriding with more efficient energy utilization. The main objective of this project is to develop the conventional, atmospheric-pressure, low-cost surface hardening treatments for the case hardening of carbon, alloy and stainless steel. The possibility of plasma activation of atmosphere and metal surface to shorten processing time and save energy and time is investigated in this research. The process atmosphere is safer, more efficient, less toxic and less flammable. "

"The CHTE surface hardening simulation tools, CarboNitrideTool© and CarbTool© have been enhanced to improve the accuracy of the simulation and to predict the microstructure and microhardness profiles after the heat treatment process. These tools can be used for the prediction of both gas and low pressure carburizing processes. The steel alloys in the data base include 10XX, 48XX, 51XX, 86XX, 93XX and Pyrowear 53. They have been used by CHTE members to design efficient carburizing cycles to maximum the profit by controlling the cost and time. In the current software, the model has successfully predicted the carbon concentration profiles for gas carburizing process and many low pressure carburizing processes. In some case, the simulation toll may not work well with the low pressure carburizing process, especially with AISI 9310 alloy. In the previous simulation, a constant carbon flux boundary condition was used. However, it has been experimentally proven that the flux is a function of time. The high carbon potential may cause soot and carbides at the outer edge. The soot and carbides will impede the diffusion of carbon during the low pressure carburizing process. The constant carbon flux cannot be appropriately used as the boundary condition. An improved model for the process is proposed. In the modeling, carbon potential and mass transfer coefficient are calculated and used as the boundary condition. CarbonitrideToolⒸ has been developed for the prediction of both carbon and nitrogen profiles for carbonitriding process. The microstructure and hardness profile is also needed by the industry. The nitrogen is an austenite stabilizer which result in high amount of retained austenite (RA). RA plays important role in the hardness. The model has been developed to predict the Martensite start temperature (Ms) which can be used for RA prediction. Mixture rule is used then to predict the hardness profiles. Experiments has been conducted to verify the simulation. The hardness profile is also predicted for tempered carburized alloys. Hollomon-Jaffe equation was used. A matrix of tempering experiments are conducted to study the Hollomon Jaffe parameter for AISI 8620 and AISI 9310 alloy. Constant C value is calculated with a new mathematical method. With the calculation result, the hardness profile can be predicted with input of tempering time and temperature. Case depth and surface hardness are important properties for carburized steel that must be well controlled. The traditional testing is usually destructive. Samples are sectioned and measured by either OES or microhardness tester. It is time consuming and can only be applied on sampled parts. The heat treating industry needs a physics based, verified simulation tool for surface hardening processes to accurately predict concentration profiles, microstructure and microhardness profiles. There is also a need for non-destructive measurement tool to accurately determine the surface hardness and case depth. Magnetic Barkhausen Noise (MBN) is one of the promising way to test the case depth and hardness. MBN measures the pulses generating by the interaction between magnetic domain walls in the ferromagnetic material and the pinning sites such as carbides, impurities and dislocation. These signals are analyzed to evaluate the properties of the carburized steel. "

High rolling contact fatigue parts are vital for the long service life of fuel pumps. Cummins Fuel Systems are currently using an M2 tool steel for one of the most important roller bearing application in their pumps, namely the cam follower. The future design of the cam follower is a pin-less tappet roller. The wear and fatigue properties of the roller is vital to ensure reliability of the fuel system. M2 tool steel is an expensive material and becomes even more so if diamond like coating (DLC) is needed to decrease the friction coefficients. To cut costs of the fuel pump it might be possible to replace the M2 tool steel with 52100 steel (100Cr6). Competitive methods have proven that carbonitrided 52100 can reach excellent wear and fatigue properties making it a candidate to replace M2 tool steel. How the properties of hardness, toughness and compressive residual stresses are developed in 52100 and how they affect the fatigue and wear resistance has been researched from the literature. A big part of this project was to do an extensive analysis of a roller bearing that was believed to have gone through one of these competitive methods that produce excellent wear and fatigue resistance. The analysis was done with background to the knowledge gathered from the literature. Finally process trials were set up to carbonitride 52100 steel samples. The trials were done to develop a better understanding of how adding carbon together with nitrogen to the surface of 52100 steel will influence the metallurgical parameters that results in good wear and fatigue resistance. From this analysis Cummins hope to create a process recipe that can be used for carbonitriding the cam follower and maybe other components in their fuel systems.

"Gas carburizing is an important heat treatment process used for steel surface hardening of automotive and aerospace components. The quality of the carburized parts is determined by the hardness and the case depth required for a particular application. Despite its worldwide application, the current carburizing process performance faces some challenges in process control and variability. Case depth variability if often encountered in the carburized parts and may present problems with i) manufacturing quality rejections when tight tolerances are imposed or ii) insufficient mechanical properties and increased failure rate in service. The industrial approach to these problems often involves trial and error methods and empirical analysis, both of which are expensive, time consuming and, most importantly, rarely yield optimal solutions. The objective for this work was to develop a fundamental understanding of the mass transfer during gas carburizing process and to develop a strategy for the process control and optimization. The research methodology was based on both experimental work and theoretical developments, and included modeling the thermodynamics of the carburizing atmosphere with various enriching gasses, kinetics of mass transfer at the gas-steel interface and carbon diffusion in steel. The models accurately predict: 1) the atmosphere gas composition during the enriching stage of carburizing, 2) the kinetics of carbon transfer at the gas-steel surfaces, and 3) the carbon diffusion coefficient in steel for various process conditions and steel alloying. The above models and investigations were further combined to accurately predict the surface carbon concentration and the carbon concentration profile in the steel during the heat treatment process. Finally, these models were used to develop a methodology for the process optimization to minimize case depth variation, carburizing cycle time and total cycle cost. Application of this optimization technique provides a tradeoff between minimizing the case depth variation and total cycle cost and results in significant energy reduction by shortening cycle time and thereby enhancing carburizing furnace capacity."

"The effects of rust on the carburization behavior of AISI 8620 steel have been experimentally investigated. AISI 8620 steel samples were subjected to a humid environment for time of 1 day to 30 days. After the exposure, a part of the samples was cleaned by acid cleaning. Both cleaned and non-cleaned samples have been carburized, followed by quenching in mineral oil, and then tempered. To determine the effect of rust on gas carburizing, weight gained by the parts and the surface hardness were measured. Surface carbon concentration was also measured using mass spectrometry. Carbon flux and mass transfer coefficient have been calculated. The results show that acid cleaning removes the rust layer effectively. Acid cleaned samples displayed the same response to carburization as clean parts. Rusted parts had a lower carbon uptake as well as lower surface carbon concentration. The surface hardness (Rc) did not show a significant difference between the heavily rusted sample and clean sample. It has been observed that the carbon flux and mass transfer coefficient are smaller due to rust layer for the heavily rusted samples. These results are discussed in terms of the effects of carbon mass transfer on the steel surface and the resulting mass transfer coefficient."

"The effects of heat treating process parameters on the microstructure, residual stress, and distortion of a vacuum carburized, quenched and cold treated ring gear made of Pyrowear 53 has been determined using Abaqus and DANTE software. The data from these finite element method simulations was compared to measured values from physical testing. It was found that the heat treating process of the ring gear could be simulated and provide similar results to the measured and specified values for hardness, carbon content, and distortion. The simulations and distortion in this paper provide a detailed view of the mass transfer, heat transfer, and stress that occur during heat treating. These simulations suggest nonuniform cooling of a ring gear leads to greater distortion than uniform cooling. Simulations compared the retained austenite and hardness in ring gears that were oil quenched and high pressure gas quenched."

O presente trabalho tem como objetivo principal avaliar o efeito da nitretação e da carbonetação por plasma na liga Ti-6Al-4V sob condições de fluência na faixa de 500 a 600°C. Os resultados permitiram concluir que a microestrutura da liga Ti-6Al-4V é constituída pela configuração de Widmanstätten, com uma dureza média de 300 ± 17 HV para a condição como recebida e 334±18 HV para a amostra tratada termicamente. Após os tratamentos termoquímicos por plasma observou-se para a nitretação a plasma a formação de uma camada nitretada de espessura média de 4?m e dureza de 1539 ± 157 HV. A dureza está relacionada com a espessura e a presença das fases Ti2N e TiN identificadas por difratometria de raios X. Para a carbonetação a plasma verificou-se a formação de uma camada constituída pelo carboneto TiC0,66, de espessura média igual a 1,5 ?m e dureza de 809 ± 79 HV. A nitretação a plasma reduziu os valores de rugosidade média de 1,29?m do material como recebido para 1,18?m e a carbonetação aumentou esse valor de 1,28 ?m da amostra tratada termicamente para 2,02 ?m. Os testes demonstraram que a nitretação reduziu a taxa de fluência e aumentou a vida em fluência a 600 °C. A carbonetação contribuiu para reduzir a taxa estacionária. Com base nos valores de energia de ativação na faixa de 272 a 309 kJ/mol e nos valores do expoente de tensão de 4,82 a 7,28, estima-se que o mecanismo dominante de deformação por fluência está associado à escalagem de discordâncias para todas as condições experimentais. A análise fractográfica revelou que os mecanismos de fratura estão associados principalmente à nucleação e coalescência de microcavidades e decoesão intergranular. / This work aims to evaluate the effect of plasma nitriding and carburizing on the Ti-6Al-4V alloy subjected to creep in the range 500-600 °C. The results showed that the microstructure of Ti-6Al-4V consists of Widmanstätten morphologie, with a hardness of 300 ± 17 HV for the as received condition and 334 ± 18 HV for the sample heat-treated. After treatments thermochemical by plasma was observed for plasma nitriding the formation of a nitrided layer of hardness of 1539 ± 157 HV and of average thickness of 4 ?m. The hardness is due to the thickness and the presence of Ti2N and TiN phases identified by X-ray diffraction. For plasma carburizing was observed the formation of a carbide layer composed of TiC0,66, of average thickness equal to 1.5 ?m and hardness of 809 ± 79 HV. The treatment of plasma nitriding reduced the values of average roughness of 1.29 ?m of the as received condition to 1.18 ?m and the plasma carburizing increased this value of 1.28 ?m of the sample heat-treated to 2.02 ?m. The tests showed that the plasma nitriding reduced the rate of creep and increased the creep life at 600 °C. Based on the values of activation energy in the range 272-309 kJ / mol and the values of stress exponent from 4.82 to 7.28, it was estimated that the dominant mechanism of creep deformation is associated with the climb of dislocations for all experimental conditions. The fractographic analysis revealed that the fracture mechanisms are mainly associated with the nucleation and coalescence of microvoids and decoehasion intergranular.

La cémentation gazeuse à basse pression est un procédé de traitement de surface qui consiste à renforcer des pièces en acier par diffusion d’atomes de carbone provenant de la pyrolyse d’hydrocarbures gazeux. Une partie de l’hydrocarbure craqué est adsorbée sur le métal mais une autre partie réagit en phase gazeuse et conduit, entre autres, à la formation de HAP. Or, de nombreux HAP sont toxiques, voire cancérigènes, et les salariés en charge du nettoyage ou de la maintenance des fours de cémentation peuvent y être exposés. Des expériences de pyrolyse d’acétylène ont été réalisées à 900°C et 8 kPa, conditions proches de celles des procédés de cémentation gazeuse à basse pression. Un réacteur auto-agité par jets gazeux et des réacteurs tubulaires ont été utilisés. A la sortie de la zone réactionnelle, les produits de la pyrolyse ont été analysés. Entre autres, 16 HAP considérés comme des polluants prioritaires par l’Agence de Protection de l’Environnement aux Etats-Unis (US EPA) ont été observés. L’influence du taux de dilution du réactif en entrée et du temps de passage dans le réacteur a été étudiée. Les résultats expérimentaux ont été comparés à ceux obtenus avec un modèle cinétique détaillé. Ce modèle a été développé dans le but de décrire la formation des HAP lors de la pyrolyse d’hydrocarbures légers. Une attention particulière a été portée aux voies de formation des premiers cycles aromatiques et des 16 HAP de la liste de l’EPA. En plus des données expérimentales obtenues dans le cadre de cette étude, le modèle a été validé à partir de données expérimentales de la littérature. Le but de l’étude est de comprendre les phénomènes de formation et de croissance des HAP afin de trouver des conditions opératoires permettant de rendre plus surs les procédés de cémentation gazeuse à basse pression / Low-pressure gas carburizing is a heat treatment process used to harden surface of steel by enriching the metal with carbon atoms coming from pyrolysis of hydrocarbons. At the same time, a wide variety of molecules and radicals are also formed in the gas phase. They react together, leading to the formation of PAHs. PAHs are toxic and even carcinogenic, and activities such as furnace maintenance may thus represent a risk to workers. Experiments of acetylene pyrolysis were carried out in conditions close to low-pressure gas carburizing processes, at 900°C and 8 kPa. Two kinds of reactors were used: a jet stirred reactor and tubular reactors. At the outlet of the reaction zone, products of pyrolysis were analyzed. Among other products, 16 PAHs classified as priority pollutants by the United States Environmental Protection Agency (US EPA) were observed. Influence of residence time and of reactant dilution was studied. Experimental results were compared to those obtained with a detailed kinetic model. This model was developed in order to describe PAH formation during light hydrocarbon pyrolysis. The focus was placed on formation pathways of the first aromatic rings and of the 16 EPA-PAHs. In addition to the experimental data obtained in this study, the model was validated using experimental data from the literature. The aim of the study is to understand the phenomena of PAH formation and growth in order to find operating conditions to make safer the low-pressure gas carburizing processes

O presente trabalho tem como objetivo principal avaliar o efeito da nitretação e da carbonetação por plasma na liga Ti-6Al-4V sob condições de fluência na faixa de 500 a 600°C. Os resultados permitiram concluir que a microestrutura da liga Ti-6Al-4V é constituída pela configuração de Widmanstätten, com uma dureza média de 300 ± 17 HV para a condição como recebida e 334±18 HV para a amostra tratada termicamente. Após os tratamentos termoquímicos por plasma observou-se para a nitretação a plasma a formação de uma camada nitretada de espessura média de 4?m e dureza de 1539 ± 157 HV. A dureza está relacionada com a espessura e a presença das fases Ti2N e TiN identificadas por difratometria de raios X. Para a carbonetação a plasma verificou-se a formação de uma camada constituída pelo carboneto TiC0,66, de espessura média igual a 1,5 ?m e dureza de 809 ± 79 HV. A nitretação a plasma reduziu os valores de rugosidade média de 1,29?m do material como recebido para 1,18?m e a carbonetação aumentou esse valor de 1,28 ?m da amostra tratada termicamente para 2,02 ?m. Os testes demonstraram que a nitretação reduziu a taxa de fluência e aumentou a vida em fluência a 600 °C. A carbonetação contribuiu para reduzir a taxa estacionária. Com base nos valores de energia de ativação na faixa de 272 a 309 kJ/mol e nos valores do expoente de tensão de 4,82 a 7,28, estima-se que o mecanismo dominante de deformação por fluência está associado à escalagem de discordâncias para todas as condições experimentais. A análise fractográfica revelou que os mecanismos de fratura estão associados principalmente à nucleação e coalescência de microcavidades e decoesão intergranular. / This work aims to evaluate the effect of plasma nitriding and carburizing on the Ti-6Al-4V alloy subjected to creep in the range 500-600 °C. The results showed that the microstructure of Ti-6Al-4V consists of Widmanstätten morphologie, with a hardness of 300 ± 17 HV for the as received condition and 334 ± 18 HV for the sample heat-treated. After treatments thermochemical by plasma was observed for plasma nitriding the formation of a nitrided layer of hardness of 1539 ± 157 HV and of average thickness of 4 ?m. The hardness is due to the thickness and the presence of Ti2N and TiN phases identified by X-ray diffraction. For plasma carburizing was observed the formation of a carbide layer composed of TiC0,66, of average thickness equal to 1.5 ?m and hardness of 809 ± 79 HV. The treatment of plasma nitriding reduced the values of average roughness of 1.29 ?m of the as received condition to 1.18 ?m and the plasma carburizing increased this value of 1.28 ?m of the sample heat-treated to 2.02 ?m. The tests showed that the plasma nitriding reduced the rate of creep and increased the creep life at 600 °C. Based on the values of activation energy in the range 272-309 kJ / mol and the values of stress exponent from 4.82 to 7.28, it was estimated that the dominant mechanism of creep deformation is associated with the climb of dislocations for all experimental conditions. The fractographic analysis revealed that the fracture mechanisms are mainly associated with the nucleation and coalescence of microvoids and decoehasion intergranular.