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. "

Nitriding is a plasma based processing technique that is used to improve the surface properties of components and products in many areas including the aerospace, automotive and biomedical industries to name a few. Active Screen Plasma Nitriding (ASPN) is a relatively new nitriding technique which has potential advantages over the more traditional nitriding techniques such as Direct Current (DC) plasma nitriding where high substrate biases can be problematic. However, there is considerable debate as to the mechanism for nitriding in ASPN. This thesis focuses on investigating the mechanism for nitriding in a commercial ASPN system. Commercial ASPN treatments of nitrideable alloy steels were found to be unsatisfactory unless a sufficient bias was applied. The level of bias required to produce a satisfactory nitriding response, in terms of the cross sectional hardness, was found to depend on the concentration of strong alloy nitride forming elements present in the steel. Although active screen material was found to be transferred to the workload, no evidence was found that this process played a significant role in enhancing the nitriding response. The primary mechanism for nitrogen mass transfer in ASPN was found to be dependent on the active screen/workload separation distance. When this separation is small (less than approximately 10cm for the conditions used in this study) then nitrogen mass transfer in the form of energetic ions or neutrals can occur between the active screen and the workload. This allows samples to be treated without a substrate bias. On the other hand, when the active screen/workload separation distance is large (greater than approximately 10cm) as is normally the case in a commercial environment, this mechanism for nitrogen mass transfer breaks down and a substrate bias is essential. In this latter case, nitrogen ions attracted to the workload using a bias is the primary nitrogen mass transfer mechanism and the role of the active screen is primar ily to uniformly heat the workload.

The factors influencing the direct nitridation of silicon, including the effects of the native oxide layer covering the surface of silicon, the effects of hydrogen contained in the nitridation gas and the catalytic effects of metals added to the raw material silicon, were investigated, using a tubular flow reactor and a fluidized-bed reactor operated at temperatures ranging from 1150��C to 1390��C in a stream of nitrogen containing 10% hydrogen. The nitridation of silicon is not initiated until the native oxide is removed by an assistance of hydrogen contained in argon during the pretreatment or in the nitridation gas mixture. An induction period is observed before the initiation of the nitridation and depends on the nitridation temperature as well as the pretreatment time, which is associated with the removal of the oxide layer. The presence of hydrogen in the nitridation atmosphere is crucial for the nitridation of silicon. When pretreated silicon grains are exposed to nitrogen without hydrogen for a time period as short as 5 minutes, the subsequent nitridation, even with hydrogen, becomes extremely slow. The concentration of hydrogen as low as 0.3% is effective for sustaining the reactivity of silicon for the nitridation. The results suggest the formation of a protective layer on the surface of silicon when silicon grains are exposed to nitrogen without hydrogen. The protective film is suspected to be silicon oxynitride, or a mixture of silicon oxynitride and silicon dioxide or silicon nitride formed from the reaction of silicon with oxygen and nitrogen, depending on the temperature of its formation. However, the protective film does not form on the native oxide layer, and the reactivity of silicon is resumed upon the removal of the native oxide. An addition of calcium (as low as 0.125%) or yttrium (1.0-2.0%) to silicon suppresses the formation of ��-silicon nitride while iron enhances the formation of silicon nitride. Copper promotes not only the nitridation but also the formation of ��-silicon nitride at 1200��C, but enhances the ��-phase formation at higher temperatures. The role of liquid phases on the formation of ��-/��-silicon nitride was also discussed based on the nitridation of silicon impregnated with copper, calcium, silver, chromium and tungsten. / Graduation date: 2003

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: nitriding; contaminants. Includes bibliographical references (p.58).

Os aços de ultra resistência com médio carbono e baixa liga apresentam excelentes propriedades de resistência mecânica e à fadiga além de uma elevada temperabilidade. No entanto, tais aços apresentam o fenômeno da têmpera ou de tratamentos superficiais. O aço 300M foi desenvolvido visando a ampliação desta faixa, permitindo tratamentos em temperaturas em torno de 350ºC viabilizando o emprego da técnica de nitretação do mesmo por plasma sem a consequente fragilização. Neste trabalho comparou-se as características e propriedades do aço 300M nitretado por plasma à 350ºC e 550ºC com os aços de alta-resistência AISI 4340, 4140 e 5140 nitretados à 550ºC. Variou-se as condições de tratamento de forma a verificar a influência do tempo de nitretação e de frequência de pulso nas camadas nitretadas. Verificou-se que a nitretação por plasma demostrou ser bastante efetiva para conferir resistência ao desgaste abrasivo para todos os aços nitretados. O aço 300M nitretado em 350ºC apresentou no ensaio de desgaste abrasivo comportamento similar aos demais aços nitretados em 550ºC, enquanto que o melhor resultado geral foi alcançado pelo 300M nitretado à 550ºC em CC durante 6 horas, devido à formação de uma camada dupla de levada dureza e espessura. / The ultrahigh-strength steels with medium carbon and low alloy, like AISI 4340, show excellent tensile strength and fatigue properties further higher hardenability. However, these steels have the phenomenon of tempering embrittlement, that restrict the temperature of treatments after quench or surfaces treatments. The 300M steel was developed aiming the enlargement this area of application, this steel permits treatments in temperatures about 350°C what allowed to use the plasma nitriding without problems with the embrittlement. In this production were compared the properties of the nitrited layers of the 300M nitriding by plasma in 350°C and 550°C with the steels AISI 4340, AISI 4140, AISI 5140 nitriding in 550°C. Plasma nitriding conditions like time and pulse frequency were varied. It was checked that the plasma nitriding showed good performance to improve the resistance to the abrasive wear in ali the steels tested. The 300M steel nitrited in 350°C showed results similar to the others in the pin-on-disk test and the better result occurred to the 300M steel nitrited in 550°C in dC plasma during 6 hours, because of a duble compound lawyer that arouse with a high thickness and high hardness.

"Surface contamination during the heat treatment process can greatly affect the quality of the heat treated parts. Although cleaning the post-heat treated parts is considered a value added process in heat treatment, cleaning parts prior to heat treatment is also important and can influence the outcome of subsequent processes. A series of experiments has been designed to determine the effects of the surface contaminants on nitriding. Nitralloy-135 steel was selected to evaluate the cleaning methods and the effects on the result of subsequent nitriding process. The samples were contaminated with a variety of contaminants, including rust preventive oil, cutting fluid and rust. The contaminated samples were cleaned using either acid or alkaline cleaner. Both cleaned and contaminated samples were nitride in the furnace. To determine the effect of contaminants on gas nitriding, weight gain and the surface hardness were measured. Nitrogen flux was also calculated. The results show that the acid cleaning effectively removes the rust layer. Rusted parts had lower nitrogen absorption than that of the cleaned samples. The parts contaminated by the oil did not reveal any effect on the nitrogen uptake. The surface hardness (Rc) did not show a significant difference between the heavily rusted sample and the clean sample. It has been observed that the nitrogen flux is smaller due to the rust layer on the heavily rusted samples."

Os aços de ultra resistência com médio carbono e baixa liga apresentam excelentes propriedades de resistência mecânica e à fadiga além de uma elevada temperabilidade. No entanto, tais aços apresentam o fenômeno da têmpera ou de tratamentos superficiais. O aço 300M foi desenvolvido visando a ampliação desta faixa, permitindo tratamentos em temperaturas em torno de 350ºC viabilizando o emprego da técnica de nitretação do mesmo por plasma sem a consequente fragilização. Neste trabalho comparou-se as características e propriedades do aço 300M nitretado por plasma à 350ºC e 550ºC com os aços de alta-resistência AISI 4340, 4140 e 5140 nitretados à 550ºC. Variou-se as condições de tratamento de forma a verificar a influência do tempo de nitretação e de frequência de pulso nas camadas nitretadas. Verificou-se que a nitretação por plasma demostrou ser bastante efetiva para conferir resistência ao desgaste abrasivo para todos os aços nitretados. O aço 300M nitretado em 350ºC apresentou no ensaio de desgaste abrasivo comportamento similar aos demais aços nitretados em 550ºC, enquanto que o melhor resultado geral foi alcançado pelo 300M nitretado à 550ºC em CC durante 6 horas, devido à formação de uma camada dupla de levada dureza e espessura. / The ultrahigh-strength steels with medium carbon and low alloy, like AISI 4340, show excellent tensile strength and fatigue properties further higher hardenability. However, these steels have the phenomenon of tempering embrittlement, that restrict the temperature of treatments after quench or surfaces treatments. The 300M steel was developed aiming the enlargement this area of application, this steel permits treatments in temperatures about 350°C what allowed to use the plasma nitriding without problems with the embrittlement. In this production were compared the properties of the nitrited layers of the 300M nitriding by plasma in 350°C and 550°C with the steels AISI 4340, AISI 4140, AISI 5140 nitriding in 550°C. Plasma nitriding conditions like time and pulse frequency were varied. It was checked that the plasma nitriding showed good performance to improve the resistance to the abrasive wear in ali the steels tested. The 300M steel nitrited in 350°C showed results similar to the others in the pin-on-disk test and the better result occurred to the 300M steel nitrited in 550°C in dC plasma during 6 hours, because of a duble compound lawyer that arouse with a high thickness and high hardness.

No description available.

Thesis (PhD)--University of South Australia, 2002.