Machining of steels not only affects the shape of the workpiece, but also influences surface structure and composition. At elevated temperatures and pressures during machining or the subsequent cleaning process cooling lubricants, hydraulic oils, machine greases, and cleaning residues can form stable adsorption or reaction layers. These layers can work as barriers and therefore inhibit surface modification reactions, as can be seen in the example of gas nitriding. The aim of the present investigation was to explore possible mechanisms of reaction inhibition and develop a nitriding process more reproducible in terms of reaction temperature and residence time. Samples prepared with residues of cleaning and machining processes on different alloy surfaces were nitrided under normal conditions and with extremely short residence times (1 < t < 20 min), in order to detect the effect of reaction and barrier layers on nitrogen acceptance during the initial stages of the nitriding process. For all alloys examined the samples with adsorbed cleaning agents showed the worst nitriding result. Volatile contaminations of industrial cleaners such as tens ides desorbed from the surface during gas nitriding. Non-volatile contamination and reaction layers such as phosphates and silicates are stable and hindered gas nitriding during the entire process period. The nitriding hindrance of non-volatile contaminants was found to increase with increasing chromium and aluminium content of the steel and decreasing nitriding time.
The article contains sections titled: 1. Introduction 2. Cleaning 2.1. Cleaning in Aqueous and Nonaqueous Solvents 2.1.1. Cleaning and Degreasing in Aqueous Media 2.1.2. Cleaning Agents Based on Organic Solvents 2.1.3. Cleaning Process Technology 2.2. Cleaning and Pickling with Salt Baths 2.2.1. Bath Composition and Properties 2.2.2. Descaling of Metals 2.2.3. Casting Cleaning 2.2.4. Coatings Removal 3. Phosphating 3.1. Chemistry 3.2. Methods of Application 3.3. Uses of Phosphate Coatings 4. Case Hardening of Ferrous Metals 4.1. Case Hardening without Chemical Reaction via Phase Boundary 4.1.1. Flame Hardening 4.1.2. Induction Hardening 4.1.3. Laser Hardening 4.1.4. Electron Beam Hardening 4.2. Thermochemical Treatment 4.2.1. Gas Carburizing 4.2.1.1. Principles 4.2.1.2. Properties of Carburized Components 4.2.1.3. Carburizing Methods 4.2.2. Nitriding and Nitrocarburizing 4.2.2.1. Principles 4.2.2.2. Properties of Nitrided and Nitrocarburized Components 4.2.2.3. Nitriding and Nitrocarburizing Methods 4.2.3. Boriding 4.2.3.1. Principles 4.2.3.2. Boriding Methods 4.2.4. Other Thermochemical Processes 5. Thermal Spraying 5.1. Methods of Thermal Spraying 5.1.1. Flame Spraying 5.1.2. Electric Arc Spraying 5.1.3. Plasma Spraying 5.1.4. High‐Velocity Oxy – Fuel Spraying (HVOF) 5.2. Properties of Thermally Sprayed Coatings 5.3. Materials 5.3.1. Spray Coating Materials 5.3.2. Base Materials 5.4. Applications 6. Coloring 7. Acknowledgement
Minimising the health and environmental risks resulting from surface cleaning involves the design of suitable hardware but also the choice of a cleaning agent. As an alternative to chlorinated hydrocarbons, chlorine free hydrocarbons or surfactant cleaners in aqueous solution can be used. Whereas hydrocarbons work as solvents and remove mainly soluble oil or fat contamination, surfactant cleaners dissolve water soluble contamination and displace oils, fats, and insoluble particles, swarf, chippings, and cuttings. While it is advantageous to work with water as a solvent, many alkaline surfactant cleaners contain inorganic salts which leave non-volatile residues at metal surfaces ifthe parts are not rinsed sufficiently with desalinated water. Laboratory scale cleaning experiments with both types of cleaning agent have been carried out. Results were obtained by analytical means and modern surface analysis; in addition, gas nitriding, a surface modification process which is known to be sensitive to surface composition, was used to assess cleaning potential. For the total process of machining, cleaning, and nitriding, it is shown that it is neither possible nor sensible to define a standard of surface purity without considering the special demands of the surface modification process following.
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