Thermal Diffusion Process The thermal diffusion process is based on the fact that molecular diffusion can be caused by a temperature gradient.In general, two gases are exposed to a temperature gradient between surfaces, the gas with the lower molecular weight will tend toward the hotter surface.The two gases will separate until a concentration gradient occurs resulting in concentration diffusion of equal magnitude in the opposite direction.Under equililbrium conditions the rate of transfer of the light molecules towards the hot surface resulting from thermal diffusion will be exactly counterbalanced concentration diffuslon In a thermal diffusion column the relatively small separation is multiplied by the effect of convection currents resulting from temperature gradient.The heavy molecules, which tend toward cold surface, are swept to the bottom of the column while the molecules at the hot surface are swept toward the top The maximum separation is limited by the concentration the axial direction as a result Of The separation itself .

Nitriding - Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. - These processes are most commonly used on low-carbon, low-alloy steels. They are also used on medium and high-carbon steels, titanium, aluminum and molybdenum. known to be associated with strongly enhanced mechanical properties. - The nitriding process involves the diffusion of nitrogen into the base steel. This diffusion takes place at relatively low temperatures (typical process temperature is 975° F) and the hardening occurs without quenching. Core properties are not affected by the nitriding process provided the final tempering temperature for the product was higher than the nitriding process temperature

Why we use nitride in surface hardening Why we use nitride in surface hardening ? Nitride surfaces are highly wear resistant, Fatigue life is improved, and the process improves the corrosion resistance of the part. An additional advantage of nitriding is that the surface hardness is resistant to softening by temperatures up to the process temperature. Type Of Nitriding 1- Gas nitriding 2- Salt bath nitriding 3- Plasma nitriding

Mechanism Of Gas Nitriding

2) Salt bath nitriding In salt bath nitriding the nitrogen donating medium is a nitrogen-containing salt such as cyanide salt HCN. The salts used also donate carbon to the work piece surface making salt bath a nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550–570 °C. The advantages of salt nitriding is that it achieves higher diffusion in the same period of time compared to any other method. 3) Plasma nitriding Plasma nitriding, also known as ion nitriding In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called plasma. The gas used for plasma nitriding is usually pure nitrogen, since no spontaneous decomposition is needed (as is the case of gas nitriding with ammonia). Application Of Nitriding gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm components and plastic mold tools.

Carburizing It is a heat treatment process. Increasing carbon on the surface of iron or steel followed by heat treatment. Absorbs carbon liberated when the metal is heated in the presence of carbon bearing materials . Carbon bearing materials can be charcoal, carbon Monoxide or carbon gases.

Carburizing Importance Types of carburizing Effect factors Longer Carburizing time and higher temperatures lead to greater diffusion of carbon into the metal/alloy Rapid Cooling/Quenching results transformation of austenite to martensite. While core remains soft and tough a ferritic pearlitic microstructure. Importance It is used to increase . Ductility Toughness Hardness Strength It is also used to relieve internal stresses Types of carburizing Gas Carburizing ,Vacuum Carburizing ,Pack Carburizing , Liquid Carburizing.

GAS CARBURIZING Process Description: Surface chemistry process. Main carburizing agent can be either Methane, Propane, Natural Gas or any other Carbon carrying gas. Work pieces are heated with carbon carrying gas. between 800°C 950°C Then held for period of time at s pecific temperature After that quench the sample.

VACUUM CARBURIZING PACK CARBURIZING . It is done under very low pressure. Sample is heatein vacuum above transformation temperature. Then exposed to carbon carrying gas or mixture under partial pressure. Temperaure Range: 800°C to 1100°C . Pressure Range: l to 20 toms. PACK CARBURIZING . Sample is paced in a steel container& sample is completely surrounded by granules of charcoal . Then heate in a furnace for 12 to 72 hours at 900C. Due to high temperature, carhon diffuses into the surface of sample.

LIQUID CARBURIZING . Sample plbced in mohen cyanide's ath so that carbon will diuse into the sample Diffusion of carbon nto sample is greater than ntrogen. Low temperature salt hath contains 20 % cyanide & operate between 1550PF to 1650F. High temperature salt bath contains 10 % , cyanide & operate between 1650PF to 1750F.

nitride carbon carbon nitride compounds with a general formula near to C3N4 (albeit typically with non-zero amounts of Hydrogen) . carbon nitride can also be prepared by electrodeposition on(Si) substrate from a saturated acetone solution of cyanuric trichloride (C3H6N6)and melamine (C3H6N6) (ratio =1: 1.5) at room temperature Recently, a new method of syntheses of graphitic carbon nitrides by heating at 400-600 °C of a mixture of melamine and uric acid (carbon, nitrogen, oxygen, and hydrogen with the formula C5H4N4O3) in the presence of alumina has been reported. Alumina favored the deposition of the graphitic carbon nitrides layers on the exposed surface. This method can be assimilated to an in situ chemical vapor deposition (CVD)

Properties Due to the special semiconductor properties of carbon nitrides, they show unexpected catalytic activity for a variety of reactions Uses: it can be used for tribological coatings, biocompatible medical coatings, chemically inert coatings, insulators and for energy-storage solutions

Carbonitriding Carbonitriding introduces both carbon and nitrogen into the austenite of the steel. The process is similar to carburizing in that the austenite composition is enhanced and the high surface hardness is produced by quenching to form martensite. This process is a modified form of gas carburizing in which ammonia is introduced into the gas-carburizing atmosphere. As in gas nitriding, elemental nitrogen forms at the workpiece surface and diffuses along with carbon into the steel. Typically, carbonitriding takes place at a lower temperature and a shorter time than gas carburizing, producing a shallower case.

The process results in the formation of a thin white layer or compound layer, with an underlying diffusion zone of dissolved nitrogen in iron, or alloy nitrides. The white layer improves surface resistance to wear, and the diffusion zone increases the fatigue endurance limit, especially in carbon and low-alloy steels. Alloy steels, cast irons, and some stainless steels can be treated. The process is used to produce a thin, hard skin, usually less than 25 μm (1 mil) thick, on low-carbon steels in the form of sheet metal parts, powder metallurgy parts, small shaft sprockets, and so forth.