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case hardening Another word for surface hardening. In order to harden low-carbon steel, it is necessary to increase the carbon content of the surface of the steel so that a thin outer case can be hardened by heating the steel to the hardening temperature and then quenching it. The process, therefore, involves two separate operations. The first is the carburizing operation for impregnating the outer surface with sufficient carbon, and the second operation is that of heat treating the carburized parts so as to obtain a hard outer case and, at the same time, give the core the required physical properties. The term case hardening is ordinarily used to indicate the complete process of carburizing and hardening.
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ceramic rod flame spraying A thermal spraying process in which the material to be sprayed is in ceramic rod form.
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cold forming Same as cold working.
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cold spraying A kinetic spray process utilizing supersonic jets of compressed gas to accelerate near-room temperature powder particles at ultra high velocities. The unmelted particles, traveling at speeds between 500 to 1,500 m/s plastically deform and consolidate on impact with their substrate to create a coating. Cold spraying is a type of thermal spraying. Cold spray coatings exhibit improved adhesion, reduced material loss by vaporization, low gas entrapment, insignificant grain growth and recrystallization, low residual stress, phase and compositional stability, reduced masking requirements, and improved surface finishes. Disadvantages to the process are: the use of high gas flows and increased gas costs.
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detonation flame spraying A thermal spraying process in which the controlled explosion of a mixture of fuel gas, oxygen, and powdered coating material is utilized to melt and propel the material to the work piece. It is a complicated process in which shock waves accelerates the powders to the material. Detonation coatings are designed for applying hard materials, especially carbides, on surfaces subject to aggressive wear.
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electric arc spraying A thermal spraying process in which an arc is struck between two consumable elctrodes of a coating material. Compressed gas is used to atomize and propel the material to the substrate. This process differs from other thermal spray processes in that there are no external heat sources as in any of the combustion gas/flame spray processes. Electric arc spraying has the advantage of not requiring the use of oxygen and/or a combustible gas. It has demonstrated the ability to process metals at high spray rates and is, in many cases, less expensive to operate than either plasma and/or wire flame spraying.
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hardfacing A process of spraying and fusing metal powder onto a work piece or substrate to give it specific hardness, wear resistance, or corrosion resistance properties.
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high velocity oxy-fuel (HVOF) spraying A high-velocity flame spray process. The process utilizes a combination of oxygen with various fuel gases including hydrogen, propane, propylene, hydrogen, and even kerosene. Disadvantages of this process include low deposition rates and in-flight oxidation of particles.
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hot forming Same as hot working.
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hot hardness Hot hardness is the ability of a material to retain its hardness at high temperatures. This is required because of the high-temperature environment in which the tool operates.
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powder flame spraying A thermal spraying process in which the material to be sprayed is in powder form. This process involves feeding a powder through the center bore of a nozzle where it melts and is carried by the escaping oxy-fuel gases to the work piece.
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molten metal flame spraying A thermal spraying process in which the metallic material to be sprayed is in the molten condition. Advantages of this process are: cheap raw materials, use of inexpensive gases, and basic gun design. Disadvantages of this process are: gun is cumbersome to use in the manual mode, can only be held in horizontal plane, high maintenance due to high temperature oxidation and molten metal corrosion, and useful only with low melting temperature metals. Uses include the fabrication of molds, masks and forms for the plastics industry, using low melting point bismuth based alloys, the deposition of solder alloys to joints that would be coalesced using torches and ovens, and the production of metal powders.
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nontransferred plasma arc spraying A thermal spraying process in which a nontransferred arc is a source of heat that ionizes a gas which melts the coating material and propels it to the work piece.
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RF plasma spraying A system in which the torch is a water-cooled, high-frequency induction coil surrounding a gas stream. On ignition a conductive load is produced within the induction coil, which couples to the gas, ionizing it to produce a plasma. Coatings produced using RF plasma have shown to be generally homogeneous and not porous. This method, using neutral atmospheres, can deposit reactive and toxic metals including calcium, uranium, niobium, and titanium.
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thermal spraying Thermal spraying is a group of coating processes in which finely divided metallic or nonmetallic materials are deposited in a molten or semimolten condition to form a coating. The coating material may be in the form of powder, ceramic-rod, wire or molten materials. Different thermal spray processes are: molten metal flame spraying, powder flame spraying, wire flame spraying, ceramic rod flame spraying, detonation flame spraying, high velocity oxy-fuel (HVOF) spraying, cold spraying, nontransferred plasma arc spraying, electric arc spraying, and RF plasma spraying.
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wear resistance Hardness is the single most important property needed to resist abrasive wear. All cutting-tool materials must be hard. However, wear resistance in metal cutting depends on more than just tool hardness, because of the other tool-wear mechanisms. Other characteristics affecting wear resistance include surface finish on the tool (a smoother surface means a lower coefficient of friction), chemistry of tool and work materials, and whether a cutting fluid is used.
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wire flame spraying A thermal spraying process in which the feed stock is in wire or rod form. This process involves pushing or pulling a wire into a flame, melting and atomizing it, and depositing the molten droplets to form an adherent coating.
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