Heating is one of the main processes in the heat treatment of metals. The selection of reasonable heating methods can ensure and improve the quality of metal heat treatment (refer to quality control of metal heat treatment). Defects of some parts after heat treatment are due to improper heating method. When heating, the temperature must be kept suitable and uniform to prevent or reduce oxidation and decarburization of the metal surface. At the same time, the heating rate must be controlled. All of these are related to the proper choice of heating method.
Early heating was carried out in an open stove with charcoal or charcoal as fuel. Later, the position of the combustion chamber was changed to make different shapes of reverberatory furnaces, which improved the heating efficiency. In order to change the oxidation and decarburization of the surface caused by the direct contact of the flame with the workpiece, some medium and small workpieces often use indirect heating methods, such as burying the workpiece in a molten salt solution. and other means, ie “bath oven”. heating, which basically can prevent oxidation and reduce decarburization.
The use of liquid and gaseous fuels and the expanded application of electric heating make the heating method of metal heat treatment more perfect, the heating temperature is easier to control, and at the same time, environmental pollution is avoided. Before and after the First World War companies specialized in the manufacture of heat treatment furnaces appeared.
In the early 1930s, the advent of controlled atmosphere brilliant heating and mechanized continuous heat treatment equipment made the heat treatment method of heating a step forward. After the advent of vacuum heat treatment in the 1960s, the expanded application of controllable atmospheres, the transplantation of new heat sources, the application of oxygen probes and microprocessors, etc., have brought new developments in heat treatment. and heating methods.
According to different heat sources, metal heat treatment heating methods can be roughly divided into three categories: fuel combustion heating method, electric heating method, and high energy density heating method.
Fuel combustion heating method?The fuel used can be solid (coal), liquid (oil) and gas (coal gas, natural gas, liquefied petroleum gas).
Coal heating: Coal is rich in resources and coal reverberatory furnaces have played a role in heat treatment heating methods. The nature of the coal and the structure of the reverberatory furnace determine that the coal is not easy to burn completely, so the thermal efficiency of the coal furnace is low, the heating quality and working conditions are poor, and the soot pollutes the environment. These shortcomings have caused the coal heating method to be gradually replaced by other heating methods.
Liquid fuel heating?Mainly uses heavy gas oil as fuel, which is suitable for heating in large heating furnaces and also for heating in externally heated salt bath furnaces. Generally, nozzles are installed on one or both sides of the outer heating wall of the furnace. When liquid fuel is used to heat an externally heated salt bath furnace, the nozzle is installed in the furnace shell outside the crucible. The liquid fuel is mixed with air in the nozzle and atomized under the action of compressed air, and then sprayed out of the nozzle and burned in the heating chamber (or outside the crucible of the salt bath furnace) to heat the workpiece (or crucible). ).
Reasonable design and arrangement of nozzles play a key role in keeping the furnace temperature uniform and saving fuel. The atomized oil sprayed from the nozzle can also be burned in the radiant tube of the furnace, and the radiant tube is heated to indirectly heat the workpiece. Fuel oil is easier to control heating temperature than coal, and it is suitable for heating large parts as a whole and areas with sufficient fuel supply.
Fuel gas heating:?In the nozzle, the gas is mixed with a certain proportion of air, and then blown out and burned. This method can directly heat the workpiece placed in the heating chamber, or spray the flame on the radiant tube installed in the heating chamber to indirectly heat the workpiece. When used in a salt bath furnace, the nozzle is installed on the furnace shell outside the crucible, and the flame is shot out of the crucible to heat the molten salt. Gaseous fuels used for heating include coal gas, natural gas, and liquefied petroleum gas.
Adjusting the air to gas ratio can obtain an oxidized or reduced combustion atmosphere, thus reducing the degree of oxidative decarburization when the workpiece is heated. This heating method is suitable for areas with high general heating and sufficient gas supply.
??Another way is to use the nozzle flame to directly heat the surface of the workpiece. At this time, the nozzle and the workpiece move relative to each other. The gas used is oxygen-acetylene, oxygen-propane, oxygen-methane, etc. This heating method is flame quenching, which is suitable for surface cooling of workpieces.
“Electric heating method” uses electricity as a heat source and uses various methods to convert electrical energy into thermal energy to heat the workpiece. When electric heating, the temperature is easy to control, there is no environmental pollution, and the thermal efficiency is high. There are many methods of electric heating.
??Electric heating element heating Using power frequency alternating current (50-60 Hz) through the electrical resistance heat generated by the heating element to heat the work piece. Electric heating elements are often placed around or on both sides of the heating chamber to ensure an even temperature in the heating chamber; some are installed on the radiant tube to indirectly heat the work piece. For external heating salt bath furnaces or metal bath furnaces, electrical heating elements are arranged in the outer space of the crucible and inside the casing. This heating method can also be used in a floating particle furnace for alumina particles.
It has enough electrical power.
??The resistance of the workpiece that heats?the alternating current after the voltage drop passes directly through the workpiece, and the resistance of the workpiece itself generates heat to increase the temperature of the workpiece of work. This method is suitable for general heating of workpieces with uniform cross sections. Another method is to use a rolling copper wheel to press on the metal workpiece, pass a low voltage and high current alternating current, and use the contact resistance between the copper wheel and the workpiece to generate heat to heat the surface of the workpiece.
??Induction heating of?the workpiece puts the workpiece into a spiral coil, the coil is passed with a certain frequency (generally higher than the power frequency) alternating current, so that the workpiece of work placed in the coil generates parasitic current, using the resistance of the work. part itself to generate heat to be heated. The depth of this heating can decrease as the current frequency increases, which is called induction heating heat treatment.
Induction heating is mainly used to heat the surface of the workpiece, but it can also be heated as a whole with lower frequency and small diameter of the workpiece. This heating method has high efficiency and low power consumption, and is mainly used for heating and cooling of medium and small parts.
Resistance heating of heating medium “?Introducing power-frequency low-voltage alternating current into the electrode buried in the medium, using the resistance heat generated when the current flows through the medium to make the medium reach a high temperature . The work piece is heated in this high temperature medium, which can reduce or prevent oxidative decarburization. This type of medium is a conductor, such as salt, graphite particles, etc. Furnace types of heating furnaces include internal heating type salt bath furnace and graphite floating particle furnace.
This heating method is mainly used to heat and cool small and medium-sized parts.
??A high energy density heat energy?to heat a large workpiece surface power density, heating time in milliseconds, power density up to 10 . 6 ~ 10 . 8 W / cm 2 , using a solar heat, electron beams and the laser beam.
Solar heating uses a concentrating solar heater to heat the workpiece.
??Laser beam heating?Using the laser light generated by continuous CO 2 laser generator, the high temperature beam is generated after focusing to irradiate the workpiece, so that the local thin layer on the surface of the workpiece instantly reaches cooling temperature or melting temperature. After the irradiation is stopped, the heat from the surface is quickly transferred to the base material to make the surface quickly harden or solidify.
Processes using laser beam heating include hardening-phase change hardening, surface “glazing”-rapid solidification, surface alloying, etc. The mirror can change the direction of the beam, so this method is more suitable for heating the inner wall (such as the cylinder liner), but the thermal efficiency is low.
Electron beam heating?uses high-speed electrons to bombard the surface of the workpiece, so that the high kinetic energy is quickly converted into thermal energy, and the surface temperature of the workpiece is quickly raised to cooling temperature or melting temperature. After the irradiation is stopped, the surface heat can be transferred to the cold base material in a short time to quickly harden or solidify. Like laser heating, the electron beam heating process also includes phase change hardening, surface “glazing”, and surface alloying. Since heating must be done in a vacuum chamber,