What is pit furnace carburization

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What is pit furnace carburization

the chemical heat treatment of iron and steel can be divided into the chemical heat treatment of ferrite and the chemical heat treatment of austenite according to the basic structure during diffusion. The diffusion temperature of the former is lower than the eutectoid temperature of iron and nitrogen, such as nitriding, sulfurizing, sulfurizing and oxynitriding. These processes can also be called low temperature chemical heat treatment; The latter is diffusion above the critical temperature, such as carburizing, boriding, aluminizing, carbonitriding, etc. these processes belong to the scope of high-temperature chemical heat treatment

carburizing is a chemical heat treatment process that enables carbon atoms to penetrate into the surface of steel workpiece. After carburizing, the carbon content on the surface of the workpiece is generally higher than 0.8%. After quenching and low-temperature tempering, while improving hardness and wear resistance, the core can maintain quite high toughness, can withstand impact load, and has high fatigue strength. But the disadvantage is that the processing temperature is high and the workpiece distortion is large

well type furnace carburizing treatment is a specific process:

well type gas carburizing furnace (nitriding furnace) DL series microcomputer controlled well type multipurpose furnace SL series controllable well type nitriding furnace

1. Preparation before opening the furnace

(1) check whether the lifting mechanism of the furnace cover and the fan are in good operation and lubrication

(2) check whether the electrical parts of the equipment are normal, whether the furnace cover is well grounded, and whether the electric heating elements are short circuited or open circuited

(3) check whether the furnace temperature instrument and thermocouple are normal

(4) check whether the oil dripper or gas flowmeter is in good condition

(5) if the fan bearing of the furnace cover has been modified into a circulating cooling water cooling structure, check whether the cooling water circulation is normal

(6) clean the dust in the furnace tank, check whether the furnace tank has cracks and other abnormal phenomena, and clean other parts on the pipeline

(7) check whether all valves are closed and whether there is leakage

(8) check whether the lifting equipment and lifting appliances are complete and intact

(9) check whether the sealing materials of the furnace cover are complete

(10) prepare tools and fixtures

(11) reserve auxiliary materials, such as kerosene, methanol, samples and other materials

(12) prepare fire extinguishing equipment

(13) purge the furnace tank with compressed air before temperature rise

(14) bolts of furnace cover shall not be tightened during temperature rise

2. Oven drying and temperature rise

(1) adjust the instrument to the temperature specified by the process, turn on the small switch and switch on the power supply

(2) the new furnace or the furnace after overhaul shall be dried according to the drying curve or process specified in the equipment manual

(3) for the furnace with short-term shutdown, there are two types of temperature rise process curves

1) for the furnace below 60kW, generally, it can be directly heated to the working temperature

2) furnaces above 60kW can be placed at room temperature day and night. After the electrician checks that the resistance of three-phase electric heating elements to the ground (furnace shell) should be greater than 0.5m Ω with a 500V megger, the power can be transmitted, and the following process is followed for power on and baking:

1) ℃ h furnace door opens

2) ℃ h furnace door opens

3) ℃ 8h furnace door closes and opens the fan

4) ℃ 8h furnace door closes and opens the fan

(3) when drying and heating up, the furnace begins to heat up.After, The fan bearing shall be supplied with cooling circulating water

3. Pit furnace work

(1) carburizing process of new furnace or furnace tank after overhaul shall be carried out with reference to gas carburizing process

(2) refer to the gas carburizing process for the carburizing process of furnace tank during short-term shutdown

(3) technical requirements for charging

1) before charging, cut off the power supply, close the oil dripper or inlet pipe flowmeter valve, stop the supply of dripping agent or other infiltrating atmosphere, and open the furnace cover

2) during furnace loading, the basket ears shall be lifted and placed stably in the middle of the furnace tank, aligned up and down, without inclination and clearance

3) after charging, cover the furnace cover and restore the sealing of the furnace as soon as possible, connect the power supply, drop the carburizing agent, and put the middle sample in the furnace in time

(4) carburizing, workpiece carburizing process, refer to the "gas carburizing process" for installation that does not require much time

(5) technical requirements for discharging

1) prepare lifting appliances for discharging and wear labor protection articles

2) close all valves on the oil dripper and furnace gas pipeline, turn off the fan and cut off the power supply

3) open the sample hole and vent valve

4) start the furnace cover, crane and lifting appliance, and lift the basket by aligning it with the ears. It is not allowed to lift the basket diagonally

4. Shut down

(1) for the furnace directly cooled, the fan shall be shut down when the furnace temperature drops to 400 ℃, or the cooling circulating water shall be shut down when the furnace temperature drops below 200 ℃

(2) the furnace to be used for thermal insulation can be reduced to 300 ℃ for thermal insulation, and a small amount of carburizing agent or protective gas can be dropped into the furnace

5. Operating precautions

(1) regularly check whether the indicated temperature of the instrument is normal, and the inspection cycle is 15min

(2) frequently check the pressure of the furnace, the amount of liquid dripping agent, whether the exhaust pipe is blocked, and whether the circulating cooling water is unblocked

(3) the maximum operating temperature of the furnace shall not exceed the design temperature

(4) when the furnace temperature is lower than 750 ℃, it is forbidden to drop liquid dropping agent or send protective gas into the furnace

(5) add lubricating oil regularly, at least once a day

(6) the furnace tank, electric heating element and fan guard shall be inspected regularly, and the oil dripper and exhaust pipe shall be cleaned regularly

(7) check the temperature control instrument once a quarter and the thermocouple once a half year

(8) the furnace discharge must be stable, and it is strictly prohibited for the charging basket to collide with the furnace tank

there are two main forms of carbon in steel. One is to dissolve into iron to form a solid solution (a homogeneous solid phase formed by the mutual dissolution of more than two chemical components); The other is to form iron carbon compound with iron, called cementite (Fe3C), which has high hardness and brittleness. Carbon dissolved in α The solid solution formed in iron is called ferrite; Dissolve in γ The solid solution formed in iron is called austenite, and its maximum solubility is 2.11%. During the cooling process of steel, the supersaturated austenite will decompose to form ferrite and cementite. Pearlite is a mixture of ferrite and cementite which are arranged in flakes. The metallographic structure of general carbon steel at room temperature consists of ferrite, pearlite and cementite

The molecular formula of cementite is Fe3C, which is a kind of interstitial compound with complex lattice structure

the carbon content of cementite is 6.69%; The melting point is about 1227 ℃; No allotropic transformation; However, there is magnetic transformation, and the surface of the wire rod shall not be damaged when the cementite is straightened. It has weak ferromagnetism below 230 ℃, and loses ferromagnetism above 230 ℃; The hardness of cementite is very high (equivalent to HB800), while the plasticity and impact toughness are almost equal to zero, and the brittleness is very great

cementite is not easy to be corroded by nitric acid alcohol solution and appears white and bright under the microscope, but it is corroded by alkaline sodium picrate and appears black under the microscope. There are many microstructures of cementite, which are lamellar, granular, lamellar or plate-like when coexisting with other phases in steel and cast iron

cementite is the main strengthening phase in carbon steel. Its shape and distribution have a great influence on the properties of steel. At the same time, Fe3C is a meso (sub) stable phase, which will decompose under certain conditions. (end)

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