Detailed Explanation of Gas Carburizing Process

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1. Ready

Check whether the equipment and instruments are normal, check whether the screws and sealing rings that clamp the furnace cover are in good condition, and arrange the work site in front of the furnace.

Check the kerosene in the oil storage tank, its reserve should be sufficient.

Regularly remove carbon black and ash in the muffle tank and exhaust pipe.

Check whether the supply of cooling water is normal.

Check whether the material of the workpiece is consistent with the drawing, and understand the technical requirements and process regulations of the parts.

Adjust the meter to the specified temperature of the process, turn on the small switch of the meter, switch on and send power.

Choose suitable tools and fixtures, and consider how to install and release the furnace.

Check whether the anti-seepage part of the part is coated with anti-seepage paint.

Prepare the carburizing sample and put the workpiece in the carburizing basket.

2. Method of operation

Gas carburizing and carbonitriding are heated by pit carburizing furnace. The uniformity of the furnace temperature in the uniform temperature zone of the heating furnace should be controlled at (10-15)°C, and the instrument technician is responsible for the calibration of the instrument. For carburizing, kerosene is used as the carburizing agent, which is directly dropped into the carburizing furnace. The carbon concentration on the surface of the part is mainly controlled by adjusting the number of droplets. For carbonitriding, kerosene and ammonia are used as co-permeating agents. See below for specific operations:

Adjust the instrument to the temperature specified by the process, turn on the small switch of the instrument, close the power supply, turn on the fan power, and turn on the circulating cooling water.

Adjust the pointer of the temperature meter to the carburizing temperature according to the requirements of the process specification.

When the empty furnace is heated up to 850°C, kerosene starts to drip. When the temperature is reached, cut off the power supply, start the furnace cover, put the material basket into the furnace, cover the furnace cover, and tighten the bolts.

Drop carburizing agent according to the process specification to ensure the drop volume during the heating stage.

After the furnace temperature rises to the carburizing temperature, add the carburizing agent according to the drops of each stage of carburizing.

After reaching the carburizing temperature, insert the sample into the furnace, usually two, to prevent air leakage.

The exhaust gas should be ignited. The normal flame is dark gray and the length is 80-250mm.

When placing the sample, the color of the combustion exhaust gas should be apricot yellow.

Check the stability of the drop volume of the penetrating agent.

After 3 hours of strong infiltration, draw a sample to check the depth of the infiltration layer to determine the holding time.

The amount of penetrant drops should be adjusted in time to ensure that the process requirements are met.

After the homogenization stage is completed, cut off the power supply, close the infiltrant valve, open the furnace cover, and use the spreader to lift the carburizing basket to directly quench.

After the furnace is shut down, the fan can be stopped when the furnace temperature drops below 600°C, and the cooling circulating water can be stopped when the temperature drops below 200°C.

3. Process Specification

Control of process parameters:

Carburizing temperature: generally 900-950°C, the most commonly used is 920-930°C, because the carburizing speed increases greatly with the increase of temperature, when the depth of the carburized layer is required to be shallow, thin-layer carburizing is required It should be controlled by lowering the temperature instead of shortening the time; the carbonitriding temperature is generally 820-880°C, most commonly 850-860°C.

Holding time: Determined by the depth of the sample penetration layer and combined with Tables 1 and 2.

Carburized layer depth/mm	Holding time/h
	Carbon carburized steel	Alloy carburized steel
0.4-0.6	2-3	1.5-2.5
0.6-0.8	3-4	2.5-3.5
0.8-1.0	4-5	3.5-4.5
1.0-1.2	5-6	4.5-5.5
1.2-1.4	6-7	5.5-6.5

Tables 1 The relationship between carburizing time and depth of carburizing layer at 960℃

Co-osmosis time	1-1.5	2-3	4-5	6-8
Infiltration depth	0.2-0.3	0.4-0.5	0.6-0.7	0.8-1.0

Tables 2 The relationship between carbonitriding time and layer at 860℃

Local anti-seepage method:After degreasing and cleaning the parts, apply anti-carburizing paint to the place where carburization (co-carburization) is not required, and coat with an anti-seepage layer of 0.4-0.5mm, generally dry at room temperature for 4-6 hours, and then put it into the basket furnace. Asbestos rope can be used to plug the threaded holes that are not allowed to carburize.

Heat treatment specifications, mechanical properties and uses of carburized steel:The final heat treatment process after carburizing of parts is mainly quenching and low temperature tempering. The quenching and tempering process is based on the quenching and tempering process of high-carbon steel and high-carbon alloy steel, and the core performance is guaranteed by the pre-heat treatment before carburizing. The characteristics and uses of carbonitriding are shown in Table 3.

Types of	Temperature range	Features and uses
High temperature carbonitriding	750-950	Treated above the ACI point of the steel, the carbon content in the carburized layer decreases with the increase of temperature, and the hardenability and wear resistance of the carburized layer are better than carburized treatment
Low temperature carbonitriding	500-650	It is processed below the ACI point of steel, and has similar surface hardness, wear resistance and adhesion resistance to gas nitriding. It is suitable for wear-resistant parts with thin hardened layers and small loads.

Tables 3 Characteristics and uses of carbonitriding

Extreme heat treatment of carbonitriding steel:The requirements on the mechanical properties, process performance and steel quality of carbonitriding steel are basically the same as those of carburizing steel, so general carburizing steel can be used for carbonitriding. Carbonitriding heat treatment specifications, characteristics and scope of use are shown in Table 4.

Process plan	Heat treatment process	Features and scope of application
Direct quenching	Quench directly from the co-infiltration temperature, temper at low temperature	The most common heat treatment scheme for high temperature carbonitriding workpieces. For medium and low carbon steels and low-alloy steels, satisfactory surface and core structures can be obtained, quenching medium stress and slightly improved steel toughness, for those with specific hardness requirements The workpiece can be selected according to the tempering temperature of high carbon steel
Direct quenching	Pit furnace 930 gas carburizing, cooling with the furnace to 860 heat preservation for 30 minutes, direct quenching, low temperature tempering	After the parts of carbon steel, quenching and low temperature tempering are mainly used, and the quenching and tempering process is formulated with reference to the quenching and tempering process of high carbon steel

Table 4 Characteristics and application range of carbon steel co-infiltration heat treatment