What is carburizing? Common problems and experience summary of carburizing process

Massive or reticular carbides appear in carburized layer

Causes of defects: the surface carbon concentration is too high
1. Drip feed carburizing, the amount of drops is too large
2. Control atmosphere carburization, too much rich gas.
3. Liquid carburization, too high cyanide content in salt bath
4. The carburized layer is discharged from the furnace for air cooling, and the cooling rate is too slow.

Method:
1. Reducing the surface carbon concentration, reducing the dripping amount during the diffusion period and appropriately increasing the humidity during the diffusion period can also appropriately reduce the dripping amount during the carburizing period.
2. Reduce the carbon catalyst for solid carburization
3. Reduce the cyanide content of liquid carburizing.
4. If the room temperature is too high in summer, the air cooled parts can be cooled by blowing air after infiltration.
5. Raise the quenching heating temperature by 50 ~ 80 C and extend the holding time appropriately.
6. Twice quenching or normalizing+quenching, or normalizing+high temperature tempering, and then quenching and tempering.

A large amount of retained austenite appears in the carburized layer

Causes of defects:
1. Austenite is stable, and the content of carbon and alloy elements in austenite is high.
2. Austenite is thermally stabilized if tempering is not timely.
3. Cooling after tempering is too slow

Method:
1. Surface carbon concentration should not be too high.
2. Reduce the quenching temperature of direct quenching or reheating, and control the grade of ferrite in the center ≤3.
3. Quick cooling after low temperature tempering
4. It can be reheated and quenched, cold treated, or quenched after high temperature tempering.

Surface decarbonization

Causes of defects:
1. In the later stage of gas carburization, the carbon potential of furnace gas is low.
2. After solid carburizing, the cooling rate is too slow.
3. The air cooling time after carburizing is too long.
4. Unprotected cooling in cooling wells
5. Air furnace heating quenching unprotected gas
6. Salt bath furnace heating quenching, salt bath deoxidation is not complete.

Method:
1. Replenishing infiltration in a medium with suitable carbon potential.
2. Shot peening after quenching.
3. Grinding allowance, a certain decarburization layer (≤0.02mm) is allowed for larger pieces.

Troostite structure (black structure) appears after quenching of carburized layer

Causes of defects:
The oxygen content in carburizing medium is high: oxygen diffuses to the grain boundary to form oxides of Cr, Mn and Si, which makes alloy elements depleted and hardenability reduced.
Method:
1. Control the composition of furnace gas medium and reduce the oxygen content.
2. Shot peening can be used for remedy.
3. Improve the cooling capacity of quenching medium.

Too much ferrite in the center leads to insufficient hardness

Causes of defects:
1. Low quenching temperature
2. The reheating quenching time is not enough, and the quenching cooling speed is not enough.
3. There is undissolved ferrite in the center.
4. There are austenite decomposition products in the heart.

Method:
1. Reheat and quench according to the normal process.
2. Properly raise quenching temperature and prolong holding time.

Insufficient carburized layer depth

Causes of defects:
1. Low furnace temperature and short heat preservation time.
2. Low concentration of penetrant
3. The stove leaks air
4. The composition of salt bath carburizing is abnormal
5. Too much furnace charging
6. There is scale or carbon deposit on the surface of the workpiece.

Method:
1. According to the reasons, adjust carburizing temperature, time, dripping amount and furnace sealing.
2. Strengthen the identification of new salt and the inspection of working conditions.
3. Parts should be cleaned up.
4. If the carburized layer is too thin, it can be re-infiltrated, and the speed of re-infiltration is 1/2 of that of normal carburizing, which is about 0.1 mm/h.

Uneven infiltration depth

Causes of defects:
1. Uneven furnace temperature
2. Bad atmosphere circulation in the furnace.
3. Deposition of carbon black on the surface
4. The temperature difference in the solid carburizing box is large and the carburizing agent is uneven.
5. There are rust spots and oil stains on the surface of parts.
6. The surface roughness of parts is inconsistent.
7. Uneven hanging density of parts
8. Raw materials have banded structures.

Method:
1. Strictly clean the parts before carburizing.
2. Clean up the carbon deposit in the furnace
3. Parts should be evenly distributed when clamping, and the gap should be equal in size.
4. Regularly check the uniformity of furnace temperature.
5. Raw materials shall not have banded structures.
6. Regularly check the furnace temperature, furnace gas and furnace charging.

Low surface hardness

Causes of defects:
1. Low surface carbon concentration
2. There are many retained austenite on the surface.
3. troostite structure is formed on the surface.
4. The quenching temperature is high, and the amount of carbon dissolved in austenite is large, and a large amount of residual austenite is formed after quenching.
5. The quenching heating temperature is low, the amount of carbon dissolved in austenite is not enough, and the carbon content in quenched martensite is low.
6. Tempering temperature is too high

Countermeasures:
1. The carbon concentration is low, so it can be replenished.
2. There are many retained austenite, which can be tempered at high temperature and then quenched by heating.
3. There is a Toroidal structure, which can be reheated and quenched.
4. Strict heat treatment process discipline.

Surface corrosion and oxidation

Causes of defects:
1. Infiltrating agents are not pure water, sulfur and sulfate.
2. When the gas carburizing furnace leaks gas, the carburizing agent melts on the surface of the workpiece, and after liquid carburizing, the surface of the workpiece is stuck with residual salt.
3. High temperature furnace, air cooling protection is not enough
4. The salt furnace is not completely calibrated, the air furnace is heated without protective atmosphere, and it is not cleaned in time after quenching.
5. The surface of the parts is not clean

Method:
1. Strictly control carburizing agent and salt bath composition.
2. Always check the sealing condition of the equipment.
3. Clean and clean the surface of parts in time.
4. Strictly implement process discipline.

Cracking of carburized parts

Causes of defects:

1. The cooling rate is too slow, and the tissue transformation is uneven.

2. The alloy steel is air-cooled after infiltration, and a layer of untransformed austenite remains under the surface layer of Trollite. When it is subsequently cooled or placed at room temperature, it is transformed into martensite, and its specific volume is increased, resulting in tensile stress.

3. When quenching for the first time, the cooling speed is too fast or the shape of the workpiece is complicated]

4. The material contains too many trace elements (Mo, B) to improve hardenability, etc.

Method:

1. Slow down the cooling rate after infiltration, so that the infiltration layer is completely eutectoid during the cooling process.

2. After infiltration, the cooling rate is accelerated to obtain martensite and retained austenite. Tensile stress caused by the transformation of relaxed inner layer structure

3. Quenching cracking should slow down the cooling rate, contain trace elements for process test, or increase the temperature of quenching medium.

Hydrogen embrittlement of high alloy steel

Causes of defects:
1. The hydrogen content in the furnace gas is too high.
2. Too high carburizing temperature is beneficial to hydrogen diffusion.
3. Directly quenched after infiltration, hydrogen can not be precipitated and exists in steel in supersaturated state.

Method:
1. Slow cooling after carburizing
2. After direct quenching, temper quickly above 250 C.
3. Stop supplying permeating agent before the parts are discharged from the furnace, and directly quench after introducing nitrogen to remove hydrogen.

Low carbon concentration in carburized layer

Causes of defects:
1. The carbon potential in the furnace is low, the temperature is low, the dripping amount is small, and the furnace leaks.
2. Carbon black is formed on the surface of the workpiece or covered by carbon black, which leads to too much furnace charging.
3. The atmosphere of the furnace is uneven, and the furnace pressure is too low, which causes dead corners in some parts of the furnace.
4. The distance between workpieces is too small, and the furnace circulation is not smooth.
5. Decarburization during cooling after infiltration

Method:
1. When carburizing, always check the furnace temperature and the amount of dripping agent.
2. Pay attention to the furnace gas and pressure.
3. Prevent the furnace from leaking and the fan from stopping and reversing.
4. The distance between workpieces is greater than 1cm.
5. Burning carbon black frequently, cleaning the carbon accumulated in the furnace, infiltrating it into a cooling well for cooling, and pouring kerosene or methanol into the well for protection.

Too thick carburized layer

Causes of defects:

1. The carburizing temperature is too high and the holding time is too long.

2. If the dropping amount is too large, the carbon potential in the furnace is high.

3. The sample inspection is not allowed

Method:

1. According to the reasons, take technical measures.

2. If the infiltration layer exceeds the upper limit of the pattern and is unqualified, but it is 0.05mm different from the pattern, it can be qualified by arbitration or applied for reuse.

Excessive distortion of carburized parts

Causes of defects:
1. Improper selection of carburizing furnace method or fixture.
2. The carburizing temperature is too high and the furnace gas and pressure are uneven and unstable.
3. The direct quenching temperature is too high
4. Improper arrangement of twice quenching
5. Improper heating mode, quenching agent and cooling mode.
6. Too many times of quenching repair.
7. The concentration and depth of carburized layer on the parts are uneven, resulting in irregular warping during quenching.
8. The workpiece has complex shape and uneven wall thickness, some surfaces are carburized, and some surfaces are not carburized or less carburized.

Method:
1. The long rod-shaped parts should be hung vertically, the flat parts should be laid flat, the parts should be stable on the fixture and not prestressed, and the furnace operation should be stable and the furnace temperature should be appropriate.
2. Direct quenching should be precooled, primary quenching should be used instead of secondary quenching as far as possible, and the heat treatment process should be selected correctly.
3. Set aside machining allowance in advance.

The carburizing speed is very slow

Causes of defects:

1. The temperature is too low

2. Too much permeating agent leads to carbon on the surface of parts.

3. Too much sulfur in the penetrant.

4. The fan bearing is lubricated with MoS2, and the lubricating oil enters the furnace, which increases the sulfur.

5. The fan bearing leaks air and oxygen enters the furnace.

6. The cooling water of the fan shaft leaks into the furnace

Method:

Take corresponding measures against defects.

Surface spalling of carburized parts after quenching

Causes of defects:
1. The activity of solid carburizing agent is too strong
2. If the carburizing temperature is too high, a large number of carbon atoms will penetrate into the surface of the workpiece before diffusion, and the carbon concentration on the surface will be too high due to poor transition.

Method:
1. Heat high carbon potential parts in protective atmosphere (carbon potential (volume fraction) is 0.8%) for 2~4h to reduce the surface carbon concentration.
2. It can also be heated to 920~940ºC in 3%~5% soda and charcoal, and kept for 2 ~ 4 hours to reduce the surface carbon concentration.

Glass bumps appear on the parts

Causes of defects:
1. During solid carburization, the mass fraction of SiO2 _ 2 is more than 2%.
2. When SiO _ 2 reacts with Na2CO3 at high temperature, a glassy substance is generated and adheres to the surface of the workpiece, forming a bump.

Method:
1. When solid carburizing, the permeating agent should be pure.
2. The old carburizing agent is thoroughly screened to remove dust.
3. Remove the sand and gravel in the penetrant and the refractory clay for sealing.

Abnormal structure (free ferrite, free cementite or reticular ferrite around secondary cementite) appears in carburized parts

Causes of defects:
1. The high oxygen content in steel and carburizing medium leads to soft spots during quenching, which reduces the wear resistance.
2. Carburizing agent should be dried to remove water.

Method:
1. Properly increase the quenching temperature and prolong the holding time to homogenize the structure.
2. Choose quenching medium with strong quenching intensity.

Superheat

Causes of defects:
1. Overforce during carburizing or quenching and heating makes the grain grow and brittleness increase.
2. Excessive carburization not only increases the carbon content in the surface layer, but also increases the carbides, resulting in ledeburite.

Method:
1. Normalizing is adopted to refine the crystal grain.
2. When the salt furnace is heated and quenched, the workpiece cannot be close to the electrode.
3. Check whether the instrument is out of order.

 

 

 

 

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