Common Defects and Countermeasures for Carburized Parts
Large or network-like carbides appear in the carburized layer
Causes of Defects:
Excessive surface carbon concentration
1. Excessive droplet volume in drip carburizing
2. Excessive enrichment gas in controlled atmosphere carburizing
3. Excessive cyanide content in the salt bath during liquid carburizing
4. Too slow cooling rate after air cooling the carburized layer
Countermeasures:
1. Reduce surface carbon concentration, reduce droplet volume during the diffusion phase, and appropriately increase humidity during the diffusion phase. Alternatively, reduce droplet volume during the carburizing phase.
2. Reduce the catalyst used in solid carburizing.
3. Reduce the cyanide content in the liquid carburizing phase.
4. In summer, if the room temperature is too high, air can be blown to aid cooling of air-cooled parts after carburizing.
5. Increase the quenching temperature by 50-80°C and appropriately extend the holding time.
6. Perform a double quenching process, or normalize followed by a quench, or normalize followed by high-temperature tempering, followed by a quench and tempering process.
Large amounts of retained austenite appear in the carburized layer.
Defect Causes:
1. Austenite is relatively stable, with high carbon and alloying element contents.
2. Tempering is delayed, resulting in thermal stabilization of the austenite.
3. Cooling too slowly after tempering.
Countermeasures:
1. Surface carbon concentration should not be too high.
2. Reduce the direct quenching or reheating quenching temperature to control the core ferrite level to ≤ Grade 3.
3. Rapid cooling after low-temperature tempering.
4. Reheat quenching and cold treatment are possible, or high-temperature tempering followed by requenching is also possible.
Surface decarburization.
Defect Causes:
1. Low carbon potential of the furnace gas in the late stages of gas carburizing.
2. Excessively slow cooling after solid carburizing.
3. Excessively long air cooling after carburizing.
4. Unprotected cooling in a cooling well.
5. Air furnace quenching without protective gas.
6. Incomplete salt bath deoxidation during quenching in a salt bath.
Countermeasures:
1. Recarburize in a medium with an appropriate carbon potential.
2. Shot peening after quenching.
3. Grinding allowance: A certain decarburized layer (≤0.02mm) is permitted for larger parts.
After quenching, the carburized layer develops a troostite structure (black structure).
Cause of Defect:
High oxygen content in the carburizing medium: Oxygen diffuses into the grain boundaries, forming oxides of Cr, Mn, and Si, depleting the alloying elements and reducing hardenability.
Solutions:
1. Control the furnace gas composition to reduce the oxygen content.
2. Shot peening can be used as a remedy.
3. Improve the cooling capacity of the quenching medium.
Excessive ferrite in the core results in insufficient hardness.
Defect Causes:
1. Low quenching temperature
2. Insufficient reheating and quenching holding time, insufficient quenching cooling rate
3. Undissolved ferrite in the core
4. Austenite decomposition products in the core
Solutions:
1. Reheat and quench according to normal process
2. Appropriately increase the quenching temperature and extend the holding time
Insufficient carburized layer depth
Defect Causes:
1. Low furnace temperature and short holding time
2. Low carburizing agent concentration
3. Furnace leaks
4. Abnormal carburizing salt bath composition
5. Excessive furnace loading
6. Oxide scale or carbon deposits on the workpiece surface
Solutions:
1. Adjust carburizing temperature, time, dripping rate, and furnace sealing according to the cause.
2. Strengthen inspection of new carburizing salt and working conditions.
3. Clean parts thoroughly.
4. If the carburized layer is too thin, it can be repaired. The repair rate is half that of normal carburizing, approximately 0.1 mm/h.
Uneven carburized layer depth
Defect Causes:
1. Uneven furnace temperature
2. Poor furnace atmosphere circulation
3. Carbon black deposition on the surface
4. Large temperature differences in the solid carburizing chamber and uneven distribution of the catalyst
5. Rust, oil stains, etc. on the part surface
6. Inconsistent part surface roughness
7. Uneven hanging density of parts
8. Banded structure in the raw material
Solutions:
1. Thoroughly clean parts before carburizing
2. Remove carbon deposits in the furnace
3. Ensure even spacing and equal gaps between parts when clamping
4. Regularly check furnace temperature uniformity
5. No banded structure in the raw material
6. Regularly check furnace temperature, atmosphere, and loading conditions
Low surface hardness
Defect Causes:
1. Low surface carbon concentration
2. Excessive retained austenite on the surface
3. Troostite formation on the surface
4. High quenching temperature results in a high amount of carbon dissolved into austenite, resulting in a large amount of retained austenite after quenching.
5. Low quenching heating temperature results in insufficient carbon dissolved into austenite, resulting in a low carbon content in the quenched martensite.
6. Tempering temperature is too high.
Countermeasures:
1. Low carbon concentration; re-infiltration is possible.
2. Excessive retained austenite: Temper at high temperature followed by re-quenching.
3. Troostite structure present: Re-quenching is possible.
4. Strict heat treatment process discipline.
Surface corrosion and oxidation.
Defect causes:
1. Impure carburizing agent containing water, sulfur, and sulfates.
2. Gas carburizing furnace leaks. During solid carburizing, the accelerator melts on the workpiece surface. After liquid carburizing, residual salt adheres to the workpiece surface.
3. High-temperature exit from the furnace with insufficient air cooling.
4. Incomplete salt furnace calibration; heating in an air furnace without a protective atmosphere; and failure to clean promptly after quenching.
5. Unclean part surface.
Countermeasures:
1. Strictly control the composition of the carburizing agent and salt bath.
2. Regularly check equipment seals.
3. Clean and rinse part surfaces promptly.
4. Strictly adhere to process disciplines.
Carburized parts cracking
Causes of defects:
1. Too slow cooling rate, uneven microstructural transformation.
2. When alloy steel is air-cooled after carburizing, a layer of untransformed austenite remains beneath the surface troostite. During subsequent cooling or when left at room temperature, this layer transforms into martensite, increasing the specific volume and generating tensile stress.
3. Too fast cooling rate during the first quench or complex workpiece shape.
4. Excessive hardenability-enhancing trace elements (Mo, B).
Countermeasures:
1. Slow cooling rate after carburizing to allow the carburized layer to undergo complete eutectoid transformation during cooling.
2. Speed up cooling rate after carburizing to obtain martensite + retained austenite, thus relaxing the tensile stress generated by the microstructural transformation in the inner layer.
3. For cracking during quenching, slow cooling rate, perform process tests with trace elements, or increase the quenching medium temperature.
Hydrogen embrittlement in high-alloy steel
Causes of defects:
1. Excessive hydrogen content in the furnace gas.
2. Too high a carburizing temperature facilitates hydrogen diffusion.
3. Direct quenching after carburizing prevents hydrogen from precipitating and leaves the steel in a supersaturated state.
Countermeasures:
1. Slow cooling after carburizing.
2. After direct quenching, quickly temper at above 250°C.
3. Stop the carburizing agent supply before removing the part from the furnace, purge the hydrogen with nitrogen, and then quench directly.
Low carbon concentration in the carburized layer.
Defect Causes:
1. Low carbon potential in the furnace, low temperature, low droplet volume, or furnace leaks.
2. Carbon black formation or coating on the workpiece surface, excessive furnace loading.
3. Uneven furnace atmosphere or low furnace pressure, resulting in dead spots.
4. Too little distance between workpieces, resulting in poor furnace circulation.
5. Decarburization during post-carburizing cooling.
Countermeasures:
1. Frequently check furnace temperature and carburizing agent droplet volume during carburizing.
2. Pay attention to furnace gas and pressure.
3. Prevent furnace leaks and fan stalls or reverse rotation.
4. Distance between workpieces greater than 1 cm.
5. Frequently burn carbon black, clean carbon deposits in the furnace, and cool the carburized parts in the cooling well after carburizing. Pour kerosene or methanol into the well for protection.
Excessively thick carburized layer
Defect Causes:
1. Carburizing temperature too high, holding time too long
2. Excessive drop volume, high carbon potential in the furnace
3. Inaccurate sample inspection
Countermeasures:
1. Implement process measures based on the cause
2. Carburized layers exceeding the upper limit of the drawing are unqualified. However, if the deviation is within 0.05mm from the drawing specification, they can be accepted by arbitration or applied for reuse.
Excessive distortion of carburized parts
Defect Causes:
1. Improper carburizing furnace loading method or fixture selection
2. Carburizing temperature too high, uneven and unstable furnace gas and pressure
3. Direct quenching temperature too high
4. Inappropriate arrangement of double quenching
5. Improper heating method, quenching agent, and cooling method
6. Excessive quenching rework
7. Uneven concentration and depth of carburized layer on the part, causing irregular warping during quenching
8. The workpiece has a complex shape and uneven wall thickness, resulting in some surfaces being carburized while others are not or only partially carburized.
Countermeasures:
1. Long rod-shaped parts should be hung vertically, and flat parts should be placed flat. Parts should be stable in the fixture and not subject to prestressing. The furnace temperature should be maintained smoothly when removing them from the furnace.
2. Direct quenching should be pre-cooled. Try to use a single quench instead of a double quench. Select the correct heat treatment process.
3. Allow for machining allowances.
The carburizing rate is very slow.
Defect Causes:
1. Too low a temperature.
2. Excessive carburizing agent, resulting in carbon buildup on the part surface.
3. Excessive sulfur content in the carburizing agent.
4. The fan bearing is lubricated with MoS2, and lubricating oil enters the furnace, increasing the sulfur content.
5. Air leakage from the fan bearing allows oxygen to enter the furnace.
6. Cooling water from the fan shaft leaks into the furnace.
Countermeasures: Take appropriate measures for the defect.
The surface of the carburized part peels after quenching.
Defect Causes:
1. Excessive activity of the solid carburizing agent.
2. The carburizing temperature is too high, causing a large number of carbon atoms to penetrate the workpiece surface without sufficient time to diffuse. This poor transition results in an excessively high surface carbon concentration.
Countermeasures:
1. Heat the high-carbon-potential part in a protective atmosphere (carbon potential (volume fraction) 0.8%) for 2-4 hours to reduce the surface carbon concentration.
2. Alternatively, heat the part to 920-940°C in a 3%-5% by mass mixture of soda and charcoal and hold for 2-4 hours to reduce the surface carbon concentration.
Glass-like nodules appear on the part.
Defect Causes:
1. During solid carburizing, this is caused by a SiO₂ content exceeding 2% by mass in the carburizer.
2. The SiO₂ reacts with Na₂CO₃ at high temperatures to form a glassy substance that adheres to the workpiece surface, forming nodules.
Countermeasures:
1. During solid carburizing, the carburizing agent should be pure.
2. Thoroughly sieve out dust from the old carburizing agent.
3. Remove sand and gravel from the carburizing agent and seal with refractory clay.
Abnormal microstructure of carburized parts (free ferrite, free cementite, or network ferrite surrounding secondary cementite).
Defect Causes:
1. Excessive oxygen content in the steel and carburizing medium, resulting in soft spots during quenching and reduced wear resistance.
2. The carburizing agent should be dried to remove moisture.
Countermeasures:
1. Appropriately increase the quenching temperature and extend the holding time to achieve uniform microstructure.
2. Select a quenching medium with a high quenching intensity.
Overheating
Defect Causes:
1. Overheating during carburizing or quenching, causing grain growth and increased brittleness.
2. Overheating during carburizing increases not only the carbon content in the surface layer but also carbides, resulting in the formation of ledeburite.
Countermeasures:
1. Normalize to refine the grain size.
2. When quenching in a salt furnace, avoid placing the workpiece close to the electrodes.
3. Check for instrument malfunctions.
