Large Blocky or Reticulated Carbides in Carburized Layers
Causes of the Defect:
Excessive surface carbon concentration:
Excessive drip rate in drip-type carburizing.
Excessive enrichment gas in controlled atmosphere carburizing.
High cyanide content in salt bath liquid carburizing.
Slow cooling rate after furnace discharge.
Countermeasures:
Reduce surface carbon concentration:
Reduce the drip rate during the diffusion stage and slightly increase the humidity in the diffusion period.
Reduce the drip rate during the carburizing stage.
Decrease carbonizing agent in solid carburizing.
Reduce cyanide content in liquid carburizing.
During summer, use airflow cooling to aid in faster cooling after discharge.
Raise quenching temperature by 50–80°C and extend the holding time.
Perform double quenching, normalizing + quenching, or normalizing + high-temperature tempering followed by quenching and tempering.
Excess Residual Austenite in the Carburized Layer
Causes of the Defect:
Stable austenite with high carbon and alloy element content.
Delayed tempering causes thermal stabilization of austenite.
Slow cooling after tempering.
Countermeasures:
Avoid excessive surface carbon concentration.
Reduce quenching or reheating temperatures to ensure ferrite content in the core is ≤ grade 3.
Perform rapid cooling after low-temperature tempering.
Reheat, quench, or perform cryogenic treatment after high-temperature tempering.
Surface Decarburization
Causes of the Defect:
Low carbon potential in furnace gas during the later stages of gas carburizing.
Slow cooling rate after solid carburizing.
Prolonged air cooling after carburizing.
Lack of protective cooling in the cooling pit.
Air heating in furnaces without a protective atmosphere during quenching.
Incomplete deoxidation in salt bath heating.
Countermeasures:
Use a medium with an appropriate carbon potential for replenishment.
Conduct shot peening after quenching.
Grind off decarburized layers. Larger parts may allow decarburized layers up to 0.02mm.
Bainite (Black Structure) Formation in the Carburized Layer
Causes of the Defect:
High oxygen content in carburizing media: oxygen diffuses into grain boundaries, forming Cr, Mn, and Si oxides, depleting alloying elements and reducing hardenability.
Countermeasures:
Control the furnace gas composition to reduce oxygen content.
Perform shot peening as a remedy.
Enhance quenching medium cooling capacity.
Excessive Ferrite in the Core, Leading to Insufficient Hardness
Causes of the Defect:
Low quenching temperature.
Inadequate reheating or insufficient holding time during quenching.
Presence of undissolved ferrite in the core.
Formation of austenite decomposition products in the core.
Countermeasures:
Reheat and quench as per standard procedures.
Slightly raise the quenching temperature and extend the holding time.
Insufficient Carburized Layer Depth
Causes of the Defect:
Low furnace temperature and insufficient holding time.
Low carburizing agent concentration.
Furnace leakage.
Abnormal salt bath composition during liquid carburizing.
Excessive furnace load.
Oxidation or carbon deposit on part surfaces.
Countermeasures:
Adjust carburizing temperature, time, drip rate, and furnace seal.
Regularly check salt composition and equipment.
Thoroughly clean parts before carburizing.
Perform supplementary carburizing if the layer is too thin, with a replenishment speed of 0.1mm/h.
Uneven Carburized Layer Depth
Causes of the Defect:
Uneven furnace temperature.
Poor atmosphere circulation in the furnace.
Carbon black deposits on the surface.
Temperature differences in solid carburizing boxes or uneven carburizing agent.
Rust, oil stains, or inconsistent surface roughness on parts.
Uneven suspension or packing density of parts.
Band structure in raw materials.
Countermeasures:
Thoroughly clean parts before carburizing.
Remove furnace carbon deposits.
Arrange parts evenly with equal spacing during loading.
Regularly check furnace temperature uniformity.
Ensure raw materials are free from banded structure.
Monitor furnace atmosphere and conditions consistently.
Low Surface Hardness
Causes of the Defect:
Low surface carbon concentration.
High residual austenite content on the surface.
Formation of bainite structure.
High quenching temperature, causing excessive carbon dissolution in austenite.
Low quenching temperature, resulting in insufficient carbon in martensite.
Excessive tempering temperature.
Countermeasures:
Replenish carbon concentration as needed.
Reheat and quench parts with high residual austenite after high-temperature tempering.
For bainite structures, perform reheating and quenching.
Adhere to strict heat treatment procedures.
Surface Corrosion and Oxidation
Causes of the Defect:
Impurities like water, sulfur, and sulfates in carburizing agents.
Furnace gas leakage or molten salt residue after liquid carburizing.
High-temperature discharge without adequate protection.
Inadequate cleaning after salt bath treatment.
Dirty part surfaces.
Countermeasures:
Use pure carburizing agents and monitor salt bath composition.
Regularly inspect furnace seals.
Clean and wash parts promptly after treatment.
Follow strict operational procedures.
Cracking in Carburized Parts
Causes of the Defect:
Uneven cooling causes stress during transformation.
Retained austenite beneath bainite transforms to martensite at later cooling stages, causing tensile stress.
Excessive cooling rates during quenching or complex part shapes.
High trace elements (Mo, B) increase hardenability excessively.
Countermeasures:
Slow cooling after carburizing to ensure complete pearlitic transformation.
Accelerate cooling post-carburizing to obtain martensite + retained austenite while relieving stress.
Reduce cooling rate during quenching or increase quenching medium temperature for parts prone to cracking.
