Most heat treatment defects can be traced back to five reasons: temperature, atmosphere, time, quenching and furnace loading. This article summarizes the phenomena, possible causes and countermeasures of common defects in table form: for example, insufficient surface hardness is mostly related to decarburization or low carbon potential; network carbides are mostly caused by too high carbon potential or insufficient diffusion; layer depth fluctuations are often caused by carbon potential or temperature out of control; deformation and cracking are related to furnace loading, quenching intensity and materials.
- Checking sequence: temperature → atmosphere → time → quenching → furnace loading → material
- Low surface hardness, soft spots: common in decarburization, low carbon potential, and insufficient quenching
- Excessive network carbides and paralympics: too high carbon potential or insufficient diffusion
- Fluctuation of layer depth: unstable carbon potential/temperature control
- Internal oxidation: selective oxidation of oxidizing components in the atmosphere, Cr/Mn/Si
Common Defects Comparison Table
| Defect | Typical phenomenon | Possible reasons | Countermeasures |
|---|---|---|---|
| Oxidation | Surface oxide scale, blue and black | The furnace body leaks, the atmosphere is highly oxidizing, and the exit flame curtain fails. | Check seals, atmosphere flow and dew point |
| decarbonization | Low surface hardness and reduced fatigue life | The carbon potential is lower than the carbon content of the material | Match carbon potential, check probe and carrier gas |
| Insufficient surface hardness | Hardness is lower than the lower limit | Decarburization, low carbon potential, insufficient quenching cooling rate, and high tempering temperature | Calibrate carbon potential, check quenching medium and temperature |
| Soft spot | Low local hardness | Too dense furnace loading, insufficient stirring of quenching medium, and dirt on the surface of parts | Optimize furnace loading, improve mixing and cleaning |
| Insufficient/too deep layer depth | Effective hardened layer is out of tolerance | Temperature deviation, carbon potential deviation, improper time | TUS/SAT, carbon potential calibration, diffusion simulation |
| Deep batch fluctuations | Results from furnaces are inconsistent | The carbon potential control is unstable, the carrier gas CO fluctuates, and the door opening recovery time is different. | Stable carrier gas, online diffusion calculation |
| mesh carbide | Surface layer carbides along grain boundaries | Carbon potential is too high, insufficient diffusion, slow cooling | Reduce strong carburization potential and extend diffusion |
| Too much retained austenite | Low hardness and unstable dimensions | Surface carbon/nitrogen is too high and quenching temperature is too high | Reduce carbon potential/ammonia content, cold treatment |
| Internal oxidation (grain boundary oxidation) | Superficial black microstructure | H₂O and CO₂ in the atmosphere selectively oxidize Cr, Mn and Si | Control the oxidation of the atmosphere, and if necessary, low-pressure carburizing |
| carbon black | Carbon deposits on parts and furnaces | The carbon potential exceeds saturation and there is too much enriched gas. | Control the upper limit of carbon potential and burn charcoal regularly |
| Quenching deformation | Size, shape and position are out of tolerance | Furnace loading method, quenching intensity, part structure, residual stress | Optimize furnace charging and quenching parameters and pre-stress relief |
| Quenching cracking | crack | Too fast cooling speed, sharp corners, overheating, material defects | Choose appropriate media, improve design, control temperature |
Suggestions for troubleshooting order
- temperature: Whether TUS, SAT and furnace temperature records are normal;
- atmosphere: Carbon potential/nitriding potential curve, calibration records, probe status, carrier gas composition;
- time: Whether each period of time is executed according to the process, and whether the door opening recovery is abnormal;
- Quenching: Medium temperature, stirring, transfer time, medium aging;
- Loading the furnace: Loading capacity, spacing, and parts placement;
- Material: Composition, initial microstructure, surface state.
Prevention is better than detection
- Establish continuous monitoring and alarming of key parameters;
- Use data trends to find early problems such as probe aging and generator drift;
- Conduct FMEA and sample verification before new parts and new processes are put online;
- Quality issues are formed into a case library and reviewed regularly.
Frequently asked questions
How to judge whether decarburization or insufficient quenching results in low hardness?
Do metallography: the decarburized surface layer contains ferrite or a structure with reduced carbon content; insufficient quenching shows a non-martensitic structure (such as troostite), and the surface carbon content is normal.
Can internal oxidation be completely avoided?
It is difficult to completely avoid it in an endothermic atmosphere containing H₂O and CO₂, but the depth can be reduced by controlling the oxidation of the atmosphere and selecting appropriate materials; low-pressure vacuum carburizing can basically eliminate it.
Can defective parts be reworked?
Parts can, for example, decarburization can be recarburized, and if the hardness is insufficient, it can be re-quenched. However, rework will increase deformation and cost, and must be reviewed and approved according to customer requirements.