Oxidation is when the iron on the steel surface reacts with O₂, H₂O, and CO₂ to form oxides; decarburization is when the surface carbon reacts with these oxidizing gases to form CO or CH₄ and is lost. Both result in reduced surface hardness, reduced fatigue life and dimensional loss. The core of prevention is to make the oxidation of the atmosphere in the furnace low enough, match the carbon potential with the carbon content of the steel, and ensure that the furnace body is sealed and the atmosphere is stable.
- Source of oxidation: air infiltration, high H₂O and CO₂ in the atmosphere
- The decarburization layer will cause insufficient surface hardness and is a common cause of fatigue failure of gears and springs.
- Prevention methods: controlled atmosphere, carbon potential matching, sealing, reasonable furnace installation and rapid transfer
- Testing: Metallographic method, microhardness gradient method
Oxidation: formation of oxide scale
At high temperatures, iron reacts with oxidizing gases:
- 2Fe + O₂ → 2FeO
- Fe + H₂O ⇌ FeO + H₂
- Fe + CO₂ ⇌ FeO + CO
When the ratio of H₂O/H₂ or CO₂/CO in the atmosphere exceeds a critical value at a certain temperature, the steel surface will be oxidized. This is why the protective atmosphere must have sufficient reducing components (H₂, CO) and control moisture and carbon dioxide.
Decarbonization: Loss of surface carbon
The main reactions of decarburization include:
- [C] + O₂ → CO₂ / CO
- [C] + H₂O → CO + H₂
- [C] + CO₂ → 2CO
- [C] + 2H₂ ⇌ CH₄ (may also occur in high temperature, dry hydrogen)
As long as the carbon potential in the furnace is lower than the carbon content of the steel, carbon will diffuse outward. Decarbonization can be divided into Full decarbonization(The surface layer is almost entirely ferrite) and partial decarbonization(The carbon content is lower than the matrix).
Harm
- After surface quenching, the hardness is insufficient and "soft spots" appear;
- The residual compressive stress in the surface layer is reduced or even turned into tensile stress.Fatigue life significantly reduced, especially sensitive to springs, gears, and shafts;
- Oxide scale causes dimensional loss and increased machining allowance;
- Grain boundary oxidation (internal oxidation) can become a source of fatigue cracks.
How to prevent
- Use a controlled atmosphere and match the carbon potential: Set the carbon potential near the carbon content of the material during quenching and heating;
- Ensure the furnace body is sealed: Air leakage at furnace doors, furnace curtains, thermowells, fan shafts, etc. is the main way for air to enter;
- source of stable atmosphere: Fluctuations in endothermic gas generators and nitrogen-methanol systems will directly manifest as carbon potential fluctuations;
- Monitoring and alarming: Timely detection of abnormalities through oxygen probes and analyzers;
- Craftsmanship details: Reduce the transfer time between parts coming out of the furnace and quenching, and avoid staying in the medium temperature zone for a long time.
How to detect
- Metallography: Observe the ferrite layer and structural changes after cross-section corrosion;
- microhardness test: Measure the hardness gradient from the surface to the inside to determine the depth of the decarburized layer;
- chemical analysis: Layer-by-layer peeling or spark spectrum analysis of surface carbon content.
Frequently asked questions
Can protecting with pure nitrogen prevent decarburization?
Pure nitrogen can reduce oxidation, but trace amounts of oxygen and moisture in nitrogen may still cause decarburization at high temperatures, and pure nitrogen does not have the ability to "recarb". For high carbon steel and parts with strict surface carbon requirements, it is recommended to use an atmosphere with controlled carbon potential.
Can decarburization be repaired?
The surface carbon content can be restored through recarbonization (reheating in an atmosphere with a carbon potential slightly higher than the carbon content of the material), but the cost is high and the effect needs to be verified. The best way is to prevent it at the source.
What does it mean if the outlet parts of the mesh belt furnace turn blue and black?
It usually means that air has entered or the moisture content is high at the cooling section or outlet, causing slight oxidation. Exit fire curtains, cooling section seals and atmosphere flow should be checked.