Commonly used atmospheres for heat treatment can be divided into three categories: generating atmosphere (endothermic RX gas, exothermic gas, dissociated ammonia), synthetic atmosphere (nitrogen-methanol, nitrogen-hydrogen, pure nitrogen) and vacuum/low pressure. Endothermic gas (about 20% CO, 40% H₂, 40% N₂) has stable composition and is the mainstream carrier gas for carburizing and neutral quenching; nitrogen-methanol is easy to implement but has large composition fluctuations; dissociated ammonia (75% H₂, 25% N₂) is mostly used for bright annealing and nitriding dilution.
- Endothermic RX gas: about 20% CO + 40% H₂ + 40% N₂ (natural gas raw material), suitable for carburizing and heating before quenching
- Nitrogen-methanol: simulates endothermic composition, easy to implement but affected by temperature and drip stability
- dissociated ammonia: 75% H₂ + 25% N₂, used for bright annealing and nitriding dilution
- Selection depends on process, output, safety and hazardous chemicals management costs
A table to understand the main atmosphere
| Atmosphere | Typical ingredients | Preparation method | Main purpose |
|---|---|---|---|
| Endothermic (Endo/RX) | Natural gas raw material: about 20% CO, 40% H₂, 40% N₂, a small amount of CO₂, H₂O, CH₄ | Air and natural gas (or propane) are cracked on high-temperature catalysts, requiring external heating | Carburizing, carbonitriding carrier gas, quenching and heating of medium and high carbon steel |
| Exothermic (Exo) | Mainly N₂, containing CO₂, H₂O and a small amount of CO, H₂ | The gas partially burns with more air and releases heat by itself | Bright annealing of low carbon steel, annealing of copper and other occasions that do not require high carbon potential |
| Nitrogen-methanol | Common 40/60 ratio is about 20% CO, 40% H₂, 40% N₂ | Nitrogen gas is directly introduced into the furnace and methanol is dripped into the furnace for cracking | integral-quench furnace, mesh belt furnace carburizing and quenching |
| Pure methanol cracking | Theoretical value 33% CO, 67% H₂ | Injecting methanol into the furnace | Small furnace, high permeability requirements (difficult to control) |
| dissociated ammonia | 75% H₂、25% N₂ | Ammonia decomposes at high temperature over a catalyst | Stainless steel, silicon steel bright annealing, nitriding diluent gas |
| Nitrogen-Hydrogen Mix | N₂ + small to moderate H₂ | Pipe network gas or liquid nitrogen + hydrogen | Bright annealing, brazing, powder metallurgy sintering |
| vacuum/low pressure | Extremely low pressure, acetylene, etc. are introduced during low-pressure carburizing. | vacuum furnace | Aerospace, high-end gear low-pressure carburizing + high-pressure gas quenching |
Why are endothermic atmospheres most commonly used?
The endothermic gas is produced by "directional cracking" in the generator and is stable in composition, especially providing a relatively constant CO ratio, which is critical for carbon potential calculations (carbon potential calculations require known CO%). Cracking is an endothermic reaction, and the heat is provided by the generator. The gas will not disturb the furnace temperature after entering the heat treatment furnace.
Characteristics of nitrogen-methanol
The nitrogen-methanol system has low investment and fast start-up. The common ratio is about 1.1 m³ nitrogen corresponding to 1 L of methanol, which results in a composition similar to the endothermic type. However, the degree of methanol cracking is affected by the furnace temperature and furnace type, and the CO content will fluctuate. The on-site monitoring case of Gosi Control shows that the CO of a certain nitrogen-methanol integral-quench furnace changes between about 19% and 22% within a month, which can bring about a carbon potential deviation of about ±0.12.
Selection ideas
- Process requirements: Carburizing and quenching require controllable carbon potential → endothermic or nitrogen-methanol; bright annealing → ammonia decomposition, nitrogen-hydrogen or exothermic;
- Production volume and number of furnaces: Multiple furnaces, continuous production → endothermic gas generators with centralized air supply are more economical;
- Security and Compliance: Methanol and liquid ammonia are hazardous chemicals, and the storage and management costs cannot be ignored;
- Quality system: Automotive industry CQI-9 has strict requirements on carbon potential accuracy, and atmosphere stability is a prerequisite.
Frequently asked questions
What is the difference between propane and natural gas for endothermic gas?
Propane feedstock produces gas with higher CO (approximately 23%) and lower H₂ (approximately 31%), with slightly different carbon transfer efficiencies than natural gas feedstock. The choice usually depends on local gas supply.
Will vacuum low pressure carburizing replace gas carburizing?
Low-pressure carburizing has grown significantly in the high-end field, but equipment investment and operating costs are high. For large quantities of automobile parts, controlled atmosphere carburization is still the mainstream, and the two will coexist for a long time.
Which is cheaper, nitrogen shielding or endothermic atmosphere?
Depends on local nitrogen and gas prices, gas usage and number of furnaces. In large-volume, multi-furnace applications, centralized gas supply from endothermic gas generators usually has lower operating costs; in small-volume applications, nitrogen-based atmosphere is more flexible.