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Points to keep in mind when using nitrogen gas
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Overview.
Nitrogen gas, a neutral gas, is an important heating medium in the heat treatment field because it is virtually inert to steel and is the cheapest and most readily available.
01 Impurities in nitrogen gas
Argon gas (Ar), a representative inert gas, is effective as an atmosphere for bright heating of steel, but it is very expensive, so nitrogen gas ( N2 ), which is inexpensive, is used in industry.
However, since oxidation-producing gases such asO2,H2O, and CO2 are present in N2 as impurities, their content has a significant effect on the surface brightness of the treated product. Incidentally, in the case of the least expensive industrial cylinder-filled N2, the H2O content is about -70°C as the dew point, and the O2 content is 1~50 ppm.
When N2 is used in large quantities, vaporized liquid N2 is most effective. This is because it is the least expensive of the various types of N2 and has a high degree of purity. However, liquid N2 easily absorbs oxygen when exposed to air, so it must be handled with care.
02 Effect of oxygen content on luminosity
N2Inside.Bright heat treatmentIF THE O2The content has a significant effect, but carbon and low-alloy steels with as little as 2 ppm will have excellent brightness.
However, even with that level of oxygen content, brightness of stainless steels containing large amounts of chromium (Cr) deteriorates rapidly at temperatures above 550°C. This is due to preferential oxidation of Cr. This is due to preferential oxidation of Cr.2It is difficult to use alone.
At 850°C, which is the quenching and heating temperature of carbon steel, bright heat treatment is possible if the O2 content is about 5 ppm, but if the O2 content is higher than that, the brightness deteriorates extremely, making industrial handling very difficult.
03 Use of N2-based atmosphere
When N2 gas is used as a stand-alone protective gas for heating, even if high-purity N2 gas is used, it is completely impossible to maintain high purity as a heating atmosphere if the surface of the workpiece, tray, inner wall of the furnace, or inner surface of piping material is contaminated.
As shown in the table, the use of N2 base gas with the addition of reducing or carburizing gas is effective to the extent that the effects of these impurities and contaminants can be eliminated.
| Temperature (°C) | Heating atmosphere | steel grade | Gloss (%) | |
|---|---|---|---|---|
| 900 | N2-CH4 system | CH4: 2.09 | SK105 | 87.9 |
| SKS3 | 88.8 | |||
| CH4: 2.7 | SK105 | 84.1 | ||
| N2-C3H8 series | C3H8: 0.45 | SK105 | 90.6 | |
| SKS3 | 88.8 | |||
| N2-CO-CO2-H2system | CO: 20.8 CO2: 0. 20 H2: 39.5% C |
SK105 | 79.0 | |
| Temperature (°C) | Heating atmosphere | steel grade | Gloss (%) | |
|---|---|---|---|---|
| 1000 | N2 only | O2: 2.3ppm | SKD11 | 66.0 |
| O2: 6.0ppm | 57.5 | |||
| N2-CH4 system | CH4: 2.09 | SK105 | 68.8 | |
| SKS3 | 79.6 | |||
| SKD11 | 79.2 | |||
The addition of hydrocarbon gases ( CH4,C3H8, etc. ) is also effective for bright quenching of carbon and low-alloy steels.