Carburizing process is a special process in the production of carburized steel bearings.
The length of its processing cycle and the consumption of feed gas affect the cost of bearing production.
For a long time, the carburized steel bearing parts of our company have been carburized with propane gas as the rich gas, which has a high production cost.
The use of natural gas in the controllable atmosphere heat treatment industry to replace the original propane gas or propane decabutane mixture for carburization can not only save the cost of raw gas consumption for enterprises, but also alleviate the increasingly serious energy problem today.
The research on the use of natural gas as carburizing agent for carburizing treatment in China began in the 1980s.
After more than 30 years of development, the relevant atmosphere control and process technology have been basically mature.
But at present, the common application field is mainly the carburizing treatment of gear products.
Although some enterprises have begun to use natural gas as the carburizing agent for bearing products, this technology has not been popularized in the industry.
Our propane station was built in the 1980s.
There are two 43m3 propane storage tanks and a 10m3 residual liquid tank, a compressor room, two gasification devices, two sets of pressure relief devices, about 2000m (DN125) outdoor buried mains, and eight propane pressure relief chambers.
The total gas consumption is 11.5~24.5t/month.
In recent years, the pipeline network often leaks due to corrosion during operation, causing potential safety hazards for the company.
At the same time, with the development and production demand of the company and the commercial development of surrounding land, it is urgent to solve the replacement problem of heat treatment gas.
Preparation of controllable atmosphere by natural gas instead of propane gas
1. Comparison of chemical reaction equations of two feed gases
The process principle of carburizing heat treatment with natural gas is similar to that with propane gas.
The main reaction equation for preparing endothermic atmosphere with natural gas is:
CH4＋2.38air→CO+2H2+1.88N2 （1）
In the formula, NiO2 is the catalyst, CH4 is the enriched gas, and the reaction temperature is ≥ 1000 ℃.
It can be seen from the above formula that the ratio of natural gas carburized natural gas to air is 1:2.38, while the actual situation is generally 1:2.5, or even higher, because natural gas contains not only methane, but also propane and other compounds with higher carbon content and harmful impurities.
The reference of RX atmosphere preparation is shown in Table 1.
In addition, the carburizing mechanism of natural gas is CH4 → Cad+H2 Cad activated carbon.
The main reaction equation for preparing endothermic atmosphere with propane gas is:
C3H8＋7.14air→3CO+4H2+5.64N2 （2）
In the formula, NiO2 is the catalyst, C3H8 is the enriched gas, and the reaction temperature is ≥ 1000 ℃.
In order to achieve a higher carbon potential in the carburizing furnace, feed gas (gaseous alkanes such as propane or methane) is added to the furnace as the enriched gas.
When propane is added to the endothermic controlled atmosphere for enrichment, propane will react immediately at the working temperature of the heat treatment furnace (800~950 ℃):
C3H8→2[C]+CH4+2H2 （3）
CO2, H2O, CO and H2 in endothermic atmosphere react with water gas:
CO+ H2O=CO2+H2 （4）
During carburizing, CO and H2 are consumed and CO2 and H2O are generated.
CO+H2=[C]+H2O （5）
2CO=[C]+CO2 （6）
Adding rich gas (CH4) will in turn consume CO2 and H2O, supplement CO and H2, and promote carburization reaction.
The reaction formula is:
CH4+CO2=2CO+H2 （7）
CH4+H2O=CO+3H2 （8）
Add rich gas (C3H8), propane will finally form methane under high temperature, and then participate in carburization reaction.
The reaction formula is:
C3H8=2[C]+2H2+CH4 （9）
C3H8=[C]+2CH4 （10）
2. Comparison of products of two gases
It can be seen from Table 2 that compared with propane gas, natural gas and propane gas have the same reaction principle, and the composition of the atmosphere produced is not much different, except that the proportion of air introduced is different.
Therefore, after switching to natural gas, the workload of equipment transformation and process adjustment is less, which is conducive to improving product quality and shortening production cycle.
The results show that when natural gas is used as feed gas, the thermal efficiency is significantly improved, from 45%~68% to 54%~89%.
Therefore, it is decided to use natural gas to replace liquefied propane gas as the endothermic atmosphere in the production of heat treatment.
Table 2 shows the gas source consumption required for different gas production.
Table 1 Reference Data of RX Atmosphere Preparation
| Gas production/m3. h-1 | 25 | 32 | 37 | 42 | 56 | 61 | 66 |
| Natural gas/m3. h-1 | 5.0 | 6.5 | 7.5 | 8.5 | 11.5 | 12.5 | 13.5 |
| Air/m3. h-1 | 12 | 16 | 18 | 2.1 | 28 | 30 | 33 |
Remarks:
- Gas production temperature: 1000~1040 ℃;
- Natural gas: air=1: 2.4;
- Dew point: – 2~5 ℃.
Table 2 Composition of endothermic gas prepared from different feed gases (Volume fraction) (%)
| Type of atmosphere | CO2 | CO | H2 | CH4 | H2O | N2 |
| natural gas | 0.3 | 20.7 | 38.7 | 0.4 | 0.6 | 39.8 |
| propane | 0.3 | 23 | 32 | 0.4 | 0.6 | 39.8 |
Natural gas carburizing heat treatment of parts
Natural gas is used for carburizing heat treatment.
The feeding cycle of continuous gas carburizing furnace is 46min, and the material is G20CrNi2MoA.
The process is shown in Table 3. The inspection results of physical samples of carburized parts (samples cut from ferrules) are shown in Table 4.
It can be seen from Table 4 that all indicators of natural gas after carburization meet the technical requirements.
Table 3 Carburizing Process Parameters
| Carburizing zones | Heating zone | Strong permeability zone 1 | Strong permeability zone 1 | Diffusion zone | Quenching cooling zone |
| Temperature/℃ | 920 | 930 | 930 | 925 | 880 |
| Carbon potential (%) | – | 1.3 | 1.3 | 1.2 | 0.90 |
Table 4 Inspection Data of Physical Samples
| Serial No | Hardened layer depth/mm | Carbon content on surface (%) | Coarse carbide on surface/grade | Surface network carbide/grade | Depth of decarburization layer/mm |
| 1 | 2.4 | 0.85 | 1 | 1 | 0 |
| 2 | 2.4 | 0.85 | 1 | 1 | 0 |
| 3 | 2.3 | 0.8 | 1 | 1 | 0 |
| 4 | 2.4 | 0.85 | 1 | 1 | 0 |
| 5 | 2.5 | 0.85 | 1 | 1 | 0 |
| 6 | 2.4 | 0.85 | 1 | 1 | 0 |
| 7 | 2.4 | 0.85 | 1 | 1 | 0 |
| 8 | 2.5 | 0.85 | 1 | 1 | 0 |
| 9 | 2.3 | 0.8 | 1 | 1 | 0 |
| Technical requirement | 2.3~2.9 | 0.80~1.05 | 1~2 | 1~3 | ≤0.06 |
Comparison of energy conservation and emission reduction between propane and natural gas
1. Energy saving effect
The price of propane gas is about 4000 yuan/t (provided by ENN Gas), and the gasification rate is about 550Nm3/t, equivalent to 7.27/Nm3.
The price of natural gas is expected to be 2.88 yuan/Nm3, and the price converted into propane according to the substitution ratio is 6.05 yuan/Nm3.
The annual propane consumption of our company is 220t, and the cost is 220 × 550 × 7.27=879670 yuan.
If the propane gas is replaced by natural gas, the annual natural gas consumption will be 220 × 550 × 2.1=254100Nm3, cost 254100 × 2.88=731808 yuan, and the annual saved gas cost is 879670-731808 ≈ 148000 yuan.
And with the constant tension of oil resources, the price has a trend of continuous increase.
Therefore, from the perspective of feed gas consumption composition, the use of natural gas as the gas source for the preparation of controllable carburizing gas can greatly reduce the cost of feed gas consumption.
2. Emission reduction effect and safety
The combustion of propane gas emits 3.1kg of carbon dioxide per kilogram, and the combustion of natural gas emits 2.3kg of carbon dioxide per kilogram.
The use of natural gas for carburizing heat treatment reduces carbon dioxide emissions by about 25% compared with propane gas.
In addition, in case of leakage, natural gas will not pile up on the ground when floating to the sky, which is not likely to cause potential safety hazards.
The explosion limit of natural gas is higher than that of propane gas.
Conclusion
Natural gas is used to replace the original propane gas in the controllable atmosphere heat treatment industry for carburization.
Its equipment transformation and process commissioning work is small, which not only saves the cost of raw gas consumption of the enterprise, but also alleviates the increasingly serious energy problem today.
At the same time, it can reduce human and material costs for safety protection, save the planned land, and improve the company’s civilization construction in the surrounding areas.
