The carburizing process is a special process in the production of case-hardened steel bearings.
The length of your processing cycle and feed gas consumption affect the production cost of the bearing.
For a long time, our company's carbureted steel bearing parts have been carbureted with propane gas as 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 decabutane propane mixture for carburization can not only save the cost of raw gas consumption for enterprises, but also alleviate the energy problem every time more serious today.
Research on the use of natural gas as a carburizing agent for carburizing treatment in China began in the 1980s.
After more than 30 years of development, relevant atmosphere control and process technology are basically mature.
But at present, the common application field is mainly carburizing treatment of gear products.
Although some companies have begun to use natural gas as a 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 43m 3 propane storage tanks and 10m 3 waste liquid tank, a compressor room, two gasification devices, two sets of pressure relief devices, around 2,000 m (DN125) of buried air electrical network free and eight propane pressure relief chambers.
The total gas consumption is 11.5~24.5t/month.
In recent years, the pipeline network frequently leaks due to corrosion during operation, causing potential safety risks for the company.
At the same time, with the development and demand for the company's production and the commercial development of the surrounding land, it is urgent to solve the problem of replacing heat treatment gas.
Preparation of atmosphere controllable by natural gas instead of propane gas
1. Comparison of chemical reaction equations of two feed gases
The principle of the natural gas carburizing heat treatment process is similar to that of propane gas.
The main reaction equation for preparing an endothermic atmosphere with natural gas is:
CH 4 ＋2.38air→CO+2H 2 +1.88N 2 (1)
In the formula, NiO 2 is the catalyst, CH 4 is the enriched gas, and the reaction temperature is ≥ 1000 ℃.
It can be seen from the above formula that the ratio of carbureted natural gas to air is 1:2.38, while the actual situation is generally 1:2.5, or even more, because natural gas contains not only methane, but also propane and other compounds with a higher carbon content and harmful impurities.
The RX atmosphere preparation reference is shown in Table 1.
Furthermore, the carburizing mechanism of natural gas is CH 4 →C of Ads +H 2 C of Ads activated carbon.
The main reaction equation for preparing an endothermic atmosphere with propane gas is:
C 3 H 8 ＋7.14ar→3CO+4H 2 +5.64N 2 (2)
In the formula, NiO 2 is the catalyst, C 3 H 8 is the enriched gas, and the reaction temperature is ≥ 1000 ℃.
To achieve a higher carbon potential in the carburizing furnace, the feed gas (gaseous alkanes such as propane or methane) is added to the furnace as enriched gas.
When propane is added to the endothermic controlled atmosphere for enrichment, the propane will immediately react to the working temperature of the heat treatment furnace (800~950℃):
C 3 H 8 →2(C)+CH 4 +2H 2 (3)
CO 2 H 2 O, CO and H 2 in the endothermic atmosphere react with water gas:
CO + H 2 O=CO 2 +H 2 (4)
During carburizing, CO and H 2 are consumed and CO 2 and H 2 O are generated.
CO+H 2 =(C)+H 2 Ó (5)
2CO=(C)+CO 2 (6)
Adding rich gas (CH 4 ) will in turn consume CO 2 and H 2 O, supplement CO and H 2 and promote the carburetion reaction.
The reaction formula is:
CH 4 +CO 2 =2CO+H 2 (7)
CH 4 +H 2 O=CO+3H 2 (8)
Add rich gas ( C3H8 ), propane will finally form methane under high temperature and then participate in the carburization reaction.
The reaction formula is:
C 3 H 8 =2(C)+2H 2 +CH 4 (9)
C 3 H 8 =(C)+2CH 4 (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 lower, which contributes to improving product quality and shortening the 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 an endothermic atmosphere in heat treatment production.
Table 2 shows the gas source consumption required for different gas productions.
Table 1 RX Atmosphere Preparation Reference Data
| Gas production/m 3 . h -1 | 25 | 32 | 37 | 42 | 56 | 61 | 66 |
| Natural gas/m 3 . h -1 | 5.0 | 6.5 | 7.5 | 8.5 | 11.5 | 12.5 | 13.5 |
| Air/m 3 . h -1 | 12 | 16 | 18 | 2.1 | 28 | 30 | 33 |
Comments:
- 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 | H 2 O | N 2 |
| 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 |
Heat treatment of parts with natural gas carburizing
Natural gas is used for carburizing heat treatment.
The feeding cycle of the continuous gas carburizing furnace is 46 minutes and the material is G20CrNi 2 MoA.
The process is shown in Table 3. The inspection results of physical samples of carbureted parts (samples cut from tips) are shown in Table 4.
It can be seen from Table 4 that all natural gas indicators after carburization meet the technical requirements.
Table 3 Carburization Process Parameters
| Carburizing zones | Heating zone | Strong Permeability Zone 1 | Strong Permeability Zone 1 | Diffusion zone | Quench Cooling Zone |
| Temperature/℃ | 920 | 930 | 930 | 925 | 880 |
| Carbon potential (%) | – | 1.3 | 1.3 | 1.2 | 0.90 |
Table 4 Physical Sample Inspection Data
| Serial number | Hardened layer depth/mm | Surface carbon content (%) | Coarse carbide in surface/grade | Carbide/Surface Network Grade | Decarburization layer depth/mm |
| 1 | 2.4 | 0.85 | 1 | 1 | 0 |
| two | 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 Emissions Reduction Between Propane and Natural Gas
1. Energy saving effect
The price of propane gas is about 4,000 yuan/t (supplied by ENN Gas), and the gasification rate is about 550Nm 3 /t, equivalent to 7.27/Nm 3 .
The price of natural gas is expected to be 2.88 yuan/Nm 3 and the price converted to propane according to the replacement rate is 6.05 yuan/Nm 3 .
Our company's annual propane consumption is 220 tons, and the cost is 220 × 550 × 7.27 = 879,670 yuan.
If propane gas is replaced by natural gas, the annual consumption of natural gas will be 220 × 550 × 2.1 = 254100Nm 3 costs 254,100 × 2.88 = 731,808 yuan, and the annual cost of gas saved is 879,670-731,808 ≈ 148,000 yuan.
And with the constant stress on oil resources, the price has a continuous upward trend.
Therefore, from the perspective of the composition of feed gas consumption, the use of natural gas as a gas source for the preparation of controllable carburizing gas can greatly reduce the cost of feed gas consumption.
2. Emission reduction and safety effect
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.
Using natural gas for carburizing heat treatment reduces carbon dioxide emissions by around 25% compared to propane gas.
Additionally, in the event of a leak, natural gas will not accumulate on the ground when it floats into the sky, which is unlikely 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 original propane gas in the controllable atmosphere heat treatment industry for carburizing.
Its work on transforming equipment and commissioning processes is small, which not only saves the cost of the enterprise's raw gas consumption, but also alleviates today's increasingly serious energy problem.
At the same time, it can reduce human and material costs for security protection, save planned land, and improve the company's civilization construction in the surrounding areas.
