Direct atmosphere heat treatment
Before the development of carrier gas technology and the introduction of carbon potential control technology, the direct introduction of fuel gas and air into the working furnace for carburizing is still a major gas carburizing method, but without control, production is only based on experience. Until the early 1980s, Gohring et al. re-tested the direct atmosphere. The results show that although the composition of the direct atmosphere is far from reaching the thermodynamic equilibrium, although the atmosphere contains higher methane (CH4), this non-equilibrium atmosphere is still controllable. In the carbon transfer process, the main role is still the decomposition of CO, namely CO-→COad-→[C]+Oad
Carbon activity can still be expressed as:
Since then, this technology has attracted attention in many countries and has been conducting experimental research. So far, the technology has begun to apply industrial production, and gradually shows its advantages of energy saving and high efficiency.
1. Characteristics of direct-fired atmospheres (1) Types of fuels Industrially applied fuels include gases and liquids, typically the following:
1 natural gas; 2 propane; 3 acetone; 4 isopropanol.
(2) Gas composition The composition of the direct atmosphere is related to the type of fuel, the ratio of fuel to air, and the temperature inside the furnace.
Table 1 shows the composition of the atmosphere at 950 ° C, 1.15% carbon potential, and 850 ° C, 0.90% carbon potential, respectively.
Table 1 Direct-type atmosphere composition (volume fraction)
The straight-type atmosphere contains a high content of CH4, and the composition of the atmosphere, especially the CH4 content, is greatly affected by temperature. Direct gas atmospheres of natural gas/air type have not been practically applied at temperatures below 850 °C due to excessive CH4 content.
(3) Carbon transfer coefficient 950 ° C CO and H 2 content and carbon transfer coefficient () in different types of atmosphere are shown in Table 2.
Table 2 CO and H2 content in the atmosphere and carbon transfer coefficient
1 The percentage of gas is the volume fraction.
The direct atmosphere has a higher carbon transfer coefficient than other atmosphere types, which means that under the same carbon potential atmosphere, the direct atmosphere can obtain a higher carbon concentration on the surface of the workpiece and obtain a higher carbon in the surface layer. The concentration gradient increases the carburization rate and increases the depth of the layer. In the case of continuous or pusher furnace gas carburizing, when the pushing cycle is 15 min, the carburizing atmosphere is used for carburizing, and the effective layer depth of the workpiece is 1.09 mm. When the natural gas/air type direct atmosphere is used for carburizing, the effective layer depth of the workpiece can reach 1.17 mm, which is 7.3%. When the effective layer depth is required to be the same, the pushing cycle of the direct-fired atmosphere carburizing can be reduced to 14min, saving about 400h per year.
(4) Carbon availability The direct atmosphere has a higher carbon availability than the conventional endothermic atmosphere, which is mainly due to the high CH4 content in the direct atmosphere. High carbon availability ensures carburization uniformity, especially in the case of complex part shapes and high workpiece loading density, and poor atmospheric circulation. The available amount of carbon is the determining factor for carburization uniformity. The test proves that when the sealed box type multi-purpose furnace adopts the direct-fired atmosphere carburizing, the large difference in the carburizing depth zui is about 0.12 mm.
High CH4 and high carbon availability contribute to rapid recovery of the atmosphere and rapid adjustment of the carbon potential. When a sealed box type multi-purpose furnace with a furnace capacity of 1t adopts a direct-fired atmosphere, the settling time of re-opening does not exceed 1.5 hours, while in the case of an endothermic atmosphere, it usually takes more than 3 hours.
2. Carbon potential control in a straight-through atmosphere The test confirmed that natural gas/air, acetone/air and isopropanol/air have good controllability in the range of 850-1000 °C. The propane/air type atmosphere has good controllability in a lower temperature range (800 to 880 ° C), but has a tendency to produce carbon black under conventional temperature carburization.
The direct-fired atmosphere carbon potential control system is shown in Figure 1, using O2-CO-t three-parameter microcomputer carbon potential control.
Generally, the adjustment of the air flow rate has a better carbon potential control effect than the adjustment of the fuel flow rate. On the one hand, the composition change of the atmosphere is sensitive to the air flow rate; on the other hand, when the fuel flow rate is adjusted, the risk of generating carbon black is greater.
To control the carbon atmosphere of the direct atmosphere, a special oxygen probe is required, and its structure is shown in Fig. 2. Since the direct atmosphere contains a high CH4, the electrode material in the conventional oxygen probe (such as a platinum electrode) has a strong catalytic effect on the decomposition of CH4, resulting in an excessive output millivolt value. Special probes use special alloy electrodes and compensation electrodes.
When using the oxygen probe for direct atmosphere carbon potential control, the carbon black precipitated at the probe end should be periodically purged.
3. Direct-type atmosphere requirements for equipment The direct-fire atmosphere is suitable for sealed box type multi-purpose furnaces, pit furnaces, continuous furnaces and other heat treatment furnace types. However, in the case of a direct-fired atmosphere, the design of the heat treatment furnace should consider the following factors: First, it should have sufficient heating capacity to meet the additional energy consumed by the heat absorption of the fuel and air; second, the circulation of the furnace atmosphere should be strengthened. And choose the appropriate atmosphere injection structure and population location to ensure that the atmosphere entering the furnace is fully mixed; the third is to ensure that the atmosphere has been more fully reacted before touching the workpiece.
An intake structure for continuous protection is shown as 囹3. The air with higher pressure enters from the middle thin tube, the ends of the two tubes are about 300mm apart, and the tube ends are sealed, forcing the airflow to change direction for the purpose of thorough mixing.
Using the guiding action of the muffle in the sealed box furnace (Fig. 1), the atmosphere entering the furnace is thoroughly mixed, then passed through the heating element and then contacted to the workpiece.
The Beijing Institute of Mechanical and Electrical Engineering has extended the residence time of the atmosphere in the furnace by controlling the atmospheric flow rate on the ordinary multi-purpose furnace, so that the reaction can be fully carried out, and the carbon potential control and carburization effect are also achieved.
4. Atmosphere consumption and economy The fuel consumption of the direct atmosphere is significantly reduced. Below are a few examples from actual production.
Furnace size is 760mm × 1220mm × 760mm, multi-purpose furnace with a capacity of 1000kg. When using the traditional endothermic atmosphere, it needs to consume 12m3/h of the suction efficiency and 1.0m3/h of natural gas under standard conditions. . When a direct atmosphere is used, only 1.3 m3/h of natural gas or 0.7 m3/h of propane or 1.0 L/h of acetone is required, and the raw material consumption is reduced by 70% to 75%.
The rotary hearth furnace with a diameter of about 2 m is heat-treated to protect heat. When an endothermic atmosphere is used, the atmosphere consumption (in the state of the standard) is 15 m3/h. When using a natural gas/air direct atmosphere, the natural gas consumption is 1.5m3/h, saving 91%.
The Beijing Institute of Mechanical and Electrical Engineering conducted direct gas gas carburizing test on a multi-purpose furnace. Compared with the endothermic atmosphere, it can save 89% of raw material gas and reduce production cost by 49%.
5. The limitations of the direct atmosphere In summary, the direct atmosphere has obvious advantages in many aspects, but it is not difficult to see that under 850 °C, the direct atmosphere (especially natural gas / air type) It contains too much CH4, and the CO content is also lowered due to the hindered reaction of CH4 and O2, and its application is limited. Many processes, such as carbonitriding, protective heating, and the temperature of the workpiece into the furnace are about 850 °C. In order to solve the problem of gas supply below 850 °C, the German Ipsen company developed the atmosphere system shown in Figure 4. That is, the methanol supply system is added to the original system, and the supply of methanol is controlled by detecting the CO content in the furnace.
6. Fuel/CO2 type direct atmosphere When CO is used instead of air to prepare a direct atmosphere, the CO in the atmosphere will be greatly improved. The transfer coefficient and carburization speed will also increase.
Before the development of carrier gas technology and the introduction of carbon potential control technology, the direct introduction of fuel gas and air into the working furnace for carburizing is still a major gas carburizing method, but without control, production is only based on experience. Until the early 1980s, Gohring et al. re-tested the direct atmosphere. The results show that although the composition of the direct atmosphere is far from reaching the thermodynamic equilibrium, although the atmosphere contains higher methane (CH4), this non-equilibrium atmosphere is still controllable. In the carbon transfer process, the main role is still the decomposition of CO, namely CO-→COad-→[C]+Oad
Carbon activity can still be expressed as:
Since then, this technology has attracted attention in many countries and has been conducting experimental research. So far, the technology has begun to apply industrial production, and gradually shows its advantages of energy saving and high efficiency.
1. Characteristics of direct-fired atmospheres (1) Types of fuels Industrially applied fuels include gases and liquids, typically the following:
1 natural gas; 2 propane; 3 acetone; 4 isopropanol.
(2) Gas composition The composition of the direct atmosphere is related to the type of fuel, the ratio of fuel to air, and the temperature inside the furnace.
Table 1 shows the composition of the atmosphere at 950 ° C, 1.15% carbon potential, and 850 ° C, 0.90% carbon potential, respectively.
Table 1 Direct-type atmosphere composition (volume fraction)
The straight-type atmosphere contains a high content of CH4, and the composition of the atmosphere, especially the CH4 content, is greatly affected by temperature. Direct gas atmospheres of natural gas/air type have not been practically applied at temperatures below 850 °C due to excessive CH4 content.
(3) Carbon transfer coefficient 950 ° C CO and H 2 content and carbon transfer coefficient () in different types of atmosphere are shown in Table 2.
Table 2 CO and H2 content in the atmosphere and carbon transfer coefficient
1 The percentage of gas is the volume fraction.
The direct atmosphere has a higher carbon transfer coefficient than other atmosphere types, which means that under the same carbon potential atmosphere, the direct atmosphere can obtain a higher carbon concentration on the surface of the workpiece and obtain a higher carbon in the surface layer. The concentration gradient increases the carburization rate and increases the depth of the layer. In the case of continuous or pusher furnace gas carburizing, when the pushing cycle is 15 min, the carburizing atmosphere is used for carburizing, and the effective layer depth of the workpiece is 1.09 mm. When the natural gas/air type direct atmosphere is used for carburizing, the effective layer depth of the workpiece can reach 1.17 mm, which is 7.3%. When the effective layer depth is required to be the same, the pushing cycle of the direct-fired atmosphere carburizing can be reduced to 14min, saving about 400h per year.
(4) Carbon availability The direct atmosphere has a higher carbon availability than the conventional endothermic atmosphere, which is mainly due to the high CH4 content in the direct atmosphere. High carbon availability ensures carburization uniformity, especially in the case of complex part shapes and high workpiece loading density, and poor atmospheric circulation. The available amount of carbon is the determining factor for carburization uniformity. The test proves that when the sealed box type multi-purpose furnace adopts the direct-fired atmosphere carburizing, the large difference in the carburizing depth zui is about 0.12 mm.
High CH4 and high carbon availability contribute to rapid recovery of the atmosphere and rapid adjustment of the carbon potential. When a sealed box type multi-purpose furnace with a furnace capacity of 1t adopts a direct-fired atmosphere, the settling time of re-opening does not exceed 1.5 hours, while in the case of an endothermic atmosphere, it usually takes more than 3 hours.
2. Carbon potential control in a straight-through atmosphere The test confirmed that natural gas/air, acetone/air and isopropanol/air have good controllability in the range of 850-1000 °C. The propane/air type atmosphere has good controllability in a lower temperature range (800 to 880 ° C), but has a tendency to produce carbon black under conventional temperature carburization.
The direct-fired atmosphere carbon potential control system is shown in Figure 1, using O2-CO-t three-parameter microcomputer carbon potential control.
Generally, the adjustment of the air flow rate has a better carbon potential control effect than the adjustment of the fuel flow rate. On the one hand, the composition change of the atmosphere is sensitive to the air flow rate; on the other hand, when the fuel flow rate is adjusted, the risk of generating carbon black is greater.
To control the carbon atmosphere of the direct atmosphere, a special oxygen probe is required, and its structure is shown in Fig. 2. Since the direct atmosphere contains a high CH4, the electrode material in the conventional oxygen probe (such as a platinum electrode) has a strong catalytic effect on the decomposition of CH4, resulting in an excessive output millivolt value. Special probes use special alloy electrodes and compensation electrodes.
When using the oxygen probe for direct atmosphere carbon potential control, the carbon black precipitated at the probe end should be periodically purged.
3. Direct-type atmosphere requirements for equipment The direct-fire atmosphere is suitable for sealed box type multi-purpose furnaces, pit furnaces, continuous furnaces and other heat treatment furnace types. However, in the case of a direct-fired atmosphere, the design of the heat treatment furnace should consider the following factors: First, it should have sufficient heating capacity to meet the additional energy consumed by the heat absorption of the fuel and air; second, the circulation of the furnace atmosphere should be strengthened. And choose the appropriate atmosphere injection structure and population location to ensure that the atmosphere entering the furnace is fully mixed; the third is to ensure that the atmosphere has been more fully reacted before touching the workpiece.
An intake structure for continuous protection is shown as 囹3. The air with higher pressure enters from the middle thin tube, the ends of the two tubes are about 300mm apart, and the tube ends are sealed, forcing the airflow to change direction for the purpose of thorough mixing.
Using the guiding action of the muffle in the sealed box furnace (Fig. 1), the atmosphere entering the furnace is thoroughly mixed, then passed through the heating element and then contacted to the workpiece.
The Beijing Institute of Mechanical and Electrical Engineering has extended the residence time of the atmosphere in the furnace by controlling the atmospheric flow rate on the ordinary multi-purpose furnace, so that the reaction can be fully carried out, and the carbon potential control and carburization effect are also achieved.
4. Atmosphere consumption and economy The fuel consumption of the direct atmosphere is significantly reduced. Below are a few examples from actual production.
Furnace size is 760mm × 1220mm × 760mm, multi-purpose furnace with a capacity of 1000kg. When using the traditional endothermic atmosphere, it needs to consume 12m3/h of the suction efficiency and 1.0m3/h of natural gas under standard conditions. . When a direct atmosphere is used, only 1.3 m3/h of natural gas or 0.7 m3/h of propane or 1.0 L/h of acetone is required, and the raw material consumption is reduced by 70% to 75%.
The rotary hearth furnace with a diameter of about 2 m is heat-treated to protect heat. When an endothermic atmosphere is used, the atmosphere consumption (in the state of the standard) is 15 m3/h. When using a natural gas/air direct atmosphere, the natural gas consumption is 1.5m3/h, saving 91%.
The Beijing Institute of Mechanical and Electrical Engineering conducted direct gas gas carburizing test on a multi-purpose furnace. Compared with the endothermic atmosphere, it can save 89% of raw material gas and reduce production cost by 49%.
5. The limitations of the direct atmosphere In summary, the direct atmosphere has obvious advantages in many aspects, but it is not difficult to see that under 850 °C, the direct atmosphere (especially natural gas / air type) It contains too much CH4, and the CO content is also lowered due to the hindered reaction of CH4 and O2, and its application is limited. Many processes, such as carbonitriding, protective heating, and the temperature of the workpiece into the furnace are about 850 °C. In order to solve the problem of gas supply below 850 °C, the German Ipsen company developed the atmosphere system shown in Figure 4. That is, the methanol supply system is added to the original system, and the supply of methanol is controlled by detecting the CO content in the furnace.
6. Fuel/CO2 type direct atmosphere When CO is used instead of air to prepare a direct atmosphere, the CO in the atmosphere will be greatly improved. The transfer coefficient and carburization speed will also increase.
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