The nitriding furnace technology was analyzed and compared in terms of layout, furnace charging method, cooling method, temperature control scheme, nitriding process scheme and environmental protection and energy saving characteristics, and pointed out the advantages and disadvantages of various schemes.
Nitriding refers to a heat treatment process in which a nitrogen atom is infiltrated into the surface layer of a workpiece in a certain medium at a certain temperature. This process can significantly improve the surface hardness of the steel workpiece, wear resistance, fatigue resistance and corrosion resistance, the power, machine tools, petrochemical, machinery, tooling, etc. is widely used. At present, there are three kinds of domestic nitriding furnaces: well type furnace, hood type furnace and horizontal type furnace. In this paper, the arrangement, cooling method, nitriding process and control strategy of different furnace types are analyzed and compared.First, the arrangement of comparison
The domestic nitriding furnace is currently commonly used in well furnaces. The main structural components of the pit furnace consist of the furnace shell, furnace lining, heating elements, corrugated muffle, corrugated air duct, furnace cover and other auxiliary facilities. The furnace body is arranged underground, saving space, but the maintenance cost is high. The main structural parts of the hood furnace are composed of a heating cover, an inner cover and a guide tube, a furnace table and a base, a cooling cover and other auxiliary facilities, and are arranged above the ground, and the furnace body has a strong bearing capacity. The horizontal furnace adopts the drawer type furnace charging method, and the maximum amount of the furnace is not more than 2.5 tons, but it is easy to realize the automatic operation of loading and unloading the furnace. From this point of view, the advantages and disadvantages of different furnace types are more obvious, the specific furnace type selection needs to be determined according to the use requirements.
Second, the comparison of furnace loading methods
The pit furnace must first load the mold on the ground on the rack, and then hoist the rack as a whole into the well. The hood type furnace is operated on the ground. It only needs to use a crane or a forklift to lift the mold one by one onto the rack, and then cover the nitriding furnace cover. The furnace is simpler and more convenient than the well type furnace. The horizontal furnace can realize the automatic loading and unloading furnace by mechanization. It only needs to load or lift the workpiece out of the workbench. The operation is the easiest, but the auxiliary facilities occupy a small area.
Third, the comparison of cooling methods
The domestic pit furnace mainly adopts the furnace cooling mode, stops heating and heat preservation after the completion of nitriding, naturally cools with the furnace body, and has no nitrogen protection measures during the cooling process, the cooling efficiency is low, and it is easy to occur during the cooling process. The oxidation phenomenon affects the hardness and wear resistance of the mold surface and reduces the life of the mold after nitriding. In the cooling process, the cover furnace is first filled with nitrogen gas to protect the inner cover, and then the cooling fan cools the inner cover along the tangential line. When the temperature is lowered to 200 ° C, the inner cover is cooled to a specified temperature by cooling water, and the cooling is performed. More efficient. In the case of the same weight of the same workpiece, the pit furnace requires 48 hours of cooling, and the hood furnace requires 10 hours of cooling. Due to the small amount of furnace loading, horizontal furnaces use nitrogen-filled direct cooling, which has the highest cooling efficiency and best protection for workpieces.
Fourth, the temperature control program analysis and comparison
(1) Using classical control theory
The scheme needs to identify the equation of state of the entire nitriding system, and analyze the equations of state for different heating regions by appropriate mathematical methods. For the system with multiple input and multiple output of the furnace, the mutual coupling of each input has great influence. It is necessary to obtain the equation of state for each temperature region, and decoupling the coupling effects between the segments to find the segments. Control decisions in the temperature zone. At present, there are only two decouplings that are relatively successful, so the specific implementation of this scheme is very difficult.
(B) using traditional PID control
Many existing nitriding furnaces use this control strategy for segmentation control. The actual operation proves that this control scheme can not meet the requirements of both overshoot and steady state error, and because of the mutual coupling of multiple temperature zones. It is very difficult to set the PID parameters.
(C) the use of intelligent temperature controller
The temperature controller of this kind of nitriding furnace adopts microprocessor digital control and adopts two-degree-of-freedom PID control law. It can automatically identify the system online, automatically perform optimal PID parameter tuning, and provide two-stage target value migration function. Automatic adjustment of the two target values, with a variety of self-diagnosis and alarm functions, can be adjusted to a variety of input forms as needed.
From the above analysis, the intelligent temperature controller has better control performance than the other two control schemes.
V. Comparison of nitriding processes
Domestic manufacturers mainly control the ammonia decomposition rate for nitriding. The ammonia decomposition rate is generally controlled at 20% to 41%. It belongs to the input terminal control and can only control the decomposition index of the input ammonia gas, and cannot control the specific nitrogen atom absorption of the workpiece. Therefore, the control precision is low, and defects such as unstable depth of the nitride layer or low hardness of the nitride layer are apt to occur during nitriding. Different workpiece weights, surface areas and materials have different ammonia decomposition rates. It is difficult to establish a more accurate mathematical model, mainly relying on empirical accumulation to determine various process parameters.
The hood furnace and the horizontal furnace mostly adopt the German stange control system, the expert system is used to determine the process parameters, the hydrogen probe is used to accurately control the nitrogen potential, and various mass flow meters are used to control the flow of various process gases, so that the nitriding effect can be precisely controlled. It belongs to the consumption end control. In order to detect Kn and Ko in the furnace, a double-line hydrogen-oxygen probe is installed on the nitriding furnace. During the heat treatment process, the hydrogen probe delivers the directly measured volume concentration of hydrogen and oxygen gas to the intelligent controller, and calculates the current nitrogen potential and oxygen potential through the mathematical model in the SE607 intelligent controller, which is set according to Kn and Ko. The value, mass flow control system inputs the ammonia mass flow rate (in the standard state), the mass flow rate after ammonia decomposition, and the air mass flow rate into the furnace according to the required ratio. Thereby, automatic control of the dynamic process Kn and Ko values ​​is realized. The depth deviation of the nitride layer can be controlled to be ≤610%, the white bright layer depth is below 0.02 mm, and the nitriding quality is stable.
Sixth, energy saving comparison
The hood type furnace is equipped with a heating hood and a cooling hood respectively. After the nitriding heating and heat preservation process is completed, the high temperature heating hood can be hoisted to another hood type furnace and heated by the injury to maximize the utilization of waste heat, and the well type furnace The part of the residual heat cannot be recycled due to the limitation of the structure of the furnace body, so the hood type furnace is much better than the well type furnace and the horizontal type furnace in terms of energy saving.
Seven, environmental comparison
The ammonia gas added during the nitriding process cannot be completely consumed. Therefore, the treatment of nitriding exhaust gas is also very important. At present, there is no strict distinction between the furnace types in the treatment of exhaust gas. The main treatment methods include cracking combustion and water absorption. A small number of manufacturers use high temperature pyrolysis combustion to treat the exhaust gas generated during the nitriding process. The exhaust gas combustion system includes an ignition tube, a shut-off valve, an igniter, a ball valve and the like. The igniter can be automatically and manually controlled by a selection switch, and the igniting gas is used for civil liquefied gas. The high temperature generated by the exhaust gas discharged from the furnace completely decomposes the residual ammonia in the furnace, ensuring that the gas discharged into the atmosphere is a harmless gas, and can ensure the safety and environmental protection requirements in the workshop. At the same time, the furnace is equipped with a complete and feasible nitrogen protection system: in the event of an electrical failure, the protective gas can not be supplied in time, and the safe nitrogen can automatically flush the furnace to eliminate the potential danger of the furnace gas explosion and prevent the workpiece from oxidizing. Most domestic manufacturers use water to treat waste gas, that is, the waste gas is passed into the water, and the water is used to absorb the waste gas to form ammonia water. The ammonia water is discharged after being diluted and treated. This method is not thorough and easy to form secondary pollution, which is sometimes smelled in the workshop. The pungent smell of ammonia damages the operator's respiratory tract.
Eight, capacity comparison
Since the hood type nitriding furnace adopts a hood structure, after the nitriding treatment is finished, the heating hood is removed, replaced with a specially designed cooling hood, protected by nitrogen gas, cooled by a strong exhaust air to below 250 ° C, and sprayed by a water spray system. Water cooling, cooling time can be significantly shortened, greatly shortening the cooling process cycle; at the same time, the high temperature heating cover can be used immediately on other furnaces, improving the utilization efficiency of waste heat, saving energy and shortening the heating process cycle, so the overall production efficiency of the equipment Greatly improved, and the production capacity is large. However, if the number of hood furnaces is small or a single furnace configuration, the advantage is not obvious.
Although the combined nitriding furnace also adopts forced air cooling, due to the cooling method with the furnace, the furnace body has a large heat storage capacity, the cooling time is still relatively long, and the production efficiency is lower than that of the hood furnace.
Nine, conclusion
Through the above comparison, various types of nitriding furnaces have great differences in furnace type, operation mode, control means, environmental protection and energy saving. The nitrogen potential control method and the high-temperature pyrolysis combustion method of exhaust gas have great technical advantages, and in future technical applications. Will be gradually promoted. In terms of specific furnace type selection, the well type furnace is more suitable for the production capacity, the nitriding effect is not high, and the number of furnaces is small, the cost performance advantage is obvious; and the hood furnace is more suitable for large-scale processing, and the number of furnaces is large. Production efficiency and energy saving advantages are obvious; in short, the choice of specific furnace type should be determined in combination with the actual production situation.
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