I. Project Background and Equipment Specifications

This case study involves a fully automatic intelligent ion nitriding furnace, model WDL-IV-75A, employing a vertical single-bell-jar single-station structure. The equipment is capable of ion nitriding and soft ion nitriding of alloy steels, mold steels, and stainless steels.

The effective working zone of the furnace body measures 600 mm in diameter and 800 mm in height, with a maximum loading capacity of 400 kg. The common operating temperature range is 500 to 600°C, with a maximum operating temperature not exceeding 650°C. The ultimate vacuum degree is no greater than 6.7 Pa, the pressure rise rate is no greater than 5 Pa/h, and the time required to evacuate from atmospheric pressure to ultimate vacuum does not exceed 30 minutes. The furnace shell surface temperature rise does not exceed 55°C.

In the model designation, IV represents the fourth-generation digital control system, 75 represents the maximum average current of the pulse power supply at 75 amperes, and A represents the fully automatic intelligent type.

II. Matching Relationship Between Large-Volume Chamber and Power Supply Capacity

From the 30A model to the 75A model, the maximum average current of the pulse power supply increases by a factor of 1.5, while the effective chamber volume expands from approximately 28 liters to approximately 226 liters, a factor of about 8. This non-proportional change reveals a design principle of ion nitriding equipment: expansion of chamber volume does not require the power supply current to increase proportionally with volume, because the heating power for ion nitriding depends primarily on the workpiece surface area and the required current density, rather than the chamber spatial volume.

The abnormal glow discharge current density range given in the proposal is 0.1 to 5 mA/cm². Based on this range, for a workpiece combination with a loading capacity of 400 kg and a relatively large surface area, the maximum average current of 75 A provides a clear design margin for the upper limit of supportable workpiece surface area. Meanwhile, the pulse power supply output voltage is continuously adjustable from 0 to 800 V with no sudden jump above 200 V, and the duty cycle is continuously adjustable within the range of 10% to 85%.

III. Duty Cycle Adjustment Logic of the Pulse Power Supply

Gas discharge, as a power source, presents a load with negative resistance characteristics. The workpiece, acting as the cathode, has its surface shape and stacking method both affecting the surface current density. Ion nitriding has a minimum threshold condition: to satisfy the current density required for uniform glow coverage, the heating power may exceed the power required to maintain workpiece temperature. This contradiction is particularly prominent for thin-walled, lightweight workpieces.

After the pulse power supply introduces an adjustable duty cycle, the average heating power equals the peak voltage multiplied by current, then multiplied by the duty cycle. By reducing the duty cycle, the average heating power can be lowered while maintaining sufficient peak voltage to sustain glow coverage, thereby resolving the aforementioned contradiction. This mechanism is the core reason why this series of equipment can simultaneously process heavy workpieces and thin, lightweight workpieces.

The implementation path for fast arc extinguishing is as follows: utilizing the principle of electromagnetic induction in the discharge circuit to rapidly detect the arcing signal, then immediately controlling the high-speed electronic switch of the main current circuit through high-speed digital logic devices, extinguishing the arc within 2 μs.

IV. Thermal Insulation and Temperature Uniformity Design of the Large-Volume Furnace Body

The bell cylinder adopts a water-cooled double-wall structure and multi-channel water cooling system to ensure furnace temperature uniformity. The inner insulating liner of the cylinder is made of stainless steel, while the inner and outer cylinder walls are made of high-quality carbon steel plates.

The furnace body is equipped with two layers of 304 stainless steel heat shields, each layer 2 mm thick, and two layers of 304 stainless steel heat shields are also installed on the upper part of the furnace bottom plate. Compared with the six-layer heat shield configuration of the previous case, this model has fewer heat shield layers. This difference stems from the change in the ratio of chamber volume to surface area – a large-volume chamber has a relatively smaller heat dissipation area, and the heat loss proportion decreases accordingly, so the number of insulating layers can be appropriately reduced while ensuring long-term high-temperature operation.

The cathode plate is supported on the furnace base by three sets of high-quality cast mica, with adjustable height. The cathode plate is made of carbon steel plate with a thickness of no less than 30 mm; the material tray fixture frame in contact with the workpiece is also made of high-quality carbon steel. The cathode connection device is designed as a rigid connection to avoid overheating problems caused by loose quick-connect couplings. The cathode structure includes cathode PTFE insulation, cathode insulating mica, cathode conductor connection socket, cathode connection fitting, and M10 stud bolt fixed connection cable lug.

Two thermocouples are introduced from the furnace bottom plate and positioned in the middle of the cathode plate to simulate the temperature measuring head. One is used for temperature control by the ion heating pulse power supply, and the other is connected to the alarm instrument for multi-directional temperature monitoring. Three observation windows are arranged on the outer circumference of the furnace shell for multi-directional observation of conditions inside the furnace.

V. Vacuum System and Pressure Closed-Loop Control

The vacuum system is used for pre-vacuuming of the furnace body and for atmosphere replacement and exhaust gas discharge during the nitriding process. The pump set consists of one 2X-15 rotary vane pump, one electromagnetic vacuum pump with an inflation valve, and one high-vacuum butterfly valve. The rotary vane pump is a Guangdong Xunda 2X series unit, resistant to process gas corrosion, controlled by a frequency converter, and equipped with a GI-50 high-vacuum butterfly valve as the main control valve.

The pressure closed-loop control principle is as follows: the pressure transmitter measures furnace pressure and outputs a standard current signal to the digital controller. The controller performs PID calculations based on the set pressure value and real-time sampled values, then outputs a standard current signal to the frequency converter. By adjusting the vacuum pump motor speed, the pumping speed is adjusted, thereby keeping the furnace pressure constant at the process set value. This control method achieves a pressure control accuracy of ±1 Pa. The combined use of the vacuum pump and frequency converter greatly reduces mechanical wear of the pump body during startup, and long-term low-speed operation can significantly extend service life.

Vacuum measurement employs a ZJ-1C type capacitive diaphragm absolute pressure transmitter, with a range of 0 to 1000 Pa, output of 4 to 20 mA, and power supply of 15 to 30 VDC. This vacuum gauge is not affected by the type and composition of the measured medium, and the measured value is displayed by a digital display instrument and participates in furnace pressure control.

VI. Gas Distribution and Cooling Water Systems

The gas distribution system is used to control the introduction of process gas into the furnace. Gas sources include nitrogen, hydrogen, and other process gases, provided by the user. The gases pass through mass flow meters and are then introduced into the furnace body through a dedicated mixing tank according to the process formula. This model is equipped with two dedicated mass flow meters, controlling nitrogen and hydrogen flow separately – this is the main difference from the single flow meter configuration of the 30A model, corresponding to the higher requirements for atmosphere composition adjustment precision in a large-volume chamber.

The mass flow meters are Japanese HORIBA S500 series units, with a response time of less than 0.3 ±2% seconds from fully closed state to reaching the setpoint when control begins and when the setpoint is changed during control. The control range covers 1 to 100% full scale, operating from a single universal DC24V power supply, with the internal power circuit isolated from input and output circuits.

Cooling water requirements: inlet water temperature not exceeding 31°C, inlet water pressure of 0.15 to 0.25 MPa, and total flow rate of 20 m³/h. Each cooling water branch is equipped with separate inlet and outlet valves. All cooling water pipes, valves, and fittings attached to the equipment are made of stainless steel. An electrical contact pressure gauge is installed on the main inlet pipe for alarms in case of abnormal inlet pressure. A backup water inlet valve is installed at the main water inlet pipe, connected to the user's tap water pipe, so that the furnace body can be cooled by tap water in the event of an unexpected power outage or water outage.

VII. Three-Parameter Closed-Loop Architecture of the Control System

The control system uses an FX3G series programmable logic controller and a Kunlun Tongtai TPC1231Ni touch screen as the main control center. The PLC has a program memory capacity of 32K steps, a basic instruction processing speed of 0.21 μs, and two high-speed communication interfaces (RS422 and USB). The touch screen is a 12-inch TFT LCD with a resolution of 1920×1080, four-wire resistive touch, a main frequency of 1 GHz, and pre-installed McgsPro configuration software.

The system adopts a multi-task centralized management and distributed control fully digital architecture, capable of simultaneously achieving closed-loop control of three key parameters in ion nitriding: temperature control accuracy of ±1°C, pressure control accuracy of ±1 Pa, and flow control accuracy of ±0.5%.

Temperature control employs Japanese Shimaden SRS13 series digital controllers with PID regulation, forming a temperature closed-loop regulation system with the temperature control circuit and thermocouple. Pressure control also employs Shimaden SRS13 digital controllers, forming a pressure closed-loop regulation system with a frequency converter and pressure sensor.

In automatic operation mode, the system automatically starts the maintaining pump and pre-evacuation pump, opens the baffle valve and solenoid valve to extract a vacuum. Once the vacuum reaches the working pressure for glow discharge activation, the glow discharge voltage and resistance-assisted heating are automatically activated to perform the initial workpiece arcing and cleaning process. After arcing ends, the system automatically enters the heating stage, automatically adjusting flow rate, pressure, voltage, duty cycle, and current according to the process curve design steps. When the temperature reaches the process temperature, the system enters the temperature maintenance stage, maintaining according to the set process holding time. After the holding period, the system shuts down the vacuum pump and all valves on the extraction pipeline. Once the furnace temperature drops to the set temperature, all switches automatically turn off, and the system enters a stop state.