I. Project Background and Equipment Specifications
This case study involves a fully automatic intelligent ion nitriding furnace, model WDL-IV-30A, employing a vertical single-bell-jar single-station structure. The equipment is capable of ion nitriding and soft ion nitriding of titanium alloys, alloy steels, mold steels, and stainless steels.
The effective working zone of the furnace body measures 300 mm in diameter and 400 mm in height, with a maximum loading capacity of 100 kg. The common operating temperature range is 500 to 600°C, with a maximum operating temperature of 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 pump 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, 30 represents the maximum average current of the pulse power supply at 30 amperes, and A represents the fully automatic intelligent type.
II. Physical Constraints of the Ion Nitriding Process
Ion nitriding is conducted in a low-pressure gas discharge environment, and its volt-ampere characteristic curve is non-linear. Ion nitriding of metallic materials generally operates in the "abnormal discharge" segment of the volt-ampere characteristic curve, and the upper limit point G of this segment is a dangerous inflection point. Gas discharge, as a power source, presents a load with negative resistance characteristics. The workpiece, acting as the cathode, has its surface shape and surrounding environment (workpiece stacking) both affecting the surface current density.
These physical characteristics determine that the ion power supply must overcome three technical difficulties. The first is arc discharge: when the workpiece surface is not clean, a non-conductive oil film can easily form. When charge accumulates on the surface to a certain extent, it breaks down and forms a momentary high-current discharge, producing high-energy sparks. If not cut off quickly, this will cause power failure and workpiece damage. The second is the hollow cathode effect: when workpieces have holes or slots, gaps between workpieces, or uneven bends, a critical state of glow discharge overlap occurs at certain gas pressures. This manifests as uneven glow discharge brightness across the workpiece surface, corresponding to uneven surface current density, causing uneven temperature distribution. The third is the coupling relationship between current density and gas pressure: gas pressure affects glow thickness, which in turn affects current density – higher pressure produces a thinner glow, beneficial for temperature uniformity; however, when pressure is high enough to uniformly cover all groove and hole surfaces, the current will greatly exceed the power required for workpiece insulation.
III. Technical Response Logic of the Pulse Power Supply
The pulse power supply configured for this equipment has an output frequency of 20,000 Hz, a square wave output waveform, an output voltage continuously adjustable from 0 to 900 V with no sudden jump above 200 V, a duty cycle continuously adjustable within the range of 10% to 85%, a maximum pulse output average current of 30 A, and an arc extinguishing time of no more than 2 μs.
The duty cycle is defined as the ratio of on-time to pulse period. The average heating power equals the peak voltage multiplied by current, then multiplied by the duty cycle. The introduction of an adjustable duty cycle in the pulse power supply effectively resolves the contradiction between the average heating power and the threshold conditions required for workpiece processing – the essence of this contradiction being that, to satisfy the current density required for uniform glow coverage, the heating power may exceed the power required to maintain workpiece temperature, and duty cycle adjustment allows these two to be decoupled.
Fast arc extinguishing is achieved by utilizing the principle of electromagnetic induction in the discharge circuit to rapidly detect the arcing signal (with almost no delay), 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.
The application of the pulse power supply also yields several process effects: the absolute off-time of the pulse is relatively short, and after power is turned off, the concentration of charged particles does not immediately drop to zero but decreases slowly. Before it has decreased significantly, the next pulse arrives, resulting in a higher average electron concentration and increased ionization rate; the hollow cathode effect is effectively suppressed; and because the pulse power supply widens the adjustment range of nitriding process parameters, with each parameter independently adjustable, it can improve the uniformity of glow coverage on workpieces with complex surface shapes, thereby enhancing infiltration layer quality and surface smoothness.
IV. Three-Parameter Closed-Loop Control System
The control system uses a programmable logic controller and touch screen as the main control center. The PLC is an FX3G series unit with 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. Flow control is configured with one dedicated mass flow meter to control gas flow.
The pressure closed-loop implementation path 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 precision of ±1 Pa, while 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.
Flow control employs Japanese HORIBA S500 series mass flow meters, 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.
V. Furnace Body System Structural Features
The furnace body adopts a vertical bell-jar structure, consisting of two parts: the bell and the base. The bell cylinder adopts a water-cooled double-wall structure and multi-channel water cooling system to ensure furnace temperature uniformity. Six layers of 310S stainless steel heat shields are installed inside the cylinder, and two layers of stainless steel heat shields are installed on the upper part of the furnace base, enabling long-term high-temperature operation and significant energy savings. A layer of aluminum silicate insulation cotton with a thickness of 20 mm is added inside the insulation layer near the furnace wall.
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 310S stainless steel plate with a thickness of no less than 30 mm; the material tray fixture frame that contacts the workpiece is also made of 310S stainless steel. The cathode connection device is designed as a rigid connection to avoid overheating problems caused by loose quick-connect couplings.
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. The base center has a vacuum extraction port and a cathode inlet, connected by flanges, thick-walled stainless steel pipe, and stainless steel bellows, with sealing provided by high-temperature-resistant fluororubber sealing rings.
VI. Vacuum and Gas Systems
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 system is equipped with one 2X15 rotary vane pump as the vacuum unit. The pump set includes 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.
Vacuum measurement employs a ZJ-1C type capacitive diaphragm absolute pressure transmitter. This vacuum gauge is not affected by the type and composition of the measured medium, providing accurate and reliable measurement, with the measured value displayed by a digital display instrument and participating in furnace pressure control.
The gas distribution system is used to control the introduction of process gas into the furnace, consisting of a gas source, flow meter, and corresponding pipelines. Process gases (such as ammonia) are provided by the user, passing through a mass flow meter and then introduced into the furnace body through a dedicated mixing tank according to the process formula. Gas pipelines are made of stainless steel, with each gas line equipped with a pressure reducing valve, manual shut-off valve, filter, pressure gauge, and solenoid valve.
VII. Cooling Water System
The cooling water system adopts a circulating cooling method, with the water source provided by the cooling tower and water pump (buyer's responsibility). 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 for individual flow adjustment. 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, and the control system will take corresponding protective actions. A backup water inlet valve is installed at the main water inlet pipe, connected to the user's tap water pipe – in the event of an unexpected power outage or water outage, the furnace body can be cooled by tap water.
VIII. Safety Interlock and Alarm Logic
The system has alarm functions for over-temperature, over-pressure, thermocouple damage, water shortage, and phase loss, with both audible and visual alarms and control interface alarms, taking corresponding protective actions such as prohibiting power-on, prohibiting heating, cutting off or resuming heating according to specific problems.
The control system can realize self-diagnosis of equipment faults, prompting users to check and repair the corresponding fault points. When a fault occurs, the system automatically shuts off all valves. The vacuum system has sequential start and stop functions, equipped with a complete interlocking and mutual interlocking mechanism, with all valves being power-off shut-off type.
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