A workshop in our company is equipped with a variety of heat treatment systems, including Ipsen’s pusher furnace, Ai Xiulin multi-purpose furnace, circular rotary hearth furnace, and secondary limited quenching line. These systems are responsible for carrying out the company’s heat treatment processes. The temperature control of the atmosphere furnaces and the accuracy and stability of the carbon control system directly impact product quality. Thermocouple verification and temperature control instrument checks are handled by the company's inspection department, while the carbon control system is managed internally by the workshop. This article outlines the procedures for completing carbon calibration according to established protocols and provides guidance on quickly addressing carbon control system failures during production.
**1. Carbon Calibration Steps and Precautions**
After the furnace has completed the atmosphere recovery process, it is ready for carbon fixation. Based on our company’s product process requirements, the multi-purpose furnace uses two-point carbon correction, while the continuous furnace uses a single-point correction (taking the multi-purpose furnace as an example).
(1) The first carbon setting target is 850°C with a carbon potential of 0.85%. Once the target is reached, the carbon potential should stabilize for about one hour.
The carbon fixation method used is the "steel foil determination of carbon potential method." Our company currently uses steel foils weighing approximately 100mg and with a thickness of 0.02mm. The electronic balance model is FPG1.0, which measures between 95-105mg with a precision of 0.01mg. These technical specifications are outlined in JB/T 10312-2011, the national standard for determining carbon potential using steel foil.
Precautions during the process include: handling the foil carefully with tweezers to avoid damage; ensuring the balance is warmed up and calibrated before use; rolling the steel foil into a cylinder and placing it at the front end of the carbon rod without making it too loose; checking the oxygen probe purge cycle before carbon fixation and closing the flowmeter during the process to ensure accurate readings; blowing out any carbon black buildup in the fixed carbon hole using furnace pressure to prevent contamination; and avoiding oxidation of the steel foil, which should remain bright. If it turns blue, the process must be repeated.
The carbon control instrument (European 2604) is then adjusted based on the carbon measurement results. For example, if the set temperature is 850°C with a carbon potential of 0.85%, and the actual reading from the Aichelin FOCOS PC program shows 0.9% with a CO content of 20%, the oxygen probe millivolt value is calculated as 1123 mV. Adjusting the carbon potential to 0.9% gives a millivolt value of 1127 mV, resulting in a 4 mV difference. Enter the “LEVEL3†menu on the 2604 meter, navigate to “ZIRCONIAPROBE,†and adjust the “probe offset†parameter accordingly. After correction, the measured and displayed values should be within ±0.03%.
(2) The second carbon setting target is 920°C with a carbon potential of 1.25%. Repeat the same steps, adjusting the parameters in the 2604 meter under the “INPUT OPERS†menu. For example, if the measured value is 1.27%, the input is adjusted to 1.25%, and the output becomes 1.27%. Multi-point calibration can be performed as needed.
[Image: An image of a carbon calibration setup is included here.]
**2. Examples of Carbon Control Failures**
(1) Fault Phenomenon 1: After completing carbon adjustment, the offset increases rapidly after rechecking. First, check for carbon deposits inside the furnace. If present, the carbon control accuracy may be lost. If no deposit is found and the furnace is well-sealed, proceed with the following steps:
Step 1: Check for carbon deposits on the oxygen probe head. Open the manual purge switch on the MP270 panel, set the purge gas flow to 200–300 L/h, and observe the temperature rise and millivolt drop. If the millivolt value drops below 700–800 mV within 1 minute, the probe is clean. Otherwise, repeat purging until the value decreases. Be cautious not to overheat the probe, and turn off the purge when the temperature reaches 960–980°C to extend its life.
Step 2: Check the reference gas flow. The reference gas flow for our oxygen probe is typically 6–10 L/h. Unplug the reference gas tube and insert it into a water cup to check for bubbles. If no bubbles appear, check for leaks in the line.
Step 3: Check the 2604 meter’s input module. The oxygen probe data is sent to the 6-channel dual-probe input module. Use the “MODULE IO†menu to compare the measured voltage with the multimeter reading. If the deviation exceeds 10 mV, replace the module.
Step 4: Leak check. Turn off the reference gas for 30 seconds and observe the millivolt drop. If it exceeds 5 mV, the probe may be damaged and needs replacement.
Step 5: Check the internal resistance of the oxygen probe. At 850°C with a carbon potential of 0.85%, measure the probe’s output voltage and the voltage across a 10kΩ resistor. If E0/Es > 6, the internal resistance is above 50kΩ. Monitor this monthly, as a new probe should have less than 1kΩ.
(2) Fault Phenomenon 2: The carburizing layer is not deep enough despite normal host computer monitoring. This could be due to incorrect methanol or nitrogen flow ratios. If the methanol flow is low, the CO value will be too low, leading to an overestimated carbon potential. Regular operator adjustments are crucial to maintain product quality.
(3) Fault Phenomenon 3: Carbon buildup causes system failure. For example, if the acetone solenoid valve doesn’t close properly, excess acetone enters the furnace. Even though the carbon potential curve looks normal, the actual value may be higher, leading to excessive carbides. To fix this, increase inspections and consider connecting two solenoid valves in series to improve reliability.
(4) Fault Phenomenon 4: The 2604 meter displays “Sbr.†This occurs when the oxygen probe is open. Navigate to the “ZIRCONIA PROBE†menu, change the Temp Src parameter to PVIn.Val, and recalibrate the system.
**3. Maintenance of the Carbon Control System**
(1) Regular carbon burning improves measurement accuracy and extends the life of the oxygen probe and equipment.
(2) Perform regular carbon calibration to ensure system accuracy and support product quality.
(3) Replace the oxygen probe strictly according to procedures to avoid unnecessary damage.
(4) Operators should regularly inspect and adjust process parameters such as reference gas, purge gas, methanol, nitrogen, and acetone.
(5) Equipment managers should test the probe’s internal resistance and verify the accuracy of the instrument input module for preventive maintenance.
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