Common problems with muffle furnace tempering

Muffle furnace tempering is a key step in controlling material properties in heat treatment processes, but problems often arise in practical operations due to factors such as temperature control, atmosphere management, and cooling methods. The following are common problems and solutions, covering four categories: abnormal hardness, tissue defects, cracking risks, and operational errors:

1. Abnormal hardness issue
a. High hardness after tempering (insufficient tempering)
Phenomenon: The hardness of the workpiece is higher than the process requirements, for example, the hardness of GCr15 bearing steel after tempering is still greater than HRC65 (target HRC60-62).
Reason:
The tempering temperature is too low (such as setting 180 ℃ but actually only 160 ℃).
Insufficient insulation time (e.g. thin parts should be insulated for 2 hours but only 1 hour).
Poor uniformity of furnace temperature (local temperature below the set value).
Solution:
Re calibrate the thermocouple to ensure accurate temperature measurement.
Extend the insulation time by 30-60 minutes, or increase the tempering temperature by 10-20 ℃.
Check the furnace airflow and optimize the placement of workpieces (avoiding the direct radiation area of heating elements).
b. Low hardness after tempering (over tempering)
Phenomenon: The hardness of the workpiece is lower than the standard, for example, the hardness of 45 steel shaft parts after tempering is only HRC22 (target HRC25-28).
Reason:
The tempering temperature is too high (such as setting 500 ℃ but actually reaching 530 ℃).
The insulation time is too long (for example, thick parts should be insulated for 4 hours but up to 6 hours).
The cooling rate is too fast (such as direct water cooling after high-temperature tempering).
Solution:
Reduce the tempering temperature by 10-20 ℃ or shorten the holding time by 30 minutes.
Adjust the cooling method (air cool high-temperature tempered parts to below 300 ℃ before processing).
If necessary, re quench and then temper according to the modified process.

2. Organizational deficiency issues
a. Tempering brittleness
Phenomenon: The impact toughness of the workpiece significantly decreases, and the fracture surface exhibits intergranular fracture characteristics.
Reason:
Temper in the brittle range of 250-400 ℃ (such as 35CrMo steel tempered at 300 ℃).
The cooling rate after tempering is too slow (such as air cooling to room temperature causing the second type of tempering brittleness).
Solution:
Avoid the brittle temperature range (such as using low-temperature tempering at 180 ℃ or high-temperature tempering at 500 ℃).
Quickly pass through the brittle range (such as oil cooling to below 100 ℃ after high-temperature tempering).
Adding alloying elements (such as Mo, W) to improve the resistance to tempering brittleness.
b. Incomplete transformation of residual austenite
Phenomenon: The workpiece size is unstable and undergoes aging deformation after storage (such as a change in precision ball diameter>0.002mm).
Reason:
Insufficient tempering temperature (such as GCr15 steel only being tempered at a low temperature of 150 ℃).
The cooling rate is too fast (such as direct air cooling after tempering, insufficient transformation of residual austenite).
Solution:
Raise the tempering temperature to 180-200 ℃, or add secondary tempering (such as tempering high-speed steel twice at 560 ℃).
Slow cooling after tempering (such as furnace cooling to below 100 ℃ before removal).

3. Cracking risk issue
a. Tempering cracking
Phenomenon: Cracks appear on the surface or core of the workpiece, commonly found in large bearing rings or thick section parts.
Reason:
The quenching stress has not been completely eliminated (such as not tempering in time after quenching).
Sudden change in tempering temperature (such as loading the furnace before the furnace temperature stabilizes).
There are microcracks inside the workpiece (such as raw material defects or quenching crack propagation).
Solution:
Immediately perform pre tempering (100-150 ℃ stress relief treatment) after quenching.
Slowly increase the temperature (heating rate ≤ 30 ℃/h) to ensure consistent temperature inside and outside the workpiece.
Strengthen non-destructive testing (such as magnetic particle inspection) and remove workpieces with cracks.
b. Hydrogen embrittlement
Phenomenon: After tempering, the toughness of high-strength steel (such as 30CrMnSiA) decreases, and brittle fracture occurs under static load.
Reason:
The tempering atmosphere contains hydrogen (such as a mixture of unpurified hydrogen gas).
The tempering temperature is too low (such as<200 ℃ without removing hydrogen atoms).
Solution:
Protect with high-purity nitrogen or argon gas, with a hydrogen ratio of ≤ 4%.
Raise the tempering temperature to 250-300 ℃ and extend the holding time to 4 hours.

4. Operational errors
a. Temperature control error
Phenomenon: The furnace temperature fluctuates by more than ± 5 ℃, resulting in poor process repeatability.
Reason:
Temperature control system malfunction (such as PID parameters not optimized).
Heating element aging (such as partial breakage of resistance wire).
Frequent opening of the furnace door (such as temperature drop caused by sampling inspection).
Solution:
Regularly calibrate the temperature controller and optimize PID parameters (such as P=50, I=300, D=10).
Replace aging heating elements to ensure uniform power.
Reduce the number of furnace door openings and use observation windows to monitor the process.
b. Improper atmosphere management
Phenomenon: Oxidation or decarburization on the surface of the workpiece (such as the surface turning blue after tempering GCr15 steel).
Reason:
Insufficient nitrogen flow rate (such as<5L/min leading to excessive oxygen content). The furnace door is not tightly sealed (such as aging sealing strips). The atmosphere humidity is too high (such as dew point>-40 ℃).
Solution:
Increase the nitrogen flow rate to 8-10L/min, with an oxygen content of ≤ 10ppm.
Replace the sealing strip to ensure that the furnace door is tightly closed.
Use desiccants or molecular sieves to reduce atmospheric humidity.

5. Typical Case Analysis
Case 1: Tempering cracking of large self-aligning roller bearings
Problem: Radial cracks appear in the 200mm diameter bearing ring after tempering.
Reason:
After quenching, no pre tempering was carried out, resulting in residual stress concentration.
The tempering temperature rises too quickly (from room temperature to 500 ℃ in 1 hour), and the temperature difference between the core and the surface is too large.
Solution:
Immediately conduct a pre tempering at 150 ℃ for 2 hours after quenching.
During tempering, segmented heating is used (room temperature → 150 ℃ → 300 ℃ → 500 ℃, with each segment held for 1 hour).
Case 2: Insufficient tempering hardness of high-speed steel cutting tools
Problem: After tempering at 560 ℃, the hardness of W18Cr4V tool is only HRC60 (target HRC67).
Reason:
After tempering, it was directly air-cooled, and the carbides were not fully analyzed.
Insufficient tempering times (only 1 time, 2-3 times are needed).
Solution:
After tempering, cool the oil to below 100 ℃ and then remove it for air cooling.
Add secondary tempering (560 ℃ × 1 hour × 2 times) to restore the hardness to HRC66-68.

6. Summary of preventive measures
Process validation: Conduct small-scale tests before the first tempering to confirm temperature and time parameters.
Equipment maintenance: Check heating elements, thermocouples, and sealing strips monthly, and replace aging components in a timely manner.
Process monitoring: Use a data logger to track the temperature curve and ensure consistency with the process card.
Personnel training: Regularly assess operators’ mastery of knowledge points such as tempering brittleness range and cooling methods.