How to solve the problem of large hardness fluctuations after tempering of H13 material?

Optimizing the tempering process control, standardizing the furnace loading procedure, and bolstering the pre-inspection of raw materials to guaranty consistent temperature, appropriate tempering cycles, and traceable data are the fundamental solutions to the significant hardness fluctuations following tempering of H13 material.

 

1. Fundamental Reasons and Related Remedies

Causes and Remedies

Inconsistent Temperature

Make sure the temperature differential inside the furnace is ≤±5°C by using a tempering furnace with multi-point temperature measurement and PID temperature control.

Stacking or Overcrowding

To prevent impeding heat circulation, keep workpieces at least five centimeters apart. Using a fixture for fixing is advised.

Inadequate Tempering Cycles

Adhere strictly to two rounds of tempering (550~600℃×2h). Before the second tempering, allow the first to cool to room temperature.

Deviation in Raw Material Composition

Before tempering, use XRF to determine whether Cr (5.0%~5.5%), Mo (1.1%~1.75%), and V (0.8%~1.2%) fulfill the requirements.

Unreliable Cooling Rate

To guaranty that thick and thin sections are heated alternately, change the direction of the workpiece location. To improve convection, turn on the furnace fan.

Real-world Example: A tempering furnace fan breakdown caused uneven airflow in a Wuhan company. The hardness difference dropped from 3.7 HRC to 1.5 HRC upon repair.

 

2. Measures for On-Site Optimization

Pre-calibrate Equipment: To prevent systematic mistakes, check the furnace temperature and hardness tester on a standard block prior to each batch.

Multi-point Inspection for Every Piece: Determine the range ≤ 2 HRC by testing three to five locations each piece (center, edge, thickness boundary).

Keep Process Records: To ensure quality traceability, keep test results, operator data, and furnace temperature curves.

Choose Local Compliant Service Providers: Wuhan Heat Treatment Plant and Jingmo Technology, for example, may offer comprehensive reports and enable fast processing.

Advice: To quickly spot trend deviations, control charts can be created using data from several successive batches.

 

3. Suggestions for Managing Non-conforming Items

Reworkable Circumstances:

Slightly High Hardness (53–54 HRC): Add one more tempering (580°C × 2 hours).

Slightly Low Hardness (45–47 HRC): Re-quench and temper after verifying that the quenching was not complete.

Non-reworkable problems include: Hardness < 45 HRC and incomplete hardening revealed by metallographic investigation; Hardness > 54 HRC and the presence of microcracks; Items that are still unstable after several reworks are advised to be scrapped.

"One-time rework is the limit, microstructure verification is the standard," is a safety principle that guards against grain coarsening via repeated heating.