Why Workpieces Have Uneven Hardness After Box Tempering & Fixes

Uneven hardness of the same batch of workpieces is one of the most common problems in box type tempering furnace production, which directly leads to workpiece scrap and economic losses for heat treatment enterprises. Many operators only solve problems from the perspective of process parameters but ignore equipment and loading factors. This article analyzes all root causes and targeted solutions from multiple dimensions.

1. Unreasonable Workpiece Loading Method (Primary Cause)

Most uneven hardness problems are caused by improper loading. Dense stacking and overlapping placement of workpieces will block the internal hot air circulation of the furnace, resulting in insufficient heating of inner-layer workpieces and overheating of outer-layer parts. The solution is to adopt layered spaced placement, use special material racks to separate workpieces, and control the single-batch loading quantity according to the furnace chamber volume.

2. Improper Tempering Process Parameters

Too fast heating rate and insufficient heat preservation time will cause inconsistent internal and external temperature of thick workpieces. For metal parts with a thickness greater than 20mm, it is necessary to slow down the heating speed and appropriately extend the constant temperature holding time to ensure uniform heat penetration. In addition, frequent temperature parameter modification during operation will also affect the tempering effect.

3. Equipment Abnormalities Lead to Temperature Difference

The internal temperature difference of the furnace chamber exceeding ±3℃ will directly cause uneven hardness. The main equipment faults include aging furnace door sealing strips leading to heat loss, partial aging of heating elements resulting in uneven heating, and offset thermocouple installation leading to inaccurate temperature feedback. Operators need to regularly detect the temperature difference of the furnace chamber and replace damaged accessories in time.

4. Inconsistent Pre-tempering Workpiece State

The quenching hardness and surface cleanliness of raw workpieces are inconsistent, which will also affect the final tempering effect. It is recommended to classify workpieces according to material and quenching hardness before feeding, and carry out separate tempering production to avoid mixing different types of parts in the same furnace.

Conclusion

Uneven hardness of tempered workpieces can be completely avoided by optimizing loading methods, adjusting process parameters and regular equipment inspection. Our technical team can provide free tempering process parameter guidance for global customers to increase workpiece yield.