How Does Tempering Furnace Eliminate Quenching Residual Stress

Quenching residual stress is the biggest hidden danger of quenched steel parts. During rapid cooling of quenching, the surface and core of the workpiece have inconsistent cooling speed and volume shrinkage, resulting in uneven internal stress distribution. This residual stress is in a long-term unstable state, which will cause workpiece deformation, warping, cracking and size deviation in the subsequent processing and service process. The tempering furnace is the only professional equipment to completely and safely eliminate quenching residual stress, and its stress relief mechanism is based on precise low and medium temperature thermal metallurgy principles.

The whole process of tempering furnace eliminating quenching residual stress is divided into three core stages: stress relaxation, microstructure adjustment and dimensional stabilization, all completed under precise constant-temperature heating conditions set by the tempering furnace. Unlike natural aging which takes weeks or months to release stress, tempering furnace heating can complete efficient stress relief within a few hours, greatly improving production efficiency while ensuring thorough stress elimination.

First stage: Thermal stress relaxation (low-temperature heating stage)

When the tempering furnace heats the workpiece to 150°C–300°C, the internal molecular activity of the steel increases significantly. The concentrated quenching residual stress is gradually released through micro plastic deformation of the metal microstructure. In this stage, the hardness of the workpiece basically remains unchanged, but the local stress concentration points are effectively dispersed, avoiding sudden cracking of high-stress parts. This stage is the core process of low-temperature tempering stress relief for tool steel and mold steel.

Second stage: Microstructure reconstruction (medium-temperature heat preservation stage)

When the furnace temperature rises to 350°C–500°C, the unstable martensite structure formed by quenching begins to decompose and transform into stable tempered troostite. The irregular lattice distortion caused by rapid quenching is repaired, and the internal microscopic defects are eliminated. With the optimization of microstructure, the internal residual stress of the workpiece is further released and balanced, and the brittleness of the steel is significantly reduced. This stage is suitable for elastic parts such as industrial springs.

Third stage: Complete stress elimination and stabilization (high-temperature tempering stage)

At 500°C–650°C high-temperature tempering state, the steel microstructure is completely transformed into uniform tempered sorbite. The lattice stress generated by quenching is completely eliminated, the internal stress distribution of the workpiece tends to be uniform and stable, and the dimensional tolerance of the workpiece is fixed. Workpieces processed by high-temperature tempering will not deform or change size during subsequent machining and long-term service, which is the standard processing for structural steel parts.

The reason why tempering furnaces can achieve thorough stress relief is their precise uniform temperature field. Only when the whole workpiece is heated uniformly can the internal stress be released synchronously without new secondary stress. Ordinary heating equipment has uneven temperature, which will lead to incomplete stress relief or secondary stress concentration. Professional tempering furnaces adopt multi-zone balanced heating and PID precise temperature control to ensure 100% uniform stress relief of each workpiece.

In industrial production, standardized tempering furnace processing can eliminate more than 95% of quenching residual stress, completely solve the problems of workpiece deformation and cracking, and greatly improve the dimensional stability and service life of heat-treated parts.