What are the purposes of muffle furnace annealing and tempering?

The purpose of muffle furnace annealing and tempering is related to improving material properties, but there are significant differences between the two in terms of process objectives, application scenarios, and effects. The following is a specific analysis:

1. The purpose of muffle furnace annealing
Eliminate internal stress
During the processing of materials such as casting, forging, welding, and cold working, residual stresses are generated, leading to deformation, cracking, or dimensional instability. Annealing enhances atomic activity and releases internal stress by heating to an appropriate temperature and holding for a certain period of time.
Application scenario: Annealing of welded parts can prevent crack formation; Annealing cold-rolled steel plates can eliminate work hardening and avoid cracking during subsequent forming.
Improve organizational structure
Annealing can adjust the grain size, phase composition, and distribution of materials, optimizing their performance. For example:
Complete annealing: Heat the steel above the austenitizing temperature, slowly cool it, obtain coarse and large pearlite, reduce hardness, and improve machinability.
Spheroidization annealing: spheroidizing carbides in high carbon steel, improving cutting performance and microstructure uniformity during quenching.
Stress relief annealing: Low temperature heating eliminates internal stresses generated by mechanical processing and maintains dimensional stability.
Reduce hardness and improve plasticity
Softening the material through annealing facilitates subsequent processing such as cutting, stamping, and stretching. For example, the hardness of cold drawn steel wire decreases after annealing, and it can be further drawn into finer wire.
Prepare for subsequent heat treatment
Annealing can adjust the material structure and provide a good initial state for processes such as quenching and tempering. For example, high carbon steel after spheroidization annealing is more likely to obtain a uniform martensitic structure during quenching.

2. The purpose of muffle furnace tempering
Eliminate quenching stress
After quenching, there are high residual stresses inside the material, leading to increased brittleness, unstable dimensions, and even cracking. Tempering is achieved by heating to a lower temperature (usually below the Ac1 point) and keeping it warm, which relaxes the stress and avoids sudden fracture of the workpiece during use.
Application scenario: Tool steel must be tempered after quenching, otherwise it may break or fracture under impact load.
Adjust organizational structure and balance performance
Quenched structures (such as martensite) have high hardness but high brittleness. Tempering transforms them into more stable structures (such as tempered martensite, martensite, bainite) through phase transformation, thereby balancing hardness and toughness.
Low temperature tempering (150-250 ℃): Obtain high hardness tempered martensite, suitable for cutting tools and measuring tools.
Medium temperature tempering (350-500 ℃): forms tempered martensite, improves elastic limit, suitable for springs.
High temperature tempering (500-650 ℃): Obtain tempered martensite, optimize the combination of strength and toughness, suitable for shafts and gears.
Stabilize dimensions and improve accuracy
Tempering can reduce the dimensional changes caused by residual stress in materials, ensuring that the workpiece maintains accuracy during long-term use. For example, precision molds can avoid deformation due to stress release after tempering.
Improve comprehensive mechanical performance
Tempering can significantly improve the impact toughness, fatigue strength, and corrosion resistance of materials. For example, after high-temperature tempering, car gears can withstand high loads and resist pitting and wear.

3. Comparison of Application Examples
Annealing application
Cold rolled steel plate: After cold rolling, it has high hardness and poor plasticity. It is restored to plasticity through recrystallization annealing (650-700 ℃), which is convenient for stamping and forming.
Casting aluminum alloy: Eliminating segregation in the as cast structure and improving mechanical properties through homogenization annealing.
Tempering application
High speed steel cutting tools: After quenching, the hardness reaches HRC63 or above, but the brittleness is high. After three rounds of tempering at 560 ℃, the hardness drops to HRC60-62, and the toughness is significantly improved.
Automotive crankshaft: After quenching and high-temperature tempering, it obtains a martensitic structure that combines high strength and fatigue resistance.