What are the effects of the holding time of the tempering furnace on the tempering effect of metals?

The holding time of the tempering furnace is one of the key parameters in the metal tempering process, which directly affects the internal microstructure transformation and final performance of the material. The following provides a detailed analysis from three aspects: organizational changes, performance impact, and process optimization:

1. The impact of insulation time on organizational transformation
a. Precipitation and aggregation of carbides
Short insulation time:
Carbides (such as Fe-C) are only initially precipitated, unevenly distributed, and retain a large amount of supersaturated carbon (especially carbon in quenched martensite).
Example: After quenching and tempering for a short period of time (such as 30 minutes), the martensite decomposition is not sufficient, residual stress is not completely eliminated, and the hardness decrease is limited.
Sufficient insulation time:
Carbides are analyzed and aggregate to form stable “tempered martensite” (medium temperature tempering) or “tempered pearlite” (high temperature tempering), improving the uniformity of the microstructure.
Example: By extending the insulation to 2 hours, the carbides in 45 steel changed from fine dispersed to granular, significantly improving its toughness.
Excessive insulation time:
Excessive aggregation and coarsening of carbides result in the formation of “coarse pearlite”, leading to a decrease in strength and hardness (i.e. “over tempering”).
b. Transformation of residual austenite
High alloy steel (such as Cr12MoV, high-speed steel) contains a large amount of residual austenite after quenching, and needs to be gradually decomposed through multiple tempering and sufficient insulation time:
Short insulation: Residual austenite only undergoes partial transformation, which may lead to dimensional instability (continued transformation during subsequent use).
Adequate insulation: Residual austenite is fully decomposed into martensite or bainite, combined with cooling processes (such as air cooling), reducing stress and stabilizing the structure.
c. Adequacy of stress relief
Insufficient insulation time:
The internal stress of the workpiece is only partially released, especially for complex structural components or large parts, which may cause deformation and cracking during subsequent processing (such as grinding) or use.
Adequate insulation:
Atomic diffusion is sufficient, lattice distortion is eliminated, and stress release is thorough. For example, insulation of cast iron parts for 2-4 hours can effectively reduce casting stress.
2. The influence of insulation time on metal properties
a. Hardness and strength
law:
When the insulation time is insufficient, the hardness decreases with the extension of time (due to martensitic decomposition), but does not reach an equilibrium state;
After reaching the critical insulation time, the hardness tends to stabilize;
If the insulation is too long, the hardness will decrease slightly due to the coarsening of carbides.

b. Resilience and plasticity
Short insulation:
Insufficient decomposition of martensite results in high internal stress retention and low toughness (impact absorption energy).
Example: After low-temperature tempering and insulation for 1 hour, the impact toughness of T10 steel is about 20% lower than that after insulation for 3 hours.
Adequate insulation:
The uniform distribution of carbides reduces stress concentration at grain boundaries and significantly improves toughness. For example, the impact toughness of 40Cr steel after quenching and tempering treatment (insulation for 2 hours) is more than 30% higher than that after insulation for 1 hour.
Excessive insulation:
If there is no accompanying temperature increase, the toughness usually no longer changes significantly; If the temperature is too high at the same time, it may lead to a decrease in toughness due to grain coarsening.
c. Dimensional stability
Precision parts (such as measuring tools and bearings) need to be insulated sufficiently to eliminate residual stress and avoid dimensional drift during use.
Example: Insufficient insulation during tempering of GCr15 bearing steel may result in dimensional deviations after grinding. Insulation for more than 3 hours can control dimensional changes within ± 0.002mm.
3. Principle of process optimization for insulation time
a. Determine the base time based on material properties
Carbon steel:
Small items (≤ 50mm): 1-2 hours;
Large items (≥ 100mm): 2-4 hours (with an extension of 0.5 hours for every 20mm increase in thickness).
Alloy steel:
Due to slow atomic diffusion, the insulation time is 30% to 50% longer than that of carbon steel. For example, when 35CrMo steel is quenched and tempered, workpieces of the same size need to be insulated for 2.5 to 5 hours.
Cast iron:
Stress relief annealing takes 3-6 hours (up to 8 hours for large castings) to ensure sufficient stress release around the graphite.
b. Consider the thickness of the workpiece and the furnace loading method
Single piece vs batch furnace loading:
When batch loading the furnace, the temperature uniformity inside the furnace decreases, and the insulation time needs to be extended by 10% to 20% (to avoid insufficient heating of the central workpiece).
Difference in cross-sectional dimensions:
For stepped shaft parts, the insulation time is calculated based on the maximum cross-sectional thickness, or “segmented insulation” is used (first based on the time for small parts, and then supplemented based on the time for large parts).
c. Avoid blindly extending the insulation time
Cost perspective: Excessive insulation increases energy consumption (such as 5-10 kW · h for every additional hour of insulation in a box furnace) and labor costs.
Performance risk: For certain age strengthened alloys (such as aluminum alloys), excessive insulation may lead to “over aging” and a decrease in strength.
4. Auxiliary measures shorten the effective time
Improving heating uniformity: Using a circulating fan and a multi zone temperature controlled tempering furnace can shorten the insulation time by 20% to 30%.
Preheating treatment: For large workpieces, “stress relief annealing” is performed after quenching to reduce the holding time during tempering.

Summarize
The holding time is the key to the “qualitative change” in the tempering process – if it is insufficient, the structural transformation may not be sufficient, and if it is too long, the performance may deteriorate. In practical applications, it is necessary to comprehensively set the material composition, workpiece size, and equipment characteristics, and verify the effect through hardness testing and metallographic analysis. For example, for newly processed materials, a “step insulation test” (such as insulation for 1 hour, 2 hours, and 3 hours respectively) can be used to compare performance and determine the optimal time, balancing efficiency and cost while ensuring quality.