Can metal heat treatment furnaces be tempered?

08-12-2025       Author: KJ technology

Metal heat treatment furnaces can be fully used for tempering processes. Tempering is an essential key step in metal heat treatment, usually carried out after quenching or cold working. By precisely controlling the heating temperature, holding time, and cooling method, internal stress is eliminated, hardness and toughness are adjusted, and microstructure dimensions are stabilized, thereby optimizing the comprehensive mechanical properties of the metal. The following is a detailed explanation:

1. Selection of furnace type for tempering process

Metal heat treatment furnaces can be flexibly selected according to the tempering temperature range, part size, and production batch. Common furnace types include:

Box type tempering furnace

Applicable scenarios: Batch tempering of small and medium-sized parts such as gears, shafts, molds, and cutting tools.

characteristic:

Good temperature uniformity (within ± 3 ℃) ensures consistent performance of parts.

Equipped with a controllable atmosphere system (such as nitrogen, methanol cracking gas) to prevent oxidation and decarbonization.

Integrated temperature recorder for process traceability.

Example: After low-temperature tempering in a 200 ℃ box furnace, the surface hardness of automotive gears remains at 58-62 HRC, while eliminating quenching stress and reducing the risk of cracking.

Well type tempering furnace

Applicable scenarios: Vertical tempering of long axis and annular parts (such as drive shafts and bearing rings).

characteristic:

Vertical heating design reduces part bending deformation.

Supporting protective atmosphere or vacuum environment to avoid surface oxidation.

Example: After tempering in a 180 ℃ pit furnace, the residual stress of a bearing ring with a diameter of 500mm is reduced by 80%, and the dimensional stability is improved to within ± 0.05mm.

Continuous tempering furnace

Applicable scenarios: Large scale production lines (such as continuous tempering of springs, steel wires, and strips).

characteristic:

The parts are continuously transported through the heating zone, insulation zone, and cooling zone via a conveyor belt, resulting in high production efficiency.

The cooling method can be adjusted (such as wind cooling, water mist cooling combination) to meet different tempering needs.

Example: After tempering at medium temperature in a continuous furnace at 450 ℃, the tensile strength of the car suspension spring remains above 1600 MPa, while the elongation rate increases to 12%.

tempering vacuum furnace

Applicable scenarios: high-precision, high surface quality parts (such as aviation blades, precision molds, medical devices).

characteristic:

Heat in a vacuum environment (to avoid oxidation and decarburization), then cool with inert gas.

High surface smoothness (Ra ≤ 0.2 μ m) and minimal deformation.

Example: After high-temperature tempering in a 650 ℃ vacuum furnace, the structural stability of aviation turbine blades is significantly improved, and the high-temperature endurance strength reaches over 800 MPa.

2. Key parameter control of tempering process

temperature

Principle: Select temperature range based on material type, quenching process, and performance requirements:

Low temperature tempering (150-250 ℃): eliminates quenching stress and maintains high hardness (such as tool steel and mold steel with a hardness of ≥ 60 HRC after tempering).

Medium temperature tempering (350-500 ℃): Obtain high elastic limit and yield strength (such as tensile strength ≥ 1500 MPa after tempering of spring steel).

High temperature tempering (500-650 ℃): significantly improves toughness (such as impact energy ≥ 30 J after tempering of structural steel), suitable for parts that can withstand impact loads.

Example:

After quenching, Cr12MoV mold steel can achieve a low temperature tempering hardness of 60-62 HRC at 200 ℃, and has excellent wear resistance.

After quenching, the elastic modulus of 60Si2Mn spring steel remains above 200 GPa after tempering at 450 ℃.

Holding time

Principle: Calculate based on the effective thickness of the part (usually 15-30 minutes/25mm) to ensure sufficient tissue transformation.

Example: For shaft components with a diameter of 100mm, the insulation time should be ≥ 1 hour; Thin walled parts (such as cutting tools) can be shortened to 30 minutes.

Cooling method

Air cooling: suitable for most low-temperature tempering and medium temperature tempering, with a slow cooling rate to avoid tissue stress.

Oil cooling/water cooling: only used in scenarios where rapid cooling is required after special high-temperature tempering (such as certain high alloy steels), but the cooling rate must be strictly controlled to prevent cracking.

Graded cooling: First air cool to below 300 ℃, then slowly cool in an insulated box, suitable for complex shaped parts to reduce deformation.

3. Typical microstructure and properties after tempering

Low temperature tempering microstructure

Organization: Martensite decomposes into a supersaturated carbon alpha solid solution (tempered martensite)+ε carbides.

Performance: High hardness (58-64 HRC), good wear resistance, but low toughness, suitable for cutting tools, molds, etc.

Medium temperature tempering microstructure

Organization: Tempered martensite (fine lamellar carbides distributed on the ferrite matrix).

Performance: High elastic limit (such as spring steel with an elastic modulus of 200 GPa after tempering), excellent yield strength, suitable for springs, leaf springs, etc.

High temperature tempering microstructure

Organization: Tempered martensite (granular cementite distributed on the ferrite matrix).

Performance: Balance of strength and toughness (such as tensile strength ≥ 800 MPa and impact energy ≥ 30 J after tempering of structural steel), suitable for parts such as shafts and gears that withstand alternating loads.

4. Precautions for tempering process

Prevent temper brittleness:

The first type of tempering brittleness (250-400 ℃): irreversible, suppressed by avoiding this temperature range or adding Mo and W elements.

The second type of tempering brittleness (500-650 ℃): reversible, eliminated by rapid cooling (such as water cooling) or adding Al or Ti elements.

Example: 40CrNiMo steel needs to be water-cooled after tempering at 600 ℃ to avoid the decrease in toughness caused by the second type of tempering brittleness.

Control deformation and cracking:

When placing parts, avoid local stress (such as hanging or stacking too tightly).

For thin-walled parts, they can be fixed with fixtures or cooled in the furnace to below 150 ℃ before being removed from the furnace.

After quenching, the parts need to be tempered in a timely manner (usually within 24 hours) to prevent delayed cracking.

Organizational uniformity:

Ensure sufficient heating temperature and holding time to avoid incomplete tissue transformation.

For large section parts, segmented heating or preheating treatment can be used.

Process connection:

If cold processing (such as grinding or polishing) is required after tempering, the processing amount should be controlled (usually ≤ 5%) to avoid the generation of internal stress again.

When tempering multiple times, it is necessary to cool to room temperature after each tempering before the next heating.