How to choose the temperature for metal tempering furnace?

The temperature selection of metal tempering furnace needs to comprehensively consider the material type, microstructure transformation requirements, mechanical performance requirements, and process economy. The following is a systematic method and specific case analysis for temperature selection:

1. The core principle of temperature selection
a. The temperature of material phase transition determines the basic range
Iron carbon alloy:
Low temperature tempering (150-250 ℃): Quenched martensite decomposes into tempered martensite, retaining high hardness (HRC55-64), suitable for cutting tools and measuring tools.
Medium temperature tempering (350-500 ℃): precipitation of ε – carbides, formation of tempered martensite, hardness HRC35-45, suitable for springs and molds.
High temperature tempering (500-650 ℃): Carbides aggregate and grow into cementite, forming tempered martensite with a hardness of HRC25-35, suitable for shafts and gears.
Nonferrous metals:
Aluminum alloy (such as 6061): Tempered at 170-200 ℃ (T6 treatment), Mg ₂ Si strengthening phase is precipitated, and the tensile strength is increased by 20%.
Copper alloy (such as H62): Tempered at 250-300 ℃ to eliminate cold work hardening, hardness HB80-100.
b. Target performance extrapolated to temperature range
Hardness priority: Low temperature tempering (such as Cr12MoV cold work die steel tempered at 180-220 ℃, hardness HRC58-62).
Priority of toughness: High temperature tempering (such as tempering 40Cr structural steel at 550-600 ℃, impact toughness ≥ 40J/cm ²).
Red hardness priority: High speed steel (such as W18Cr4V) needs to be tempered three times at 540-560 ℃, and the hardness remains above HRC60 at 600 ℃.

2. Engineering methods for temperature selection
a. Material Temperature Performance Matching Table
Typical tempering temperature target performance application scenarios for material types
Tool steel (T10) 180-200 ℃ high hardness (HRC60-62) punching die, cold extrusion die
Spring steel (60Si2Mn) 450-500 ℃ elastic limit ≥ 1800MPa automotive suspension spring
Bearing steel (GCr15) 150-180 ℃ contact fatigue life ≥ 10 times high-precision rolling bearings
Cast iron (QT400-18) with elongation at 500-550 ℃ ≥ 18% for crankshafts and gears
Aluminum alloy (7075-T6) 190 ± 5 ℃ yield strength ≥ 503MPa aviation structural components

b. Temperature calibration and verification
Thermocouple accuracy: It is recommended to use S-type or B-type thermocouples with an accuracy of ± 1 ℃ and to calibrate them regularly.
Temperature uniformity test: Under no-load and full load conditions, measure the temperature difference in the furnace using multi-point thermocouples, which should be ≤± 5 ℃.
Sample verification: Conduct metallographic analysis (such as martensitic decomposition degree), hardness testing (HV or HRC), and mechanical property testing (tensile, impact) on the first workpiece.

3. Special scenarios for temperature selection
a. Temperature gradient control of complex processes
Tempering of carburized parts:
20CrMnTi carburizing steel: quenched at 860 ℃ and tempered at 180 ℃, with a surface hardness of HRC58-62 and a core hardness of HRC30-35.
Graded tempering:
High alloy steel (such as 3Cr2W8V) is first pre tempered at 260 ℃, and then finally tempered at 520 ℃ to reduce the residual austenite content.
b. Collaborative control of atmosphere and temperature
Protective atmosphere tempering:
Stainless steel (such as 316L) is tempered at 250 ℃ in a nitrogen atmosphere to avoid the formation of oxide scale, and the surface smoothness Ra is ≤ 0.4 μ m.
Vacuum tempering:
Titanium alloys (such as TC4) are tempered at 500 ℃ under a vacuum degree of 10 ⁻ Pa to eliminate the risk of hydrogen embrittlement and increase fatigue strength by 15%.

4. Engineering case of temperature selection
Case 1: Tempering process for automotive gears
Material: 20CrMnTi carburizing steel
workmanship
920 ℃ carburizing (carbon potential 0.8% -1.0%).
860 ℃ quenching (oil cooling).
Low temperature tempering at 180 ℃ (2 hours), surface hardness HRC58-62, and core hardness HRC30-35.
Effect: Contact fatigue life ≥ 5 × 10 ⁶ times, bending fatigue strength increased by 30%.
Case 2: Tempering process for high-speed steel cutting tools
Material: W18Cr4V
workmanship
Quenching at 1270 ℃ (salt bath graded cooling).
Tempering at 560 ℃ for three times (1 hour each time), hardness HRC62-64, red hardness (600 ℃) ≥ HRC60.
Effect: The tool life is doubled compared to single tempering, and the cutting efficiency is increased by 15%.

5. Precautions for Temperature Selection
Avoid temperature overshoot:
Heating rate ≤ 10 ℃/min, temperature fluctuation during insulation stage ≤ ± 2 ℃, to prevent tissue overburning or insufficient hardness.
Cooling rate matching:
It is recommended to air cool high carbon steel after tempering to avoid cracking caused by oil cooling; After tempering, aluminum alloy requires rapid water cooling to fix the structure.
Equipment calibration:
Verify the furnace temperature with a standard thermocouple every month, and adjust the temperature control parameters when the deviation exceeds ± 3 ℃.

6. Summary and Recommendation
General principles:
Low temperature tempering maintains hardness, medium temperature tempering adjusts toughness, and high temperature tempering provides comprehensive performance.
Material orientation:
Tool steel prioritizes low temperature, structural steel chooses high temperature, and non-ferrous metals require precise temperature control.
Process validation:
Combining metallographic analysis, hardness testing, and fatigue testing to ensure the scientific selection of temperature.
By scientifically selecting the tempering temperature, the comprehensive performance of metal materials can be significantly improved to meet the stringent requirements of different industrial scenarios.