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Reasons for Uneven Hardness of DIN 1.2379 Steel

You purchase a batch of DIN 1.2379 steel. After heat treatment, hardness test shows edge at 61 HRC while core only 57 HRC. Soft spots can also be found. This issue does not stem from a single factor. It results from combined effects of four aspects: material, heat treatment, section size and inspection. Keyspark Steel analyzes each cause step by step.

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DIN 1.2379 Steel
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1.2379 Tool Steel

Material and hot working defects of DIN 1.2379 Steel

Carbide Segregation of 1.2379 steel

  1. DIN 1.2379 steel is a ledeburitic steel, with carbon of 1.40%-1.60% and chromium of 11.00%-13.00%. During solidification, chromium and carbon form coarse eutectic carbides, distributed in network or band shapes
  2. In carbide segregation zones, high concentration of carbon and chromium leads to higher hardness after quenching. In carbide lean zones, insufficient alloy elements result in lower hardness after quenching.

Insufficient Forging Ratio of DIN 1.2379 Steel

Forged is the key process to break the carbide network of DIN 1.2379 steel. When the forging ratio is below 4:1, the network carbides at the ingot center cannot be fully crushed. The carbides remain coarse and strongly directional, and uneven hardness becomes inevitable. For this reason, high-grade 1.2379 steel requires a forging ratio above 6:1.

Heat Treatment Process Defects of DIN 1.2379 Steel

Incomplete Spheroidizing Annealing of 1.2379 Material

  1. Spheroidizing annealing turns coarse plate carbides into fine spherical carbides. If annealing temperature is too low, holding time is not enough, or cooling speed is too fast, DIN 1.2379 steel will have incomplete carbide spheroidization, with plate pearlite left in the structure.
  2. During quenching, plate pearlite causes uneven austenitization. Zones with slow-dissolving plate carbides have low carbon in austenite and low hardness after quenching. Zones with fully dissolved carbides get high hardness. This finally leads 1.2379 tool steel to uneven hardness.

Uneven Heating Temperature and Holding Time of 1.2379 Tool Steel

  1. Uneven heating temperature: In zones with low temperature, austenitization is incomplete, non-martensite structure forms after quenching and hardness is low. In zones with high temperature, grains become coarse and hardness is high. For box furnace heating, temperature difference can reach 20–30°C near furnace door and furnace wall, causing 2–3 HRC hardness difference.
  2. Insufficient holding time: For large-section blocks, the core needs enough time to reach austenitization temperature. If holding time is not enough, structure transformation at core is incomplete, and core hardness is lower than surface after quenching.

Uneven Cooling Rate of DIN 1.2379 Steel

  1. Oil quenching: Bubbles attached to workpiece surface, insufficient oil agitation, excessive oil temperature or aged oil will make local cooling rate lower than critical cooling rate. In areas with insufficient cooling, supercooled austenite decomposes in pearlite transformation zone and forms troostite or pearlite, whose hardness is much lower than martensite.
  2. Gas quenching: Insufficient gas pressure, dead airflow zones or too dense loading in vacuum gas quenching also lead to poor local cooling. DIN 1.2379 steel is air-hardening steel and sensitive to gas quenching pressure. If pressure is below 5 bar, core hardness of large-section workpieces may fail to meet requirements.

Insufficient Tempered

  1. Retained austenite: DIN 1.2379 steel has 10%-20% retained austenite after quenching. Austenite and martensite differ in hardness and volume. With insufficient tempering, retained austenite transforms into martensite to varying degrees at different positions and causes hardness fluctuation.
  2. Insufficient tempering cycles: DIN 1.2379 steel needs at least two tempering runs. Single tempering only removes about 70% internal stress, and retained austenite cannot decompose fully. After double tempering, retained austenite mostly decomposes and hardness becomes stable.
  3. Uneven tempering temperature: Uneven temperature inside tempering furnace. A 10–20°C actual temperature difference on different workpiece positions brings 1–2 HRC hardness difference.

Section Size Reason of 1.2379 Steel

  1. Cause: 1.2379 mold steel has good hardenability. But the cooling rate at the core of large-section blocks is naturally lower than the surface. If the actual cooling rate is below critical cooling rate, the core cannot be fully hardened, so its hardness is lower than the surface layer.
  2. Size influence: The larger the section size, the more obvious this effect. For DIN 1.2379 steel blocks below 100 mm, hardness uniformity is easy to control. Blocks over 200 mm show more hardness difference between core and surface.
  3. Improvement measures: To reduce uneven hardness of 1.2379 tool steel, increase quenching cooling rate (such as using better cooling liquid), or select steel with higher hardenability.

Inspection Errors of 1.2379 Material

  1. Decarburized layer: If you test on the black skin surface of DIN 1.2379 steel, residual decarburized layer on surface will give low hardness value.
  2. Segregation band: Test point on carbide rich zone gets high hardness; test point on carbide lean zone gets low hardness. Hardness test for DIN 1.2379 steel should be done at 1/2 radius of the cross section, away from segregation bands and decarburized layer.
  3. Hardness tester error: Uncalibrated tester, damaged indenter or uneven sample surface will cause reading fluctuation.

FAQ of DIN 1.2379 Steel

Q1: Can forging remove network carbides in DIN 1.2379 steel?

A: No, it cannot remove them completely in 1.2379 material. Forging only breaks the formed carbide network. If segregation remains in finished material and forging ratio is insufficient, re-forging cannot fully fix it. Return to supplier and request qualified metallurgy report.

A: The improvement is obvious. 4:1 ratio only breaks the network basically. Ratio above 6:1 can refine carbides fully. The HRC difference of large blocks can drop from 3–4 points to 1–2 points.

A: Qualified annealing hardness does not mean complete spheroidization. If plate pearlite remains, carbides in these zones dissolve slowly, austenitization is incomplete. Non-martensite structure forms after quenching and soft spots appear.

A: Keep the thickness of 1.2379 steel more than 100 mm away from furnace walls and door. Place in single layer, use circulating fan. Heating rate ≤200°C/h. Loading capacity ≤1/3 of furnace chamber. Temperature difference can be controlled within 10°C.

A: The core cooling rate falls below critical cooling rate. Troostite or pearlite forms, with much lower hardness than martensite. Core hardness of sections over 200 mm may be less than 58 HRC.

A: Remove 0.02–0.05 mm white layer by light grinding or light cutting. Or carry out stress relief tempering at 170–200°C before test. Avoid cutting surface for hardness test points.

Conclusion

  1. Carbide segregation — inherent material defect, detectable by metallographic test
  2. Insufficient forging ratio — MTC required
  3. Incomplete spheroidizing annealing — annealed hardness requirement ≤255 HB
  4. Uneven heating temperature and holding time — check furnace temperature uniformity and loading method
  5. Uneven cooling rate — check oil quench agitation, gas quench pressure and loading method
  6. Insufficient tempering — confirm at least two tempering runs
  7. Section size effect — low core hardness of large blocks is normal
  8. Inspection error — test point position, hardness tester calibration, decarburized layer

More DIN 1.2379 Steel Resources

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