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Cr12MoV Tool Steel - Three Core Indicators After Nitriding

Cr12MoV tool steel has high hardenability, high wear resistance and good size stability. It is widely used for nitriding treatment of stamping dies, cold extrusion dies and drawing dies. This article will explain three key indicators you must check for Cr12MoV nitrided dies: surface hardness, nitrided case depth and nitrided layer brittleness.

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Cr12MoV Tool Steel
Cr12MoV Steel
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Cr12MoV

Why Does Nitrided Cr12MoV Tool Steel Need Special Inspection

Cr12MoV is a typical ledeburite cold work die steel. Its carbon content is about 1.45% ~ 1.70%, and it contains carbide-forming elements such as chromium, molybdenum and vanadium. High chromium content makes Cr12MoV tool steel easy to form a hard but brittle white layer (compound layer) after nitriding. The state of base metal (quenched and tempered condition) directly decides the support strength of the nitrided layer. This means two Cr12MoV steel may look the same on the surface, but their real service life can differ by 3 to 5 times.

Three Core Indicators

  1. Hardness: decides the die’s wear resistance
  2. Case depth: decides wear life and fatigue strength
  3. Brittleness: decides if the die will peel or crack early under impact load.

Hardness Acceptance

Surface Hardness Requirement

The typical surface hardness of Cr12MoV after nitriding ranges from 700 to 1200 HV. The surface hardness after gas nitriding is usually 700~860 HV. In actual production, ion nitriding can get higher surface hardness, with typical values up to 950~1200 HV.

Core Hardness

Before nitriding treatment, the core hardness of Cr12MoV die is generally required to be ≥55HRC (about 550HV). The nitriding process has little influence on the core hardness.

Hardness Test Method

  • Load selection: The surface hardness shall be measured by HV0.2~HV0.3 (0.2~0.3kgf) low-load Vickers hardness tester, to prevent excessive indentation from affecting the hardness reading of thin nitrided layer.
  • Hardness gradient: Carry out point-by-point measurement from the surface to the core on the cross section perpendicular to the nitrided surface, and draw the hardness gradient curve. The hardness gradient of typical Cr12MoV nitrided layer shows that the surface hardness is the highest, and it gradually decreases to the core hardness with the increase of depth.
  • Test position: The measurement shall be carried out at representative positions on the working surface of the die, and at least 3 points shall be taken at each position to get the average value.

Nitrided Layer Depth of Cr12MoV Tool Steel

Definition of Layer Depth

The nitrided layer of Cr12MoV steel has two parts:

  1. White layer (compound layer): ε phase (Fe₂~₃N) + γ′ phase (Fe₄N). It has very high hardness but high brittleness. It is generally controlled at 5～15 μm. Thinner thickness (≤8 μm) can be used for precision dies.
  2. Diffusion layer: It is solid solution strengthening layer and nitride precipitation layer formed by inward diffusion of nitrogen atoms. It is the main part for wear resistance and fatigue resistance. The diffusion layer of Cr12MoV material nitrided die is usually required to be 0.10～0.25 mm.

Test Method

  • Diffusion layer depth test: Use metallographic method. After sample etching with 4% nital, measure the depth where hardness drops to core hardness +30～50 HV. This value is diffusion layer depth. Specific values follow technical agreement, common range is 0.10～0.25 mm.
  • White layer thickness test: Measured by metallographic method. Criterion: ≤15 μm, ≤8 μm for precision dies.
  • Layer depth uniformity test: Take samples on cavity surface and corner areas. The layer depth deviation of different positions on one die shall not exceed 20%.

Selection Suggestion

Deeper layer depth is not always better. Larger layer depth means longer nitriding time and higher cost. For Cr12MoV mold steel with high carbon and high alloy, deep nitriding is easier to cause brittleness. In general:

  • Thin sheet blanking dies (punch, die matrix): Diffusion layer 0.08～0.15 mm, pursue high hardness;
  • Cold extrusion dies, drawing dies: Diffusion layer 0.15～0.25 mm, balance load capacity and wear resistance;
  • Dies with complex cavity and many sharp corners: Prefer shallow layer instead of deep layer, control brittleness first.

Brittleness Acceptance of Cr12MoV Tool Steel

Acceptance Standard

  • The vickers hardness indentation rating method (GB/T 11354): Make vickers indentations on the nitrided layer surface with specified test force. Brittle grades are divided from grade Ⅰ to grade Ⅴ according to the chipping degree of indentation corners. Cr12MoV tool steel nitrided dies generally require brittleness grade ≤ grade Ⅱ (indentation edge is complete or only slight corner chipping).
  • Metallographic method for nitride morphology test: No network or vein-shaped nitrides shall appear in the nitrided layer. Nitrides shall be fine and dispersed. Continuous network nitrides are judged as unqualified.
  • Actual working condition verification: For dies supplied in batches, sampling blanking or extrusion test can be carried out to check early peeling or galling on the cavity surface.

Typical Causes Of Excessive Brittleness

  • High nitriding temperature or low ammonia decomposition rate leads to too thick white layer;
  • Serious banded segregation of raw material and uneven carbide distribution, nitrogen accumulates along segregation bands;
  • Insufficient substrate hardness or decarburized layer before nitriding, which cannot provide rigid support for nitrided layer;
  • Two-stage nitriding (strong nitriding + diffusion) process is not adopted, or denitriding treatment is not performed.

FAQ of Cr12MoV Tool Steel

Q1: Why Cr12MoV tool steel substrate hardness below 55HRC before nitriding causes nitrided layer failure?

A: The nitrided layer of Cr12MoV steel needs support from the substrate. When the substrate is below 55HRC, it is too soft. The nitrided layer collapses and peels under impact, and service life drops.

A: The white layer is hard and brittle. When the white layer of Cr12MoV material exceeds 15μm, cracks easily form and peel under impact load. It shall be ≤8μm for precision dies.

A: Take samples on cavity and corners of Cr12MoV tool steel die, test layer depth by metallography. It is qualified if the deviation at different positions ≤20%.

A: Stress concentrates at complex sharp corners. Deep nitriding of Cr12MoV steel increases brittleness, and sharp corners are easy to crack. Shallow layer shall be selected to control brittleness.

A: It means improper nitriding process for Cr12MoV tool steel, and nitrogen gathers along grain boundaries. Vein-shaped nitride has high brittleness and easily becomes crack source, judged as unqualified.

A: Too high nitriding temperature and low ammonia decomposition rate lead to excessive thick white layer; banded segregation in Cr12MoV tool steel raw material; no two-stage nitriding or denitriding treatment.

Conclusion

The acceptance of Cr12MoV tool steel nitrided dies is essentially the verification of the triangular balance of “hardness — layer depth — brittleness”: hardness guarantees wear resistance, layer depth guarantees service life, and brittleness control ensures reliability.

More Cr12MoV Tool Steel Resources

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    These are our related articles if you want to learn more tool steel comparison:

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