Nitriding is a widely applied surface strengthening treatment for mold steel, significantly improving hardness, wear resistance, fatigue resistance, and corrosion resistance.A high‑quality nitrided layer effectively extends mold service life and reduces maintenance costs.The thickness of the nitrided layer is a critical quality indicator that must be measured accurately using standardized methods.

Each testing method has unique advantages and suitable scenarios, allowing flexible selection based on inspection requirements.

Metallographic Method

The metallographic method is the most common and standard approach for mold inspection.It is intuitive, cost‑effective, and suitable for batch quality control.The process includes sampling, mounting, grinding, polishing, and etching with a 4% nitric acid alcohol solution.Under a metallurgical microscope, the nitrided layer and substrate show clear color contrast.Measurements are taken at multiple points and averaged for accuracy.This method complies with national standards and supports clear record‑keeping through microstructure photography.It is widely used in factory inspection and delivery acceptance.

Hardness Gradient MethodThe hardness gradient method provides high‑precision measurement for critical molds and third‑party certification.Microhardness is measured from the surface inward using a low load.The effective nitrided layer depth is defined where hardness is 50HV higher than the substrate.This method produces accurate thickness values and hardness distribution curves.

However, it requires expensive equipment, skilled operation, and longer testing time, making it suitable for sampling rather than full batch inspection.

Chemical Layer Analysis

Chemical layer analysis is used for advanced process research and material development.The surface is stripped layer by layer, and nitrogen content is measured at each depth.This generates a precise nitrogen concentration profile, revealing diffusion behavior during nitriding.Although highly accurate, the method is time‑consuming, costly, and unsuitable for daily production inspection.It mainly supports process optimization and scientific research.

Non-Destructive Testing MethodsNon‑destructive methods, including eddy current and magnetic testing, enable fast on-site inspection without damaging finished molds.Eddy current testing uses conductivity differences between the nitrided layer and substrate.Magnetic testing relies on permeability changes in ferritic mold steels.These methods are fast and convenient for preliminary screening.However, accuracy is lower than destructive methods and can be affected by surface roughness.They are ideal for initial filtering, with suspicious samples rechecked by metallographic testing.