Decarburization Depth Analysis Using Microindentation Hardness Testing

Importance of the Topic

Decarburization reduces carbon content at the steel surface, leading to reduced hardness, wear resistance and fatigue life. Accurately measuring the depth of this softened layer is critical for quality control in heat-treated components and for ensuring mechanical performance in industrial applications.

Objectives and Overview of the Study

This application note demonstrates a microindentation hardness testing method for quantifying decarburization depth in heat-treated steel fasteners. The study details sample preparation, testing parameters and data analysis procedures to establish a reproducible protocol using the LECO AMH55 instrument.

Methodology and Instrumentation

Sample Preparation and Sectioning:

• Specimen: Heat-treated steel fastener
• Sectioning: MSX255M2 saw with 12"×0.062" rubber-bonded aluminum oxide blade
• Mounting: MX400 press in black epoxy (1.5" mold)

Grinding and Polishing:

• Rough grinding on PX400 (8" wheel) using Platinum #1 media, 40 lb, 300 FPM for 2 minutes
• Pre-polishing on FAS magnetic system with 6 µm diamond suspension, 40 lb, 200 FPM for 2 minutes
• Final polishing with 3 µm and 1 µm diamond pastes on separate disks, 35–40 lb, 200 FPM for 1–3 minutes

Microindentation Testing:

• Instrument: LECO AMH55 with LM248AT microscope and 10×/50× objectives
• Calibration: Certified microindentation hardness standards (LECO, NIST or equivalent)
• Etching: 2% Nital solution for approximately 15 seconds to reveal decarburization zone
• Test Pattern: Automated panoptic imaging followed by a single vector of ten equally spaced Knoop indentations along a fastener thread

Main Results and Discussion

The hardness profile shows a gradual increase from the decarburized surface toward the core. Surface hardness was measured at 137 HK and core hardness at 197 HK. Defining the decarburized layer limit at 75% of the hardness difference yields a threshold of 182 HK. This corresponds to a decarburization depth of approximately 110 µm. Data can be exported for customized reporting and further analysis.

Benefits and Practical Applications

Microindentation hardness profiling offers precise, localized measurement of decarburization depth without requiring extensive sample removal. This method supports:

• Quality assurance of heat-treatment processes
• Verification of surface integrity in critical components
• Development of process controls to minimize decarburization

Future Trends and Potential Uses

Advances in automated imaging, combined hardness and microstructural analysis, and integration with AI-driven data interpretation are expected to enhance throughput and reliability. Emerging techniques may allow real-time monitoring of decarburization during heat treatment or adoption of nanoindentation for ultra-shallow case depth evaluation.

Conclusion

This protocol demonstrates that the LECO AMH55 microindentation hardness tester provides a robust and repeatable approach to quantify decarburization depth in heat-treated steel. Implementing this method enables more rigorous quality control and helps ensure component performance in demanding applications.

Reference

LECO Corporation. Decarburization Depth Analysis Using Microindentation Hardness Testing, Application Note, Form No. 203-821-511, Rev 1, September 2019.