Hardness Determination in Decarburized Steel

Significance of the Topic

Decarburization is the surface loss of carbon in steel exposed to reactive atmospheres at elevated temperatures. It produces a softened layer on the surface that can compromise wear resistance, fatigue life and dimensional stability of components. Quantifying the depth and severity of decarburization is critical in metallurgical quality control and failure analysis.

Objectives and Study Overview

This application note presents a complete methodology for determining decarburization depth in steel using Vickers microindentation on the LECO AMH43 system. The goal is to establish a reliable, automated workflow that delivers reproducible hardness profiles from the surface through the transition zone to the core.

Methodology and Instrumentation

The procedure combines automated and manual sample preparation, hardness testing and data analysis.

• Sample Preparation
  • Sectioning with precision saw and coolant to isolate specimens
  • Mounting in bakelite under defined pressure and cycle
  • Grinding sequence: 180, 320 and 600 grit SiC wheels
  • Polishing sequence: magnetic pre-polish, diamond suspensions (3 µm, 1 µm) on Ultrafine cloth
  • Etching with 2 % Nital for microstructure visibility
• Hardness Testing Parameters
  • Instrument: AMH43 Vickers Microhardness Tester with 50× and 10× objectives
  • Load: 300 g; dwell time: 13 s; indenter: diamond pyramid
  • Patterns: individual and segment arrays spaced 75–250 µm, reaching 1 200 µm depth
• Data Analysis
  • Calibration using certified reference blocks (LECO, NIST etc.) matching test load and indenter
  • Automated macro sequence in AMH software: sample input, pattern alignment, indentation and measurement
  • Calculation of decarburization boundary as 88 % of core average hardness

Main Results and Discussion

The method yields consistent hardness profiles across multiple specimens. Individual pattern arrays deliver precise point-depth data, while segment patterns facilitate mapping of the entire decarb layer. For complex geometries or dense indentation grids, the color hardness contour plot provides a clear, visual representation of hardness distribution and decarburization transition.

Benefits and Practical Applications

This protocol offers:

• High reproducibility via automated software routines
• Quantitative depth determination using a standardized formula
• Rapid reporting through built-in Excel template export
• Visual mapping for intuitive interpretation of decarb severity

Future Trends and Opportunities

Emerging directions include integration of AI-driven image analysis to refine transition boundary detection, high-throughput indentation arrays for rapid screening, and adaptation of the workflow to other alloy systems. Advanced contour plotting algorithms and remote monitoring of hardness data may further enhance process control in manufacturing environments.

Conclusion

The described Vickers microindentation approach on the AMH43 system establishes a robust, automated solution for decarburization depth measurement in steel. By combining precise sample preparation, calibrated hardness testing and advanced data visualization, laboratories can achieve reliable quality assessments and support life-cycle management of critical components.

Instrumentation

• LECO AMH43 Vickers Microindentation Tester
• 50× and 10× objectives
• GPX200 grinding unit, FAS magnetic pre-polisher, Ultra Silk cloths
• Diamond suspensions (3 µm, 1 µm) and 2 % Nital etchant