Quantitative Depth Profile (QDP) Analysis of Carburized Steel

Significance of the Topic

Carburization strengthens steel surfaces by creating a carbon concentration gradient that enhances wear, friction, and impact resistance. Precise characterization of case depth and chemical composition is vital for quality control and lifetime prediction in automotive, tooling, and aerospace components.

Study Objectives and Overview

This study demonstrates the use of Glow Discharge Atomic Emission Spectrometry (GD-AES) to obtain quantitative depth profiles of carbon and alloying elements in a carburized PW53 steel rod. Correlation of chemical gradients with hardness measurements and metallographic analysis provides a comprehensive case characterization.

Used Instrumentation

• LECO GDS850A GD-AES with DC lamp, 4 mm anode, 40 mA, 1200 V
• LECO LV700AT Vickers Hardness Tester
• LECO LCR500 Rockwell Hardness Tester
• LECO CS600 Carbon/Sulfur Determinator
• LECO MSX250A Sectioning Machine and PR-25 Mounting Press
• LECO GPX300 Grinder/Polisher with colloidal silica (0.05 μm)
• LECO GX51 Metallograph (50×) with PAX-it™ imaging
• LECO LM247AT Knoop Microhardness Tester and AMH43 Automatic Indenter (500 g load)

Methodology

• Quantitative Depth Profiling: Layer-by-layer sputtering of the carburized surface on GDS850A, data acquired every 10 s over up to 1200 s per segment, removing 80–120 μm per run.
  • Eleven subsequent ground surfaces analyzed to exceed 2 mm total depth.
• Hardness Correlation: Vickers (HV) and Rockwell C (HRC) hardness measured at each depth; HV converted to HRC for comparison.
• Metallography: Sectioning, mounting, polishing, etching with 2% Nital, imaging cross section to verify case depth and microstructure.
• Microhardness: Knoop indentations performed across the case and substrate on a polished cross section to map hardness variation.

Main Results and Discussion

• Carbon Profile: Surface carbon peaked at ~2.9 wt% at 0.5 μm, decreasing to ~0.61 wt% by 5 μm, and stabilizing at ~0.11 wt% by 100 μm and beyond, matching substrate composition.
• Hardness Profile: Surface hardness reached HRC ~60 and HV ~636, gradually dropping to HRC ~40 and HV ~389 at 2.28 mm depth, mirroring the carbon decline.
• Correlation: Strong linear relationship observed between carbon content and both HRC and HV values, confirming chemical control of mechanical properties.
• Metallographic Confirmation: Optical micrograph at 50× distinctly revealed the hardened case boundary corresponding to chemical and hardness transitions.
• Microhardness Mapping: Knoop hardness decreased from ~680 HK near the surface to ~400 HK in the substrate, in agreement with macrohardness data.

Benefits and Practical Applications

• Non-preferential sputtering ensures accurate depth resolution without matrix effects.
• Simultaneous multi-element analysis with hardness correlation accelerates process optimization.
• Applicable for routine QA/QC of case-hardened components and failure analysis.

Future Trends and Applications

• Integration of in-situ monitoring during heat treatment to dynamically adjust carburizing parameters.
• Coupling GD-AES data with machine learning for predictive modelling of diffusion and case properties.
• Miniaturized glow discharge sources for analysis of small or complex-shaped parts.
• Expansion to other surface treatments such as nitriding or coating performance evaluation.

Conclusion

Quantitative depth profiling by GD-AES, combined with hardness testing and metallography, provides a robust toolkit for detailed assessment of carburized steel. This approach yields precise case depth, chemical gradients, and mechanical property correlations essential for ensuring component performance and reliability.

References

• Spectroscopy Performance Note: Quantitative Depth Profile Analysis of Carburized Steel, LECO Corporation, Form No. 209-076-036 (2007).