Duplex Thermochemical Treatment of AISI 420 ESR

Duplex Thermochemical Treatment of AISI 420 ESR: Boriding and Nitriding Effects

Innovación y tecnología Boriding, Contexto, Damián Gutierrez Sosa, Dr. Melvyn Álvarez Vera, Dr. Pablo Moreno Garibaldi, Dr. Rafael Carrera Espinoza, Francisco Javier Romero Álvarez, Ismael López Rodríguez, Nitriding, Programa de Honores, Stainless Steel, UDLAP, Valeria López López

This study investigates the effect of thermochemical treatments—boriding, nitriding, and sequential boronitriding—on AISI 420 ESR stainless steel. Samples were treated and analyzed via microscopy, microhardness testing, adhesion evaluation, and energy-dispersive spectroscopy (EDS). Boriding produced the highest surface hardness (~1250 HV₀.₀₅), representing an increase of approximately 733% over the base material, but showed poor adhesion. Nitriding showed lower hardness (~1050 HV₀.₀₅), a 600% increase, with excellent adhesion. Boronitriding offered balanced results (~842 HV₀.₀₅), increasing hardness by ~461%, and demonstrated good adhesion while forming a duplex diffusion zone. EDS confirmed effective diffusion of boron and nitrogen into the surface. Findings suggest boronitriding as a promising compromise when both wear resistance and coating integrity are required in mechanical applications.

Introduction

Thermochemical surface treatments are among the most effective techniques for enhancing the surface properties of metallic materials (Gale & Totemeler, 2004). These treatments introduce elements such as boron (Juijerm, 2020) and nitrogen into the surface through diffusion (Shi et al., 2023a), enhancing hardness (Shi et al., 2023b), wear resistance (Taktak & Kayali, 2017), and overall performance without affecting the material’s core structure (Barut et al., 2014; Abbasi-Khazaei & Mollaahmadi, 2017). Among them, boronitriding has emerged as a promising approach for achieving a balance between surface hardness and coating adhesion in martensitic stainless steels such as AISI 420 ESR (Carrera Espinoza et al., 2024; Kayali & Taktak, 2015; Brnic et al., 2011).

Thermochemical Treatments and Methodology

Four cylindrical specimens (10 mm thick) of AISI 420 ESR stainless steel were subjected to thermochemical treatments. The sequential boronitriding involved pack boriding at 900 °C for 4 hours (Thermo Scientific FD1540M), followed by gas nitriding at 560 °C for 8 hours using NH₃ (KGO furnace). After treatment, the samples were sectioned, mounted, ground, and polished to a mirror finish.

Surface characterization included Vickers microhardness tests (HV₀.₀₅, 50 g load, Micromet 6010), adhesion testing based on VDI 3198 (OMAG 206 EX-2), and structural and compositional analysis through scanning electron microscopy (SEM; Tescan Mira 3) and energy-dispersive spectroscopy (EDS; Bruker).

Results and Surface Performance Comparison

The borided sample displayed a surface hardness of ~1250 HV₀.₀₅ due to the formation of FeB and Fe₂B phases but showed poor adhesion (VDI 3198: HF6) with evident cracking and spallation. High boron content (18.28 wt%, 43.49 at%) confirmed significant diffusion during the boriding process.

Fig 1. Borided layer of AISI 420 ESR steel (50×), b) VDI test of borided AISI 420 ESR, c) Vickers microhardness of the borided material, d) Energy-dispersive spectroscopy (EDS)

Nitriding produced a hardness of ~1050 HV₀.₀₅ and excellent adhesion (HF1–HF2), with minimal cracking and no delamination. Nitrogen was identified as the dominant alloying element after iron, with 12.26 wt% and 22.83 at%, confirming successful surface incorporation.

Fig 2. Nitrided layer of AISI 420 ESR steel (50×), b) VDI test of nitrided AISI 420 ESR, c) Vickers microhardness of nitrided AISI 420 ESR, d) Energy-dispersive spectroscopy (EDS)

The boronitrided sample showed ~842 HV₀.₀₅ surface hardness and moderate adhesion (HF2–HF3), combining advantages of both processes. The EDS spectra indicated the presence of both boron (5.73 wt%) and nitrogen (18.60 wt%), verifying the effectiveness of the combined treatment.

Fig 3. a) Boronitrided layer of AISI 420 ESR steel (50×), b) VDI test of boronitrided AISI 420 ESR, c) Vickers microhardness of boronitrided AISI 420 ESR, d) Energy-dispersive spectroscopy (EDS)

Conclusion

Boriding significantly improves surface hardness but compromises adhesion. Nitriding offers a balanced performance with excellent coating integrity. Boronitriding provides a compromise, combining moderate hardness and good adhesion. Treatment selection should align with performance requirements—boriding for maximum hardness, nitriding for wear resistance and toughness, and boronitriding for applications requiring both attributes.

References

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