Plasma Nitriding of Steels, Titanium and Aluminum Alloys for Manufacturing

Dear Colleagues,

Plasma nitriding has been widely utilized to harden dies and molds, as well as mechanical and structural parts, instead of using case hardening and gas-/liquid-phase nitriding processes.  Various nitriding systems have been developed in recent years, e.g., DC-plasma, DC-pulsed plasma, RF-plasma, beam-assisted plasma, RF/DC plasmas and plasma-enhanced CVD (Chemical Vapor Deposition) systems.  In parallel with these instrumental developments, plasma chemistry related to plasma nitriding was also investigated via plasma diagnosis.  Various methods and devices have been proposed to improve nitrogen ion and electron density.

In particular, a number of studies have reported on plasma nitriding on stainless steels as well as tool and high-chromium-content steels.  In the case of plasma nitriding processes at higher temperatures (greater than 673 K or 400 °C), a thick nitrided layer, up to 0.1 to 0.2 mm, was successfully formed and the surface hardness increased to 1100 to 1200 HV, using nitride precipitation hardening, together with the formation of a nitride-rich layer (or white layer).  This inner nitriding process is governed by the nitrogen diffusion process with a precipitation reaction at the nitriding front end.  In those high-temperature plasma nitriding experiments, the nitrided layer thickness is proportional to the root of nitriding duration time.  The maximum unbound nitrogen content is limited by 0.1 to 0.2 mass%, which is close to the nitrogen solubility limit in the equilibrium phase diagram.  This nitrding behavior is also common in the nitriding processes of titanium and aluminum, as well as their alloys.

In the case of plasma nitriding at, or below, 673 K, no nitrides are synthesized as a precipitate in the matrix materials.  Each cell in the pure iron and steels expands due to the nitrogen super-saturation process, where the nitrogen interstitial atoms into the cells  and occupy their octahedral vacancy sites.  The nitrogen solute diffuses from the surface to the depth of the matrix, together with this occupation process; the classical theory does not hold on this inner nitriding behavior.  Owing to the elastic lattice expansion in each cell in the inside of the nitrided layer, the elastic strain energy increase contributes to the enhancement of free energy and to drive phase transformation.  In nitrided AISI304 and AISI316 steels, the original austenite transforms to martensite; the original martensite, to the austenite in the nitrided AISI420 steels.  These nitrided steels have a higher surface hardness than 1400 HV due to solid solution hardening.  Their nitrided layer thickness reaches 80–100 mm even at 673 K for 14.4 ks.

The above merits of low temperature plasma nitriding are applied to manufacturing.  Nitrided NAK 80 and SKD11 punches at 673 K for 14.4 are utilized in the dry stamping of the zero-clearance piercing process and in the progressive stamping process.  Higher engineering durability is demonstrated in comparison to case-hardened punches.  The die life for progressive stamping is prolonged 10 times in comparison to the quenched SKD11 punches. 

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• Plasma nitriding system, Plasma chemistry, Plasma diagnosis
• Nitride synthesis, Fe3N, Fe4N, CrN, AlN, TiN
• Precipitate hardening, Diffusing zone, Nitrided layer thickness
• Nitrogen super-saturation, Fe (N), Ti (N)
• Solid solution hardening, Nitrogen interstitials, Vacancy site occupation
• Lattice expansion, Lattice straining, Phase transformation
• Plastic straining, grain spinning, Grain size refinement
• Stainless steels, Tool steels, Nitrogen steels, Dies and molds
• Pure titanium, Titanium alloys, Mechanical parts
• Pure aluminum, Al-Cu alloys