Effect of Ratio of Nitrogen to Hydrogen on Microstructure and Wear Properties of Plasma Nitrided H13 Steel

Abstract

Aiming at the problem of early fatigue cracks in plasma nitrided H13 steel precision hot forging dies， plasma nitriding process tests were conducted under a nitriding temperature of 520 ℃， a pressure of 250 Pa and different nitrogen to hydrogen ratio conditions to reveal the effect of nitrogen hydrogen ratio in a nitriding atmosphere on the microstructure and wear properties of coating.The phase structures， microstructure， microhardness and wear properties of the nitrided coatings with different nitrogen hydrogen ratios were examined and analyzed using X-ray diffraction (XRD)， confocal microscopy， scanning electron microscopy (SEM)， microhardness tester and a friction and wear test machine.Results showed that the nitrided coatings with N ∶H ＝1 ∶3，1 ∶4 and 1 ∶5 after 3 h mainly consisted of ε-Fe2-3 N， γ′-Fe4 N and αN-Fe(N) phases， respectively， while nitrided coatings with N ∶H＝1 ∶8 mainly consisted of γ′ and αN.The depth and hardness of the nitrided layers decreased with the reduction of N concentration in the nitriding atmosphere.The depth of the nitrided layer with N ∶H ＝1 ∶8 increased with nitriding time， and the hardness reached 800～900 HV0.1 at a depth of 60 μm.The wear mechanisms were mainly fatigue wear for the nitrided coatings with N ∶H＝1 ∶3，1 ∶4， and abrasive wear for the coatings with N ∶H＝1 ∶5，1 ∶8.When ion nitriding H13 steel with a nitrogen to hydrogen ratio of 1 ∶8 was carried out， there was no compound layer or vein like structure in the nitriding layer， and the nitriding layer could obtain moderate hardness and better toughness， with good wear resistance.

Keywords

• h13 steel； plasma nitriding； ratio of nitrogen to hydrogen； microstructure； wear resistance