M.F. Yan

Microstructure and mechanical properties of surface layer of M50NiL steel plasma nitrided

Abstract The quenched M50NiL steel specimens have been plasma nitrided at temperatures from 460 to 590°C for 4 h under a constant mixture gaseous supply of 0·05N2–0·4H2 L min−1. The effects of the treatment temperature on the microstructure, microhardness and wear resistance of the surface nitrided layers are studied. The results show that the plasma nitrided layer includes only the diffusion layer without conventional compound layer. The surface hardness is significantly enhanced. α′-Fe and γ′-Fe4N phases in surface layer are formed at relatively low nitriding temperatures (460–560°C), while a low nitrogen phase FeN0·076 forms when the nitriding temperature exceeds 575°C. With the increase in nitriding temperature, the nitrided layer thickness increases, whereas both the surface and core microhardness of the nitrided specimens decreases. The wear resistance of specimens can also be improved significantly by plasma nitriding.

Improvement of wear resistance for carburised steel by Ti depositing and plasma nitriding

ABSTRACT In order to improve the wear resistance of carburised M50NiL steel, the surface layer was deposited with Ti coating by magnetron sputtering and then plasma nitrided for 2 h with hollow cathode equipment. The influence of nitriding temperature (750, 800 and 850°C) on microstructure and mechanical properties of the coating were investigated. Results show that the coating of specimen 750 containing Ti-based-nitrides possesses higher nanohardness (13.769 ± 0.738 GPa) than the carburised layer (7.695 ± 0.338 GPa) and the load bearing capacity reaches 18 N due to the supporting carburised layer. The friction coefficient and wear rate of the specimen 750 decrease 23.1 and 72.2%, respectively, compared with the carburised specimen. The hardness and wear performance of the specimen 800 and 850 were underestimated by traditional test criterion because a loose and porous a′C(N) layer was formed on the surface due to sputtering effect of steel hollow cathode.

Insights into plasma nitriding behaviour of M50NiL steel with different initial microstructures

Abstract Annealed and quenched M50NiL steel specimens were plasma nitrided at 460°C for 4 h under a mixture gaseous supply of 0·05N2+0·4H2 L min−1 to investigate the effects of the substrate microstructure on the features of nitrided layer, such as microstructure, microhardness and wear resistance. The results show that the nitrided layer is mainly consisted of α′-Fe, γ′-Fe4N and α′N (nitrogen expanded martensite) phases. The surface hardness is significantly enhanced. For the quenched specimens, the nitrided depth decreases with increasing quenched temperature. Compared to the quenched specimens, the nitrided depth of the annealed specimen is thicker under the same nitriding condition. The mechanism may be the variation in the amounts of interstitial carbon atoms and substitution alloying elements in the lattice, and the compressive residual stresses in the substrate. It is also demonstrated that the wear resistance for both the annealed and the quenched specimens can be significantly improved by plasma nitriding treatment.

Phase Field Simulation for Grains Evolution of 17-4PH Steel During Cyclic Heat Treatment

A phase field model is developed to simulate the grain evolution of 17-4PH steel during cyclic heat treatment (CHT). Our simulations successfully reproduce the grain morphologies of every CHT. In the process of every CHT, phase transformation recrystallization happens. The recrystallized grains appear mainly on the original grain boundaries. The average grain size of 13.2 µm obtained by 1040 °C×1 h solution treatment for this experimental steel can be refined to 2.2 µm after five CHT’s. Furthermore, the effects of phenomenological parameters in our model are discussed.

Kinetics and wear behaviour of M50NiL steel plasma nitrided at low temperature

The quenched M50NiL steel was plasma nitrided at 460°C for different time to investigate the effects of the duration time on the microstructure, microhardness and wear resistance of the nitrided layers. The results show that the plasma nitrided layer depth increases with increasing nitriding time. The plasma nitrided layer includes only the diffusion layer without compound layer. The main phases in the nitrided surface layer are nitrogen expended α′-Fe and γ′-Fe4N. The microstructure of the nitrided layer is refined. The wear resistance of the nitrided samples can be improved significantly by plasma nitriding. The sample nitrided for 4 h possesses the highest wear resistance, due to its relatively smooth surface and ultra-fine grains in the nitrided layer.

Effects of rare earths on nanocrystalline for nitrocarburised layer of stainless steel

ABSTRACT Solution-treated stainless steel was plasma-nitrocarburised at the precipitation-sensitive temperature with and without rare earth (RE) addition. The nitrocarburised layers were characterised by means of X-ray diffraction, transmission electron microscope, nanoindentation and anodic polarisation test. Experimental results show that the depth of plasma-nitrocarburised layer can be apparently increased from 60 to 75 μm after RE addition. More importantly, microstructure of stainless steel surface is refined into nano-sized grains after plasma nitrocarburising. At the depth of 20 μm, the hardness and the modulus of the nanocrystallised layer are as high as 13.6 and 218 GPa. After RE addition, the hardness and the modulus of the nanocrystallised layer increase to 17.5 and 255 GPa, respectively.

Plasma Nitriding of 2024 Al Alloy Deposited with Ti Film: Effects of N 2 –H 2 Ratio on Microstructure Evolution and Mechanical Properties

2024 Al alloys were firstly deposited with pure Ti film (~5.0 μm in thickness) using magnetron sputtering, then plasma nitrided for 8 h at 490 °C under N2–H2 gas mixtures with different N2 content (15, 40 and 50%). The Ti–N and Ti–Al diffusion reactions were activated simultaneously on the surface of Al alloy to obtain duplex coatings. The results showed that the duplex coatings were composed of nitride/aluminide layers (TiN0.3/Al3Ti/Al18Ti2Mg3 layers). With increasing the N2 content in the gas mixture, the peak ratio I(002)/I(100) of the outer TiN0.3 phase increased and the nitride particles grew larger. The surface hardness of coatings increased with the increasing N2–H2 ratio, reaching the maximum value of 630 HV under 50%N2 + 50%H2. The coatings obtained under gas mixture with high N content showed larger fluctuation at the initial stage during friction tests. All the obtained coatings showed the similar friction coefficient (~0.30) at the stable stage, much lower than the untreated Al alloy (~0.45). The sample nitrided under 50%N2 + 50%H2 exhibited the lowest wear rate, 60% lower than that of the uncoated one. The nitrided samples exhibited predominant abrasive wear and adhesive wear.