Microstructure And Properties Of Medium Carbon Low-alloy Steels Treated By Plasma Nitriding And Laser Quenching

The surface modification with enhanced layer depth is a challenging task to be resolved in the field of surface engineering. Relatively deeper modified layer can be obtained by increasing the temperature or the nitriding times of thermochemical treatments. However, these stategies result in the coarsening of the microstructure and deterioration of the mechanical properties. In this work, the low-temperature nitriding and low-temperature nitrocarburizing of the medium carbon low-alloy steels are investigated to explore the correlation between the microstructure and properties. The rare earth element is applied to the nitrocarburizing processes to further improve the case depths due to its catalytic and micro-alloying effects. In addition, the duplex treatment of plasma nitriding(nitrocarburizing) prior to laser quenching process for the steels 38 Cr Mo Al/30 Cr Mn Si is investigated as a noval deep surface modification technique. The mechanism involved is proposed.The low-temperature nitriding is conducted at 460~500°C to suppress the veinlike structure and enhance the case depth. The low-temperature nitrided layer shows a “white layer” of 30mm thickness without obvious amount of compound layer. The XRD is carried out by removing the outer surface gradually to obtain the diffraction patterns of different depth. Significant broadening diffraction peaks of e-Fe2-3N and g￠-Fe4 N phases can be seen on top of the nitrided layer, showing an amorphous characterization. The diffraction peaks at the position of 20 mm deep from the nitrided surface are basically identical with that of the matrix, which shift to lower 2 theta angles. Both the XRD and TEM results demonstrate the refinement of the nitrided layer。The grain size is about tens nanometers. The nano-crystallites are identified as a￠N by SEAD.A precipitation free modified layer with single-phase structure in the form of bcc type solid solution(a￠N) is prepared on steel 38 Cr Mo Al by plasma nitriding under low nitrogen containing atmosphere(N2:H2, 1:8 vol.%) at a high temperature of 590°C. TEM observation shows that the substructure of a￠N phase is high density dislocation. Compared with g￠-Fe4 N and e-Fe2-3N phases, a￠N has a much higher hardness and a slightly increased Young’s modulus. The ratio of H/E for a￠N is 25.5% and 28% higher than that for g￠-Fe4 N and e-Fe2-3N phase, and the ratio of H3/E2 for a￠N phase is 141.67% and 132% higher than that for g￠-Fe4 N and e-Fe2-3N, respectively. In addition, the ratios of H/E and H3/E2 for a￠N phase obtained by low temperature nitriding are further improved a factor of 4.7% and 25.9%, respectively. That is, the nitrogen expanded martensite(a￠N) shows a good combination of strength and toughness even superior to the nitrides, which is attributed to the strengthening effects of grain refinement, solid solution and high dislocation density.The crystallographic texture of g￠ in the compound layer and its influences on the tribological properties are investigated. The preferred orientation of(200) g￠ is produced under the nitriding condition of low temperature and low nitrogen containing atmosphere and increases with the nitriding time. The preferred orientation of(220)g￠ appears in the surface layer after 72 h cyclic nitriding. Such orientation relationships(ORs),(0001)e//(101)a￠ and [110]e//[111]a￠,(111)g￠//(0001)e and [011]g￠//[1 2 10]e,(200)g￠//(110)a￠ and [011]g￠//[111]a￠, as well as(1 1 03)e//(220)g￠ and [0100]e//[110]g￠ are established. Further, the misfit of interatomic distance(δ) is calculated. The value of δ is an indicator of phase transition resistance. Therefore, two kinds of reaction pathways during nitriding: a￠N®g￠ and a￠N®e®g￠ can be assumed. In addition, the observed preferred orientations have close relationship with the reaction pathways. Results of mechanical tests show that the preferred orientation of(200)g￠ usually results in lower frictional coefficient and wear rate in comparison with the preferred orientation of(220) g￠.(111)g￠ texture usually relates to the lower frictional coefficient but higher wear rate, due to the slip system parallel to the sliding plane. Therefore, the preferred orientation of(200)g￠ has a positive significance in improving the wear properties of steels.Nitrocarburizing with rare earth(REs) addition is successfully employed as an effective method for acquiring thicker case depth in comparison with the ordinary nitrocarburizing process. The case depths for RE nitrocarburized layers are enhanced remarkably(19.71%~44.23%) compared with the ordinary nitrocarburized layers. Based on experiments, the merits of RE addition during nitrocarburizing lies in the formation of La Fe O3 and g￠-(Fe,La)4N. The high catalysis sensitivity of the La Fe O3 can promote the decomposition of the nitrides (usually prevent the N diffusion), thus providing more diffusion paths for N atoms. La atoms can diffuse into a distance of 5mm in the surface layer and form g￠-(Fe,La)4N in the surface layer. The expanded lattices for g￠-(Fe,La)4N due to the larger atomic size of La increase the solution of N, which also hamper the formation of e-Fe2-3N(C). TEM images demonstrate the refinement of the surface layer and the diffusion of La atoms to the depth of 25 mm. Therefore, the strong influence of the RE elements makes them favorable catalysts for the nitrocarburizing of steels.Laser quenching(LQ) is utilized as subsequent procedure of typical plasma nitriding(PN) or plasma nitrocarburizing(PNC) processes to improve the surface properties of two kinds of low-alloy steels of 38 Cr Mo Al and 30 Cr Mn Si. Laser quenching of plasma nitrided(nitrocarburized) specimens shows a great increase in the thickness and hardness of the modified layer in comparison with the PN or LQ treatment. The nitrides, such as ??-Fe4 N and ?-Fe2-3N(C), decomposed after laser quenching. Instead, quenched martensite ?? and iron oxides(Fe3O4 with trace of Fe2O3 and Fe O) with certain amount of low nitrogen compound(retained austenite) form on the surface. The wear property for the duplex treated layer is better than the PN/PNC layer, which is attributed to the higher hardness in the subsurface layer, lubrication action of the Fe3O4 as well as the improved impact toughness due to the retained austenite.The great increase in case depth by the duplex treatment of PN/PNC+LQ is attributed to the addition of nitrogen element, leading to a reduction of the austenitic transition temperature from 727?C to 565?C according to the phase diagrams. In other words, the depth can be quenched effectively increases under the same temperature distribution in the surface layer.