Mechanism Of Microstructural Superrefinement And Deep Diffusion Of Plasma Rare Earth Nitrocarburized Iron-base Alloys

M50Ni L martensitic steel is classified into a new generationhigh-strength bearing steel due to its inimitable fracture toughnessand fatigue life, which has been widely used for special purpose including aerospace engines. Generally speaking, the most common failures of bearings are due to surface damage such as contact fatigue and wear. Therefore the excellent surface properties are deserved. However, there are few researches have been carried out on the surface modification of M50 Ni L steel so far. How to get a super-refiment and deep diffusion layer by thermochemical treatment has become the hot topic. In this study, in order to achieve the requirements of high strength and toughness of nitrocarburized layer of M50 Ni L steel, plasma nitrocarburizing with rare earth(RE) addition was carried out. The cyclic nitrocarburizing with varying temperature was designed to obtain the super-finement nitrocarburized layer. Meanwhile, the catalytic mechanism of RE elements during low-temperature RE nitriding/nirocarburizing for various iron-base alloys was discussed.M50Ni L steel was plasma nitrocarburized in a and g zones with RE addition. The effects of temperatre, N-H ratio and duration on microstructure and phase composition of nitrocarburized layer were discussed. The results show that the a zone nitrocarburzied layer is characterized by a diffusion layer without the compound layer. The coarse martensite laths are observed by TEM. T he surface layer mainly consists of a′N、g ′-Fe4 N and e-Fe2-3N. The phase composition is mainly depends on temperature rather than N-H ratio. When increase the temperature to g zone, the cross-sectional microstucture is similar as the a zone nitrocarburzied layer. The phase composition of g zone nitrocarburzied layer is determined by temperature as well as N-H ratio. The proportion of g ′-Fe4 N decreases with the temperature increasing. The g-Fe N0.076 phase can be detected after nitrocaburizing at 650°C for 1h with N-H ration of 0.3:0.1 L/min, which indicates the beginning of austenitizing.Based on the law of phase transition of a and g zone nitrocarburzied layers and thermodynamic calculation, the cyclic RE nitrocarburizing with varying temperature was designed to obtain the super-refined nitrocarburzied layer. The results show that the super-fined crystal with nano size forms on the surface of RE nitrocarburized layer and the local inner nitrocarburized layer(30 mm) is also nanocrystallized. The surface of g?a cyclic RE nitrocarburizd layer is composed of super-fined a′N+g ′-Fe4 N or single g ′-Fe4 N nano-grains, while the surface of g cyclic RE nitrocarburizd layer is composed of super-fined a′N+amorphous. The mechanism of super-refinement can be described as two steps. Step 1: Austenitizing and the formation of g-Fe N0.076. Step 2: phase transition of g-Fe N0.076®a′N + g ′-Fe4 N and g-Fe N0.076®a′N(martensitic transition) occur during cyclic with decresing and increasing temperature, respectively. On the other hand, the stability of e and g ′ nitrides is decreased with alloy elements addition, which restricts the growth of nitrides and maintains the small size.The hardness and wear resistance of M50 Ni L steel were dramatically improved by plasma RE nitrocarburizing. The wear mechanism transfers form oxidation to abrasive and adhesive with sliding speed increasing. The g?a cyclic RE nitrocarburizd layer treated with 2 times presents the best wear resistance. The RE addition obviously improves the toughness of nitrocarburized layers which brings excellent wear properties. The super-fined microstructure also results in high strength and toughness as well as higher wear resistance.The catalytic effects of RE elements have been observed in many experiments. However, the mechanism is still unclear, especially in plasma thermochemical treatments. In this study, the catalytic mechanism of RE elements will be investigated systematically. The results of experiments and thermodynamic calculations prove the attractive nature between La and N atoms during plasma RE nitriding. The catalytic mechanism of RE elements can be available in two aspects of physics and chemistry. Firstly, under the co-works of La and La Fe O3, the treated surface becomes rough and the specific surface dramaticly increases, wh ich is beneficial for the adsorption of N atoms. Secondly, the attractive nature between La and N makes a higher N concentration on the treated surface. Meanwhile, the separation between La and N occurs through the O adsorption function of La Fe O3. The unbonded N atoms diffuse deeply into the specimens.Based on the calculated results of the formation energy of oxygen defects in the alloyed La Fe O3, the non-catalytic phenomenon of high alloy steel has been explained and the design idea of deep-nitrided steels has been proposed. Ni could decrease the formation energy of oxygen defects while Cr, Mo, V and Mn show the opposite tendency. The deep-nitrided steels should contain suit amount of Ni and decrease the amount of Cr, Mo and V as much as possible.