Effects of lanthanum and cerium on low temperature plasma nitrocarburizing of nanocrystallized 3J33 steel

Plasma nitrocarburizing of nanocrystallized （NC） 3J33 steel were carried out at 400 and 430 ℃ for 4 h in a mixed gas of N2：3H2 and different flow rates of rare earths （RE） La and Ce reagents in this paper. Effects of temperature, rare earth addition and its addition amount on the microstructure and hardness of the nitrocarburized layer of NC 3J33 steel were also investigated. Surface phase composition of the nitrocarburized samples was analyzed by X-ray diffraction. Metallurgical structure, La and Ce concentration and microhardness profiles of cross-sectional nitrocarburized samples were studied using an optical microscope, a scanning electron microscope equipped with an energy dispersive X-ray analyzer and Vickers microhardness tester, respectively. The results showed that the surfaces of the nitrocarburized samples were mainly composed of γ＇-Fe4N and α＇-Fe （α-Fe dissolved with N and C） when the NC 3J33 steel was nitrocarburized at 400 ℃. As the temperature was enhanced up to 430 ℃, the surfaces consisted of γ＇-Fe4N, α＇-Fe and low nitrogen compound FeNx （x=0.0324–0.0989）, and simple substance La was presented when RE flow rate was 0.1 L/min. The addition of La and Ce into nitrocarburized gas increased the thickness and hardness of the nitrocarburized layers. The samples nitrocarburized at 400 ℃ with RE flow rate of 0.025 L/min and 430 ℃ of 0.05 L/min possessed the thickest nitrocarburized layer, highest proportion of nitrides and hardness profile. RE elements could diffuse into the nitrocarburized layer and their concentration increased with temperature. The excess RE impeded the permeation of N, C elements and led to thinner compound layer as well as the diffusion layer.

Diffusion of lanthanum in nanocrystallized surface layer during plasma rare earth nitriding of 3J33B steel

Plasma rare earth nitriding of nanocrystallized surface layer of 3J33B steel at 350 and 410℃ for different time was studied. The microstructure observation and X-ray diffraction（XRD） analysis show that the nitriding layer consists of compound layer （γ′-Fe4N） and diffusion layer （α-Fe）. Lanthanum content profiles in nanocrystallized surface layer were measured using glow discharge spectometry（GDS）. The results show that lanthanum can diffuse into the surface layer of the steel to a large depth. Based on the experimental results mentioned above, the diffusion coefficients and activation energy of lanthanum in γ′ phase are calculated to be 1.03×10 -15 cm2/s （350℃）, 1.75×10 -15 cm2/s （410℃） and 31.313kJ/mol, respectively.

纳米化3J33钢低温渗氮生成相性能第一性原理表征

采用固溶处理、热轧、冷拔变形和电加热的复合技术实现了3J33马氏体时效钢纳米化,平均晶粒尺寸约为70nm。对纳米化的马氏体时效钢分别在390℃和360℃进行8h脉冲等离子体渗氮,利用XRD、显微硬度计和纳米硬度计对渗氮层生成相和性能进行了测试,并且基于第一性原理对渗氮相的性能进行了表征。结果表明,两个温度下渗层中生成的氮化相分别为γ′-Fe4N和FeN0.076,二者均具有较高的硬度和良好的塑性。计算结果表明,γ′-Fe4N和FeN0.076相中N与Fe原子的成键作用较强,且两相都具有延性。

纳米化表面360℃脉冲等离子体渗氮研究

本文主要研究了3J33马氏体时效钢表面纳米化后,在360℃下脉冲等离子体渗氮0.5～24h之不同时间下的渗氮行为。通过硬度法测量了渗层的厚度。结果发现,在360℃下即可实现快速渗氮。当渗氮时间达8h时,渗层可达50μm以上,硬度为800HV0.05左右;但8h后时间再延长渗层没有明显的增加。用X射线衍射法标定了渗氮层中相的成分。结果表明,渗层中并不含Fe4N或Fe3N,而是含氮量较低的低氮化物FeN0.0324。纳米压痕试验表明,渗氮后渗层硬度可达13GPa左右,比未渗氮试样硬度高3倍以上。

纳米化3J33钢稀土氮碳共渗生成相第一性原理表征

通过稀土化学热处理和第一性原理计算方法研究了纳米化3 J33钢在500℃脉冲等离子体稀土氮碳共渗4h时共渗层的相结构和硬度分布以及共渗相的性质。结果表明,共渗层由厚度约5μm的化合物层和90μm的扩散层组成;共渗相主要由γ′-Fe4N和含碳氮的α′-Fe相组成;与纳米化的3 J33钢相比,渗层表面硬度提高约1倍,基体时效后硬度也有所提高。计算结果表明,在共渗过程中γ′-Fe4N相较α′-Fe相更容易形成,但α′-Fe相更加稳定;γ′-Fe4N相的硬度高于α′-Fe相的硬度的主要原因是γ′-Fe4N相的N-Fe键较α′-Fe相的C-Fe键更强;此外,γ′-Fe4N相的体模量与剪切模量的比值大于1.75,因此该相具有延性。