冷却方式及时效处理对580℃气体渗氮层的影响

The Effect of Cooling Methods and Subsequent Ageing on the Nitrided Layer of Pure-iron by Gas Nitriding at 580 ℃

工业纯铁（厚度为1mm）经580℃气体渗氮4h后进行不同方式（油淬,炉冷）的冷却,然后对油淬的试样进行自然时效和100-220℃的人工时效.采用X射线衍射仪、扫描电镜、透射电镜、维氏显微硬度计和拉伸疲劳实验机等研究了不同热处理条件下渗氮层的微观组织结构和力学性能.结果表明：渗氮试样完全由化合物层和扩散层组成,冷却方式不改变化合物层的厚度和最外层的相结构,却显著影响扩散层的微观组织和力学性能.油淬试样的扩散层在自然时效过程中形成大量的氮原子团簇从而产生了自然时效强化;人工时效可以控制油淬试样扩散层中析出相的种类和数量,低温时效的析出相主要为α′′-Fe16N2相,较高温度时效的析出相主要为γ′-Fe4N相;自然时效样品具有很高的硬度和强度,人工时效样品的硬度和强度随时效温度的升高而降低,较高温度时效有助于提高延伸率;化合物层对渗氮层的强度没有很大影响,但却显著降低了渗氮层的拉伸疲劳强度.

A pure iron （1 mm in thickness） was gas nitrided at 580 ℃ for 4h, followed by oil quenching or furnace cooling. The oil-quenched samples were first naturally aged （NA）, and then artificially aged between 100 ℃ and 140 ℃. The microstructural examinations and property characterization were investigated using X-ray di？ ractometer, scanning electron microscope, transmission electron microscope, Viekers hardness tester and fatigue tester. It was found that the nitrided samples consisted of compound layers （CLs） and diffusion layers （DLs）. The cooling method did not change the thickness of the CLs and the phase constitutes of the upper--most layer, but had remarkable effect on the microstructure and mechanical property of the DLs. A great quantity of rich-N clusters precipitated from the oil-quenched DLs after natural ageing for 200h, and the oil-quenched DLs had a great natural ageing strengthening phenomenon. During artificial ageing （AA）, the type and number of precipitates could be controlled by the ageing tem perature and ageing time. The main precipitates are the α＂-Fe16 N2 during lower temperature ageing while it is the γ＇-Fe4 N at higher temperature ageing. The NA samples could get very high hardness and strength. The hardness and strength of the AA samples decreased with the increase of ageing temperature. The high temperature ageing greatly improves the elongation of the oil-quenched samples. The CLs had little effect on the tensile stress of the nitrided samples, but greatly decreased the fatigue strength under the tensile.