Plasma Nitriding of ISO 5832-1 Stainless Steel at 425 ºC with Intermittent Nitrogen Flow

The aim of this work was to study the influence of varying nitrogen potential during plasma nitriding of stainless steel ISO 5832-1. The control of nitrogen potential was achieved by pulsing the nitrogen flow for different times (01/19, 02/18, and 05/15), where the numbers represent the time (in minutes) of nitrogen flow on/off, thus creating an intermittent flow of nitrogen during the treatment. For all tested conditions - continuous and pulsed flow of nitrogen - the treatment temperature was kept at 425ºC during 2 and 8 hours. Specimens were characterized by means of X-ray diffraction, scanning electron microscopy, optical microscopy, energy-dispersive X-ray spectroscopy, and nanohardness. Results showed that the layer thickness increases with the increase of total treatment time, and decreased for shorter times of pulsed nitrogen flow. Smaller expansion of the austenite phase, as well as less precipitation of chromium nitrides, were also observed for shorter times of pulsed nitrogen flow. Hence, the use of intermittent nitrogen flow appears to be an efficient approach in order to control the nitrogen concentration within the layer, reducing its brittleness.

本研究旨在探讨ISO 5832-1不锈钢等离子渗氮（plasma nitriding）过程中可变氮势的影响。氮势控制通过对氮气流量进行脉冲调控实现，脉冲参数采用01/19、02/18与05/15三组配比，其中每组数字依次对应氮气导通与断开的时长（单位：分钟），由此在处理过程中形成间歇性氮气供给。所有测试工况（含连续式与脉冲式氮气流量模式）下，渗氮处理温度均维持在425℃，保温时长分别为2小时与8小时。研究采用X射线衍射（X-ray diffraction）、扫描电子显微镜（scanning electron microscopy）、光学显微镜、能量色散X射线光谱（energy-dispersive X-ray spectroscopy）及纳米硬度测试对试样进行表征。结果表明，渗层厚度随总处理时长的增加而增大，且随脉冲氮气供气时长的缩短而降低；采用更短脉冲氮气供气时长的试样，其奥氏体相（austenite phase）膨胀程度更低，同时氮化铬（chromium nitrides）析出量也更少。因此，采用间歇性氮气供气工艺可有效调控渗层内的氮浓度，进而降低渗层脆性，是一种高效的工艺优化路径。