Influence of process time on microstructure and properties of 17-4PH steel plasma nitrocarburized with rare earths addition at low temperature
Introduction
17-4PH martensitic stainless steel has high strength, excellent corrosion resistance and relatively simple heat treatment and has been widely utilized in industries [1]. However, it has relatively low surface hardness and poor tribological properties and cannot meet challenging design requirements of high strength, high toughness and good resistance to both corrosion and wear in some applications [2]. To solve these problems, many surface modifications have been conducted on 17-4PH steel, including gas nitriding [3], [4], plasma nitriding [2], [5], ion implantation [6], [7], laser alloying [8], and salt bath nitrocarburizing [9]. More recently, the mechanical properties study has been done on this steel plasma nitrocarburized at 430 °C [10].
The thermochemical treatments with rare earths (RE) addition were investigated at the beginning of the 1980s [11]. In the following many years, a number of advantages of the new treatments have been ascribed to RE addition [12], [13], [14], [15]. They concluded that the RE elements could diffuse into a significant depth of steels during gas carburizing, gas carbonitriding, gas nitrocarburizing, and plasma nitriding. The incorporated RE elements can help interstitial species (such as carbon and nitrogen) diffuse into a significant depth of the steel, and help improve the microstructure and enhance mass transfer and kinetics, and then improve the property of the modified layer.
Low temperature plasma nitriding and low temperature plasma/gas carburizing of austenitic stainless steel have been extensively studied by researchers in recent years [16], [17], [18]. Moreover, plasma nitrocarburizing is another well-established plasma nitriding method [19]. More recently, low temperature plasma nitrocarburizing of austenitic stainless steel has also been studied by many researchers [20], [21], [22], [23]. In contrast to the very hot research and development activities on austenitic stainless steel, much less attention is given to thermochemical processing of martensitic stainless steel particularly low temperature nitrocarburizing with and without RE addition [10], [24]. Based on the previous study of low temperature plasma nitrocarburizing of 17-4PH steel [10], the microstructure and properties of this steel plasma nitrocarburized at low temperature with RE addition were systematically investigated.
Section snippets
Experimental procedure
The solution treated [10] 17-4PH steel with the chemical composition (wt.%) 0.06C, 17.22Cr, 3.91Ni, 3.01Cu, 0.16Nb, 0.81Si, 0.76Mo, and balance Fe was used. Prior to plasma nitrocarburizing, the steel specimens for wear tests, corrosion tests and other detections were machined into 28 mm × 14 mm × 3 mm, Ø 15 mm × 4 mm and 13 mm × 13 mm × 6 mm, respectively. The flat faces of the specimens were ground from 240 to 1200 grades by SiC papers, and then ultrasonically cleaned with alcohol and distilled water in
Optimization of the amount of RE addition
As shown above, the RE reagents containing different RE content were added in plasma nitrocarburizing atmosphere. Fig. 1 displays the microstructure of the nitrocarburized layer obtained under different RE reagent content. Table 2 gives relevant results of the surface hardness and layer depth measurements. It can be seen that the depths of the modified layers are about 18, 18, 19, 21, 22, and 21 μm, respectively. That is to say, the depths of the modified layers first increase and then decrease
Conclusions
- (1)
A significant layer depth can be obtained by plasma nitrocarburizing at 430 °C with RE addition for different time on 17-4PH steel surface. The microstructure of plasma RE nitrocarburized layer is featureless under optical microscopy at proper process time, except for traces of ‘dark’ phases formed. The optimal RE reagent content is 0.100 L/min in the present test condition. The modified layer depths increase with process time and the layer growth conforms approximately to the parabolic law.
- (2)
The
Acknowledgements
The authors gratefully acknowledge the National Natural Science Foundation of China (Grant No. 50871035 and 70701012) for financial support of this research work. In addition, the authors would like to thank, Master D.L. Wu for her experimental assistance.
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