فایل ورد کامل تغییرات فاز در فولاد کربن ساده واکنش کروم ۰?۴۵ wt? C در دمای پایین

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بخشی از مقاله انگلیسیعنوان انگلیسی:Phase transformations in low-temperature chromized 0.45 wt.% C plain carbon steel~~en~~

Abstract

The phase transformations occurring in a 0.45 wt.% C plain steel subjected to plasma nitriding at 540–۵۶۰ °C for 5.5 h, followed by a salt bath thermoreactive deposition and diffusion (TRD) chromizing process at 500 °C or 550 °C (a process referred to as low-temperature chromizing or duplex chromizing) was investigated by means of optical microscopy(OM), scanning electron microscopy(SEM), X-ray energy dispersive spectroscopy(EDS), and X-ray diffraction. It was found that a CrN compound layer with an average thickness of 7.4 m and an average microhardness of 1476 HV0.01 was formed in the prior plasma nitrided compound layer by low-temperature chromizing at 550 °C for 6 h. The chromized coating as a whole was found consisting of three sub-layers, namely the outer CrN layer, the intermediate diffusional layer, and the inner residual nitrided compound layer, all formed in the prior nitrided compound layer, and with the inner sub-layer vanishing by prolonging the chromizing time. The intermediate diffusional layer formed at the initial stages of TRD was seen “black” under OM (hence is called “black zone”), and found consisting of -Fe as a major phase. The self-exhaustion of the “black zone” promoted the chromium atom diffusion deeper into the substrates. The transformation paths involved in the decomposition of the prior nitrided compound layer was likely to be -Fe2–۳N- Fe4N-Fe; and the high hardness of the chromized coating was attributed to a large amount of nano-sized and evenly distributed CrN grains generated in the compound layer.

۱ Introduction

As a widely used surface coating technology, chromizing is employed to economically develop corrosion- and/or wearresistant layer on metal or alloy substrates [1]. Due to the limited diffusivity of chromium at low temperatures, however, all the conventional chromizing processes are usually conducted at temperatures above 1000 °C for a duration of 6–۱۰ h [2]. In view of the susceptibility of the substrate microstructure to coarsening and deteriorating at such high temperatures, a lower temperature chromizing process was developed by the authors in order to avoid these negative effects of the conventional chromizing processes on microstructure, and to promote the application of chromizing techniques to various substrate materials. Duplex surface technique [3], involving the consecutive application of two or more established surface engineering techniques to a component, so as to produce a surface layer with multiple properties which can not be obtained by any single technique, is gaining increasing attentions in surface modification engineering. There is a duplex process which combines nitriding or nitrocarburizing with thermoreactive deposition and diffusion (TRD) process. Chicco et al.[4] assessed the effects of various pre-treatments (gas nitriding, liquid nitrocarburizing, or liquid carburizing) on the TRD of vanadium on AISI H13 steel at 1000 °C. High temperature TRD performed above 700 °C and low-temperature TRD conducted below 700 °C are classified in the present study, based on the Ac1 critical temperature of the steel. In the high temperature TRD processes, carbideforming elements such as chromium, niobium, titanium, and vanadium, while reacting with carbon diffusing from the interior of the substrate, are deposited on the surface of substrates by a thermo-chemical process. A 4–۷ m thick carbide layer is produced in 10 min to 8 h [5], depending on the bath temperature and the type of steels used. For a low-temperature coating, however, a rather long time is needed to form a significant carbide layer because of the lower diffusivity of carbon at low temperatures, so that a plain TRD process is useless for the practical purposes. However, Arai et al. [6] pointed out that compound layer consisting of iron nitrides produced by nitriding is very effective in producing a vanadium carbonitride layer with adequate thickness at 550–۷۰۰ °C for practical applications. There is at present some evidence that a hard coating consisting of nitride or carbonitride of chromium may be produced at temperatures below 700 °C by a duplex process involving nitriding and low-temperature TRD. Arai et al. [7] claimed that a CrN layer of about 8 m in thickness was formed on a nitrided substrate after the nitrided substrate was further chromized at 570 °C for 50 h in a fluidized bed reactor. King et al. carried out nitrocarburizing plus a fluidized bed low-temperature TRD process on AISI H13 [8] and plain carbon steels [9] at 570 °C, and found that a “cover layer” consisting of porous iron nitride was formed during the nitrocarburizing which significantly influenced the microstructure of the Cr(N,C) coating, thus suggesting using a higher temperature or prolonging the treatment time to achieve a thicker Cr deposition. Fabijanic et al. [10] developed a duplex process involving a precursor nitrocarburizing plus a fluidized bed low-temperature TRD on AISI H13 at 575 °C, claiming that while producing a good chromized coating, the low-temperature treatment did not alter the properties of the substrate interior. A chromizing process conducted in a salt bath at temperatures ranging from 500 to 550 °C via TRD with specimens subjected to a precursor plasma nitriding at 540–۵۶۰ °C is referred to as low-temperature chromizing or simply as duplex chromizing. To the authors’ knowledge, little has been reported in the literatures regarding the duplex chromizing of steels at temperatures below 550 °C. Therefore, the purposes of this study are to further explore the duplex process, and to clarify the transformations occurring in the prior plasma nitrided compound layer during the low-temperature chromizing process, based on the previous studies [11,12].

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