Research of Electrolytic-plasma Carbonitriding and Nitriding Influence on Phase Composition of the Stainless Steel

Маzhyn Skakov 1a, Sherzod Kurbanbekov 1b, Almira Zhilkаshinova2, 1 D.Serikbayev East Kazakhstan State Technical University, Ust-Kamenogorsk, Kazakhstan

2 S.Amanzholov East Kazakhstan State University, Ust-Kamenogorsk, Kazakhstan

1askakovmk@mail.ru, 1bsherzod05_88@mail.ru, 2almira_1981@mail.ru

Keywords: Phase structure, Modified surface, Optimal regime.

Abstract. In the present work we have studied the phase structure of surface modified layer of

austenitic steel 12Cr18Ni10Ti after electrolytic-plasma carbonitriding and nitriding. It was

determined that the carbonitriding and nitriding with the subsequent hardening formed carbide and

nitride phase. Also it is revealed that steel 12Cr18Ni10Ti after the electrolyte-plasma processing has

high hardness. The microstructure of samples surface is presented by martensite and residual

austenite. Optimum modes of steel 12Cr18Ni10Ti carbonitriding and nitriding by electrolytic-

plasma way have been identified.

Introduction

The analysis of modern condition of the problem shows that effective way of surface strengthening

of steels and alloys with prospects for further development is a chemical-thermal processing (CTP)

[1]. The diversity of types and variants of CTP gives the possibility to select the optimum method

of strengthening on the basis of economic and operational tasks [2]. Methods of high-speed heating,

cathode heating in electrolytes included, has a number of advantages from the point of view of its

use for thermal or chemical-thermal process [3,4]. Chemical-thermal hardening, in particular

carbonitriding and nitriding, allows to increase as mechanical (hardness, strength, endurance) and

corrosion-resistant properties. Carbonitriding constantly expanding use is due to a number of this

hardening method advantages as compared to others: the lower temperature of saturation process,

more high strength properties of parts. At the same time, the advantages of electrochemical-thermal

nitriding are significant at few minutes short-term treatment allowing to get low nitrogenous layers

with thickness up to 30 µm [5]. With simultaneous diffusion of carbon and nitrogen in the austenite

the diffusion of carbon is accelerated, despite this, nitrogen increases the content of residual

austenite in the structure of hardened modified layer. Residual austenite delays the fatigue

violations occurrence and increases the fatigue strength of nitrocarbonized parts. The presence of

nitrogen in the solid solution increases the stability of the supercooled austenite. To combine short-

term nitriding with hardening, electrolytic heating is especially useful because of the hardening

possibility in the same solution, on the same equipment, simply by deenergization.

The purpose of this work is the phase composition study of steel 12Cr18Ni10Ti surface under

electrolytic-plasma carbonitriding and nitriding with the subsequent hardening.

For the object of study we have chosen steel of austenitic grade 12Cr18Ni10Ti, the chemical

composition of which is presented in Table 1 [6].

Materials and methods of research (Experimental)

Table 1. The chemical composition of austenitic steel 12Cr18Ni10Ti (wt.%)

Fe C Cr Ni Ti Si Mn P S

base 0.12 17.00 10.66 0.50 0.34 1.67 0.032 0.013

For research we produced plate samples 30х30х5 mm3 by size of 12Cr18Ni10Ti steel rolled

sheets. Electrolytic-plasma carbonitriding and nitriding was carried out on the developed by us

equipment [7] as follows: first, heated sample at a voltage of 320 V and current 20-25 A within a

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temperature range of 700-900 0C and withstand voltage for 10-12 seconds. The sample heating was

made by plasma, with the sample partially immersed in the electrolyte to a depth of 4-5 mm, then

lowered the voltage up to 170 V and current up to 10-12 A and kept at a temperature during 5-7

minutes, after which we carried out quenching in cooled electrolyte flow. For the electrolyte we

used: for carbonitriding aqueous solution containing carbamide 15%NH2CONH2, 10 % glycerol

(С3Н8О3) and 10 % sodium carbonate (Na2CO3), for nitriding: solution containing urea

20%NH2CONH2, and 10 % sodium carbonate (Na2CO3). Modes are shown in Table 2.

Table 2. Processing modes Type of treatment Holding temperature

0C Exposure time, min

Carbonitriding 850 7

Nitriding 750 7

The electrolyte temperature was supported as 25±5 0C at the entrance to the chamber. Heating

temperature was measured with a digital temperature meter ATA-9380.

X-ray structural analysis of the surface layer of the samples were collected at the diffractometer

XRD-6000 using CuKα- radiation in the center of collective use of TSU in March 2013. The

morphology of the surface structure was studied by the optical microscope Altami MET 1M in

scientific research Institute «Nanotechnology and new materials» at D.Serikbayev EKSTU.

Research Results and their Discussion

Figure 1 shows a fragment of the surface microstructure of steel 12Cr18Ni10Ti after the etching in

10% oxalic acid solution for 30-90 sec. The results of metallographic research testify to the fact that

in the initial state the microstructure is austenite (Fig.1a). The surface of the sample after the

treatment of electrolytic-plasma by carbonitriding at a temperature of 8500С within 7 minutes is

presented on Figure 1b. It is seen that after the processing near grain boundaries iron carbides were

formed. It is known that the formation of carbides on the limits of austenite grains has a direct effect

on the mechanical properties of steel. On Fig.1с microstructure is presented after electrolyte-plasma

nitriding at a temperature of 7500C within 7 min. Particles of nitrides are visible. Thus, the

microstructure of the modified surface in both cases represents by itself martensite structure and

residual austenite.

Fig.1. The microstructure of steel 12Cr18Ni10Ti surface (a) the initial, and b) after the electrolyte-plasma

carbonitriding at a temperature of 850 0C within 7 min, c) after the electrolyte-plasma nitriding at a

temperature of 750 0C for 7 minutes.

50 50

c

50

ba

25 25

grain boundary

γ- austenite α′- martensite

particles nitride

α′- martensite

Applied Mechanics and Materials Vol. 404 41

Fig.2 displays the x-ray diffraction pattern showing changes in phase composition of steel

modified surface. After electrolytic-plasma carbonitriding in the surface microstructure of the

samples there are reinforcing phase Fe3C (carbides), Fe2-3N (nitrides), and α′-phase on the basis of

Fe, which testifies to the emergence of martensite hardening (2b). After electrolyte-plasma nitriding

on the surface of the samples phase Fe2-3N (nitrides), CrN (chromium nitride), α′-martensite and

residual γ-austenite (Fig. 2c) have also been found. According to the results of x-ray structural

analysis, we may speak about the best mode for carbonitriding and nitriding. Optimal parameters of

the electrolytic-plasma carbonitriding are - temperature 8500С with an exposure for 7 minutes, and

for electrolyte-plasma nitriding - the temperature of 750 0C for 7 minutes.

Fig.2. X-ray diffraction patterns of steel 12Cr18Ni10Ti a) in the initial state, and b) after carbonitriding

within 7 min at the temperature of 850 0C c) after nitriding within 7 min at a temperature of 750

0C.

Conclusions

On the basis of the analysis results made by optical microscopy and x-ray diffraction studies of the

electrolytic-plasma carbonitriding impact and nitriding over samples surface of steel 12Cr18Ni10Ti

you may come to the following conclusions.

It has been found that in the electrolytic-plasma carbonitriding at a temperature of 8500С within

7 minutes and electrolytic-plasma nitriding at a temperature of 750 0C within 7 minutes in the

microstructure of steel 12Cr18Ni10Ti surface allocated carbides, nitrides and martensitic structure

as well as residual austenite appeared.

Inte

nsi

ty

42 Engineering Decisions for Manufacturing Systems

It is established, that after the electrolyte-plasma processing particles of hardening phases of

carbides Fe3C, nitrides Fe2-3N, CrN appeared. The main structure is represented by γ-phase, also

contains the individual particles of oxides Fe3O4, and - martensite. The emergence of a new phase

contributes to hardening of steel 12Cr18Ni10Ti modified surface.

Acknowledgements

This work was financially supported by the JSC «NADT» of the Republic of Kazakhstan in 2012-

2014.

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Applied Mechanics and Materials Vol. 404 43

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