Hardening Roll Surface by Plasma Nitriding with Subsequent Hardfacing

The wear of the surface layer of rolls after ion nitriding in glow discharge, followed by a coating of TiN -TiAlN plasma arc are studied and simulated. stress-strain state of the material rolls under asymmetric rolling with ultra-high shear deformations is simulated. The effect of thermal fields, formed upon contact of the tool and a deformable sheet, the structure of aluminum alloys, are considered.


Introduction
The purposeful asymmetry in the process of plastic deformation is gained due to the roll velocity misalignment by proportions from 1:2 to 1:4, when the cold rolling is provided under the high contact friction with severe single deformations (no less than 50%) [1]. The asymmetry results in the significant increase of the friction force which provides the high shear deformations of the material, and in the surface wear of rolls.
Currently, the development of the new ion-plasma techniques for hardening the roll surface which provide the purposeful change of the structure and phase composition of the surface layer and the physical and mechanical characteristics of the tool [2,3] is an urgent issue. The stimulating techniques of ionnitriding with the crossed electric and magnetic fields [4][5][6][7][8] due to the charged particles densification followed by the TiN-TiAlN coating are considered to be perspective. The material modification is achieved through the increase in the number of the ionizing events, the ion mixing, sputtering, flash heat, atom and molecule deposition, defect formation, chemical interaction, and the radioactive diffusion stimulation.

Materials and Research Methods
The numerical simulation of the asymmetric thin sheet rolling was performed as a two-dimensional problem using the DEFORM 2D software. Also, the metal heating was considered, so the thermal and mechanical problems were solved as well. To solve the thermophysical problem, the thermophysical properties of the Al 5083 alloy were predefined as follows: the heat conductivity coefficient was 180.2 N/s/°С, heat capacity coefficient was 2.433 N/mm 2 /°С, emissivity was 0.7. Also, the thermophysical properties of the roll material were predefined -AISI D2 from the DEFORM 2D library: the heat conductivity coefficient was 50.71 N/s/°С; the heat capacity was 3,81 N/mm 2 /°C. The curve of the Al 5083 yield was predefined from the DEFORM 2D library within the temperature band 20 -500 °С.
The experimental setup to research the glow discharge characteristics in crossed electrical and magnetic fields was developed using the commercial ELU-5 equipment. The setup uses the pulsed power supply with the duty cycle S=80 % and the frequency of 50kHz.
The vacuum chamber was equipped with a standard magnetron with a length of 450 mm, a height of 50 mm, and a width of 100 mm and the magnetic field induction of 0.03 T, which was attached to the regular place of the vacuum chamber. As the working gas, argon as well as the mixture of nitrogen, argon and acetylene in the proportion of N2 50% -80%, Ar 25% -10%, C2H2 25% -10% were used. After the pressure got below 2 Pa, the chamber was blown with argon and filled with the working gas. The working gas pressure was altered within 5 -200 Pa. The experimental circuit of the experimental ion nitriding is displayed in figure 1. The research was conducted with the cylindrical-shaped samples of the shear steel Р6М5 and Х12, each was 12 mm in diameter and 4 mm high. The samples were mechanically polished before nitriding. After nitriding, the samples were coated by TiN and TiAlN compositions using the NNV-6.6-I1 machine. The process sequence was as follows. Firstly, the sample surface was cleaned and activated with argon plasma for 20 min when the bias voltage of -700 V was applied to the specimens. When cleaning, the samples were heated up to 440 ºС. Then, they were coated with the TiN and TiAlN compositions. The following coating modes were applied: the nitrogen pressure was 0.11 -0.13 Pa, the discharge current was 15A supported by altering the heating current of the emission cathode, the arc current of the titan cathode was 90A, of the aluminum one 60A. The stoichiometric compound of the TiN and TiAlN coating was obtained checking the arc current of the titan and aluminum cathodes. Simultaneously, the bias voltage of -200 V was applied to the materials. The temperature of the specimens under coating did not exceed 500 ºС.

Results and Discussion
The analysis of the numerical simulation in DEFORM 2D revealed that, comparing to the symmetric distortion, the rolling force decreased by 2.4 -3.2 times under 50 -75% spellerizing and the misalignment of roll velocities Vi/V2 = 2 -4. However, there was a significant torque increase on the rolls: by 1.5 -3.5 times on the bottom roll, by 1. The results of the electron microscopical study of samples demonstrate that the nitriding in crossed electrical and magnetic fields of Р6М5 and Х12 steel results in the formation of nitride layer under which a diffusion sublayer with microtexture is formed.
When nitriding the Р6М5 steel in a glow discharge plasma for 4 hours, a hardened layer of 200 µm is formed ( figure 2). It is known [2] that the hardened layer height reaches 25 µm after 3 hours of gas nitriding.
The analysis of X12 microtexture revealed the thin nitride zone of 10 -15 µm (figure 3) with minimal grain boundary nitride precipitate in the diffusion zone. The nitride layer in the images of microtexture looks to be completely texture-free, it includes the iron nitride and the alloying element nitride. Both the nitride and diffusion layers observed microscopically are considered as the common thickness of the nitrogenized layer. The research of the microtexture and the phase composition of the diffusion sublayer (figure 7) revealed the nitrogenous solid solution of the parent metal, its nitrides and the nitrides of alloying elements [7]. During the alpha-phase diffusion saturation, the CrN phases in the shape of fine substances of 1 -2 µm are separated ( figure 3).
There is no clear boundary between the nitride layer and the diffusion sublayer in the microtexture images. The transition from the nitride layer to sublayers is smooth, that is one of the key requirements to the nitrogenized layer [3].
The thickness of the nitride layer in X12 steel was 10 µm after the 4 hour nitriding in crossed electrical and magnetic fields (figure 3) and in P6M5 it was 80 µm (figure 2). The high percentage of Cr in X12 steel provides the thinning of the hardened layer, as may be supposed, due to the chrome nitride layer which prevents the nitrogen diffusion as well as the diffusion layer formation.
There is no parent phase on the sample surface after nitriding in the crossed electrical and magnetic fields. Instead of that, a range of phases is formed as a result of interaction of the glow discharge plasma with the matrix material.  The X-ray pattern (figure 4) of the P6M5 surface after the ion nitriding revealed that around the 40º-50º angles, there is a broadening at the Fe3(N,C) and СrN peak bases which can be caused by several phases with the similar values of interplanar distance and the retained compression stress.
The analysis of the X12 surface after the nitriding in crossed electrical and magnetic fields demonstrated the reflexes of the Fe3N ε-phase as well as the phases which was composed of CrN. The significant fall in intensity of the α-iron peaks (figure 5) after nitriding indicates the decreasing α-phase fraction in the surface layer as a result of forming the Fe3N ε-phase. After the treatment in the nitrogenargonacetylene mixture with proportions of N2 50% -80%, Ar 25% -10%, C2H2 25% -10%, the sample surfaces revealed the reaction to iron carbide (Fe5С2) as well. Using the nitrogenargonacetylene mixture with proportions of N2 50% -80%, Ar 25% -10%, C2H2 25% -10% in nitriding provides the deactivation of the oxygen remained and the diffusion saturation.
After the 180 minutes of coating deposition, the coating of the TiN composition of 9 µm (figure 6) as well as the TiAlN coating with thickness of 6 µm ( figure 7). Then, the materials were cooled to 100 ºС under vacuum. The micro-hardness dependence on depth is presented in Figure 8.
The high wearing is provided by the TiN and TiAlN coating as well as by hard flexible nitrogenized layers under coating. Moreover, smoothing the sharp transition between the "soft" material and the hard coating contributed to the wearing quality and decreased the hardness gradient between two different textures as well.
Compared to the conventional nitriding, the advantage of the ion nitriding in magnetic field in the microhardness distribution through the obtained layer is obvious. The thickness of the hardened layer was increased from 54 µm to 84 µm, i.e. by 1.5 times. The samples placed in magnetic field demonstrated smoother microhardness depth distribution (figure 8a).

Conclusion
 The analysis of the numerical simulation in DEFORM 2D revealed that comparing to the symmetric distortion the rolling force decreased by 2.4 -3.2 times when 50 -75% spellerizing and the misalignment of roll velocities Vi/V2 = 2 -4. However, there was a significant torque increase on the rolls: by 1.5 -3.5 times on the bottom roll, by 1.1 -2.6 times om the top roll.  The crossed electrical and magnetic fields provide the energy and density increase of the charged particles current to the treated surface.  It was experimentally proved that the 4 hour ion nitriding in the crossed electrical and magnetic fields of the Р6М5 and Х12 steel under pressure of 80 Pa resulted in the surface modified layer which was compounded of Fe3N, СrN nitride phases of increased hardness. The CrN phase appeared as fine particles of 1-2 nm.  The crossed electrical and magnetic fields increase the thickness of the hardfaced layer of the Р6М5 sample up to 200 µm for 4 hours in comparison with 25 µm for 3 hours under gas nitriding.  The ion nitriding in glow discharge with magnetic field is an efficient technique to prevent the TiN and TiAlN coating from excessive plastic deformation of the support.