Surface Modification of Carbon Steel by Atmospheric-Controlled IH-FPP Treatment Using Mixed Chromium and High-Speed Steel Particles

Shumpei Ota; Hiroyuki Akebono; Shoichi Kikuchi; Kazue Murai; Jun Komotori; Kengo Fukazawa; Yoshitaka Misaka; Kazuhiro Kawasaki

doi:10.2320/matertrans.M2016141

Abstract

First, in this paper, a new atmospheric-controlled induction heating and fine particle peening treatment system (vacuum AIH-FPP system) which reduces the oxygen concentration in the chamber to the order of ppm, much less than a conventional processing apparatus was presented. Next, in order to examine the effect on the formation of the surface modified layer of (i) mixing hard particles, (ii) the processing temperature, and (iii) the particle velocity, carbon steel AISI 1045 was treated with this system in conjunction with high-frequency induction heating, by peening Cr particles and mixed particles of Cr and high-speed tool steel. From the observation results by a scanning electron microscope and an energy dispersive X-ray spectrometer, it is clear that for the formation of a Cr diffused layer, using a mixture of Cr particles and high-speed tool steel particles is important. The treatment must be conducted at a higher temperature of approximately 1273 K to form a Cr diffused layer. Furthermore, by increasing the particle velocity, a thicker Cr transfer layer is formed at the surface under process. Therefore, an increased particle velocity accelerates the transfer of Cr.

This Paper was Originally Published In Japanese in J. Japan Inst. Met. Mater. 79 (2015) 491–496.

In order to establish an effective method of the surface treatment proposed in this paper, some parts of the contents were revised and Figure 10 was added. The sentences of abstract, conclusions, and references were slightly modified. Two contributed authors were also added.

1. Introduction

Materials used in machine parts and structural members must possess multiple characteristics depending on their specific application such as fatigue and tribological characteristics or resistance to corrosion and abrasion. To improve these properties, high-strength materials are often subjected to surface modification. Fine particle peening (FPP) is expected to enhance corrosion resistance and tribological characteristics in addition to applying compressive residual stress^1–5) and improving fatigue strength through grain refinement^6–11) because the shot particle elements are transferred onto the surface to be treated^12–17). Consequently, this treatment is a promising surface modification method for metals.

By combining FPP with high-frequency induction heating (IH), we recently developed a device capable of performing FPP on substrates heated to 873 K–1473 K in a controlled atmosphere. In addition, we conducted a study on the improvement effect and a series of experiments attempting to elucidate the mechanism underlining the effect of atmospheric-controlled induction heating and fine particle peening (AIH-FPP)^18–25). For example, it was previously reported that by performing AIH-FPP on carbon steel using Cr particles, the components of the Cr particles were transferred onto the surface to be processed and diffused inside the substrate to form a Cr diffusion layer, whereby the corrosion resistance of the carbon steel was improved²²⁾. However, in that processing apparatus, atmosphere substitution is conducted by continuous supply of an inert gas to peen the particles, such as nitrogen or argon, into a chamber at constant pressure till the system is exhausted. Therefore, a small amount of oxygen is not evacuated but remains in the chamber. This trace oxygen may form oxides on the surface to be processed, which are likely to inhibit the transfer and diffusion of Cr^20–23).

In the present study, we first develop a system that reduces the oxygen concentration in the chamber to the order of ppm, much less than a conventional processing apparatus. Hereinafter, this will be referred to as the vacuum AIH-FPP processing system. Then, processing is performed with this treatment system on carbon steel using Cr particles and a mixture of Cr particles and hard high-speed tool steel particles (hereinafter, referred to as mixed particles). On the basis of the processing results, we investigate and discuss (i) the effect of mixing hard particles, (ii) the effect of the processing temperature, and (iii) the particle velocity on the formation of the surface modified layer.

2. Vacuum AIH-FPP Treatment System

Figure 1 shows the structure of the vacuum AIH-FPP treatment system. In this processing system, after the pressure in the stainless steel vacuum chamber is reduced to 130 Pa using a rotary pump, the atmosphere is replaced by supplying an inert gas. Thus, reducing the oxygen concentration in the chamber to 10 ppm is possible. The oxygen concentration is measured by a zirconia- type oxygen analyzer mounted on the exhaust system. The chamber is further equipped with a particle peening nozzle (suction jet system) and an IH coil to perform the FPP treatment while heating the material under treatment in a predetermined atmosphere.

Fig. 1

Schematic illustration of vacuum AIH-FPP treatment system.

Figure 2(a) shows a macroscopic view of the specimen surface, after treatment with this system and shooting Cr particles for 90 s onto carbon steel specimens heated to 1273 K. Figure 2(b) shows the macroscopic structure of the specimen surface after treatment under the same conditions using conventional treating equipment. In the latter case, the test piece surface is green owing to the reaction of Cr with a small amount of oxygen remaining in the chamber to form Cr₂O₃²²⁾. In contrast, when the vacuum AIH-FPP treatment system is used, independent X-ray diffraction (XRD) analysis does not indicate the presence of an oxide on the specimen surface although the metallic luster is lost because the peened surface is uneven. These results confirm that the present vacuum AIH-FPP treatment system facilitates processing while suppressing the oxidation of the specimen surface.

Fig. 2

Macroscopic observation of specimen. (a) using vacuum AIH-FPP treatment system and (b) using traditional one (1273 K, using Cr particle).

3. Experimental Method

The material to be processed was a structural carbon steel (AISI 1045) with the chemical composition shown in Table 1, machined to disk-shaped test pieces 15 mm in diameter and 4 mm in thickness. This material was subjected to quenching and tempering under the conditions shown in Fig. 3. The one-end face of the test piece was polished with emery papers (#240–#1200), and that surface was subjected to AIH-FPP treatment.

Table 1 Chemical composition of AISI 1045.

C	Si	Mn	P	S	Ni	Cr	Mo	Cu	Fe
0.45	0.20	0.71	0.02	0.03	0.04	0.13	0.01	0.09	Bal.

Fig. 3

Quenching and tempering condition of AISI 1045.

Cr particles were sieved up to 45 µm in diameter, and a mixture of these particles and hard high-speed tool steel (AISI M42) particles (particle size up to 45 µm) were used as shot particles. Three types of mixed particles were prepared, each containing, 75, 50, and 25 mass% Cr particles by mass ratio. Henceforth, these are referred to as 75Cr particles, 50Cr particles, and 25Cr particles, respectively. Furthermore, in comparison, 100% Cr particles (100Cr) were also prepared. Figure 4 shows scanning electron microscopic (SEM) images of Cr particles and high-speed tool steel particles. It is observed that the diameter of Cr particles used in this study was from several micro-meters to several tens of micro-meters as shown in this figure.

Fig. 4

SEM image of particle. (a) chromium (600 HV) and (b) AISI M42 (900 HV).

AIH-FPP treatment was conducted as follows. First, a prepared material to be treated was placed inside the IH coil in the vacuum chamber; then, the vacuum pump was operated for approximately 10 min to reach a vacuum of 130 Pa or less in the chamber. Then, nitrogen gas (99.99%) was supplied. After confirming that the oxygen concentration in the gas that was discharged from the chamber became 10 ppm or less, the test piece was heated to a predetermined temperature (1073 K, 1273 K). Immediately after reaching this predetermined temperature, the particles were peened for 30 s. Meanwhile, to maintain the temperature at the predetermined level, the IH output was successively adjusted. Thereafter, the test piece was held for another 60 s at the predetermined temperature while only the gas was injected. The test piece was cooled down by a continuous supply of nitrogen gas, after switching OFF the output of the IH. Figure 5 shows the thermal history of specimen during this process which was measured by using the thermocouple attaching the center of the treated surface. The FPP treatment conditions are presented in Table 2. Furthermore, to investigate the impact of changes in the particle velocity during AIH-FPP treatment, a commercial particle peening nozzle of 6 mm in internal diameter and a second nozzle obtained by fitting the tip of the same nozzle with a stainless steel circular pipe of 4 mm internal diameter were prepared. Figure 6 shows the cross-sectional view of the nozzles. Table 3 summarizes the results of the flow rate measurements using the particle peening nozzles at 0.5 MPa, which is the maximum peening pressure of the vacuum AIH-FPP treatment system.

Fig. 5

Thermal history of specimen during the AIH-FPP treatment.

Table 2 FPP treatment condition.

Shot particle	Cr
	Cr and AISI M42 (Cr75, 50, 25 mass%)
Particle diameter	< 45 µm
Particle supply rate	0.2 g/s
Peening pressure	0.5 MPa
FPP nozzle	Inside diameter	6 mm/4 mm
	Nozzle distance	100 mm/50 mm

Fig. 6

Cross sectional view of FPP nozzle. (a) ϕ6 mm nozzle and (b) ϕ4 mm nozzle.

Table 3 Condition of gas flow rate and particle velocity.

Inside diameter of FPP nozzle d	Gas flow rate Q	Particle velocity
6 mm	134 L/min	Low
4 mm	111 L/min	High

To investigate the characteristics of the surface-modified layer in the prepared test pieces, longitudinal sections of the specimens were analyzed using SEM and energy dispersive X-ray spectrometry (EDX). In EDX analysis, Fe and Cr elemental mapping was performed in the same field of vision of the SEM image. In addition, line analysis was conducted to compare the detected relative intensity of these elements. On the basis of the results of the line analysis, the distance between the surface and the point where Cr detection intensity assumes half of the maximum intensity was defined as the thickness of the modified layer.

4. Results and Discussion

4.1 Influence of peening mixed particles and treatment temperature on the formation of the surface modified layer

Figure 7 shows the results of the SEM and EDX analysis on the longitudinal section of specimens subjected to AIH-FPP treatment with Cr particles (100Cr particles) and mixed particles (50Cr particles), at the treatment temperatures of 1073 K and 1273 K. In this case, the commercially available particle-peening nozzle with an inner diameter of 6 mm is used. The treatment temperature does not appear to affect the structure and composition of the carbon steel surface, when only Cr particles are used. Nevertheless, we observe a state wherein Cr is transferred onto the surface of the substrate. In contrast, when 50Cr particles are shot at 1273 K treatment temperature, a layer is observed near the surface where Fe and Cr co-exist. To investigate this difference in the surface modified layer further, line analysis by EDX was performed, targeting Fe and Cr on the lines shown in the SEM image of Fig. 7. Figure 8 shows the results. On the basis of the results, the detection intensity gradient for both elements is smaller when 50Cr particles are projected at 1273 K as compared with peening Cr particles alone. In addition, Co and Mo are hardly detected near the surface of the high-speed tool steel. This result agrees with previous studies^23,26), which did not confirm any particle transfer deposition after AIH-FPP treatment by projecting high-speed tool steel particles onto the steel material. Therefore, the reduced gradient of the Fe and Cr detection intensities is probably owing to the diffusion of Cr, which was transferred onto the surface to be treated, inside the substrate to form a Cr diffused layer. Next, experiments were performed at a temperature of 1273 K using particles with different mixing ratios and the effect of the particle mixing ratio on the formation of surface modified layers was examined. Figure 9 shows the EDX line analysis results on the longitudinal section of the specimens after peening 75Cr and 25Cr particles, respectively. The same changes in detection intensity are clearly observed after peening 75Cr and 100Cr particles. Therefore, in this case, we hypothesize that only a Cr transfer layer is formed. In contrast, when peening 25Cr particles, Fe and Cr elements coexist and a layer with a small gradient of detection intensity can be obtained. Thus, it is believed that in this case, only a Cr diffusion layer is formed without generation of a Cr transfer layer.

Fig. 7

SEM images and EDX maps observed at longitudinal section of specimen (ϕ6 mm nozzle).

Fig. 8

EDX line analysis. (a) using 100Cr particle and (b) using 50Cr particle (1273 K, ϕ6 mm nozzle).

Fig. 9

EDX line analysis. (a) using 75Cr particle and (b) using 25Cr particle (1273 K, ϕ6 mm nozzle).

The above results reveal that within the scope of this study, at a treatment temperature of 1273 K, a Cr diffused layer is formed at the treated surface, by peening mixed particles with a Cr particle mixing ratio of 50 mass% or less. This may be attributed to the effect of the concurrent peening of hard high-speed tool steel particles (Vickers hardness = 900 HV), which are harder than Cr particles (600 HV), causing larger dislocations in the substrate^6,7), followed by refinement of the crystalline grains^6–11,26). Note that the treatment temperature, in the present study, is in a temperature range that allows recovery of the dislocations in carbon steel. Thus, although dislocation recovery also occurs in an AIH-FPP treatment, other dislocations are constantly introduced during the particle peening process. At this time, fine crystal grains are probably formed by dynamic recrystallization²⁶⁾, resulting in the formation of a microstructure that promotes diffusion. Although grain growth may occur also while the temperature remains constant after particle peening, the time interval is short (even 60 s). Therefore, near the treated surface, dislocation and grain boundary diffusions occur remarkably faster than bulk diffusion in the crystal grains, which enhances the diffusion of Cr.

The reason that the diffusion layer was only formed at the high treating temperature can be attributed to the temperature dependence of the diffusion coefficient. Furthermore, only Cr is transferred to the treated surface, even when peening mixed particles because of the difference in size and shape between the two types of particles. Whereas the high-speed tool steel particles comprise spherical shape of approximately 40 µm in diameter, as shown in Fig. 4, Cr particles have an angular shape and contain many fine particles with a size of several µm. Since Cr particles can be easily pressed onto the treated surface, we assume that only Cr is transferred and diffused, even when peening mixed particles.

On the basis of the above results, to form a Cr diffusion layer on the surface of the material under AIH-FPP treatment, increasing the number of introduced dislocations due to the collision of hard particles and to induce grain refinement is necessary. Accordingly, in the following examination, AIH-FPP treatment is performed on high-hardness Cr particles subjected to the above heat treatment, to generate a Cr diffusion layer with Cr particles only. The Cr particles are heat-treated using an electric furnace in the atmosphere at 1273 K for 10 min. XRD analysis of the Cr particles is conducted after the heat treatment, and we confirm the formation of high-hardness Cr particles with chromium nitride (Cr₂N) and chromium (III) oxide (Cr₂O₃) on their surface. Using the resulting high-hardness Cr particles, carbon steel AISI 1045 was subjected to AIH-FPP treatment in a nitrogen atmosphere at 1273 K, using a particle-peening nozzle with a 6-mm inner diameter. The SEM-EDX elemental mapping results for the test piece cross-section after the AIH-FPP treatment are shown in Fig. 10. A Cr transfer layer is formed on the substrate surface, and a Cr diffusion layer is not observed. This indicates that the formation of the Cr diffused layer is difficult by AIH-FPP treatment using high-hardness Cr particles alone. Thus, to form a Cr diffusion layer, it is important to use a mixture of Cr particles and high-speed tool steel particles. This is probably because the compound layer formed by the heat treatment on the surface layer of the Cr particles inhibits the diffusion of Cr. However, on this point, conducting further examinations in the future is necessary.

Fig. 10

SEM images and EDX maps observed at cross-section of specimen. (AISI1045, 1273 K, Peening 30 sec, Heat keeping 60 sec, ϕ6 mm nozzle, N₂atmosphere).

4.2 The effect of the particle velocity on the formation of the surface modified layer

In this section, we examine the influence of the particle velocity on the formation of the surface modified layer, by performing AIH-FPP treatment with a peening nozzle fitted with a 4 mm inner diameter circular tube.

Figure 11 shows SEM and EDX results on the cross-section of specimens peened with 100Cr and 50Cr particles at 1073 K and 1273 K treatment temperatures. The figure confirms the presence of a Cr transfer layer under both treatment conditions. However, when the 4-mm–inner diameter nozzle is used, instead of the commercially available 6-mm–inner diameter nozzle (Fig. 7), a Cr diffusion layer cannot be confirmed, even when peening 50Cr particles at 1273 K. Moreover, by comparing the results of Figs. 7 and 11, we confirm that under all peening conditions, the Cr transfer layer becomes thicker with increasing particle velocity. Furthermore, Fig. 12 shows the results of measuring the thickness of the surface modified layer, when peening with 100Cr, 75Cr, 50Cr, and 25Cr particles at 1273 K, respectively. By increasing the particle velocity, the thickness of the Cr transfer layer increases substantially, irrespective of the type of particles used. For example, for 100Cr particles, the thickness of the Cr transfer layer increases by approximately four times, when the nozzle inner diameter is reduced from 6 to 4 mm.

Fig. 11

SEM images and EDX maps observed at cross-section of specimen (ϕ4 mm nozzle).

Fig. 12

Relation between type of shot particle and thickness of modified layer (1273 K).

These results indicate that Cr transfer is accelerated by increasing the particle velocity. This is probably because as the particle velocity increases, the bonding effect between the Cr particles in the treated surface is enhanced. Furthermore, if the particle velocity is large, a Cr diffusion layer is not formed, even when peening mixed particles. This can probably be explained by the fact that in the presence of an excessively transferred Cr layer, dislocations, introduced by the collision of hard high-speed tool steel particles into the substrate, are inhibited.

5. Conclusion

In this study, a new vacuum AIH-FPP treatment system was presented. Carbon steel AISI 1045 was treated with this system in conjunction with high-frequency induction heating, by peening Cr particles and mixed particles of Cr and high-speed tool steel. We examined the effect on the formation of the surface modified layer of: (i) mixing hard particles, (ii) the processing temperature, and (iii) the particle velocity. The main conclusions of the present study are the following.

(1) When peening with Cr particles alone at 1273 K treatment temperature, and when peening with Cr particles with increased hardness imparted by further thermal treatment, only a Cr transfer layer is formed and no diffused layer is formed. In contrast, when peening by mixing hard high-speed tool steel particles, a Cr diffusion layer is formed. This result suggests that for the formation of a Cr diffused layer, using a mixture of Cr particles and high-speed tool steel particles is important.
(2) When the treatment temperature is 1073 K, a Cr diffused layer is not observed, even when peening with mixed particles. This is because the diffusion rate is temperature-dependent and to form a Cr diffused layer, the treatment must be conducted at a higher temperature of approximately 1273 K.
(3) By increasing the particle velocity, a thicker Cr transfer layer is formed at the surface under process. Therefore, an increased particle velocity accelerates the transfer of Cr.

These results suggest that in the AIH-FPP treatment of carbon steel, it is highly possible to control the formation of diffused layer or adjust the thickness of the surface modified layer by varying the type of shot particles, the processing temperature, and the flow velocity at the particle-peening nozzle point.

Acknowledgement

This work has been supported by the Grant-in-Aid for Scientific Research (B) No.15H03894 of JSPS KAKENHI Grant from 2015 and The Light Metal Educational Foundation, Inc. We are grateful for their support.

REFERENCES

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