ISIJ International
Online ISSN : 1347-5460
Print ISSN : 0915-1559
ISSN-L : 0915-1559
Regular Article
Influence of Oxide Scale Formed on Chrome Steel Surface in Steam Atmosphere on Deformation Behavior of Chrome Steel in Hot Ring Compression
Ryo MatsumotoShohei HaradaHiroshi Utsunomiya
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2015 年 55 巻 8 号 p. 1711-1720

詳細
Abstract

Hot forging characteristics of chrome steel and carbon steel covered with oxide scale formed in steam atmosphere were investigated with the ring compression test. In oxidation of the steels in steam atmosphere, the oxide scale with high porosity was formed in oxidation of the steels, while the oxide scale with thick FeCr2O4 layer around the oxide scale–chrome steel interface was formed in oxidation of the chrome steel. The nominal coefficient of shear friction of the steels covered with oxide scale was derived from the plastic deformation behavior of the steels covered with oxide scale during the ring compression test. The oxide scale of the chrome steel formed in steam atmosphere provided low friction characteristic, compared with the oxide scale formed in air atmosphere. The mechanism of reduction of friction of the steels covered with oxide scale was discussed from the viewpoints of heat transfer and interfacial friction at oxide scale–chrome steel interface by the experiment and finite element analysis.

1. Introduction

Formation of oxide scale on surface of metals is inevitable phenomenon at elevated temperatures in the presence of air. Since the oxide scale is located between tool and metal workpiece during hot working processes, the oxide scale affects to the hot working characteristics. Many investigations on the formation of oxide scale of metals have been conducted,1) while the investigations on the behavior of the oxide scale during hot working processes have not been conducted sufficiently, especially during secondary working processes such as sheet metal forming and forging processes. In primary working process, the plastic deformation behavior and crack initiation,2) heat transfer characteristic3) and friction characteristic4) have been investigated in hot rolling processes. In secondary working process, friction characteristic in hot forging5) and hot stamping6) processes and heat transfer characteristic in hot forging7) and hot stamping8) processes have been investigated.

Concerning the friction characteristic in hot forging process, the influence of the composition and thickness of the oxide scale of carbon-manganese steel on the friction during hot ring compression has been investigated by experiment.5) We reported the influence of the thickness and thermal property of the oxide scale of chrome steel on the friction during hot ring compression by experiment and finite element analysis.9) The oxide scale was confirmed to provide low friction characteristic during hot forging process in above studies.5,9) It has also been suggested that the oxide scale of metals has a potential to play a role of a lubricant in hot working process.10) If the oxide scale is used instead of the lubricant such as graphite and glass in hot working process, dry hot metal working process without using lubricant may be realized. This is essentially important technique for environmental issue.

On the other hand, recently, hot working products with complicated shape and high dimensional accuracy have been desired. To realize the demands, novel hot working processes such as precision flashless hot forging11) and hot stamping12) processes have been developed. Since the oxide scale is located at the interface between the tool and the workpiece as above mentioned, the oxide scale strongly affects to the dimensional accuracy and surface quality of the hot formed product. Due to this, the control of the deformation behavior of the oxide scale is one of effective solutions to realize such novel hot working processes.

To utilize the oxide scale of steels in hot working process from tribological aspect, the thickness, composition and structure of the oxide scale are typical factors to be controlled in formation of the oxide scale. The control of oxidation atmosphere is one of well-known methods to change the characteristics of the oxide scale of metals such as chemical composition, structure and mechanical properties of the formed oxide scale. The formation behaviors of the oxide scale of steel in steam atmosphere13,14) and the oxide scales of iron and low carbon steel in atmosphere controlled oxygen and nitrogen concentrations15,16) have been investigated. The deformability of the steel oxidized in steam atmosphere has been enhanced in wire drawing process.17) The oxidation in steam atmosphere may have a potential to change the forming characteristics in hot working process.

As mentioned in our previous study,9) the oxide scale of chrome steel formed in air atmosphere reduces the friction in hot forging process. For the purpose of further reduction of friction of steel in hot forging process, the oxide scale of steels formed at elevated temperature in steam atmosphere is focused in this study. The formation characteristics of the oxide scale of chrome steel and carbon steel are compared between oxidation in air and steam atmospheres. To investigate the effects of the oxide scale formed in steam atmosphere on the hot forging characteristics, the ring compression test of the steels covered with oxide scale is carried out at elevated temperature. The coefficient of shear friction of the steels covered with oxide scale is derived from the plastic deformation behavior of the steels covered with oxide scale. The influence of heat transfer at the tool–oxide scale and oxide scale–steel workpiece contacts and the interfacial friction at oxide scale–steel interface on the plastic deformation of the steels covered with oxide scale is discussed through the finite element analysis.

2. Experimental Set-up

2.1. Oxidation Test

The formation behaviors of oxide scale of chrome steel JIS SCr420 and carbon steel JIS S25C were investigated. The chemical compositions of the steels are shown in Table 1. The steel workpiece was heated and oxidized at a temperature of 1273 K in air or steam atmosphere in an electric furnace as shown in Fig. 1. Oxidation of the steels in steam atmosphere is expected to change the chemical composition and structure of the oxide scale.13,14) The steam was generated by boiling water with a generation rate of 2.0 L/min. The generated steam was injected to the electric furnace through a tube. The oxidized steel workpiece was taken out from the electric furnace after oxidation for oxidation duration to = 0–7.2×103 seconds. To preserve the oxide scale formed in the electric furnace, the surface of the oxidized steel workpiece was then immediately coated with a PbO-B2O3-based glass powder.18)

Table 1. Chemical compositions of chrome steel and carbon steel workpieces used in this study (mass%).
CSiMnPSNiCrMoFe
Chrome steel
(JIS: SCr420)
0.190.230.770.0180.0250.021.14<0.01Bal.
Carbon steel
(JIS: S25C)
0.230.220.380.0100.0180.050.060.01Bal.
Fig. 1.

Illustration of oxidation of steel workpiece in steam atmosphere.

2.2. Ring Compression Test

The plastic deformation behaviors of the chrome steel and carbon steel covered with oxide scale during hot forging process were investigated with the ring compression test. In the test, a ring-shaped workpiece was compressed between flat parallel tools and the friction was estimated by changing in the inner diameter of the workpiece.19,20) Since this test does not necessitate measurement of load, it has been frequently employed for estimating friction during forging without large expansion of a workpiece surface. The initial ring-shaped workpiece having a ratio of outer diameter (D0): inner diameter (d0): height (h0) = 6:3:2 is usually employed. The obtained coefficient of shear friction was described as nominal coefficient of shear friction in this study because the obtained coefficient of shear friction was determined not by the normal and shear stresses at the workpiece–tool contact but the plastic deformation behavior of the workpiece during the ring compression test. This means the obtained nominal coefficient of shear friction is affected not only by the friction at the workpiece–tool contact but also by other factors such as thermal characteristics of the workpiece or tool, especially in the test at elevated temperature.

In this study, a steel workpiece with an initial shape of D0 = 18.0 mm, d0 = 9.0 mm and h0 = 6.0 mm was compressed between the flat parallel tools with tungsten carbide (WC-20 mass%Co) under dry (unlubricated) condition. The surface roughness values of the steel workpiece and the tools were Ra = 0.30–0.50 μm and 0.02–0.04 μm, respectively. The ring-shaped steel workpiece was heated and oxidized in an electric furnace at a temperature of 1273 K in air or steam atmosphere, and the oxide scale thickness was controlled according to the oxide scale thickness-oxidation duration relationship (see Fig. 6) prior to applying the ring compression test. The oxidized steel workpiece having oxide scale with a thickness of hs0 = 0–300 μm on the surface was compressed with an average strain rate of 2.8 s−1 with a maximum reduction in height (Δh/h0) of 60% at an initial workpiece temperature of 1273 K. To prevent secondary oxidation of the upper and lower surfaces of the forged steel workpiece, the upper tool was held at the bottom dead center of the press for 60 seconds immediately after forging. The steel workpiecce sandwiched between the upper and lower tools was cooled to a temperature below 323 K by contacting to the tools at room temperature. Owing to this press ram motion control with a servo press, further oxidation of the steel workpiece hardly occurred after forging. The minimum inner diameter of the steel workpiece at the center of the ring-shaped workpiece (along the vertical direction) was measured using a microscope to estimate the coefficient of shear friction of the steel workpiece.

Fig. 6.

Relationship between net thickness of oxide scale on steel workpiece surface and oxidation duration.

3. Experimental Results

3.1. Formation Behavior of Oxide Scale

Figure 2 shows the cross-sections of the oxide scale–chrome steel workpiece interface oxidized in air or steam atmosphere. Irrespective of oxidation atmosphere, the oxide scale was composed of four phases of FeO, Fe3O4, Fe2O3 and FeCr2O4. The Fe2O3 phase was formed at the surface of the oxide scale, while the FeCr2O4 phase was formed at the oxide scale–chrome steel interface. Thicker FeCr2O4 layer and larger voids were observed in the oxide scale formed in steam atmosphere from the energy dispersive x-ray spectrometry (EDX) results.

Fig. 2.

Cross-sections of oxide scale–chrome steel workpiece interface.

The thickness distribution of the chemical compositions of the oxide scale is shown in Fig. 3. Irrespective of oxidation atmosphere and thickness of the oxide scale, the primary chemical composition of the oxide scale was FeO. The Fe3O4 layer formed in steam atmosphere was slightly thinner than that formed in air atmosphere. The FeCr2O4 layer was not formed in the oxide scale of the carbon steel, while it was formed in the oxide scale of the chrome steel, especially thick FeCr2O4 layer was observed in the oxide scale formed in steam atmosphere.

Fig. 3.

Thickness distribution of chemical compositions of oxide scale film on steel workpiece surface.

Figure 4 shows the scanning electron microscope (SEM) photographs of the cross-section of the oxide scale–carbon steel workpiece interface. Some large voids were observed in the oxide scale formed in steam atmosphere. Figure 5 shows the relationship between the porosity and the thickness of the oxide scale in the cross-section. The porosity of the oxide scale formed in steam atmosphere was much higher than that formed in air atmosphere. This is because hydrogen dissociated from steam causes reduction reaction with FeO, and then some voids are formed in the oxide scale.21)

Fig. 4.

Scanning Electron Microscope (SEM) photographs of cross-section of oxide scale–carbon steel workpiece interface (thickness of oxide scale hs0 = 190–200 μm).

Fig. 5.

Relationship between porosity and thickness of oxide scale film on steel workpiece surface.

The oxide scale thickness increase (Δhs0) could be generally described according to the following equation:   

Δ h s0 =K t o 1/2 (1)
where K is the oxidation rate coefficient. Figure 6 shows the relationship between the oxide scale thickness increase (Δhs0) on the chrome steel surface and the oxidation duration. The oxide scale thickness increase linearly increased with an increase in oxidation duration, and the oxidation rate in steam atmosphere was higher than that in air atmosphere. This result is reasonable because aggressive oxidation of steel is well known to be caused in steam atmosphere.22)

3.2. Deformation Behavior of Steel Covered with Oxide Scale

Figure 7 shows the calculated and experimental results of the changes in the inner diameter (d/d0) of the steel workpiece covered with the oxide scale during the ring compression test. In comparison with the changes in the inner diameter of the steel workpiece having same thickness of the oxide scale formed in air and steam atmospheres, the change in the inner diameter oxidized in steam atmosphere was higher than that oxidized in air atmosphere.

Fig. 7.

Change in inner diameter of ring-shaped workpiece covered with oxide scale during ring compression test (marks: experiment, lines: finite element analysis).

Figure 8 shows the estimated nominal coefficient of shear friction (mnom) of the steel workpiece covered with the oxide scale during the ring compression test. The nominal coefficient of shear friction was calculated using the calibration curves for the relationship between the coefficient of shear friction and the change in the inner diameter plotted as dashed lines in Fig. 7. Irrespective of oxidation atmosphere, a high friction value (mnom > 0.6) was obtained for the steel workpiece covered with a very thin oxide scale (hs0 = 6 μm), while the nominal coefficient of shear friction decreased with an increase in thickness of the oxide scale. The difference of nominal coefficients of shear friction of the steel workpiece covered with the oxide scale in steam and air atmospheres (Δmnom) was calculated as follows:   

Δ m nom = m nom-steam m nom-air (2)
where mnom-steam and mnom-air are the nominal coefficients of shear friction of the steel workpiece oxidized in air and steam atmospheres, respectively. The difference of the nominal coefficient of shear friction is shown in Fig. 9. In both of chrome steel and carbon steel workpieces, the nominal friction of the steel workpiece oxidized in steam atmosphere was lower than that oxidized in air atmosphere. In addition, Δmnom was closed to zero as the reduction in height increased. This means that the influence of the oxidation atmosphere on the nominal friction is small at high reduction in height.
Fig. 8.

Nominal coefficient of shear friction for steel workpiece covered with oxide scale during ring compression test.

Fig. 9.

Difference of nominal coefficient of shear friction for steel workpiece covered with oxide scale formed in steam and air atmospheres during ring compression test (Δmnom=mnom-steammnom-air).

3.3. Deformation Behavior of Oxide Scale

Figure 10 shows the photographs of the cross-section of the oxide scale–steel workpiece interface after the ring compression test. The oxide scale was confined between the tool and the steel workpiece, and extended to be radial direction with the steel workpiece. The thickness of the confined oxide scale at the outer and inner peripheries was thinner than that at the center part. Then the steel workpiece directly contacted with the tool near the outer and inner peripheries as shown in Fig. 10(b). The average thickness of the oxide scale (hs), area fraction ((Ds2ds2)/(Dw2dw2)) and expanded ratio ((Ds2ds2)/(D02d02)) of the oxide scale on the steel workpiece surface during the ring compression test are shown in Fig. 11. Here, Ds, Dw, ds, dw are the outer diameter of the oxide scale surface, the outer diameter of the steel workpiece surface, the inner diameter of the oxide scale surface, the inner diameter of the steel workpiece surface, respectively (see Fig. 10). In case of the oxide scale of the chrome steel, it is seen that the area fraction and the expanded ratio of the oxide scale formed in steam atmosphere are higher than those formed in air atmosphere at Δh/h0 > 40%. In case of the oxide scale of the carbon steel, it is seen that the area fraction and the expanded ratio are almost same behaviors in the oxide scale formed in air and steam atmospheres.

Fig. 10.

Photographs of cross-section of oxide scale–steel workpiece interface after ring compression test.

Fig. 11.

Deformation behavior of oxide scale on steel workpiece surface during ring compression test (ds: diameter of oxide scale surface, d0: initial diameters of steel workpiece and oxide scale, dw: diameter of steel workpiece surface, see Fig. 10).

From above deformation behaviors of the oxide scale, the oxide scale of the chrome steel formed in steam atmosphere has a high ductility, compared with the oxide scale formed in air atmosphere. The Fe3O4 layer has been reported to have low friction characteristic and enhance the deformability of the steel in wire drawing process,17,23) however, the Fe3O4 layer formed in steam atmosphere was slightly thinner than that formed in air atmosphere as mentioned in section 3.1. High ductility of the oxide scale of the chrome steel is considered to be caused by thick FeCr2O4 layer at the oxide scale–chrome steel workpiece interface as shown in Fig. 3. High ductility of the oxide scale prevents to the steel workpiece from contacting the tool directly, especially at high reduction in height of the steel workpiece. The nominal friction of the oxide scale–tool contact is lower than that the steel workpiece–tool contact as shown in Fig. 8. For these reasons, the nominal coefficient of shear friction of the steel workpiece with the oxide scale formed in steam atmosphere is lower than that formed in air atmosphere as shown in Fig. 9.

Furthermore, since the nominal coefficient of shear friction determined by the ring compression test is affected not only by friction but also by other factors as mentioned in section 2.2, nominal friction of the steel workpiece covered with the oxide scale may be affected by the porosity of the oxide scale. It is generally known that thermal conductivity decreases with an increase in porosity of material and structural component.24) For this reason, the thermal conductivity of the oxide scale and the heat transfer at the oxide scale–steel workpiece contact are expected to be low during the ring compression test of the steel workpiece oxidized in steam atmosphere. Low heat transfer at the oxide scale–steel workpiece contact prevents the temperature of the steel workpiece from dropping during the ring compression test. The temperature change of the steel workpiece affects to the deformation behavior during the ring compression test. As the result, the coefficient of shear friction determined from the plastic deformation of the steel workpiece is estimated to be low. The relationship between the temperature change and the estimated nominal friction in the ring compression test have also been indicated in our previous study.9)

Figure 12 shows the SEM photographs of the cross-section of the oxide scale–chrome steel workpiece interface oxidized in steam atmosphere during the ring compression test. The voids in the oxide scale gradually got smaller as the reduction in height increased, and the voids were seen to be almost closed at Δh/h0 > 30%. As the result, the coefficient of shear friction of the steel workpiece with the oxide scale formed in steam atmosphere was nominally lower than that formed in air atmosphere at Δh/h0 < 30%.

Fig. 12.

Scanning Electron Microscope (SEM) photographs of cross-section of oxide scale–chrome steel workpiece interface during ring compression test (oxidation atmosphere: steam, hs0 = 300 μm).

As mentioned in section 2.2, plastic deformation of the ring-shaped workpiece is considerably simple in the ring compression test, compared with actual hot forging process, and large surface expansion of the ring-shaped workpiece does not occur. Due to this, the application of the test results in sections 3.2 and 3.3 is limited to hot forging without large plastic deformation such as upsetting, forging with simple shape product.

4. Discussions by Finite Element Analysis

4.1. Finite Element Analysis Conditions

Since many factors besides the friction affect the nominal coefficient of shear friction determined by the ring compression test as mentioned in section 2.2, the frictional characteristics of the oxide scale are discussed by a finite element analysis. The finite element analysis of the plastic deformation of the chrome steel covered with oxide scale layer during the ring compression test was conducted on the following assumptions as described in sections 3.2 and 3.3:

• The porosity in the oxide scale causes the change in the heat transfer at the tool–oxide scale and oxide scale–steel workpiece contacts.

• The FeCr2O4 at the oxide scale–steel workpiece interface causes the change in the friction at the oxide scale–steel workpiece contact.

On these assumptions, the influence of the heat transfer and interfacial friction at oxide scale–steel workpiece interface on the plastic deformation of the chrome steel covered with oxide scale layer during the ring compression test was discussed. A commercial three-dimensional finite element code DEFORM-3D ver.10.2 SP1 (Scientific Forming Technologies Corporation) was employed. In the simulation, a rigid-plastic finite element method for plastic deformation and a heat conduction finite element method for temperature change were employed to calculate the stress, strain states, and temperature distributions of the steel workpiece and the oxide scale at each calculation step. The tools were treated as rigid bodies. The constitutive relation used in the finite element analysis was a multilinear isotropic hardening determined from the stress–strain relationships of the steel workpiece and the oxide scale. The simulation model is illustrated in Fig. 13. A 30 degrees section of the ring-shaped workpiece was analyzed with consideration for the symmetry of the ring compression geometry. An oxide scale with an initial thickness of hs0 = 300 μm was allocated onto the upper and lower surfaces of the steel workpiece, while the oxide scale was not allocated onto the side surfaces of the steel workpiece. A tetrahedral mesh was employed to model the steel workpiece and the oxide scale. The average volume of the initial mesh was about 3.0×10−3 mm3. The elements were automatically re-meshed, depending on the plastic deformation of each element. The occurrences of crack of the steel workpiece and the oxide scale were not considered in the analysis.

Fig. 13.

Finite element analysis model for ring compression test of steel workpiece having oxide scale layers. (Online version in color.)

Table 2 shows the analysis conditions used to simulate the ring compression test. The initial geometries and temperatures of the chrome steel workpiece, oxide scale layer, and tools used for the simulation were identical to the experimental values. The oxide scale layer was simply assumed to be FeO single-phase in the simulation because the primary composition of the scale was FeO shown as Fig. 3. The material properties of chrome steel and FeO such as flow stress, density, specific heat and thermal conductivity were employed as the values described in our previous study.9) The temperature dependency of these material properties were considered. The boundary conditions among the tool, steel workpiece, oxide scale and air such as heat transfer coefficient and coefficient of shear friction were also followed as our previous study.9)

Table 2. Finite element analysis conditions for ring compression test of steel workpiece having oxide scale layers.
Initial temperature of chrome steel workpiece/K1273
Initial temperature of oxide scale/K1273
Tool temperature/K293
Air temperature/K293
Heat transfer
coefficient/W/(m2·K)
Tool–steel workpiece contact30000
Tool–oxide scale contact20000–40000
Oxide scale–steel workpiece contact20000–40000
Free surface150
Coefficient of
shear friction
Tool–steel workpiece contact mt-w0.8
Tool–oxide scale contact mt-s0.4
Oxide scale–steel workpiece contact ms-w0.4–1.0

In this study, the heat transfer coefficients at the tool–oxide scale and oxide scale–steel workpiece contacts were varied in the range with 20000–40000 W/(m2·K) to examine the influence of the porosity of the oxide scale on the plastic deformation of the steel wrokpiece. On the other hand, the coefficient of shear friction at the oxide scale–steel workpiece contact (ms-w) was varied in the range with 0.4–1.0 to examine the influence of the FeCr2O4 layer on the plastic deformation of the steel workpiece. The nominal coefficient of shear friction for the steel workpiece covered with oxide scale layer was calculated from the change in the inner diameter of the steel workpiece using the calibration curves describing the relationship between the coefficient of shear friction and the change in the inner diameter.

4.2. Influence of Heat Transfer on Plastic Deformation

Figure 14 shows the relationship between the heat transfer coefficient and the calculated nominal coefficient of shear friction for the steel workpiece covered with oxide scale layer during the ring compression test. Low heat transfer coefficient provided low nominal coefficient of shear friction. This is because the plastic deformation of the steel workpiece during the ring compression test is merely affected by the heat transfer. Low heat transfer coefficient prevents the steel workpiece from cooling down by contacting to the tools (room temperature) during the ring compression test. The temperature change in steel workpiece causes the change in the plastic deformation behavior of the steel workpiece. As the result, the coefficient of shear friction was nominally changed because the nominal coefficient of shear friction was determined from the plastic deformation behavior of the steel workpiece. This means that the heat transfer does not affect to the friction for the steel workpiece covered with oxide scale. Thus low nominal friction for the carbon steel workpiece oxidized in steam atmosphere shown as Fig. 8(b) is not true friction change but nominal friction change. High porosity of the oxide scale formed in steam atmosphere does not contribute to the friction reduction of the carbon steel and chrome steel workpieces covered with oxide scale. The reduction of the difference of nominal coefficient of shear friction (Δmnom) in the carbon steel workpiece with oxide scale layer shown as Fig. 9 was not caused by the frictional characteristics of the oxide scale of the carbon steel formed in steam atmosphere.

Fig. 14.

Influence of heat transfer coefficients of tool–oxide scale and oxide scale–chrome steel workpiece contacts on nominal coefficient of shear friction of chrome steel workpiece covered with oxide scale layer in ring compression test (ms-w = 1.0).

4.3. Influence of Interfacial Friction at Oxide Scale–steel Workpiece Interface on Plastic Deformation

The calculated area fraction and expanded ratio of the oxide scale on the chrome steel workpiece surface during the ring compression test is shown in Fig. 15. Low coefficient of shear friction at the oxide scale–chrome steel workpiece interface provided high expanded ratio of the oxide scale. Since high expanded ratio of the oxide scale was obtained by thick FeCr2O4 layer formed at the oxide scale–chrome steel workpiece interface from the experimental results described in section 3.2, thick FeCr2O4 layer resulted in low friction at the oxide scale–chrome steel workpiece interface. Figure 16 shows the relationship between the coefficient of shear friction at the oxide scale–chrome steel workpiece interface and the nominal coefficient of shear friction for the chrome steel workpiece with the oxide scale layer during the ring compression test. Low interfacial friction at the oxide scale–chrome steel workpiece interface provided low nominal coefficient of shear friction. This means that thick FeCr2O4 layer formed in steam atmosphere contributes to the friction reduction of the chrome steel workpiece covered with oxide scale. The reduction of the difference of nominal coefficient of shear friction (Δmnom) in the chrome carbon steel workpiece with oxide scale layer shown as Fig. 9 was caused by the porosity and frictional characteristics of the oxide scale of the chrome steel formed in steam atmosphere.

Fig. 15.

Influence of coefficient of shear friction at oxide scale–chrome steel workpiece contact (ms-w) on area fraction and expanded ratio of oxide scale on chrome steel workpiece surface in ring compression test (heat transfer coefficient: 30000 W/(m2·K)).

Fig. 16.

Influence of interfacial friction at oxide scale–chrome steel workpiece contact (ms-w) on nominal coefficient of shear friction of chrome steel workpiece covered with oxide scale layer in ring compression test (heat transfer coefficient: 30000 W/(m2·K)).

5. Conclusions

The hot ring compression test of chrome steel covered with oxide scale was carried out to examine the effects of the oxide scale formed in steam atmosphere on the plastic deformation behavior and friction characteristics. The nominal coefficient of shear friction of the steels covered with oxide scale was derived from the plastic deformation behavior of the steels covered with oxide scale. The plastic deformation behaviors of the oxide scale and the steel covered with the oxide scale were discussed through the experiment and finite element analysis. The following conclusions were obtained.

(1) The oxide scale with high porosity is formed in oxidation of the chrome and carbon steels in steam atmosphere. The oxide scale of the chrome steel formed in steam atmosphere has thick FeCr2O4 layer around the oxide scale–chrome steel interface and exhibits high ductility between the chrome steel and the tool during hot ring compression.

(2) The porosity in the oxide scale layer formed in steam atmosphere is effective to keep the temperature of the steels to be high during hot forging because of decrease in the heat transfer of the oxide scale–steel interface, however, the porosity does not contribute to the friction reduction of the steel workpiece covered with oxide scale during hot ring compression.

(3) The oxide scale of the chrome steel formed in steam atmosphere exhibits low friction characteristic in hot ring compression because the FeCr2O4 layer formed around the oxide scale–chrome steel interface provides high ductility of the oxide scale and prevents the chrome steel from contacting to the tool during hot forging.

Acknowledgement

This work was financially supported in part by the Research Committee for Oxide Scale Behavior in Steel Manufacturing Processes, Iron and Steel Institute of Japan.

References
 
© 2015 by The Iron and Steel Institute of Japan
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