**4. Discussion**

348 Corrosion Resistance

**50%N2, 50%C2H2 after Corrosion** 

**20 μm** 

**20 μm**

**100%C2H2 after Corrosion**

Fig. 9. SEM pictures of the surface of untreated 304 AISI and treated samples, respectively, in comparison with pictures of treated samples after corrosion test. The last image is for treated sample at 100 % C2H2 scanned after removing few tens nanometers from the

Two distinguishable regions are observed on the surface of the samples, beside the white points and the black base. For 100% N2, one observes a very fine white uniform deposition all over the surface. On the surface of sample treated with 100% C2H2 the white deposition is larger. For 50% N2, surface fractures and little particle deposition can be seen. EDAX analysis shows that the white precipitations contain more carbon than the black ones. The elemental composition of the untreated, treated samples and selected parts was analysed by EDAX and the results are listed in Table 1. For nitrocarburizing with 50 % N2 and 50 %

C2H2, the nitrogen concentration is higher than that of carbon.

surface.

**100%C2H2**

20 μm

**50%N2 50%C2H2**

**20 μm** 

The plasma efficiency may be increased due to creation of more plasma species such as H, CH, NH, HCN or CN by adding C2H2 to N2 gas during rf plasma carbonitriding. The microstructure of the modified layers depends on the pressure ratio between nitrogen and acetylene plasma gas. The nitride phases and their intensities increased by adding C2H2. The effect of adding acetylene has been also observed in [18] where 0.7 % of C2H2 was used in addition to the N2 gas. Even though Blawert et. al. [8, 19] has observed the nitrogen and carbon expanded austenite phases nearly at the same peak positions as in our case. However, we can not ignore that at high N-content by XRD the γn phase can not be easily distinguished from the Fe4N phase. Both nitride and carbide phases contribute to the improvement of the mechanical properties of the treated samples. Nitride phases (γn) are harder than the carbide phases (γc) [8]. The interplay between the temperature and gas composition might be caused by the effect of hydrogen from the acetylene gas. Compared to carbon and nitrogen the mass difference between plasma species and the ionization potential of atoms can play an important role in the resulting plasma temperature (electron

Corrosion Performance and Tribological Properties of Carbonitrided 304 Stainless Steel 351

are responsible for the breakdown in the corrosion resistance [20]. The degradation in corrosion resistance of the modified layers is mostly related to the concentration of CrN and CrC. It is well known that the nature of the corrosion reactions depends on the microstructure of the treated surface which controlled by gas composition and treatment temperature. Using XPS, Borges et al. [18] have recently observed a decrease in the chromium concentration on the nitride surface by adding small amount of acetylene. The authors interpreted this decrease in chromium concentration on the surface by the chemical formation of Cr-Hx (x = 1, 2) which is partly removed by the vacuum system. Former experiments involving the reaction of chromium atoms with H2 and matrix isolation of the products have provided the spectroscopic evidence for the molecules CrH2 and CrH [22]. In our case the intensity of CrN decreases with the increase of C2H2 up to 30 %, onward it increases again. This is in a good agreement with the sharp increase in the corrosion resistance of the same sample in comparison with the pure nitrided or pure carburized layer. Maybe the formation rate of the CrH2 and CrH on the surface is higher than the precipitation rate of CrN in the grain boundaries on the surface of the sample prepared at 30 % C2H2. But by the increase of the substrate temperature higher than 525 °C, the balance in

The gas composition has a significant influence on the microstructure of the modified layers. At nitrocarburizing, most of phases related to nitride phases (such as γn, Fe2N, Fe3N, Fe4N and CrN) and carbide phases (such as γc, Fe3C and CrN or CrC) for samples treated at high nitrogen content and high carbon content, respectively. In dependence on the gas composition ratio (N2/C2H2), the sample temperatures varied from 475 °C to 600 °C. The surface roughness was found to increase as the C2H2 content increases up to 50 % but it decreases for higher ratios. The amount of fine precipitations of carbon on the surface is responsible for the gradually decrease in the surface roughness and friction coefficient for samples prepared at high and pure carbon content. In comparison to the untreated samples, the wear rate is reduced by more than two orders. The carbonitrided layer exhibits higher corrosion resistance in comparison to the layers obtained after pure nitriding or pure carburizing treatment. The lower content of the CrN phase leads to a good corrosion resistance. Pure nitriding samples are exposed to a strong biting corrosion surrounded by

intergranular corrosion where the carburized layer is exposed to general corrosion.

[4] Jan Macák, Petr Sajdl, Pavel Kučera, Radel Novotný, Jan Vošta, Electrochim. Acta

[7] E. Richter, R. Günzel, S. Parascandola, T. Telbizova, O. Kruse, W. Möller, Surf. Coat.

[2] Y.F. Liu, J.S. Mu, X.Y. Xu, S.Z. Yang, Mater. Sci. Eng. A 458 (2007) 366. [3] Ajit K. Roy, Vinay Virupaksha, Mater. Sci. Eng. A 452–453 (2007) 665.

[6] E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116-119 (1999) 199.

the two rates has been changed.

**5. Conclusions** 

**6. References** 

[1] N. Yasumaru, Mater. Trans. 39 (1998) 1046.

[5] Z. L. Zhang, T. Bell, Surf. Eng. 1 (1985) 131.

Technol. 128-129 (2000) 21.

51(2006) 3566.

and ion temperature) which has an influence on the temperature of the substrate. In this case, the ionization potential of hydrogen is 6.4 % lower than of that nitrogen and the light hydrogen ions are easily to accelerate by the rf plasma field. These ions itself contribute to the plasma heating due to secondary electrons generation by elastic collisions with the plasma species.

Most probably, the high increase in the surface roughness, especially for the samples treated at high nitrogen content, is correlated with the increase in the sample temperature beside some physical reactions between the heavy plasma species such as CN and HCN and the surface. In a comparable study concerned with nitriding of stainless steel by plasma immersion ion implantation, the formation of expanded austenitic phase in the matrix was accompanied by a high improvement of the microhardness. However, the surface roughness increased by a factor of 4.6 and 7 at 450 °C and 520 °C, respectively [20]. Blawert et. al. [21] has attributed to the increasing in the surface roughness of treated 304-AISI samples to the sputtering surfaces caused by ion bombardment during the treatment. Even though, an increase in the substrate temperature has been observed for samples prepared at high and at pure carbon content, the surface roughness is sharply decreased. It might be due to the decrease in the physical reactions and the high amount of fine precipitations from carbon on the surface.

The decrease in the friction coefficient for samples treated at high C2H2 content can be attributed to the fine precipitation from carbon on the surface, which works as solid lubricant between sliding surfaces in wear experiment. After that, as the wear path increased, the effect of fine precipitation of carbon nearly disappeared. Therefore, the friction coefficient increases with the number of wear tracks. This suggestion is supported experimentally by imaging the carburized surface after removing a few hundred nanometres by a fine mechanical polishing (Fig. 9). The high number of carbon precipitations is nearly removed after polishing. Obviously, the wear behavior is influenced by the microstructure of the first sublayer which is created in dependence of the gas composition by the process of nitriding, carburizing or nitrocarburizing. Blawert [8] has reported that nitrogen expanded austenite layers are harder than those of the carbon expanded and therefore this results in smaller wear depth at low load (5 N). The role of the oxide layer is completely different in the wear behavior of untreated and treated samples. In untreated samples the oxidized wear particles lead to severe wear resulting in a high friction coefficient for the sliding of surfaces. However, for all treated samples, the oxide layer works in the opposite direction. The oxide layer of samples treated at high nitrogen content or high carbon content contains low or high amount of fine precipitations from carbon on the surface, respectively. This oxide layer acts as a lubricating layer, which prevents metalto-metal contact, decreases the friction coefficient, reduces adhesive wear and therefore generally reduces the wear [17].

The high concentration of nitrogen detected by EDAX on the surface of the nitrocarburized sample is supported by the XRD results, which show that more nitride phases are created in the compound layer.

The solid solution phase γN should maintain the good corrosion resistance of stainless steel [5, 21]. But in our case, microstructure reveals that high parts of chromium nitride or chromium carbide are detected on the surface precipitated at the grain boundaries, which are responsible for the breakdown in the corrosion resistance [20]. The degradation in corrosion resistance of the modified layers is mostly related to the concentration of CrN and CrC. It is well known that the nature of the corrosion reactions depends on the microstructure of the treated surface which controlled by gas composition and treatment temperature. Using XPS, Borges et al. [18] have recently observed a decrease in the chromium concentration on the nitride surface by adding small amount of acetylene. The authors interpreted this decrease in chromium concentration on the surface by the chemical formation of Cr-Hx (x = 1, 2) which is partly removed by the vacuum system. Former experiments involving the reaction of chromium atoms with H2 and matrix isolation of the products have provided the spectroscopic evidence for the molecules CrH2 and CrH [22]. In our case the intensity of CrN decreases with the increase of C2H2 up to 30 %, onward it increases again. This is in a good agreement with the sharp increase in the corrosion resistance of the same sample in comparison with the pure nitrided or pure carburized layer. Maybe the formation rate of the CrH2 and CrH on the surface is higher than the precipitation rate of CrN in the grain boundaries on the surface of the sample prepared at 30 % C2H2. But by the increase of the substrate temperature higher than 525 °C, the balance in the two rates has been changed.

#### **5. Conclusions**

350 Corrosion Resistance

and ion temperature) which has an influence on the temperature of the substrate. In this case, the ionization potential of hydrogen is 6.4 % lower than of that nitrogen and the light hydrogen ions are easily to accelerate by the rf plasma field. These ions itself contribute to the plasma heating due to secondary electrons generation by elastic collisions with the

Most probably, the high increase in the surface roughness, especially for the samples treated at high nitrogen content, is correlated with the increase in the sample temperature beside some physical reactions between the heavy plasma species such as CN and HCN and the surface. In a comparable study concerned with nitriding of stainless steel by plasma immersion ion implantation, the formation of expanded austenitic phase in the matrix was accompanied by a high improvement of the microhardness. However, the surface roughness increased by a factor of 4.6 and 7 at 450 °C and 520 °C, respectively [20]. Blawert et. al. [21] has attributed to the increasing in the surface roughness of treated 304-AISI samples to the sputtering surfaces caused by ion bombardment during the treatment. Even though, an increase in the substrate temperature has been observed for samples prepared at high and at pure carbon content, the surface roughness is sharply decreased. It might be due to the decrease in the physical reactions and the high amount of fine precipitations from carbon on

The decrease in the friction coefficient for samples treated at high C2H2 content can be attributed to the fine precipitation from carbon on the surface, which works as solid lubricant between sliding surfaces in wear experiment. After that, as the wear path increased, the effect of fine precipitation of carbon nearly disappeared. Therefore, the friction coefficient increases with the number of wear tracks. This suggestion is supported experimentally by imaging the carburized surface after removing a few hundred nanometres by a fine mechanical polishing (Fig. 9). The high number of carbon precipitations is nearly removed after polishing. Obviously, the wear behavior is influenced by the microstructure of the first sublayer which is created in dependence of the gas composition by the process of nitriding, carburizing or nitrocarburizing. Blawert [8] has reported that nitrogen expanded austenite layers are harder than those of the carbon expanded and therefore this results in smaller wear depth at low load (5 N). The role of the oxide layer is completely different in the wear behavior of untreated and treated samples. In untreated samples the oxidized wear particles lead to severe wear resulting in a high friction coefficient for the sliding of surfaces. However, for all treated samples, the oxide layer works in the opposite direction. The oxide layer of samples treated at high nitrogen content or high carbon content contains low or high amount of fine precipitations from carbon on the surface, respectively. This oxide layer acts as a lubricating layer, which prevents metalto-metal contact, decreases the friction coefficient, reduces adhesive wear and therefore

The high concentration of nitrogen detected by EDAX on the surface of the nitrocarburized sample is supported by the XRD results, which show that more nitride phases are created in

The solid solution phase γN should maintain the good corrosion resistance of stainless steel [5, 21]. But in our case, microstructure reveals that high parts of chromium nitride or chromium carbide are detected on the surface precipitated at the grain boundaries, which

plasma species.

the surface.

generally reduces the wear [17].

the compound layer.

The gas composition has a significant influence on the microstructure of the modified layers. At nitrocarburizing, most of phases related to nitride phases (such as γn, Fe2N, Fe3N, Fe4N and CrN) and carbide phases (such as γc, Fe3C and CrN or CrC) for samples treated at high nitrogen content and high carbon content, respectively. In dependence on the gas composition ratio (N2/C2H2), the sample temperatures varied from 475 °C to 600 °C. The surface roughness was found to increase as the C2H2 content increases up to 50 % but it decreases for higher ratios. The amount of fine precipitations of carbon on the surface is responsible for the gradually decrease in the surface roughness and friction coefficient for samples prepared at high and pure carbon content. In comparison to the untreated samples, the wear rate is reduced by more than two orders. The carbonitrided layer exhibits higher corrosion resistance in comparison to the layers obtained after pure nitriding or pure carburizing treatment. The lower content of the CrN phase leads to a good corrosion resistance. Pure nitriding samples are exposed to a strong biting corrosion surrounded by intergranular corrosion where the carburized layer is exposed to general corrosion.

#### **6. References**


**16** 

Adam Grajcar

*Poland* 

*Silesian University of Technology* 

**Corrosion Resistance of High-Mn** 

**Austenitic Steels for the Automotive Industry** 

Significant progress in a field of development of new groups of steel sheets for the automotive industry has been made in the period of the last twenty years. From the aspect of materials, this development has been accelerated by strong competition with non-ferrous aluminium and magnesium alloys as well as with composite polymers, which meaning has been successively increasing. From the aspect of ecology, an essential factor is to limit the amount of exhaust gas emitted into the environment. It is strictly connected to fuel consumption, mainly dependent on a car weight. Application of sheets with lower thickness preserving proper stiffness requires the application of sheets with higher mechanical properties, keeping adequate formability. Figure 1 presents conventional high-strength steels (HSS) and the new generations of advanced high-strength steels (AHSS) used in the automotive industry. Steels of IF (Interstitial Free) and BH (Bake Hardening) type with moderate strength and high susceptibility to deep drawing were elaborated for elements of body panelling. However, the increasing application belongs to new multiphase steels consisting of ferritic matrix containing martensitic islands (DP – Dual Phase) or bainiticaustenitic regions (TRIP – Transformation Induced Plasticity). These steels together with CP (Complex Phase) and MART steels with the highest strength level are the first generation of advanced high-strength steels (AHSS) used for different reinforcing elements (International

Nowadays, apart from limiting fuel consumption, special pressure is placed on increasing the safety of car users. The role of structural elements such as frontal frame members, bumpers and other parts is to take over the energy of an impact. Therefore, steels that are used for these parts should be characterized by high product of UTS and UEl, proving the ability of energy absorption. It is difficult to achieve for conventional HSS and the first

The requirements of the automotive industry can be met by the second generation of advanced high-strength steels combining exceptional strength and ductile properties as well as cold formability (Fig. 1). These TWIP (Twinning Induced Plasticity) and L-IP (Light – Induced Plasticity) steels belong to a group of high-manganese austenitic alloys but are much cheaper comparing to Cr-Ni stainless steels (AUST SS). Their mean advantage over first generation steels with a matrix based on A2 lattice structure is the great susceptibility of austenite on plastic deformation, during which dislocation glide, mechanical twinning

generation AHSS because the ductility decreases with increasing strength (Fig. 1).

**1. Introduction** 

Iron & Steel Institute, 2006).

