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Mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling C. Medrea1, G. Negrea2 1 Department of Physics, Chemistry and Materials Technology, Technological Education Institute of Pireaus,250 , Thivon &P. Ralli Street, 12244 Aegaleo, Greece URL: www .teipir .gr e-mail: medrea@ internet.gr; 2 Faculty of Materials Science and Engineering, Technical University of Cluj-Napoca, Muncii Avenue 103105, 3400 Cluj-Napoca e-mail:[email protected] URL: www.utcluj.ro ABSTRACT: Nitriding is usually applied to alloyed steels with the scope of increasing their surface hardness and wear resistance. Warm working has been found to produce a fine-grained microstructure, which makes possible further treatment of low carbon steels. In combination with a low temperature thermochemical treatment, such as nitriding, warm working can be used to produce machine parts with a though core and with a hard, wear resistant surface layer. This paper presents a study of mechanical and structural properties of AISI 1015 carbon steel nitrided after warm rolling. The rolling was performed in the following conditions: temperature 670 – 550 oC, rolling speed 1.39 s-1 and deformation ratio 36.4%. After rolling, the samples were reheated to 550 oC for a duration varying from a few minutes to 10 hours. The microstructural changes were assessed by light microscopy and quantitative microscopy analysis. Warm rolled samples were ion nitrided at 510-520 oC in dissociated ammonia. The microstructure was analyzed by scanning electron microscopy and the mechanical properties were evaluated by tensile testing, surface hardness and friction coefficient measurements. Prior application of warm rolling makes possible (in the sense that is a viable solution) the ion nitriding of low carbon steels in order to produce machine parts with improved mechanical properties in the core (due to warm rolling) and longer service life (due to ion nitriding). Key words: carbon steels, warm rolling, nitriding, microstructure, mechanical properties. 1 INTRODUCTION Generally, low carbon steels are delivered in as rolled, annealed or normalized condition and, for a given chemical composition, their mechanical characteristics depend on their microstructure. At room these steels consists of ferrite and pearlite. The mechanical characteristics of ferrite-pearlite microstructure are strongly influenced by the ferrite grain size [1]. A series of methods are applied in the industry in order to refine the ferrite-pearlite microstructure: modification of the chemical composition [2], normalizing [3], plastic deformation by controlled rolling [4], rapid cooling [5] , warm working [6,7]. Situated between the cold and hot working, the warm-working process corresponds to a temperature range in which, after the plastic deformation, the material is partially strain hardened and partially recrystallized [8]. In several works have been studied the microstructure development and mechanical behavior during forging [9], stamping [10], rolling[11,12], caliber rolling and drawing [13] at warm temperatures. For industrial applications, warm working is very attractive because it offers certain advantages. Thus, compared to cold working, it requires lower deformation forces, can be applied to a broader range of steels, allows for higher deformation ratios, generates a more uniform deformation across the transversal section and leads to a less strained microstructure [14].Compared to hot working, it leads to a finer microstructure with superior mechanical properties, better surface quality and better dimensional control, lower material losses due to decarburization and oxidation. Attempts to apply thermochemical treatments premise that inclusions are uniformly distributed, have the same geometrical shape and differ only in size. Based on Cavalieri-Aker principle [16] and by using statistical analysis of results [17], the frequency histograms were determined. As a result of this study it was possible to define the nitriding regime that can be suitable to warm worked products. Investigated surface b b/2 to some carbon steels, previously subjected to warm working, have resulted in superior results as compared to those subjected to hot working [15]. However, desirable results can only be achieved if the heat-treating regime is properly defined. In the case of warm rolled steel products, the heating temperature must be limited below the pearlitic reaction temperature (Ac1) and the soaking time needs to be established such as to preserve the fine microstructure produced by warm working. The present study focuses on nitriding a low carbon steel after warm rolling and the evaluation of the final mechanical properties and microstructure. y Rolling direction x Fig. 1. Schematic draw of a plastically deformed sample and 2 EXPERIMENTAL DETAILS The AISI 1015 carbon steel bars were warm rolled in the following conditions: the temperature at the beginning and at the end of rolling - 670 oC and 550 oC, respectively, the rolling speed 1.39 s-1 and the deformation ratio 36.4%. A number of 12 samples were cut from warm rolled steel and were heated in an electrical laboratory furnace. Each sample was heated separately and then cooled in still air. The heating temperature and soaking time for each sample are given in table 1. Table 1. Parameters of the heat treatment applied to AISI 1015 steel after warm rolling. Sample No. Temperature[ o C] Time [min.] 1 1 2 3 3 5 4 10 5 15 6 20 550 7 30 8 60 9 120 10 180 11 300 12 600 Microstructural changes that took place during the heat treatment were assessed by light microscopy. Quantitative microscopy analyses were also performed by using an automated image analyzer type Epiquand. The analyses were made in two directions: parallel (x) and perpendicular (y) to the rolling direction as shown in fig. 1. The field of investigation had dimensions 4x4 mm. The quantitative microscopy analysis was based on the location of the specimen cut for microscopic analysis. . For determination of mechanical properties, standardized samples were machined from warm rolled steel and then subjected to ion nitriding in dissociated ammonia for 10 h at 510-520 oC by using a Nitrion 10 type equipment. Tensile characteristics, surface hardness and dry friction behavior were evaluated. Friction coefficient was determined by using a home made ring-on-block tribometer. The block (10x10x10 mm) was made from warm rolled steel and the ring from gray cast iron. The tests were performed under dry friction by using a constant normal force (Fn=181,85N) and recording the variation of the friction force as a function of time. After 3 … 4 sec. the friction force has stabilized (Ff max). The friction coefficient was calculated by the relationship: Ff max (1) μ= Fn The fracture surface of the samples subjected to tensile testing was investigated by scanning electron microscopy (SEM). 3 RESULTS AND DISCUSSIONS After warm rolling the microstructure of the steel consists of flattened and partially strain hardened ferrite grains and very fine pearlite particles distributed in lines parallel to the rolling direction (Fig. 2 a). Figs. 2 and 3 show the microstructure and the distribution curves, respectively, for significant soaking times. By heating to 550 oC, the microstructure is completely recrystallized after 10 min and displays well defined grain boundaries (fig. 2 b ). The ferrite has a fine grain size, close to the dashed line). The stability of the microstructure to heating after warm rolling allows for thermochemical treatment by nitriding of steel. 10 min 2h 10 h 100 μm 15 Frequency, % 100 μm 10 5 100 μm Increasing of the soaking time to 2 hours does not affect essentially the ferrite grain size (fig. 2 c). The microstructure appears fine and homogenous. In the rolling direction the distribution curve shifts slightly to the right and the maximum frequency increases from (8.0 … 11.3) μm to (11.3 … 16.0) μm (fig. 3 a, continuous line). Large and medium size grains grow very slightly on the expense of fine grains and lead to a homogenous microstructure. The pearlite is distributed in lines in the form a spheroidal separations (fig. 2 c). The distribution curves shift slightly to the left and the dispersion degree decreases (fig. 3 b, continuous line).Increasing of the soaking time to 10 hours leads only to a slight increase of the ferrite grain size (fig. 2 d ). It can be noticed a decrease of the frequency values in the very fine grain classes. The distribution curves remain in the same grain size field but the central parts of the curves shift slightly to the right. The dispersion degree of the grain size decreases further (fig. 3 a, dashed line). The pearlite particles have a spheroidal shape and the line distribution is preserved (fig.2 d). The dispersion of the distribution curves decreases significantly indicating a dimensional leveling of the particles (fig. 3 b, 181.0 128.0 64.0 90.5 32.0 45.2 22.6 16.0 8.0 11.3 4.0 5.5 Grain size, μm 10 min 2h 10 h 15 Frquency, % 10 5 128.0 181.0 Grain size, μm 90.5 64.0 45.2 32.0 16.0 22.6 8.0 11.3 5.5 4.0 (b) 0 2.0 initial grain size, with uniformly distributed grains (fig. 3 a, doted line). In the rolling direction the dispersion is larger and the average grain size values are shifted to the right. The maximum grain dimension is situated in the range [45.2 – 90.5] μm. Immediately after recrystallization, the ferrite is very fine, with slightly elongated grains in the rolling direction and uniformly distributed in the microstructure. The pearlite grain size is little affected by the soaking time (fig. 3 b doted line). 2.0 Fig. 2. The microstructure of samples warm rolled (a) and after different soaking times at 550 oC ( b - 10 min , c - 2 h , d – 10 h). 2.8 (a) 0 2.8 100 μm Fig. 3.Grain size distribution curves of the ferrite (a ) and pearlite (b) after heating to 550 oC with different soaking times. (Parallel to the rolling direction) Table 2 shows the mechanical properties of the ion nitrided samples after warm rolling. Table2. Mechanical properties of samples after warm-rolling and in normalized condition. AISI1015 Warm rolled Nitrated after warm rolling Friction in area HV5 coefficient [%] μ Yield strength [MPa] Tensile strength [MPa] 359 493 24,7 59 188 0,342 398 538 18,6 49,5 304 0,206 Elongation [%] Reduction The application of nitriding after warm rolling determines a slight increase of the strength (10% for yield strength and 20% for tensile strength, respectively). The surface hardness increases significantly (62%), while the friction coefficient decreases much (by 40%). The significant modifications of the hardness and friction coefficients lead to the improvement of the wear resistance of the parts treated in this way and to the increase of their lifetime. Figure 4 shows the fracture surfaces of warm rolled and nitrided samples subjected to tensile testing. Due to nitriding, the fracture surface losses the coneand-cup aspect. A clear delimitation appears between the white layer (8 μm) and the diffusion zone (0.2 mm). There is a good link between the diffusion zone and the core (Fig. 4a). In the white layer, the fracture surface is rather smooth indicating a brittle fracture due the fine and very hard nitrides (Fig. 4b). In the diffusion zone, the fracture surface presents shallow hollows. In the core (Fig. 4c), with fine hollows which preserves the aspect and dimensions of those obtained by simple warm rolling (Fig. 4d). The similarity of the two images (Fig. 4c and 4b) confirms the stability during nitriding of the microstructure obtained by warm rolling. carbon steel and implicitly to increasing their lifetime. REFERENCES 1. 2. 3. 4. 5. 6. 7. 50 μm 10 μm 8. 9. 10. 5 μm 5 μm Fig. 4. SEM micrographs showing the morphology of the fracture surfaces of nitride after warm-rolling (a,b,c) and warm rolled (d) samples subjected to tensile testing. 11. 12. 4 CONCLUSIONS Warm rolling determines the refining of the ferritepearlite microstructure and, implicitly, an improvement of mechanical properties. 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