Comparison of Impact Properties for Carbon and Low Alloy Steels

Transcription

1 J. Mater. Sci. Technol., 2011, 27(10), Comparison of Impact Properties for Carbon and Low Alloy Steels O.H. Ibrahim Metallurgy Dept., Nuclear Research Centre, Atomic Energy Authority, Egypt [Manuscript received March 29, 2010, in revised form June 21, 2010] The impact properties of hot rolled carbon steel (used for the manufacture of reinforcement steel bars) and the quenched & tempered (Q&T) low alloy steel (used in the pressure vessel industry) were determined. The microstructure of the hot rolled carbon steel contained ferrite/pearlite phases, while that of the quenched and tempered low alloy steel contained bainite structure. Impact properties were determined for both steels by instrumented impact testing at temperatures between 150 and 200 C. The impact properties comprised total impact energy, ductile to brittle transition temperature, crack initiation and propagation energy, brittleness transition temperature and cleavage fracture stress. The Q&T low alloy steel displayed much higher resistance to ductile fracture at high test temperatures, while its resistance to brittle fracture at low test temperatures was a little higher than that of the hot rolled carbon steel. The results were discussed in relation to the difference in the chemical composition and microstructure for the two steels. KEY WORDS: Ferritic/pearlitic steel; Bainitic steel; Impact properties 1. Introduction Impact properties of steels are primarily dependent on its microstructure which is determined by the chemical composition and heat treatment. Microstructural parameters of steels incorporate dislocation density, grain size as well as the volume fraction and size of second phase particles (carbides and inclusions). Low alloy steels are candidate materials for pressure vessel industry which require adequate amount of strength and toughness. The main micro alloying elements, in low alloy steels, used to ensure the amount of hardenability required to obtain bainitic steels are Cr, Mo and Ni [1]. The microstructure of such steels is of a complex nature and is characterized by highly dislocated lath structure arranged in packets subdividing the prior austenite grains in addition to the carbides that precipitate during the tempering process [2]. Carbon steels in the hot rolled condition have been the main structural materials used for the manufacture of reinforcement steel bars. These steels have a nominal carbon level of 0.2% 0.4% with Ph.D.; address: omyma essam@yahho.com. 1.4% Mn which is considered optimum when high strength with good ductility is required. Mn addition has a beneficial effect on the impact toughness because it raises the cleavage fracture stress through the refinement of grain size [3]. The microstructure of hot rolled carbon steels normally comprises a mixture of ferrite and pearlite which is a lamellar structure consisting of ferrite and cementite. Both the strength and toughness are increased as the pearlite proportion is increased [4]. In the present study, a comparison has been made between the impact properties of the above-mentioned two types of steels. The determined properties were further correlated to the steels microstructures. 2. Materials and Test Procedures The reinforcement steel bars used in this investigation were hot rolled at 900 C and then air cooled. The heat treatment for the low alloy pressure vessel steel included austenitizing at 880 C for 8 h, quenching in water and then tempering at 660 C for 6 h followed by air cooling. The chemical composition of the two steels is presented in Table 1.

2 932 O.H. Ibrahim: J. Mater. Sci. Technol., 2011, 27(10), Fig. 1 Microstructure of carbon steel (a) and low alloy steel (b) Table 1 Chemical composition of investigated steels, wt% Element C Si Mn Cr Mo Ni P S Cu Fe Carbon steel Bal. Low alloy steel Bal. Table 2 Impact test values of investigated steels Material TT/ C USE/J E i/j E p/j E i/e t/% E p/e t/% Hot rolled carbon steel Q&T low alloy steel Standard Charpy V-notch specimens were impact tested using an instrumented impact machine (AMSLER-RKP 300) with a total energy of 300 Joule and a hammer velocity of 5.2 m/s. Tests were conducted in a temperature range between 150 and 200 C to generate full transition curves. The loadtime traces produced from the tests were utilized to obtain dynamic fracture loads and crack initiation and propagation energies. Optical micrographs were obtained by etching polished specimens with a solution of 4% picric acid in methanol. Fracture surface was examined using a scanning electron microscope (SEM, Joel, JSM-400). 3. Results 3.1 Microstructure The microstructure of the investigated steels is shown in Fig. 1. The hot rolled steel shows ferriticpearlitic structure with a grain size of about 10 µm while the low alloy steel displays bainitic structure with a grain size of about 30 µm. 3.2 Impact results The impact test results for both steels showed typical ductile-to-brittle transition behavior characteristic of ferritic steels. The variation of impact energy with testing temperature for the two steels is shown in Fig. 2. The upper shelf energy (USE) and the ductile to brittle transition temperature (DBTT), as evaluated at the intersection point of half the value of Absorbed energy / J Low alloy steel Carbon steel Temp. / o C Fig. 2 Impact transition curves of investigated steels upper shelf energy with the transition curve, were determined for both steels. The low alloy Q&T steel has much higher USE than that of the hot rolled carbon steel (200 vs 80 J), Table 2. The DBTT of the low alloy Q&T steel shows lower value than that of the hot rolled carbon steel ( 25 C vs 25 C), Table 2. This indicates that the low alloy Q&T steel has superior resistance to ductile fracture and relatively better resistance to brittle fracture than the hot rolled carbon steel. Examination of the load-time traces of the impact tests of low alloy Q&T steel in upper shelf temperature range showed that the ductile fracture initiation energy, E i, was 25% of that of the total energy (50 vs 200 J), Table 2. This means that the ductile crack

3 O.H. Ibrahim: J. Mater. Sci. Technol., 2011, 27(10), Fig. 3 Impact test load-time curves at room temperature: (a) carbon steel, (b) low alloy steel Fig. 4 SEM fractographs of investigated steels (ductile fracture): (a) carbon steel (100 C, 80 J); (b) low alloy steel (RT, 200 J) initiation process has consumed less energy than that for the ductile crack propagation process, i.e. most of the total fracture energy was expended in the crack propagation process. In contrast, the ductile fracture initiation energy value of the hot rolled carbon steel (at upper shelf temperature range) was about 45% that of the total energy (35 vs 80 J). This means that the total fracture energy was shared almost equally between the initiation and propagation fracture processes. Figure 3 shows typical load-deflection curves as derived from the instrumented impact tests for both steels at room temperature. As shown, the low alloy Q&T steel displays fully ductile mode of fracture (upper shelf behavior) with a total energy of 200 J, while the hot rolled carbon steel exhibits semi-ductile mode of fracture (ductile to brittle transition behavior) with a total energy of only 37 J. In Fig. 3, the P m and P y denote maximum dynamic and yield loads, respectively. The area under the curve up to the point of P m represents the fracture initiation energy E i, while the remaining area represents the fracture propagation energy E p. The load-deflection curves demonstrate that, at room temperature, the low alloy Q&T steel experienced much greater amount of deflection (strain) than the hot rolled carbon steel (40 vs 10 mm). 3.3 Fractography Figure 4 shows fracture surface of specimens tested in upper shelf temperature range (ductile fracture mode). As shown, fracture proceeded by microvoid coalescence manner. Both large voids, in the case of hot rolled carbon steel (Fig. 4(a)) and rather finer dimples, in the case of low alloy Q&T steel (Fig. 4(b)), are clearly observed. In the case of brittle fracture condition (lower shelf temperature range), the hot rolled carbon steel displays cleavage mode of fracture characterized by small cleavage facets (Fig. 5(a)). Alternatively, the low alloy Q&T steel fracture surface contains larger facet-like arrangements with river patterns formed by cleavage lines and steps (Fig. 5(b)). The cleavage facet size in both cases can be compared to their grain size. 3.4 Cleavage fracture stress (σ f ) An important parameter that can be determined from the variation of the dynamic load against test temperature is the local fracture stress σ f which is directly related to the micromechanism of cleavage [5] ; therefore, it is often called the microscopic cleavage fracture stress. Cleavage fracture will take place when a combination of load and plastic constraint at the

4 934 O.H. Ibrahim: J. Mater. Sci. Technol., 2011, 27(10), Fig. 5 SEM fractographs of investigated steels (brittle fracture): (a) carbon steel ( 50 C, 11 J); (b) low alloy steel ( 100 C, 9 J) Local fracture stress, yy / MPa Carbon steel Low alloy steel f =2200 MPa f =1950 MPa Temp. / o C Fig. 6 Variation of local fracture stress with test temperature notch tip raises the maximum value of the local stress ahead of the notch, σ yy, to σ f. It has been found that σ yy =2.52σ [6] yd, where σ yd is the dynamic yield strength and related to yield load by the relationship: σ yd = P y L CB(W A) 2 where: B is the specimen width; W is the specimen depth; a is the notch depth; L is the bend span and C is the constraint factor ( 1.25). P y in the above equation is the yield load at test temperature at which fracture surface of the specimen is completely brittle, i.e. plastic deformation is no longer operative. This specific temperature is called the brittleness temperature. This temperature was determined as 50 and 100 C for the hot rolled carbon steel and low alloy Q&T steel, respectively from the examination of the fracture surface of tested specimens. The corresponding P y values at these test temperatures were about 19 and 22 kn, for the hot rolled carbon steel and low alloy Q&T steel, respectively. Using the above P y values for the two tested steels yields a cleavage fracture stress (σ f ) values of 1950 and 2200 MPa for the hot rolled carbon steel and low alloy Q&T steel, respectively. This indicates that the low alloy Q&T steel has a relatively higher resistance to cleavage fracture than that of the hot rolled carbon steel. The relationship between the local fracture stress and test temperature is shown in Fig Discussion 4.1 Microstructure The difference in the microstructure between the two tested steels can be correlated to the difference in chemical composition and heat treatment. The low alloy Q&T steel contained higher percentage of the alloying elements of Cr, Mo and Ni than the hot rolled carbon steel, Table 1. On the other hand, the low alloy Q&T steel heat treatment involved quenching and tempering while that of the hot rolled carbon steel involved hot rolling and normalizing. This was reflected upon the produced microstructure of both steels. The low alloy Q&T steel had higher hardenability than the hot rolled carbon steel due to its higher percentage of alloying elements. This has led to the formation of bainite structure and the precipitation of alloy carbides during tempering. Figure 1(a) shows a mixture of ferrite and pearlite (a lamellar structure consisting of ferrite and cementite). The fine grain size of hot rolled carbon steel could be attributed to the simultaneous deformation and recrystallization processes (dynamic recrystallization) during the hot rolling operation [3]. The bainitic microstructure of low alloy Q&T steel contained lath structure with considerable amount of carbides precipitated during the tempering process (Fig. 1(b)). 4.2 Impact properties The results have shown that the low alloy Q&T

5 O.H. Ibrahim: J. Mater. Sci. Technol., 2011, 27(10), steel with its bainitic microstructure exhibits higher resistance to both ductile and brittle fracture than the hot rolled carbon steel with its ferritic pearlitic microstructure. This difference in fracture behavior between these two investigated steels can be related to the difference in their chemical composition and microstructure. The main microstructural parameters that would control the fracture properties of both steels are second phase particles (carbides and inclusions) and grain size. The role played by each of these parameters during ductile and brittle fracture processes is discussed. 4.3 Ductile fracture The micromechanism operated during ductile fracture involves crack initiation through nucleation and growth of voids around carbides and/or inclusions and crack propagation through plastic deformation of the matrix [7]. The higher upper shelf energy value of the low alloy Q&T steel compared to that of the hot rolled carbon steel (200 vs 80 J) indicates higher energy expenditure during ductile crack initiation and propagation processes. As shown in Table 1, the carbon content of the hot rolled carbon steel is twice as much as that of the Q&T low alloy steel (0.4% vs 0.18%), while the S content of the hot rolled carbon steel is ten times that of the Q&T low alloy steel (0.043% vs 0.004%). C and S have been known to have detrimental effect on toughness of steels through formation of carbides and inclusions. Since the hot rolled carbon steel contains higher carbon content than the Q&T low alloy steel it is then expected that it will form higher amount of carbide particles. The type of carbides formed in the hot rolled carbon steel is expected to be cementite (Fe 3 C) which is incorporated in the pearlite structure. The cementite carbide particles would provide sites for easy nucleation of voids through cracking of these particles [8]. In addition, the much higher S content of the hot rolled carbon steel would lead to the formation of high density manganese sulphide (MnS) inclusions. This provides greater nucleation sites for voids by debonding the inclusion matrix interface under the stress and strain fields ahead of the notch tip of the impact specimens. In addition, the higher density of MnS inclusions provides less inter-particle spacing which would facilitate the process of void coalescence subsequent to the stage of void growth [9]. This is manifested as the observed less initiation and propagation energy values of the hot rolled carbon steel as compared to that of the Q&T low alloy steel; (35 vs 50 J) and (45 vs 150 J), respectively. The noticeable difference in the values of initiation and propagation energy especially of the Q&T low alloy steel can be explained in view of the difference between the two processes. Initiation is fundamentally a two-dimensional process where crack blunting takes place effectively in the straight ahead direction. In such case, only those parameters associated with this direction will be important. Propagation, however, occurs by linkage of voids which are spatially distributed, so that second phase particle parameters of all three dimensions must be taken into account [10]. The result is that the state (size, density, distribution) of these parameters will affect propagation process to a much higher degree than crack initiation process. 4.4 Brittle fracture Compared to hot rolled carbon steel, the Q&T low alloy steel showed lower ductile to brittle transition temperature ( 25 C vs 25 C), less brittleness transition temperature ( 100 C vs 50 C) and higher cleavage fracture stress (2200 MPa vs 1950 MPa). The fracture surface of hot rolled carbon steel was characterized by smaller cleavage facets than the Q&T low alloy steel. The smaller grain size of the hot rolled carbon steel proposes higher resistance to cleavage brittle fracture since grain boundaries are effective barriers to the propagation of brittle cracks [11]. However, the probably larger size and higher volume fraction of carbide particles (cementite) of the hot rolled carbon steel microstructure might have masked the toughening effect of its fine grain size. The presence of carbides such as cementite (Fe 3 C) in the microstructure of carbon steel provides sites for easy nucleation of cleavage microcracks particularly at the ferrite/cementite interfaces. Investigations on the mechanisms of microcrack nucleation proposed that there is a critical carbide size above which it becomes susceptible to cracking with the consequence that the impact transition temperature is raised. This critical size was found to be in the range of 2 5 µm in the case of cementite [12]. On the other hand, when alloy carbides replace cementite in alloy steels during tempering, the probable size of carbide particles is reduced [12]. Consequently, alloy carbides formed upon tempering are finer and much more resistant to coarsening than cementite. This fact can account for the observed higher resistance to cleavage brittle fracture of Q&T low alloy steel which was manifested by less ductile to brittle transition temperature and higher cleavage fracture stress value despite the larger grain size of the microstructure. 5. Conclusions (1) At high test temperatures, the Q&T low alloy steel exhibits much higher resistance to ductile facture than the hot rolled carbon steel as indicated by its higher upper shelf energy value (200 J vs 80 J). (2) At low test temperatures, the resistance to brittle fracture of Q&T low alloy steel is a little higher than that of the hot rolled carbon steel, as indicated by its lower ductile to brittle transition temperature ( 25 C vs 25 C), lower brittleness transition temperature ( 100 C vs 50 C) and its higher cleavage fracture stress (2200 MPa vs 1950 MPa).

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