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Effects of Heat Treatment on the Mechanical and MK0400029 EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND ... EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND MICROSTRUCTURAL PROPERTIES OF IMPROVABLE STEEL C. 1531 (Ck 45) Jon S. Magdeski, Dragan S. Slavkov, Sveto T. Cvetkovski University "Sv. Kiril and Metodij" - Skopje, Republic of Macedonia Faculty of Technology and Metallurgy, Rudjer Boskovic 16, Skopje, Republic of Macedonia Abstract An analysis of the mechanical properties of improvable steel C. 1531 and relationship with micro structural features in the present work is made. The mechanical properties (hardness and impact toughness) are related to the parameters of improvement process (the temperature of austenitization Ta, the temperature of tempering To, as well as the time of tempering to). Optical microscopy analysis was applied to examine the microstructure characteristics of as-received and heat-treated specimens. INTRODUCTION Heat treatment can appreciably change the properties of steel, its effect on mechanical properties being most important. Steel as annealed, normalized or tempered (Ttemp>400 °C) consists of lamellar ferrite and carbide (cementite) inclusions. Ferrite has a low strength and high ductility, whereas cementite has a high hardness (around HBS 800) and zero values of elongation and reduction. The fact that iron-base alloys with more than 0.01% carbon have an increased strength and reduced plasticity should evidently be attributed to the strengthening effect of carbide inclusions. The quantity of carbide particles of a constant size depends on the carbon content in steel (directly proportional to the carbon content in carbon steels). This is why the values of strength in steels increase and the values of ductility diminish with increasing carbon content [1]. In order to obtain the best combination of mechanical properties in hardened steel, it is essential to form a fine-acicular martensitic structure, which is possible only if the original austenitic structure is also fine-grained [1]. Two-stage heat treatment, in which the final structure is formed from martensit rather than from austenite, i.e. hardening followed with tempering, makes it possible to vary the strength properties of steel within a wide range from the maximum in as-hardened state to the minimum in as-annealed state; it is important that a steel thus treated has better plasticity and ductility properties than ordinary treated (products of decomposition of austenite) [2]. Obviously, a two-stage heat treatment procedure appreciably improves the whole complex of mechanical properties of steel. Therefore this method can be called improvement (or more properly, thermal improvement) and it is the principal type of heat treatment for medium-carbon structural steels. EXPERIMENTAL A cold drawn steel rod with 20 mm diameter was used. The chemical composition of steel is given in Table 1 and the mechanical properties in delivery state (guaranteed by manufacturer) are given in Table 2. rd Proceedings of 3 BMC-2003-Ohrid, R. Macedonia w „„„.- -.^A.,-™^ 109 t^ /\ u ."I >J f; EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND ... Table 1. Chemical composition of the investigated steel Type of steel C Mn Si S P CΓ Ni Mo N C.1531 0.470 0.560 0.240 0.017 0.016 0.050 0.030 0.005 0.006 Table 2. Mechanical properties of steel in initial (cold drawn) state Type of steel Reh(MPa) Rm(MPa) A(%) C.1531 682.00 701.00 12.20 Standard procedure for heat treatment, according to literature data [3, 4], was applied to the specimens of investigated steel i.e.: - austenitizing at temperatures ranging from 820 to 850 °C and constant time of austenitization-30 min.; quenching in water. - tempering at temperatures ranging from 550 to 650 °C and time of tempering 30 to 120 min. A resistance chamber furnace for annealing for austenitization before quenching and tempering was used. The hardness of the specimens was determined using the Rockwell hardness test and the Charpy impact test for impact toughness values. The microstructure was investigated using light microscopy. RESULTS AND DISCUSSION At the beginning, the hardness and impact toughness values in initial state were determined and the results are presented in Table 3. The microstructure of the steel in cold drawn state (typical ferrite-pearlitic structure) is shown on Figure 1. Table 3. Hardness (HRB) and impact toughness (KV) values of steel in initial state Measurement 1 2 3 Mean value HRB 95 92 95 94 KV (J), 20 °C 12 15 14 13.67 110 Proceedings of 3rd BMC-2003-Ohrid, R. Macedonia EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND ... In order to establish the dependence of austenitizing temperature on the hardness, three temperatures of austenitization (820, 835 and 850 °C) were chosen. The results for hardness measurements of quenched specimens are shown in Table 4. It's obvious that the temperature of austenitization in the investigated region has no great influence on the hardness of quenched specimens, i.e. approximately same hardness values are obtained for different temperatures of Figure 1. Microstructure of investigated steel in austenitization. Figure 2 shows the initial state (cold drawn) ferrite-pearlitic structure, microstructure (tetragonal martensite) of the X 500. quenched specimen. Table 4. Hardness values (HRC) of specimens quenched from three different temperatures WBffl&L Temperature of austenitization, °C Quenched 820 835 850 specimens Rockvell hardness, HRC 1 56 55 61 2 57 59 60 3 56 55 59 4 53 58 59 5 59 60 58 Figure 2. Microstructure of quenched specimen - 6 59 56 58 tetragonal martensite, X 500. 7 57 58 60 8 61 59 62 9 57 56 53 10 57 59 53 Further, previously quenched 11 58 60 58 specimens were subjected on tempering 12 55 58 59 process (different temperatures and different times) and the obtained hardness values are Mean 57.08 57.75 58.33 presented in Table 5 and the values for impact Value toughness in Table 6. Proceedings of 3rd BMC-2003-Ohrid, R. Macedonia 111 EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND ... Table 5. Rockwell-hardness values (HRC) Table 6. Impact toughness values (KV) of of tempered specimens tempered specimens Treatment Mean Treatment Mean T/TQ/TO 1 2 3 value Ta/To/xo 1 2 3 value 820/550/30 22 26 25 24.33 820/550/30 111 115 115 113.67 820/550/120 23 23 25 23.67 820/550/120 122 119 116 109.67 820/650/30 19 18 18 18.33 820/650/30 142 130 137 136.33 820/650/120 15 15 15 15.00 820/650/120 147 155 142 148.00 835/600/5 24 25 24 24.33 835/600/5 112 114 114 113.33 835/600/30 21 20 21 20.67 835/600/30 135 138 138 137.00 835/600/75 20 19 19 19.33 835/600/75 140 140 140 140.00 835/600/120 18 19 19 18.67 835/600/120 144 146 140 143.33 850/550/30 25 23 24 24.00 850/550/30 118 114 112 114.67 24 850/550/120 105 110 112 109.00 850/550/120 25 L_ 23 24.00 850/650/30 16 18 17 17.00 850/650/30 142 157 145 148.00 850/650/120 13 14 15 14.00 850/650/120 142 156 147 148.33 The microstructure of tempered steel is shown in Figure 3. As one can see, an acicular The obtained results for hardness and structure is retained even at this, relatively impact toughness values are graphically high, tempering temperature. presented on Figure 4 and 5. Figure 4 clearly shows the softening effect. It is obvious that increasing the tempering temperature results in increasing of impact toughness values and decreasing of hardness values. This is expected behaviour, for this type of steel, due to the diffusion and lattice relaxation processes occurring during tempering process. Namely, diffusion of carbon atoms through supersaturated α-solution (martensitic structure) and precipitation of intermediate carbides takes place at the beginning of the tempering process and a disturbance of coherence at carbide-matrix boundaries and relief of elastic micro stresses, later. At higher tempering temperatures the formation of cementite particles and their coarsening and Figure 3. Microstructure of tempered specimen, spheroidization takes place. Figure 5 shows structure of tempering, X 500. the time dependence of mechanical properties at constant (600 °C) tempering temperature. rd 112 Proceedings of 3 BMC-2003-Ohrid, R. Macedonia EFFECTS OF HEAT TREATMENT ON THE MECHANICAL AND 160 • o 25 150 D 23 140 a o O 21 •—. 130 I 19 120 "N^ HRC ~-Q^ KV, j B 110 o 100 560 600 620 640 660 To, oC Figure 4. Dependence of hardness and impact toughness values on tempering temperature at TO=30 min. 26 150 D \ \—— -—- \ 24 ;: 140 \ a • ^ 22 \ 130 20 / 120 I ~^—^-S^ o -—•— D - ;• 18 • 110 — •N^ HRC "•D^ KV.J } 16 100 20 40 60 80 100 120 140 to, mm Figure 5. Dependence of hardness and impact toughness values on time of tempering at To=600 °C CONCLUSION Heat treatment of C. 1531 (Ck 45) carbon steel shows normal regularity in mechanical properties (hardness and impact toughness values) changes. So, hardness values steadily decrease with tempering (softening effect); the influence of tempering temperature being more pronounced compared to the time of tempering process. This is result of the diffusion and lattice relaxation processes occurring during tempering. The same holds also for impact toughness value changes during tempering. Impact toughness remarkably increases with tempering (KV 100 -=-150 J) compared to cold drawn state (KV 12 -f 15 J) and normalized specimens (KV 48 -4- 52 J). Literature 1. A. P.
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