Effect of Heat Treatment on the Microstructure and Mechanical Properties of Steel

Transcription

1 金属材料及热处理 学生实验报告范例课程编号 : 课程主讲及实验课指导教师 : 刘国权黄鹏 Effect of Heat Treatment on the Microstructure and Mechanical Properties of Steel Song Zhen University of Science and Technology Beijing, Dept.of M.S.E Abstract: Effect of heat treatment on the microstructure and mechanical properties of steel 45, steel 40CrNi and steel T8 was investigated in this experiment. Specimens undergoing different quenching/tempering temperature reveal various microstructure and hardness. The result shows that hardness decreases with increase in tempering temperature. Quenched at different temperature, austenite may decompose into martensite, troosite with martensite, ferrite with cementite, and so on. Alloying elements can increase hardenability and tempering stability of steel. Keywords: heat treatment; steel; quenching; tempering; hardness; hardenability 1. Introduction Metals and alloys develop requisite properties by heat treatment which plays a critical role in achieving appropriate microstructure that imparts the desired characteristics in a given material. Heat treatment consists of annealing, normalizing, quenching and tempering. Different phase transformations take place during heat treatment. This experiment is executed to probe the effect of different heat treatment parameters on the microstructure and mechanical properties of steels. 2. Specification of desired Heat Treatment processes 2.1 Coarse-grained Martensite and fine-grained Martensite In order to obtain full Martensite, the quenching temperature should be set C above Ac3 for hypoeutectoid steels. The grain size of Martensite is determined by that of Austenite. Austenite appears during heating of a ferrite-carbide mixture, growth centers of the austenite phase are very numerous, and initially austenite grains are extremely small, on the order of μ m.but with an increase in the heating temperature or holding time in the austenite range, the grains begin to grow intensively, which means the coarsening of martensite grain.[1]. The quenching temperature of Coarse-grained Martensite should be higher than that of Fine-grained Martensite. 2.2 Troosite and Martensite With a lower cooling rate, not all the austenite transforms to martensite. Partial retained austenite transforms to troosite according to the TTT diagram. Microstructure after quenching consists of Martensite and troosite. 2.3 Ferrite and Pearlite Ferrite-Pearlite can be obtained by slow cooling (usually in the air) after Austenitized, which is usually referred as Normalizing.

2 2.4 Ferrite and Martensite As to low carbon steels, when held at temperature between A c1 and A c3, Ferrite and Austenite are obtained. If quenched at higher cooling rate, Austenite transforms to Marstenite and Ferrite remains in the steel. 2.5 Three stages of Tempering [2] (a) The first stage of tempering is also referred to as low temperature tempering. This results in the formation of a low carbon martensite and a carbide by transformation of high carbon martensite. The low carbon martensite is usually referred as tempered-martensite. As far as mechanical properties are concerned, marginal decreases in hardness value takes place at this stage. Excellent wear resistance and reduced internal stresses are the characteristics of hardened steel tempered in this stage. (b) The second stage of tempering consists of heating steels in the temperature range varying from C. During this stage, retained austenite transforms to tempered-troosite. Another name given to this stage is medium temperature tempering. Ductility and toughness increase by this treatment with a corresponding decrease in hardness and strength. (c) The third stage of tempering is also popularly known as high temperature tempering. It consists of heating steel within a temperature range of C. Heating to such high temperature results in the formation of ferrite-cementite mixture which is referred as tempered sorbite. The cooling rate has no effect on the microstructure of steels after being tempered. 2.6 Identification of desired heat treatment process As to desired microstructure, the whole heat treatment processes are listed in Table.2. Relative standard heat treatment parameters, such as critical temperatures, commonly used quenching temperature and quenching media, is shown in Table Material and specimen preparation Three kinds of specimens were prepared for Heat treatment and Hardness test: steel 45, steel 40CrNi and steel T8. The chemical composition and geometrical size are show in Table.1. Of each kind, ten specimens were prepared for various Heat treatment operations. Table.1 Chemical Composition and Geometrical Size[3] Steel Diameter/ mm Chemical Composition (wt%) C Si Mn Cr Ni Cu S P Mo CrNi T Heat Treatment Heat treatment processes of all the specimens are identified as show in Table.2

3 Tabel.2 Microstructure & Heat Treatment Process (without tempering) & Hardness Microstructure Steel Coarse-grained Martensite Fine-grained Martensite Troosite and Martensite Ferrite and Pearlite Ferrite and Martensite Hardness/HRC Heat Treatment Temperature/ C Cr Ni Water Water Oil Air Water(Agitating) Media Hardness/HRC Temperature/ Heat Treatment C Media Oil Oil Air Oil Air Oil Hardness/HRC T8 Heat Treatment Temperature/ C Media Water Water (Agitating) Oil Air Tempered Martensite Tempered Troosite Tempered Sorbite 45 Temperature: 840 C Time: 30min Hardness/HRC Tempering Temperature/ C Media: Water Cooling Media Air Air Air Air Air 40CrNi Hardness/HRC

4 Temperature: 840 C Time: 30min Media: Oil Tempering Temperature/ C Cooling Media Air Air Air Air Air T8 Temperature: 770 C Time: 30min Hardness/HRC Tempering Temperature/ C Media: Water Cooling Media Air Air Air Air Air Table.3. Standard Heat Treatment Parameters (Critical Temperature, Parameters, Hardness)[4] Steel Critical Temperature/ C Ac1 Ac 3 Ar1 Ar 3 M s Tempering (temp/ C ) Hardness after Tempering at different Hardness/ temperatures/hrc Temp/ C Media HRC M r Water/Oil CrNi Oil T8 800 Water/Oil Three groups of all the specimens are divided according to their quenching temperature and are held at temperature 1000 C,840 C,770 C individually in three furnaces. process is executed after holding time 30min. The specimens required tempered are tempered at various temperatures in another five furnaces. After holding time 40min and cooled to ambient temperature all the specimens are ready for hardness test. 5. Hardness Test In this experiment, Rockwell Hardness Test is used to measure hardness of specimens. Before hardness test, the surfaces of the specimen are smoothed on the sand paper for the upper and lower surfaces should be parallel for hardness test. The hardness of all the specimens are recorded in Table.2 6. Metallography Metallographic investigations are carried out after the specimens corroded by nitric acid. The specimens are pictured with magnification 100 and 400.

5 7. Results and Discussions 7.1 Effect of quenching temperature on microstructure and mechanical properties [5][6] When hypoeutetoid steels are heated to about C above the upper critical temperature A c3, Ferrite and pearlite transform to austenite. This austenite transforms to martensite on rapid quenching from hardening temperature. Fine-grained martensite can be obtained from fine-grained austenite, as depicted in Fig.1. The presence of martensite accounts for high hardness of quenched steels. Fig.1. Fine gained martensite in steel 45 But with an increase in heating temperature austenite grains grow intensively and coarse-grained martensite is obtained instead of fine-grained martensite, as depicted in Fig.2. Coarse-grained steel provides less nucleating sites for pearlite transformation, and hardness increases. But the increase in hardenability is associated with poor impact properties, quench crack susceptibility, and loss of ductility. Fig.2. Coarse grained martensite in steel 45

6 Ac1 A c 3 If hypoeutectoid steel is heated to a hardening temperature between and, the structure will consist of ferrite and austenite. This will transform to ferrite and martensite on quenching. Ferrite, a very soft phase, lowers the hardness of hardened steel considerably. This is also known as incomplete hardening. Table.4 shows the hardness variation against quenching temperature for hypoeutectoid steels. Table.4. Hardness variation against quenching temperature in steel 45 temperature/ C Microstructure Fine-grained Martensite Ferrite and Martensite Hardness/HRC The preferred hardening temperature for eutecoid and hypereutectoid steel lies between the lower critical temperature ( A1 Acm ) and the upper critical temperature ( ),about C above the lower critical temperature. The advantage gained from hardening temperature in this range is two-fold. The first is related to the presence of cementite in hardened steel. The cementite in a martensite matrix accounts for several desirable properties. Wear resistance is one of them. The fact that both martensite and cementite are hard constituents is responsible for high wear resistance of the resulting microstructure. Cementite is harder than martensite and so wear resistance of the two phase microstructure is better than what is achieved by martensite alone. The second advantage of this hardening temperature is the attainment of fine martensite in the final structure. In fact, heating of hypereutectoid steel above the upper critical temperature for hardening is detrimental because such a high temperature will result in coarsening of austenitic grains and decarburization at the surface. Coarse austenite will transform to coarse acicular martensite which has poor mechanical properties. Decarburized surface responds poorly to hardening treatment. In addition to these factors, quenching from such a high temperature will introduce severe internal stresses into the hardened steel. 7.2 Effect of quenching media on microstructure and mechanical properties must be severe enough to avoid any pearlite formation on cooling from the austenitizing temperature to the temperature of the austempering bath. As in the TTT curve, which is depicted in Fig.3, the cooling rate curve should surpass the nose of C shape curve. Different quenching media means different cooling rate. Of water, oil and air as quenching media, water is the most severe and oil the least. Sometimes agitation is added for more severe cooling rate.

7 Fig.3 TTT diagram of steel 45 Fig.4 shows microstructures of three specimens undergoing the same quenching temperature but different quenching media. With a more severe cooling rate with water as quenching media, fine-gained martensite is obtained. When oil is used for quenching, the cooling rate becomes slower and pearlite appears with martensite. As the cooling rate decrease with air cooling, the cooling rate is too slow to obtain martensite, only ferrite and cementite is obtained. Variation in hardness is also depicted in Fig.4. Fig.4. Microstructure and hardness of steel 45 under different quenching media 7.3 Effect of tempering temperature on micro structure and mechanical properties. [7] A number of structural changes take place during tempering treatment. These changes include isothermal transformation of retained austenite, ejection of carbon from body centered tetragonal lattice of martensite, growth and spheroidization of carbide particles and formation of ferrite-carbide mixture. Depending on the range of tempering temperature, the treatment proceeds to various stages. The first stage of tempering is also referred to as low temperature tempering. The maximum

8 temperature to which steel is heated is restricted to about 250 C at this stage. This results in the formation of a low carbon martensite and a carbide by transformation of high carbon martensite. The carbide precipitated from the high carbon martensite during the first stage of tempering is not cementite. This carbide is known as ε -cabide which has a hexagonal closed packed structure. The carbon content of ε -cabide is more than that of cementite (Fe 3 C), and the chemical formula is approximately Fe 2.4 C. As far as mechanical properties are concerned, marginal decrease in hardness value takes place at this stage. Strength and toughness improves. Excellent wear resistance and reduced internal stresses are the characteristics of hardened steel tempered in this range. Fig.5.shows the microstructure of steel T8 being tempered at 200 C. Fig.5. Microstructure of steel T8 being tempered at 200 C The second stage of tempering consists of heating steels in the temperature range varying from 350 to 500 C. During this stage, retained austenite transforms to bainite. This bainite differs from conventional bainite in the sense that it consists of ferrite and ε -cabide. Another name given to this stage is medium temperature tempering. Ductility and toughness increase by this treatment with a corresponding decrease in hardness and strength. The steel develops maximum elastic properties during this stage. Fig.6 shows the microstructure of steel T8 being tempered at 400 C. Fig.6. Microstructure of steel T8 being tempered at 400 C. The third stage of tempering is also popularly known as high temperature tempering. It consists of heating steel within a temperature range of 500 to 680 C. Heating to such high temperatures results in the formation of ferrite-cementite mixture. Martensite changes to ferrite by

9 losing its carbon. Carbon thus released combines with ε -cabide which in turn transforms to cementite. All these changes occur with the help of diffusion and nucleation. Steel, thus treated, has better tensile, yield and impact strength than annealed or normalized steel and is free from internal stresses. Fig.7.shows the microstructure of steel T8 being tempered at 600 C. Fig.7.Microstructure of steel T8 being tempered at 600 C Hardness decreases as tempering temperature increases for the consumption of martensite. Fig.8. shows this tendency. Fig.8. Hardness against Tempering Temperature 7.4 Effect of alloying elements on hardenability and tempering stability An important role of alloying elements (except cobalt) is to shift the nose of the C-curve to the right in the TTT diagram. This increases the hardenability of steel even at slow cooling rates. The reason for this is obvious for austenite stabilizing elements. Ferrite stabilizers do the same job by forming carbides. Alloy carbides are more stable than cementite, and hence they retard the diffusion of carbon which in turn decrease the rate of decomposition of austenite. Strong carbide formers have more pronounced effect on the retardation of austenite decomposition than the weak carbide formers. Since pearlitic transformation involves diffusion of both carbon and metallic

10 atoms, the effect of alloying elements is much more pronounced in pearlitic region. The effect is less pronounced in bainitic region as bainitic transformation involves diffusion of carbon atoms only. To be effective, the alloying elements must be dissolved in austenite. The presence of cobalt is helpful for nucleation and growth of pearlite. Therefore, steels containing this element are difficult to harden. [8] Fig.9 shows the comparison of TTT diagrams of steel 45 and steel 40CrNi., Ni is an austenite-forming element while Cr is a medium carbide former. With effect of Cr and Ni, the nose of the C-curve is shifted to the right on the TTT diagram of steel 40CrNi in comparison to that of Steel 45. Fig.9.Comparison of TTT diagrams of steel 45 and steel 40CrNi Martensite precipitation is strongly influenced by a number of alloying elements during tempering. They retard the growth of carbide particles, and consequently supersaturation of the α -solution with carbon is preserved. Thus the state of tempered martensite is retained up to temperatures of C. A delay in martensite precipitation can be explained by two factors. First, one of the alloying elements lowers the rate of carbon diffusion in the α -solution. Second, the other elements can increase the strength of interatomic bonds in the α -solution lattice. This will prevent the atoms from crossing the α -solution carbide interface. Both factors impede precipitation of martensite [9]. 8. Conclusions 1. temperature of hypoeutetoid steel is C above the upper critical A c3 temperature when fine-grained martensite is obtained. Coarse-grained martensite will take place of fine-grained martensite with an increase in quenching temperature. The preferred hardening temperature for eutectoid and hypereutectoid steel is about C above the lower critical temperature. Higher wear resistance can be obtained with appearance of cementite. 2. Different quenching media results in different cooling rate. Martensite is more easily

11 obtained with water as the quenching media. 3. Hardness decreases as tempering temperature increases for the consumption of martensite. 4. Alloying elements can retard precipitation of martensite, which will increases the hardenability and stability in tempering of steel ACKNOWLEDGEMENT With all my thanks to Professor. Liu Guoquan, for his charming teaching and insightful explanation. During the experiment, great help is received from staffs of laboratory, and here I give my thanks as well. Hardness data and metallographic photos come from all my classmates. This experiment essay cannot be completed without their help. REFERENCES [1] George E.Totten. Steel heat treatment: metallurgy and technologies.crc Press, Tayor & Francis Group [2] T.V.Rajan,C.P.Sharma,Ashok Sharma. Heat Treatment Principles and Techniques. Revised edition. New Delhi: Prentice-Hall of India private Limited, [3] 樊东黎, 徐跃明, 佟晓辉. 热处理技术数据手册. 第二版. 北京 : 机械工业出版社,2006. [4] 樊东黎, 徐跃明, 佟晓辉. 热处理技术数据手册. 第二版. 北京 : 机械工业出版社,2006. [5] T.V.Rajan,C.P.Sharma,Ashok Sharma. Heat Treatment Principles and Techniques.Revised edition.new Delhi: Prentice-Hall of India private Limited, [6] 宋维锡. 金属学. 第二版. 北京 : 冶金工业出版社, [7] T.V.Rajan,C.P.Sharma,Ashok Sharma. Heat Treatment Principles and Techniques. Revised edition.new Delhi: Prentice-Hall of India private Limited, [8] T.V.Rajan,C.P.Sharma,Ashok Sharma. Heat Treatment Principles and Techniques. Revised edition.new Delhi: Prentice-Hall of India private Limited, [9] George E.Totten. Steel heat treatment: metallurgy and technologies. CRC Press, Tayor & Francis Group