Microstructure investigation on inner crack thermal healing in Q235 steel with La addition

Microstructure investigation on inner crack thermal healing in Q235 steel with La additionZhongli ZhaoBeijing University of Chemical TechnologyBeijing ,Chinazhongli5167@Abstract. Thermal healing experiment to inner crack made by compressing a drilled hole on samples of Q235 steel with various La addition was carried out. The effects of various heating temperature on the appearance and morphology evolution of inner crack healing in Q235 steel were investigated. Optical microscope and SEM observation indicate that artificial inner crack samples of Q235 steel with La addition can occur healing phenomenon at 650 ć, while inner crack samples have apparent healing at 800 ć and the microstructure in healing area is mainly ferrite. La in the solute solution decreases the critical healing temperature, makes the microstructure in host appear obvious variety during crack healing, not only decreases the quantity of pearlite and its shape becomes like-pearlite or granular but also increases the quantity of ferrite and the concentration of carbon in ferrite. The healing process was controlled by the diffusion and migration of Fe atoms from steel host to crack zone.Keywords: Q235 steel; inner crack; thermal healing; LaI.I NTRODUCTIONMetals remain the most important engineering materials. However, internal cracks occur readily in metals during the manufacturing process and their application. The propagation of internal cracks will reduce the strength, rigidity, toughness and residual life of the metal. Therefore, research on internal micro-crack healing in metals, particularly in steel, is necessary. The research of inner crack healing of metal material is one ramification of the research o1n the reverse course of the material failure. By the research on the mechanism and methods of inner crack healing in metal material, the conception of crack healing is introducedto the course of the design, manufacture and application of the material et al. and these measures will help to improveThis study is financially supported by the Specialized Research Fund for Young Teachers of Beijing University of Chemical Technology (QN0808).reliability of the material and keep higher anti-failure ability of the material.Up to now, researches have deeply investigated the problem of inner crack healing in metal material from theory, physical and numerical simulations to application. The primary mechanism of inner crack healing in metals is diffusion thermal healing [1~7].It is well known that RE addition will become a very important way in the 21st century for improving the quality of steel. The addition of RE can not only purify molten steels and metamorphose inclusions, but also has the obvious microalloying effects in purified steels [8~10].At present, most researches of crack healing focus on change external condition, there is no report about the effect on crack healing with micro-addition element. In the present paper, heat-healing experiment by artificial inner crack samples of Q235 steel with various La addition was done. The effects of various heating temperatures on the appearance and morphology evolution of inner crack healing in Q235 steel were also investigated.II.E XPERIMENTAL METHODSIn the present investigation, Q235 steel and La were chosen as host and addition elements, respectively. The chemical composition of tested steel was shown in table 1.Table 1 Chemical composition of the tested steel( wt %)C Si Mn P S La Fe0.21 0.26 0.46 0.014 0.014 0.1 balance0.21 0.26 0.46 0.014 0.014 0.2 balance0.21 0.26 0.46 0.014 0.014 0.3 balanceA.The preparation of compressed samples with innercrackThe sample was made in two steps˖(1) The steel bar was turned into a number of cylinders of diameter of 10 mm and length of 8 mm, and a blind hole of diameter of 2 mm978-1-4244-7739-5/10/$26.00 ©2010 IEEEand length of 7 mm was drilled in every cylinder (Fig.1a). Then, holes was cleaned with liquid acetone and ethanol, and then sealed by vacuum welding. (2)The cylinders are heated at 1100 ć for 5 min, after that, upset to a height of 3.5 mm in a 600 kN hydraulic press. Then, the hot metal microstructure was maintained by water quenching. Therefore, the inner crack was obtained, as shown in Fig. 1b.B. The Healing TreatmentThe compressed sample was cut along axial direction, one part was used to heat healing treatment and the other one was used to observe the initial shape of crack. Healing treatment was carried out at 650 ć and 800 ć for 60 min, then furnace-cooling to room-temperature.Fig.1 Sample for healing experiment at elevated temperature˖(a) Initial sample˗(b) Sample with inner crack afterIII. 3.R ESULTS AND DISCUSSIONA. The morphology observation on initial inner crack ofcompressed sample by SEMFor observing shape of initial inner crack, the sample is cut in axial direction and is made into metallographic specimen. Due to initial inner crack is similar among Q235 steel with various La addition the SEM images represent Q235 steel with 0.1% La addition. Results of SEM observation show that initial crack morphology before healing, such as micro-crack left tip(Fig.2a), middle part, right tip(Fig.2b). The width of initial crack is the widest at the middle part of crack, as shown in Fig.2b.B. The microstructure around inner cracks in Q235 steelbefore and after healing by optical microscope and SEM Q235 steel is pro-eutectoid steel whose structure is ferrite and pearlite at room temperature. Fig.3 shows the morphology of cracks and the structure around cracks by optical microscope and SEM. Fig.3a,d,g,k.shows the morphology of cracks before healing and the others show the morphology of cracks after healing. It can be seen: (1)The structure of steel host heated at 650 ć for 60 min is representative white ferrite and dark pearlite, but the microstructure of pearlite has changed from the lamella to pearlite-like or granula. Near the end area of closed hole inner crack appears obvious healing phenomenon, near the middle area of closed hole local interface of crack have joined, as shown in Fig.3b,e,i. (2) Whole crack in specimen heated at 800 ć for 60 min appears obvious healing phenomenon. Local interface between joined interfaces were formed spherical micro-holes, some grain boundary passed interface, as shown in Fig.3c,f,h,j. The structure in healing area is made up of ferrite, which is obviously different from steel host. The grain size in healing area is smaller than that of host. The healing process resulted fromthe diffusion of Fe atoms from steel host to crack area.C. Effect of La on crack healingRE mainly exists as inclusion and dissolved in steel, both of which determine the change of total content of RE in steel. RE has certain solubility in steel. Dissolved RE will effect on the grain boundary, phase transformation and texture etc [11].On the one hand, La addition can change the microstructure in host. From Fig.3 we can see that the La addition increases the amount of pearlite, changed its shape from pearlite to like-pearlite or granular and decreases the amount of ferrite. La in the solute solution shortens the incubation period of proeutectoid ferrite, increases its quantity and carbon content which results from La attracting some carbon atoms to or even getting into Į-Fe-La cell, decreases the quantity of pearlite. Because La lowers the diffusion ability of C atoms and causes the serious lattice distortion of cementite and ferrite, some pearlite will become like-pearlite or granular. The shape of pearlite is changed.On the other hand, La addition can change the healing critical temperature in materials. Sun[12] based on EET (empirical electron theory of solids and molecules) theory points out that there is an effect of forcible solid solutioning in solid diffusion reaction. The entrance of RE into Fe lattice inevitably causes severe distortion, weaken the bond strength, promotes vacancy formed and raises vacancy concentration. At present mechanisms for vacancy diffusion is considered the main diffusion mechanism in solid metals.Therefore theself-diffusion of Fe atoms is increased and the healing process of inner crack is accelerated.Fig.2 SEM image of prepared inner crack for the sample (a)left tip (b)middle part (c)right tipFig.3 The microstructure of inner crack before and after healing˖(a)before healing, La=0.1%˗(b) 650 ć, La=0.1%˗(c) 800 ćˈLa=0.1%˗(d) before healing, La=0.2%˗(e) 650ć, La=0.2%˗(f) 800 ćˈLa=0.2%˗(g)before healing, La=0.3%˗(h)800 ćˈLa=0.3%˗(i) 650ć, La=0.1%˗(j)800 ć, La=0.1%˗(k) before healing, La=0%˗(l) 800 ć, La=0 %ˊThirdly, the activation energy for diffusion of Fe atoms is reduced. Fig.3 shows that there are better healing ability in Q235 steel with La than without La addition. Therefore it shows that there is a greater diffusion coefficient in Q235 steel with La than without La addition. While the diffusion coefficient D is related to temperature by an Arrhenius-type equation,¸¹·¨©§ RT Q D D exp 0.˄1˅Where D 0 is the diffusion constant, Q is the activation energyfor diffusion, T is the diffusion temperature, R is the gas constant. From the equation (1) we can know that when T is not changeable the diffusion speed is strongly dependent on Q . Therefore the better healing ability in Q235 steel with La than without La addition shows the decreasing of activation energy for diffusion of Fe atoms.D. The essence of crack healingThe crack healing system is an open system that has both energy exchange and substance exchange with the surroundings. The shape change of crack healing can be considered as a process that diffusing atom from the host nucleates and grows up in the crack surface.Due to the internal surface of the crack exists inhomogeneous such as energy, structure and composition fluctuation therefore crack healing is nonuniform. Namely, the crack healing is multi-scales that exists macro-scale healing, mesoscale healing, micro-scale healing at the same time. Based on large quantity of experimental investigation research, authors consider that the healing process was controlled by the diffusion of Fe atoms from steel host to crack zone. Some defects such as dislocations and voids appeared during healing process. Their movement leads to micro-healing dissipative structure, but the appearance of macro-healing dissipative structure should be accomplished through diffusion.In short, La addition can promote obviously crack healing, but its effect mechanism needs deep and systematic research and the optimal La content needs more experimental investigation.IV.C ONCLUSION(1) La in the solute solution decreases the critical healing temperature, makes the inner crack samples of Q235 steel with La addition occur healing phenomenon at 650 ć and can be healed at 800 ć.(2)La in the solute solution increases ferrite quantity and the concentration of carbon in ferrite, decreases the quantity of pearlite and granular pearlite can be obtained.(3) The microstructure in healing area is main ferrite. The healing process was controlled by the diffusion and migration of Fe atoms from steel host to crack zone.[1]Han J.T., Zhao G., Cao Q.X.㸪“Internal crack recovery of 20MnMo steel 㸪”Science in China(Series E), vol. 27, pp. 23.-27, January 1997. [2]Han J.T., Zhao G., “Discovery of inner crack recovery and its structurechange in20MnMo steel,”Acta Metall., vol. 32, pp. 723-727, July 1996. [3]Wei D.B.㸪Research on the Discip l inarian of inner crack healing inmetal [Dissertation]㸪University of Science and Technilogy Beijing, Beijing, 2001.[4]Wei D.B., Han J.T., Xie J.X.㸪et al, “Study on Inner Crack Healing atElevated Temperature in Metals,” J. Univ. Sci. Technol. Beijing, vol. 22, pp. 245-248, March 2000.[5]Zhang Y.J., Xue L., Han J.T, “Inner Variable in Healing Processing,”J.Univ. Sci. Technol. Beijing, vol. 25, pp. 143-146, February 2003.[6]Zhang Y.J., Xue L., Han J.T, “Morphology Change during CrackHealing of Impacted sample”㸪J. Iron Steel Resear., vol. 15, pp. 52-55, June 2003.[7]Zhang Y.J., Han J.T., Ren X.P., Zhao Z.L., “Mico-analysis of thehealing area of inner cracks in a 16 Mn steel,” J. Univ. Sci. Technol. Beijing, vol. 28, pp. 129-132, February 2006.[8]Zhou G.H., Gao K.W., Wan F.R., “Molecular dynamics simulation ofmicrocrack healing in aluminium,” Pro. Natu. Sci., vol. 11, pp. 215-218, March 2001.[9]Lin Q., Fu T.L., Yu Z.S., “Study on the interaction between rare erathand carbon in high carbin steel,” J. Rare Earth , vol. 13, pp. 190-195, March 1995.[10]Lin Q., Ye W., Li S.L., “Rare Earth Dissolved in solid solution of steeland its effect on micro structure,” J. Chin. RE. Soc., vol. 7, pp. 54-58, February 1989.[11]Zhu Y.G.㸪Liu Y.㸪Liu Z.L., “Analysis of Valence Electron Structureof RE in Solid Solution in Medium and Low Carbon Steel,” J. Rare Earth, vol. 24, pp. 282-287, February 2004.[12]Sun X.H., Wu W., Yang H.S., “Research of rare earth-boron bathmultiple permeation,” J. Shenyang Polytech. Uni., vol. 32, pp.1-5, February1987.R EFERENCES。