Advances in Systems Science and Applications (2012) Vol.12 No.2 153-161 Effect of Laser Shock Processing Residual Stress on Pellet Traveling Grate Surface Crack X.D.Ren, Y.K.Zhang, D.W.Jiang, A.X.Feng and T.Zhang Jiangsu Key Laboratory of Laser Manufacture Science and Technology Ministry, Jiangsu University,Zhenjiang,Jiangsu 212013 Abstract The effect of the laser shock processing on Pellet Traveling Grate surface fatigue crack growth performance were investigated from the theory of the fracture me- chanics. A stress formula was set up to indicate the function of the residual stress on fatigue life. The effect of the compressive stresses was deemed responsible for increasing the resistance to fatigue crack growth of the Pellet Traveling Grate. It was observed that the dynamic stress intensity factor of the crack and the stress ratio were declined due to laser shock processing, which would make the spread doorsill of the fatigue crack increasing. The results indicate a significant reduction in fatigue crack growth rates using laser shock processning specimens. It is shown that the near-surface microstructures, which in Pellet Traveling Grate consist of a layer of work hardened nanoscale grains, play a critical role in the enhancement of fatigue life by mechanical surface treatment. Keywords Laser shock, Residual stress, Pellet Traveling Grate, Crack expand 1 Introduction In recent years, the pellet has obtained favor and been taken seriously as the high quality raw material, and has the tendency of replacing the agglomerate gradually. The chain fire grate machine is a core equipment in the production of pelletizing mineral aggregate, and as a non-sign design large-scale equipment, which core part movement chain system operates in an alternation temperature environment from normal temperature to 1050◦C for a long time. The main bottleneck question is that the alternation high temperature environment forms the complex heat expansion and the alternation thermal load, which causes the heat-resisting service life to be shorter and the pelletizing chain fire grate ma- chine requires extremely high performances of heat shock fatigue, strength at high temperatures and wear-resisting ability of spare part material[1]. Therefore, enhancing the fatigue resistance ability of the crucial element and guaranteeing the reliability of the movement are the difficult problems of the entire equipment technology. Laser shock processing is to use the high power density (GW/cm2 magnitude), short pulse (ns magnitude) to impact the energy conversion body in the met- al surface. After laser energy absorption, the temperature is elevated rapidly, forming the shock-wave with a high peak-to-peak value, which means that the 154 X.D.Ren:Effect of Laser Shock Processing Residual Stress on Pellet Traveling... energy of light is transformed into the shock-wave mechanical energy. The shock- wave pressure which reaches as highly as counts GPa causes that the microscopic plastic deformation happens in the material surface layer, and forms the residual compressive stress layer, LSP improves the mechanical property of the metallic material effectively, which would enhance the fatigue life and the anti-stress cor- rosion performance of the material in a large scale specially. Compared with the conventional method, this kind of high rate of strain strengthening technology has a unique superiority [2-3]. Regarding to the stress destruction in the surface of essential spare part axis in chain fire grate machine, this article uses the technology of laser shock processing to enhance the fatigue life and the wear resistance of the axis, and the result- s indicate that using advanced surface treatment technology of the shock-wave mechanics effect induced by the strong laser can effectively improve the surface layer stress condition of the axis material, and specially can enhance the fatigue life performance of the material obviously. 2 Residual Stress Analysis In the laser shock processing, the coating raises the laser energy coupling effi- ciency, and the coating is able to enhance the absorption of the laser energy, which causes the absorption coating gasification to form the plasma to explode, producing the high-pressured shock-wave separately disseminates to the metal target and the restraint level. The high-pressured shock-wave plays the strength- ening and the distortion roles to the metal target, and the superficial residual compressive stress is enhanced greatly. Researching the fatigue short fracture growth problem in laser shock processing residual compressive stress field has also solved the problems of the crack rate of expansion caused by residual stress and life prediction[4-6]. the model of metal surface’s influence layer depth and superficial residual compressive stress after laser shock processing proposed by Ballard[7] is based on in this article, and the shock wave parameters of peak pressure is optimized in condition of ideal surface stress. A residual stress model which is used to calculate the longitudinal and plane shock-wave system’s ideal elastic-plasticity response for metallic material is proposed, and some basic suppositions are made as fellow: (1) Distortion induced by the laser shock processing is single axle and plane; (2) Distribution of pressure pulse induced by laser is even in space; (3) The ma- terial obeys Von the Mises yield criterion, and the elastic deformation and the strain strengthening effect of the material are neglected. The residual stress’s production has two stages, as shown in Fig.1. (a) During the time of laser pulse, a pure uniaxial stress is generated along the shock-wave propagation direction because of rapid spraying by plasma, while a tensile stress is generated in the plane parallel to the material surface ; (b) After laser pulse Advances in Systems Science and Applications (2012) Vol.12 No.2 155 vanishing, the plastic strain happens to the laser impact zone’s volume, and t- wo axle compressive stress field is produced in the plane parallel to the impact surface due to that the plastic strain is limited by the metallic material around. Fig.1 Schematic of the residual stress induced by the LSP According to the hooke’s law, the metal surface plasticity strain εp can be written as[8], εp = −2HEL 3λ+ 2µ ( P HEL − 1 ) (1) WhereHEL is the Hugoniot limit of elasticity, P is the laser shock-wave pressure, and λ and µ are the Lame’s parameters. To determine the residual stress field in the materials impact area, regarding any impact condition assigned, the plastic influence depth was calculated as, Lp = CelCplτ Cel − Cpl (P −HEL 2HEL ) (2) Cel and Cpl represent the elasticity speed and the plastic speed separately, and τ is the duration of laser pulse. And ρ represents the target material’s density, while Cel and Cpl can be defined as, Cel = √ λ+ 2µ ρ ,Cpl = √ λ+ 2µ/3 ρ (3) Therefore, according to the influence level depth, the compressive residual stress produced in the elastic-plasticity semi-infinite body can be shown as, σsurf = σ0 − [ µεp 1 + ν 1− ν + σ0 ][ 1− 4 √ 2 π (1 + ν) Lp r0 √ 2 ] (4) Where ρ0 represents the laser facular’s radius, and σ0 represents the initial superficial residual stress which may be zero in the ordinary circumstances. The formula (4) indicates that the metal surface residual stress’s peak-to-peak value increases with the laser peak pressure in the laser impact process, while the peak pressure is related to the incident laser power density. 156 X.D.Ren:Effect of Laser Shock Processing Residual Stress on Pellet Traveling... 3 Laser Shock Experiment The laser treatment was carried out at the Jiangsu university strong laser lab- oratorys high power and Nd:Glass laser implement, with its laser pulse wave is 1.06µm,and syntory laser stick is made by ϕ6mm × 80mm yttrium aluminum garnet. One stair Nd:Glass laser magnify beforehand and four steps main ampli- fier by two ways, and big caliber quarter wave piece and high strength big caliber polarize film was adopt, Q switch, the output laser film wave at half maximum is about 20ns, the pulse repletion rate is 0.5Hz and the pulse energy was 28J to 36J corresponding to laser energy densities at the metals surface of 56J/cm2 and 72 J/cm2. The experiment material is 12CrMoV, whose thickness is 5mm. Before the experiment, the test sample surface was polished with the ethyl alcohol. The axis material was made into the standard test specimen. Fig.2 Residual stress of the staff in different conditions The depths of the impact zone indicate that after laser shock processing, its superficial residual stress is transformed from tensile stress to compressive stress, as shown in Fig.2. The surface residual stress is qualitatively changed from the 135.7MPa tensile stress to - 230.6MPa compressive stresses, which scope is reached as highly as 3 times. And the hardness is obviously enhanced, which has an important value to enhance fatigue life of the work piece. 4 Results and Discussions 4.1 Material Performance Influence After laser shock processing, the residual stress of the 12CrMoV is qualitatively changed from the 135.7MPa tensile stress to 230.6MPa compressive stress, cor- respondingly surface hardness is enhanced from HV252 to HV530. Fig.3 shows the result of metallography observed, and the big arrow shows the crack that the naked eye sees, which assumes the bending shape and branches out many small cracks; the small arrow shows the small crack which branches out from two sides Advances in Systems Science and Applications (2012) Vol.12 No.2 157 of the big crack. By magnifying part of the crack that naked eye sees in Fig.3, it can be seen that the small cracks along with both sides of the crack are along crystal cracks. Obviously these along crystal small cracks indicate that the crack that can be seen by naked eye is also to expand along the crystal. It is only because the latter is a little wider, and its characteristic of along crystal crack is not clearer than the small cracks.Fig.4 shows the microstructure of the axis test specimen by chemical reagent corrosion. It is the axis high-temperature steel normal tissue. And Fig.4 demonstrates that crack is to expand along the crystal. Fig.3 Metallographic grinding in the surface morphology of the cracks Fig.4 Microstructure of the small axis 4.2 Influence to Crack Growth After laser shock processing, residual compressive stress remains in the surface, and the residual stress will weaken in the process of test specimen surface with- standing cyclic loading, and the residual stress on the test sample section is a 158 X.D.Ren:Effect of Laser Shock Processing Residual Stress on Pellet Traveling... distribution rather than a definite value. In order to analyze that simply, the effect of the residual stress was estimated with mean stress’s viewpoint, and the influence of mean stress to fatigue limit is often described with the Goodman relations [9], which is shown in Fig.5. In Fig.5, σm is the mean stress, and σb is the corresponding tensile strength, while σ0 p is fatigue limit whenσm = 0 . As average pressure exists, the fatigue limit may be represented as. σm p = σ0 p − (σ0 p/σb)/σm = σ0 p −mσm (5) Fig.5 Goodman sketch map of the relationship between stress and metal fatigue limit In the formula(4.1),m = σ0 p/σb denotes the slope of the line between σ0 p and σb in Fig.5, which is called the mean stress sensitive coefficient. When the residual stress σr exists and also it is thought to be equivalent with mean stress, the formula (5) can be rewritten as. σr+m p = σ0 p −m(σm + σr) (6) Compare formula (5) with formula (6), it is known that the change of materials fatigue limit induced by the residual stress is. ∆σr p = σr+m p − σm p = −mσr (7) It could be seen that the materials fatigue limit would decrease if the residual tensile stress remains in test specimens surface, while the materials fatigue limit would increase when the residual compressive stress remains in test specimens surface, and m may also be called as the residual stress function coefficient. Therefore, the residual stresss influence may be estimated quantificationally if Advances in Systems Science and Applications (2012) Vol.12 No.2 159 the materials residual stress function coefficien m and the test samples residual stress value are known. The problem of crack growth in residual compressive stress field, besides the influence of residual stress to fatigue limit, and also influence of the residual stress to the crack growth threshold should be considered. According to the revision relations proposed by EI Haddad [10] and so on, the fatigue limit of the short crack in the metal surface can be calculate as ∆σ = ∆Kth y[π(R+ a0)]0.5 (8) Where ∆σ is the fatigue limited stress, and ∆Kth is the cracks threshold stress intensity factor, and y is the scoop channel’s shape factor, and a0 is the materials critical crack length. ∆Kth = ∆σy[π(R+ a0)] 0.5 (9) After laser shock processing, the existence of the residual compressive stress in the metallic material surface greatly enhances the fatigue limit in the formula (9), and its result will cause the threshold value to obtain the enhancement, and in this condition the crack is not easily generated; the cracks that have extended will stop extending in the high residual compressive stress region and become the non-extension cracks, and a more universal influence is that the fatigue crack growth rate is reduced obviously in the residual compressive stress field. This is because that the residual compressive stress reduces the mean stress in alternat- ing load, and reduces the alternating tensile stress that the test sample surface actual withstands. Meanwhile, laser shock processing enhances the plastic de- formation resistance of the sample surface hardened layer, which enhances the fatigue strength of the material. When ratio of materials fatigue strength (σR) and value of test sample surface withstanding stress (σω)σR/σω > 1, he proba- bility of the formation of fatigue crack is greatly reduced [11]. The influence of the residual stress to the fatigue crack growth rate can be described with Forman formula, da dN = C(∆K)m/[(1−R)K −∆K)] (10) In the formula (10), ∆K is the cracks threshold stress intensity factor, and R is the stress ratio, while C and m are constants. When residual compressive stress exists after laser shock processing, it can be known that the stress ratio R is smaller than the stress ratio RW when residual compressive stress doesnt exist. Obviously the residual compressive stress not only could reduce the expansion speed of the fatigue cracking, but also enhance the fatigue cracking expansion resisting force of the test sample. 160 X.D.Ren:Effect of Laser Shock Processing Residual Stress on Pellet Traveling... 5 Conclusions The compressive residual stress is formed in the impact metal surface during laser shock processing,. The residual compressive stress is equivalent to negative aver- age residual stress, and it can enhance the anti-fatigue strength of the workpiece. The compressive residual stress increases the locking force of the crack and reduces the expansion speed of the fatigue cracking obviously, and also can close the metal plate crack effectively.After a large number of experiments, the empir- ical datum is summarized and theories are consummated unceasingly, and the high rate of strain strengthening theory and the laser shock processing database are established. Acknowledgements Support provided by the State Key Program of National Natural Science of China (Grant No. 50735001) and Project supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 07KJB460012 ) and the Ph.D. Programs Foundation of Jiangsu University(CX07B-02X). References [1] Zhang Xiliang, Zhang Jian, Feng Ai-xin, Zhu Shen-cai, Wu Ti-chang, Wan Xue-gong. (2007), “Reliability design on key parts of chain grate”, Sintering and Pelletizing, Vol.32, No.4, pp.5-9. 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(1997), “Effect of laser hardening treatmeng on fatigue stength of 34CrNiMo steel”, Ordnance Material Science and Engi- neering, Vol.20 No.4, pp.29-34. Corresponding author Corresponding author: renxd@ujs.edu.cn