Dissertation
Dissertation > Industrial Technology > Metallurgy and Metal Craft > Welding, metal cutting and metal bonding > Welding process > Special welding > Laser welding

Effects of Laser Shock Processing on Cavitation Erosion Resistance of Stainless Steel Weldment

Author ZhangLei
Tutor ZhangYongKang; LuJinZhong
School Jiangsu University
Course Mechanical Manufacturing and Automation
Keywords Laser shock processing (LSP) Stainless steel weldment Cavitationerosion Electrochemical corrosion Residual stress field Cavitation microfluidization
CLC TG456.7
Type PhD thesis
Year 2013
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Liquefied natural gas (LNG) is considered as superior, efficient, economical and clean energy. And LNG vehicle is considered as the better treatment measure, which promotes the development and application of the matched LNG fueling station. The phenomenon of cavitation erosion usually occurred on the surface of stainless steel cryopump during the working process of the LNG installation. Based on our long-term research, this thesis presented the effects of laser shock processing (LSP) on cavitation erosion resistance of stainless steel weldments of skid-mounted LNG fueling station. Four different aspects were involved, i.e. the choice of the appropriate fiber laser welding parameters, the strengthening technology and mechanism on cavitation erosion resistance of stainless steel weldments by LSP, the effects of LSP on electrochemical corrosion resistance of stainless steel weldments after cavitation erosion and the preparation technology of electrochemical corrosion samples, and the comparison between the simulation of the stress fields induced by LSP and that by cavitation microfluidization. Some important conclusions and innovative achievements of this work were listed as follows:(1) The laser welding technology of304austenitic stainless steel was designed, and the appropriate fiber laser welding parameters were gained.The effects of laser power, defocusing distance and the welding speed were studied by the orthogonal test, when304austenitic stainless steel thick sheet was welded using the fiber laser. The most appropriate fiber laser welding parameter combination has been obtained, and it was described as follows:the fiber laser power was4000W, the defocusing distance was-3mm and the welding speed was35mm/s. At the same time, the feasibility of the above-mentioned parameter combination was verified from both microscopic morphologies and mechanical properties. The widths of laser welding zone (LWZ) and heat-affected zone (HAZ) were smaller when compared with CO2and Nd:YAG stainless steel weldments through metallographic test. The compressive residual stress occurred in the middle of the LWZ of the fiber stainless steel weldments. In addition, the maximum of tensile residual stress appeared at the interface between the LWZ and the base metal, and the tensile residual stress of the fiber stainless steel weldments was the lowest. Micro-hardness and tensile strength of the fiber stainless steel weldments were the highest among these three laser welding technologies. The crystal solidification process induced by the fiber laser welding was schematically illustrated and systematically revealed. The crystal solidified microstructure of the LWZ was completely different from that of the base metal.(2) The strengthening technology and mechanism on cavitation erosion resistance of304austenitic stainless steel weldments by LSP was researched. The effects of LSP on cavitation erosion resistance of stainless steel weldments were obtained, and the microscopic mechanism was developed to improve cavitation erosion resistance by LSP.Undulations and upheavals appeared on the surface of stainless steel weldments after cavitation erosion. Thus, stainless steel weldments were treated through LSP. With the increment of LSP pulse energies after cavitation erosion, the martensite intensity was improved relatively and the removal of martensite was restrained, the hardness increased, and the cumulative mass loss and damaged surface areas of stainless steel weldments decreased. Meanwhile, the martensite grains were fined, undulations and upheavals became shallower and sparser, so the growth and propagation of the cracks on the surface were restrained. Thus, cavitation erosion resistance of stainless steel weldments was enhanced. In addition, cavitation erosion resistance of the LWZ was better than that of the HAZ. The surface appearances in the LWZ were different from that in the HAZ after cavitation erosion. Undulations and upheavals which presented dendrite grain appeared on the surface in the LWZ, while common undulations and upheavals occurred in the HAZ. The damage degree of the surface appearances in the LWZ and HAZ without LSP impacts was more severe than that with LSP impacts after cavitation erosion. Because the plastic deformation of stainless steel weldments was generated by LSP impacts, and the refinement of the grains and the increment of the slip systems were induced during this process.(3) The effects of LSP on electrochemical corrosion resistance of304austenitic stainless steel weldments after cavitation erosion were explored. The corrosion fatigue properties of stainless steel weldments by LSP were getten.LSP impacts remarkably changed residual stresses from tensile to compressive in the LWZ and HAZ of stainless steel weldments. The top surface had maximum values of compressive residual stresses. On the whole, compressive residual stresses in the LWZ were higher than those in the HAZ with LSP impacts. Grains of the columnar ferrite and the original austenite with LSP impacts after cavitation erosion were refined in the LWZ. For the plastic deformation in the LWZ and HAZ with LSP impacts after cavitation erosion, many micro steps were generated due to the slip. Through LSP impacts, slip systems became more and denser. The surface roughness in the LWZ and HAZ with LSP impacts was smaller than that without LSP impacts after cavitation erosion. The surface roughness in the LWZ was smaller than that in the HAZ after cavitation erosion as a whole. Electrochemical testing was carried out to investigate the electrochemical corrosion behavior of stainless steel weldments without and with LSP impacts after cavitation erosion in3.5wt.%NaCl solution. Electrochemical corrosion resistance of stainless steel weldments after cavitation erosion was improved by LSP. The free-corrosion potential of potentiodynamic polarization curves increased and the free-corrosion current decreased with LSP impacts after cavitation erosion. Obvious passivation areas occurred on polarization curves of stainless steel weldments with LSP impacts after cavitation erosion. Pitting corrosion occurred in the LWZ without and with LSP impacts after cavitation erosion in3.5wt.%NaCl solution. Compared with corroded morphologies in the LWZ without LSP impacts after cavitation erosion in3.5wt.%NaCl solution, the distribution and size of corrosion pits were uniform, and the size of corrosion pits was smaller after LSP impacts. Corroded morphologies in the HAZ were different from those in the LWZ. The multicorrosion process appearing in the LWZ was not seen in the HAZ. There was a severely corroded region in the HAZ without LSP impacts after cavitation erosion in3.5wt.%NaCl solution. But the severely corroded region became smaller and shallower, with some big corrosion pits left in the HAZ with LSP impacts.(4) The stress fields induced by LSP and cavitation microfluidization were analyzed. The generation and collapse of cavitation bubbles were also introduced. The stress field simulation of304austenitic stainless steel induced by LSP and cavitation microfluidization was compared.The contour of stress distribution, such as Von Mises stress, residual stress and stress based on the strength theory, was received according to the finite element model with the shock waves induced by LSP. Due to the plastic loading wave after LSP impacts, the plastic deformation along the depth direction was induced to a greater extent, when the stress of the shock waves exceeded the dynamic yield strength. The compressive residual stresses of several hundred MPa existed in near-surface regions. The depth of the strengthening layer was determined on the basis of LSP parameters, which played a key role in the improvement of the material mechanical properties. The equilibrium system of a single spherical cavitation bubble was analyzed in the stationary flowing fluid with unbounded domain. It was deduced that the phenomenon of cavitation erosion mainly related to the vaporization pressure, and this phenomenon was caused easily during the working process of the skid-mounted LNG fueling station. The generation and collapse process of cavitation bubbles presented the oscillation curves like sinusoidal shape. Meanwhile, the collapse process presented more obvious model of damped oscillation. The effects of cavitation microfluidization generated by the collapse of cavitation bubbles on the material surface were simulated by FLUENT software. The contour of the Von Mises stress and every principal stress was gained. By contrast, it was found that the stress trend induced by cavitation microfluidization was similar to that by LSP. Nevertheless, the stress values generated by cavitation microfluidization on the material surface were much lower than that by LSP, which was not sufficient to make the plastic deformation and grain refinement happen. So the strengthening effects of LSP on the mechanical properties of the materials were much more prominent, and the compressive residual stress layer was primarily attributed to the induction effects of high strength impact waves during LSP impacts.

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