Study On Crack Growth Mechanism And Microstructure Variation Law Of Welded High-strength Steel With A Bi-metal Weld Seam

Along with the advance of science and technology, and with the further extension andapplication of welding technique, the demand for manufacturing accuracy, fatigue resistanceand service life of welding component is increasingly high. Many key-components of steelstructure are connected by welding procedure, and need to be working under the condition offatigue loading. The mechanical properties of the widely used high-strength low-alloy (HSLA)welded joints are obviously lower than those of the parent metal, thus it become the “weaklink” of the whole welded structure. The welded joints often generate premature fatiguefailure when welded structures serve under fatigue loading. The study on improvement of thetraditional welded joints has stepped into its “bottleneck” stage which has seriously affectedthe performances of HSLA steels. As a result, prolonging fatigue life and increasing safetyand reliability of the welded joints is a hot topic being discussed in the academic field andengineering field, and is also a fundamental key problem for increasing fatigue resistance andprolonging fatigue life of welded structures.Aiming at the technical problem that the mechanical properties of the welded joints oftendeteriorate seriously, this thesis proposed an alternative welded joint with a bi-metal seam(soft+hard) to improve the mechanical properties of the welded HSLA steels byincorporating a relative soft buffer layer (BL) between the parent metal and the weld metal.The advantages of the good weldability, toughness, thermal conductivity and cold-crackresistance of the relative soft BL were combined with the advantages of the high strength andgood fatigue resistance of the hard weld metal. In addition, the alloy element diffusionmechanism between the BL and the two adjacent metals (weld metal and parent metal) wasmade full use together with the “heat treatment” effect of subsequent weld thermal cycle onthe BL. The microstructure, hardness profile, residual stress-induced deformation, residualstress distribution, overloading-induced deformation and fatigue crack growth performance ofwelded joints were systematically studied by combing the theory analysis with the experimentstudy. This thesis innovatively proposed an alternatively welded joint with a bi-metal seam(soft+hard), new methods for analyzing overloading-induced deformation and residualstress-induced deformation, and fatigue crack growth rate model taking into account load amplitude variation, which achived good effects according to the experimental results. Themajor research and innovative conclusions are as follows:(1) An alternatively welded joint with a bi-metal seam (soft+hard): Aiming at thetechnical problems of poor weldability, heterogeneity and mechanical properties deterioration(weak link) of welded HSLA steel, this thesis proposed an alternative welded joint with abi-metal seam (soft+hard) by incorporating a relative soft BL between the parent metal andthe weld metal to combine the advantages of soft metal with the advantages of hard metal.The alternative welded joint solved the problems that the weldability and the toughness ofovermatching or equalmatching weld joints were poor, and the fatigue strength ofundermatching weld joint was low. This kind of welded joint increased the adhesive forcebetween the BL and the two adjacent metals and reduced the welding cold crack, thusimproved the welding quality. The experimental results under constant amplitude fatigueloading showed that the welded HSLA with a4mm BL gained good results as for the studiedwelded joints. The fatigue crack growth rate in weakest zone of welded joint specimen wasreached to that of the parent metal, and the fatigue life of the welded HSLA specimen with a4mm BL under maximum load of25kN increased by101%as compared with the traditioinalwelded joint specimen under maximum load of24kN.(2) An alternative method for analyzing plastic deformation and overloading fatiguelife: Because the compressive residual stress and strain hardening at the crack tip resultedfrom the plastic deformation, the relationship between the overload-induced plasticdeformation and the fatigue crack growth rate was proposed by testing the thickness variationalong the fatigue crack growth path of the specimens. Based on fatigue fracture mechanism,the fatigue crack growth rate variation under overloading was explained reasonably. Duringoverloading test, as for the alternatively welded joint with a bi-metal seam (soft+hard), theweld metal with high strength increased the fracture resistance, thus reduced the acceleratedeffect of overload-induced fracture zone on fatigue crack growth; the thick BL with lowstrength increased the overload-induced plastic deformation, thus increased the retardationeffect on the fatigue crack growth by increasing the compressive residual stress and strainhardening. The experimental results under overloading showed that the welded HSLA with a10mm BL gained good results as for the studied welded joints. The fatigue life of the welded HSLA specimen with a10mm BL after application of the1stoverload (70kN) increased byapproximately6times as compared with the traditioinal welded joint specimen as the fatiguecrack propagated form10mm to17.5mm.(3) An alternative method for analyzing hardness and residual stress-induceddeformation: The residual stress-induced deformation was measured by changing the stiffessand boundary conditions of the welded HSLA steel. The amplitude and nature of weldingresidual stress in different welded joints were compared according the partial release(accumulated value) of residual stress. A novel method for determining the location and itswidth of the tensile residual stress zone was proposed by using hardness profile of weldedjoint. The experimental results of fatigue and hardness showed that welding softening zonecorresponded to the tensile residual stress zone in the direction of vertical weld and alongspecimen length, and that the locations and the widths of the two zones were similar. Thetested method is of practical engineering value because of easy operatioin and low cost.(4) A novel fatigue crack growth rate mode suitable for heterogeneous zone ofwelded joint: A novel fatigue crack growth rate mode taking into account the stressamplitude effect was proposed according to the fatigue date resulted from the welded jointspecimens. The mode was suitable for heterogeneous zone of welded joint, and explained theinherent phenomenon that welded joint existed “weakest link” zone of fatigue performance.The sudden increase in the fatigue crack growth rate within the “weak link” zone wasexplained based on the fatigue fracture mechanism, and the fatigue fracture mechanismvariation during fatigue crack propagatin was gained for different welded joints. The fatiguedata showed that the curve could reflect the variation of the fatigue crack growth rate if thenovel fatigue mode taking into account the stress amplitude effect was chose to descripe thefatigue data resulted from heterogenous zone of welded joint, and that the fitting error wasdecreased one order of magnitude as compare with Paris mode from1.0295×10-12to2.8964×10-13, the correlation coefficient was increased from0.7470to0.9316.