Effect Of Microstructure On Fatigue Life And Impact Toughness Of Austenitic Stainless Steel Welds Made By SMAW

The major portion of stainless steel family which covers almost 60-70%of its structural application range due to most suitable mechanical properties,corrosion resistance and an excellent level of fabricability,is austenitic stainless steel(ASS).Shielded Metal Arc Welding(SMAW)is the most frequently used welding process in different AISI grades of stainless steel family due to its less expensive,strong suitability and a pretty good portable nature.Major industries of Pakistan,especially Heavy Mechanical Complex and Heavy Industries Taxila,are relying on SMAW for most of their fabrications.This welding type is a common practice in maintenance units of other industries(sugar mills,cement factories crushing plants)in Pakistan.Thus,the author used SMAW to make ASS weld joints to carry out this study.Under cyclic and impact loading conditions,welded joints are always vulnerable to fatigue and toughness failure.Microstructure evolved under different welding conditions influences enormously the ultimate life of the joint.It is of great significance to reveal how microstructural evolution affects the fatigue life and impact toughness of ASS welds made by SMAW.Solidification and re-solidification with subsequent phase transformation in different weld zones of multi pass weldment play a vital role in defining mechanical properties of weld joints.One of the objectives of this research is to investigate the effect of multiple welding passes on microstructural pattern and different mechanical properties.Metallurgical microscope is used to examine the delta ferrite phase in austenite.Instead of depending on general ferrite number(F.N.),a MATLAB(?)code for image processing is developed to estimate the local percentage of delta ferrite within the austenitic phase.To reveal delta ferrite effects,its local percentage is discussed in relation to various properties.With the help of this technique,a good estimation is also made to detect the localized delta ferrite morphologies.The fusion zone(FZ)solidified in ferritic-austenitic mode(FA mode)comprises two basic delta ferrite morphologies i.e.,lacy and vermicular.Grain growth behavior and ferritic percentage of BM,HAZ and FZ are discussed in single,double and triple pass weldments.Effects of delta ferrite percentage and morphology on fatigue crack initiation and propagation are also investigated in different zones of each pass weldment.Fatigue crack propagation rate(FCPR)in different orientation with respect to weld zones is checked,and it is noticed that propagation rate is much lower in perpendicular direction as compared to parallel one with respect to direction of welding.Due to specific microstructural pattern,HAZ has greater FCPR in comparison to BM and FZ.For fatigue crack propagation,there is highest resistant in triple pass weldment due to high concentration of lacy ferrites.However,in some case it is found that crack mechanics is actually responsible instead of microstructural effects.To attain desired microstructural pattern which yields optimum fatigue and impact strength,deep investigations on the percentage of major austenitizer and ferritizers elements(Nickel and Chromium)in molten metal pool of SS304L weldments are made,and their effects on delta ferrites morphology,number,their localized precipitation and fatigue crack propagation rate(FCPR)are also examined in this work.Estimation of localized delta ferrite number(F.N.)for a particular weld zone is assessed through aforementioned image processing methodology.Reduction in Ni percentage in filler alloy yields randomly scattered granular ferrites in FZ and equiaxed grain growing in HAZ with blocky frites at grain boundaries.While enhancement of Chromium causes the precipitation of dendritic clusters and vermicular ferrites in FZ and HAZ of weldments respectively.Effect of ferrite numbers and their morphology at crack tip are investigated by developing a relation between crack opening displacements(COD)and corresponding FCPR.Influence of ferrite number on variation of FCPR is also determined for all weldments,and it is found that the ferrite number range betweenl5-19 provides optimum fatigue strength for SS 304L.Paris curves are plotted,and it is found that the weldment attained with filler alloy having Cr 24.60%and Ni 10.92%shows the highest fatigue life as compared to other studied weldments.In addition to evolved microstructure,the mechanics of the crack also affects the fatigue life of the weldment.For better understanding of mechanics influence,crack tip plastic zone size at various crack lengths is calculated analytically by simplified Sih’s model and numerically by Irwin’s models.Influence of plastic zone size is explained in term of COD and elastic stress intensity factor within valid range of small scale yielding(SSY)approximation.The role of variations in elastic stress intensity factor in governing local plastic flow at crack tip,and thus FCPR,is determined experimentally for high cycle fatigue situation.Up to plastic zone size range of 4-5 mm,a good agreement between numerically and analytically calculated plastic zone size of all weldments is observed for a specific elastic stress intensity factor.For high loads and greater crack lengths,experimentally obtained COD values were found 15-19%more than calculated ones due to rapidly induced plasticity at high crack dimensions.Due to huge thermal variations and microstructural inhomogeneity,sudden losses of toughness at interfaces of different weld zones lead to serious failures of joints.Under impact loading,the stability of shielded metal arc welded(SMAW)SS304L is investigated by localized control of ferrite morphology and precipitation of delta ferrite as well as ferrite number via suitable chemical composition and cooling rate of weld deposit.Based on the ferrite morphology and ferrite numbers of different weld zones and interfaces at varied chemical composition and cooling rate of weld deposits,the optimum values of these parameters are achieved for good toughness at the interfaces.Localized δ/γ is discussed with corresponding toughness values across the weld to predict its suitable range.The recommended range of delta ferrite-austenite ratio,chemical compositions and cooling rates are 0.2-0.25,Ni-9.75%-Cr-21.20%-Mn-7.20%and 6-7℃/sec,respectively.Influence of minor alloying elements on morphology and nucleation of phases is examined.Effects of Si and C were detrimental towards toughness while the effect of Mo is beneficial in this regard.