Failure analysis of an economizer tube sheet & its cover due to corrosionand imperfection

The boiler's economizer was found to be inefficient. Corrosion is suspected to be the source of the economizer's pressure reduction. The examinations included visual inspection, stereomicroscope, metallography, elemental microanalysis by EDS, and chemical composition analysis. The economizer failed due to a weld crater caused by improper welding, which induced residual welding stress. The slight sulfur content of the condensed or flue gas that causes rusting on the economizer's outer cover is not considered a severe failure issue. Material SUS304 stainless steel is the right selection for economizer materials.

Failure Analysis for Girth Weld of Gathering Pipelines

Through failure generalization, fracture feature analysis and material performance test, a comprehensively analysis was made on the fracture failure analysis for girth weld of gathering pipelines containing H2S gas. The results showed that the fracture failure might be mainly due to sulfide stress cracking in the girth weld. The crack originated from the fusion line on the inner surface of girth weld and extended along the girth weld to outside closed to bends. The sulfide stress cracking of the girth weld was caused by the intersection of multiple factors. The service condition was located in SSC 3 zone and the SSC risk of girth weld was high. The girth weld itself was not been stress-relieved, and its ability to resist SSC was poor. Due to low wall thickness, welding defects, welding stress and additional load, the actual stress of weld was higher.

Prediction and numerical simulation of residual stress in multi-pass pipe welds

A numerical simulation procedure is presented to predict residual stress states in multi-pass welds in oil transportation pipes. In this paper, a two-dimensional thermo-mechanical finite element model is used to calculate the temperature distribution, hardness, and the distribution of residual stresses during multi-pass welding of pipes of dissimilar metals and varying thicknesses. In this model, the temperature dependence of the thermal and mechanical properties of the material was considered. The present model was validated using the hardness measurement. Good agreement was found between the measurement and the numerical simulation results. The simulated result shows that the two-dimensional model can be effectively used to simulate the hardness test and predict the residual stress in the pipe weld. The simulation results and measurements suggest that the model with moving heat source can obtain a good prediction of residual welding stress. Both the two-dimensional and the three-dimensional modeling can be used to estimate the residual stresses in different weld regions and help saving time.

FEM assessment of the local side compression technique efficiency as applicable for notched prismatic specimens

A crack front straightness is one of the test result validity criteria for fatigue precracked static fracture toughness specimens. Actually, the ideally straight crack front cannot be reached due to the presence of residual stress. This is particularly actual for specimens cut out of welded joints containing the residual welding stress (RWS). One of the techniques allowing to lower the RWS effect is a local side compression of specimens. Its efficiency has been proved in physical testing however no quantitative assessments are known in the literature. This work comprises FEM simulation of welding, sampling and side compression processes. The effect of local compression on base metal containing no residual stress is also investigated.It has been found that in the course of local side compression the initial residual stress field caused by welding and specimen making is replaced by another field showing stress gradients more favourable for getting the fatigue crack shape meeting the validity criteria of test results as per approved test methods. The calculation results show that the complete removal of residual stress as in base metal as in welded specimens is not feasible in the range of actual practicable degrees of compression.

Analysis of the Crack of 304 Stainless Steel and Improvement Measures

Stainless steel is more and more widely used in industry and life. 304 stainless steel has been widely used because of its lower price and better corrosion resistance. In this paper, according to the phenomenon of the crack of the adapter strip made of 304 stainless steel when used on the ship, the reason of the crack is analyzed from the aspects of design, technology and use, and the reasons of the crack are finally determined. In this paper, the mechanism of welding stress and chlorine embrittlement are analyzed and the corresponding improvement strategies are developed.

Effect of welding sequence on stress and deformation field of Invar alloy multi-layer and multi-pass welding: A simulation study

The welding sequence has an important influence on the formation of stress and deformation in multi-layer and multi-pass welding (MLMPW). In this paper, a 3D finite element model of Invar alloy welding that can calculate the welding stress and deformation has been established using MSC. Marc. The simulation results show that there is high residual tensile stress in the welding zone. For six-layer twelve-pass welding, the welding sequence has little effect on maximum stress value. Meantime, welding deformation is mainly distributed far from the weld metal area and is mainly affected by welding sequence under similar other welding conditions. It is found that the deformation angle of welding sequence D is the smallest, with a value of [Formula: see text].