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.

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.

Effect of the welding parameters on residual stresses in pipe weld using numerical simulation

The objective of this article is to predict the residual welding stress in a dissimilar pipe weld. The 2D model, instead of 3D was used to reduce the time and cost of the numerical calculation. The 2D numerical simulation MSC MARC code is used to predict the residual stress developed during pipe welding. The present model was validated using hardness measurement. Good agreement was found between the measurement and numerical simulation results. The effects of welding parameters on residual stress field on the outer and inner surface were assessed. The effect of welding parameter (welding current) is examined. The axial and hoop residual stresses in dissimilar pipe joints of different thickness for pipe weld were simulated in outer and inner surfaces. When the other parameters remain fixed, and the current has great effect on the weld shape and size, and then affects the residual stress level significantly.

PREVENTION OF METAL BRIDGE SPANS FROM FATIGUE CRACKING

At present, more than four thousand metal bridge spans are in operation all over the Russian railway network. Through the years of operation, about thirty types of fatigue cracking were identified. The dynamics of the types Т-9 and Т-10 cracks formation has increased significantly in recent years. The formation and growth of fatigue cracking is influenced by such factors as residual welding stress, stress-strain state of the bridge structure, defects, damages, and non-observing the operation and maintenance standards, bridge location and bridge span structure. At present, neither measures nor repair are performed to prevent fatigue cracking not exceeding 20 mm in length. Only when a creak reaches a certain length, the regulatory documents require to drill a hole at a crack mouth in order to prevent its further development. The hole diameter should be equal approximately to the doubled wall thickness. In order to prevent further crack growth and cover the hole, a high-tensile bearing type bolt is fixed into it, creating a volume stress. As a rule, such a repair is not enough to stop the cracking process. The article suggests a number of measures based on induction heating that allows to prevent cracking (including forging reducing gaps between connecting plates and horizontal sheets in a truss), to carry out repair (crack mouth soldering) and reinforcement (fixing metal plates onto a wall with a hole).

Control of Cold Welding Cracks in 30CrMnSi Steel by Welding with Trailing Impacting and Rolling

As a kind of the high strength low alloy steel (HSLA), 30CrMoSi has been used widely in some industrial fields. However, just like some HSLA, this steel also faces some problems when it is welded, especially such problem as cold cracking in the welded joints. In this paper, the cold welding cracks and microstructure of the joints of 30CrMoSi steel were studied by scanning electron microscopy (SEM) and optical microscopy (OM), and a method called welding with trailing impacting and rolling (WTIR) was utilized to solve the problem mentioned above by decreasing the residual welding stress which is one main factor to lead to cold welding crack in the joint of HSLA. The crack-free joints of 30CrMoSi steel were obtained by using proper parameters.