Sayyed H. Hashemi

Multi-objective Optimization of Welding Parameters in Submerged Arc Welding of API X65 Steel Plates

Submerged arc welding (SAW) is one of the main welding processes with high deposition rate and high welding quality. This welding method is extensively used in welding large-diameter gas transmission pipelines and high-pressure vessels. In welding of such structures, the selection process parameters has great influence on the weld bead geometry and consequently affects the weld quality. Based on Fuzzy logic and NSGA-II (Non-dominated Sorting Genetic Algorithm-II) algorithm, a new approach was proposed for weld bead geometry prediction and for process parameters optimization. First, different welding parameters including welding voltage, current and speed were set to perform SAW under different conditions on API X65 steel plates. Next, the designed Fuzzy model was used for predicting the weld bead geometry and modeling of the process. The obtained mean percentage error of penetration depth, weld bead width and height from the proposed Fuzzy model was 6.06%, 6.40% and 5.82%, respectively. The process parameters were then optimized to achieve the desired values of convexity and penetration indexes simultaneously using NSGA-II algorithm. As a result, a set of optimum vectors (each vector contains current, voltage and speed within their selected experimental domains) was presented for desirable values of convexity and penetration indexes in the ranges of (0.106, 0.168) and (0.354, 0.561) respectively, which was more applicable in real conditions.

Measurement and Analysis of Impact Test Data for X100 Pipeline Steel

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components, i.e. fracture and non-related fracture energy, are of great importance for pipeline engineers. This paper presents the results of instrumented Charpy impact experiments on high-grade pipeline steel of grade X100. First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the data-recording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are presented and discussed. These include the test data from full and sub-sized Charpy V-notched specimens. The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The results showed that there was a significant drop in hammer velocity during the impact event. This resulted in a change in the fracture mode from dynamic to quasi-static which was more appreciable for full-size Charpy test samples. As a result, sub-sized specimens might be preferable for impact testing of this steel in order to guarantee the conditions of dynamic crack propagation in the specimen ligament. Accurate analysis of the instrumented impact test data showed that the ratio of crack initiation energy to propagation energy was around 30% for the X100 steel. It can be concluded that in impact testing of high-grade pipeline steel a significant portion of overall fracture energy is consumed in non-related fracture processes. This high fracture initiation energy should be accounted for if the current failure models are going to be used for toughness assessment of highstrength low-alloy gas pipeline steels.

Evaluation of Fracture Initiation Energy in API X65 Pipeline Steel

In this paper, energy absorption characteristics of spiral welded gas pipeline steel are investigated under impact loading. Emphasise is given to energy consuming processes before fracture propagation in tested linepipe steel. The API X65 grade pipe was produced (by Sadid Pipe and Equipment Company) from thermo-mechanical controlled process (TMCP) coils supplied by a Korean steel mill. To measure material impact toughness, an instrumented Charpy machine was used. Experiments were conducted at room temperature on different sets of standard full size Charpy V-notched specimens taken from the pipe material, seam weld and heat affected zone. The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The numerical results showed good agreement between the instrumentation data and those read from dial indicator. From fracture energy plots it was found that the maximum and minimum fracture energy was associated with the pipe material and seam weld, respectively. In all test samples, a significant amount of energy was consumed in non-fracture related processes including indentation at the support anvils and at the impact point, bending of test specimen and crack initiation. From this finding, correction factors were suggested to account for considerable energy level of non-fracture related processes. This energy had been ignored apparently in conventional pipeline failure models calibrated in the past on low toughness pipe materials in which fracture initiation energy was negligible. The paper concluded with a comparison of suggested correction factors with those obtained by full-scale burst experiments on tough pipeline steels.Copyright

Experimental Study of Charpy Impact Characteristics of High-Strength Spiral Welded Gas Pipeline

Charpy upper shelf energy is widely used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. The measurement of this fracture criterion particularly for modern steels and its apportion into different components (i.e. fracture and non-related fracture energy) are of great importance for pipeline engineers in order to transfer laboratory data from Charpy experiment to real structure. As the conventional Charpy impact test has only one output (i.e. the overall fracture energy) the instrumented test has been used to achieve full failure information from impact test samples. In this paper the results of instrumented Charpy impact experiments on high-strength spiral welded pipeline steel of grade API X70 are presented. First, the instrumentation technique including the design and implementation of a strain gauge load-cell and the details of the data-recording scheme are reviewed. Next, the experimental data obtained from the Charpy impact machine so instrumented are given. These include test data obtained at room temperature from different sets of standard full size Charpy V-notched specimens taken from the pipe material, seam weld and heat affected zone (HAZ). The instrumented Charpy machine was able to capture the load history in full during the fracture process of the test specimens resulting in a smooth load-time response. This eliminated the need for filtering used in similar test techniques. From the recorded test data the hammer displacement, impact velocity and fracture energy were numerically calculated. The numerical results showed good agreement between the instrumentation data and those read from dial indicator. From fracture energy plots it was found that the maximum and minimum fracture energy were associated with the pipe material and seam weld (in average), respectively. In all test samples a significant amount of energy was consumed in non-related fracture processes including crack initiation, bending and gross deformation of test specimen, and indentation at the support anvils and at the impact point. This non-related fracture energy should be accounted for if the current failure models are going to be used for toughness assessment of high-strength low-alloy gas pipeline steels.Copyright

An Experimental and Finite Element Study of the Ductile Tearing Characteristics of High-Toughness Gas Pipeline Steel

This paper reports recent results from a set of experimental and computational studies of ductile flat fracture in modern gas pipeline steel. Experimental data from plain and notched cylindrical tensile bars and standard C(T) specimens together with damage mechanics theories have been used to capture the flat fracture characteristics of a gas pipeline steel of grade X100. The modelling was via finite element analysis using the Gurson-Tvergaard modified model (GTN) of ductile damage development. The assumption of effective material damage isotropy was sufficiently accurate to allow the transfer of data from the notched bars to predict, in a 2D model, the crack growth behaviour of the C(T) specimen. This was in spite of the considerable ovalisation of the bars at the end of their deformation. However, it was not possible to obtain similar accuracy with a 3D model of the C(T)test, even after a large number of attempts to adjust the values of the GTN parameters. This, and the anisotropic shape change in the tensile bars, suggests very strongly that the damage behaviour is so anisotropic that conventional models are not good enough for a full engineering description of the flat fracture behaviour. Suitable averaging (of shape) in the modelling of the notched bar data, and the companion averaging associated with the 2D model of the C(T) data provide a relatively fast way of transferring engineering data in the tests. There is a discussion of potential ways in which to incorporate 3D effects into the modelling for those purposes where the considerable increase in computational time (due to the microstructurally-sized finite elements needed to capture the damage behaviour) is acceptable in order to include through-thickness effects.

Estimation of Slant Tearing Energy for High-Grade Pipeline Steel From Instrumented Charpy Test Data and its Transferability to Large Structures

For several decades, the Charpy upper shelf energy has been used as a fracture controlling parameter to estimate the crack arrest/propagation performance of gas transportation pipeline steels. However, significant discrepancies have been observed between the results of full-scale burst experiments on modern pipeline steels and those predicted by Charpy-based fracture models. This indicates that fracture models calibrated in the past on lower-grade pipeline steels (Charpy toughness below about 100J) cannot be extrapolated beyond their calibration range to assess the fracture behaviour of higher-strength high-toughness steels. One reason for this is the high level of energy often required for crack initiation in these steels. Accordingly, in the short term different correction factors ranging from 1.4 to 2 have been proposed to refine these fracture prediction models. The use of alternative failure parameters like CTOA is currently under review. In this paper a novel experimental technique is given to apportion the upper shelf Charpy fracture energy into its different components, i.e. crack initiation energy and flat and slant tearing energy. The experimental data from instrumented Charpy tests on standard impact specimens made from an X100 grade pipeline steel is used to estimate crack initiation and propagation energy. The areas associated with flat tearing in the centre and slant shearing at the edges of the fracture surface of Charpy test specimens are estimated optically using a fine measurement grid with 0.5 mm spacing. The energy required for generating the flat and slant fracture areas is calculated by the use of associated multipliers, i.e. the specific flat and slant fracture energy (in terms of J/mm2 ). These are measured separately using flat and slant crack growth data from fracture tests on standard C(T) and modified DCB like specimens. The results showed that the Charpy energy from a test is dominated by non-crack propagation energies. Around 36% of the measured impact energy appeared to be associated with flat and slant tearing processes. As the latter is the important failure micro-mechanism in pipeline steel only that part of the overall Charpy shelf energy which is associated with slant shearing might be used to evaluate the crack growth resistance of modern steels. This suggests the possible use of correction factors for high toughness pipeline steels of the order of 1.7 to transfer the slant fracture energy measured on small-scale specimens to the real structures for predicting their crack arrest/propagation behaviour. The correction factor proposed here from the laboratory test programme agrees with those obtained from costly full-thickness burst experiments on similar class of pipeline steel.Copyright

Experimental Investigation of Slant Crack Propagation in X100 Pipeline Steel

Failure information from full-thickness burst experiments on long distance gas transportation pipelines has shown that unstable fractures propagating in the pipeline axial direction are dominated by ductile slant shearing [1]–[3]. Different test samples, e.g. Charpy impact [4], drop weight tear test (DWTT) [5] and double cantilever beam (DCB) [6] have been proposed to study the ductile shear crack growth in pipeline steels in a laboratory scale experiment. Because of the design geometry of these specimens, slant crack growth is often preceded by flat fracture in the specimen un-cracked ligament.