| Author: | Jin, Hao |
| Title: | Effects of welding on structural performance of high strength steel thick welded sections |
| Advisors: | Chung, K. F. (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Steel, High strength Steel, Structural -- Welding Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | xix, 258 pages : color illustrations |
| Language: | English |
| Abstract: | Motivation Structural steel sections have been extensively employed in the construction of buildings, bridges, offshore structures and heavy industrial machines for many years. Nowadays, with the development of modern metallurgical technology, high strength steels (HSS), especially Grade 690 steel, and the corresponding welding consumables become available for engineering applications. Compared with normal grade steel, it is seen that the use of high-strength structural steel leads to a reduction in the plate thickness of components and the overall weight of the structure. Consequently, this reduction in weight results in a decrease in the quantity of steel required, as well as a decrease in the dimensions of the weld and the materials used for welding. A reduction of 50% in plate thickness brings along a savings of about 67 to 75% in welding manpower and electrodes, depending on joint details. However, strength reductions in high strength steel welded sections have been reported by many researchers, and the heat input energy should be controlled into a small value during welding. Such a restriction will severely limit the application of high strength S690 steel in the construction industry. Hence, it is highly desirable to examine and quantify such effects in these Grade 690 steel thick sections for applications in construction. In general, there is a lack of scientific understandings and engineering data on the effects of welding onto the structural performance of the welded sections of these Grade 690 steel with plate thicknesses up to 70 mm. Objectives and scope of work In the present research, it is aimed to investigate the effects of welding onto the mechanical properties, and hence, the structural performance of the high strength steel welded sections with thicknesses up to 70 mm through systematic experimental and numerical investigations. The scope of work includes: a) To investigate both thermal and thermomechanical responses of typical welded sections of thick high strength steel plates during and after welding, and hence, to establish through thickness distributions of welding-induced residual stresses within these welded connections. b) To assess any reduction in the mechanical properties of these welded sections of thick high strength steel plates which are prepared with a practical range of heat input energy during welding, to establish advanced finite element models for accurate prediction to the structural behaviour of these welded sections. c) To perform physical measurements and structural tests on these welded sections of thick high strength steel plates to provide test data for calibration of various finite element models. Key research findings • Task 1. Mechanical properties of high strength steel thick welded sections In order to determine the effects of welding onto the mechanical properties of these thick welded sections, a total of 22 standard coupons extracted from the S690-QT 50 and 70 mm thick welded sections, and 22 standard coupons extracted from the Q620-TM 28 and 44 mm thick welded sections were tested under monotonic loadings. The range of heat input energy in preparing these thick welded sections varied from 2.4 to 5.0 kJ/mm, and it was considered to cover the heat input energy commonly used in practice. Images of Scanning Electron Microscope (SEM) were captured to illustrate microstructural evolutions within the heat affected zones of the welded sections. In general, the effects of welding are demonstrated to be significantly less severe than anticipated as there is little or even no reduction in the mechanical properties of these thick welded sections. In many cases, the engineering stress-strain curves of these thick welded sections follow closely with those of the base plates along the entire deformation ranges with a discrepancy smaller than 3 to 5%. • Task 2. Through thickness residual stresses of thick welded sections The through thickness residual stresses of thick welded sections of these high strength steel plates were assessed as follows: • Firstly, the hole-drilling method was applied to the top and the bottom surfaces of a S690-QT 50 mm thick welded section to establish a reference set of residual stress distribution. • Secondly, as the residual stresses distributed through the thickness of the welded section could not be measured directly, a large amount of the steel was removed from the top and the bottom parts of the welded section with milling and the hole-drilling method was then applied onto these newly formed surfaces. Owing to significant stress release in the welded section during removal of the steel, correction to the second set of residual stress distribution was necessary. • Thirdly, an advanced finite element model was developed using ABAQUS, and both the processes of welding and removal of the steel were simulated. The heat transfer analyses were first calibrated with the measured temperature history during welding. Then, sequentially coupled thermomechanical analyses were conducted to predict the residual stresses of these welded sections. After careful calibration against the two sets of measured residual stress distributions, the finite element model was able to simulate the through thickness residual stresses of the welded sections with a high degree of accuracy. In general, the effects of welding are demonstrated to be significantly less severe than anticipated as the residual stresses in these 50 mm thick welded sections are found to be significantly smaller than those 16 mm thick welded sections. • Task 3. Advanced modelling approach with critical sub-zones In order to establish a rational finite element model to simulate the structural performance of a thick welded section, critical sub-zones were proposed to be adopted in the heat affected zones of the welded section. Based on the predicted transient temperature history, the elements having a cooling time of t8/5, i.e. a time for the temperature to decrease from 800°C to 500°C, were identified as the critical sub-zones. Hence, both the locations and the dimensions of the critical sub-zones were readily determined. The mechanical properties of these elements in the critical sub-zones were then assigned conservatively to lower bound values according to their t8/5. It should be noted that these lower bound values of mechanical properties were obtained from a series of tensile tests on heat-treated coupons after a physical welding simulation process. Finite element models have been established to simulate structural behaviour of: i) welded sections of the S690-QT and of the Q620-TM steel with 2 heat input energy under tension; and ii) stocky columns of the Q620-TM steel under compression. Good comparison between the predicted and the measured test data was achieved. Hence, the proposed modelling approach with the use of critical sub-zones are demonstrated to be highly effective in simulating the structural behaviour of these thick welded sections of high strength steel plates. It should be noted that the proposed approach is able to simulate the structural behaviour of the welded sections under tension with a small level of strength reductions at 1%, and also of the stocky columns under compression with a small level of strength reductions at 2 to 3 %. Research significance This research is an in-depth investigation into the effects of welding onto both S690-QT and Q620-TM high strength steel thick sections. Both scientific understanding and engineering data on the mechanical properties, and hence, the structural performance of these welded sections have been obtained. An advanced finite element modelling approach is also proposed with the use of critical sub-zones in the heat affected zones of these welded sections, and finite element models have been established to simulate the structural behaviour of these welded sections under both tension and compression. Consequently, these models are able to quantify the effects of welding onto the structural behaviour of these welded sections in a rational manner. |
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| Access: | open access |
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