Imre Török

Assessment of Methods in Girth Welds of Steel Pipelines

This paper presents two basic methods for the assessment of failed girth welds of steel hydrocarbon transporting pipelines. One of them is based on the principles of linear elastic fracture mechanics (LEFM) and stress intensity factor conception for planar material discontinuities, and the other can be used for the complex assessment of all kinds of occurring defects. The results of the presented methods are compared to the results of burst test of pipeline sections containing a failed girth weld and cut from a Hungarian gas pipeline.

Resistance Spot Welding of 7075 Aluminium Alloy

New generations of high strength aluminium alloys are widely used in automotive industry. 7075 (AlZn5.5MgCu) is an aluminium alloy with zinc as the primary alloying element (according to EN 485-2 standard). The 7075 has outstanding strength properties, but the elongation is limited between 5 and 11%. This means an obstacle when this alloy is aimed to use in car body elements. The goal of the present research work is to analyse the weldability of this alloy through resistance spot welding (RSW) tests. Experiments are performed on 1 mm thick 7075 sheets in T6 condition with different surface pre-treatment methods and the weld lobe was determined for constant electrode force. A Tecna 8007 RSW equipment with a TE 550 control unit was applied, which has been recently developed with a tool for the application of post pressing force. The spot welded joints were examined by macro testing, tensile shear and hardness tests. The properties of RSW joint from 7075 were compared with the 6082 alloy.

The Effect of Solution Annealing and Ageing During the RSW of 6082 Aluminium Alloy

In the automotive industry there is a growing tendency for the application of high strength aluminium alloys. In spite of their significant role in weight reduction there are still obstacles for their wider use due to their limited formability and weldability. Hot forming and in-die quenching (HFQ) process was recently developed for the forming of car body sheets. During the HFQ technology the sheet metal forming should be performed in a solution annealed condition. In the solution annealed condition the aluminium alloys have lower strength and better formability properties. The forming process is followed by a precipitation hardening which is generally connected with the painting of body parts (bake hardening). Besides the formability the implementation of HFQ has an effect on the weldability properties, too. HFQ must have an effect on the resistance spot welding (RSW) of aluminium sheets since the weld nuggets are produced after the HFQ, in the assembly part of the production chain, when the aluminium alloy is in a solution annealed and formed condition. The final properties of the welded joints are determined by the precipitation hardening which is the final step of the whole production process. The present research work aims to investigate the effect of the HFQ process on the weldability of AA6082-T6 aluminium alloy. The properties of the RSW joints are examined in different conditions (T6 delivery condition, solution annealed, precipitation aged). The materials tests include conventional macro testing, hardness tests and tensile-shear tests extended with EDS (Energy Dispersive Spectroscopy) and EBSD (Electron Backscatter Diffraction) tests in order to characterize the distribution of alloying elements and to analyze the grain structure.

Certain Weldability Problems of 6082-T6 Aluminium Alloy and the Mechanical Properties of the Welded Joints

The usage of modern high strength aluminium alloys are getting more remarkable in several industry sectors mostly the fabrication of light weight structures, such as vehicles, railway transport systems, aerostructures and building constructions. The weldability problems of these alloys are more complex than the steels with similar strength. Therefore weldability problems must be analyzed very accurately, by the help of the modern physical simulation. By knowing the difficulties of the weldability of high strength aluminium alloys the proper parameters of the welding technology can be defined. This article represents the investigation of a certain weldability problem of 6082T6 aluminium alloy with the aim of physical simulation and welding experiments with gas metal arc welding and pulsed current technology.

Investigation of Dynamic Recrystallization on the Root Side of FSW Joints

The grain size is basically influences the mechanical properties of welded joints. During friction stir welding (FSW) different grain sizes occur on the welded joint as a function of wall thickness and joint zones [1] [2]. The main goal is to reach the fine grained structure in whole joint. According to the literature [3] [4] [5] the dynamic recrystallization (DRX) can occur during friction stir welding and it can cause extra fine grains. This can be determining for the aspects of tool design and technological parameter optimization. The weakest part of a friction stir welded joint is the root because the circumstances are not always good for dynamic recrystallization, so coarser grains are appeared there. Therefore the investigation of dynamic recrystallization on the root side is important for technological parameter optimization.

Effect of Temperature Field and Pressure Force on the Inhomogeneity of 5754-H22 and 6082-T6 FSW Joints

The Friction Stir Welding (FSW) differs from the conventional welding process inter alia that the joint properties are not symmetric to the axis of joint line. Basically, it follows from the principle of the method because a rotary tool is used for welding. This asymmetry is due to created temperature field during welding, and the pressure force distribution along the thickness, so the investigation of these effects is important to create good quality welds. Experiments were performed on FSW joints made of non-heat treatable (5754-H22) and heat treatable (6082-T6) aluminium alloys, because there are clear difference between the reacts to these effect and show different changing on the microstructure. A special FSW tool was developed for the experiments and lowest inhomogeneity. Dynamic recrystallization and dynamic recovery can occur during welding so these were investigated by Gleeble material simulating system. During the experiments the grain sizes and grain shapes were also investigated.

The Role of the External and Internal Reinforcing on the Structural Integrity of Industrial and Transporting Steel Pipelines

The lifetime management of different engineering structures and structural elements is one of the important technical-economic problems nowadays. On the one hand, the aim of our research work is to develop an integrity management plan for pipelines and pipeline systems, and afterwards a Pipeline Integrity Management System. Material databases play important role both on the integrity management and on the engineering critical assessment of the pipeline systems. On the other hand, the aim of our research work is to establish the Pipeline Integrity Management System with different data, frequently with experimental data. The direct purpose of the paper is to present the role of the external and internal reinforcing on the structural integrity of industrial and transporting steel pipelines, based on own examinations. External and internal reinforcement was developed using carbon fibre and glass fibre polymer matrix composites, respectively. Fatigue and burst tests were performed on pipeline sections containing natural and artificial metal loss defects, and girth welds including weld defects. Both unreinforced and reinforced pipeline sections were examined. The burst pressures belonging to the unreinforced and the reinforced pipelines, and belonging to the passed and not passed girth welds were compared.