Gerhard Posch

Effect of High-Strength Filler Metals on Fatigue

Fatigue life of welded joints is in general independent from the material strength. High-strength materials are only beneficial in the low-cycle-fatigue region due to their increased yield limit. This property leads to their application, for instance, in welded mobile crane structures. The high-cycle fatigue limit, however, depends mostly on the geometry and the metallurgy of the notch. Therefore, an optimized weld process is required to achieve a certain fatigue strength. This paper contributes to the obtainable fatigue limits for thin-walled, high-strength joints regarding an optimization of the gas metal arc weld process for fillet welds without additional post-treatment. A methodology is designed to manufacture welded specimens with minimized production scatter. The specimens were carefully analysed by metallographic studies, hardness, distortion and geometric weld toe measurements. The detailed analysis enables a profound link between experimental fatigue life and weld process settings. For the assessment of the fatigue life of thin-walled specimens, the nominal stress approach and the notch stress method are used. The thin wall thickness is considered in the nominal approach by a thickness correction factor. The experimental results showed that the highest fatigue strength for the specific specimen design in ‘as-welded’ condition can be obtained when using a high-strength metal-cored wire filler in combination with a three-component mixed gas.

Effect of high-strength filler metals on the fatigue behaviour of butt joints

Welded structures made of high-strength steel offer benefits in fatigue strength for finite life applications. The high-cycle fatigue limit, however, depends mostly on the geometry, and the metallurgy of the notch is of little account. Therefore, an optimised weld process is required to achieve an improvement in the fatigue strength. This paper contributes to the field of fatigue behaviour of thin-walled, high-strength steel butt joints, with regard to an optimisation of the gas metal arc weld process. An existing methodology was extended to manufacture welded specimens with minimised production scatter. The majority of the butt joint samples were dynamically tested, with the root surface ground flush to plate, to study the effect of the weld process on fatigue. The investigated specimen were carefully analysed by metallographic studies along with hardness, distortion, and weld toe topography measurements. This facilitated in finding a relation between experimental fatigue life and the weld parameters. The nominal stress approach, including a benign, nonconservative thinness correction, and the recommended notch stress concept, were applied to assess the fatigue behaviour of the thin-walled, high-strength steel butt joints. The experimental results showed that in case of high-quality welds with negligable geometric notch factor, a small, but distinct influence of the filler metal on fatigue is observable. The highest fatigue strength for the investigated butt joint design was obtained with a high-strength metal-cored wire filler in combination with a three-component shielding gas.

Welding Advanced Martensitic Creep-Resistant Steels With Boron Containing Filler Metal

Within the last years, the beneficial effect of boron addition in order to increase the creep strength of martensitic 9–12 % Cr steels has been widely investigated and demonstrated. Several boron-strengthened alloys Care the outcome of an intensive European research activity within the scientific framework of COST. The most promising alloy for castings with a targeted application temperature of 650 °C for use in future ultra-super critical steam power plants is a modified 0.1 C-9Cr-1.5Mo steel with the addition of approximately 110 ppm boron. For joining of this advanced steel grade, new “chemically matching” welding consumables have to be developed to provide similar creep strength in the weld. Within this paper, the development of an advanced flux-cored wire with a good “out of position weldability” — based on a rutile slag system is presented. The work discusses the optimization of the chemical composition of the filler, the influence of various post-weld heat treatments on short term mechanical properties, first results of creep tests and results of thick section welds. The boron content of the consumable was optimized to achieve highest creep strength combined with acceptable toughness and lowest hot cracking sensitivity. For a butt weld of a 34 mm-thick casting, the optimum welding parameters and layer sequence were determined. A post-weld heat treatment consisting of stress relieving twice at 730 °C for 12 h seemed to be the best compromise between applicability in the manufacturing of large turbine casings and achievement of acceptable weld toughness.

Investigation of HAC-Susceptibility of Multi-Layer Welds with the “Bead Bend Test” Procedure and Examples

A new test method called “Bead Bend Test” for evaluation of the hydrogen-assisted cold cracking — (HAC) susceptibility of multi-layer welds will be presented. The main advantage of this test is the possibility of evaluation of weldments that are established by using real welding procedures. Also, different on-site stress conditions can be taken into account by variation of the clamping system. The test results can be used for theoretical investigations, but also for practical recommendations regarding the necessary minimum interpass temperatures to prevent HAC under consideration of the plate thickness, the materials strength level, and the diffusible hydrogen content of the weld metal. As an example, the HAC behaviour of high strength cellulosic pipeline weldments are discussed. An additional feature of this test procedure is the HAC evaluation of joints built up by a combination of different electrode types; e.g., cellulosic and basic types which are shown in a second example.

Metallurgical approach for the development of a hot crack-resistant metal-cored wire

During the last years, the automotive industry has striven to decrease the emission rates by raising combustion temperatures in the engine. As the exhaust temperature increases, this places higher demands on the exhaust system components and the filler metals used for welding. Due to the ever increasing requirements placed on welding efficiency, the use of metal-cored stainless steel wires continues to grow. High quality can be achieved at high welding speeds with minimum amount of rework, when these wires are used. As hot cracking can occur when welding high-temperature stainless steel grades, the weldability is restricted by the resistance to solidification cracking. Within the scope of this paper, two types of metal-cored wires with significant difference in δ-ferrite and manganese contents were compared to each other concerning hot crack susceptibility. Programmable Deformation Cracking and Modified Varestraint Transvarestraint tests were performed to determine the hot cracking liability. The wire with the higher amount of δ-ferrite showed better resistance than the wire with higher manganese content. Good weldability at high welding speed was confirmed by robotic welding. Additionally, the influence of the high temperature on the formation of σ-phase and its effect on the impact toughness was investigated.