Fatigue mechanism of medium-carbon steel welded joint: Competitive impacts of various defects

This paper presents an experimental investigation into mechanical properties and fatigue performance of butt-welded high strength steel plates. Four typical high-strength steels (HSSs) were selected, with steel grades of 460, 550, 690, and 960 MPa. Mechanical properties were measured through monotonic tensile tests, cold bending tests, Charpy V-notch impact tests and hardness tests. A total of 114 fatigue tests were conducted on butt-welded high-strength steel plates. Comparisons of mechanical properties and fatigue performance are made among HSSs tested and conventional strength steels (CSSs) with a yield strength around 235 to 355 MPa. The natural slope of S–N curves for HSS butt-welded connections varied from 3 to 5. Two fatigue design approaches are recommended with one based on natural slope and one based on a fixed slope of 3 as for CSSs.

Experimental investigation on fatigue behavior of butt-welded high-strength steel plates

In the laser welding process using a focused Gaussian beam, high thermal gradient and molten pool instability usually lead to coarse columnar grain, poor surface quality and degradation of mechanical properties. In this paper, a beam shaping strategy that coaxially combines fiber laser and diode laser is applied in laser welding of austenitic stainless steel. Effects of the diode laser power on both the microstructures and mechanical properties are systematically studied by experiments and simulations. Results show that a fiber-diode hybrid heat source could effectively avoid unstable states of the keyhole, and has significant advantages in controlling spatter and surface morphology. Equiaxed grains tends to form on both sides of the junctions in synergistic and dominant regions due to the reduction in the thermal gradient produced by the hybrid beam. EBSD results show that the addition of diode beam weakens the formation of columnar grains along the easy slip texture <100> due to the disturbance in the liquid metal by a transverse distribution of the diode laser. Compared with the Gaussian heat source, the joint welded by the fiber-diode hybrid laser welding provides higher strength and elongation, with typical fracture morphologies of deeper and larger dimples. The optimum mechanical properties of the joint can be obtained by 2000 W fiber laser power and 1000 W diode laser power, where cracks occur in the base metal. 

Microstructure characteristics and mechanical properties of fiber-diode hybrid laser welded 304 austenitic stainless steel

Fine-grained steels belong to the progressive materials, which are increasingly used in the production of welded structures subjected to both static and dynamic loads. These are unalloyed or microalloyed steels hardened mainly by the grain-boundary strengthening mechanism. Such steels require specific welding procedures, especially in terms of the heat input value. At present, there are studies of the welding influence on the change of thermomechanically processed steels’ mechanical properties, however mainly under static loading. The paper is therefore focused on the assessment of the welding effect under dynamic loading of welded joints. In the experimental part was determined the influence of five different heat input values on the change of weld fatigue life. As a result, there is both determination of five S-N curves for the double-sided fillet welds from the thermomechanically processed fine-grained steel S460MC and the quantification of the main influences reducing the fatigue life of the joint.

Heat Input Influence on the Fatigue Life of Welds from Steel S460MC

Microstructure and mechanical properties were studied in fusion welded AZ31B-alloy to DP590-steel at different weld conditions such as with varying laser power, and Sn powder added to lap joint with prefabricated width-settled concave groove. Temperature field and flow field for laser welding including chemical potential of each element are also calculated. The results suggested that Al atom tends to diffuse toward Fe side which contributes to form a FeAl compound reaction layer at interface. Meanwhile, Al accelerates the diffusion of Sn and Fe as a catalyst. The shear force of joint climbs at first, then declines, and the maximum value reaches 1711 N when laser power is 1 kW, which is improved by 38% compared with that of 0.9 kW. The improved joint properties are dominated by metallurgical regulation of Sn power, which benefits from the accelerated flow of molten pool and increased temperature leading to the phase composition transformation from α-Mg + Mg2Sn + FeAl to α-Mg + Mg2Sn + FeSn + Fe3Sn + FeSn2 + FeAl in direct laser irradiation region. However, under the condition of excessive laser power, overreaction occurs at side boundary interface, which leads to the formation of thick IMC and the production of hot crack defect, thus the joint performance decreases. 

Influence of laser-Sn powder coupling on microstructure and mechanical properties of fusion welded AZ31B-alloy to DP590-steel

The hybrid laser-tungsten inert gas welding technology was applied to butt weld 3-mm-thick S460MC and S700MC high-strength low-alloy steel sheets. The intent of low-current arc addition was to preheat the material to reduce extremely fast cooling rate accompanying laser welding. High-cycle fatigue tests were performed to evaluate the effect of welding conditions on fatigue behavior of S460MC and S700MC laser and hybrid welds. Both base materials exhibit approximately the same fatigue limit. However, corresponding samples with the weld behaved significantly different under cyclic loading. We demonstrated that the drop of their fatigue lifetime is caused by the combination of microstructural changes and especially by the presence of surface notches resulting from the welding process. The two series of both steels were tested to reveal the contribution of induced surface roughness and microstructural changes itself. The first one was in the as-welded condition. The second one had a smooth surface resulting from surface layer grinding after the welding. It was found, that the geometric notches created by the welding process are the determining factor in case of both tested steels. Testing of the ground samples showed, that in case of S460MC steel, the application of pre-heat did not improve the fatigue properties. On the other hand, the performed fatigue tests showed improved fatigue limit of ground series samples of the pre-heated S700MC steel, when compared with the conventional laser welding.

Fatigue properties of laser and hybrid laser-TIG welds of thermo-mechanically rolled steels

Abstract Color metallography of aluminum alloys can provide new structural knowledge or extend and refine structural knowledge compared to a black and white image. The basic principle of color metallography is to create a suitable film by etching. On samples coated with a suitable film, light interference is caused by the splitting of the incident light into components reflected at the air-layer interface and the layer-metal interface. The thickness of the film essentially determines at which colors the interference occurs. For films of suitable thickness, interference occurs in the blue, green and yellow regions. The film thickness depends on the chemical composition of the sample material, the chemical composition of the etchant and the etching time. If the etching conditions are kept constant (etchant type, etching time) and the chemical composition of the individual micro-areas changes significantly, it will be possible to distinguish the microareas by color contrast in the bright field. The results of color etching of aluminum alloys in different types of etchants will be presented. Using an automated apparatus, the same etching conditions will be maintained, including time and repeatability of movements in the etching, cleaning and drying process.

Automated color etching of aluminum alloys

Conventional long-term creep test (CCT) to the rupture and so called accelerated creep test (ACT) of the dissimilar weld joint made of FB2 and F martensitic steels and of the base materials were carried out at temperatures ranging from 550 °C to 650 °C in the stress range from 70 to 220 MPa. Assessment of microstructure development and changes of hardness was correlated with the creep strength. During creep at temperatures above 575 °C Laves phase precipitated in all parts of the weld joint and especially in the heat affected zones. Coarse Laves phase particles and their clusters with chromium carbides served as nucleation centers for cavities. As the fine grained heat affected zone of F steel was the softest part of the weld joint, many cavities originated and cause failure of samples. The aim of this paper is to compare results and possibilities of the “standard” methods and advanced scanning electron microscopy performed by instrument equipped with a concentric backscatter electron detector (CBS). Filtering of the signal enables improving and/or diminishing of selected type of contrast caused by various types of particles of secondary phases. The images were used as an input data for image analysis and developments of microstructures during CCT and ACT were compared. Results have shown that specimens after ACT contains significantly lower content of the Laves phase.

Characterization of Microstructure of Crept Samples of Dissimilar Weld Joint Using Standard and Advanced Electron Microscopy Techniques

Chemical etching is an integral part of metallographic sample preparation. Maintaining precise etch times can be difficult and therefore repeatability is limited. The aim of this work is to improve the repeatability of sample preparation using robotization. Prior to etching, metallographic samples of S355J2 (1.0577) structural steel were finely mechanically polished. For verification, 15 specimens were prepared using an in-house designed automated etching machine with a built-in 5-axis robotic arm and 15 specimens prepared manually by an expert metallographer. The samples were etched with Kourbatoff no. 4 reagent for 8 seconds in a beaker placed in an ultrasonic cleaner at 80 kHz. The samples were then cleaned in 7 beakers of cleaning fluid also placed in the ultrasonic cleaner. The robotic etching and cleaning process was optimized and the quality of the resulting surface is at least as good as that of the samples prepared by an expert metallographer. The surfaces were compared using a light optical microscope (LOM) and a confocal laser scanning microscope (CLSM). The repeatability of the preparation process is a key aspect for obtaining a large dataset of steel microphotographs for training a deep neural network that will be used in future research.

Ondřej Ambrož

Papers

Fatigue properties of laser and hybrid laser-TIG welds of thermo-mechanically rolled steels

Automated color etching of aluminum alloys

Characterization of Microstructure of Crept Samples of Dissimilar Weld Joint Using Standard and Advanced Electron Microscopy Techniques

Automation of Metallographic Sample Etching Process

Automation of metallographic sample cleaning process