Showing papers on "Weldability published in 2008"

Laser assisted Friction Stir Welding of drawable steel-aluminium tailored hybrids

Abstract: Steel aluminium Tailor Welded Hybrids are still mentioned to be difficult to be joint as intermetallic phases appear during melting welding techniques. These phases are the reason for failure of the joint during loading or forming. As conventional friction stir welding, a solid phase welding technology, is not feasible to join steel and aluminium, laser assistance for preheating the steel sheet is adapted in order to enhance the weldability as well as the welding feed and to reduce the wear at the tool. Tensile tests are performed to achieve mechanical properties of joints, which were welded by systematic variation of process parameters. Finally deep drawing tests are conducted to demonstrate the formability of laser assisted friction stir welded steel aluminium joints.

The Mechanism of Ductility Dip Cracking in Nickel-Chromium Alloys : Subsolidus cracking results from global stresses produced during fusion welding and local stresses generated when coherent or partially coherent second phases form

Abstract: High-chromium (-30 wt-%) nickel-alloy filler metals are desirable for use in nuclear power systems due to their outstanding resistance to corrosion and stress corrosion cracking. However, these alloys are susceptible to welding defects, especially to subsolidus intergranular cracking commonly known as ductility dip cracking (DDC). In order to develop a high-chromium filler metal that is resistant to as-welded defects, a series of Ni-Cr alloys between 16 wt-% and 34 wt-% chromium were assessed for their susceptibility to cracking. Each alloy was evaluated by fabricating a restrained, multipass, automatic gas tungsten arc, V-groove weld, and counting the number of cracks per unit area observable at 50x. The type of cracking (subsolidus DDC or solidification cracking) was further differentiated via scanning electron microscopy. The results from these welds, coupled with microstructural characterization, chemical analyses, mechanical testing, microstructural modeling, and finite element modeling indicate that DDC in Ni-Cr alloys is caused by the combination of macroscopic thermal and solidification stresses induced during welding and local grain boundary stresses generated during precipitation of partially coherent (Cr,Fe) 23 C 6 carbides. Cracking can be mitigated by alloying to minimize (Cr,Fe) 23 C 6 precipitation (e.g., by Nb and Ti additions), lessening the misfit between the matrix and these precipitates (lowering the Cr and Fe concentration), and by minimizing welding-induced stresses. This mechanism of precipitation-induced cracking (PIC) via misfit stresses is consistent with subsolidus cracking in other alloy systems including superalloys, nickel-copper alloys, titanium alloys, and ferritic steels where ductility loss corresponds to the time/temperature regime where partially coherent or fully coherent second phases form.

Process modeling and parameter optimization using neural network and genetic algorithms for aluminum laser welding automation

Abstract: In the automotive industry, applying aluminum alloys to car chassis have become an important concern in order to reduce car weight. In aluminum laser welding, the strength of weld is typically reduced by porosity, underfill, and magnesium loss. In order to overcome these problems, laser welded with filler wire was suggested. In this study, experiments on the laser welding AA5182 of aluminum alloy with AA5356 filler wire were performed with respect to laser power, welding speed, and wire feed rate. The experiments showed that the tensile strength of the weld was higher than that of the base material under certain conditions. Using the experimental results, a neural network model was proposed to predict the tensile strength. To optimize the process parameters, a fitness function was formulated, taking into account weldability and productivity. A genetic algorithm was used to optimize the laser power, welding speed, and wire feed rate. The optimal value of these parameters was considered to be the proper process conditions in terms of weldability and productivity.

High-strength hot-dip galvanized steel sheet and method for producing the same

Abstract: A high tensile-strength galvanized steel sheet, comprising: C: at least 0.05% but less than 0.12%, Si: at least 0.01% but less than 0.35%, Mn: 2.0% to 3.5%, P: 0.001% to 0.020%, S: 0.0001% to 0.0030%, Al: 0.005% to 0.1%, N: 0.0001% to 0.0060%, Cr: more than 0.5% but not more than 2.0%, Mo: 0.01% to 0.50%, Ti: 0.010% to 0.080%, Nb: 0.010% to 0.080%, and B: 0.0001% to 0.0030%, the remainder being Fe and unavoidable impurities, wherein the high tensile-strength galvanized steel sheet has a microstructure that contains 20% to 70% by volume ferrite having an average grain size of 5 µm or less. The high tensile-strength galvanized steel sheet has a tensile strength of at least 980 MPa, and excellent formability and weldability.

Role of zinc coat in friction stir lap welding Al and zinc coated steel

Abstract: AC4C cast Al alloy and Zn coated steel were successfully lap welded using friction stir welding technology. Full strength joints could be obtained and the joints fractured at Zn coated steel base metal side, while Al alloy and unzinced steel could not be welded in the same welding conditions. The joining mechanism and the role of Zn coat on friction stir lap welding of Al alloy and Zn coated steel were put forward. The intervention of Zn coat promoted the formation of Al–Zn low melting point eutectic structure at the interface, which significantly improved the weldability of Al and steel.

Effect of post weld aging treatment on tensile properties of electron beam welded AA2219 aluminum alloy

Abstract: Aluminum alloy 2219 (Al-6.5%Cu) is a favourite age hardenable alloy for aerospace applications because of its excellent welding characteristics. Though AA2219 has got an edge over its 6000 and 7000 series counterparts in terms of weldability, it also suffers from poor joint strength when welded. In this investigation an attempt has been made to improve the welded joint strength through post weld aging treatment. This paper presents the effect of post-weld aging treatment on tensile properties of electron beam welded AA2219 aluminum alloy. Square butt joints were fabricated using an electron beam welding (EBW) machine of 100 kV capacity. The joints were given post-weld artificial aging treatment. Tensile tests were carried out using 100 kN, electro-mechanical controlled universal testing machine. It is found that the post-weld aging treatment is beneficial for improving weld metal hardness and tensile properties. This is mainly due to the uniform distribution of CuAl2 precipitates in the weld metal region in post-weld aged joints compared to as welded joints as evident from weld metal microstructure.

Effect of Weld Quality and Postweld Improvement Techniques on the Fatigue Resistance of Extra High Strength Steels

Abstract: High strength steel (HSS) used in lifting equipment structure and metallic framework are known to have high static strength, good stiffness and very good weldability however knowledge of fatigue behaviour of HSS in welded structure is not sufficient in order to have representative and probabilized S-N curves to modify the design codes. In this study, a large fatigue test campaign was performed on cruciform and symmetrical butt welded joints of high and very high strength structural steel plates. Numerous fatigue tests were carried out for each configuration in as welded condition. To improve the fatigue performance, post-welding improvement techniques (burr grinding and TIG dressing) were performed on the welded specimens. All fatigue results were analysed using a statistical approach defined in the Eurocode 3 standard. It was noticed the impact of weld quality on fatigue performance. The application of post-weld improvement techniques (TIG dressing and burr grinding) leads to an additional improvement of the fatigue strength and reduces the scattering (COV), in comparison with the current welding standards or recommendations. The limited influence of the base metal yield stress on fatigue performance is explained by the local microstructure at the weld toe where fatigue cracks initiate; at this location, the mechanical properties are similar whatever the steel grades.

Joining of thick section steels using hybrid laser welding

Abstract: A recently completed project called Economical and Safe Laser Hybrid Welding of Structural Steel (HYBLAS) has developed the use of hybrid laser welding for thicker section steels up to 690 MPa yield strength. The full project involved several European organisations, was part funded by the European Research Fund for Coal and Steel (ERFCS) and was led by Corus RD&T. This paper presents an outline of those parts of the project which relate to those developments which resulted in being able to laser hybrid weld, in a single pass, up to 25 m plate thickness using 20 kW of laser power at speeds of ∼1 m min-1. Multipass and dual sided welding techniques have also been developed up to 30 mm plate thickness and fillet welds up to 20 mm steel thickness. The project examined the weldability of steels from 180-690 MPa and operational windows for defect free welding were defined. In addition various NDE methods were studied for their efficiency in regard to the defect types which can occur in laser hybrid weld...

Surface laser alloying of 17-4PH stainless steel steam turbine blades

Abstract: As a known high-quality precipitation hardening stainless steel with high strength, high antifatigue, excellent corrosion resistance and good weldability, 17-4PH has been widely used to produce steam turbine blades. However, under the impact of high-speed steam and water droplets, the blades are prone to cavitation, which could lead to lower efficiency, shorter life time, and even accidents. In this article, the 17-4PH blade's surface was alloyed using a high power CO 2 laser. The microstructure and microhardness of hardened 17-4PH were tested by scanning electronic microscope (SEM), X-ray diffraction (XRD), energy disperse spectroscopy (EDS) and a microhardness tester. After laser alloying, the surface layer was denser and the grain refined, while the microhardness of the surface (average 610HV 0.2 ) was about one times higher than that of the substrate material (330HV 0.2 ). The friction coefficient of the laser-alloyed 17-4PH layer was much lower than that of the substrate.

Recent Development in Microstructural Control Technologies through the Thermo-Mechanical Control Process (TMCP) with JFE Steel's High-Performance Plates

Abstract: Thermo-mechanical control process (TMCP) is a microstructural control technique combining controlled rolling and cooling. Thermo-mechanical control process is used to obtain excellent properties for steel plates, such as high strength, excellent toughness, and excellent weldability. JFE Steel has been developing TMCP technologies ever since it started operating its accelerated cooling facility, OLAC® (On-Line Accelerated Cooling), in its plate mill at West Japan Works (Fukuyama) in 1980 (the world s first industrial accelerated cooling system ever built). In 1998, JFE Steel developed Super-OLAC, an advanced accelerated cooling system capable of cooling plates homogeneously at high cooling rates close to the theoretical limits. In 2004, the epoch-making on-line induction heating facility, HOP® (Heat-treatment On-line Process), was also installed in the plate mill at West Japan Works (Fukuyama). High-strength steels, a grade of steel usually produced by the quenching and tempering (Q-T) process, can be toughened by refining the component carbides through rapid tempering by HOP. Because Super-OLAC is capable of accurately controlling the stop cooling temperature before tempering, JFE has managed to develop a new set of microstructural control techniques using M-A (martensite-austenite constituent) as the hard phase. These are unique techniques unachievable with the conventional Q-T process or conventional TMCP. These techniques have already been applied to various advanced products. In this paper, the fundamentals of microstructural control by TMCP, and the recent development of TMCP are described. Examples of the advanced high-strength plates produced in JFE Steel are also presented.

Metallurgical and Mechanical Properties of Fusion Zones of TRIP Steels in Laser Welding

Abstract: Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and high plastic strain capacity that they offer. Si and Al are two important alternatives for alloying of TRIP steels in order to suppress carbide precipitation in the bainite holding temperature range during steel manufacture. Weldability of TRIP steel is one of the key factors governing its application in auto industry. In this paper, Al-alloyed TRIP steel was investigated with the diode laser welding process in terms of fusion zone metallurgical and mechanical properties, with Si-alloyed TRIP steel also included for comparison. It was found that the fusion zone of the Al-alloyed steel has a multiphase microstructure, containing skeletal ferrite, bainitic ferrite, martensite and retained austenite of two different morphologies. In contrast, the Si-alloyed steel fusion zone consists almost entirely of martensite. The high martensite content results in low fusion zone ductility in the Si-alloyed steel, only providing half the tensile elongation of the Al-alloyed steel. The Si-alloyed steel shows a greater decrease of the strength‐ductility balance (ultimate tensile strength times elongation) due to welding, i.e., 62.9% compared to 45.2% for the Al-alloyed steel in quasi-static tensile testing. High strain rate tensile testing with a Hopkinson Bar apparatus shows no significant effect of strain rate on the fusion zone ductility for either steel. The fusion zone of the Al-alloyed steel does not exhibit a detectable TRIP effect probably due to the low carbon content in the retained austenite. Al and Si are both relevant as agents to suppress cementite precipitation, but they are found to exert very different influences on steel weldability.

Weld Solidification Cracking: Critical Conditions for Crack Initiation and Growth

Abstract: A perspective will be given that outlines important considerations in evaluating and predicting weldability. An examination will be made of the local conditions necessary for solidification crack initiation and growth in a weld. This will be done in light of two prominent thermo-mechanical approaches involving critical strain and critical strain rate. Critical conditions will be identified based upon values available in the literature. Methods used to measure strain and strain rate will be compared. The interpretation of crack length measurements commonly used to quantify weldability will be questioned, based upon our current understanding of the problem. Complications and problem areas needing better definition will be identified and discussed, including strain distribution in the mushy zone, segregation at grain boundaries, effect of impurities, and effect of cooling rate on solidification path. Finally, a suggestion will be made for a new approach to weld development using in-situ strain rate measurement and new composition-strain rate maps that define the boundary between crack and no-crack conditions.

Weldability in dissimilar welds between Type 310 austenitic stainless steel and Alloy 657

Abstract: Microstructural evolution and solidification cracking susceptibility of dissimilar metal welds between Type 310 austenitic stainless steel and Inconel 657, a nickel-based alloy, were studied using a combination of electron microscopy analysis and Varestraint testing techniques. In addition, the effect of filler metal chemistry on the fusion zone composition, microstructure, and resultant weldability was investigated. The good cracking resistance of welds prepared with Inconel A was due to a small amount of secondary phase (NbC) and narrow solidification temperature range. The relatively poor cracking resistance of welds prepared with Inconel 82 and Type 310 stainless steel (310 SS) was a result of a wide solidification temperature range and an increase in the amount of secondary phases. Consequently, it is concluded that for the joint between Inconel 657 and 310 SS, filler material of Inconel A offers the best weldability.

Study of the Simulated Heat Affected Zone of Creep Resistant 9–12% Advanced Chromium Steel

Abstract: Global warming leads to research-development attempts worldwide with a view to reduce CO2 emission. Therefore, in the energy supply sector, a special focus has been directed to further development of steels that can endure the ultra-supercritical steam conditions. Our article presents the basic study on weldability of the advanced 9–12% chromium steel applying the thermomechanical simulation of the heat affected zone (HAZ). The changes of microstructures and material properties of HAZ before and after postweld heat treatment (PWHT) have been analyzed and compared by light microscopy, scanning electron microscopy (SEM), hardness measurements, and impact toughness testing. Microstructures at representative points during typical welding cycle and PWHT were studied in details.

Modelling of CO2 laser welding of magnesium alloys

Abstract: Laser welding is an important joining process for magnesium alloys These materials are being increasingly used in different applications such as in aerospace, aircraft, automotive, electronics, etc To date, carbon dioxide (CO 2 ) neodymium-doped yttrium aluminum garnet (Nd:YAG) and the high power diode laser have been extensively used to investigate the weldability of magnesium alloys The present work describes an analytical thermal model for the weldability of magnesium alloys (WE43) using an industrial (CO 2 ) laser source The main target of the project is to present to the industrial community a simple and rapid tool for the determination of the penetration depth and the bead width as a function of both the incident laser power and welding speed The proposed model is based on the Davis thermal approach, largely considered for the characterization of the average radius of the liquid zone, aiming at predicting the joint shape Moreover, since during the welding process considered in this study, a protecting gas is used to avoid joint oxidation, both thermal convection and radiation phenomena in the welding area have been estimated and introduced in our model for a better characterization of the welding process The obtained results have been compared to the experimental ones and a satisfactory correlation has been observed, indicating the reliability of the model developed in this study

High-strength hot-dip galvanized steel sheet excellent in workability and weldability, and its manufacturing method

Abstract: PROBLEM TO BE SOLVED: To provide a high-strength hot-dip galvanized steel sheet having a high tensile strength of TS≥980 MPa and being excellent in workability and weldability. SOLUTION: The high-strength hot-dip galvanized steel sheet has a composition comprising not less than 0.05% and less than 0.12% C, not less than 0.01% and less than 0.35% Si, 2.0-3.5% Mn, 0.001-0.020% P, 0.0001-0.0030% S, 0.005-0.1% Al, 0.0001-0.0060% N, more than 0.5% and 2.0% or less Cr, 0.01-0.50% Mo, 0.010-0.080% Ti, 0.010-0.080% Nb, and 0.0001-0.0030% B with the balance being Fe and unavoidable impurities; has a structure containing a ferrite phase having a volume fraction of 20-70% and an average crystal grain diameter of 5 μm or less; and is coated with a hot-dip galvanized layer of 20-150 g/m 2 coating weight (per one surface) on the steel sheet surface. COPYRIGHT: (C)2009,JPO&INPIT

Modern High Strength Steels for Oil and Gas Transmission Pipelines

Abstract: Increasing world demand for energy has resulted in plans to expand the oil and gas transmission pipeline infrastructure in many countries utilizing higher strength steels of API grade X70 and X80. Traditional transmission pipeline steels, up to grade X70, relied on a ferrite/pearlite microstructural design generated through traditional TMCP rolling of a niobium microalloyed C-Mn steel design. Increasing strengths up to X70 and X80 for transmission pipelines has resulted in a shift toward a ferrite/acicular ferrite microstructure designs. Traditionally, to generate the ferrite/acicular ferrite microstructure design for X70 or X80, TMCP rolling is applied to a C-Mn-Si-Mo-Nb alloy system. The Nb content is typically less than 0.070% in this alloy system. With the rising cost of alloys over the past three years, steel and pipe producers have been working with different alloy designs to reduce total costs to produce the ferrite/acicular ferrite microstructure. In recent developments it has been determined that an optimized low-C-Mn-Si-Cr-Nb alloy design (usually referred as NbCr steel), utilizing an Nb content between 0.080 – 0.11% can produce the same ferrite/acicular ferrite microstructure with either no, or minimal, use of molybdenum. This approach has been successfully used in several transmission pipeline projects such as the Cantarell, Cheyenne Plains and Rockies Express. Recognizing the success of previous projects around the world, the large ∼ 4500 Km 2nd West-East Pipeline Project specification in China has been modified to allow for the use of this NbCr design for both plate and coil for conversion to long seam or spiral pipe. The NbCr design allows the steel producer to utilize niobium’s unique ability to retard recrystallization at higher than normal TMCP rolling temperatures, hence the term for the alloy design High Temperature Processing (HTP), producing the desired ferrite/acicular ferrite microstructure with excellent strength, toughness and weldability. This paper will discuss the technical background, rolling strategy, mechanical properties, welding, specific projects, and specification modifications with practical examples.Copyright © 2008 by ASME

Heat affected zone liquation cracking in electron beam welded third generation nickel base superalloys

Abstract: The weldability of directionally solidified nickel base superalloy TMS-75 and TMS-75+C was investigated by autogenous bead-on-plate electron beam welding. The analysis of microsegregation that occurred during solidification of the as-cast alloys indicated that while W and Re segregated into the γ dendrites of both the alloys, Ta, Hf and C were rejected into the interdendritic liquid in the TMS-75+C. Heat affected zone intergranular liquation cracking was observed in both the materials and was observed to be closely associated with liquated γ–γ′ eutectic microconstituent. The TMS-75+C alloy, however, exhibited a reduced extent of HAZ cracking compared to TMS-75. Suppression of terminal solidification reaction involving non-invariant γ–γ′ eutectic transformation due to modification of primary solidification path by carbon addition is suggested to be an important factor contributing to reduced susceptibility of TMS-75+C alloy to HAZ liquation cracking relative to the TMS-75 superalloy.