Showing papers on "Heat-affected zone published in 1995"

Microstructural changes in HSLA-100 steel thermally cycled to simulate the heat-affected zone during welding

Abstract: The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone (HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region, the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer. The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent upon peak temperature(i.e., position within the HAZ), although the cooling rate(i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.

Method of production of workpieces by welding equipment

Abstract: Starting on the surface of a foundation body (2) weld material is applied in runs to form layers by an equipment for profiling building-up welding and hence a workpiece (1) is manufactured, where the foundation body (2) may be a component of the workpiece (1). The course of runs to form layers as well as the approximate rate of application of weld material are calculated by a computer system (11), starting from a two- or three-dimensional data model of the workpiece (1) in the form of software, and given over to the system control (10) which positions the welding torch (7) by means of the robot (5) and the workpiece (1) located on a turntable (4) by a clamping mechanism (3) with respect to one another, and regulates the rate of application of weld material applied by the welding apparatus (8) in such a way that the liquid weld material adheres to the layer lying underneath without running down and irregularities in the runs to form layers are levelled out. Hence metal workpieces having manifold bodily shapes and theoretically of any size and having any thickness of wall, even consisting of different metallic materials, may be manufactured by profiling building-up welding without auxiliary cores or other devices supporting the liquid weld material.

Determining the constitutive properties of the heat-affected zone in a resistance spot weld

Abstract: A methodology is described for determining the mechanical properties of the heat-affected zone (HAZ) in a resistance spot weld. To perform detailed finite-element analyses of spot weld overload failures, it is necessary to have the mechanical properties of the various HAZ subzones. To this end, simulated HAZ material samples were fabricated in a process that utilized a resistance spot welding machine fitted with specially modified electrodes. Tensile tests at several loading rates were performed on miniature notched-bar specimens machined from the simulated HAZ material. These tests provided the empirical data from which the strain hardening capacities and strain rate sensitivities of the weld HAZ were determined through detailed finite-element analyses of the notched-bar tensile tests.

Laser welding with filler wire

Abstract: New applications such as welding of material combinations and the ability to fill opening gaps between the workpieces offer new prospects for laser beam welding processes with filler wire. To guarantee good quality, vertical distance variations between wire tip and weld pool are, above all, not permissible as this causes globular metal transfer and would accordingly result in strongly rippled, unclean welds. A process-internal signal, recorded by a sensor, helps to solve this problem. The automatic tracking of the vertical wire position is possible on-line via a controller. In this way, the running process can be optimized and a consistently good weld quality can be achieved.

Infrared Sensing for On-Line Weld Geometry Monitoring and Control

Abstract: This paper describes a method for on-line weld geometry monitoring and control using a single front-side infrared sensor. Variations in plate thickness, shielding gas composition and minor element content are known to cause weld geometry changes. These changes in the weld geometry can be distinctly detected from an analysis of temperature gradients computed from infrared data. Deviations in temperature gradients were used to control the bead width and depth of penetration during the welding process. The analytical techniques described in this paper have been used to control gas tungsten arc and gas metal arc welding processes.

Mathematical modeling of key‐hole laser welding

Abstract: Laser welding is a unique way of joining materials with less thermal distortion and minimum metallurgical damage to the workpiece. The molten pool formed during welding determines the shape of the final welded region. At high laser intensities, the molten material vaporizes and a key hole is formed during the welding process. This vapor and the shape of the molten pool affect the absorption of laser at the liquid surface. The forces generated at the liquid‐vapor interface due to surface tension gradient induce thermocapillary convection in the weld pool. This paper presents a mathematical model by considering these surface forces and the energy balance at the liquid‐vapor and solid‐liquid interfaces. The model is used to predict the surface velocity and temperature distributions, weld pool shape, key‐hole depth and diameter. The velocity field is found to be large in the radial and azimuthal directions before the key hole is formed, and it changes to a radially and axially dominant field after the formation of the key hole. The results of this model are also compared with experimental data.

A study on the effect of contact tube-to-workpiece distance on weld pool shape in gas metal arc welding

Abstract: Computer simulations of the three-dimensional heat transfer and fluid flow in gas metal arc (GMA) welding have been studied for analyzing the effect of contact tube-to-workpiece distance on the weld pool shape by considering the driving forces for weld pool convection, the electromagnetic force, the buoyancy force and the surface tension force at the weld pool surface, and also the effect of molten electrode droplets In the numerical simulation, difficulties associated with the irregular shape of the weld bead have been successfully overcome by adopting a boundary-fitted coordinate system that eliminates the analytical complexity at the weld pool and bead surface boundary The method used in this paper has the capacity to determine the weld bead and penetration profile by solving the surface equation and convection equations simultaneously The experiments are performed to show the variation of the weld bead geometry due to the change of the contact tube-to-workpiece distance The calculated weld shapes correspond well with those of experiments, and both these resu Its demonstrate that the contact of tube-to-workpiece distance exerts a considerable influence on the formation of the weld pool and the resulting weld shape by affecting the arc length and welding current

Formation of molten droplets at a consumable anode in an electric welding arc

Abstract: A theoretical dynamic model has been developed to predict the formation of molten droplets on a moving wire electrode in a gas metal welding arc. Calculations have been made of the droplet shape and size as a function of the welding current by accounting for the electromagnetic pinch effect, surface tension, gravitation and momentum transfer due to motion of the solid wire electrode. Our calculations start with an artificial cylindrical liquid column which, for low currents, develops into a droplet which is close to spherical. However, for currents above about 250 A, the magnetic pinch constricts the column such that a smaller elongated droplet is formed.

Heat treatment of welded 13%Cr-4%Ni martensitic stainless steels for sour service

Abstract: For many years, the petroleum industry has employed martensitic stainless steels for wellhead and valve applications, and increasing use has been made of 13%Cr-4%Ni alloys. This material type was originally developed as a cast alloy (e.g., ASTM A487/A487M-89a Grade CA6NM). The combination of a low-carbon content and the addition of 3.5 to 4.5% nickel produces a fine, lath martensite structure which, after a tempering heat treatment, can exhibit superior mechanical properties. Thus, CA6NM and its forged variant ASTM A182/A182M-91 F6NM find application for production fluids containing CO{sub 2} and H{sub 2}S environments, particularly when hardening occurs, as is the case with fusion welds. Sensitivity to sulfide SCC increases at high material hardness levels, and the NACE MR0175 standard limits 13%Cr-4%Ni alloys to HRC 23 maximum for sour service. Attainment of such a hardness level requires careful consideration of tempering procedure. In this paper, the roles of welding procedure, material composition and postweld heat treatment are examined in relation to producing the minimum hardness levels in the weld zone.

A Chart Method to Determine Necessary Preheat Temperature in Steel Welding

Abstract: Preheating is carried out to avoid cold cracking in steel welding. The occurrence of cold cracking is considered to be governed by accumulating diffusible hydrogen, welding residual stresses and hardness at the crack initiation site. The hydrogen accumulating at the crack site depends on the initial diffusible hydrogen content in weld metal, the weld heat input and the wall thickness. The local residual stress is governed by the weld metal yield strength, the joint restraint and the notch stress concentration factor. The HAZ hardness is influenced by the steel chemical composition, the weld heat input and the plate (wall) thickness.These influential factors affect cold cracking independenly or in an interacted manner. It must be, thus, difficult to predict the necessary preheat temperature by a theoretical method or simple formulae. The method presently proposed is completely based on the empirical results by y-groove weld cracking tests. The present method determines the necessary preheat temperature through the charts describing the following respective effects : 1) steel composition ; 2) diffusible hydrogen content of weld metal ; 3) weld heat input ; 4) wall thickness ; 5) weld metal yield strength ; 6) joint restraint. As to the steel composition, this method uses CEN carbon equivalent that preferably assesses weldability of a wide variety of steels. Also, this method considers a logarithmic dependence of the weld metal hydrogen and the analysis of hydrogen diffusion in a weld has proved that the hydrogen effect on cold cracking must be logarithmic.

Hybrid laser and arc process for welding workpieces

Abstract: A welding process in which laser radiation is trained on the seam area fod between two sheet-metal workpieces, one of which has an edge projecting above the other In addition, the arc is guided along this edge to melt the edge and contribute molten metal to the pool formed by the laser

MAG arc welding apparatus

Abstract: An MAG arc welding method and apparatus is capable of achieving a welding bead in a regular ripple pattern or in a suitable sectional form The welding power source generates the first welding current I1 and a second welding current I2 larger than the first welding current The wire melting speed is changed by switching between the first and the second welding currents The welding method or apparatus according to the invention generates the first arc length more than 2 mm and the second arc length more than the first arc length and switches between both arc length at a switching frequency F of 05 to 25 Hz The ratio of the second to the first welding currents is made to be in 103 to 120 In addition to a welding method to change the arc length by switching the first and the second welding currents at a constant wire feeding rate, the present welding method makes it possible to carry out the lap welding or butt welding even when there is a large gap The large gap requires a large amount of molten metal which is prepared by increasing the wire melting speed caused by an increase in the wire feeding rate by 5 to 20% The resultant reinforcement has a beautiful appearance

HAZ Softening and Creep Rupture Strength of High Cr Ferritic Steel Weldments

Abstract: The creep rupture strengths of welded joints of Modified 9Cr-1Mo steel and 12Cr steel tend to decrease as compared with those of base metals. Therefore, the softening behavior at the heat affected zone (HAZ) was examined by means of mainly welding thermal cycle simulation, and the improvement of the creep rupture strength of GTAW joint was investigated from the standpoint of precipitation strengthening and/or solid solution strengthening. Consequently, the maximum HAZ softening was found at the peak temperature around 900°C, which was closely associated with the lack of fine Nb and V carbonitrides coherent with the matirix in addition to the fine grained microstructure formed by recovery and polygonalization of the tempered martensite. When conducting creep rupture test for their welded joints, the fracture location shifted from the base metal or the weld metal into the softened HAZ with increasing the Larson Miller parameter.As far as the improvement of the creep rupture strength of GTAW joints is concerned, the control of morphology of Nb and V carbonitrides in the base metal prior to welding by using the thermo-mechanical control process (TMCP) was not effective. On the contrary, the creep rupture properties tended to be improved by increasing W and Cu contents similar to the case of base metal.In conclusion, since the effect of precipitation strengthening is reduced at softened HAZ, the decrease in the creep rupture strength of weldment to some extent must be taken into account, and it is of importance to improve the strength of weldment by increasing that of base metal resulting from mainly solid solution strengthening.

Ultrasonic vibration aided laser welding of Al alloys: improvement of laser welding‐quality

Abstract: Using a pulsed YAG laser, meltability of Al‐Mg and Al‐Mg‐Si alloys were investigated by a single‐pass irradiation. In order to improve the quality in laser welding, the effectiveness of the Ultrasonic Vibration Laser Welding (UVLW) method proposed in this paper was investigated experimentally. The proposed method was also compared with the traditional welding methods of Normal Laser Welding (NLW) and preHeating Laser Welding (HLW). The welding methods were evaluated from the geometry in the melt zone generated by a single pulse of the laser beam. It was suggested that ultrasonic vibration suppressed welding defects and improved the melt characteristics due to cavitation effects and dispersion of particles in the molten pool during laser welding. The influence on melt characteristics of the melt zone by preheating was also investigated. In these experiments, UVLW was the most useful laser welding method from the point of view of improving the laser welding quality of Al alloys.

Study of creep rupture strength in heat affected zone of 9Cr‐1Mo‐V‐Nb‐N steel by welding thermal cycle simulation

Abstract: Summary This paper describes an investigation of the creep rupture strength (CRS) of 9Cr‐1Mo‐V‐Nb‐N steel welded joints by welding thermal cycle simulation. The reduction in the CRS of the welded joints is successfully reproduced, and the reasons for this reduction are discussed from the perspective of fine precipitates in the matrix. The CRS of the simulated HAZ (heat affected zone) varies with the peak temperature of welding thermal cycle simulation (PT). It falls sharply beyond the Ac1 temperature, gradually reaching the minimum at the Ac3 temperature of 925 °C. At any higher temperature than Ac3, it rises steadily, showing almost the same CRS as the base metal at 1100°C. The weakest CRS of the simulated HAZ lies at the lowest limit of the CRS data band of various welded joints prepared by GTA (TIG), MMA, and submerged‐arc (SA) welding with the same base metal. The simulated HAZ is useful for evaluation and analysis of the CRS reduction of welded joints. Heating to the Ac3 temperature by welding change...

Fatigue crack initiation life prediction in high strength structural steel welded joints

Abstract: The local approach method is used to calculate the fatigue crack initiation/early crack growth lives (N i ) in high strength structural steel weldments. Weld-toe geometries, welding residual stresses and HAZ (heat affected zone) cyclic mechanical properties are taken into account in the N i estimation procedure. Fatigue crack initiation lives are calculated from either a Basquin type or a Manson-Coffin type equation. The local (HAZ) stress and strain amplitudes and the local mean stress are determined from an analysis based on the Neuber rule and the Molski-Glinka energy approach. The accuracy of the different methods is evaluated and discussed. Finally the previous methods are used with HAZ cyclic mechanical properties estimated from hardness measurements.

Method of repairing a discontinuity on a tube by welding

Abstract: The wall of a pressure vessel tube having cracks or other discontinuities is repaired by localized melting with the addition of an alloying material The melting reforms or eliminates the discontinuity while the alloying material improves the physical and chemical properties of the reformed tube wall The alloying material may be added as an insert or as a weld wire The insert may be melted in only its center section to ensure that the entire weld is properly alloyed Sections of the insert which are not melted may be expanded to be in intimate contact with the tube The weld wire feed means may be integral with the welding head A post-weld stress relief process may be applied to the welded area to reduce residual stress generated by the welding operation

Method of welding a carbon steel and an austenitic stainless steel together and resultant structure

Abstract: Method of welding a carbon steel and a austenitic stainless steel involves using high density energy beam like a laser beam or electron beam Welding using high density energy beam is effective to obtain a high precision welding For the purpose of obtaining both high precision and no cracks, and no deformation, the method of the invention controls the structure of a weld portion to be a mixed structure of an austenitic structure and not greater than 20 wt % of a ferritic structure

Ultrasonic welding of metallized plastic

Abstract: A method for ultrasonically welding two plastic parts together One part (30) has a weld joint feature (23) that is covered with a metal layer (27), and the other plastic part (25) has a complimentary weld joint feature The two parts are joined together by ultrasonically welding the two weld joint features to each other A force (40) is applied to the first and second plastic parts prior to the step of welding The force is substantially in excess of that required to cause the two weld joint features to contact each other This method creates an ultrasonic welded metallized plastic assembly Each part of the assembly has a weld joint feature, at least one of which was covered with a metal layer prior to welding Portions of the each of the two weld joint features are deformed to form a plastic weld joint through the metal coating

Explosive welding: principles and potentials

Abstract: Explosive welding is a solid-state process in which controlled explosive detonations force two or more metals together at high pressures. The resultant composite system is joined with a high-quality metallurgical bond. Explosive welding (or explosive bonding) is a high-pressure process in which contaminant surface films are plastically jetted off the base metals as a result of the collision of two metals. The time duration involved in the explosive welding event is so short that the reaction zone (or heat affected zone) between the constituent metals is microscopic. During the process, the first few atomic layers of each metal become plasma because of the high velocity of the impact (200 to 500 m/s, 660 to 1,640 ft/s.) The angle of collision causes the plasma to jet in front of the collision point, effectively scrub-cleaning both surfaces, and leaving clean metal behind.

Process for the welding of work pieces with laser beams

Abstract: A process for welding work pieces (10) with a laser beam that is focused n the work pieces (10) that are moved relative to it, and causes vaporization of the material, such that apart from the laser beam (11), an arc (12) is used in the area of the work pieces (10) that is struck by the laser beam. In order to improve the welding result, the process is undertaken in such a way that a work piece (10) with a laser-formed surface pathway that is free of dielectric substances, which pathway guides the arc (12) within a dielectric layer (15) that completely covers the remainder of the area of the weld (14), is used.

Theoretical investigation of penetration characteristics in gas metal-arc welding using finite element method

Abstract: In the modeling of the gas metal-arc (GMA) welding process, heat inputs to the workpiece by the arc and the metal transfers have been considered separately. The heat energy delivered due to the metal transfer has been approximated in the form of a cylindrical volumetric heat source, whose dimensions of the radius and the height are dependent on the molten metal droplet characteristics. The pinch instability theory (PIT) and the static force balance theory (SFBT) of drop detachment have independently been used to obtain the expressions for various characteristics of the drop,i.e., the drop radius, the drop velocity, and the drop frequency at various welding parameters. The occurrence or the nonoccurrence of finger penetration, routinely found in the GMA welding at high welding currents, has been satisfactorily explained by the cylindrical heat source model. The effect of various welding parameters,e.g., the welding current, the wire radiusetc., on the weld bead penetration characteristics has been investigated. In this modeling effort, the heat conduction equation has been solved in three dimensions.