Roseana da Exaltação Trevisan

An SEM-Based Method for the Evaluation of the Cavitation Erosion Behavior of Materials

The ultrasonically induced vibratory cavitation test is used to compare the performance of materials against the cavitation erosion. This article describes a new method to evaluate the results of such tests, using scanning electron microscopy.

Analytical solutions for heat flow in multiple pass welding

Abstract The purpose of this paper is to present analytical solutions for the heat distribution in the base metal during and after serial heating in multiple pass arc welding. Recent literature on welding heat transfer has covered extensively numerical models for different aspects of the process. Most of these aspects are non-linear problems in heat flow, e.g. varying thermal properties, inclusion of radiative transfer in the arc, and/or convective flow in the welding pool. The basic analytical models in use were established long ago in classical papers, but these refer to single heat cycles. The present paper addresses the circumstances of multiple cycles. In these cases, depending on the bead length and the interpass time, as shown by the proposed solution, the thermal effects of each successive pass will accumulate. When modelling the heat transfer in each pass, the effects of previous passes enter into the differential equations as non-homogeneous initial conditions. The complexity introduced by these conditions requires special treatment: an analytical solution employing the Green’s function method was used. The main focus in this work is on welding of thin plates. Simple changes to the models take account of the convective heat loss to the surroundings and enable temperature dependent properties to be calculated. The solutions were checked against experimental results obtained from a specially designed laboratory setup. Thermal cycles provided by the analytical solutions compare well with temperature histories measured at different locations during three pass gas metal arc welding of a 0.5 in (∼1 cm) AISI 304 stainless steel plate. Measured data and model results show very good agreement. The analytical solutions are also extended to the geometries of moderately thick plates.

Fracture modes and acoustic emission characteristics of hydrogen-assisted cracking in high-strength low-alloy steel weldment

The aim of the present paper is to study the relationship between the fracture modes in hydrogen-assisted cracking (HAC) in microalloied steel and the emission of acoustic signals during the fracturing process. For this reason, a flux-cored arc weld (FCAW) was used in a high-strength low-alloy steel. The consumable used were the commercially available AWS E120T5-K4 and had a diameter of 1.6 mm. Two different shielding gases were used (CO2 and CO2+5% H2) to obtain complete phenomenon characterization. The implant test was applied with three levels of restriction stresses. An acoustic emission measurement system (AEMS) was coupled to the implant test apparatus. The output signal from the acoustic emission sensor was passed through an electronic amplifier and processed by a root mean square (RMS) voltage converter. Fracture surfaces were examined by scanning electron microscopy (SEM) and image analysis. Fracture modes were related with the intensity, the energy and the number of the peaks of the acoustic emission signal. The shielding gas CO2+5% H2 proved to be very useful in the experiments. Basically, three different fracture modes were identified in terms of fracture appearance: microvoid coalescence (MVC), intergranular (IG) and quasi-cleavage (QC). The results show that each mode of fracture presents a characteristic acoustic signal.

Influence of preheating on API 5L-X80 pipeline joint welding with self-shielded flux-cored wire

Abstract The present work refers to the characterization of the mechanical properties of API 5L-X80 pipeline joints welded with self-shielded flux-cored wire. This process was evaluated under preheating conditions, with a uniform and steady heat input. All joints were welded in a flat position (IG), with the pipe turning and the torch still. Tube dimensions were 762 mm in external diameter and 16 mm in thickness. Welds were applied on single V-groove, with six weld beads, along with three levels of preheating temperatures (room temperature, 100°C and 160°C). These temperatures were maintained as the inter-pass temperature. The filler metal E71T8-K6, whose mechanical properties differ from those of the parent metal, was used in undermatched conditions. The weld characterization is presented according to the mechanical test results for tensile strength, hardness and impact testing. These tests were conducted according to API 1104, AWS and ASTM standards. API 1104 and API 5L standards were used as screening criteria. Based upon the results obtained, it was possible to note that it is appropriate to weld API 5L-X80 steel piping with self-shielded flux-cored wire, in conformance with API standards and with no need to use a preheating temperature.

Stress Relaxation in Aluminum Welding Using Ultrasonic Method

Stress relaxation of welded joints can increase the safety of structures. Although it is not absolutely clear why it happens, several researches have proved the phenomenon. This work presents the application of the ultrasonic technique to evaluate the stress relaxation in welds of 5052 aluminum. A special geometry was developed, so each welded plate had uni-axial stresses in the measurement region. Twenty-two plates were tested, eleven in the rolling direction and the remaining in the perpendicular direction. The plates were stress relief using a heat treatment to set an initial stress free reference state. After that, they were welded to create the stress field in the area of investigation. A final step was to cut each of the samples after different periods of time from the welding. The stress was measured in the uni-axial stress region. A calendar of the process was established to record precisely the time for each step. Longitudinal critically refracted waves were used to measure the stresses. The results showed a clear effect of relaxation. The magnitude was not as pronounced as in steel, but kept the same relation with the ultimate strength of material.Copyright

Evaluation of the Rolling Direction Effect in the Acoustoelastic Properties for API 5L X70 Steel Used in Pipelines

Ultrasonic evaluation of stresses has experienced great development in the last decade, mostly because of the new electronic instruments and high-speed data acquisition systems now available. Past applications have been applied mostly using shear waves and the birefringence technique. Longitudinal critically refracted (skimming) waves have proved to be more sensitive to stress variation, and have been applied recently to oil steel pipelines. Oil pipe failures have caused a lot of environmental damage in Brazil. The environmental and economical costs are very high. This work describes part of a research effort to further apply longitudinal critically refracted waves to inspect the welded region in pipes in Brazil through a study of the acoustoelastic constants. Twenty-eight samples were cut from a flat plate of the oil industry class steel (API 5L X70) which would be typically formed into pipe. The samples were prepared for tensile testing. Half of those were cut from what would be the longitudinal direction of the pipe, and the other half from the perpendicular direction. The samples are bars of 760×70×10.8-mm3 . Some bars were stress relieved before the test. The results show that there are significant differences between the acoustoelastic constants for the rolling direction and the perpendicular direction. It is clearly important to have the correct value for the acoustoelastic coefficient when applying the technique to stress measurement. There is a minor, but important influence of the stress relief process.Copyright

Susceptibility to hydrogen-induced cracking in H2S corrosion environment of API 5L-X80 welding metal

The susceptibility to hydrogen-induced cracking in hydrogen sulphide (H2S) environment of welded API X80 steel was studied. The flux-cored arc welding process was employed with E71-T1 and E71-T8K6 wires. The welding parameters were kept constant, but the samples were welded using different preheat temperatures (room temperature and 100°C). The gapped bead-on-plate (G-BOP) test was used. The specimens of modified G-BOP tests were exposed to an environment saturated with H2S, as recommended by the NACE TM0284 standard. The weld beads were characterized by optical microscopy and the level of residual hydrogen in the samples was measured. The fracture surface areas of hydrogen-induced cracking were calculated and the fracture mode was discussed. It was found that the preheating temperature of 100°C was enough to avoid cracking, even in the presence of H2S. It was also found that the E71-T8K6 wire was more susceptible to cracking, and the typical mixed-mode fracture was predominant in all samples.

Effect of Interpass Temperature on Morphology, Microstructure and Microhardness of Welded API 5L X65 Steel

The overwhelming bulk of oil and gas in pipeline construction is done by welding the individual joints of pipe together. In a broad sense, welding is a metal-joining process wherein coalescence is produced by heating to a suitable temperature. In pipeline construction, this temperature has to be sufficient to render fusion of the joint. The mechanical and metallurgical properties and distortions usually present in weld structures are strongly influenced by preheating and interpass temperatures that are applied during the welding process. Basically, interpass temperatures depend on two factors: composition of the material and cooling rate. It is very important to choose the correct interpass temperatures, however, this is not a completely dominating matter. The objective of this paper is to present a study on the effect of different interpass temperatures on morphology, microstructure and consequently on microhardness of welded API 5L X65 steel. The welds were deposited by a Flux Cored Arc Welding Process and the heat input was held constant during all welding production. The interpass temperatures were calculated by different methods. Such temperatures were later verified experimentally. Temperature data were collected via a data acquisition system. The geometry and microstructure characterizations were performed via light optical microscopy and image analysis. These data were related to the different thermal cycles obtained. The results showed that the morphology, the microstructure and the microhardness of welded API 5L X65 steel were strongly influenced by the interpass temperature, revealing how important it is to choose the appropriate value.Copyright

Use of the implant test and acoustic emission technique to investigate hydrogen‐assisted cracking in the fusion zone of welded HSLA‐80 steel

Summary Hydrogen‐assisted cracking in the fusion zone (FZ) was studied, using flux‐cored wire (AWS E 70T5 and AWS E 120 T5‐K4) and a CO2 + 5% H2 gas mixture to induce high values of diffusible hydrogen in high strength, low alloy steel (HSLA‐80) welds. An acoustic emission measurement system (AEMS) based on a root mean square (RMS) voltmeter was coupled to the implant test apparatus (NF 89–100) to determine energy, amplitude and number of signals. Using optical microscopy and scanning electron microscopy (SEM), it was observed that, in all tests, cracks were initiated in the partially molten zone and propagated in the coarse‐grained region of the heat affected zone (HAZ) when E 70 T5 wire was used and quasi‐cleavage (QC) fracture mode was predominant. When E 120 T5 K.4 wire was used, the cracks were propagated vertically across the FZ and the mixed fracture mode was predominant. A significant relationship between acoustic emission (AE) parameters and fracture modes was found in the cracking mechanism.

Arc fusion of self-fluxed nickel alloys

Self-fluxed nickel alloys are usually flame fused after thermal spraying. However, due to the practical aspects of high temperatures reached during flame fusing, large structures such as the hydraulic turbines for power generation, can not be efficiently coated. An alternative is to fuse the sprayed coating with a gas tungsten electric arc. In this case, heating is much more intensive and substrate temperature during and after the fusing operation is much lower, thus reducing the possibility that any problem will occur. In this work, coatings of self-fluxed nickel alloy fused by flame and gas tungsten arc were evaluated as protection of hydraulic turbines against cavitational damage. Several tests were performed, including the ASTM ultrasonically vibration-induced cavitation, optical and scanning electronic microscopic metallography, and hardness tests. The results showed that the arc-fused coating presented better cavitation damage resistance, probably due to its finer microstructure. A field application of this new technique is also described. A self-fluxed Ni alloy was flame sprayed in critical regions of Francis-type hydraulic turbine blades and fused by a gas tungsten arc after spraying. The blades will be inspected during the next two years.