Search

This research investigates the feasibility of using the thermoelectric power to monitor the thermal embrittlement in 2507 super duplex stainless steel (SDSS) exposed to a temperature between 280 °C and 500 °C. It is well known that the precipitation of Cr-rich α′ phase as a result of the spinodal decomposition is the major cause of the embrittlement and the loss of corrosion resistance in this material. The specimens are thermally aged at 475 °C for different holding times. A series of mechanical testing including the tensile test, Vickers microhardness measurement, and Charpy impact test are conducted to determine the property changes with holding time due to the embrittlement. The mechanical strengths and ferrite hardness exhibit very similar trends. Scanning electron microscopy images of impact-fractured surfaces reveal a ductile to brittle transition in the fracture mode as direct evidence of the embrittlement. It is shown that the thermoelectric power is highly sensitive to the thermal embrittlement and has an excellent linear correlation with the ferrite hardness. This paper, therefore, demonstrates that the thermoelectric power is an excellent nondestructive evaluation technique for detecting and evaluating the 475 °C embrittlement of field 2507 SDSS structures. Keywords: Thermoelectric power, Super duplex stainless steel, 475 °C embrittlement, Nondestructive evaluation

Dual phase steel DP980 sheets were joined by resistance spot welding (RSW) process. Mechanical resistance of the welds was characterized by microhardness, tensile shear and fatigue tests. A significant hardness decreases was observed in the RSW lap joints with respect to the base material, which was attributed to phase transformations during the heating and cooling of the steel. Fatigue Wohler curves using a fixed load ratio of 0.1 were obtained. It was found that the spot weld at the nugget interface close to the fusion zone induced a critical stress concentration factor, which decreased the fatigue life of the joints in the as-welded condition. Failure of the welds was initiated at the interface between welded sheets. Two predominant fatigue fracture modes were observed associated with mode I/III cyclic loading, which were correlated with the fatigue crack initiation and propagation stages. Compressive residual stresses were induced by a loading-unloading cycle on the spot welds, which tends to increase the fatigue life of the joints when compared to the as-welded condition.

Heat moving source models along with transient heat analysis by finite element method were used to determine weld thermal cycles and isothermal sections obtained from the application of a gas tungsten arc welding beads on Inconel 718 plates. Analytical (Rosenthal's thick plate model) and finite element results show an acceptable approximation with the experimental weld thermal cycles. The isothermal sections determined by numerical simulation show a better approximation with the experimental welding profile for double-ellipse model heat distribution than Gauss model. To analyze the microstructural transformation produced by different cooling rates in the fusion and heat affected zones, Vickers microhardness measurements (profile and mapping representation) were conducted. A hardness decrement for the heat affected zone (∼200 HV0.2) and fusion zone (∼240 HV0.2) in comparison with base material (∼350 HV0.2) was observed. This behavior has been attributed to the heterogeneous solubilization process of the γ″ phase (nickel matrix), which, according to the continuous-cooling−transformation curve, produced the Laves phase, δ and MC transition phases, generating a loss in hardness close to the fusion zone.

The fatigue and crack growth behavior of Inconel 718 and AL6XN alloys joined with an ERNiFeCr-2 filler by the gas tungsten arc welding process was evaluated. A comparison between Wohler curves for the monolithic IN718 and AL6XN alloys and the IN718-AL6XN dissimilar welded joint were obtained. Although Vickers microhardness profiles revealed the presence of a soft region in the heat affected zone of the IN718 base material, the fatigue crack nucleated at the fusion zone near to the AL6XN and propagated across the weld bead. The welded joint and the AL6XN base material presented a similar high cycle fatigue behavior despite the presence of an unmixed zone at the AL6XN/weld metal interface. The results of the fatigue crack growth test indicated that the weld metal has the highest growing rate owing to the presence of brittle secondary phases such as Laves and NbC. The fatigue crack growth rates obtained from experimental data were compared with measurements of the distance between striations on the fracture surfaces, both were very close.

The modified indirect electric arc welding (MIEA) and the conventional gas metal arc welding (GMAW) processes were used to weld 7075-T651 aluminum alloy plates. The fatigue life behavior was studied for three different zones in the welded joints (base metal, heat affected zone and weld metal). Compact-type (CT) specimens manufactured from the 7075-T651 aluminum alloy welded plates were subjected to fatigue tests with constant amplitude cyclic loading. The CT specimens corresponding to MIEA weld metal shown the largest fatigue life (number of cycles to reach the stable crack propagation), which was four times larger than the weld metal obtained by GMAW. This improvement was attributed to the high tensile residual stresses induced by the application of three welding beads of the GMAW process in contrast with the stresses induced by the application of only one welding bead in the MIEA. The tensile residual stresses were of no consequence in terms of fatigue life for the heat affected zone (HAZ). At this zone, the microstructural transformation (soft zone formation in the HAZ) induces a similar fatigue life behavior. For the weld metal produced by the MIEA welding process, the fatigue crack growth tends to increase in comparison with the GMAW, whereas for the HAZ no difference was detected. Fracture surfaces of all materials were analyzed by using scanning electron microscopy to correlate the fatigue crack growth condition with the striations formation.

Thermal cycles and isothermal sections were determined analytically by using the Rosenthal thin plate model, as well as by means of finite element method. The simulation model provides an acceptable approximation to thermal cycles measured during the welding process. A discrepancy for the analytical model with respect to the experimental results was found. Due to the thermal effect a hardness decrement of 43% (60 HV 0.1 ) with respect to the base material (107 HV 0.1 ) at a distance of 18 mm from the center of the welding bead was observed. The weld thermal cycle measured experimentally at this distance traverses the C-curve for the β′ precipitation phase at a peak temperature of 380 °C, with a dwell-time of approximately 10 s. The presence of rod-shape β′ precipitates was observed on a transmission electron microscopy image. A correlation between isothermal sections and the hardness distribution for different zones of the welded joint was identified.

By using an instrumented impact pendulum, the force versus time curves of 7075-T651 aluminum welds were obtained from standard Charpy-V samples. Considering the force–time curves and constant impact velocity, the fracture energies for different zones were quantified. A fracture energy improvement for the HAZ (33.6 J) was observed in comparison with the weld metal (7.88 J), and base metal (5.37 J and 7.37 J for longitudinal and transverse directions, respectively). This toughness increment was attributed to the microstructural transformation caused by the thermodynamic instability of η′ precipitates during the welding. Fracture energy for weld metal was higher than that for base metal, probably due to pores created during solidification. Regarding the dynamic yielding force obtained from the force–time curves, an approximation to the dynamic yield strength for weld, HAZ and base metal was determined. Fracture surfaces revealed an intergranular failure for base metal in longitudinal direction, whereas a predominately brittle failure (cleavage) with some insights of ductile characteristics was observed for the transverse direction. In contrast, a ductile failure was observed for weld metal and HAZ.

The influence of variable load spectra in the fatigue crack growth (FCG) was analyzed. A 4.1% interference was pre-set to some specimens by an expansion operation. As-drilled hole and hole expanded specimens were subjected to FCG tests under a combination of baseline cyclic loading and periodic overloads or underloads applied. Fatigue life was of approximately 1 million of cycles for the as-drilled hole, and 3.5 million of cycles for the hole cold expanded. The pre-setting operation and variable load spectra interaction exhibited a synergy, which was associated with multiple periods of retardation and arresting observed in the crack growth rate.

Compact test specimens were extracted from a 6061-T6 aluminum alloy welded plate with a thickness of 9 mm to analyze the cold hole expansion effect on fatigue crack growth tests conducted in mode I cyclic loading. At R = 0.1, a sharp crack in base metal, weld metal and heat affected zone was propagated from 17 to 24 mm. The fatigue crack growth at 24 mm ( α = a / W = 0.3) was delayed by drilling a hole at the crack tip and applying a cold hole expansion of 4.1%. The residual stress fields due to cold hole expansion were determined with the finite element method. The fatigue crack growth testing was continued up to a crack length of 35 mm ( α ∼ 0.43) at the same R , and crack opening displacements of the post-expansion crack were also determined with the finite element method. The results were expressed in terms of crack length versus number of cycles, as well as, fatigue crack growth rate as a function of applied and effective stress intensity factor range. The cold hole expansion contributed to delay the fatigue crack growth in base metal, and to a lesser extent in the weld metal and heat affected zone. A crack closure effect was determined by means of load versus crack opening displacement curves of the post-expansion crack, which was, completely or partially closed, in welded zones with compressive residual stress fields. The fracture surfaces of each welded zone were analyzed to elucidate the crack nucleation zone and its relation with the residual stress field. In all cases the crack was initiated at the surface of the specimen where the residual stresses were positive.

This paper is concerned with the effect of strain rate on the tensile properties of 6061-T6 aluminum alloy welds obtained by a novel welding method called the modified indirect electric arc technique. Experimental impact tests in welded joints and base metal were carried out by means of an inertial flywheel at a constant speed of 10 m s −1 . From the force-time evolution, dynamic behavior and impact fracture energy were determined in the case of base metal and welded joints. Additionally, by means of an ultra-high speed camera synchronized to the force signal, and employing digital image correlation method and monitoring markers, the true strain-time curves were obtained. These results enabled determination of the stress-strain behavior under dynamic conditions for the base metal and welded joints, which, in turn, were compared with those reported in quasi-static conditions. The strain rate does not have a strong influence on the mechanical properties of yield and tensile stress, contrasted with energy absorption capacity, which increased significantly. Finally, the effect of strain rate on the fracture surfaces was investigated.

The microstructural transformation of ferrite into secondary austenite and sigma phase during long term exposure to high-temperatures (650–900 °C) in a 2205 duplex stainless steel has been investigated using the thermoelectric power (TEP) technique, scanning electron microscopy (SEM), Charpy-impact test (CIT), equivalent ferrite and sigma phases content measurements. The possibility of using the TEP coefficient as a nondestructive assessment technique to characterize the aging kinetics of 2205 duplex stainless steel is discussed. Experimental results indicate that TEP coefficient is sensitive to the gradual microstructural transformation of ferrite phase experienced by the 2205 duplex stainless steel during the aging treatments.

International audience; The local mechanical properties of a weld zone, in a 6061-T6 aluminium alloy subjected to the modified indirect electric arc technique have been studied. The mechanical properties of the base metal, the weld metal and the heat affected zone were determined by means of usual and instrumented indentation testing, as well as micro-traction testing. To analyse the heat input effect resulting from the welding process, the evolution of the weld zone size was evaluated by means of classical indentation under a constant applied load. The results were presented using a Vickers hardness map representation. This allows monitoring exact hardness variation while leading to the identification of the different zones of the welded joint. Instrumented indentation testing was carried out to determine the local mechanical properties, such as the yield stress, the bulk modulus and the strain-hardening exponent. Obtained results are compared to those derived from tensile tests conducted on micro-specimen cuts taken from the weld zone. It was observed that yield stress values are directly comparable for indentation and micro-traction experiments. As for the elastic properties, no comparison was possible since the bulk modulus is measured by indentation, whereas it is the Young's modulus by tensile test. The micro-traction testing seems to be more sensitive to represent the work hardening of a material since the corresponding exponent is found to be constant by instrumented indentation.

Plates of Al-6061-T6 with a thickness of 12.7 mm were welded with a non-heat treatable ER4043 electrode in four welding passes using the typical single V groove butt joint (at room temperature) and in one pass using the novel modified indirect electric arc (MIEA) joint (at 50, 100 and 150 C). Microstructural characterization of the weld metal revealed that the MIEA joint design yielded microstructures in which columnar growth was restricted and a grain refining effect occurred without the addition of inoculants, even with preheating of the joints. The self-refining effect in the MIEA joint was discussed in terms of the cooling behavior and the large dilution of base material. Vickers microhardness profiles revealed that the high dilution rates obtained with the MIEA joint caused alloying of the weld pool with magnesium producing a weld metal composition sensible to heat treating due to the precipitation of Mg2Si precipitates. The grain self-refining effect of the MIEA joint along with alloying of the weld metal were found to have a profound impact on the mechanical properties of the welded joints.