Yield Strength Of Base Metal Research Articles

Postweld Heat Treatment of 15Cr-6Ni-2Mo-1Cu Supermartensitic Stainless Steel Welds

Supermartensitic stainless steels with 15Cr-6Ni- 2Mo-1Cu and 135 ksi minimum yield strength (15Cr- 135 SMSS) offer high strength and good toughness through a complex hierarchical microstructure of nanoscale precipitates, tempered martensite, and reverted austenite. Upon welding and postweld heat treatment, substantial changes to heat-affected zone microstructure and hardness may occur. Here, we reveal spatially heterogeneous microstructure and microhardness in the HAZ of 15Cr-135 SMSS; correlate these changes to measured phase transformation temperatures; and demonstrate that HAZ hardness changes depend strongly on postweld heat treatment (PWHT) temperatures. Furthermore, we demonstrate that PWHT may lead to undesired reductions in base metal yield strength and formulate a design guide for PWHT that quantifies the trade-offs in desired reductions of HAZ hardness with undesired changes to base metal yield strength. These findings have important practical implications for welding procedure design and qualification.

Development of 3-Dimensional (3D) response surfaces to maximize yield strength and elongation of InterPulsed TIG welded thin high temperature alloy sheets for jet engine applications

Alloy 718 is a high temperature nickel base superalloy, broadly used in the aerospace sector due to its exceptional strength at elevated temperature. It is more vulnerable to the segregation of alloying elements and development of laves phase in fusion zone (FZ) owing to the high heat input in Tungsten Inert Gas (TIG) welding which drastically deteriorates the yield strength and ductility of welded joints. To obviate this problem, a newly emerged InterPulsed TIG welding process, principally characterized by electromagnetic constriction and high frequency pulsation of arc, was employed to make the joints of thin Alloy 718 sheets. The three-dimensional (3D) response surface graphs were developed using Response Surface Methodology (RSM) to identify the InterPulsed TIG welding parameters giving maximum yield strength and elongation. The yield strength and elongation of joints were predicted from the parametric mathematical models. The mathematical relationships were also developed to predict yield strength and elongation from volume fraction (%) of laves phase. The InterPulsed TIG welded joints exhibited a maximum of 94.45% and 82.89% of base metal yield strength and elongation. It is attributed to the grain refinement in FZ and substantial evolvement of laves phase in finer discrete morphology.

Metallography and Computed Tomography Analysis of the Shrinkage Cavity Formed in Advanced High-Strength Steel Resistance Spot Welds

Abstract Among various defects formed in resistance spot welds (RSW), voids or cavities formed at the center of the nugget are scarcely studied. However, it is widely accepted that such cavities significantly influence the mechanical properties of the spots, especially in advanced high-strength steels (AHSS). Nonavailability of a proper characterization technique to quantify these cavities is one of the reasons for less research in this area. Although conventional metallographic analysis portrays cavities in the nugget, the associated destructive sampling results in erroneous interpretation. To date, there is no reliable technique to quantify the size, shape, and distribution of the voids in the nugget. In this context, the present study uses a computed tomography (CT) technique to visualize and quantify the cavities in spot-welded AHSS nuggets. The images obtained using a CT scan provide a new insight into the cavity and its characteristics, such as volume and surface topology. According to the CT results, the voids formed in AHSS spots increase with the increase in base metal yield strength. Furthermore, the CT results also reveal that the conventional critical diameter of 4 × thickness1/2 to obtain plug failure in RSW is not valid for AHSS spots because of the influence of the cavity in the faying interface.

Estimation of plastic eta factors for clamp-end SE(T) specimens with undermatched weld metal based on 3D finite element analyses

To ensure the corrosion resistance property of girth welds of clad or lined pipes, nickel based alloy consumable is often utilized, indicating that the yield strength of base metal is higher than that of weld metal. To test the fracture toughness of the undermatched weld metal, the plastic eta factors for clamp-end single-edge tension specimens with various configurations and different undermatched levels are determined based on detailed 3D finite element analyses. Finally, a set of equations of the plastic eta factor is developed.

Effects of material type, preheating and weld pass number on residual stress of welded steel pipes by multi-pass TIG welding (C-Mn, SUS304, SUS316)

A numerical procedure is provided to analyze temperature and residual stress fields in multi-pass TIG welds with different pipe and electrode material. Based on the ABAQUS software and FORTRAN codes, to appraise the temperature fields and the residual stresses over the TIG process, an uncoupled thermo-mechanical, finite element model (FEM) is developed. For considering volumetric thermal flux on the workpiece, the “GOLDACK” method is used. The silent element method is utilized to model the weld deposit. To validate the presented simulation model, the TIG process is performed in two passes on SUS304 stainless steel pipe. Then, the process is modeled for three and four passes to consider the effect of pass number on axial and hoop residual stresses in the outer and inner surfaces of the welded pipes. Additionally, the effects of base material and preheating temperature on the internal and external surface stresses of pipe are investigated. Results show that, the base material type has a greater effect on the tensile axial residual stress of the outer surface rather than that of the inner surface. However, the welding pass number shows no significant influence on the peak of the tensile axial residual stress on the inner surface of the pipe. In terms of preheating, the stress magnitude is reduced by raising the preheating temperature but with more intense in the inner surface of the pipe. Additionally, to keep the tensile axial residual stress below the base metal yield strength, the preheating at least up to 325 °C is essential.

Residual Stress, Microstructure and Mechanical Properties in Thick 6005A-T6 Aluminium Alloy Friction Stir Welds

Plates (37 mm thick) of 6005A-T6 aluminum alloy were butt joined by a single-sided and double-sided friction stir welding (FSW). The 3D residual stresses in the joints were determined using neutron diffraction. The microstructures were characterized by a transmission electron microscope (TEM) and electron backscatter diffraction (EBSD). In the single-sided FSW specimen, there were acceptable mechanical properties with a tensile strength of 74.4% of base metal (BM) and low residual stresses with peak magnitudes of approximately 37.5% yield strength of BM were achieved. The hardness is related to the grain size of the nugget zone (NZ), and in this study, precipitations were dissolved due to the high heat input. In the double-sided FSW specimen, there were good mechanical properties with a tensile strength of 80.8% of BM, but high residual stresses with peak magnitudes of approximately 70% yield strength of BM were obtained. The heat input by the second pass provided an aging environment for the first-pass weld zone where the dissolved phases were precipitated and residual stresses were relaxed.

Influence of Cu/Li ratio on the microstructure evolution of bobbin-tool friction stir welded Al–Cu–Li alloys

Two modern aluminum lithium alloys were welded by semi-stationary bobbin tool friction stir welding. The influence of the Cu/Li ratio on precipitation phenomena under process heat impact was investigated by comparing the response of low Cu/Li alloy 2196-T8 and high Cu/Li alloy 2060-T8. Identical process parameters with a weld pitch of one rotation per mm were used to conduct flawless weldments. The thermal history and microstructural features were studied and correlated to the resulting mechanical properties of the welds. Analysis of microstructure using differential scanning calorimetry and high energy X-ray diffraction technique showed significant differences in the precipitation sequence of the base metal and in the welded samples of the two alloys of interest. A low Cu/Li ratio led to a higher softening resulting in a reduction of 43% of base metal yield strength while the high Cu/Li ratio alloy AA 2060 could demonstrate more thermal stability (38% reduction). Severe dissolution of the T1 precipitate and presence of equilibrium phases were confirmed for the stirred zone of both alloys. The heat affected zone suffered dissolution and overaging reactions leading to a mechanically unfavorable microstructure. The low Cu/Li alloy 2196 developed a higher process temperatures and exhibited a more evolved precipitation sequence.

Effect of thermomechanical parameters on mechanical properties of base metal and heat affected zone of X65 pipeline steel weld in the presence of hydrogen

In this study, the effect of different microstructures resulting from thermochemical processes of X65 pipeline steel on mechanical properties of welding in the presence of hydrogen was investigated. According to studies conducted, due to the charge of hydrogen, the hardness of base metal (BM) increased by 10%, the hardness of heat affected zone (HAZ) samples increased by about 13%, the yield strength of base metal increased by an average of 5% and the yield strength of HAZ increased by about 7%, indicating hardening. Elongation was reduced by 32–70% and the percentage of ductile fracture decreased by an average 46% which shows that the fracture of samples in the presence of hydrogen is brittle. In the study of hydrogen defect and concentration of hydrogen permeated to the material it was determined that, the ferrite-pearlite banded microstructure of base metal has a higher hydrogen defect sensitivity than HAZ with an acicular ferrite microstructure and hard phase. In base metal samples under various rolling conditions, the two-pass rolled and the final rolling temperature of 800 °C sample (2BM800), with a smaller grain microstructure and higher interface, had the most diffusive hydrogen content and therefore had the highest hydrogen defects. In the case of HAZ samples, the effect of hydrogen on the increase in hardness and strength properties is greater than that of the base metal samples due to the different microstructure (of the acicular ferrite) and the variable grain size (i.e. the presence of two coarse-grained and fine grained areas). Overall, the results of this study showed that the microstructure has a significant effect on mechanical properties, the amount of diffusive hydrogen and hydrogen defects in welds.

Effect of treatment in weld surface on fatigue and fracture behavior of titanium alloys welded joints by vacuum electron beam welding

The effect of treatment in weld surface on four-point bend fatigue properties and fracture characteristic of Ti–6.5Al–2Zr–1Mo–1V alloy by vacuum electron beam welding were tested and analyzed. When the excess weld metal and excess penetration are removed, the high fatigue strength of joints can be obtained. When the excess weld metal and excess penetration were not removed, the fatigue strength of joints is about 40% of yield strength of base metal. Two kinds of fatigue crack initiations can be observed under the upper surface of joint, which show the obvious local tearing fracture morphology and bulk inclusions or compounds morphology.

Microstructural Properties and Four-Point Bend Fatigue Behavior of Ti-6.5Al-2Zr-1Mo-1V Welded Joints by Electron Beam Welding

Abstract With the help of a four-point-bend of fatigue rig, high-cycle fatigue tests were carried out on an Ti–6.5Al–2Zr-1Mo-1V titanium alloy at room temperature, 20 Hz and R = 0.1 in ambient air. The test results indicated that the fatigue strength of base metal, 888 MPa, is about 120% of yield strength. The fatigue strength of joints is 814 MPa. It is about 110% of yield strength of base metal. When the loading stress is higher, the fatigue failure region is located in middle weld zone of weld face, which the cracks are propagated along coarse β phase’s grain boundary. When the loading stress is lower, the fatigue failure region is located between the incomplete recrystallization zone and base metal. The crack nucleation resistance gradually increases from the WN to HAZ with the variable of loading stress and β phase (little α′ phases)→ α + β phase→ α phase.

Mechanical and metallurgical properties of DP 1000 steel square butt welded joints with GMAW

In this work, Gas Metal Arc Welding (GMAW) was used to study the influence of heat input (i.e. cooling rate) on mechanical/metallurgical properties of square butt welded joints of DP 1000 sheets. The influence of filler metals of different strengths on the mechanical properties of joints was also tested. A significant decrease in hardness was observed in the Heat Affected Zone (HAZ) due to martensite tempering, in regions where peak temperature was close to isotherm AC1 (calculated in 735 oC for these steel); coincidently, fracture in every tensile test occurred in regions where martensite was tempered, even when a wire of less strength was used. It was noticed that the decrease in ultimate tensile and yield strength of base metal was inherent to welding. When minimum heat input was used, deterioration in mechanical properties was less pronounced, once degree of HAZ softening was smaller. Elongation of joints increased with increasing heat input.

Numerical analysis of welding residual stress and distortion in laser+GMAW hybrid welding of aluminum alloy T-joint

A 3-D finite element model is developed to predict the temperature field and thermally induced residual stress and distortion in laser+GMAW hybrid welding of 6061-T6 aluminum alloy T-joint. And the characteristics of residual stress distribution and deformation are numerically investigated. In the simulation, the heat source model takes into account the effect of joint geometric shape and welding torch slant on the heat flux distribution and a sequentially coupled thermo-mechanical method is used. The calculated results show that higher residual stress is distributed in and surround the weld zone. Its peak value is very close to the yield strength of base metal. Besides, a large deformation appears in the middle and rear part of the weldment.

Local Fatigue Properties of Al-Zn-Mg Alloy Welded Joint

Local tensile properties and fatigue properties of Al-Zn-Mg alloy welded joint were investigated. Experiment results show that A7N01 aluminium alloy welded joint is highly inhomogenous both in microstructure and mechanical performances. Ultimate tensile strength and yield strength of base metal are superior compared to HAZ and weld metal. U-shape notched specimens were used in fatigue tests to study local fatigue properties of the joint. Fatigue tests results demonstrate that the difference of local fatigue life is conspicuous in the three regions. The difference of yield strength is believed to result from the diversity of fatigue life for each region in A7N01 aluminium alloy welded joint.

Effects of Different Fillers on Microstructure and Tensile Properties of Welded AA6061-T6

Several different groups of filler metal are known to be compatible in welding of AA6xxx series. The aim of this investigation is to study the effects of different filler alloys toward mechanical properties and microstructure of welded AA6061 aluminium alloy using gas metal arc welding. Single V butt joint configuration has been used for joining the plate’s. Butt-joint welds were made on 6 mm thick plates using 21 – 22 V arc voltage. Tensile test was carried out using 100kN, electro-mechanical controlled hydraulic Instron 8801 universal testing machine. The results showed that the yield strength of base metal were 330 MPa and while the yield strength of ER5356 joints and ER4043 joints were 200 MPa and 235 MPa, respectively. The difference properties of strength in both weld metal was due to the difference major element in the both filler composition. The amount of silicon content in ER4043(Al-Si5%) filler is believed to play a role in the mechanical strength on weld metal. Microstructural examination was carried out using a light optical and electron microscope. The different filler alloys give different weld metal microstructure.

Effect of inner diameter on friction torque in the friction welding of pipes

To clarify the joining mechanism of friction welding, the authors have previously investigated joining phenomena in the first phase of friction welding using solid round bars.1–5 The results obtained in these studies have led to the proposal of a method for simulation of friction torque in the first phase of friction welding with the friction welding conditions and the physical values of the base metals adopted as key parameters of interest. Results have also led to the development of a method for simple estimation of the initial torque based on the base metal yield strength at the seizure temperature of the faying surfaces.6

Overload failure curve and fatigue behavior of spot-welded specimens

The mechanical behavior of a spot-welded specimen is generally approached in angles of overload and fatigue failures. The primary issue in an overload failure is to establish an overload failure criterion. Fatigue failure of spot-welded specimens can be dealt with a fracture parameter, since a spot-weld forms a singular geometry of external crack type. In this work, we express the limit loads in terms of base metal yield strength and specimen geometries. We then present a master overload failure curve for a single spot-welded specimen in a mixed-mode load domain. The coordinates of the domain are normalized by the limit loads of single spot-welded specimens. Recasting the load vs. fatigue life relations experimentally obtained, we attempt to predict the fatigue life of various spot-weld specimens with a single parameter denoting the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and load type in an inclusive manner. The suggested fatigue life formula for a single spot-weld can be used in the assessment of spot-welded panel structures as the fatigue strength of multi-spots is eventually determined by that of each single spot-weld.

Structure, properties, and residual stress of 304L stainless steel friction stir welds

Alloy 304L stainless steel sheets, 3.2 mm thick, were joined by friction stir welding at a single welding speed using two different tool rotation rates. The resulting welds were overmatched relative to the base metal. Longitudinal residual stresses (tensile) were near the base metal yield strength.

정사각 모자형 박판튜브의 에너지흡수특성 및 최적 용접간격

In this work, energy absorption characteristics and optimal welding space of spot-welded square hat type tube are investigated via quasi-static crush experiments and finite element (FE) analyses. A FE model reflecting the crush characteristics is established based on the experimentally observed crush mechanisms of specimens with welding spaces (20, 30 & 45 mm) and (25,40 & 55 mm) respectively for two specimen widths (60, 75 mm). The established FE model is then applied to other crush models of widths (50, 60 & 75 mm) with various welding spaces (20, 25, 30, 40, 45, 55, 75, 150, 300 mm) respectively. We examine the energy absorption characteristics with respect to the welding space for each specimen width. The outcome suggests an optimal spot welding space of square hat type thin-walled tube. Energy absorption is also presented in terms of yield strength of base metal, specimen thickness, width, and mean crushing force of spot-welded square hat type thin-walled tube.

An Experimental Investigation of Limit Loads and Fatigue Properties of Spot Welded Specimens

The study on the mechanical behavior of a spot-welded specimen is largely divided into the quasi-static overload failure analysis and the fatigue failure prediction. The main issue in an overload analysis is to examine the critical loads, thereby providing a generalized overload failure criterion. As the welding spot forms a singular geometry of an external crack type, fatigue failure of spot-welded specimens can be evaluated by means of a fracture parameter. In this study, we first present the limit loads of 4 representative types of single spot-welded specimens in terms of the base metal yield strength and specimen geometries. Recasting the load vs. fatigue life relationships experimentally, obtained here, we then predict the fatigue life of spot-weld specimens with a single parameter denoted the equivalent stress intensity factor. This crack driving parameter is demonstrated to successfully describe the effects of specimen geometry and loading type in a comprehensive manner. The suggested fatigue life formula for a single spot weld can play a key, role in the design and assessment of spot-welded panel structures, in that the fatigue strength of multi-spots is eventually determined by the fatigue strength of each single spot.

Finite element analysis on relationships between the J-integral and CTOD for stationary cracks in welded tensile specimens

In this study the effects of weld strength mismatching and geometry parameters on the relationship between the J-integral and the crack tip opening displacement (CTOD) are investigated. Numerical analysis was carried out by an ABAQUS two-dimensional elastic–plastic analysis mode. The work was performed for center-cracked welded specimens with uniform tensile load. In the specimens the weld strength mismatch, M, was from 0.8 to 1.2, the crack length, a/ W, from 0.1 to 0.5, the weld width, h/ c, from 0.1 to 0.5. The main results indicate that weld strength mismatching has only a weak influence on the relationship between the J-integral and CTOD at low load levels, but there is a strong effect at high load levels. The yield strength of weld metal may be used for low load levels and the yield strength of base metal may be used for high load levels, when the basic relationship of J-integral versus CTOD is utilized to treat the problem of welded joints. The results also show that the crack size and weld width have an influence on the relationship between the J-integral and CTOD at high load levels. Because the equivalence between the J-integral and CTOD breaks down at high load levels, the relationship between J-integral and CTOD becomes more complex, and the weld strength and geometry mismatching factors must be included.