Heat Affected Zone Material Research Articles

Stress analysis and lifetime prediction for Ti–6Al–4V welding joint under fatigue loading

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

Study on the Effect of Heat Treatment on Microstructures and High Temperature Mechanical Properties of Welding Spots of Hot Stamped Ultra-High Strength Steel Patchwork Blanks

Insufficient strength of welding spots is a common problem in the hot stamping process of ultra-high strength steel patchwork blanks (UHSSP). In this paper, the welding spots of 22MnB5 boron steel with thicknesses of 1.2 and 1.5 mm were austenitized and then air-cooled to 650–850 °C for high temperature tensile shear tests and high temperature cross-tension tests, respectively. To study the mechanical properties of the welding spots at room temperature after heat treatment, the austenitized welding spots were quenched in cold water to room temperature, and microhardness tests and microstructure observations were performed. The results indicated that compared to the original welding spots, the heat-affected softening zone disappeared after heat treatment, and the hardness values of the fusion zone, heat-affected zone and base material were basically the same, at about 500 HV. After heat treatment, the welding spots were mainly martensite. With the increase in deformation temperature, the peak loads of the tensile shear and the cross tension of the welding spots decreased. At 750 °C, the peak loads of the welding spots decreased less, energy absorption was larger, and the welding spots had the comprehensive mechanical properties of strength and ductility.

Impact Wear of Flash- and Aluminothermic-Welded Hypereutectoid Steel Rail Joints

High-speed, high-load railway development has necessitated a deeper understanding of the wear behavior and mechanism of welded steel railway joints subjected to repeated impact. Under different simulated impact loads and impact cycles, the impact damage evolution and wear mechanism of flash- and aluminothermic-welded joints in the weld and heat-affected zones and base material were compared and analyzed. The results show that the impact wear severity increased with impact cycle and load. With increasing impact load and cycle, the impact wear in the welded rail joints progressed through plastic deformation, partial pitting failure, fatigue crack propagation, and spalling from the synergistic effect of fatigue and oxidation wear. The degree of impact wear increased in the order of the softening part of the heat-affected zone, welding seam, and base material. Comparison of the wear performance of the two kinds of welded joints showed that the fatigue resistance of the flash-welded joint was higher than that of the aluminothermic-welded joint under low impact loads and cycles. In contrast, under high impact loads and cycles, the wear degree of the flash-welded joint increased sharply upon reaching a critical value and was more severe than that of the aluminothermic-welded joint.

Dynamic tensile behaviors of welded steel joint material

Q355 structural steel has been widely used in the construction industry. A typical Q355 welded steel joint consists of three portions, i.e. Q355 base metal material, heat affected zone (HAZ) material and weld metal material. Although the static material properties of the Q355 base metal and weld metal have been intensively investigated, the dynamic material properties of Q355 base metal, HAZ and weld metal are not well understood yet. The lack of understanding of the strain rate effect on the base metal, HAZ and weld metal material may lead to improper design of welded steel structure against dynamic loading. In this study, quasi-static and dynamic tensile tests on the widely used Q355 base metal, HAZ and weld metal materials were carried out to study their material properties with the strain rate up to 234.90 s−1. The failure modes were investigated and compared with the aid of scanning electron microscope (SEM) images. The strain rate effects on the tensile strength, failure strain and elastic modulus of base metal, HAZ and weld metal materials were revealed. With the testing data, empirical formulae based on the Cowper-Symonds model and Johnson-Cook model were fitted by incorporating the dynamic increase factors (DIFs) of tensile strength and dynamic stress flow, respectively. The proposed empirical formulae can be used in numerical modelling of welded steel structures under dynamic loading.

High-carbon steel: microstructure and abrasive wear resistance of heat affected zone after welding with fast cooling

The goal of this work is to estimate the abrasive wear resistance of heat affected zone (HAZ) after welding high-carbon low-alloy steel 120Mn3Si2 with fast cooling. The following benchmark data were used: microstructures of HAZ of 120Mn3Si2 steel after welding with cooling in water; abrasive wear resistance of different microstructure constituents of 120Mn3Si2 steel in two-body abrasive wear conditions. It is shown that high abrasive wear resistance of material in HAZ is provided in the vicinity of fusion line. The reason is unstable retained austenite which appears in HAZ as a result of quenching at fast cooling right after welding. The wear resistance of material in HAZ is altered by microstructural changes from austenite to austenite+martensite and finally martensite. The martensitic zone is about 0.5 mm wide and it is followed by zone of tempering of initial structure of steel. Zone of tempering is 1.5-2.0 mm wide and is the only zone of low abrasive wear resistance in HAZ of 120Mn3Si2 steel welded with fast cooling. Welding of 120Mn3Si2 steel with fast cooling in water results in welding joints which have as high wear resistant as the base material or even higher. This enables manufacturing flat welded elements with high wear-resistance and large surface area

Study on local failure conditions of nuclear containment steels

Tensile tests were performed for notched round-bar and plate specimens to acquire the local failure characteristics of SGV480 and SPV490 steels for nuclear containment structures. The notched round-bar specimens were sampled from base metals, weld joints including heat affected zone (HAZ), and simulated-HAZ materials. The simulated-HAZ materials were made using multiple heating and cooling processes with induction heating and forced air cooling. The limit strain was calculated using the Bridgman’s solution with the notch cross-sectional dimensions of the specimens, which were continuously measured with a profile projector. All notched round-bar specimens caused the tensile type ductile fracture, and there was no significant difference in the limit strains among the base metal, HAZ, and simulated-HAZ material. The limit strains were higher than the ASME VIII-2 design curve, and showed conservative results. Tensile tests using the full or partial width notched plate specimens were performed for the base metals of the two steels. In the cases of small curvature notched specimens of both full and partial widths, cracks occurred on the surface of the notch bottom and propagated into the depth of the material, until the specimens finally fractured. The limit strain plots were concentrated in a low strain and stress triaxiality position. In the cases of large curvature notched specimens of both types, no cracks occurred, and the specimens would suddenly fracture in shear after a slight shrinking of thickness. The limit strains of the notched plate specimens were lower than those of the notched round-bar specimens.

Effect of welding on microstructure and mechanical response of X100Q bainitic steel through nanoindentation, tensile, cyclic plasticity and fatigue characterisation

This paper presents an experimental characterisation of fatigue at welded connections for the next-generation high-strength low-alloy offshore riser steel, X100Q. An instrumented girth weld is conducted with a parallel programme of physical-thermal simulation (Gleeble) to develop heat affected zone (HAZ) test specimens. X100Q is shown to exhibit superior fatigue performance to the current state of the art offshore riser steel, X80. Significant differences are demonstrated between the parent material and simulated HAZ in terms of hardness, monotonic strength and cyclic plasticity response, which can be related to the observed microstructural transformations: the refined grain and bainitic block size in the fine-grained HAZ are shown to give a harder and stronger response than parent material, whereas the coarsened bainitic lath structure in the intercritical HAZ gives a softer and weaker response. The simulated HAZ materials exhibit superior fatigue performance to the parent material and weld metal. A significant reduction in life is shown for cross-weld specimens, indicating susceptibility to failure due to HAZ softening for matched or over-matched X100Q welds.

Optimizing the Shape of Welded Constructions Made through the Technique “Temper Bead Welding”

Welded constructions are subject to high stresses during operation. One solution for improving the behavior in exploitation of welded constructions in various cases is to use the welding technique “temper bead welding” (TBW). In the paper, the optimization of the geometry of the welded joints by the TBW technique was performed. Thus, corner welded joints made of S355 steel were analyzed. To make the welded joints, three layers of welding seams were deposited, and the intermediate layers were processed through cutting with various radii. To analyze the influence of the size of these rays on the behavior of welded constructions, a research program based on factorial experiences was designed. The samples were tested in terms of fatigue behavior by applying loads between ±8 kN and ±12 kN. The research also focused on determining the hardness of the materials in the joints welded and on determining the microstructure of the materials in the heat affected zone (HAZ). Research has shown that it is possible to improve the characteristics of joints made by the TBW technique in the sense that it can be achieved an improvement in fatigue stress, a decrease in the hardness of the HAZ material and an improvement in the metallographic structure of the HAZ material, meaning that it has a structure made of ferrite and fine pearlite.

Research on the corrosion behavior of double-side friction stir welded 6082Al alloy thick plate

In order to study the influences of microstructures on corrosion behavior of double-side friction stir welded 6082Al thick plate. The corrosion behavior was firstly clarified by potentiodynamic polarization and exfoliation corrosion (EXCO) tests. The microstructures before and after corrosion were identified by optical microscopy (OM), SEM and transmission electron microscope (TEM). The results showed that the corrosion resistance of heat-affected zone (HAZ) and base material (BM) was lower than those of thermo mechanically affected zone (TMAZ), top of NZ and middle of NZ. Bottom part of NZ was the most severe corroded zone. Intergranular corrosion along grain boundaries was the dominant corrosion type in BM, HAZ, TMAZ and bottom of NZ, while the top and middle part of NZ were susceptibility to pitting. The corrosion resistance decreased with the increase of weld pass. The precipitates mainly precipitated in NZ of the first pass and HAZ of both two passes during double side friction stir welding (FSW) due to the over aging process, which deteriorated the corrosion resistance.

Metal Inert Gas (MIG)–Cold Metal Transfer (CMT) and Fiber Laser–MIG Hybrid Welds for 6005A T6: Experimental and Numerical Comparison

The microstructural and hardness properties of welded joints of AA 6005-T6 aluminum alloy obtained with metal inert gas (MIG), fiber laser–MIG hybrid, and cold metal transfer (CMT) welding were analyzed immediately after welding and after 3 years of natural aging. The fine hardening precipitation within the heat-affected zone, fusion zone, and base material was characterized by differential scanning calorimetry for the three different welding techniques. The elastic–plastic properties of the welded joints were characterized by means of finite element. For all investigations, the analysis on AA6005 aluminum alloy confirms a better behavior of CMT and fiber laser–MIG hybrid joints with respect to the conventional MIG processes.

The effect of temperature distribution and cooling rate on microstructure and microhardness of laser re-melted and laser-borided carbon steels with various carbon concentrations

Laser heat treatment was arranged as a laser remelting and laser boriding. Materials with different carbon concentration were used. Three zones characterized the microstructure after laser treatment: re-melted zone (MZ), heat-affected zone (HAZ) and substrate material. The differences in carbon content and physical properties of used materials influenced the obtained microstructure, temperature distribution and hardness. In case of laser re-melted specimens, the limited martensite transformation proceeded only in C20, C45 and C90 steels. As a result of laser boriding the microstructure of MZ contained additionally iron borides. The specimens with higher dilution ratio value were characterized by higher hardness of re-melted zone. The aim of this study was to indicate the possibility of predicting the microstructure and depths of MZ and HAZ with the usage of Ashby and Esterling model. The influence of carbon concentration in steel on the thickness, dilution ratio and hardness of laser-borided layers was determined.

Experimental Study on Welding Dynamic Mechanical Properties of Q460JSC and HQ600 High Strength Steel at High Strain Rate

In order to study the high strain rate effect of the high-strength steel structure weld and its heat-affected zone material under the action of explosion, in this paper, the Q460JSC and HQ600 high-strength steel welded specimens were quasi-static and high strain rate tensile test of 1500/s to 5000/s using electronic universal testing machine and Hopkinson rod tester. The dynamic mechanical properties of high strength steel welded joints were studied. The results show that both the weld and its heat affected zone have strain rate sensitivity, and the strength and ultimate strain are higher than quasi-static under high strain rate. The center weld strength is higher than that of the base metal but the plastic deformation ability is weak. The strength and ultimate strain of the Q460JSC heat-affected zone are about 10% lower than that of the base metal, while the HQ600 high-strength steel has a smaller drop and better weldability.

Fatigue Modeling Containing Hardening Particles and Grain Orientation for Aluminum Alloy FSW Joints.

The macro-mesoscopic joint fatigue model containing hardening particles and crystal characteristics is established to study the effect of the hardening particles and the grain orientation on fatigue properties of an aluminum alloy friction stir welding (FSW) joint. The macroscopic model is composed of the weld nugget zone, thermo-mechanically affected zone, heat-affected zone, and base material, according to the metallurgical morphology and hardness distribution of the joint. Cyclic stress and strain data are used to determine the material properties. The fatigue parameters used in the calculation of cyclic stresses and strains are obtained with the four-point correlation method. The mesoscopic models of different zones are inserted into the joint macroscopic model as submodules. The models containing the information of hardening particles and grain orientation are established with crystal plasticity theory for the grains and isotropic hardening rule for the hardening particles. The effects of hardening particles and grain orientation on the stress and strain responses are discussed. The simulation results show that high-angle misorientation of adjacent grains hinders the stress transfer. The particle cluster or cracked particles intensify the stress and strain concentrations.

Algorithm for automatic fatigue crack growth simulation on welded high strength steels

Nowadays the demand for high strength steels is increasing. In order to design and develop high performance products, it is essential to understand the fatigue behavior of these materials. When considering welded components, the fatigue behavior is even more complex. The material parameters may change along the crack growth and mixed mode crack propagation may also occur. To assess welded high strength steel fatigue behavior, different welded CT specimens were tested. The Paris law material constants were obtained for the heat affected zone material. Fatigue crack growth life predictions were made using the obtained parameters and different automatic techniques. Previous work showed that the ABAQUS extended finite element method can predict fatigue crack growth, but as the implementation of the Paris law is not straight forward, to many conversions must be made and the results are too computer intensive. A simpler and more intuitive Python algorithm was developed, to enable the use of the experimental material parameters, to predict the crack propagation path. The obtain results show a good agreement with both the experimental Paris curves, and the analytical solution.

Assessment of the Mechanical Integrity of a 2 mm AA6060-T6 Butt Weld Produced Using the Hybrid Metal Extrusion & Bonding (HYB) Process – Part II: Tensile Test Results

The present investigation is concerned with solid state butt welding of 2 mm thin AA6060-T6 profiles at room temperature (RT), using the HYB process and AA6082-T4 as filler material (FM). In Part II of this investigation the tensile properties of the butt weld have been evaluated as a follow-up of the three-point bend testing done in Part I. In the HYB case the weld can be sub-divided into two distinct zones, i.e. the Extrusion Zone (EZ) and the Heat-Affected Zone (HAZ). The EZ, which is a composite material consisting of a mix of a strong FM and a soft HAZ material, displays superior tensile properties compared to the HAZ. By allowing for this in the subsequent finite element (FE) simulations of the load-bearing capacity of the joint, the observed stress-strain response of the HYB weld has been fully reproduced and rationalized. These results show that the tensile properties are comparable with those reported for FSW of 2 mm AA6082-T6 profiles and surpass those reported for FSW of 2 mm 6061-T6 profiles as far as the yield strength is concerned.

Effect of heat input in pulsed Nd:YAG laser welding of titanium alloy (Ti6Al4V) on microstructure and mechanical properties

In the present investigation, titanium alloy Ti6Al4V of 1.4 mm thickness has been laser-welded in butt joint configuration using pulsed Nd:YAG laser system. The effects of heat input on weld bead shape, fusion zone width (top, middle, and bottom), heat-affected zone (HAZ) width (top, middle, and bottom), and fusion zone area have been studied. The microstructure and mechanical properties of laser-welded specimens at various heat inputs (43.7–103.5 J/mm) have also been investigated. Microstructures of the fusion zone, HAZ, and parent material have been compared at various heat inputs using optical microscope and field emission scanning electron microscope (FESEM). The mechanical properties such as microhardness and tensile strength of the welded joints at varying heat inputs have been studied. Tensile tests of the welded specimen and base metal have been conducted for analyzing ultimate tensile strength and percentage elongation. Surface topography of the tensile fractured specimen of the welded joints and base metal has been examined to analyze the ductile and brittle behavior. EDS analyses of base metal and fusion zone of the welded specimen have been studied. XRD of the as-received base metal and welded specimen have been measured in the range of 30 to 85° to study the crystallographic structure.

Study of fracture resistance of a weldment with a propagating crack

The overall fracture resistance of a weldment with a propagating crack across the weld, HAZ and base material has been evaluated for a AISI type 316LN stainless steel (SS), based on coupled experimental-finite element analysis. Compact type (CT) specimens with initial crack tip in the weld, heat affected zone (HAZ) and base material have been tested to evaluate J-integral resistance curve (J-R curve) across the weldment. Towards evaluating the J-integral for a heterogeneous material, elastic component of J was determined using equivalent Young's modulus (Eeq) in terms of instantaneous compliance and crack depths. Simultaneously, finite element (FE) analyses have been carried out to estimate J-integrals for various contours across the weldment and for nodal singularities along interfaces, leading to a modified J-integral for overall weldment. The reasonable agreement among experimental J values from different regions with the modified J-integral values gives credence to this procedure to predict overall fracture resistance across the weldment over a heterogeneous microstructural gradient.

Microstructure, mechanical properties and stress corrosion behavior of friction stir welded joint of Al–Mg–Si alloy extrusion

Microstructure, mechanical properties and stress corrosion behavior of friction stir welded (FSWed) Al–Mg–Si alloy were investigated. The average grain sizes of shoulder-affected zone (SAZ), nugget zone (NZ), heat-affected zone (HAZ) and base material (BM) are 6.03, 4.80, 168.30 and 127.24 μm, respectively. The thermo-mechanically affected zone (TMAZ), which is generated on the edge position between HAZ and weld nugget zone, has a narrow width of 400 μm. The ultimate tensile strength (UTS) of FSWed joint is 232.20 MPa, about 91.04% with respect to that of base material of 255.06 MPa, and the joint fracture occurs at HAZ on advancing side (AS). The FSWed joint is more susceptive to stress corrosion cracking (SCC) than base material, and the SCC susceptibility increases with the rise in temperature. The residual UTS of FSWed joints in constant loaded tests at the load levels of 90, 105 and 120 MPa is 89.97%, 67.50% and 54.75% of the UST of FSWed joint in air, respectively. The increase of the load in constant loaded tests and four-point beam-bent tests accelerates the SCC of FSWed joints.

A precipitate evolution-based continuum damage mechanics model of creep behaviour in welded 9Cr steel at high temperature

A multiaxial, physically based, continuum damage mechanics methodology for creep of welded 9Cr steels is presented, incorporating a multiple precipitate-type state variable, which simulates the effects of strain- and temperature-induced coarsening kinematics. Precipitate volume fraction and initial diameter for carbide and carbo-nitride precipitate types are key microstructural variables controlling time to failure in the model. The heat-affected zone material is simulated explicitly utilising measured microstructural data, allowing detailed investigation of failure mechanisms. Failure is shown to be controlled by a combination of microstructural degradation and Kachanov-type damage for the formation and growth of creep cavities. Comparisons with experimental data demonstrate the accuracy of this model for P91 material.

Investigation on size tolerance of pore defect of girth weld pipe.

Welding quality control is an important parameter for safe operation of oil and gas pipes, especially for high-strength steel pipes. Size control of welding defect is a bottleneck problem for current pipe construction. As a key part of construction procedure for butt-welding of pipes, pore defects in girth weld is difficult to ignore. A three-dimensional non-linear finite element numerical model is established to study applicability of size control indices based on groove shape and softening phenomenon of material in heat-affected zone of practical pipe girth weld. Taking design criteria of pipe as the basis, basic tensile, extremely tensile and extremely compressive loading conditions are determined for pipe stress analysis, and failure criteria based on flow stress is employed to perform stress analysis for pipe girth weld with pore defect. Results show that pipe girth welding stresses of pores at various radial locations are similar. Whereas, stress for pores of different sharpness varied significantly. Besides, tolerance capability of API 5L X90 grade pipe to pore defect of girth weld is lower than that of API 5L X80 grade pipe, and size control index of 3 mm related to pore defect in current standards is applicable to API 5L X80 and X90 grade girth welded pipes with radially non-sharp pore defects.