Microstructure Of Weld Metal Research Articles

Effect of TiO<SUB align="right">2 enriched fluxes on the bead geometry, grain size and hardness in submerged arc welds

This work is mainly concerned about the influence of titanium dioxide powder addition into flux on the bead geometry, grain size, and hardness in submerged arc weld of low alloy steel plates. Titanium dioxide powders were mixed with the commercial fluxes in a different proportion of 2.5, 5, 7.5, 10 and 12.5%. The welding process parameters were kept constant for different welding conditions. Bead geometry parameters show a significant improvement due to the titanium dioxide addition. 5% of titanium dioxide enrichment was showing the best result for the bead geometry parameters. Nucleation of the acicular ferrite and grain refinement of weld metal (WM) microstructure has been observed with the enrichment of titanium. Average grain size of ferrite and pearlite decreases with the increase of titanium content. Phase analysis of WMs shows that the ferrite% increased and pearlite% decreased with respect to titanium content. But, the increase of titanium content was not showing any clear trend on the hardness profile of weldments.

X‐Ray Diffraction Technique for Residual Stress Measurement in NiCrMo Alloy Weld Metal

In the present work, residual stresses in nickel‐based (Ni 625) superalloy weld metal of a 9%Ni steel‐welded joint were measured by X‐ray diffraction (XRD). This technique presents some difficulties in performing measurements in coarse and preferentially oriented weld metal microstructures. It is proposed a preliminary surface treatment by rotating steel wire brushing to perform the stress analysis through XRD technique possible for this kind of material. Stress measurements with proposed XRD technique showed that the stress state in Ni 625 weld metal on the outside surface of the welded joint is characterised by tensile stresses in the transverse and longitudinal directions, while compressive transverse and tensile longitudinal residual stresses are developed in the root pass region.

Pitting Corrosion of the Resistance Welding Joints of Stainless Steel Ventilation Grille Operated in Swimming Pool Environment

This work focuses on the pitting corrosion of ventilation grilles operated in swimming pool environments. The ventilation grille was made by resistance welding of stainless steel rods. Based on the macroscopic and microscopic examinations, the mechanism of the pitting corrosion was confirmed. Chemical composition microanalysis of sediments as well as base metal using scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS) method was carried out. The weldments did not meet the operating conditions of the swimming pool environment. The wear due to the pitting corrosion was identified in heat affected zones of stainless steel weldment and was more severe than the corrosion of base metal. The low quality finish of the joints and influence of the welding process on the weld metal microstructure lead to accelerated deposition of corrosion effecting elements such as chlorine.

Welding of Al-Mg aluminum alloy to aluminum clad steel sheet using pulsed gas tungsten arc process

Al-Mg aluminum alloy was lap joined to aluminum clad steel sheet using pulsed gas tungsten arc welding process and Al-Si filler metal. The effects of the welding heat-input were investigated on the joint microstructure and mechanical properties. Weld metal microstructure, formation of intermetallic compounds (IMCs) at the joint interface and the fracture locations were studied using stereo, optical and scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS). The joint strength of the welds was evaluated by shear-tensile test. The results showed that presence of a thin aluminum clad layer with 350 μm thickness drastically decreased the Al-Fe intermetallic thickness at the weld seam/steel interface to less than 2.5 μm. The joint strength increased with enhancement of the heat-input up to an optimum value and then decreased beyond it. This behaviour was justified in the light of the contradictory effects of the weld metal microstructure and adhesion of the weld metal to the aluminum clad layer near the weld root. In the optimum heat-input of 250 J mm−1, the joint strength reached to ∼90% of Al-Mg aluminum base metal strength. In all the welds, fracture path had an angle of 75 ± 3° with respect to the horizontal base plane. Stress analysis in the weld showed that fracture in the joint was controlled predominantly by the maximum normal stress rather than the maximum shear or von Mises effective stress.

The interface microstructure, mechanical properties and corrosion resistance of dissimilar joints during multipass laser welding for nuclear power plants

This study presents the interface microstructure, mechanical properties and corrosion resistance of dissimilar joints between Inconel 52M overlays and 316L stainless steel during multipass laser welding for nuclear power plants. The results indicate that the microstructure at the interface beside 316L stainless steel consists of cellular with the width of 30–40 μm, which also exhibits numerous Cr and Mo-rich precipitates like flocculent structure and in chains along grain boundaries as a mixed chemical solution for etching. Many dendritic structure with local melting characteristics and Nb-rich precipitates are exhibited at the interface beside Inconel 52M overlays. Such Nb-rich precipitates at the interface beside Inconel 52M overlays deteriorate the tensile strength and toughness of dissimilar joints at room temperature. The tensile strength of 316L stainless steel at 350 °C significantly decreases with the result that dissimilar joints are fractured in 316L stainless steel. The correlation between corrosion behavior and microstructure of weld metals is also discussed. The difference in high corrosion potential between Nb-rich precipitates and the matrix could result in establishing effective galvanic couples, and thus accelerating the corrosion of weld metals.

Effect of solution treatment on microstructure and cryogenic toughness of 316LN austenite stainless steel weld metal welded by NG-MAG arc welding

In this paper, the effect of solution treatment on microstructure, micro-hardness and cryogenic toughness of 316LN weld metal welded by NG-MAG arc welding was investigated by electron microscopy, hardness measurements and charpy V impact tests. After a proper solution treatment, the dendritic sub-grains could disappear, the segregation of Mn and Mo at the sub-grain boundaries could be completely eliminated and the dislocation density drop significantly. All these aspects contributed to the significant improvement of cryogenic toughness. Specifically, the fracture morphology with respect to the solution temperature was discussed. Finally, the maximum fracture toughness at cryogenic temperature was obtained after solution treatment at 1100°C for 2h.

Effect of Dynamic Reheating Induced by Weaving on the Microstructure of GTAW Weld Metal of 25% Cr Super Duplex Stainless Steel Weld Metal

The importance of the additional growth and/or transformation of the austenite phase that occurs in weld metals of super duplex stainless steel upon reheating is known. However, the effects have not been fully investigated, especially with respect to reheating induced by weaving during single-pass welding. In this work, bead-on-pipe gas tungsten arc welding (GTAW) was conducted on super duplex stainless steel to understand the effect of weaving on the microstructure of weld metal. Microstructural analysis, electron backscatter diffraction (EBSD), and focused ion beam transmission electron microscopy (FIB-TEM) were carried out to investigate the relationship between weaving and microstructural change. The weaving of GTAW produced a dynamic reheated area just before the weld bead during welding. It was revealed that extensive reheated weld existed even after one welding pass, and that the content of the austenite phase in the reheated area was higher than that in the non-reheated area, indicating the existence of a large quantity of intragranular austenite phase. In addition, the Cr2N content in the reheated area was lower than that in the non-reheated area. This reduction of Cr2N was closely related to the reheating resulting from weaving. TEM analysis revealed that Cr2N in the non-reheated area was dispersed following heating and transformed to secondary austenite.

Use of electron microprobe analysis to explain metallurgical phenomena and their effect on properties when welding stainless steel

A modified electron probe microanalysis (EPMA) instrument containing six wavelength dispersive spectrometers has been used for mapping cross-sections of stainless steel base material and welds. It made it possible to measure the composition in specific small areas and to illustrate element partitioning; ranging from centerline segregation to concealed sub-microstructure. Examples from industrial research are given to demonstrate the versatile application field for this established technology. EPMA was used to study the chemical composition of gas tungsten arc welds in the lean duplex grade UNS S32101 performed with and without nitrogen additions to the argon-based shielding gas. The level of nitrogen in the shielding gas not only determined the weld metal nitrogen content, but also affected evaporation of other elements from the weld metal. Mapping could also be used to reveal detailed information about the dendritic substructure in welds and the fusion line that could not be seen with conventional microscopy as the weld metal microstructure is controlled by nitrogen. With the measured local composition, it was possible to calculate the pitting resistance equivalent number, PRE = Cr + 3.3 × Mo + 16 × N (wt%) and thereby visualize the local variation in the microstructure. Nitrogen is strongly concentrated in the austenite and has the largest impact on the PRE. Pitting corrosion testing confirmed that the ferrite phase was most susceptible to pitting attack and that a higher average PRE does not significantly increase the critical pitting temperature. The EPMA instrument was also applied to study all-weld metal of E347T0 type flux-cored wire with and without bismuth additions. Bismuth can have a negative effect on weld metal ductility after post-weld heat treatment and/or applications at high temperature. It has been suggested that this is due to grain boundary bismuth segregation, and it has been debated whether it occurs as bismuth or bismuth oxide Bi2O3. Hot tensile testing confirmed that the bismuth content affects the weld metal ductility at 700 °C and EPMA proved that bismuth segregates as round particles after post-weld heat treatment and not as bismuth oxide.

The Study on Characteristics of Dissimilar Weld Metals between AA6061-T6 and Silafont-36 by Laser Welding

The characteristics of dissimilar weld metals between AA6061-T6 and Silafont-36 by laser welding were evaluated in terms of bead shape, microstructure, EDS analysis, hardness, and tensile strength. Laser power of 3.0, 3.5, 4.0kW and incident angle of 15, 20° were employed to make dissimilar weld metals. Bead shape of weld metal with 3.5kW, 15° was most stable. Microstructure of dissimilar weld metals consisted of columnar dendrite in area adjacent to fusion line and equiaxed dendrite in center of weld metal. Also, Mg2Si and α-AlFeMnSi were precipitated in weld metal. Microstructure in HAZ of Silafont-36 consisted of eutectic Al-Si, Mg2Si, and β-Al5FeSi. Si contents of weld metal made with 4.0kW, 15° were very high. And Mg contents of weld metals were relatively low because of vaporation of Mg. Hardness of weld metal made with 4.0kW, 15° was very high because Si contents of that were high. In tensile test, Specimens were fractured in Silafont-36 or weld metals. The fracture in base metals was due to low tensile strength of Silafont-36, and the fracture in weld metals was due to undercut or porosity. Key words: AA6061, Silafont-36, Dissimilar weld, Laser welding, Mg2Si

Role of misorientation in fatigue crack growth behavior for NG-TIG welded joint of Ni-based alloy

The fatigue crack growth (FCG) behavior and microstructure of Ni-based alloy welded joint, fabricated by narrow gap tungsten inert gas welding (NG-TIG), were systematically investigated in this paper. The microstructure of weld metal (WM) and base metal (BM) are columnar grains and equiaxed grains respectively, plentiful irregular carbides aggregate at grain boundaries and the number of twin crystals decreases distinctly in HAZ due to the thermal input. The FCG tests results indicated that a higher FCG rate (da/dN) as well as a lower FCG threshold (ΔKth) for WM were obtained compared to BM and HAZ. It was found that crack deflection and branching in the whole welded joint occurred at FCG path in near-threshold regime. The misorientation of grain boundaries is considered as the main factor to account for the deceleration and retardation behavior revealed by electron back-scattered diffraction (EBSD) analysis. EBSD results also indicate that the misorientation of 20–30° is a critical value from a tortuous path to a relative straight path. A contrasting morphology of Paris regime and near-threshold regime showed the fracture mode transition from tensile mode to intense shear mode.

Evaluation of microstructure and mechanical properties of P91 steel weldment due to addition of boron trioxide into flux during submerged arc welding

The present investigation studied the effect of boron trioxide (B2O3) addition into flux on the microstructure and mechanical properties of modified 9Cr-1Mo (P91) steel in submerged arc welding (SAW) process. Boron trioxide powder was mixed with commercial flux in different ratios to change the weld metal chemical composition. Optical emission spectroscopy results showed that weight percentage of boron varies from 0.00 to 0.0068 with the addition of boron trioxide into welding flux. Weld metal microstructure without the addition of boron reveals the presence of tempered martensite and ferrite phase, whereas boron-added microstructures highlighted the dominating nature of tempered martensite. Scanning electron microscope analysis also showed that the dispersed carbides especially M23C6 were increased with the addition of boron. Weld metal macrohardness profile reveals that boron has the significant effect on the weld metals. The maximum hardness of 342 HV was found for the 12.5% B2O3, whereas weld of without boron shows the hardness of 240.8 HV. Boron addition has a positive effect on the tensile strength of weld joints except for the 12.5% B2O3. But the toughness of weld zone decreases with the addition of boron.

Systematic Expansion of the Microstructural Characterization of Ferritic Weld Metals

Abstract This paper deals with the analytic gap during the microstructural examination of ferritic weld metals and points out which methods can be used to perform a meaningful evaluation of the weld metal microstructure. The designers of filler materials and the respective metallographs are provided with a tool to specifically search for weaknesses in the microstructural composition and invalidate them by alloying on the basis of their metallurgical context.

Effect of long-term aging on the microstructure and mechanical properties of T23 steel weld metal without post-weld heat treatment

T23 steel welds without post-weld heat treatment (PWHT) were aged for 3000h at temperatures in the range 773–923K. Microstructural evolution in the aged weld metal (WM) was observed and its influence on the hardness and Charpy impact toughness were investigated. The as-welded WM exhibits a martensitic/bainitic microstructure with moderate hardness less than 350 HB and impact toughness value above 110J/cm2. Aged WM exhibits deterioration of mechanical properties dependent on the temperature of aging. When the aging temperature is below 823K, the WM shows little change in hardness but its impact toughness drops significantly. When the aging temperature is above 823K, the hardness of the WM starts to decrease quickly while its impact toughness increases gradually. As the aging temperature increases to 923K, its impact toughness further increases and recovers to a value comparable to that of the as-welded condition. Embrittlement occurring during aging at 773–823K results from tempered martensite embrittlement (TME), which is caused by the decomposition of retained austenite. When the aging temperature exceeds 823K,M23C6 carbides precipitate rapidly inside grains and at grain boundaries and MX carbides begin to precipitate inside grains. These cause the release of carbon from the matrix, reduction of lattice distortion, and accelerated recovery of laths causing the hardness to decrease rapidly and the impact toughness to increase. W-rich carbides are observed in a minority of grain boundaries in the WM aged at temperatures higher than 823K, which may cause intergranular fracture and retard the improvement of impact toughness.

The Effect of Sodium Chloride Concentration on Corrosion Resistance of Austenitic Stainless Steel 316L and SMA Weldment

Austenitic stainless steel 316L has been widely used in marine environment which containing sodium chloride solution (NaCl). In order to provide matching properties with parent metal, filler metal SMA 316L is commonly produced with slightly over alloyed composition. This work investigated the corrosion behavior of base metal 316L and SMA 316L weld metal by using Electrochemical Impedance Spectroscopy (EIS) to evaluate the mechanism of corrosion behavior based on impedance magnitude measurement at room temperature (27°C ). Various concentrations of sodium chloride solution i.e 1%,2%,3.5%,4% ,and 5% NaCl were prepared. Optical Metallography was also conducted to compare microstructure of base and weld metal. By using Nyquist graphs and its related equivalent circuit parameters showed that impedance magnitude of weld metal was higher which compared to base metal at any NaCl concentration. Metallography examination revealed that weld metal 316L had dendritic austenitic with delta ferrite and 316L base metal had austenite with typical twin boundaries structure. Higher chromium and nickel content in weld metal 316L was the key variable that control passive film characteristic rather than its microstructure. The lowest impedance magnitude of both 316L and all-weld metal 316L at various concentration was at 3.5% NaCl. Dissolved oxygen at 3.5% NaCl reach maximum solubility which causes severe pitting corrosion.

Characterization of the underwater welding arc bubble through a visual sensing method

To evaluate the effectiveness of arc bubble, the influences of controllable arc bubble on the process stability and microstructural evolution of welded joint under different welding conditions were investigated. The main welding conditions, including onshore welding, conventional underwater wet welding (UWW) and mechanical constraint assisted UWW (MC-UWW), were characterized and compared. The exerted mechanical constraint resists the arc bubble detachment and keeps the larger arc bubble attached to the weld pool surface for providing better protective effect of welding region. The adverse effect of surrounding water with its higher heat conduction is not negligible compared with onshore welding, and the mechanical coupling between arc bubble and constraint device plays a vital role in affecting the arc stability and joint quality. Microstructure analyses revealed that under the same welding parameters, the proportion of brittle microstructure in weld metal follows the order: UWW > MC-UWW > onshore welding. A more stable wet welding process can be realized by controlling the arc bubble detachment, which provides a new orientation for wet welding technique application.

Structural metastability of “cold” repair welds in 2.25Cr-1Mo (P22) steel under elevated temperature and stress conditions

This paper investigates the effects of accelerated creep testing of a temperbead (TB) repair weld for a P22 pressure vessel component that was produced without post-weld heat-treatment (PWHT). The mechanical properties of the “cold” (non-PWHT) weld metal were superior to those of aged parent metal and the original fabrication welds for all tests, with the notable exception of creep performance. The creep performance of the TB repair weld material was inferior to that of the aged parent metal, due to the metastability of the weld metal microstructure relative to restoration processes occurring at the “accelerated” temperatures used for testing. It is concluded that the test conditions induced structural changes that are unlikely to occur under the actual service conditions and resulted in a creep life assessment that was highly conservative. The reported results are relevant to the security and integrity of non-PWHT repair welding of high-energy piping components.

Effect of interpass temperature on the microstructure and mechanical properties of multi-pass weld metal in a 550-MPa-grade offshore engineering steel

The influence of interpass temperature on the microstructure and mechanical properties of multi-pass weld joints (up to 36-mm thickness) by submerged arc welding (SAW) was studied from the perspective of offshore engineering. Optimal mechanical properties were obtained with the interpass temperature of ~130 °C. Decreasing interpass temperature from 130 to 80 °C increases the strength and hardness at the cost of impact toughness of the weld joint due to the formation of hard phases including bainite and martensite. Increasing the interpass temperature from 130 to 250 °C promotes a larger volume fraction of coarse M-A constituents and larger inter-spacing of high-angle boundaries, which, in turn, deteriorates the toughness. In addition, a large amount of M-A constituent necklacing prior austenite grains was observed in the reheated zone of all weld metals and was responsible for the low impact energy of the weld joint.

Investigations on multi-run metal made of HSLA steel – Heterogeneous microstructure and mechanical properties

Multi-run high strength low alloy (HSLA) steel plates welded by manual metal arc (MMA) process were investigated. Heterogeneous weld metal microstructure was studied accompanied with its effect on the mechanical properties of the joint. Microstructural studies revealed: (1) in the last run, there were intergranular acicular ferrite and pro-eutectoid ferrite, (2) in previous weld runs, there were refined grains with precipitated particles and (3) in the interlayer region of previous runs, there were some heterogeneous ferrite grains, grain boundary discontinuities and precipitated microalloying elements. In the last run, in fusion zone weld and heat affected zone interface, the solidification is a phenomenon of an epitaxial growth type structure. However, in previous weld runs, there was neither epitaxial growth nor typical refined grain structure. The mechanical properties varied in different parts of the welded plate due to the metallurgical characteristics of weld metal in comparison with the production procedure of HSLA steels and their metallurgical parameters.

Microstructure and mechanical properties of Ti-6Al-4V welds using α, near-α and α+β filler alloys

Effects of filler metal on the microstructure and mechanical properties of Ti-6Al-4V joints were investigated. Three specimens were prepared using α, near-α and α+β type filler materials and one specimen was welded without filler metal. The microstructural characterizations, hardness and tensile tests were applied to study the welded specimens. For the specimen welded without filler material, a coarse-grained structure of α+β was found in the microstructure of weld metal. Although this specimen showed lower hardness values compared to the base metal; the hardness of other weld metals was higher than that of the base material. The microstructure of other welds was a mixture of α, β and some amount of α´-martensite. Both acicular and basketweave morphologies of α´ were found in the weld metal. The α colonies were grown from the α lamellae which were formed at the β grain boundaries. The highest tensile strength was obtained for the specimen welded with the matching filler metal (about 91% of the base metal).

Determination of Mo-Rich Laves-Phase in Weld Metals of a Creep-Resistant 9% Cr-Steel Using Light Optical Microscopy

Creep-resistant 9% Cr-steels feature different phases in their weld metal microstructure; especially after creep-rupture testing. Laves-Phase in such alloying concepts is widely studied, but there is still a need for a simple identification technique utilizing light optical microscopy. The Lichtenegger–Bloech color tint etchant was utilized to contrast the weld metals microstructure in as-welded condition, after post-weld heat treatment and creep-rupture tested. Proof of the presence of Laves-Phase after creep-rupture tests in the weld metal of a 9% Cr-steel GX13CrMoCoVNbNB9-2-1 (CB2) was performed using Lichtenegger–Bloech and light optical microscopy. To validate Laves-Phase within the analyzed microstructure, scanning electron microscopy and energy dispersive x-ray spectroscopy were performed. For a thorough analysis, phase analysis was done in selected regions of interest. The results show that the Laves-Phase in this type of steel can be easily contrasted and quantified at comparatively low lateral resolutions by Lichtenegger–Bloech, with no further need of analysis required.