Effect of Interface Form on Creep Failure and Life of Dissimilar Metal Welds Involving Nickel-Based Weld Metal and Ferritic Base Metal

For dissimilar metal welds(DMWs)involving nickel-based weld metal(WM)and ferritic heat resistant steel base metal(BM)in power plants,there must be an interface between WM and BM,and this interface suffers mechanical and microstructure mismatches and is often the rupture location of premature failure.In this study,a new form of WM/BM interface form,namely double Y-type interface was designed for the DMWs.Creep behaviors and life of DMWs containing double Y-type interface and conventional I-type interface were compared by finite element analysis and creep tests,and creep failure mechanisms were investigated by stress-strain analysis and microstructure characterization.By applying double Y-type interface instead of conventional I-type interface,failure location of DMW could be shifted from the WM/ferritic heat-affected zone(HAZ)interface into the ferritic HAZ or even the ferritic BM,and the failure mode change improved the creep life of DMW.The interface premature failure of I-type interface DMW was related to the coupling effect of microstructure degradation,stress and strain concentrations,and oxide notch on the WM/HAZ interface.The creep failure of double Y-type interface DMW was the result of Type IV fracture due to the creep voids and micro-cracks on fine-grain boundaries in HAZ,which was a result of the matrix softening of HAZ and lack of precipitate pinning at fine-grain boundaries.The double Y-type interface form separated the stress and strain concentrations in DMW from the WM/HAZ interface,preventing the trigger effect of oxide notch on interface failure and inhibiting the interfacial microstructure cracking.It is a novel scheme to prolong creep life and enhance reliability of DMW,by means of optimizing the interface form,decoupling the damage factors from WM/HAZ interface,and then changing the failure mechanism and shifting the failure location.

THRESHOLD STRESS INTENSITIES FOR HYDROGEN-INDUCED DELAYED FAILURE OF WELD METAL OF AUSTENITIC STAINLESS STEEL

It was found that hydrogen induced delayed failure could occur in 308L and 347L weld metals,and the threshold stress intensities of 308L and 347L welds were lower than that of 304L austenitic stainless steel.When dynamically charged under load on a single edge notched specimen,the threshold stress intensities of 308L,347L and 304L decrease with the increase in the diffusible hydrogen content C 0 and the experimental results are as follows:K ⅠH =85.2-10.7 ln C 0 (308L),K ⅠH =76.1-9.3 ln C 0 (347L),K ⅠH =91.7-10.1 ln C 0 (304L).The morphology of the hydrogen induced delayed fracture in the three materials are correlated with the K Ⅰ and C 0 values.

Prediction of Ac1 temperature in P92 steel weld metal

The effects of major alloying elements on the Ac, temperature in P92 steel weld metal were evaluated by Thermo- Calc, and a formula relating Ac1 to the content of major alloying elements was developed using multiple regression method. Results show that both C and N reduce Ac, temperature in weld metal, the effect of N on Ac, is greater than that of C, but their influence on Act is not so significant when they individually vary in the specified chemical composition ranges. Si, Cr, Mo and W increase the Acl temperature, and the descending order of their effects is determined as Si, Mo, W, Cr. Mn and Ni decrease the AcI in weld metal, the decreasing effect is especially remarkable when the （ Mn ＋ Ni） ≥ 1%. The effect of Co is moderate and is much smaller compared to Ni. The variations of Acl for the ranges of micro-alloying elements Nb and V are also evaluated, the effect of V is greater compared to Nb and the effect of Nb could be ignored in the specified chemical composition range. A prediction formula for Ac, temperature in P92 steel weld metal within the specified chemical composition ranges was developed based on the comprehensive consideration of the calculated Ac, temperatures and the experimentally measured results.

Effects of alloying elements on the microstructures and mechanical properties of Ni-Fe weld metals

The effects of Ce, Ti, Nb and C on the microstructures and mechanical properties of Ni Fe weld metal and the mechanism of their strengthening are discussed. Ce has the effect on graphite morphology and weld metal with spheroidal graphite has higher mechanical properties. The proper contents of Ti and Nb obviously increase the tensile strength of Ni Fe weld metal, which is mainly attributed to the austenite grain refining and second phase (TiC and NbC) strengthening. The excess C in weld metal results in increasing the quantity of carbide and graphite precipitating along the austenite grain boundary and decreasing the mechanical properties of Ni Fe weld metal.

Mechanical properties and grain refinement by acicular ferrite in high strength high toughness weld metal

The volume fraction and morphology of acicular ferrite evolution in a high strength high toughness weld metal were studied and the mechanical properties of weld metal under heat input of 21 kJ/cm with and without fast cooling were tested. The results show the weld metal can obtain a large proportion of acicular ferrite during a wide range of cooling rate and the sizes of acicular ferrite in length and thickness decrease with cooling rate increasing. The weld metal exhibited high tensile strength （895 MPa and 870 MPa） and good low temperature toughness （average AKv-30℃ 104 J and 79. 2 J）. The higher tensile strength and better low temperature toughness of the weld metal under fast cooling are due to the more refined grain of acicular ferrite.

Effects of B and Ti on the Toughness of HSLA Steel Weld Metals

Effects of microalloying Ti and B on the microstructures and low temperature toughness of manual metal arc (MMA) deposits were investi- gated.Weld metals containing 200-300 ppm Ti and 29-60 ppm B deposited by manual coated elec- trodes provided an optimum low temperature toughness.The addition of B in weld metals low- ered the γ→α transformation temperature which promoted the acicular ferrite (AF) transformation. Solid solutioned B suppressed grain boundary ferrite as well as side plate ferrite formation and benefited the acicular ferrite formation.Titanium protected B from oxidizing as well as nitriding and formed Ti-Mn silicate inclusions.Ultra-high volt- age electron microscope analyses showed that TiO structure in the Ti-Mn silicate inclusions was the favorable nucleation site for acicular ferrite forma- tion.

INFLUENCE OF LOCAL BRITTLE ZONE ON THE FRACTURETOUGHNESS OF HIGH-STRENGTH LOW-ALLOYEDMULTIPASS WELD METALS

In this paper, toughness properties and microstructurc of low-alloyed multipass welds with yield strength above 700MPa have 6een studied using the weld thermal simulation and throughout thickness CTOD fracture mechanics tests. Impact testing of thermal simulated specimens showed that the primary weld metal and the fine gmmed weld metal had good toughness, while the coarse grained weld metal had the lowest toughness value as the local brittle zone (LBZ) in multipass weld metals. Cleavage fracture in CTOD testing of thick multipass weld metals was initiated from martensite-austenite (MA) phases in the LBZ. MA phases were distributed at the prior austenite grain boundaries and around ferrite grains. As the size of the local brittle zone along the fatigue crack front increases, CTOD frncture toughness of multipass weld metals decreases. The weakest link theory was used to evaluate effect of the local brittle zone on fracture toughness of thick multipass weld metals. The estimated curves agree well with the eaperimental data.

Microstructure in the Weld Metal of Austenitic-Pearlitic Dissimilar Steels and Diffusion of Element in the Fusion Zone

Microstructure and alloy element distribution in the welded joint between austenitic stainless steel (1Cr18Ni9Ti) and pearlitic heat-resistant steel (1Cr5Mo) were researched by means of light microscopy, scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). Microstructure, divisions of the fusion zone and elemental diffusion distributions in the welded joints were investigated. Furthermore, solidification microstructure and S-ferrite distribution in the weld metal of these steels are also discussed.

Microstructure and mechanical properties of ultralow carbon high-strength steel weld metals with or without Cu-Nb addition

Two types of ultralow carbon steel weld metals(with and without added Cu-Nb) were prepared using gas metal arc welding(GMAW) to investigate the correlation between the microstructure and mechanical properties of weld metals.The results of microstructure characterization showed that the weld metal without Cu-Nb was mainly composed of acicular ferrite(AF), lath bainite(LB), and granular bainite(GB).In contrast, adding Cu-Nb to the weld metal caused an evident transformation of martensite and grain coarsening.Both weld metals had a high tensile strength(more than 950 MPa) and more than 17% elongation;however, their values of toughness deviated greatly,with a difference of approximately 40 J at-50℃.Analysis of the morphologies of the fracture surfaces and secondary cracks further revealed the correlation between the microstructure and mechanical properties.The effects of adding Cu and Nb on the microstructure and mechanical properties of the weld metal are discussed;the indication is that adding Cu-Nb increases the hardenability and grain size of the weld metal and thus deteriorates the toughness.

Microstructure and toughness of local brittle zone of HSLA steel multipass weld metals

By using thermo-simulation,Auger analysis and Charpy Ⅴ impact test and with the observation of the microstructures in which cleavage crack was initiated,the morphology and toughness of the local brittle zone of C-Mn and Cr-Ni-Mo multipass weld metals have been investigated.The results indicated that the local brittle zone in C-Mn weld metals with low and high Mn％ and Cr- Ni-Mo weld metals is different.With statistical analysis,it has been revealed that the more the local brittle zone and the lower their toughness,the lower the toughness of the entire weld metals.The alloy elements have a noticeable influ- ence on the toughness of the local brittle zone,thereby changing the toughness of weld metals.

Effects of Ni and Co on the Ac1 temperature of P92 steel weld metal

The effects of the addition of O - 1.2wt. % Ni and 0 - 2wt. % Co in F92 steel weld metal on its A L temperature was analyzed using computational thermodynamic software （ Thermo-CalcTM ） in this paper. The results indicate that both Ni and Co decrease the A, of P92 .steel weld metal, and each elements has a pronounced linear relationship with the At. The il^fluence of Ni on A,I is much larger than that of Co. In order to evaluate effects of Co and Ni in combination on AI of P92 .steel weld metal, an equation Ni = Ni ＋ O. 096Co was developed. According to the formula, the decreaing effect of Co on A., is jun about 1 / 10 of that of Ni. For welding consumables of P92 steel, the .substitution of Co for Ni （part or whole） could be an appropriate way to avoid the strong reduction of A and improve the properties of weld metal.

Characterization of the structural details of residual austenite in the weld metal of a 9Cr1MoNbV welded rotor

The existence of residual austenite in weld metal plays an important role in determining the properties and dimensional accuracy of welded rotors. An effective corrosive agent and the metallographic etching process were developed to clearly reveal the characteristics of residual austenite in the weld metal of a 9Cr1MoNbV welded rotor. Moreover, the details of the distribution, shape, length, length-to-width ratio, and the content of residual austenite were systematically characterized using the Image-Pro Plus image analysis software. The results revealed that the area fraction of residual austenite was approximately 6.3% in the observed weld seam; the average area, length, and length-to-width ratio of dispersed residual austenite were quantitatively evaluated to be （5.5 ± 0.1）μm2, （5.0 ± 0.1）μm, and （2.2 ± 0.1）, re-spectively. The newly developed corrosive agent and etching method offer an appropriate approach to characterize residual austenite in the weld metal of welded rotors in detail.

CRACK GROWTH RATE IN WELD METAL OF HOMEMADE OFFSHORE STRUCTURE STEEL 36Z

Investigation was carried out of the crack growth rate in weld metal and heat affected zone (HAZ)of offshore structural steels,homemade steel 36Z together with foreign steel A537 of similar strength grade by comparison,in artificial sea water undergone fatigue with different frequencies.The crack growth rate in weld metal and HAZ of steel 36Z fatigued at low fre- queney may be 3—5 and 4—10 times as high as that at high frequency respectivley.The crack growth rate for steel 36Z may fairly match that of A537,The crack growth model and frac- ture of weld metal in corrosive medium were also discussed.

Kinetics of Acicular Ferrite Transformation in Weld Metal

Acicular Ferrite(AF) is beneficial in welds because it provides a tough and strong microstructure.Therefore,much works have been done in the study of the effect of alloying elements and welding conditions aimed at obtaining more AF in the weld metal.However,the knowledge about the nature of AF is limited due to the lack of kinetics researches.In this paper,the kinetic curves of AF transformation in an X65 weld metal were established by means of welding thermal simulation test.The results showed that AF transformation is a typical diffusion process of nucleation and growth with incubation period.AF transformation in weld metal obeys the relationship described by JMA Equation.The effect of welding heat input was also studied.

Analysis Microstructure of Weld Metal for HQ130+QJ63 High Strength Steel

The microstructure in the weld metals for HQ130 + QJ63 high strength steels,which are welded by Ar-CO_2 gas shielded metal arc welding,was analyzed by means of microscope and scan electron microscope(SEM).The relative content of different microstructure was evaluated with XQF-2000 micro-image analyzer.The effect of acicular ferrite content on the impact toughness was also studied.The test results indicated that the main microstructure in the weld metals of HQ130 + QJ63 high strength steels is acicular ferrite and a few pro-eutectic ferrite on the boundary of original austenite grain.Near the fusion zone there are columnar grains whose direction coefficient(X) is 3.22,but the microstructure in the center of the weld metal is isometric grain,whose direction coefficient X = 1.In order to avoid welding crack and improve welding technology the weld heat input should be strictly controlled in 10-16 kJ/cm.Thus,the main microstructure in the weld metals is fine acicular ferrite and the content of pro-eutectic ferrite is limited.The impact toughness in the weld metals of HQ130+ QJ63 steels can be ensured and can meet the requirements for application in engineering and machinery.

Size Evolution of the Surface Short Fatigue Cracks of 1Cr18Ni9Ti Weld Metal

Size evolution of the surface short fatigue cracks of lCrl8Ni'9Ti weld metal was investigated. A local viewpoint is applied to be agreement with a so-called 'effectively short fatigue crack criterion'. Attention was paid to the dominant effectively short fatigue crack (DESFC) initiation zone and the zones ahead of the DESFC tips. The results revealed that the evolutionary size shows a significant character of microstructural short crack (MSC) and physical short crack (PSC) stages. In the MSC stage, fatigue damage is due to mainly the initiation and irregular growth of the effectively short fatigue cracks (ESFCs). In the PSC stage, the damage is conversely due to mainly the DESFC growth and partially, the growth of the ESFCs and the coalescence of the ESFCs themselves with the DESFC. The process involves from a non-ordered/chaotic state in the initiation of MSC stage, gradually to an independently random state at the transition point between the MSC and PSC stages and then, to an ordered/history-dependent random state. Interactive effect of the collective cracks is stronger and shows an increase in the MSC stage, it reaches a maximum value at the transition point and then, tends to a decrease in the PSC stage. The DESFC acts as a result of the interactive cracks and thus, is deemed suitable to describe the behaviour of collective cracks.

Effect of copper on mechanical properties of high strength SMAW weld metal

Mechanical properties of SMA W （shielded metal arc welding） weld metal （ yield strength higher than 900 MPa ） with systemazic additions of copper （ up to 1.48 wt% ） were tested, The microstructure and precipitates in different regions were analyzed by optical microscope and transmission electron microscope, The results indicate that copper improves the low temperature toughness of weld metal when the copper content is low and reaches the peak value 48 J （ at - 50℃ ） with 0. 2 wt% copper additions. When the content is high the copper precipitates as 8-Cu phase in the reheat zone of middle beads. These precipitates improve the strength of the weld metal evidently （ yield strength up to 975 MPa） without obvious effect on the low temperature toughness. The copper within 1.1 wt% content can improve the strength without toughness loss.

Effect of Si, Mn and Al on the Microstructure and Mechanical Properties of ADI Weld Metal

The effects of Si, Mn and Al on the microstructure and mechanical properties of ADI weld have been studied. The microstructure of ADI weld metal mainly consists of bainitic ferrite and retained austenite. Mechanical properties of ADI weld increase with increasing Si content, but an excess of Si(3.79%) results in decreasing the austemperability owing to decreasing the carbon content of the matrix austenite. Mn increases the retained austenite volume fraction, but the ductility and impact toughness of weld obviously decrease with increasing Mn content because of increased amount of martenite and twin martenite. In the range of 0.13%-0.64%AI, increasing Al content favours improving the mechanical properties of ADI weld. Therefore, it is very important to select suitable Si, Mn and Al contents to improve mechanical properties of ADI weld.

A PROBABILISTIC ASSESSMENT OF TEMPERATURE EFFECTSON THE LOW CYCLE FATIGUE BEHAVIOUR OF 1Cr_(18)Ni_9Ti STEEL WELD METAL

Based on the test results obtained from the single-step test and the incremental-step test at room temperature and 240℃, a probabilistic assessment of temperature effects on the cyclic stress-strum response and the fatigue life of 1Cr18Ni9Ti steel weld metal is performed. In orber to assess the temperature effect on cyclic stress amplitude where there is a scatter of the material cyclic constitution, a probabilistic assessment approach on the basis of probabilistic modified Ramberg-Osgood relations is introduced.The investigation shows that the cyclic stress amplitude and the scatter of cyclic stress amplitude data are decreased at 240℃. Similarly, from the consideration of the fatigue life scatter a probabilistic assessment of temperature effect on the fatigue life is suggested on the basis of probabilistic Langer S-N relations. The investigation shows that the crack initiation life is increased and the scatter of crack initiation life data is decreased at 240℃.

Microstructure evolution in the weld metal region of a Ni-based Inconel 718 superalloy produced by tungsten inert gas welding

The microstructure evolution of the weld metal （WM） region in a Ni-based 718 superaUoy is discussed. The superalloy sheets were welded using the tungsten inert gas process at different heat inputs. The precipitates and dendrites in the WM of each joint were analyzed by optical microscopy, scanning electron microscopy, and energy dispersive spectroscopy （ EDS）. Statistics on the dendrite arm spacing and precipitates were obtained from the metallographs. Using the alloy composition determined by EDS and the phase diagram obtained using ThermoCalc, the equilibrium distribution coefficient of Nb, the temperature range of solid/liquid coexistence, and the distribution and morphology of Laves phase in different subregion of the WM were analyzed. There is lamellar segregation, regional segregation, and microsegregation in the WM. As the heat input increases, the arm spacing and the microsegregation increase. At a fixed heat input, the microsegregation rate is smallest in the crater, but with the macrosegregation seriously.