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1.
The austenitic stainless steel weld metal fabricated by multipass welding exhibits a composite microstructure. Microstructural characterization of the weld metal revealed that there are two distinct regions on either side of the weld-pass interface. The variations in dislocation substructure and delta ferrite morphology are the two microstructural attributes which delineate these regions. The generation of subsequent thermal cycles during the fabrication of multipass weld joint is the paramount factor influencing the formation of the composite microstructure. During creep exposure, the extent of creep cavitation and propagation varies substantially in these two regions due to differences in their microstructures. This results in preferential damage during creep exposure of austenitic stainless steel weld metal. 相似文献
2.
Modified 9Cr-1Mo steel is a heat-treatable steel and hence the microstructure is temperature sensitive. During welding, the weld joint (WJ) is exposed to various temperatures resulting in a complex heterogeneous microstructure across the weld joint, such as the weld metal, heat-affected zone (HAZ) (consisting of coarse-grained HAZ, fine-grained HAZ, and intercritical HAZ), and the unaffected base metal of varying mechanical properties. The overall creep–fatigue interaction (CFI) response of the WJ is hence due to a complex interplay between various factors such as surface oxides and stress relaxation (SR) occurring in each microstructural zone. It has been demonstrated that SR occurring during application of hold in a CFI cycle is an important parameter that controls fatigue life. Creep–fatigue damage in a cavitation-resistant material such as modified 9Cr-1Mo steel base metal is accommodated in the form of microstructural degradation. However, due to the complex heterogeneous microstructure across the weld joint, SR will be different in different microstructural zones. Hence, the damage is accommodated in the form of preferential coarsening of the substructure, cavity formation around the coarsened carbides, and new surface formation such as cracks in the soft heat-affected zone. 相似文献
3.
This paper analyzed the evolution of microstructure, composition, and impact toughness across the fusion line of high-strength bainitic steel weldments with different heat inputs. The main purpose was to develop a convenient method to evaluate the HAZ toughness quickly. The compositions of HAZ were insensitive to higher contents of alloy elements ( e.g., Ni, Mo) in the weld metal because their diffusion distance is very short into the HAZ. The weld metal contained predominantly acicular ferrite at any a heat input, whereas the main microstructures in the HAZ changed from lath martensite/bainite to upper bainite with the increasing heat input. The evolution of HAZ toughness in relation to microstructural changes can be revealed clearly combined with the impact load curve and fracture morphology, although the results of impact tests do not show an obvious change with heat input because the position of Charpy V notch contains the weld metal, HAZ as well as a part of base metal. As a result, based on the bead-on-plate welding tests, the welding parameter affecting the HAZ toughness can be evaluated rapidly. 相似文献
4.
Cast precipitation-hardened (PH) stainless steels 17-4 and 13-8+Mo are used in applications that require a combination of high strength and moderate corrosion resistance. Many such applications require fabrication and/or casting repair by fusion welding. The purpose of this work is to develop an understanding of microstructural evolution and resultant mechanical properties of these materials when subjected to weld thermal cycles. Samples of each material were subjected to heat-affected zone (HAZ) thermal cycles in the solution-treated and aged condition (S-A-W condition) and solution-treated condition with a postweld thermal cycle age (S-W-A condition). Dilatometry was used to establish the onset of various phase transformation temperatures. Light optical microscopy (LOM), scanning electron microscopy (SEM), and energy dispersive spectrometry (EDS) were used to characterize the microstructures, and comparisons were made to gas metal arc welds that were heat treated in the same conditions. Tensile testing was also performed. MatCalc thermodynamic and kinetic modeling software was used to predict the evolution of copper (Cu)-rich body center cubic precipitates in 17-4 and β-NiAl precipitates in 13-8+Mo. The yield strength was lower in the simulated HAZ samples of both materials prepared in the S-A-W condition when compared to their respective base metals. Samples prepared in the S-W-A condition had higher and more uniform yield strengths for both materials. Significant changes were observed in the matrix microstructure of various HAZ regions depending on the peak temperature, and these microstructural changes were interpreted with the aid of dilatometry results, LOM, SEM, and EDS. Despite these significant changes to the matrix microstructure, the changes in mechanical properties appear to be governed primarily by the precipitation behavior. The decrease in strength in the HAZ samples prepared in the S-A-W condition was attributed to the dissolution of precipitates, which was supported by the MatCalc modeling results. MatCalc modeling results for samples in the S-W-A condition predicted uniform size of precipitates across all regions of the HAZ, and these predictions were supported by the observed trends in mechanical properties. Cross-weld tensile tests performed on GMA welds showed the same trends in mechanical behavior as the simulated HAZ samples. Welding in the S-W-A condition resulted in over 90 pct retention in yield strength when compared to base metal strengths. These findings indicate that welding these PH stainless steels in the solution-treated condition and using a postweld age will provide better and more uniform mechanical properties in the HAZ that are more consistent with the base metal properties. 相似文献
5.
The reliability of steel welds becomes more critical issue with increasing steel strength,because brittle phases are more likely to form in the weld metals and heat-affected zone(HAZ) and thereby the toughness and ductility of the welds are degraded.Therefore,refinement of microstructure and minimization of the brittle phases are necessary to improve the reliability of the high-strength steel welds.In this presentation,microstructure formation that controls the toughness of weld metals and HAZ in high-strength low-alloy(HSLA) steel welds is reviewed and possible routes to the improvement of the weld microstructure and weld toughness are discussed. 相似文献
6.
The microstructural changes that occur in a commercial HSLA-100 steel thermally cycled to simulate weld heat affected zone
(HAZ) behavior were systematically investigated primarily by transmission electron microscopy (TEM). Eight different weld
thermal cycles, with peak temperatures representative of four HAZ regions (the tempered region, the intercritical region,
the fine-grained austenitized region, and the coarse-grained austenitized region) and cooling rates characteristic of high
heat input (cooling rate (CR) = 5 °C/s) and low heat input (CR = 60 °C/s) welding were simulated in a heating/quenching dilatometer.
The as-received base plate consisted of heavily tempered lath martensite, acicular ferrite, and retained austenite matrix
phases with precipitates of copper, niobiumcarbonitride, and cementite. The microstructural changes in both the matrix and
precipitate phases due to thermal cycling were examined by TEM and correlated with the results of (1) conventional optical
microscopy, (2) prior austenite grain size measurements, (3) microhardness testing, and (4) dilatometric analysis. Many of
the thermal cycles resulted in dramatic changes in both the microstructures and the properties due to the synergistic interaction
between the simulated position in the HAZ and the heat input. Some of these microstructures deviate substantially from those
predicted from published continuous cooling transformation (CCT) curves. The final microstructure was predominantly dependent
upon peak temperature (i.e., position within the HAZ), although the cooling rate (i.e., heat input) strongly affected the microstructures of the simulated intercritical and finegrained austenitized regions.
A. MATUSZESKI, formerly Summer Student, Physical Metallurgy Branch, Naval Research Laboratory. 相似文献
7.
A model of direct-drive friction welding has been developed, which can be used to predict the time-temperature histories,
the resultant microstructure, and the microhardness distribution across the weld interface of direct-drive friction-welded
AISI/SAE 1045 steel bars. Experimentally measured power and axial displacement data were used in conjunction with a finite-element
transient thermal model to predict the time-temperature history within the heat-affected zone (HAZ) of the weld. This was
then used with a microstructure evolution model to predict the volume fraction of the subsequent microconstituents and the
microhardness distribution across the weld interface of welds produced using three significantly different welding conditions:
one with optimal conditions, one with a long burn-off time, and one with high axial pressure and rotational speed but short
burn-off time. There was generally good agreement between the predicted and the measured time-temperature histories, volume
fraction of the resultant microstructures, and microhardness distribution in the HAZ of AISI/SAE 1045 steel friction welds
produced using these three significantly different welding conditions. 相似文献
8.
Microstructural degradations in the base metal adjacent to the weld pool, i.e., the heat-affected zone (HAZ), caused during welding of 2.25Cr-1Mo steel, were characterized by electron and optical microscopy
of different regions of the weldments. In order to study the influence of the microstructural degradations on scaling kinetics
in steam and the resulting subscale features, samples of the base metal, the HAZ, and weld metal specimens were extracted
from the weldment and oxidized in an environment of 35 pct steam+nitrogen at 873 K for 10 hours. Oxide scales formed in the
three regions and the underlying subscales were characterized using scanning electron microscopy (SEM) and electron probe
microanalysis (EPMA). Influence of the “free” chromium content in the three weldment regions on protective scale formation
and on the subscale features has been investigated. As the principal achievement, this study has clearly shown the occurrence
of oxidation-induced void formation in the subscale zone and grain boundary cavitation in the neighboring area during steam
oxidation of the HAZ. This article also discusses the possible role of oxidation-induced void formation and grain boundary
cavitation in the inferior service life of welds in 2.25Cr-1Mo steel components. 相似文献
9.
Welds in dual-phase (DP) steels exhibit heat-affected zone (HAZ) softening in which the tempered or subcritical HAZ exhibits
a lower hardness vs that of the parent material. The rate of this softening reaction with respect to welding heat input was determined for four
DP steels by making several bead-on-plate laser welds using a variety of heat inputs and measuring the resulting minimum hardness.
The reduction in hardness was then fit to the Avrami equation, enabling a comparison of the relative heat needed to soften
each steel. It was found that the heat input required for HAZ softening decreased as the C content of the martensite within
the DP structure increased. However, the presence of carbide forming alloying elements such as Cr and Mo was able to increase
resistance to softening. 相似文献
10.
The microstructure and the mechanical properties were studied in bead-on-plate welds in a Ti-6Al-2V-1Mo alloy. The heat affected
zone (HAZ) and the fusion zone (FZ) consisted of very large primary β grains with the β-phase transformed to martensite. A special bead-on-plate welding technique allowed independent measurement of the mechanical
properties of the HAZ and the FZ. Compared to the as-received (AR) material, the strength and ductility decreased in the weld.
The highest fatigue strength was found for the AR material followed by the HAZ and the FZ, whereas the ranking for fatigue
crack growth was opposite. 相似文献
11.
Duplex grades have, due to balanced chemical compositions of both filler and base metals, a weldability that allows for successful welding using a majority of the technically relevant techniques of today. In order to fulfil the performance requirements several aspects must be considered. In the heat affected zone (HAZ) the austenite reformation must be reasonably high and in the weld metal the microstructure must be stable so that e.g. high productivity welding and multi‐pass welding are possible, without precipitation of detrimental phases in previous passes. This paper addresses the effect of alloying elements and thermal cycles on phase balance in the high temperature HAZ (HTHAZ) of the newly developed lean duplex grade LDX 2101 (EN 1.4162, UNS S32101). Bead‐on‐plate welds and simulated weld structures have been produced and investigated using metallography, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results are analysed using the thermodynamic database Thermo‐Calc and a model for phase transformation based on a paraequilibrium assumption for ferrite‐austenite transformation. In the temperature region outside the paraequilibrium domain, growth controlled by diffusion of substitutional elements was considered. The analysis follows a model by Cahn regarding grain boundary nucleated growth and the Hillert‐Engberg model on kinetics of spherical and planar growth. 相似文献
12.
Phase transformations that occur in the heat-affected zone (HAZ) of gas tungsten arc welds in AISI 1005 carbon-manganese steel
were investigated using spatially resolved X-ray diffraction (SRXRD) at the Stanford Synchrotron Radiation Laboratory. In situ SRXRD experiments were performed to probe the phases present in the HAZ during welding of cylindrical steel bars. These real-time
observations of the phases present in the HAZ were used to construct a phase transformation map that identifies five principal
phase regions between the liquid weld pool and the unaffected base metal: (1) α-ferrite that is undergoing annealing, recrystallization, and/or grain growth at subcritical temperatures, (2) partially transformed
α-ferrite co-existing with γ-austenite at intercritical temperatures, (3) single-phase γ-austenite at austenitizing temperatures, (4) δ-ferrite at temperatures near the liquidus temperature, and (5) back transformed α-ferrite co-existing with residual austenite at subcritical temperatures behind the weld. The SRXRD experimental results were
combined with a heat flow model of the weld to investigate transformation kinetics under both positive and negative temperature
gradients in the HAZ. Results show that the transformation from ferrite to austenite on heating requires 3 seconds and 158°C
of superheat to attain completion under a heating rate of 102°C/s. The reverse transformation from austenite to ferrite on
cooling was shown to require 3.3 seconds at a cooling rate of 45 °C/s to transform the majority of the austenite back to ferrite;
however, some residual austenite was observed in the microstructure as far as 17 mm behind the weld. 相似文献
13.
The present study has been carried out to investigate the coarse-grained heat-affected zone (CGHAZ) microstructure and crack
tip opening displacement (CTOD) toughness of grade StE 355 Ti-microalloyed offshore steels. Three parent plates (40-mm thick)
were studied, two of which had Ti microalloying with either Nb + V or Nb also present. As a third steel, conventional StE
355 steel without Ti addition was welded for comparison purposes. Multipass tandem submerged arc weld (SAW) and manual metal
arc weld (SMAW) welds were produced. Different heat-affected zone (HAZ) microstructures were simulated to ascertain the detrimental
effect of welding on toughness. All HAZ microstructures were examined using optical and electron microscopy. It can be concluded
that Ti addition with appropriate steel processing, which disperses fine TiN precipitates uniformly, with a fine balance of
other microalloying elements and with a Ti/N weight ratio of about 2.2, is beneficial for HAZ properties of StE 355 grade
steel. 相似文献
14.
Three low carbon structural steels of different plate thickness have been investigated for hydrogen assisted cold cracking by the IRC weldability test at different restraint intensities. At diffusible hydrogen levels of 10–15 N ml/100 g Fe (ISO 3690), cracking decreases at increasing heat inputs due to a drop in restraint stress and hardness as well as an increase in hydrogen diffusion times. Critical heat inputs for crack prevention range from 0.95 to 1.4 kJmm ?1. Higher restraints enforce higher cracking stresses as well as final stresses of uncracked test welds. Higher restraints and lower heat inputs also induce faster stress increase during cooling which, for the steels containing Ni and Cu, shift the location of cracking from the HAZ to the weld metal. The steel without Ni and lower maximum HAZ hardness reveals weld metal cracking only, regardless of welding conditions. It can be concluded that for weld metal cracking, the relation between stress increase- and hydrogen effusion rates but also the relation between weld metal and HAZ microstructure and mechanical properties are responsible. 相似文献
15.
A new high strength, high toughness steel containing Cu for precipitation strengthening was recently developed for naval,
blast-resistant structural applications. This steel, known as BlastAlloy160 (BA-160), is of nominal composition Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V
(wt pct). The evident solidification substructure of an autogenous gas tungsten arc (GTA) weld suggested fcc austenite as
the primary solidification phase. The heat-affected zone (HAZ) hardness ranged from a minimum of 353 HV in the coarse-grained
HAZ (CGHAZ) to a maximum of 448 HV in the intercritical HAZ (ICHAZ). After postweld heat treatment (PWHT) of the spot weld,
hardness increases were observed in the fusion zone (FZ), CGHAZ, and fine-grained HAZ (FGHAZ) regions. Phase transformation
and metallographic analyses of simulated single-pass HAZ regions revealed lath martensite to be the only austenitic transformation
product in the HAZ. Single-pass HAZ simulations revealed a similar hardness profile for low heat-input (LHI) and high heat-input
(HHI) conditions, with higher hardness values being measured for the LHI samples. The measured hardness values were in good
agreement with those from the GTA weld. Single-pass HAZ regions exhibited higher Charpy V-notch impact toughness than the
BM at both test temperatures of 293 K and 223 K (20 °C and –50 °C). Hardness increases were observed for multipass HAZ simulations
employing an initial CGHAZ simulation. 相似文献
16.
An experimental plate steel with the composition Fe-1.39Cu-2.7Ni-0.58Al-0.48Mn-0.48Si-0.065Nb-0.05C (wt pct) or alternatively Fe-1.43Cu-2.61Ni-1.21Al-0.48Mn-0.98Si-0.039Nb-0.23C at. pct has been recently produced at Northwestern University for use in Naval hull and deck applications—it is designated NUCu-140. To understand the microstructural changes occurring in NUCu-140 steel after gas-metal arc welding (GMAW), a detailed study of the heat-affected and fusion zones was performed throughout the weld cross section using microhardness, metallographic, chemical, and atom-probe tomographic analyses. Local-electrode atom-probe (LEAP) tomography was employed to measure the morphology and compositions of Cu-rich precipitates from each region. The mean radius, number density, volume fraction, and compositions of the precipitates, as well as the interfacial concentration profiles, are measured. The Cu precipitates dissolve partially from the heat-affected zone (HAZ) thermal cycle, and freshly formed sub-nanometer radius Cu-rich precipitates nucleate in both the HAZ and fusion zone (FZ) during cooling; however, the precipitation of Cu during cooling in the HAZ and FZ is not sufficient to restore the lost strength. The precipitation in the FZ is reduced compared to the HAZ due to a mismatched Cu composition of the weld. Multi-pass welding is suggested to restore strength in the GMAW sample by promoting Cu precipitate nucleation and growth in the HAZ and FZ. 相似文献
17.
The microstructure of the heat-affected zone (HAZ) in bead-on-plate welded 17–4 PH stainless steel was studied with special reference to the roles of prior heat treatment and heat input during welding. The HAZ in solution-annealed condition consists of three different microstructural zones containing: (i) retransformed martensite and reformed austenite; (ii) overaged martensite; and (iii) under-aged martensite. In aged condition the HAZ consists of zones (i) and (ii), while in overaged condition it consists almost entirely of zone (ii). The HAZ in solution-annealed and aged conditions is characterised by steep gradients in hardness, while in overaged condition it has uniform hardness throughout. A good correlation was obtained between the calculated temperature distribution in the HAZ and the observed microstructural features. 相似文献
18.
Theoretical and experimental investigations were carried out to determine the effect of process parameters on weld metal microstructures
of austenitic stainless steels during pulsed laser welding. Laser welds made on four austenitic stainless steels at different
power levels and scanning speeds were considered. A transient heat transfer model that takes into account fluid flow in the
weld pool was employed to simulate thermal cycles and cooling rates experienced by the material under various welding conditions.
The weld metal thermal cycles and cooling rates are related to features of the solidification structure. For the conditions
investigated, the observed fusion zone structure ranged from duplex austenite (γ)+ferrite (δ) to fully austenitic or fully
ferritic. Unlike welding with a continuous wave laser, pulsed laser welding results in thermal cycling from multiple melting
and solidification cycles in the fusion zone, causing significant post-solidification solid-state transformation to occur.
There was microstructural evidence of significant recrystallization in the fusion zone structure that can be explained on
the basis of the thermal cycles. The present investigation clearly demonstrated the potential of the computational model to
provide detailed information regarding the heat transfer conditions experienced during welding. 相似文献
19.
BlastAlloy 160 (BA160) is a low-carbon martensitic steel strengthened by copper and M 2C precipitates. Heat-affected zone (HAZ) microstructure evaluation of BA160 exhibited softening in samples subjected to the
coarse-grained HAZ thermal simulations of this steel. This softening is partially attributed to dissolution of copper precipitates
and metal carbides. After subjecting these coarse-grained HAZs to a second weld thermal cycle below the A
c1 temperature (at which austenite begins to form on heating), recovery of strength was observed. Atom-probe tomography and
microhardness analyses correlated this strength recovery to re-precipitation of copper precipitates and metal carbides. A
continuum model is proposed to rationalize strengthening and softening in the HAZ regions of BlastAlloy 160. 相似文献
20.
Numerical calculations were used to study the diffusion and thermotransport of hydrogen during deposition and diffusion of hydrogen during desorption of in-service carbon steel pipeline fillet welds prone to hydrogen cracking. During deposition, only a small amount of hydrogen migrates into the HAZ and little migrates into the pipe. Thermotransport decreases the hydrogen concentration in the HAZ and regions of the weld closer to the HAZ, and increases the concentration toward the outer edge of the weld. This effect increases the rate of desorption of hydrogen from the weld, which increases with weld size/pipe wall thickness. The activation energy for desorption varies with the fraction of hydrogen removed and appears to be a function of the activation energy for diffusion and mass fraction of hydrogen in the weld, HAZ, and pipe. The differences in the desorption rate due to thermotransport appear to be due to differences in the activation energy for desorption and hydrogen concentration gradients. Maintaining a 121 °C preheat/interpass temperature during welding significantly reduces post-weld heating times necessary to reach hydrogen levels considered to be safe from hydrogen cracking. Calculated results and experimental results from another study show good agreement for desorption at 132 °C for up to 30 minutes. 相似文献
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