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1.
Sunil Goyal K. Laha K. S. Chandravathi K. Bhanu Sankara Rao 《Transactions of the Indian Institute of Metals》2010,63(2-3):461-466
Creep tests were carried out on 2.25Cr-1Mo ferritic steel base metal and its fusion welded joint at 823 K over a stress range of 100–240 MPa. The weld joint possessed lower creep rupture strength than the base metal and the reduction was more at lower applied stresses. The failure occurred in the intercritical region of heat-affected zone (HAZ) of the joint, commonly known as Type IV cracking. Type IV cracking in the joint was manifested as pronounced localization of creep deformation in the soft intercritical region of HAZ coupled with preferential creep cavitation. The creep cavitation in intercritical HAZ was found to initiate at the central region of the creep specimen and propagate outwards to the surface. To explain the above observations, the stress and strain distributions across the weld joint during creep exposure were estimated by using finite element analysis. For this purpose creep tests were also carried out on the deposited weld metal and simulated HAZ structures (viz. coarse-grain structure, fine-grain structure, and intercritically annealed structure) of the joint. Creep rupture strength of different constituents of joint were in the increasing order of intercritical HAZ, fine-grain HAZ, base metal, weld metal and coarse-grain HAZ. Localized preferential creep straining in the intercritical HAZ of weld joint as observed experimentally was supported by the finite element analysis. Estimated higher principal stress at the interior regions of intercritical HAZ explained the pronounced creep cavitation at these regions leading to Type IV failure of the joint. 相似文献
2.
K. Laha K. S. Chandravathi P. Parameswaran Sunil Goyal M. D. Mathew 《Metallurgical and Materials Transactions A》2012,43(4):1174-1186
Evaluations of creep rupture properties of dissimilar weld joints of 2.25Cr-1Mo, 9Cr-1Mo, and 9Cr-1MoVNb steels with Alloy
800 at 823 K were carried out. The joints were fabricated by a fusion welding process employing an INCONEL 182 weld electrode.
All the joints displayed lower creep rupture strength than their respective ferritic steel base metals, and the strength reduction
was greater in the 2.25Cr-1Mo steel joint and less in the 9Cr-1Mo steel joint. Failure location in the joints was found to
shift from the ferritic steel base metal to the intercritical region of the heat-affected zone (HAZ) of the ferritic steel
(type IV cracking) with the decrease in stress. At still lower stresses, the failure in the joints occurred at the ferritic/austenitic
weld interface. The stress-life variation of the joints showed two-slope behavior and the slope change coincided with the
occurrence of ferritic/austenitic weld interface cracking. Preferential creep cavitation in the soft intercritical HAZ induced
type IV failure, whereas creep cavitation at the interfacial particles induced ferritic/austenitic weld interface cracking.
Micromechanisms of the type IV failure and the ferritic/austenitic interface cracking in the dissimilar weld joint of the
ferritic steels and relative cracking susceptibility of the joints are discussed based on microstructural investigation, mechanical
testing, and finite element analysis (FEA) of the stress state across the joint. 相似文献
3.
K. Laha K. S. Chandravathi K. Bhanu Sankara Rao S. L. Mannan D. H. Sastry 《Metallurgical and Materials Transactions A》2001,32(1):115-124
The evaluation of the creep deformation and fracture behavior of a 2.25Cr-1Mo steel base metal, a 2.25Cr-1Mo/2.25Cr-1Mo similar
weld joint, and a 2.25Cr-1Mo/Alloy 800 dissimilar weld joint at 823 K over a stress range of 90 to 250 MPa has been carried
out. The specimens for creep testing were taken from single-V weld pads fabricated by a shielded metal arc-welding process
using 2.25Cr-1Mo steel (for similar-joint) and INCONEL 182 (for dissimilar-joint) electrodes. The weld pads were subsequently
given a postweld heat treatment (PWHT) of 973 K for 1 hour. The microstructure and microhardness of the weld joints were evaluated
in the as-welded, postweld heat-treated, and creep-tested conditions. The heat-affected zone (HAZ) of similar weld joint consisted
of bainite in the coarse-prior-austenitic-grain (CPAG) region near the fusion line, followed by bainite in the fine-prior-austenitic-grain
(FPAG) and intercritical regions merging with the unaffected base metal. In addition to the HAZ structures in the 2.25Cr-1Mo
steel, the dissimilar weld joint displayed a definite INCONEL/2.25Cr-1Mo weld interface structure present either as a sharp
line or as a diffuse region. A hardness trough was observed in the intercritical region of the HAZ in both weld joints, while
a maxima in hardness was seen at the weld interface of the dissimilar weld joint. Both weld joints exhibited significantly
lower rupture lives compared to the 2.25Cr-1Mo base metal. The dissimilar weld joint exhibited poor rupture life compared
to the similar weld joint, at applied stresses lower than 130 MPa. In both weld joints, the strain distribution across the
specimen gage length during creep testing varied significantly. During creep testing, localization of deformation occurred
in the intercritical HAZ. In the similar weld joint, at all stress levels investigated, and in the dissimilar weld joint,
at stresses ≥150 MPa, the creep failure occurred in the intercritical HAZ. The fracture occurred by transgranular mode with
a large number of dimples. At stresses below 150 MPa, the failure in the dissimilar weld joint occurred in the CPAG HAZ near
to the weld interface. The failure occurred by extensive intergranular creep cavity formation. 相似文献
4.
P. Parameswaran K. Laha K. S. Chandravathi K. Bhanu Sankara Rao 《Transactions of the Indian Institute of Metals》2010,63(2-3):479-483
Fusion welded joints of Cr-Mo steels fail prematurely under creep condition at the heat affected zone (HAZ) close to the base metal, termed as type IV cracking. Optical metallography and hardness testing across the joint establish that the type IV cracking occurs in the soft intercritical HAZ. Based on detailed microstructural studies carried out to understand the evolution of the microstructure and its role in determining the tendency for type IV cracking, the factors that lead to deterioration of creep strength in intercritical HAZ in weld joint of Cr-Mo steels are:
- fine grained structure
- coarse M23C6 carbides at grain and sub-grain boundaries
- dissolution of M2X and MX types of intragranular precipitates.
5.
6.
Ferritic steels are often used in thick-plate form. The feasibility of electron-beam welding such thick plates and the mechanical
properties of these welds were examined in a recent study. In this investigation, the microstructures of these thick-plate,
electron-beam welds were evaluated. The study was carried out on a 3Cr-1.5Mo-0.1V steel. Weld simulations were used to aid
in the study of the heat-affected zone (HAZ) microstructure. Such simulations allowed for a more reliable and detailed evaluation
of the variation in microstructure with distance from the fusion line. The structures were related to microhardness measurements
made across the width of the weld and the HAZ. The fusion zone and the immediately adjacent HAZ consisted of bainite platelets
with narrow films of retained austenite at many of the bainite platelet boundaries. Farther away from the fusion zone, the
structure was a two-phase mixture of bainitic platelets and ferrite produced by heating base metal between theAc
1 and theAc
3 temperatures. Still farther from the weld, the structure consisted of tempered bainite, with the degree of tempering decreasing
with distance from the fusion line. The bainite plus ferrite region and the tempered bainite section are associated with a
soft zone in the hardness profile across the weld. A postweld heat treatment (PWHT) was found to reduce the hardnesses of
the fusion zone, HAZ, and base material to relatively uniform levels. The structure across the weld and HAZ after a PWHT is
tempered bainite except in one section of the HAZ in which tempered bainite and ferrite coexist. 相似文献
7.
Vani Shankar K. Mariappan R. Sandhya M. D. Mathew T. Jayakumar 《Metallurgical and Materials Transactions A》2014,45(3):1390-1400
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
8.
C. R. Das S. K. Albert J. Swaminathan S. Raju A. K. Bhaduri B. S. Murty 《Metallurgical and Materials Transactions A》2012,43(10):3724-3741
The roles of boron and heat-treatment temperature in improving the type IV cracking resistance of modified 9Cr-1Mo steel weldment were studied. Two different heats of P91 steel, one without boron, designated as P91 and the other with controlled addition of boron with very low nitrogen, designated as P91B, were melted for the current study. The addition of Boron to modified 9Cr-1Mo steel has increased the resistance against softening in fine-grained heat-affected zones (FGHAZ) and intercritical heat-affected zones (ICHAZ) of the weldment. Creep rupture life of boron containing modified 9Cr-1Mo steel weldment, prepared from 1423?K (1150?°C) normalized base metal, was found to be much higher than that prepared from 1323?K (1050?°C) normalized base metal because of the stabilization of lath martensite by fine M23C6 precipitates. This finding is in contrast to the reduction in creep rupture life of P91 weldment prepared from 1423?K (1150?°C) normalized base metal compared with that of the weldment prepared from 1323?K (1050?°C) normalized base metal. The trace of failure path from the weld metal to ICHAZ in P91B weldment was indicative of type II failure in contrast to type IV failure outside the HAZ and base metal junction in P91 weldment, which suggested that boron strengthened the microstructure of the HAZ, whereby the utilization of boron at a higher normalizing temperature seemed to be significantly greater than that at the lower normalizing temperature. 相似文献
9.
V. D. Vijayanand J. Ganesh Kumar P. K. Parida V. Ganesan K. Laha 《Metallurgical and Materials Transactions A》2017,48(2):706-721
Effect of electrode size on creep deformation and rupture behavior has been assessed by carrying out creep tests at 923 K (650 °C) over the stress range 140 to 225 MPa on 316LN stainless steel weld joints fabricated employing 2.5 and 4 mm diameter electrodes. The multi-pass welding technique not only changes the morphology of delta ferrite from vermicular to globular in the previous weld bead region near to the weld bead interface, but also subjects the region to thermo-mechanical heat treatment to generate appreciable strength gradient. Electron backscatter diffraction analysis revealed significant localized strain gradients in regions adjoining the weld pass interface for the joint fabricated with large electrode size. Larger electrode diameter joint exhibited higher creep rupture strength than the smaller diameter electrode joint. However, both the joints had lower creep rupture strength than the base metal. Failure in the joints was associated with microstructural instability in the fusion zone, and the vermicular delta ferrite zone was more prone to creep cavitation. Larger electrode diameter joint was found to be more resistant to failure caused by creep cavitation than the smaller diameter electrode joint. This has been attributed to the larger strength gradient between the beads and significant separation between the cavity prone vermicular delta ferrite zones which hindered the cavity growth. Close proximity of cavitated zones in smaller electrode joint facilitated their faster coalescence leading to more reduction in creep rupture strength. Failure location in the joints was found to depend on the electrode size and applied stress. The change in failure location has been assessed on performing finite element analysis of stress distribution across the joint on incorporating tensile and creep strengths of different constituents of joints, estimated by ball indentation and impression creep testing techniques. 相似文献
10.
Shaju K. Albert Masayuki Kondo Masaaki Tabuchi Fuxing Yin Kota Sawada Fujio Abe 《Metallurgical and Materials Transactions A》2005,36(2):333-343
New ferritic steels with a controlled addition of boron have been developed recently for ultrasuper-critical fossil power
plants. These steels possess excellent creep resistance compared to conventional steels like P91, P92, P122, etc., and this has been attributed to the delay in coarsening of the carbides during creep owing to partial replacement of carbon
by boron in these carbides. However, the susceptibility of the weld joints of the boron-containing ferritic steels to type
IV cracking, which significantly brings down the rupture life of the weld joints, has not been investigated so far. In the
present work, the creep properties of recently developed 9Cr-3W-3Co-NbV steels with boron contents varying from 47 to 180
ppm and of their weld joints have been studied. Creep tests were carried out at 923 K in the stress range of 140 to 80 MPa.
Specimens were examined for particle coarsening using field-emission scanning electron microscopy, and the boron content in
the precipitates was estimated using field-emission auger electron spectroscopy (FE-AES). The grain size of the parent metal
and the heat-affected zone (HAZ) were estimated using electron backscattered pattern (EBSP) imaging. Results showed that the
creep properties of the steels with 90 and 130 ppm boron and of their weld joints are superior to those of the P92 steels
and its weld joints. Further, no weld joints exhibited type IV cracking. No significant coarsening of the carbides was observed,
not only in the parent metal but also in the HAZ of the steels with ≥90 ppm of boron. In addition to the delay in carbide
coarsening, the large prior-austenite grain size of the parent metal and the absence of a conventional fine-grained HAZ (FGHAZ)
in the weld joints also seem to have a beneficial effect on improving the creep properties of these steels and their weld
joints. 相似文献
11.
Y. Liu S. Tsukamoto T. Shirane F. Abe 《Metallurgical and Materials Transactions A》2013,44(10):4626-4633
The mechanism of type IV failure has been investigated by using a conventional 9Cr ferritic heat-resistant steel Gr.92. In order to clarify the main cause of type IV failure, different heat treatments were performed on the base metal in order to change the prior austenite grain (PAG) size and precipitate distribution after applying the heat-affected zone (HAZ) simulated thermal cycle at the peak temperature of around A c3 (A c3 HAZ thermal cycle) and postweld heat treatment (PWHT). The microstructural evolution during the A c3 HAZ thermal cycle and PWHT was investigated by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). It was found that M23C6 carbides were scarcely precipitated at the newly formed fine PAG, block, and lath boundaries in A c3 HAZ-simulated Gr.92, because the carbide forming elements such as Cr and C were segregated at the former PAG and block boundaries of the base metal. On the other hand, if all the boundaries were covered by sufficient M23C6 carbides by homogenization of the alloying elements prior to applying the HAZ thermal cycle, the creep strength was much improved even if the fine PAG was formed. From these results, it is concluded that fine-grained microstructure cannot account for the occurrence of type IV failure, and it only has a small effect during long-term creep. The most important factor is the precipitate formation behavior at various boundaries. Without sufficient boundary strengthening by precipitates, the microstructure of A c3 HAZ undergoes severe changes even during PWHT and causes premature failure during creep. 相似文献
12.
13.
Hiromichi Hongo Masaaki Tabuchi Takashi Watanabe 《Metallurgical and Materials Transactions A》2012,43(4):1163-1173
Modified 9Cr-1Mo steel (ASME Grade 91 steel) is used as a key structural material for boiler components in ultra-supercritical
(USC) thermal power plants at approximately 873 K (600 °C). The creep strength of welded joints of this steel decreases as
a result of Type IV creep cracking that forms in the heat-affected zone (HAZ) under long-term use at high temperatures. The
current article aims to elucidate the damage processes and microstructural degradations that take place in the HAZ of these
welded joints. Long-term creep tests for base metal, simulated HAZ, and welded joints were conducted at 823 K, 873 K, and
923 K (550 °C, 600 °C, and 650 °C). Furthermore, creep tests of thick welded joint specimens were interrupted at several time
steps at 873 K (600 °C) and 90 MPa, after which the distribution and evolution of creep damage inside the plates were measured
quantitatively. It was found that creep voids are initiated in the early stages (0.2 of life) of creep rupture life, which
coalesce to form a crack at a later stage (0.8 of life). In a fine-grained HAZ, creep damage is concentrated chiefly in an
area approximately 20 pct below the surface of the plate. The experimental creep damage distributions coincide closely with
the computed results obtained by damage mechanics analysis using the creep properties of a simulated fine-grained HAZ. Both
the concentration of creep strain and the high multiaxial stress conditions in the fine-grained HAZ influence the distribution
of Type IV creep damage. 相似文献
14.
Vani Shankar M. Valsan K. Bhanu Sankara Rao S. D. Pathak 《Transactions of the Indian Institute of Metals》2010,63(2-3):622-627
The aim of the present paper is to study the low cycle fatigue and creep-fatigue interaction behavior of modified 9Cr-1Mo ferritic steel weld joint. Total axial strain controlled continuous cycling tests were conducted between 773 K and 873 K and at strain amplitudes ±0.25%, ±0.4%, ±0.6% and ±1%. Hold tests were also conducted at +0.6% and 823 K and 873 K temperatures to study the creep-fatigue interaction behavior of the weld joint. The alloy exhibited cyclic softening from first cycle onwards irrespective of the loading conditions. Failure location in the weld joint was correlated to the test parameters. Detailed replica study conducted on all the failed specimens revealed that most of the failures occurred in one side of the heat affected zone (HAZ) of the weld joint. Strain localization in the soft zone of the HAZ and subsurface creep cavity formation in this region and their linkage had caused enhanced crack propagation that translated into lower fatigue life of the weld joint at high temperatures. Type IV mode of failure was identified to be operative under tensile hold and high temperatures. The alloy was also found to be compressive dwell sensitive and it was ascertained that the lower life under compression hold compared to tension hold was due to the deleterious effect of oxidation. 相似文献
15.
Ultra‐fine grained ferrite steels have higher strength and better toughness than the normal ferrite steels because of their micrometer or sub‐micrometer sized grains. In this paper the ultra‐fine grained steel SS400 is welded by CO2 laser. The shape of weld, cooling rate of HAZ, width of HAZ, microstructures and mechanical properties of the joint are discussed. Experimental results indicate that laser beam welding can produce weld with a large ratio of depth to width. The cooling rate of HAZ of laser beam welding is fast, the growth of prior austenite grains of HAZ is limited, and the width of weld and HAZ is narrow. The microstructures of weld metal and coarse‐grained HAZ of laser beam welding mainly consist of BL + M (small amount). With proper laser power and welding speed, good comprehensive mechanical properties can be acquired. The toughness of weld metal and coarse‐grained HAZ are higher than that of base metal. There is no softened zone after laser beam welding. The tensile strength of a welded joint is higher than that of base metal. The welded joint has good bending ductility. 相似文献
16.
核能已经逐渐取代化石能源成为新一代能源,作为重要构件的高温气冷堆中间换热器得到了广泛关注。由于GH3128合金具有较好的焊接性、较高的高温抗氧化性能和组织稳定性,有望成为超高温气冷堆中间换热器的候选材料,但基于换热器结构复杂性以及密封性的要求,焊接是其生产和制造的关键成形手段。采用脉冲钨极氩弧焊(GTAW)对GH3128合金2 mm板材进行对接焊,研究了热处理对焊接焊接接头显微组织以及应力的影响。结果表明,在优化焊接工艺参数下,固溶态板材接头表现出最高的强塑性,室温及高温拉伸断裂位置均为母材。由于热轧态与固溶态板材接头热影响区在焊接过程中产生残余应力,导致该区硬化,在高温变形过程中残余应力诱发热影响区μ相析出,对接头持久、蠕变性能造成不利影响。焊后热处理消除了接头热影响区的残余应力,减少了持久、蠕变过程中μ相的析出,接头持久寿命得以改善。在1 200 ℃下,残余应力可为焊后热处理过程中静态再结晶提供激活能,接头热影响区发生再结晶,硬度下降,接头塑性变形能力不协调,导致室温拉伸与950 ℃拉伸断裂位置均为焊接接头。对固溶态板材试样进行不同的焊后热处理,EBSD扫描结果分析发现,接头经过1 100 ℃×10 min热处理后,残余应力明显消失,温度升高至1 140 ℃后,热影响区开始发生再结晶。 相似文献
17.
Dejun Li Kenji Shinozaki Hidemasa Harada Kenji Ohishi 《Metallurgical and Materials Transactions A》2005,36(1):107-115
This article is aimed at investigating the difference in precipitation behavior in the fine-grained heat-affected zone (FGHAZ),
coarse-grained heat-affected zone (CGHAZ), and base metal for the welded joint of high Cr ferritic heat-resistant steel (11Cr-0.4Mo-2W-1Cu-V-Nb,
normalized 1323 K×1 h and tempered 1033 K×1 h). Simulated HAZ (SHAZ) specimens were used, whose thermal cycles were controlled
to be the same as those in the actual welded joint with peak temperatures of 1523 and 1173 K to represent CGHAZ and FGHAZ,
respectively. Based on scanning electron microscopy (SEM) observation, it looks that the precipitates in FGHAZ specimens (1173
K) were fewer and larger than those in CGHAZ (1523 K) specimens and base metal specimens. This phenomenon implied that the
growth and coarsening of precipitates in FGHAZ may play a role in the deterioration of creep properties and type IV cracking,
which was observed in previous creep tests. X-ray diffraction analysis for the electrolytic extraction showed that the types
of precipitates are the same for the 1173 K specimens and base metal specimens, including M23C6, MX, Laves phase, and μ phase. Further, the elemental analysis of the extraction showed that the mass percentages of Cr, W, and Mo in the precipitates
to specimen mass were higher in the FGHAZ specimen than those in the base metal specimen, especially during the period between
600 and 2464 hours. Finally, a two-dimensional (2-D) model was proposed to simulate the precipitation behavior of the Laves
phase. 相似文献
18.
A. Foroughi M. Shamanian M. Kasiri-Asgarani J. Kangazian 《Transactions of the Indian Institute of Metals》2018,71(3):533-544
In the present research, microstructure and mechanical properties of 2205 duplex stainless steel/A517 quench and tempered low alloy steel dissimilar joint were investigated. For this purpose, gas tungsten arc welding was used with ER2209 filler metal. Characterizations were conducted by optical microscopy, scanning electron microscopy equipped with an energy dispersive spectroscopy and X-ray diffraction. Mechanical properties were evaluated in micro-hardness, tensile and impact tests. Microstructure in the weld zone included an austenitic continuous network in the matrix of primary ferrite. No brittle phases were formed in the weld metal and stainless steel heat affected zone (HAZ). The weld metal/A517 interface showed higher hardness than other regions. Tensile tests indicated that the values of the yield and tensile strength were 663 and 796 MPa, respectively. Impact tests indicated that the weld zone had almost the same impact energy as base metals. The minimum impact energy of 12 J was related to A517 HAZ. The results of scanning electron microscopy for fracture surfaces indicated that weld zone, 2205 HAZ and A517 HAZ had ductile, ductile–brittle and brittle fracture mode, respectively. 相似文献
19.
R. K. Singh Raman 《Metallurgical and Materials Transactions A》1995,26(7):1847-1858
In order to study the influence of microstructural variation on the oxidation of the weldment of 2.25Cr-1Mo steel, regions
with different microstructures were identified by optical microscopy. The weld metal, the base metal, and the heat-affected
zone (HAZ), as well as the subzones within the HAZ, i.e., the intercritical (ICR), the fine-grain bainite (FGB), and the coarse-grain
bainite (CGB) regions were separated from the weldment by precise steps of metallography. Transmission electron microscopic
examinations for the identification of the secondary phases in microstructurally different regions and subzones have suggested
that M23C6 and M7C3 pre-cipitates form predominantly in the subzones of HAZ, whereas the Mo2C type of carbide forms exclusively in the weld-metal and base-metal regions of the weldment. However, population and distribution
of the secondary phases were different in the three subzones of the HAZ. In order to understand the influence of these microstructural
variations on the oxidation behavior, the various regions and subzones were oxidized at 773 and 873 K. The HAZ and its constituents
were found to oxidize at much higher rates than the weld metal and the base metal. Relative compositions and morphologies
of the scales were compared by scanning electron microscopy with energy-dispersive analyses of X-rays (SEM/EDX), and secondary
ion mass spectrometry (SIMS). Scale formed over the weld metal shows a greater tendency for spallation, as suggested by tests
monitoring acoustic emission. X-ray diffraction (XRD) patterns of the scales over these specimens were taken. Results of the
SEM/EDX, SIMS, and XRD investigations suggest for-mation of inner scales with less Cr(i.e., less protective) over the HAZ than over the weld-metal and the base-metal regions. Variation in the Cr contents of the scales
formed over the various regions is proposed to arise from the difference in microstructural features in different regions
of the weldments.
Formerly with the Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkan, India 相似文献
20.
The aluminum alloy 6013 was friction-stir welded in the T4 and the T6 temper, and the microstructure and mechanical properties
were studied after welding and after applying a postweld heat treatment (PWHT) to the T4 condition. Optical microscopy (OM),
transmission electron microscopy (TEM), and texture measurements revealed that the elongated pancake microstructure of the
base material (BM) was transformed into a dynamically recrystallized microstructure of considerably smaller grain size in
the weld nugget. Strengthening precipitates, present before welding in the T6 state, were dissolved during welding in the
nugget, while an overaged state with much larger precipitate size was established in the heat-affected zone (HAZ). Microhardness
measurements and tensile tests showed that the HAZ is the weakest region of the weld. The welded sheet exhibited reduced strength
and ductility as compared to the BM. A PWHT restored some of the strength to the as-welded condition. 相似文献