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QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol. 5 (1987), No. 3

ISIJ International
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ONLINE ISSN:
PRINT ISSN: 0288-4771
Publisher: JAPAN WELDING SOCIETY

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QUARTERLY JOURNAL OF THE JAPAN WELDING SOCIETY Vol. 5 (1987), No. 3

Observation and Digital Control of Weld Pool in Pulsed MIG Welding

Kenji Ohshima, Masaki Morita, Kazuo Fujii, Mitsuyoshi Yamamoto, Takefumi Kubota

pp. 304-311

DOI:

10.2207/qjjws.5.304

Abstract

It is important as a basis of research in automatic arc welding to control the weld pool shape against the disturbance and the variation of system parameters. For this purpose it is useful to control the weld pool width. A basic visual robot is proposed for observing and controlling the weld pool shape by using the pulsed current.
It has been difficult to observe the weld pool with a TV camera, since the arc light is too strong to watch the molten metal under the arc. The authors tried to make periodically the current low enough to watch clearly the weld pool with CCD (Charge coupled device) camera without after image.
The weld pool is recognized by taking advantage of the feature on the brightness of the weld pool and its outskirts. A personal computer calculates the weld pool width and area in real time. The authors study the transient response of the weld pool width and area.
The welding current is determined to get a desired response of the weld pool. It is shown how to determine the digital controller characteristic. Small variations of system parameter during operation caused by the change in the base metal thickness, heat conductivity, temperature rise are allowed while the prescribed control performance is maintained.
Experiments demonstrate the good controllability of the adjustable weld pool formation with the digital controller.

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Observation and Digital Control of Weld Pool in Pulsed MIG Welding

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Estimation of Contact Tip-Workpiece Distance in GMA Welding

Hiroshi Fujimura, Eizo Ide, Hironori Inoue

pp. 311-316

DOI:

10.2207/qjjws.5.311

Abstract

The weld line tracking sensor is indispensable to improve the flexibility of the arc welding robot applications. Recently, a sensing method which utilizes the electric arc phenomenon, or more correctly the welding current, has been developed and prevalently used. The sensing accuracy, however, is directly affected by the estimate of the contact tip-workpiece distance.
The authors investigated the algorism of estimating the contact tip-workpiece distance quantitatively on the assumption that the welding arc is in an equilibrium state, with the following results.
(1) The electrode extension (LE) in each of the pulsed-current and non-pulsed-current MAG processes can be calculated using average value (Ia) and effective value (Ie) of current and wire feed rate (v).
LE=(v-K3Ia)/(K4I2e) where K3, K4=constant
(2) The voltage drop (VE) in the extended electrode can be calculated with Ia, Ie, v and LE.
VA=(4η0Ia/πd2)[(1-β/α)LE+(βv/αbI2e){exp(bI2eLE/v)-1}]
(3) Then, the arc length (LA) can be calculated as follows, from Ia and from Va which is given by subtracting VE from welding voltage (V).
LA=(VA-K6-K8Ia)/(K5+k7Ia) where K5-K8=constant
(4) The contact tip-workpiece distance (LE+LA) can be correctly estimated by the above equations.
It is possible, by judging a torch position displacement in the groove on basis of the contact tip-work-piece distance trend during the oscillation, to establish the arc sensing system capable of tracking the weld line with the higher degree of accuracy than hitherto.

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Estimation of Contact Tip-Workpiece Distance in GMA Welding

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Diffusion Bonding of A6063 Aluminum Alloy Reinforced with Alumina Short Fibers

Toshio Enjo, Kenji Ikeuchi, Yotaro Murakami, Nobuyuki Suzuki

pp. 317-323

DOI:

10.2207/qjjws.5.317

Abstract

The Al-Mg-Si A6063 alloy reinforced with alumina short fibers has been diffusion-bonded with the bond interface perpendicular to the direction of fibers, i.e., direction of the highest strength, and the effects of surface treatments, bonding parameters (temperature and pressure) and insert metals on the bond strength have been systematically investigated. The surface treatments used were (i) electropolishing, (ii) turning in a lathe and (iii) wire brushing. On the faying surface finished by (i), fibers protruded from the surface of the matrix metal were observed, and their height increased with electropolishing time. Treatment (i) resulted in bond strength considerably higher than (ii) and (iii), though the bond strength was decreased with the increase in the protruded height of fibers more than 8 μm. This decrease in the bond strength was probably due to the suppression of intimate contact between matrix surfaces by protruded fibers which were broken by bonding pressure and laid on the bond interface. In the case of (iii), a lot of fine fragments of the fiber broken by the wire brushing and an increase in iron content were observed at the bond interface. These fragments of the fiber and increase in iron content seem to have harmful effects on the bond strength, since (iii) resulted in the lowest bond strength. In the case of (ii), the fine fragment of the fiber was observed, but no increase in iron content could be found. The application of insert metals of Al-Cu-Mg A2017 alloy, copper and silver foils extended the range of bonding conditions to obtain high bond strength, and decreased the bonding deformation; e.g., in the case of (i), bond strength not less than the tensile strength of the base metal was obtained with small bonding deformation by using the insert metal of A2017 alloy or copper.

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Diffusion Bonding of A6063 Aluminum Alloy Reinforced with Alumina Short Fibers

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Effect of Copper Oxide on Joint Strength of Friction Welding of Copper-Tungsten Sintered Alloy to Tough Pitch Copper

Masatoshi Aritoshi, Kozo Okita, Toshio Enjo, Kenji Ikeuchi

pp. 323-329

DOI:

10.2207/qjjws.5.323

Abstract

The copper-tungsten sintered alloy (Cu-W) has been friction welded to the tough pitch copper (CU) and to oxygen free copper (OFC), in order to investigate the effects of copper oxides included in CU on the tensile strength of the joint. The tensile strength of the joint of Cu-W to CU (Cu-W/CU joint), which increased with the rise of the forge pressure P2, was considerably lower than that of the joint of Cu-W to OFC (Cu-W/OFC joint) at P2 below 250 MPa. The low tensile strength of Cu-W/CU joints can be attributed to uncontacted spots which could be observed on the fractured surface of the joint, since no uncontacted spot could be observed for the Cu-W/OFC joint at P2 above 150 MPa. Tensile tests at elevated temperatures and marker experiment using alumina powder show that the difference in the uncontacted spot can be explained by the difference in the degree of plastic flow which facilitated the attainment of intimate contact at the bond interface. On the other hand, at P2 above 290 MPa, the fracture of Cu-W/CU joints occurred in CU immediately next to the bond interface, in contrast to the heat affected zone of OFC for Cu-W/OFC joints, though the difference in their tensile strength became very small. On fractured surfcaes of Cu-W/CU joints, a lot of copper oxides as well as tungsten particles picked-up from Cu-W during the friction process as reported in a previous paper were observed at dimple bottoms, suggesting that they acted as nucleation sites of fracture. In CU adjacent to the bond interface, the copper oxide, which had a preferential distribution in the axial direction of the specimen in the base metal, was distributed preferentially in a direction parallel to the bond interface. The change in the distribution of copper oxides combined with the picked-up tungsten particle seems to cause the fracture in CU immediately next to the bond interface.

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Effect of Copper Oxide on Joint Strength of Friction Welding of Copper-Tungsten Sintered Alloy to Tough Pitch Copper

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Effects of Ferrite Content and Microsegregation on Solidification Crack Susceptibility of Duplex Stainless Steel Weld Metals

Fukuhisa Matsuda, Hiroji Nakagawa, Ichiro Kato, Yoshiaki Murata

pp. 329-335

DOI:

10.2207/qjjws.5.329

Abstract

Solidification crack susceptibility in weld metal of SUS329J2L steel of which ferrite content was varied in the range of about 0 to 100% was evaluated by Trans-Varestraint cracking test. The susceptibility was partly confirmed by self-restraint cracking test. Microsegregation of such detrimental impurities as P and S was studied in relation to the susceptibility. Main conclusions obtained are as follows:(1) Solidification crack susceptibility was minimum at ferrite level from 5 to 20%, and high not only near full austenite region but also in the level more than 40% ferrite region. (2) The cause of this tendency was inferred from the dependency of the microsegregation behavior of S and P on the ferrite content. That seemed to be because the degree of such interdendritic microsegregation is enhanced in high ferrite level compared with 5 to 20% ferrite level, (3) It was shown that such microsegregation behaivor resulted in making the solidification temperature range wider.

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Effects of Ferrite Content and Microsegregation on Solidification Crack Susceptibility of Duplex Stainless Steel Weld Metals

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Study on Integrity Improvement of Stainless Steel Pipe Welds in Nuclear Power Plant by Local Induction Heating Method

Tasuku Shimizu, Kunio Enomoto

pp. 335-341

DOI:

10.2207/qjjws.5.335

Abstract

The effectiveness of the IHSI method in preventing the intergranular stress corrosion cracking have been proved. This method has also been found to be effective in reducing the fatigue crack propagation rate of cracks in pipes under the repeated loading associated with plant operation.
Hydrostatic testing and external moment loading during operation may cause residual stresses to. relax. Test results showed that compressive residual stresses relax under certain circumstances, but that the compressive residual stresses on the inner surface of a pipe do not relax during operation enough to cause them to change into tensile stresses large enough to cause intergranular stress corrosion cracking.

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Study on Integrity Improvement of Stainless Steel Pipe Welds in Nuclear Power Plant by Local Induction Heating Method

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On the Trial Manufacture of Small Sodium Loop and Study on the Sodium Corro sion of Explosive Plugged Part

Seiich Kaga, Katsuhiro Fujii, Yoshihiro Fujioka, Yoshiaki Yamamoto, Kouichi Ogawa

pp. 342-348

DOI:

10.2207/qjjws.5.342

Abstract

2 1/4 Cr-1 Mo steel is used for the material of tube plate and tubing of evaporator, and SUS 321 stainless steel is used for the material of them of super heater, in steam generator of fast breeder reactor "Monju".
Supposing the case in which leakage takes place in evaporator or super heater of fast breeder reactor, the explosive plugging method was developed by Japan Welding Engineering Society etc. It was indicated by the researchers that this method is effective to the purpose.
It is not yet known whether the plugged parts keep the stable strength or not, during the long service period in the environment of fast breeder reactor.
The corrosive property of explosive plugged part in secondary sodium circuit in the reactor is studied. For this purpose, it is necessary to make the sodium loop apparatus which provides the corrosion environment which resembles that of F.B.R. as much as possible. In this study, because of practical restriction, we are obliged to aim the qualitative comparison between the corrosion characteristics of explosive bonded part in liquid sodium and that of base metal. A small and simple sodium loop is made for trial, and the soaking experiments are conducted.
A small and simple sodium loop is designed first of all. In order to model the environment of secondary sodium circuit of F.B.R., testing temperature, testing period, flow rate of Na, and material of loop are decided, respectively.
Because testing specimen for explosive bonded part can not been cut out at the explosive plugged part, a special clad plate is made. The specimen is cut out at the clad plate.
Testing result shows that explosive bonded boundary is somewhat weaker than base metal in the resistance to the sodium attack under the testing condition employed.

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On the Trial Manufacture of Small Sodium Loop and Study on the Sodium Corro sion of Explosive Plugged Part

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Effect of Surface Conditions of the Electrode on TIC Arc Starting

Shoji Imanaga, Mitsuaki Haneda, Takeshi Araya, Takayuki Kachima

pp. 353-358

DOI:

10.2207/qjjws.5.353

Abstract

Effect of surface conditions of the electrode on TIG arc starting is investigated and electrode surface is observed microscopically.
The normal arc starting decreases with the number of times of tests. The tip of its electrode (YWTh-2) is covered by solidified tungsten film and the rate of normal arc starting is reduced because particles of thorium oxide (thoria) disappear.
When slightly oxigen is added to shielding gas, the ability of arc starting is maintained at normal arc starting. Because tungsten oxide film is formed on electrode surface and its film vaporizes at lower temperature and thoria particles appear.
Electrolytical etching is conceived and thoria particles appear on tungsten surface by etching. The proportion of thoria particles is over primary content (2%), then ability of arc starting is improved by etching treatment.

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Effect of Surface Conditions of the Electrode on TIC Arc Starting

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The Effect of Cooling Rate on Mechanical Properties of Underwater Wet Welds

Yasuo Suga

pp. 358-363

DOI:

10.2207/qjjws.5.358

Abstract

Thermal cycles of welds in underwater wet gravity arc welding were measured. Then the effect of cooling rate on occurrence of underbead cracks and on the mechanical properties of welds were investigated.
Main results are summarized as follows;
(1) In case the shielding processes are used, the cooling rate and the hardness of underwater welds can be lowered certainly.
(2) Underbead cracks occurring in underwater welds of SM50 steel thinner than 9 mm in thickness may be prevented by lowering the cooling rate at 500°C below 35°C/s and the maximum hardness of the weld less than Hv360 approximately. Such lower cooling rate can be obtained by oil putty shielding.
(3) Whether the shielding process are used or not, cooling rate (R) at 500°C and cooling times (S500, S300) of underwater welds from 800°C to 500°C or 300°C may be estimated from the following experimental formula;
R=6.25×105a⋅Q-0.95⋅tβ (°C/s)
S500=455a⋅R-1.09 (s)
S300=780b⋅R-1.09 (s)
where, Q: weld heat input (J/cm), t: thickness of base metal (mm), and a, β, a and b are constants given as follows :
non shielding: α=1, β=0.17, a=1, b=1
refractory shielding: α=0.28, β=0.45, a=1, b=1
oil putty shielding : α=0.036, β=1, a=0.67, b=0.80
(4) The welded joints obtained by oil putty shielding have nearly comparable mechanical properties with those of open air welds.

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The Effect of Cooling Rate on Mechanical Properties of Underwater Wet Welds

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Applicability of Underwater Wet Plasma Welding with Developed Attachment to Horizontal Position Welding

Takeshi Fukushima, Sadao Fukushima, Junichi Kinugawa

pp. 363-369

DOI:

10.2207/qjjws.5.363

Abstract

The applicability of underwater wet plasma welding in the horizontal position was examined by using a welding attachment developed. This assembly, which is designed simply, is attached to the front face of plasma torch.
Welding was carried out in pressurized city water. Appearances of weld beads, cooling cycles at weld bonds and hardnesses of weld metals and HAZ's were investigated by varying weld rotation and welding conditions.
The use of the developed attachment stabilizes a gas cavity formed by plasma jet and protects any molten pool in this cavity from the invasion of environmental water in the horizontal wet welding. This leads to good welds.
Molten metal tends to hang down with increasing in the angle of rotation from 0 deg. (flat position) to 90 deg. (horizontal position). Therefore, from the standpoint of welding procedure, multi-pass welding with a reduced amount of weld metal may be advantageous in the horizontal position.
The ranges of adaptable plasma arc current and plasma gas flow rate in the horizontal position become narrow comparing with those in the flat position.
Cooling of welds in the horizontal position is rapid comparing with that in the flat position. This difference is caused by the fact that the dissipation of heat is accelerated by the repetition of contact with water flowing in immediately after the floating of steam bubbles formed on the backside of base plate.

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Applicability of Underwater Wet Plasma Welding with Developed Attachment to Horizontal Position Welding

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A Study on Formation of Alloyed Layer of Low Melting Temperature on Bonding Surface of IN738LC with Boron Pack Cementation

Mitsuo Kato, Takao Funamoto, Hiroshi Wachi, Takeshi Matsuzaka, Tomohiko Shida, Satoshi Kokura

pp. 370-373

DOI:

10.2207/qjjws.5.370

Abstract

Diffusioin welding method using boron coating layer on bonding surface of IN738LC with pack cementation has been studied.
In this report, effect of pack cementation temperature on formation of boron coated layer, melting temperatures and thickness of the layers were investigated.
The results were as follows: Uniform boron coated layers was obtained by pack cementating at above 873 K.
Thickness of boron coated layers increased with increase of pack cementation temperature.
Meltingtemperatureofboroncoatedlayerdecreasedwithincreaseofpackcementation temperature.
But when vacuum heat treatment for degassing was conducted to boron coated layer, melting temperature became constant regardless of pack cementation temperature.

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A Study on Formation of Alloyed Layer of Low Melting Temperature on Bonding Surface of IN738LC with Boron Pack Cementation

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Diffusion Weldability of IN738LC with Alloyed Layer on Bonding Surface by Boron Pack Cementation

Takao Funamoto, Mitsuo Kato, Hiroshi Wachi, Satoshi Kokura, Tomohiko Shida, Takeshi Matsuzaka

pp. 373-378

DOI:

10.2207/qjjws.5.373

Abstract

Diffusion welding method using boron coating layer on bonding surface of IN738LC with pack cementation has been studied.
In this report, diffusion weldability of IN738LC with boron coating surface layer was investigated.
After boron coating layers were formed on IN738LC specimens, they were separated into two groups. One group was heat treated in vacuum. The other was not treated in vacuum.
The boron coated specimen was then welded to a uncoated IN738LC specimen.
Observation of microstructures and X-ray microanalysis in cross sections were conducted, and the joint strengths were measured.
The results showed that suitable weldment was obtained by remelting boron coating layer formed at above 1073K in vacuum.

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Diffusion Weldability of IN738LC with Alloyed Layer on Bonding Surface by Boron Pack Cementation

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Joining of Alumina/Alumina Using Al-Cu Filler Metal and Its Application to Joning of Alumina/Aluminum

Masaaki Naka, Yutaka Hirono, Ikuo Okamoto

pp. 379-384

DOI:

10.2207/qjjws.5.379

Abstract

The sessile drop technique was conducted to measure the contact angle of molten aluminum-copper (0-100 mass% Cu) alloys against alumina under vacuum, and the joining strength of alumina to alumina joint using aluminum-copper filler metals was measured by fracture shear loading. Further, alumina was metallized with aluminum or 4 mass% copper-aluminum alloy filler, and brazed to A1050 under vacuum. The results obtained are summarized as follows.
(1) The work of adhesion of Al-Cu alloys to alumina at 1373 K increases from 1.31 J/m2 for pure Al to 1.35 J/m2 for 30 mass% Cu containing alloy, and decreases to 0.157 J/m2 for pure copper. This results suggest that on aluminum rich side the interaction of aluminum and copper in the alloys affects the wetting of aluminum and copper against alumina.
(2) The shear strength of alumina/alumina joint with Al-Cu fillers depends on the work of adhesion of the Al-Cu fillers, and also on the strength of the fillers. The fillers composed of (a+θ) phases are applicable to joining of alumina. In particular, alumina/alumina joint with 4 mass%Cu -aluminum filler exhibits the high strength of 157 MPa.
(3) The change in strength of Al2O3/Al2O3 joint with Al-4 mass% Cu filler with testing temperature is small up to 500 K. At further higher temperature the strength of the joint decreases with softening the alloy.
(4) After metallizing alumina with Al or Al-4 mass% Cu filler at 1373 K for 3.6 ks, the alumina is brazed to A 1050 with BA4004 at 883 K for 180 s or 900 s. The joining strength of Al2O3/Al joint where Al2O3 is metallized with Al-4 mass% Cu filler exhibits the higher value than that of Al2O3/Al joint where Al2O3 is metallized with Al.

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Joining of Alumina/Alumina Using Al-Cu Filler Metal and Its Application to Joning of Alumina/Aluminum

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Evaluation of the Wet Erosion Resistant Properties

Masahiro Fukumoto, Makoto Nakaoka, Isao Okane

pp. 385-389

DOI:

10.2207/qjjws.5.385

Abstract

In order to evaluate the erosion properties of sprayed coatings, an erosion testing machine was designed and fabricated. Some discussions are made about the erosion resistant properties of several sprayed coatings, such as Ni-base Cr-B-Si alloys, under wet erosion condition with this machine.
The results obtained are summarized as follows:
1) The reliability of this erosion testing machine for sprayed coating was verified experimentally.
2) The erosion properties were evaluated by the specific erosion rate which was defined as the gradient of erosion weight loss-period curve. And the relation between specific erosion rate and erosion periods were approximated on separate straight lines on a log scale. Each line was divided into two clearly defined portions which had different slopes, one part of the line represented the initial erosion, whereas the other part indicated the constant erosion.
3) The constant erosion resistant properties of each sprayed coating was evaluated by the comparison of the character of these specific erosion rate lines on a log scale without the effect of the initial erosion factors, e, g. surface roughness and sprayed particle's removal. This evaluation method could also be applied to the other experimental data of wet erosion testing.

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Evaluation of the Wet Erosion Resistant Properties

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Investigations on Bending Condition for Welded Carbon Steel Pipe by High Frequency Induction Heating

Toshimi Matsumoto, Teruo Matsumoto, Yasumasa Tamai

pp. 389-396

DOI:

10.2207/qjjws.5.389

Abstract

The induction heating bent pipes of carbon steel welded pipes are used for the piping in nuclear power plants, in place of elbows. This application is useful to suppress the radiation exposure at in-service inspection. The quality of the bent pipes are controlled on the technical standards of welding for electrical equipments. However, the influence of the bending condition has not been yet sufficiently understood on the mechanical nronerties of the bent nines.
The purpose of this investigation is to establish the appropriate bending dondition for the carbon steel weld pipe which corresponds to the carbon steel pipe STPT 42 in JIS G 3456, in relation to the transformation of the structures of the base metal and the weld metal durnig bending. The results are summarized as follows:
(1) The maximum heating temperature should be set in the range from 900°C to 1000°C, in order to assure the high Charpy impact properties.
(2) The maximum heating temperature which is lower than 900°C causes the imperfect transformation of the base metal and the weld metal, then is likely to spoil the Charpy impact properties.
(3) Higher heating rate causes the increase of Ac1 point, remarkably for the base metal which has higher carbon content than weld metal.
(4) Higher cooling rate causes hardening of the base metal and weld metal, however, the transformation temperature does not change remarkably, except for the Ar1 point of base metal.

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Investigations on Bending Condition for Welded Carbon Steel Pipe by High Frequency Induction Heating

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Oxygen Absorption and Oxide Inclusion of Iron Weld Metal during Arc Welding

Takeshi Kuwana, Yoshiro Sato

pp. 396-402

DOI:

10.2207/qjjws.5.396

Abstract

Pure iron plates were welded using pure iron electrode wire in a controlled arc atmosphere. Effects of the oxidizing gas partial pressure in the welding atmospheres of Ar-O2 and Ar-CO2 gas mixture and welding conditions on the oxygen contents and the nonmetallic inclusions of the iron weld mteals were systematically studied.
The results are as follows:
1. The mean temperature of the iron weld pool is about 1910 K.
2. The oxygen contents and the inclusions of the iron weld metals decrease with increasing the welding current and increase with increasing the arc voltage.
3. The oxygen contents and the inclusions of the iron weld metals decrease slightly with increaseing the traveling speed.
4. In Ar-O2 welding atmosphere, the oxygen contents and the inclusions of the iron weld metals increase with increasing the oxygen partial pressure, but the oxygen contents become constant value at about 0.26 wt.% in high oxygen partial pressure atmosphere. In Ar-CO2 welding atmosphere, the oxygen contents and the inclusions of the iron weld metals increase with increasing the carbon dioxide partial pressure. The oxide inclusions are seemed to be FeO.
5. Behavior of the oxygen absorption into the iron weld metal is discussed using thermodynamic data.

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Oxygen Absorption and Oxide Inclusion of Iron Weld Metal during Arc Welding

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Process of Phosphorus Segregation during Rapid Heating of Fe-P-C Alloy

Koreaki Tamaki, Jippei Suzuki, Shin-ichi Yada

pp. 403-409

DOI:

10.2207/qjjws.5.403

Abstract

The microsegregation of phosphorus in steel caused by a rapid heating was investigated using Fe-P-C alloys. The specimens were heated by an electric furnace or oxy-acetylene flame with heating rates ranging from 0.056 to 200 K/s. The phosphorus concentration at the prior-austenite grain boundary was examined quantitatively by the EDX analyses or the grain boundary etching method. The process of phosphorus segregation was clarified as follows. (1) Phosphorus is concentrated at the α/γ interface during the α/γ transformation. Just before completing the transformation, the phosphorus concentration at the α/γ interface reaches a maximum value, and this final interface becomes the austenite grain boundary. (2) The maximum phosphorus concentration at the grain boundary brought by the transformation is reduced by heating the austenite in a higher temperature range. (3) The grain boundary of the rapid heated alloy begins to ilquate at a lower temperature than its equilibrium liquidus temperature. The higher the phosphorus concentration at the grain boundary is, the lower the temperature of beginning the liquation becomes. The phosphorus concentration at the grain boundary is largely increased by this liquation process. (4) There is a critical value of heating rate above which the temperature of beginning the liquation falls adruptly for each alloy; for example 40 K/s for Fe-0.6P-0.3C alloy.

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Process of Phosphorus Segregation during Rapid Heating of Fe-P-C Alloy

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Factors for Formation fo Low-Hardenability Zone in Weld-Heat Affected Zone

Yoshikuni Nakao, Hiroaki Oshige, Shingo Noi, Yasuhiko Nishi

pp. 410-415

DOI:

10.2207/qjjws.5.410

Abstract

In the previous paper, it was confirmed that low hardenability zone was formed in the weld-heat affected zone. This zone was found to correspond to that heated to the peak temperature range of about 900-1100°C in weld thermal cycles.
In this paper, factors for the formation of this low hardenability zone were investigated using some high strength steels.
Main results obtained were as follows.
(1) Microstructure in low hardenability zone of all steels used was fundamentally different from that in coarse grain high hardenability zone. That is, microstructure in the low hardenability zone was characterized by both polygonal ferrite and M-A constituent.
(2) Austenite grain size had little effect on the formation of microstructure in the low hardenability zone.
(3) The formation of compounds (for example, BN) or the segregation of some elements along grain boundary during weld thermal cycles was considered to have little effect on the formation of microstructure in the low hardenability zone.
(4) It was found that carbon distribution was not uniform in the peak temperature range corresponding to low hardenability zone. It was estimated that this heterogeneous distribution of carbon in austenite was the main factor for the formation of microstructure in the low hardenability zone.

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Factors for Formation fo Low-Hardenability Zone in Weld-Heat Affected Zone

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Mechanism of Formation of Microstructure in Low-Hardenability Zone in Weld-Heat Affected Zone

Yoshikuni Nakao, Hiroaki Oshige, Shingo Noi, Yasuhiko Nishi

pp. 415-422

DOI:

10.2207/qjjws.5.415

Abstract

In the previous paper, it was shown that carbon distribution in austenite was not uniform in low hardenability zone in HAZ. This heterogeneous distribution of carbon was estimated to be the main factor for the formation of the microstructure in the low hardenability zone.
In this paper, mechanism of the formation of the microstructure in the low hardenability zone was investigated on the basis of these results.
Main results obtained were as follows.
(1) The amount of high carbon region formed during the transformation to austenite decreased as peak temperature increased. This decrease in the amount of high carbon region was found to correspond with the increase in hardenability.
(2) On cooling, polygonal ferrite was nucleated at low carbon region in austenite, when the peak temperature was within the low hardenability region (that is, high carbon region still remained). As temperature decreased, ferrite grew and the remaining austenite (including high carbon region) became M-A constituent.
(3) As the results, resultant microstructure in the low hardenability zone was composed of polygonal ferrite and M-A constituent. The distribution of M-A constituent in this zone was similar to that in the zone heated to austenite-ferrite two phase temperature range.

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Mechanism of Formation of Microstructure in Low-Hardenability Zone in Weld-Heat Affected Zone

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Mechanism of the Embrittlement in the 30 Cr-2 Mo Steel Overlay Weld

Yoshikuni Nakao, Kazutoshi Nishimoto, Kazunobu Yamazaki, Shingo Noi, Hiroshi Tsukahara, Yasuhiro Hara

pp. 422-429

DOI:

10.2207/qjjws.5.422

Abstract

A study on the mechanism of the embrittlement in the low interstitial ferritic stainless overlay welds has been undertaken using 26Cr-1Mo steels with carbon content ranging 20 to 490 ppm. Tensile test was performed on 26Cr-1Mo steels with various carbon contents and grain diameters in order to obtain the fracture stress, friction stress and Hall-Petch slope of the steels. Various mechanisms proposed in the literatures for brittle fracture were evaluated with comparison the experimental results and the calculated fracture stresses through each models. It was elucidated that the embrittlement of 26Cr-1Mo steel examined could best be explained by Almond's model involving the embrittler effect of grain boundary carbides to give lower crack initiation energy, and it was suggested that the increase in the thickness of grain boundary carbide due to pick-up of carbon from the base metal during welding might be the main cause of the overlay weld embrittlement in the low interstitial ferritic stainless steels.

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Mechanism of the Embrittlement in the 30 Cr-2 Mo Steel Overlay Weld

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