Yoshikuni Nakao

Deterioration of weldability of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W

Summary The purpose of this study is to clarify the mechanism of weld cracking in the HAZ of long-term aged HP heat-resistant cast steel containing Nb, Mo, and W during repair. The results obtained may be summarised as follows: The results obtained in butt welding cracking tests using as-cast specimens and aged specimens (aged for 247 Msec) suggest that ductility-dip cracking occurs in the HAZ of the aged specimens and that this cracking mainly propagates through the microconstituents precipitated at the dendrite grain boundaries. The results obtained in the elevated temperature high-speed tensile tests of aged specimens and as-cast specimens in the temperature range 673–1473 K suggest that the aged specimens sustain an especially heavy loss of ductility in the temperature range below 773 K. The results obtained in the elevated temperature high-speed tensile tests and small-sized U-groove restraint cracking tests at 773 K using aged specimens and specimens aged at 1323 K suggest that the hot ductility of ...

Diffusion Bondability and Joint Properties of .GAMMA.-TiAl. Diffusion Bonding of Intermetallic Compound TiAl(Part 1).

This study was carried out in order to develop a satisfactory technique for joining TiAl intermetallic compounds. Ti-38 mass% Al binary cast alloys were bonded in a vacuum of 26 mPa using the solid state diffusion bonding method by varying the bonding conditions, viz. the bonding temperature varied from 1273 to 1473 K, the bonding pressure varied from 10 to 30 MPa and the bonding time varied from 0.96 to 3.84 ks.From metallurgical point of view, the diffusion bonding diagrams with bonding conditions were produced in order to obtain a sound joint without microvoids and oxides at the bonded interface. According to these diagrams, the joints for mechanical testing were produced at the temperature of 1473 K for 3.84 ks with 15 MPa.The joint tensile strength at room temperature was about 225 MPa and the joints fractured in the base metal zone. However, the joints at the testing temperatures of 1073 and 1273 K fractured at the bonded interface and the joint tensile strength was about 40 MPa lower than that of the base metal, because minimal bonded interface migration occurred.The joints recrystallized at the bonded zone were produced in order to promote the migration of the bonded interface and sequently improved the tensile property at 1273 K for the joint with the previous bonding conditions and with the post-bonding heat-treatments. The joint tensile strength at 1273 K with recrystallized grain size of around 130 μm was about 210 MPa and the joints fractured in the base metal zone departed from the bonded zone even at 1273 K.

Transient Liquid Insert Metal Diffusion Bonding of Nickel-Base Superalloys

Transient Liquid Insert Metal Diffusion Bonding (TLIM bonding) consists of three processes, viz., a dissolution process of base metal, an isothermal solidification process and a homogenizing process. These processes are considered theoretically based on results of current researches.

Low-temperature sensitisation behaviour in weld metal in austenitic stainless steel: Low-temperature sensitisation of austenitic stainless steel welds and its improvement by laser surface melting treatment (1st Report)

Abstract This paper describes an investigation of low-temperature sensitisation (LTS) behaviour in the weld metal of type 308 stainless steel. Three type 308 stainless steels with carbon contents of 0.04, 0.06, and 0.08% were used. TIG welding was employed to prepare the corresponding weld metals, which were subjected to sensitising heat treatment in the 773–1073 K temperature range. The degree of sensitisation was examined by the EPR method and Strauss test. Chromium carbide was precipitated at the δ/γ grain boundaries in the as-welded weld metals. Corrosion test results show that a higher carbon content leads to earlier sensitisation. Sensitisation of weld metals is found to occur faster in those solution heat-treated at 1273 K before sensitising heat treatment. This suggests that the prior chromium carbides have the effect of accelerating sensitisation, clarifying the fact that the LTS phenomenon also occurs in the weld metal. Sensitisation of the weld metal occurs in a much shorter time than in the HA...

Study on bonding of heat-resisting fine ceramics. (Report 6). Microstructural analysis on bonding interface in Si3N4 to metal joints bonded using Cu-Base insert metals.

Bonding of Si3N4 to W using Cu-5%Cr, Cu-1%Nb, Cu-3%V, Cu-5%Ti and Cu-10%Zr insert metals was conducted in a vacuum furnace. The reaction layer existed at the interface between Si3N4 and insert layers in Si3N4-W joints. Cr, Nb, V, Ti or Zr was concentrated in the region adjacent to Si3N4, and Cu was enriched in the center region of insert layer for all insert metals used. From the results of X-ray diffraction analysis, it was found that Cr2N, CrN and CrSi2, NbN and Nb3Si, VN and V6Si5, TiN and Ti5Si3, ZrN and Zr5Si3 existed in thee reaction layer. The thermodynamics calculation suggested that these nitrides and silicides could be formed by the reaction between Si3N4 and Cr, Nb, V, Ti or Zr containing in the melted insert metal in the first stage of reaction layer formation.

Investigation of accelerated heat treatment conditions for low-temperature sensitisation of austenitic stainless steel weld metals: Study of low-temperature sensitisation of austenitic stainless steel welds and its improvement by laser surface melting treatment (2nd Report)

Summary This paper describes a theoretical investigation of the sensitisation behaviour of type 308 austenitic stainless steel weld metals to determine the accelerated heat treatment conditions for simulation of low-temperature sensitisation (LTS). The theoretical intergranular corrosion parameter ΔS is introduced to evaluate the extent of the Cr depleted zone adjacent to the grain boundaries. Calculation of ΔS shows that its value initially increases and then decreases for some time, which corresponds to the stainless steel weld metals loss of intergranular corrosion resistance and its healing during sensitisation heat treatment. The value of ΔS is also found to correspond fairly well with the degree of sensitisation measured by the EPR test. The sensitisation conditions for simulation of LTS are considered on the basis of the ΔS calculation results. The typical sensitisation condition is determined as 773 K × 30 sec corresponding to 573 K × 1.26 × 109 sec.

Study on Welding of the Low Interstitial Ferritic Stainless Clad Steel(Report 6). Evaluation of Corrosion Resistance of 30Cr-2Mo Steel Overlay Weld Metal.

In the previous report, it was indicated that the usage of 19Cr-2Mo-0.25Zr steel filler metal for the first layer and 30Cr-2Mo steel filler metal for the upper layers respectively was effective to ensure a good weld ductility in the overlay welds of the 30Cr-2Mo steel cladding steel.The corrosion resistance of the overlay welds is also important in the practical use of this material.Therefore, the corrosion resistance in such overlay welds has been investigated in this report.The corrosion resistance was compared with ones of 30Cr-2Mo steel base metal and cladding metal of 30Cr-2Mo clad steel. Measurement of anodic polarization curves and pitting corrosion potentials, grain boundary corrosion test, stress corrosion cracking test and crevice corrosion test were performed.The excellent corrosion resistance, which was almost same level as ones of 30Cr-2Mo steel base metal and cladding metal of 30Cr-2Mo clad steel, was obtained by using 19Cr-2Mo-0.25Zr steel filler metal for the first layer and 30Cr-2Mo steel filler metal for 2nd, 3rd and 4th layers.On the basis of these results, the overlay welding procedure by using 30Cr-2Mo steel filler metal for the upper 3 layer after using 19Cr-2Mo-0.25Zr steel filler metal for the first layer was recommended in order to ensure a good corrosion resistance of the overlay welds.

Formation Mechanism of Defects in the Reaction Layer of Si3N4 to Metal Joints. Factors Affecting Bonding Strength of Fine Ceramics to Metals(Report 3).

Formation of defects such as porous zone and crack in the reaction layer of Si3N4 to molybdenum joints was investigated using pressureless sintered (PLS-) and hot-pressed (HP-) Si3N4 with Cu-5%Cr, Cu-1%Nb and Cu-3%V insert metals.The obvious porous zone occurred in the reaction layer of PLS-Si3N4 to molybdenum joints when the reaction layer was relatively thick, while the porous zone did not occur in the reaction layer of HP-Si3N4 to molybdenum joints. The volume change with interfacial reaction between Si3N4 and the reaction layer was estimated from the analytical results of silicon content in the reaction layers. The volume after reaction was seemed to decrease to that before reaction in case that any insert metals were employed during bonding of PLS- and HP-Si3N4 to molybdenum. It was deduced that the porosity in Si3N4 substrate and the volume change before and after interfacial reaction were the important causes of the porous zone formation.The cracking in the reaction layer was appeared to be affected basically by the thermal stress generated during cooling process, and also to be promoted by the porous zone in the reaction layer which could be the stress concentration sources.

Study on welding of the low interstitial ferritic stainless clad steel. (Report 4). Mechanism of improvement of the bend ductility of the overlay weld prepared with zirconium containing low chromium type steel filler metal.

The mechanism of improvement of the bend ductility of the low interstitial ferritic stainless steel overlay welds prepared with zirconium containing low chromium type steel filler metal for the first layer has been investigated.In the previous report, it was indicated that the thickness of chromium carbide, 475°C embrittlement and grain coarsening were factors affecting embrittlement in the overlay welds.Decreasing in chromium content in the first layer of the weld metal contributed to mitigation of 475°C embrittlement and decreasing of the thickness of chromium carbide on the grain boundary in the overlay weld metal. Addition of zirconium to the first layer of the ferritic stainless steel overlay welds made the thickness of chromium carbide decrease and also the grain size refine.These resulted in an excellent weld ductility for the overlay welds.

Study on welding of the low interstitial ferritic stainless clad steel. (Report 3). Countermeasure for improvement of the bend ductility of the overlay weld.

The countermeasure for improvement of the bend ductility of the low interstitial ferritic stainless steel overlay welds has been investigated.The addition of stabilizing elements, especially zirconium, up to about 0.4% to the first layer of the ferritic stainless steel overlay welds with 30% chromium on a low alloy steel improved its bend ductility. The bend ductility of the overlay welds was also improved with decreasing the chromium content in the first layer of the weld metal.On the basis of these results, a composition of 19Cr-2Mo-0.25Zr-Fe was recommended for the filler metal for the first layer of the 30Cr-2Mo steel overlay welding on a low alloy steel in order to ensure a good weld ductility.