Ryuji Uemori

AP-FIM study on the effect of Mo addition on microstructure in Ti-Nb steel

Abstract AP-FIM technique was applied successfully to analyze the strengthening mechanism of a newly developed Ti-Nb-Mo-bearing fire-resistant steel, which maintained high yield strength at elevated temperatures. FIM observations of Ti-Nb-Mo and Ti-Nb steels before and after aging at 873 K for 0.9 ks made it clear that Nb(C, N) precipitated in ferrite matrix of Ti-Nb-Mo steel during aging was finer in size (2.3 nm) and greater in number than that of Ti-Nb steel. In the as-hot-rolled condition, corresponding to the situation before aging of those steels, a similar tendency was recognized with Nb(C, N), being coarse in size and small in number. These facts suggest that precipitation hardening of Nb(C, N) at 873 K and room temperature in Ti-Nb-Mo steel is higher than that in Ti-Nb steel. AP analyses of Mo-bearing steels, Ti-Mo steel and Ti-Nb-Mo steel, before and after aging, clearly yielded us two important facts. One was that almost all Mo atoms were in solution in ferrite matrix of both of the Mo-bearing steels, which suggested Mo solid-solution hardening was equal for both of the Mo-bearing steels. The other was that Mo atoms strongly segregated at the Nb(C, N)/ferrite matrix interface in Ti-Nb-Mo steel, which probably suppressed the diffusion of Nb atoms into Nb(C, N) from ferrite matrix. This directly results in fine precipitates in Ti-Nb-Mo steel.

A combined AP-FIM/HREM approach to the characterization of microstructure in a Mn-added TiAl intermetallic compound

Abstract Analytical techniques of atom probe-field ion microscopy (AP-FIM) and high resolution electron microscopy (HREM) are well suited to the microcharacterization of microstructures. By combining AP-FIM with HREM, it is possible to obtain more accurate structural and chemical information of microstructures. This paper reports the first HREM observation of the tip specimen for AP-FIM analysis. In the present study, the advantage of the combined AP-FIM/HREM technique is demonstrated through the characterization of microstructures in a Mn-added TiAl intermetallic compound.

Ultra-fine structure of a boron-doped Ni4Mo alloy

Abstract The ultra-fine structure of domain boundaries in a boron-doped Ni 4 Mo alloy was investigated by atom-probe field-ion microscopy. In this paper are described: (i) the contact and discontact domain boundaries, (ii) the segregation of boron atoms to three kinds of domain boundaries: anti-phase boundary (APB), anti-parallel twin boundary (APTB) and perpendicular twin boundary (PTB), (iii) the distribution of boron atoms at each domain boundary, (iv) the segregation to a node of domain boundaries and (v) the tendency of binding of boron atoms to Mo and Ni atoms.

Dynamic observation of dislocation movement across twin boundaries in the lamellar structure of TiAl intermetallic compound

Abstract Mechanisms of enhancement of plastic deformation in TiAl responsible for ductility improvement of this material, have been studied by high-voltage electron microscopy (HVEM) and atom probe field ion microscopy (AP-FIM). During cross-twinning, a dislocation interaction occurs at twin boundaries. Dislocation networks and fine Ti 3 Al particles on the pre-existing twin boundaries promote the formation and movement of twinning partial dislocations, contributing to an improvement of ductility.

Development of 6% nickel steel for LNG storage tanks

9% Ni steel has been used for LNG storage tanks for more than four decades although 5.5% Ni steel (N-TUF CR196) was developed in the 1970’s using a special heat treatment method named L-treatment. The reason why the actual application of 5.5% Ni steel has not been attained to LNG storage tanks is mainly because the requirement of fracture properties is not confirmed for the tanks. Under the circumstances of expanding demand for natural gas and double-integrity in LNG storage tanks, we restarted developing low Ni steel for LNG storage tanks by using both conventional and advanced techniques. For the application of low Ni steel to the present LNG storage tanks, both fracture initiation and propagation properties of base metal plates and welded joints should be concerned. The fracture initiation and propagation properties of base metal were compensated with the intercritical reheating process (L-treatment), and the propagation property was additionally enhanced by combining TMCP with L-treatment. In addition, the chemical composition adjustment and the homogenization treatment of solute elements were conducted for improving the fracture initiation and propagation properties of welded joints. 6% Ni steel plates were manufactured by the process of continuous casting, reheating, hot rolling, direct quenching (TMCP), L-treatment, and tempering, and their chemical composition was 0.05C-0.06Si-1.0Mn-6.3Ni-Cr-Mo. As the results of fracture property evaluation including large-scale fracture tests such as the duplex ESSO test and the wide plate tensile test, it was demonstrated that 6% Ni steel has good characteristics regarding brittle fracture initiation and propagation in base metal plates and welded joints.Copyright

High strength UOE pipe with excellent CTOD properties and deformability

New steel manufacturing technology has been developed to attain excellent HAZ toughness of high-strength steels. In this steel, the HAZ microstructure near a weld fusion line is refined markedly by utilizing the strong retardation of austenite grain growth as well as the formation of intragranular ferrite. A dual phase microstructure is very effective for obtaining both high uniform elongation and low Yield / Tensile ratio in the high strength UOE pipe body. High strength, low Y/T ratio and high uniform elongation of the steel pipe can be attained by optimizing the area fraction of ferrite and the grain size by applying accelerated cooling technology. New high strength UOE pipes called “Tough-Ace” possessing both excellent HAZ toughness and deformability have been developed, and the X60 UOE pipe has been mass-produced for the Sakhalin Project.Copyright

Estimation of Solute Carbon Concentration by Electrical Resistivity Method in Low-Carbon Martensitic Steel

The concentration of solute carbon in as-quenched tempered low-carbon martensitic steels (Fe-2%Mn-0.3%C) were estimated from the electrical resistivity. It was found that the electrical resistivity decreased gradually with the increase of the tempering period, and its decreasing rate was enlarged by raising the tempering temperature. The decrement in electrical resistivity was mainly due to the decrease in the amount of solute carbon caused by carbide precipitation. An empirical equation was then applied to convert the electrical resistivity to the solute carbon concentration, where the densities of dislocation and that of grain boundary were also taken into account. Quantitative analysis for a specimen tempered at 373 K for 3.0 ks revealed that the concentration of solute carbon was decreased by 0.005 mass% during the tempering. This estimated value agreed well with the amount of precipitated carbide (Fe2.5C) measured by TEM observation. As a result, it was concluded that the solute carbon concentration could be estimated quantitatively from the electrical resistivity measurement in as-quenched and tempered martensitic steel.

Hydrogen-assisted subcritical crack growth rate in CR-MO steels

Hydrogen substantially reduces fracture properties such as threshold stress intensity factor KIH and tearing resistance dJ/da in conventional Cr-Mo steels. In order to enable the life assessment of a reactor with a hydrogen-induced crack using a failure assessment diagram (FAD), an experimental database of hydrogen-assisted subcritical crack growth rates da/dt is requisite. However, there are very few studies concerning the effects of hydrogen- and temper-embrittlement on da/dt at ambient temperatures in 2.25Cr-1Mo steels with high and low impurity levels. In this paper, vacuum melted lab heats of 2.25Cr-1Mo steel were supplied with compositional controls. Some specimens were embrittled by step cooling heat treatment (SCHT). Subcritical crack growth rate at a constant load was obtained by means of the potential drop method for 2.25Cr-1Mo steel with initial internal hydrogen (3.2 mass ppm).Copyright

Advanced Titanium Oxide Steel With Excellent HAZ Toughness for Offshore Structures

The feature of titanium oxide steel (Ti-O steel) is that heat affected zone (HAZ) toughness is improved due to the refinement of HAZ microstructure through the formation of intragranular ferrite (IGF). This desirable microstructure, IGF, forms radially from titanium oxide particles. Recently, it has been clarified that manganese in Ti-O steel is an indispensable element for the formation of IGF. Therefore, manganese effects on Ti-O steel have been basically studied in this work, and then a new effect has been found. In Ti-O steel, manganese has the effect of suppressing the formation of ferrite side plates (FSP), which are undesirable due to their coarseness. Consequently, HAZ microstructure of Ti-O steel with high manganese content is so refined that HAZ toughness is remarkably improved. Based on the manganese effects, steel plates with excellent HAZ toughness for offshore structures have been developed and commercially mass-produced. The welded joints exhibit excellent toughness.Copyright