Yu-ichi Komizo

In situ observation of morphological development for acicular ferrite in weld metal

Abstract Acicular ferrite is regarded as the most desirable microstructural feature, in view of strength and toughness, in mild and low alloy steel weld metals. Recent evolution and diversity of mechanical property for base metal demand the same property as the weld. Therefore, understanding of the formation mechanism for acicular ferrite microstructure in weld is one of the essential problems for low alloy steel weld. In the present work, the morphological development of acicular ferrite in situ, was observed during weld cooling. The sample designed to form acicular ferrite microstructure was schematically heated and cooled by infrared imaging furnace and the morphological developments were directly observed using laser scanning conforcal microscopy. The nucleation and growing at inclusion, sympathetic nucleation and impingement event of acicular ferrite were directly shown in high time resolution.

Bioactive titanate nanomesh layer on the Ti-based bulk metallic glass by hydrothermal–electrochemical technique

Titanate nanomesh layers were fabricated on Ti-based bulk metallic glass (BMG) to induce bioactivity in the form of apatite-forming ability. Titanate nanomesh layers were prepared by hydrothermal-electrochemical treatment at 90 degrees C for 2 h, with an aqueous solution of NaOH as an electrolyte. A constant electric current of 0.5 mA cm(-2) was applied between the BMG substrate and a Pt electrode acting as the anode and cathode, respectively. A nanomesh layer, consisting of nanowires (approximately 20 nm in diameter) formed on the BMG. An immersion test in simulated body fluid for 12 days revealed that the titanate nanomesh layer on the BMG promoted the growth of bone-like hydroxyapatite.

Direct Observation that Bainite can Grow Below M-S

In situ simultaneous synchrotron X-ray diffraction and laser scanning confocal microscopy have confirmed that bainite in steels can grow below the martensite start temperature. This observation suggests that the formation curves for bainite in time-temperature-transformation diagrams should be extended below the martensite start temperature. Furthermore, the implication of this observation on the growth mechanism of bainitic ferrite is discussed.

Microscopic observation of inclusions contributing to formation of acicular ferrite in steel weld metal

The strength and toughness in low carbon low alloy steel welds is improved by the refinement of microstructures resulting from the formation of acicular ferrite. The behaviour of the nucleation and growth of acicular ferrite has been extensively studied and the weld metal of refined acicular ferrite is practically used in industry. It is known that inclusions in weld metal strongly contribute to the nucleation of acicular ferrite. The present authors made dynamic observations of the nucleation and growth of acicular ferrite at inclusion sites using a high temperature laser scanning microscope. In the steel weld metal of a Ti–B system, the formation of acicular ferrite is reported to be controlled by the weight ratio of aluminium and oxygen (Al/O ratio) which is an index of oxygen potential after the completion of deoxidisation by aluminium. The proper control of the Al/O ratio results in the formation of MnAl2O4 oxide of a spinel structure whose capability of the acicular ferrite nucleation is governed by a specific lattice misfit between nucleated ferrite and this oxide. It is considered that the addition of titanium is essential because titanium assists the formation of MnAl2O4. 8 However, there are reports depicting that there is no specific lattice orientation between ferrite and oxides of a spinel structure. As to the role of titanium, the researchers reported that it brings about catalysis or acts as the first nuclei for acicular ferrite nucleation. In spite of the fact that the acicular ferrite production technology is practically used in industry, the mechanisms of the acicular ferrite nucleation have been not completely clarified yet and thus, the development of nucleation control technologies for a further improvement of weld metal toughness is hindered. In the present study, the inclusions were formed by varying Al/O ratios. Thus formed inclusions were directly sliced into thin foils and crystallographic analyses were performed using a transmission electron microscopy (TEM). The formation of a multiphase type of inclusions and the existence of titanium enriched layers on the interface between the inclusions and nucleated ferrite were clarified by an electron diffraction analysis and an energy disperse X-ray spectroscopy (EDS). Experimental

Verification of numerical model to predict microstructure of austenitic stainless steel weld metal using synchrotron radiation and trans varestraint testing

Abstract A numerical model to predict the microstructure of austenitic stainless steel weld metal is proposed, and spatially resolved X-ray diffraction measurements using synchrotron radiation have been carried out for Fe–20Cr–(9·8–14·4)Ni weld metals, quenched in liquid Sn, to verify the validity of the numerical model. X-ray diffraction analysis of Fe–20Cr–11·5Ni quenched weld metal, solidifying in the ferritic–austenitic mode, showed that the secondary γ phase crystallised in a eutectic growth mode down to a temperature drop of 6 K from the initiation of solidification. Also, from X-ray diffraction analysis of Fe–20Cr–12·7Ni quenched weld metal, which solidified in the austenitic–ferritic mode, it was found that the secondary δ phase crystallised in a eutectic growth mode within the temperature drop range between 15 and 21 K from the initiation of solidification. The crystallisation temperatures predicted by the numerical model for secondary γ and δ phases in Fe–20Cr–11·5Ni and Fe–20Cr–12·7Ni weld metals agreed with experimental data. Furthermore, it was found that the effect of Ni content on the solidification cracking susceptibility of Fe–20Cr–(9·8–14·4)Ni weld metal, determined via trans varestraint testing, agreed with the results calculated using the model. These agreements support the validity of the developed numerical model.

Optical observation of real materials using laser scanning confocal microscopy Part 1 - techniques and observed examples of microstructural changes

Abstract Owing to the good thermal response of an infrared image furnace, the thermal cycle of welds (weld metal or heat affected zone) can be reproduced, while a laser scanning confocal microscope has a suitable light source and optical geometry for observing metal irradiated at high temperatures. The combination of an infrared image furnace and a laser scanning confocal microscope provides a useful tool for determining the microstructural changes in metals at the micrometre scale during the thermal cycle of welding. The technique of using such a combined system is called high temperature laser scanning confocal microscopy. Both macro‐ and microviews expand the understanding of the microstructural formation of welds and are useful in developing microstructure control methods. This is the first part of a report on the application of laser scanning confocal microscopy to observe microstructural changes for various types of steel samples and thermal cycles. The micro‐ or macroviews of different microstructural formations are presented, including the solidification cell, austenite from δ‐ferrite, pearlite, martensite, Widmanstätten ferrite and bainite.

In Situ Observation of Phase Transformation in Low-Carbon, Boron-Treated Steels

It is known that adding the appropriate amount of boron to steels dramatically increases their hardness and toughness as a result of the transition of the microstructure from grain boundary nucleation to intragranular nucleation. In this study, precipitation and phase transformation kinetics in heat-affected zones of low-carbon, boron-treated steels are observed directly by high-temperature laser scanning confocal microscopy. The effects of boron content and austenite grain size on the phase transformation process are investigated systematically by quantifying the transformation product, the transformation start temperature, the average length of the ferrite plates, and the average number of potent nucleation sites. Finally, detailed methods for controlling and optimizing the microstructure in the heat-affected zones of low-carbon, boron-treated steels are discussed.

In situ time resolved X‐ray diffraction using synchrotron

Abstract A new technique based on a combination of time resolved X‐ray diffraction and laser scanning confocal microscopy was developed for direct observation of morphological evolution and simultaneous identification of phases during thermal cycle of welding. Time resolved X‐ray diffraction data and laser scanning confocal microscopy images under the desired thermal cycles were measured simultaneously. As an example, the microstructural evolution in 15Cr–5Ni martensitic steel was observed to investigate the phase transformation kinetics under the thermal cycle of rapid heating and cooling.

Development of in-Situ Microstructure Observation Techniques in Welding

Unidirectional solidification for low-carbon steel weld metal was characterized by the using Time-Resolved X-Ray Diffraction (TRXRD) system. Solid-state phase transformation was also observed in situ in reciprocal lattice space. It was shown that TRXRD analysis has a potential as a comprehensive characterization technique for solidification and phase transformation processes in welding. Furthermore, it was suggested that nucleation sites in solid-state phase transformation can be observed by high-temperature laser scanning confocal microscopy (HLSCM) in order to classify the ferrite microstructure. These techniques have been designed for the dendrite alignment in unidirectional solidification and the δ-β-α phase transformation in steel weld metal to be characterized.

Influence of martensitic islands on fracture behaviour of high heat input weld HAZ

A study was made on the relation between CTOD value and M–A constituent for the single-heat-cycled weld heat-affected zone (HAZ) of YS 320–360 N/mm2 class thermomechanical control process steels with increased heat input. It was found that the M–A constituent disappeared and lost its deterioration effect on the HAZ CTOD toughness when the heat input exceeded about 15 kJ/mm, although this boundary heat input depended on the steel chemistry. On the other hand, the austenite grain size increased monotonically with increasing the heat input. But the austenite grain size could not be the controlling factor of the HAZ toughness, and its effect deferred between base metals. However, the HAZ toughness was related to the fracture facet size for the large heat input conditions. This fracture facet size, which represents the fracture toughness, is considered to be a measure of the uniformity of the transformed microstructure from austenite in the HAZ.