Time-resolved in-situ analysis of phase evolution for the directional solidification of carbon steel weld metal

Kinetic information about the phase transformation, during solidification process and solid-state transformation is essential to the material processing, such as welding. In our research group, in-situ phase identification system consisting of undulator beam and imaging plate have recently been used. The welding torch is driven by stepping-motor in the system. Those make possible that phase transformation can be identified in real-time under the condition of directional solidification and the spatial resolution of 100 × 500 μm. The time-resolution is 0.3125 seconds. In the present work, combination of analyzing method: the in-situ phase identification system, morphological observation by high-temperature laser scanning confocal microscopy and observation of microstructure at room temperature by OM, SEM and micro diffraction-system, is suggested to analyze the phase transformation during welding process. Phase transformation process of hypereutectoid carbon steel, during welding was analyzed as an example of combination observation.     

Production and properties of phytase and acid phosphatase from a sake koji mold, Aspergillus oryzae

We identified three types of acid phosphatase (ACP-I, ACP-II, and ACP-III) produced by Aspergillus oryzae in a submerged culture using only phytic acid as the phosphorous substrate. The optimum pH for the activities of the three enzymes was in the range of 4.5 to 5.5. Analysis of the substrate specificities of these enzymes revealed that ACP-I and ACP-III were acid phosphatases, and ACP-II was a phytase. These enzymes were produced during different periods of mycelial growth: ACP-II was produced during the early phase of cultivation (around 24 h), and ACP-I was produced between 24 to 72 h. ACP-III was detected after the production of ACP-I and ACP-II had ceased. The release of phosphate from phytic acid was expected to be due to the cooperative hydrolysis of these enzymes.     

Crystallographic analysis for acicular ferrite formation in low carbon steel weld metals

Inclusions contributing to acicular ferrite nucleation were investigated from a crystallographic point of view in low carbon low alloy steelweld metals. The samples from electro slag welding (ESW) and submerged arc welding (SAW) deposits with various cooling rates were prepared in this study. In those samples, intragranular acicular ferrite formation was observed from inclusions. The inclusions contributing to acicular ferrite formation were of multi-phase type consisting of amorphous phase, spinel type and MnS. They were surrounded by a Ti-enriched layer. It was confirmed by selected area diffraction patterns and energy-dispersive X-ray spectrometer analyses that the Ti-enriched layer was TiO. The acicular ferrite had a Baker–Nutting orientation relationship with the TiO layer on the inclusion surface. The misfit was 3.0% at the interface between the acicular ferrite and TiO. Therefore, it is considered that TiO on the inclusion surface contributes to the heterogeneous nucleation of acicular ferrite by small lattice misfit. However, themorphologies of ferrite growth which nucleated from inclusions were different in both samples. Whereas the growth of ferrites nucleated from TiO was enough in ESW, the size of nucleated ferrite in SAW was a few hundred millimetres in size. In the early stage of nucleation from TiO, ferrite had small deviation from Kurdjumov–Sachs orientation relationship (K–S relationship) in both ESW and SAW. However, there was a difference in the growth stage of ferrite. The ferrite orientations were gradually changed to fit to the K–S relationship in ESW. On the other hand, the nucleated ferrite in SAW stopped growing and the newly nucleated ferrite which had K–S relationship prior to austenite was formed adjacently because of large super cooling due to small heat input.     

Analytical Investigation of Prior Austenite Grain Size Dependence of Low Temperature Toughness in Steel Weld Metal

Prior austenite grain size dependence of the low temperature impact toughness has been addressed in the bainitic weld metals by in situ observations.Usually,decreasing the grain size is the only approach by which both the strength and the toughness of a steel are increased.However,low carbon bainitic steel with small grain size shows a weakening of the low temperature impact toughness in this study.By direct tracking of the morphological evolution during phase transformation,it is found that large austenite grain size dominates the nucleation of intragranular acicular ferrite,whereas small austenite grain size leads to grain boundary nucleation of bainite.This kinetics information will contribute to meet the increasing low temperature toughness requirement of weld metals for the storage tanks and offshore structures.     

The modulated structure formed by isothermal ageing in ZrO2-5.2 mol % Y2O3 alloy

The modulated structure produced by isothermal ageing of ZrO₂-5.2 mol % Y₂O₃ alloy was examined mainly by electron microscopy. It was found that the modulated structure was formed at ageing temperatures between 1400 and 1600° C, but not at 1700° C. The structure is developed by spinodal decomposition, which produces compositional fluctuation in the elastically soft 111 direction in cubic zirconia. The hardness increase caused by the development of modulated structure during ageing is larger than the hardening by precipitation of tetragonal phase in the cubic matrix.Graduate Student, Tohoku Univerisy, Sendai, Japan.     

In situ observation of the formation of intragranular acicular ferrite at non-metallic inclusions in C–Mn steel

Intragranular acicular ferrite is regarded as a most desirable microstructure feature, in view of its strength and toughness, both in weld metals and in the heat-affected zone. This paper systematically investigated the effect of Ti addition on the evolution of intragranular acicular ferrite in the heat-affected zone of C–Mn steel. We also systematically studied the effects of austenite grain size, alloy content and the characteristic of inclusions on the formation of intragranular acicular ferrite. The nucleation and growth of intragranular acicular ferrite was directly observed by laser scanning confocal microscopy. Subsequently, microscopy analysis was used to quantitatively determine and distinguish the potent and inactive inclusions with respect to the nucleation of intragranular acicular ferrite. Finally, some possible reasons are given to explain the formation of intragranular acicular ferrite in the C–Mn steel.     

<Emphasis Type="Italic">In-Situ</Emphasis> Observations of Martensitic Transformation in Blast-Resistant Steel

A hybrid in-situ characterization system, which couples the laser scanning confocal microscopy (LSCM) with the time-resolved X-ray diffraction (TRXRD) measurement with synchrotron radiation, was used to characterize the microstructure evolution during heat-affected zone (HAZ) thermal cycling of high-strength and blast-resistant steel. The combined technique has a time resolution of 0.3 seconds that allows for high-fidelity measurements of transformation kinetics, lattice parameters, and morphological features. The measurements showed a significant reduction in the martensite start transformation temperature with a decrease in the prior austenite grain size. In addition, the LSCM images confirmed the concurrent refinement of martensite packet size with smaller austenite grain sizes. This is consistent with dilatometric observations. The austenite grain size also influenced the rate of transformation (df _m /dT); however, the measurements from the hybrid (surface) and dilatometric (volume) measurements were inconsistent. Challenges and future directions of adopting this technique for comprehensive tracking of microstructure evolution in steels are discussed.     

Direct Observation that Bainite can Grow Below MS

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.     

Nitrogen distribution on rapid solidification in laser welded duplex stainless steels

Summary

Although several surfacing processes have been proposed and applied, none has been able to apply a 1 mm thick hard layer. This paper describes fundamental research to establish friction surfacing as a technique for applying a 1 mm thick hard layer.

Experiments were run to clarify the effects of operating parameters on surface quality using a conventional friction welder fitted with a sliding stage. The materials selected were a 1C‐17Cr martensitic stainless steel coating material and a low‐carbon structural steel substrate.

A stable hard layer of 20 mm width and 1 mm thickness was obtained, with no dilution being found. The dimensions of the coating depend on the friction speed (rotational speed) of the coating material. Thicker and wider coatings are produced at a lower friction speed. The deposition rate is slightly lower than that of conventional arc welding processes under the experimental conditions presently adopted.