Hiroyuki Hirata

Austenite grain growth simulation considering the solute-drag effect and pinning effect

Abstract The pinning effect is useful for restraining austenite grain growth in low alloy steel and improving heat affected zone toughness in welded joints. We propose a new calculation model for predicting austenite grain growth behavior. The model is mainly comprised of two theories: the solute-drag effect and the pinning effect of TiN precipitates. The calculation of the solute-drag effect is based on the hypothesis that the width of each austenite grain boundary is constant and that the element content maintains equilibrium segregation at the austenite grain boundaries. We used Hillert’s law under the assumption that the austenite grain boundary phase is a liquid so that we could estimate the equilibrium solute concentration at the austenite grain boundaries. The equilibrium solute concentration was calculated using the Thermo-Calc software. Pinning effect was estimated by Nishizawa’s equation. The calculated austenite grain growth at 1473–1673 K showed excellent correspondence with the experimental results.

Modeling of the deoxidization process on submerged arc weld metals

AbstractFixed welding tests were performed to investigate deoxidization during submerged arc welding and to develop a model for it. For all the chemical compositions of the fluxes used, the oxygen content of the weld metal decreased with increasing arcing time in the initial stage of welding. The oxygen content of the weld metal eventually became constant; this is assumed to be a quasi-equilibrium state. Both the rate of reduction of the weld metal oxygen content and the oxygen content of the quasi-equilibrium condition depend on the chemical composition of flux. The weld metal oxygen content in the quasi-equilibrium condition can be estimated thermodynamically. In addition, the rate of reduction of the weld metal oxygen content satisfies the following equation.1O=exp−k⋅t+lnOi−Oe+Oe

Analysis of the solidification process in Fe–36%Ni weld metal with NbC crystallization

\left[O\right]= \exp \left[-k\cdot t+ \ln \left({\left[\mathrm{O}\right]}_{\mathrm{i}}-{\left[\mathrm{O}\right]}_{\mathrm{e}}\right)\right]+{\left[\mathrm{O}\right]}_e

HIGH-STRENGTH AUSTENITIC STAINLESS STEEL FOR HIGH-PRESSURE HYDROGEN GAS

Where [O] is the weld metal oxygen content at arcing time t, [O]e is the oxygen content in the quasi-equilibrium state, [O]i is the oxygen content immediately after starting welding, and t is the arcing time. We found that the coefficient k decreases with increasing viscosity of the molten slag, which mainly depends on the chemical composition of the flux. Based on these results, we found that deoxidization in weld metal during submerged arc welding can be predicted by estimating [O]e and k and briefly determining [O]i.

NICKEL BASED HEAT-RESISTANT ALLOY

The effect of chemical compositions and microstructures on hydrogen embrittlement of austenitic stainless steel weld metals in high-pressure hydrogen gas was surveyed by using the slow strain rate test (SSRT). As a result, hydrogen embrittlement of the weld metal was hardly influenced by δ ferrite in the weld metal, but by stability of austenite phase, which was estimated by Md30 value or Ni equivalent. In the weld metal with poor stability of austenite, α′-martensite was formed near a crack induced by SSRT. In addition, although the crystal structure of α′-martensite is as same as δ ferrite, susceptibility of hydrogen embrittlement became higher with the increase in α′-martensite. The mechanism to explain the difference between δ ferrite and α′-martensite was considered as following. The hardness, which increases the hydrogen embrittlement susceptibility in bcc structure, is higher in α′-martensite than in δ ferrite. In addition, α′-martensite might be formed continuously with propagation of a crack. Therefore, the effect of α′-martensite on hydrogen embrittlement could be larger compared with δ ferrite.

Ferrite heat resistant steel

Abstract When austenitic high-alloy steel weld metal sustains single-phase solidification generally described as A-mode solidification, this is well-known to result in heightened solidification cracking susceptibility.1–3 To reduce the solidification cracking susceptibility of austenitic stainless steel, it is known to be effective to undertake component modification such as to obtain a solidification mode called the AF mode or FA mode involving the ä phase being crystallized or retained.1, 2 To obtain a complete γ solidification mode in the case of high Nibase alloys, such as Fe–36%Ni alloy, however, it is necessary to arrange for high Cr addition in order to achieve component modification facilitating AF or FA mode solidification such as affects austenitic stainless steel. The result of such addition, however, is that impairment of base metal properties also self-evidently cause heavy loss of hot workability in a way that makes this approach difficult to describe as effective.