Solidification Brittle Temperature Range Research Articles

二相ステンレス鋼の溶接割れ感受性に関する研究 ＩＩ 液体Ｓｎ急冷法によるオーステナイト系ステンレス鋼の凝固過程の検討

Solidification sequence of austenitic stainless steels, SUS31OS, SUS316L and SUS304L, was investigated by freezing the solidification microstructure with liquid-tin quenching during GTA welding. The results are as follows:(1) Liquid-tin quenching method had sufficient cooling rate to reveal the solidification sequence, even though the weld metal solidified with the primary ferrite and the subsequent austenite.(2) The solidification microstructure of SUS310S, which solidified with only austenite, can be evidently observed within Solidification Brittleness Temperature Range. Liquid droplets were confined to the intersectional site of the primary and the secondary dendrite arm boundaries due to formation of a solid bridge near the solidification front.(3) In SUS316L which solidified with the primary austenite, the subsequent ferrite formed from the same confined liquid during solidification as mentioned in (2). In the case of SUS316L which solidified with the primary ferrite, on the contrary, the subsequent austenite formed from the liquid remaining between primary ferrites. Residual liquid was found at the austenite/ferrite interphases.(4) SUS304L solidified with the primary ferrite and the subsequent austenite formed at. cellular dendritic boundaries. Residual liquid was found at the center of the austenite.With the obtained results, solidification sequence of each austenitic stainless steel during solidification could be illustrated in a more detailed fashion.

Thermal Elasto-plastic Analysis on Hot-cracking Formation in Initial-bead of Butt-welded Joints

It is well-known fact that solidification cracking frequently occurs in the initial bead of butt-welded joints. Such a solidification cracking is a troublesome problem because the one-side welding with backing materials is becoming widely used in recent years even in a relatively short welded joint. The weld metal solidification cracking occurs by rupture of liquid films of low freezing temperature segregate present between dendrites or cells during the final stages of solidification. The main object of this work is to seek for the origin of driving force and elucidate the growth mechanism for this solidification cracking. Numerical calculations are performed for four analytical models simulated to the initial pass of butt-welded joints using the finite element method for temperature analysis and the boundary element method for thermal elasto-plastic analysis. Calculated results of maximum strain rates in the middle cross section of the weld metals within the solidification brittleness temperature range are comparable to the experimental values obtained from the Trans-Varestraint test, however the amount of abrupt change of the tensile plastic strain is small compared with the value predicted from the experiments. The cooling rate at the toe of the weld metal immediately after commencement of solidification is so fast that a considerable amount of tensile plastic strain is generated suddenly in this part and its strain rate amounts to the order of 10-8/sec. It may be suggested that an effective way to prevent such a solidification cracking is to reduce the cooling rate at the toe of the weld metal during the final stages of solidification.