Hot cracking behaviour and susceptibility of extra high purity type 310 stainless steels

Abstract

Recent progress in the refining technology has enabled the production of highly pure commercial stainless steels. The hot cracking behaviour of these stainless steels was investigated with respect to type 310 stainless steel. For comparison, four types of 310 stainless steels with various amounts of minor and impurity elements such as C, P and S were used. The purity of type 310 stainless steels used was enhanced in the order of type 310,type 310S,type 310ULC,type 310EHP steels. The hot cracking susceptibility was evaluated by the transverse Varestraint test. Two types of hot cracks occurred in these steels by Varestraint test; solidification and ductility-dip cracks. The solidification cracking susceptibility was significantly reduced as the amount of C, P and S decreased, and that in type 310EHP steel reached a level so low that solidification cracking did not occur in practical welding. On the other hand, the ductility-dip cracking susceptibility adversely increased as the purity of the steels was enhanced. However, the ductility-dip cracking susceptibility of type 310EHP steel was sufficiently low as not to yield ductility-dip cracking in practical multipass welding. The experimental and numerical analyses on the solidification brittle temperature range have revealed that the reduced solidification cracking susceptibility upon decreasing the amount of C, P and S in stainless steel can be attributed to the reduced BTR due to the suppression of minor and impurity elements, such as C, P and S, in the finally solidified liquid film between the dendrite.

Quantitative evaluation of augmented strain at the weld metal during the Trans-Varestraint test

“…Therefore, it is desirable to avoid hot cracking on the material surface when measuring the strain via in-situ observation during the Trans-Varestraint test. According to a previous study [7][8][9]13] on hot cracking, solidification cracking occurs between the solidification starting temperature and solidification completion temperature. In other words, solidification cracking is less likely to occur in the case of a small solidliquid coexistence temperature range.…”

“…However, if such a method is adopted, the determination of the melting boundary when the yoke is dropped becomes unclear, and the measured crack length is inaccurate after the test [21]. Therefore, in this study, the method of dropping the yoke just after the extinction of the arc was adopted, as described by some researchers [7][8][9]13].…”

“…The Varestraint test, proposed by Savage in 1965 [1], has been used for evaluating the susceptibility of weld hot cracking. Since then, many researchers have reported the evaluation result of weld hot cracking susceptibility using the Varestraint test [2][3][4][5][6][7][8][9]. Several approaches in the Varestraint test have been reported for evaluating the type of crack.…”

Simulation of Morphologies in Solidification Microstructure and Residual Liquid by Multi-Phase Field Method

“…オーステナイト系ステンレス鋼の凝固割れは，初晶 γ 凝固 の場合に問題になる．γ 相は δ 相に比べて S および P の固溶 度が小さいため，凝固時のミクロ偏析が増大する．従って， 不純物元素を低減する，適量の δ 相を含有する成分にする 1,2) などの対策が取られている． 凝固過程の固液共存域では延性が極めて低いことが知ら れており [5][6][7] ．Fig. 1 にいくつかの凝固プロセスにおける凝固 速度と温度勾配の関係を示す 8) ．溶接プロセスは連続鋳造に 比べて温度勾配が大きいことが特徴である．1 次デンドライ トアーム間隔 λ 1 は Kurz-Fisher により，(1) 式で表わされる 9) ． λ 1 = 4.3 (ΔT 0 DΓ⁄k) 1 ⁄ 4 V -1 ⁄ 4 G - 1 ⁄ 2 (1)…”

Prediction of ^|^sigma; Phase Precipitation in Type 316FR Stainless Steel Weld Metal

“…An advanced type 316FR austenitic stainless steel which has improved creep rupture properties compared with other austenitic stainless steels is hopefully applied to the next generation of commercial fast breeder reactor (FBR, using fast neutrons that breed Pu-239 from U-238) in Japan [1]. Especially, the weld metal of type 316FR stainless steel is intentionally rendered inhomogeneity by introducing some amount of δ-ferrite in order to avoid hot cracking [2]. This issue would promote the transformation of intermetallic phases such as sigma (σ) and chi (χ) under the high temperature range from 873 K to 1173 K, which could also affect various mechanical and chemical properties [3].…”