Inconel 690 and the companion filler metal 52M (FM-52M) have susceptibility of ductility dip cracking (DDC) in welding procedure. According to test results, DDC was not only preferential to propagate on grain boundaries vertical to loading direction but also related to grain boundary structure. To reveal the relation of grain boundary angle and cracking, a three-dimensional polycrystalline model was constructed to simulate the formation of DDC. In this work, the initiation and propagation of DDC were studied based on extended Read–Shockley formula. The results indicate that grain boundaries with approximate 45° disorientation are most prone to cracking, and the simulated DDC morphology shows agreement with experimental results. The study provides an extra method to predict DDC propagation and helps to evaluate DDC susceptibility in grain level.

Ductility dip cracking mechanism of Ni-Cr-Fe alloy based on grain boundary energy

Alloy 690 is a typical Ni–Cr–Fe alloy widely used in nuclear power application. However, the companion welding filler metal 52M (FM-52M) is susceptible to an intergranular hot cracking referred to as ductility dip cracking (DDC). To investigate the effect of grain boundary (GB) characteristics on cracking mechanism of FM-52M, the extent of GB sliding and cracking was recorded based on strain-to-fracture test. By the results, it was concluded that the strength of GBs mainly affected the cracking resistance, which could be improved by increase of intergranular precipitates. During hot tensile, GB sliding was observed inducing DDC in mainly two ways: initiate cracking where GB sliding was inhibited by inflexion or triple point, and assist in developing microcrack by skipping hard propagation part of GB. Moreover, cracking was found to have a clear relation to GB misorientation. The results gave an understanding of GB sliding to DDC mechanism.

Effect of grain boundary behaviour on ductility dip cracking mechanism

To evaluate the influence of minor and impurity elements on the solidification cracking susceptibility in austenitic stainless steel welds, the transverse Varestraint test was conducted using several extra high purity type 310 stainless steels with different amounts of C, Mn, P and S. The characteristic influence on solidification cracking could be expressed by the ratio of P/S/C≈1∶1·3∶0·56, while Mn affected solidification cracking to a negligible extent. A theoretical approach revealed that the reduced solidification cracking susceptibility with decreasing C, P and S contents could be attributed to the suppression of solidification segregation.

Quantitative influence of minor and impurity elements on solidification cracking susceptibility of extra high purity type 310 stainless steel

耐硝酸性環境に使用される材料として,鋭敏化を十分に抑 制した極低炭素仕様(ULCグレード)のオーステナイト系ス テンレス鋼(SUS304ULC,316ULC,310ULCなど)が適用さ れている.しかしながら,これらの ULC鋼は Cr酸化皮膜の 保護性が低下する過不働態域で粒界腐食を生じることが問 題となっており,粒界腐食の発生原因となる P,S,Cなど の不純物元素を一層低減したステンレス鋼の開発が望まれ ていた.ごく近年,現行の ULC鋼に対して,C,P,S,N,O などの不純物元素を総量で 100 ppm以下に低減させた新し い仕様のステンレス鋼として,超高純度鋼(EHP鋼)が開発 された.この EHP鋼は,磁気浮上型高周波誘導溶解炉の還 元精錬法(CCIM)と水冷ハースを用いた電子ビーム揮発精錬 法(EB-CHR)を組合せた複合溶製法により製造されるが,ア ルカリ金属やハロゲンを含む有害不純物元素を効率的に除 去できることから,過不働態域での粒界腐食を大幅に抑制で きることが報告されている.これまで,筆者らは,超高純 度 310ステンレス鋼の溶接性(高温割れ感受性)について検 討を加え,SUS310EHP鋼では,耐粒界腐食性に加えて耐高温 割れ性も非常に優れており,完全オーステナイト組織である にもかかわらず,共金溶接(δフェライトを含有しない)が 可能であり,革新的な溶接性を有することを報告した. 一方,オーステナイト系ステンレス鋼の高温割れに関して は,これまで多くの検討がなされている.その結果,鋼中 の不純物元素(Pおよび S)が凝固割れ感受性に大きく影響 し,310系ステンレス鋼において凝固割れを抑制するために は,(P+S)含有量を 0.01 mass%以下に制御する必要があると 報告されている.一方,Pおよび Sが高温割れ(凝固割れ) に及ぼす効果に関しては,SUS304や SUS310S鋼では,Pは Sと同程度か,Sの 1.1~2.1倍とする報告 5, がある一方で, SUS308鋼では,Sは Pよりも悪影響を及ぼすとの報告も ある.また,凝固割れに及ぼす C,Mnや Bなどの影響につ いても調査されているが,微量・不純物元素の定量的な 寄与率に関しては,統一的な見解が得られていない.これら の検討では,いずれも市販鋼あるいは市販鋼相当の実験溶解 [溶接学会論文集 第 31巻 第1号 p. 56-65 (2013)]

Quantitative Influence of Minor and Impurity Elements on Hot Cracking Susceptibility of Extra High-Purity Type 310 Stainless Steel

In this study, the solidification cracking susceptibility of the weld metal of additively manufactured 316L stainless steel was investigated. Laser powder bed fusion was employed to fabricate rectangular specimens with a thickness of 4 mm, and Trans-Varestraint tests were performed to evaluate the weldability of the specimens. Subsequently, the maximum crack lengths were measured and used as indices of susceptibility. The results showed that the weld metal of additively manufactured 316L stainless steel had higher susceptibility to solidification cracking than that of conventional wrought 316L stainless steel. The high susceptibility was considered mainly due to the primary austenite solidification mode in the weld metal of the additively manufactured specimen having a relatively higher content of nitrogen.

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Solidification Cracking Susceptibility of the Weld Metal of Additively Manufactured 316L Stainless Steel

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 ...

K. Hata

Papers

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