Shigetaka OKANO*, Ryohei IHARA*, Daisuke KANAMARU** and Masahito MOCHIZUKI *

Transcription

1 (Journal of the Society of Materials Science, Japan), Vol. 64, No. 7, pp , July 2015 論文 Welding-Induced Local Maximum Residual Stress in Heat Affected Zone of Low-Carbon Austenitic Stainless Steel with Machined Surface Layer and Its Influential Factors by Shigetaka OKANO*, Ryohei IHARA*, Daisuke KANAMARU** and Masahito MOCHIZUKI * In this study, the effects of work-hardening and pre-existing stress in the machined surface layer of low-carbon austenitic stainless steel on the welding-induced residual stress were experimentally investigated through the use of weld specimens with three different surface layers; as-cutout, mechanically-polished and electrolytically-polished. The high tensile and compressive stresses exist in the work-hardened surface layer of the as-cutout and mechanically-polished specimens, respectively. Meanwhile, no stress and work-hardened surface layer exist in the electrolytically-polished specimen. TIG bead-on-plate welding under the same welding heat input conditions was performed to introduce the residual stress into these specimens. Using these welded specimens, the distributions of welding-induced residual stress were measured by the X-ray diffraction method. Similarly, the distributions of hardness in welds were estimated by the Vickers hardness test. And then, these distributions were compared with one another. Based on the results, the residual stress in the weld metal (WM) is completely unaffected by the machined surface layer because the work-hardened surface layer disappears through the processes of melting and solidification during welding. The local maximum longitudinal tensile residual stress in the heat affected zone (HAZ) depends on the work-hardening but not on the existing stress, regardless of whether tensile or compressive, in the machined surface layer before welding. At the base metal far from WM and HAZ, the residual stress is formed by the addition of the welding-induced residual stress to the pre-existing stress in the machined surface layer before welding. The features of the welding-induced residual stress in low-carbon austenitic stainless steel with the machined surface layer and their influential factors were thus clarified. Key words: Weld residual stress, Machined surface layer, Pre-existing stress, Work-hardening, Low-carbon austenitic stainless steel, X-ray stress measurement, Vickers hardness test 1 Stress Corrosion Cracking, SCC SCC SCC SUS316L SCC 1)~3) SUS316L SCC 270HV 4), 5) 4 600MPa 6) SCC heat affected zone, HAZ SCC 7), 8) SCC Received Dec. 1, The Society of Materials Science, Japan * Graduate School of Engineering, Osaka Univ., Yamada-oka, Suita * Graduate School of Engineering, Osaka Univ., Yamada-oka, Suita

2 549 TIG X X-ray diffraction method, XRD SUS316L Table 1 Table 2 150mm 100mm 10mm Table 1 Chemical composition of SUS316L used (mass %). C Si Mn P S Ni Cr Mo Fe Bal. Table 2 Mechanical properties of SUS316L used. 0.2% proof stress (MPa) Tensile strength (MPa) Elongation (%) % 0.5A/cm µm 2 2 Fig. 1 sin 2 ψ 9) X Rigaku AutoMATE X Table 3 y < 5mm y 5mm ψ ) Kr ner 11) E 220 = GPa ν 220 = Fig.1 Positions for measuring stress by X-ray diffraction method. Table 3 Conditions for X-ray stress measurement. Wave length (mm) (CrKα) Power (kv, ma) 40, 40 Beam size (mm) φ 2 Diffraction plane [γ-fe] {220} (2θ 0=128 ) Measuring time (s) 100 (for base metal) 200 (for weld metal) ψ angle (point) 10 (for base metal) 15 (for weld metal) ω axis; 5 (for base metal) Rocking ω axis; 5, x axis; 10mm (for weld metal) 2 3 HMV mN 15s Fig. 2 12mm 0.5mm 5µm 50µm 3 Fig.2 Positions for measuring hardness by Vickers hardness test.

3 Table 4 TIG 10mm 130mm Table 4 Welding conditions. Welding current (A) 120 Welding speed (mm/s) 2 Arc length (mm) 3 Shielding gas 100% Ar Gas flow rate (l/min) Fig. 3 as-cutout 300MPa -100MPa mechanically-polished -400MPa electrolytically-polished 300MPa -400MPa 3 2 Fig. 4 WM y 25mm HAZ 12)~14) (a) Longitudinal component (a) Longitudinal component Fig. 3 (b) Transverse component Distributions of pre-existing stress in specimens with different machined surface layers before welding. (b) Transverse component Fig. 4 Distributions of residual stress in specimens with different machined surface layers after welding.

4 SCC Fig % (a) Distributions of Vickers hardness around weld metal Fig. 5 Distributions of change in residual stress in specimens with different machined surface layers between before and after welding. 12)~14) Fig. 6(a) 8mm 7) 15) 0.2% X 0.2% Fig. 6(b) 0.2% (b) Distributions of dimensionless residual stress Fig. 6 Effect of 0.2% proof stress on local maximum residual stress in weld heat affected zone. Fig. 7

5 % 12)~14) (1) (2) 0.2% 0.2% (3) Fig. 7 Schematic illustration of weld residual stress distribution in low-carbon austenitic stainless steel with machined surface layer. 16) 17),18) SCC 4 1) Y. Okamura, A. Sakashita, T. Fukuda, H. Yamashita and T. Futami, Latest SCC issues of core shroud and recirculation piping in Japanese BWRs, Transaction of the 17th International Conference on Structural Mechanics in Reactor Technology, WG01-1 (2003). 2) K. Takamori, S. Suzuki and K. Kumagai, Stress corrosion cracking of L-grade stainless steel in high temperature water, Maintenology, Vol. 3, No. 2, pp (2004). 3) S. Suzuki, K. Kumagai, C. Shitara, J. Mizutani, A. Sakashita, H. Tokuma and H. Yamashita, Damage evaluation of SCC in primary loop circulation system pipe, Maintenology, Vol. 3, No. 2, pp (2004). 4) M. Tsubota, Y. Kanazawa and H. Inoue, Effect of cold work on the SCC susceptibility of stainless steels, Proceedings of the 7th International Symposium on Environmental Degradation of Materials in Nuclear Power Systems Water Reactors, Vol. 1, pp (1995). 5) N. Ishiyama, M. Mayuzumi, Y. Mizutani and J. Yani, The effect of cold rolling and aging treatment on the stress corrosion cracking of SUS 316L stainless steels in high temperature water, Journal of Japan Institute of Metals, Vol. 69, No. 12, pp (2005). 6) K. Takeda, A. Taniyama, T. Kubo, H. Uchida and J. Mizuki, SCC behavior at hardened surface layer of 316 (LC) in water on high temperature, Zairyo-to-Kankyo, Vol. 58, pp (2009). 7) R. Ihara, T. Hashimoto and M. Mochizuki, Variation behavior of residual stress distribution by manufacturing processes in welded pipes of austenitic stainless steel, Zairyo, Vol. 61, No. 12, pp (2012).

6 553 8) R. Ihara and M. Mochizuki, Effect of processing conditions on residual stress distributions by bead-on-plate welding after surface machining, Zairyo, Vol. 63, No. 7, pp (2014). 9) A. L. Christenson and E. S. Rowland, X-ray measurement of residual stress in hardened high carbon steel, Transactions of American Society for Metals, Vol. 45, pp (1953). 10) H. M. Ledbetter, Predicted single-crystal elastic constants of stainless-steel 316, British Journal of Non-Destructive Testing, Vol. 23, pp (1981). 11) E. Kröner, Berechung der elastischen konstanten des vierkristalls aus den konstanten des einkristalls, Zeiteschrift Physik, Vol. 151, pp (1958). 12) K. Satoh and T. Terasaki, Effect of welding conditions on residual stress distributions in welded structures materials, Journal of the Japan Welding Society, Vol. 45, No. 2, pp (1976). 13) K. Satoh, T. Terasaki, Y. Suita and M. Tanaka, Effect of welding conditions on residual stress distributions in basic welded joints, Journal of the Japan Welding Society, Vol. 48, No. 9, pp (1979). 14) K. Satoh, T. Toyoda, Y. Suita, M. Tanaka and T. Hirano, Controlling parameters of residual stresses and deformations in welded thin cylindrical shells, Quarterly Journal of the Japan Welding Society, Vol. 2, No. 3, pp (1984). 15) S, Matsuoka, Relationship between 0.2% proof stress and Vickers hardness of work-hardened low carbon austenitic stainless steel, 316SS, Transaction of the Japan Society of mechanical Engineers, Series A, Vol. 79, No. 698, pp (2007). 16) R, Ihara, S. Okano and M. Mochizuki, Numerical analysis of residual stress distribution generated by welding after surface machining based on hardness variation in surface machined layer to welding thermal cycle, Quarterly Journal of the Japan Welding Society, Vol. 32, No. 4, pp (2014). 17) S. Okano, M. Tanaka and M. Mochizuki, Arc physics based heat source modeling for numerical simulation of weld residual stress and distortion, Science and Technology of Welding & Joining, Vol. 16, No. 3, pp (1984). 18) S. Okano, F. Miyasaka, M. Tanaka and M. Mochizuki, Application of gas metal arc welding process model to computational welding mechanics, Quarterly Journal of the Japan Welding Society, Vol. 32, No. 4, pp (2014).