銲接製程對309L沃斯田鐵系不鏽鋼銲道之疲勞裂縫成長影響

Effect of Welding Process on the Fatigue Crack Growth Rate of 309L Austenitic Stainless Steel Weldments

陳香如(Hsiang-Ju Chen)

指導教授 ： 喻冀平 ； 黃嘉宏

國立清華大學/原子科學院/工程與系統科學系/碩士(2012年)

摘要

本研究以工業常用的銲接方式，惰性氣體鎢極電弧銲及遮蔽金屬電弧銲進行309L不鏽鋼銲道之同質金屬銲接，針對室溫與高溫環境之疲勞裂縫成長進行研究，以探討銲件的顯微組織、殘留應力與疲勞裂縫成長速率之關係，並評估顯微結構對疲勞裂縫成長的差異。本實驗希望模擬核電廠現場銲接條件，由取得核能級銲接人員進行銲接。結果顯示，309L固化模式屬於肥粒鐵-沃斯田鐵模式，氬電銲銲接微結構主要為樹枝狀結構，但因熱輸入量及冷卻速率不同而造成不同的微結構形貌。肥粒鐵含量會隨銲接次數變多而下降。因肥粒鐵較硬，硬度隨著肥粒鐵含量上升而上升。後層銲接會使前層硬度下降，使前層有回火作用。為了解殘留應力對疲勞裂縫成長的影響，所以本實驗使用X光與中子繞射法量測殘留應力。X光量測表層殘留應力，氬銲殘留壓應力較小且應力範圍較平均，電銲殘留壓應力較大且波動大，可能是熱輸入量造成冷卻速率快慢，因此對表層殘留應力影響大。中子量測內部殘留應力，氬銲殘留壓應力大於電銲。銲件分別在25°C與288°C下進行疲勞裂縫成長試驗。結果顯示，所有試片為穿晶破裂。在25°C，銲道的疲勞裂縫成長低於316L母材，可能為為結構的影響導致疲勞裂縫成長速率降低。因其銲接微結構為不均勻的，破斷面較粗糙，而阻礙疲勞裂縫成長。在288°C，所有試片結果相似。氬電銲試片分別發生動態回復以及平面滑移現象。氬銲具有較高的疊差能，造成動態回復而使疲勞裂縫成長增快。電銲有較低疊差能，發生平面滑移而延長疲勞壽命。微結構及硬度主要對疲勞裂縫成長有較大影響，殘留應力對疲勞裂縫成長無太大影響。

關鍵字

309L不鏽鋼 ； 惰性氣體鎢極電弧銲 ； 遮蔽金屬電弧銲 ； 疲勞裂縫成長

並列摘要

The purpose of the study is to understand the effect of welding process on the fatigue crack growth rate of 309L austenitic stainless steel weldments. Two welding processes, the GTAW (Gas Tungsten Arc Welding) and SMAW (Shield Metal Arc Welding) were adopted to weld 309L austenite stainless steels and test them in 25°C and 288°C.
The results show that the solidification mode of 309L austenitic stainless steel weldments is ferrite-austenitic mode and the primary microstructure is dendritic structure. The ferrite content decreases with increasing welding times. The hardness increases with the increasing ferrite content and the latter welding layer shows the lower hardness, resulting from the tempering effect. The residual stress is related to the heat input by the different welding procedures. In order to realize the effect of the residual stress on fatigue crack growth rate, this study measured the residual stress by XRD and neutron diffraction. The measurement by XRD was conducted to detect the surface residual stress. By GTAW, the compressive residual stress is small and average stress. By SMAW, the compressive residual stress is relatively large with more fluctuation. The heat input due to different process may result in different cooling rates, which play an important role on the distribution of surface residual stress. The inner residual stress is determined by neutron diffraction. The residual stress of the weldments by GTAW is higher than that by SMAW.
The weldments of fatigue crack growth was tested at 25°C and 288°C. All the specimens were transgranular fracture. The results reveal that the base metal and weld display different trends at 25°C. The fatigue crack growth of both weldment are lower than base metal due to microstructure. The inhomogeneous structure has rough fracture surface which retards the fatigue crack growth rate. At 288°C, all the specimens shows similar crack growth rate. The weldments by GTAW and SMAW have the dynamic recovery and planar slip phenomenon, respectively. The weldments by GTAW have high stacking fault energy which results in dynamic recovery and increases the fatigue crack growth rate. In contrast, the weldments by SMAW have low stacking fault energy meaning that the planar slip could occur difficultly. As a result, the weldments by SMAW have longer the fatigue life. The experimental results indicate that the residual stress has no significant effect on the fatigue crack growth rate. The microstructure and hardness have a more prominent effect on fatigue crack growth rate and the residual stress has no significant effect on fatigue crack growth rate.

並列關鍵字

309L austenite stainless steels ； Gas Tungsten Arc Welding ； Shield Metal Arc Welding ； fatigue crack growth rate

參考文獻

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[4] F.C. Hull, EFFECT OF DELTA FERRITE ON HOT CRACKING OF STAINLESS STEEL, Weld. J., 46 (1967) S399-&.

[5] ASM Handbook Vol.6 Welding, Brazing, and Soldering, 1993.