冷軋301不銹鋼在氫氣中之缺口拉伸強度及疲勞裂縫成長特性研究

Abstract

AISI 301不銹鋼屬於介穩定型（Metastable）沃斯田鐵系不銹鋼，其Md30溫度約67℃，故於室溫下進行輥軋或拉伸時，易生成α'及ε兩種麻田散鐵組織。本研究探討AISI 301不銹鋼在加工後之顯微組織變化，並對不同加工程度的試片，進行空氣及氫環境中之缺口拉伸與疲勞試驗。此外，利用鐵磁性測定儀量測拉伸過程中α'相之生成量，並利用掃描式電子顯微鏡（SEM）輔以EBSD相鑑定探討相變態對破裂模式之影響。 氫環境中之缺口拉伸試驗結果顯示，母材（CR0）試片對氫脆的敏感性高，NTS loss最大，冷輥軋（CR30）試片次之，低溫熱輥軋（HR30）試片對氫的敏感性最低；而不同氫氣壓對CR0及HR30試片影響不大，但CR30試片之NTS loss則隨外在氫氣壓的升高而增大。在氫環境下，CR0試片之破斷面為明顯的沿晶破裂，HR30為穿晶劈裂，CR30則為兩者混合之破斷面形貌。破斷面邊緣之EBSD影像顯示，CR0試片拉伸時生成的α'相集中在晶界附近，HR30試片中α'相則在晶粒內部沿滑移帶生成。缺口拉伸試驗過程中，CR0試片在過降伏強度後，α'相緩慢增加；而HR30試片中之α'則在過降伏後快速生成。 空氣中疲勞試驗，試片之裂縫成長速度，依序為CR30＞HR30＞CR0試片，但CR0試片循環硬化的現象較為明顯，因此裂縫成長加速較快（Paris law中之m值較高）。而在氫環境中疲勞裂縫成長速度則為：HR30＞CR30＞CR0試片。EBSD的影像顯示，HR30試片疲勞裂縫前端之塑性區內會沿滑移帶產生α'相，加速氫環境時的疲勞裂縫成長速度。在空氣中試驗時，所有試片之疲勞破斷面均呈現穿晶破裂的形貌；然而在氫環境中，CR0及CR30試片的破斷面產生部分沿晶破裂，而HR30試片則仍維持穿晶破裂的模式。

AISI 301 stainless steel（SS）, a metastable austenite SS, is commonly used in applications requiring severe forming operations such as screw, aircraft and rail car structural components. Since it has a high Md30 temperature（~ 67℃）, the austenite (γ) phase can be transformed partly to α'- and ε-martensites during deformation at room temperature. In this study, the effects of cold work on the microstructure, notched tensile strength and fatigue crack growth behavior were carried out using variously cold-rolled specimens. Notched tensile and fatigue tests were conducted in atmosphere as well as in gaseous hydrogen to study the influence of hydrogen embrittlement on the mechanical properties of 301 SS specimens. Additionally, the SEM fractography and phase identification using electron backscatter diffraction（EBSD）of fractured specimens were examined, and the measurement of α' content was performed on cold-rolled specimens during the tensile tests. The results of notched tensile tests indicated that the susceptibility to hydrogen embrittlement decreased in the order through the CR0 （base metal）, CR30（cold-rolled at room temperature to 30% reduction in thickness）, and HR30（cold-rolled at 150℃ to 30% reduction in thickness） specimens. The notched tensile strengths （NTSs）of the CR0 and HR30 specimens were independent of hydrogen pressure, while the NTS of the CR30 specimen reduced significantly as hydrogen pressure increased. The fracture surfaces of the CR0, HR30 and CR30 specimens in hydrogen exhibited intergranular, transgranular（cleavage）and mixed modes fractures, respectively. During the notched tensile test, the transformation of γ to α'-martensite was relatively slow in the CR0 specimens but rather fast in the HR30 specimens after yielding. EBSD phase identification in the region close to the fracture location revealed that α'-martensite was formed along the grain boundaries in the CR0 specimen and along slip bands in the HR30 specimen during the notched tensile test. The fatigue crack growth rates (FCGRs）in air were found to be the CR0, HR30, and CR30 specimens in order of increasing FCGRs. Although the FCGR of the CR0 specimen was slowest, it increased rapidly with ∆K（stress intensity factor range）due to strongly cyclic hardening. For the specimens tested in gaseous hydrogen, the CR0 specimen had the lowest FCGR, while the HR30 specimen had the highest FCGR. The transformation of γ to α'-martensite was concentrated in the plastic zone ahead of the crack tip, resulting in an accelerated FCGR of the HR30 specimen in hydrogen. The influence of hydrogen embrittlement on the fracture appearance was particularly clear for the CR0 and CR30 specimens with the presence of intergranular fracture.