Please use this identifier to cite or link to this item:
Fatigue Property and Microstructure Evolution of Superaustenitic Stainless Steel and Its Welds
|Authors: ||Chia-Chang Wu|
|Contributors: ||NTOU:Department of Mechanical and Mechatronic Engineering|
|Keywords: ||超級沃斯田不□鋼;254 SMO;UNS S31254;應變誘發麻田散鐵;應變速率;延性|
Superaustenitic stainless steel;254 SMO;UNS S31254;Strain-induced martensite;Strain rate;Elongation
|Issue Date: ||2011-06-30T07:25:16Z
|Abstract: ||超級沃斯田鐵不□鋼擁有異於常規的高鎳鉻鉬含量，其冷加工性能好、抗拉強度高，而其中以254 SMO (UNS S31254)最為著名。一般不□鋼在拉伸試驗中應變速率越快，延性越差，但在254 SMO卻發現不一樣的結果，高應變速率下其延性要比低應變速率來的好。這是與應變誘發麻田散鐵有相連的關係，在高解析微觀組織發現有應變誘發麻田散鐵變態出現，同時為高應變速率下比低應變速率會誘發較多量的交織狀ε麻田散鐵。 由於254 SMO不□鋼含有6wt%鉬，所以本文在探討254 SMO原材和銲件在不同應變速率及應變振幅下，其低週疲勞的機械性質分析。在脈衝惰性氣體電弧銲接(GTAW-P)下，原本的單一沃斯田鐵相轉變為沃斯田鐵-肥粒鐵相，為樹枝狀晶粒，其δ肥粒鐵在晶界上析出，呈現鋸齒狀的型態。原材和銲件在循環疲勞初期呈現迅速硬化而後穩定軟化現象，且在高應變振幅下，試片破裂前有所謂的二次硬化現象。低應變週期疲勞後之微觀組織顯示差排糾結的情況，而高應變週期則有麻田散鐵的出現及差排胞組織。|
In general, the strength of materials rises with increase in the strain rate, and the ductility decreases. Unexpectedly, the ductility of superaustenitic stainless steel increases significantly with an increase in the strain rate. This is associated with strain-induced ε martensite transformation. With higher the strain rates applied, more strain induced intersected ε martensite is formed. Analysis of fatigue behavior of a superaustenitic stainless steel UNS S31254 (Avesta Sheffield 254 SMO), which contains about 6wt% molybdenum, was performed to examine the cyclic hardening/softening trend, hysteresis loops, the degree of hardening, and fatigue life during cyclic straining in the total strain amplitude range from 0.2% to 1.5%. Independent of strain rate, hardening occurs first, followed by softening. The degree of hardening is dependent on the magnitude of strain amplitude. In the cyclic stress-strain curve, the material softens. That the slope of the degree of hardening versus strain amplitude is lower at high strain rate is attributed to the fast developing dislocation structures and quick saturation. The ε martensite formation either in band form or sheath, depending on strain rate, form leads to the secondary hardening at the high strain amplitude of 1.5%. The maximum hardness present at the weld metal (fusion zone) promotes both the tensile strength and fatigue strength, but it does harm to the ductility and LCF fatigue life. The dislocations piled-up at the delta ferrite grain boundary due to the essential rigid property of ferrite, which has a higher content of chromium and molybdenum. With a high fatigue strain rate, the austenite matrix of the weld metal exhibits persistent slip bands (PSB) after fatigue at 0.6% strain amplitude and evolves to dislocation cell structures at 1.5% strain amplitude. With a low fatigue strain rate, the austenite phase of the weld metal reveals the intermittent PSB at 0.6% strain amplitude and evolves to a square dislocation network at 1.5% strain amplitude. The martensite band or the martensite sheath forms at 1.5% strain, depending on the magnitude of the strain rate.
|Appears in Collections:||[機械與機電工程學系] 博碩士論文|
Files in This Item:
All items in NTOUR are protected by copyright, with all rights reserved.