| 摘要: | 中文摘要 沃斯回火球墨鑄鐵(Austempered ductile Iron；簡稱ADI)的相變態對顯微組織與機械性質有密切關係，本研究分三部份加以探討，第一部份首先以熱分析概念配合金相觀察沃斯回火溫度（360℃、320℃）與合金添加(0.9%Ni-0.6%Cu)對沃斯肥粒鐵相變態與機械性質的影響，接著第二部份探討顯微組織受超音波震盪處理之相變態，最後第三部份觀察含不同錳、矽含量之非合金ADI基地中細小介在物顆粒周圍顯微組織之相變態，並分析其對機械性質之影響。以下針對各研究主題摘要說明如下： 第一部份研究結果顯示ADI在較低沃斯回火溫度，基地中肥粒鐵板條較緻密細小，沃斯田鐵以細長形較多；較高沃斯回火溫度，基地中肥粒鐵板條較稀疏粗大，所以會增加塊狀沃斯田鐵。非合金型ADI基地中肥粒鐵板條粗短密集且易有分枝結構，細長形沃斯田鐵亦會增加；而合金型ADI肥粒鐵板條細長而稀疏，塊狀沃斯田鐵會增加且較均勻散佈在基地上。而細長形沃斯田鐵容易導致碳化物析出，所以低沃斯回火溫度與未添加合金有較高強度、硬度，但衝擊值與伸長率較低；較多塊狀沃斯田鐵會增加沃斯田鐵量，在塑性變形期間沃斯田鐵有誘起麻田散鐵變態效應(TRIP)，能吸收較多能量，所以較高沃斯回火溫度與添加合金其伸長率、衝擊值較高。以旋轉樑法求得S-N曲線與疲勞強度，顯示較高沃斯回火溫度與添加合金，因有較多沃斯田鐵量而有較佳高週疲勞強度。烤漆因為在肥粒鐵與沃斯田鐵介面會（1）析出粗大碳化物，或/且（2）由於氫擴散不均導致有微裂縫產生，致使高週疲勞強度下降。合金型ADI經360℃沃斯回火處理，由於有大量塊狀沃斯田鐵均勻散佈在基地中，所以具有良好強度與極佳伸長率、衝擊值、疲勞強度的機械性質組合。 第二部份探討超音波震盪處理對ADI表面相變態與微硬度的影響。研究結果顯示超音波震盪處理會提昇基地溫度，並促使碳擴散而進入第二階段相變態，所以非合金ADI細長形沃斯田鐵(string-type austenite)會析出碳化物，而塊狀沃斯田鐵(blocky-type austenite)則會產生應力誘起之麻田散鐵相變態。若超音波震盪處理時間加長，碳化物也會更粗大，麻田散鐵會進一步分解成肥粒鐵與碳化物。合金型ADI由於有鎳、銅會穩定沃斯田鐵，所以應力誘起之麻田散鐵變態較少，但穩定沃斯田鐵有加工硬化的能力。以EPMA測量應力誘起變態之麻田散鐵區域，發現在塊狀沃斯田鐵內若錳含量較高者，則較無微裂痕出現；反之，若錳含量較低者，超音波處理後則較容易有微裂痕的產生。測試表面微硬度變化，顯示隨超音波震盪處理時間之增加，其表面微硬度會增加，非合金型ADI因為麻田散鐵會分解為肥粒鐵和碳化物，導致微硬度有時會下降，以X光繞射分析顯示經超音波震盪處理，殘留沃斯田鐵量會下降。 第三部份探討不同錳、矽含量之高強度、非合金型ADI基地中介在物顆粒對相變態與機械性質的影響。結果顯示當錳含量增加，基地中粗大顆粒也會增加，總數目因而減少。矽含量增加時，則會使基地中的細小顆粒增加，總數目也增加。再詳細觀察基地中之介在物顆粒對周圍顯微組織的影響，發現在晶胞內不含鎂之細小介在物（Mg-deficient inclusion）周圍容易產生肥粒鐵環，但是富含鎂介在物（Mg-enriched inclusion）周圍則常會產生針狀肥粒鐵。在晶胞間顆粒是否會促進肥粒鐵形成主要受錳偏析程度影響，在晶胞間介在物顆粒周圍很難有針狀肥粒鐵的產生，此乃因為（1）晶胞間錳的偏析，和/或（2）晶胞間之富鎂介在物周圍會形成環暈狀（halo-like），結果在沃斯回火熱處理大塊狀沃斯田鐵不均勻分布在晶胞間，易產生麻田散鐵。在應力作用下，裂縫易形成並會加速沿顆粒前進，所以較多細小顆粒(<5mm)伴隨錳偏析使疲勞壽命與延韌性下降。 ABSTRACT This doctoral thesis aims to make a thorough investigation on the matrix microstructure of ADI, hoping to understand the relations between phase transformation and mechanical performance via different approaches. The whole thesis is composed of three parts. In Part I, the discussion focuses on the effects of austempered temperatures and alloy addition on the phase transformation and mechanical properties of alloyed (0.9%Ni-0.6%Cu) and unalloyed ADI. When treated at the low temperatureat (320℃), slim acicular ferrite plates were densely distributed in matrix, so the volume fraction of string-type austenite was higher. By contrast, at the high temperature (360℃), ferrite plates were coarse and loosely distributed in matrix, resulting in the increase of more blocky austenite. Also, the stubbed ferrite plates were coarse and dense in unalloyed ADI. Ausferrite transformed with a great nucleation rate and/or a pronounced sidewise growth, so more apparent branches of ferrite plates and large amounts of string-type austenite emerged in unalloyed ADI. String-type austenite tended to generate carbide precipitation. So the specimens at 320℃ or unalloyed ADI exhibited higher strength and hardness, but lower impact energy and elongation. But there were less long slim ferrite plates with more blocky austenite evenly distributed in the matrix of alloyed ADI. More blocky austenite implies more austenite content. During plastic deformation, blocky austenite was prone to absorb energy accompanied with the formation of stress-induced martensite transformation, i.e. the TRIP effect. So the elongation and impact energy were higher in the specimens at 360℃ or alloyed ADI. Fatigue properties are also significantly affected by the austenite content. Specimens at 360℃ or alloyed ADI developed excellent high-cycle fatigue strength attributed to large amounts of retained austenite. After painting and baking, the fatigue strength, however, decreased because of (1) the precipitation of coarse carbides and/or (2) the micro-cracks caused by the uneven diffusion of hydrogen along the interface of ferrite and austenite. In short, the alloyed ADI treated at 360℃ developed an optimum strength and elongation as well as superior impact toughness and high-cycle fatigue strength due to large amounts of dispersed blocky austenite. In Part II, an ultrasonic vibration treatment was adopted to investigate the effect of cavitation erosion (combining the effects of micro-jet impact and shock waves) on the phase transformation and micro-hardness changes in the matrix of alloyed/unalloyed ADI at 320℃/360℃ for 2hr. Results show that the matrix temperature would gradually increase along with the ultrasonic treatment and fostered the carbon diffusion to undergo the Stage II reaction. Specimens after subjected to the ultrasonic vibration treatment might develop phase transformation induced by plastic deformation. Some string-type austenite was found to precipitate carbides, while some island-like/blocky austenite was transformed into martensite. With increased cumulative ultrasonic time, carbide particles became coarser, and martensite might further precipitate to ferrite and carbides. However, the stress-induced martenite transformation in the austenite of alloyed ADI was obviously retarded due to the addition of Ni and Cu. The stabilized austenite behaved good work hardening. Detecting the stress-induced martensite areas after ultrasonic vibration by EPMA, we found that micro-cracks were rarely seen in the blocky austenite area with higher Mn content, but they appeared in the area with lower Mn content. After the treatment, the micro-hardness values increased with the increasing treating time, while the retained austenite content revealed a declination tested by X-ray diffraction. The occasional declination of micro-hardness in unalloyed ADI might be resulted from the decomposition of martensite into ferrite and carbides. The aim of Part III is to elucidate the influence of inclusion particles on the microstructure and mechanical properties of high strength unalloyed ADI with different contents of Mn and Si. For a given C and Si content, the total particle count decreased with the increasing Mn content (0.25-0.48%). But under a given Mn content, increasing Si content from 2.45% to 3.10% contrarily enhanced the total particle count. In addition, within the colony of eutectic cell, a Mg-deficient inclusion tended to be surrounded by ferrite ring, while the vicinity of a Mg-enriched inclusion formed acicular ferrite or ferrite lath. Inclusion particles, especially when its size is less than 5µm, were mostly found in intercellular regions. Whether an inclusion particle can induce the formation of acicular ferrite lies in Mn segregation. In intercellular region, acicular ferrite was hard to form in the vicinity of inclusion particles due to (1) serious Mn segregation, and/or (2) the Mg-enriched inclusions here in halo-like. Consequently martensite tended to form in the blocky austenite that was unevenly distributed in intercellular region after austempering treatment. Under the plastic deformation, cracks would form and accelerated their speed along the inclusions, so more fine inclusions (<5mm) accompanied with Mn segregation caused the deterioration of fatigue strength and ductility. |
