高矽鋼淬火擴散製程之微結構及其機械性質

Abstract

淬火擴散製程Q&P所利用的核心概念，最早由Matas與Hehemann在1960年所證實：麻田散鐵在回火過程，碳可能由過飽和的麻田散鐵粄條，擴散至周遭的沃斯田鐵。然其工業應用，則遲至2003年，由Edmond，Speer，Matlock所提出：利用此碳遷移的特性，來穩定化沃斯田鐵；他們並設想了Q&P的計算方法，從而決定最佳的焠火溫度、讓Q&P製程有執行依據。然而，實際上量測結果，沃斯田鐵含量與預測含量有顯著，這代表著學術界對Q&P時的相變態行為並未充分理解。其中最關鍵之因素，即為麻田散鐵散鐵在部分生成後，微結構非靜態的不變，而是持續的相變態。因此本論文專注於部分麻田散鐵生成後之相變態過程。 此論文之研究主題為高矽合金，經淬火擴散製程(Q&P)後，其顯微結構之觀察及機械性質。研究中的高矽合金則概略分為低碳及高碳合金，並分別使用一階段、二階段Q&P製程。研究發現：一階段Q&P時，會發生顯著的沃斯田鐵分解。此外，相變態的速率明顯較變韌鐵快。在此高矽合金中，變韌鐵組織為變韌肥粒鐵與沃斯田鐵所組成；而ㄧ階段Q&P，其組織由回火麻田散鐵、合併之麻田散鐵、下變韌鐵、上變韌鐵組成，並有麻田散鐵介面遷移之跡象。由此可得知文獻中所計算之Q&P最佳持溫溫度，由於未考慮到此恆溫相變態的，因此會造成計算沃斯田鐵含量的誤差。 高碳合金在二階段Q&P時，於擴散製程時會發生變韌鐵相變態。此外，先前麻田散鐵的存在，使變韌鐵相變態有約一個級數的顯著加速。此外，變韌鐵之外形發生顯著變化、非傳統束狀之外形。此方法為目前唯一可維持變韌鐵合金強度、並能同時加速的方法。 為研究奈米變韌鐵在查皮(Charpy)衝擊時高速三軸應力下之行為，進行TEM切片觀察、與同步輻射XRD實驗，發現破裂表面僅剩下麻田散鐵組織，沃斯田鐵則完全消失。代表在衝擊時，裂縫傳播時產生麻田散鐵，造成其脆性；霍普金森撞擊實驗則可發現在高速單軸壓應力，產生大量雙晶變形。此外，結果顯示，Q&P製程對查皮衝擊韌性無改善效果；然而另一方面，則發現在霍普金森撞擊實驗時，有延性破壞之證據。

The core idea of Q&P processing was proposed by Matas and Hehemann in 1960: during tempering, carbon can diffuse from supersaturated martensite to adjacent austenite. However, carbon partitioning on industrial applications was not proposed until 2003, when Edmonds, Speer and Matas indicated that stabilization of austenite can be achieved by carbon partitioning. They also proposed Q&P methodology, which determine the optimal quenching temperature for this process. However, the measurements on retained austenite volume fraction significantly deviate from predicted fractions, indicating insufficient understanding on Q&P processing. The most important cause is that microstructure is not static after partial martensite formation, and instead, austenite decompositions still occurs. Therefore, this thesis is focused on the phase transformation after partial martensite formation. In this research, the microstructures and the mechanical properties of high Si steels after quench and partitioning (Q&P) processing were studied. Low carbon grades and a high carbon steel were used, and one-step and two-step Q&P heat treatments were respectively conducted for investigation on the resulting microstructures. It was found that for low carbon alloys, during one-step Q&P processing, significant austenite decomposition occurs, and the decomposition rate is faster than bainite transformation. the microstructures after one-stage Q&P are composed of tempered coalesced martensite, lower bainite, upper bainite and interface migrations. Therefore in previous literature, the Q&P optimization methodology does not take isothermal transformation into account, leading to deviation in the predicted austenite amount. For high carbon alloy, during partitioning stage, bainite transformation occurred. The presence of martensite drastically accelerated the formation of bainite by an order of magnitude, and changed the bainite morphology. Conducting Q&P heat treatment is currently the only available method to maintained the strength of nanostructure while accelerate bainite transformation. To study the microstructural evolution during Charpy impact test which develpes tri-axial stress in nanostructured bainite, TEM observation and synchrotron radiation XRD were conducted. It was found that in the fracture surface, only martensitic structure remained and austenite disappeared. The results indicated that crack propagation lead to martensite formation, and brittleness results; in contrast, during split Hopkinson pressure bar compression, twinning deformation is dominant. For Q&P specimens, Charpy results showed no improvement in impact toughness, but split Hopkinson pressure bar shows that ductile fracture was possible even at very high strain rate.