彈簧鋼材的顯微組織及機械性質

Abstract

本實驗的主要目的在於探討三種不同釩含量的9254系列 彈簧鋼材，其三種材料分別為9254 (Fe-1.41Si-0.62Cr-0.54C (wt%))、9254LV(Fe-1.46Si-0.61Cr-0.08V-0.53C(wt%))及9254V (Fe-1.39Si-0.60Cr-0.16V-0.53C)的適宜熱處理條件，其中包含 了沃斯田鐵化溫度及持溫時間、回火溫度及持溫時間等因素的配 合。 藉由硬度、衝擊值、拉伸強度的測定，可得知其機械性質， 並藉由OM、SEM及TEM的分析，可判定其顯微組織的變化，將機 械性質的變化由回火組織的變態情形來加以解釋。故實驗的重點 為回火後其機械性質的變化及三種材料的差異性。 實驗結果得知9254系列彈簧鋼於850℃×30min沃斯田鐵化後 油淬火，再於400℃×1hr回火後其硬度可保持於HRC50~52，且於 此回火溫度可發現於9254V有碳化釩析出，故回火溫度高於400℃時，其硬度值均高於9254。回火於350℃會產生回火麻田散鐵脆性(TME)，這是由於麻田散鐵板條界面的殘留沃斯田鐵變態並析出雪 明碳鐵所致。回火於500℃~600℃衝擊值有持平的現象，是由於雪 明碳鐵的成長變慢所導致。 由回火溫度及持溫時間的硬度值可繪出回火母曲線，回火參數可用公式M=T(12+logt)表示之。要求硬度值為HRC50~52時，9254之M值為8200~8600；9254LV之M值為8300~8500；9254V 之M值為8100~8300。

The purpose of this experiment is to determine the optimal heat-treatment condition, for three spring steels containing different level of vanadium, which are 9254(Fe-1.41Si-0.62Cr-0.54C(wt%)),9254LV(Fe-1.46Si-0.61Cr-0.08V-0.53C)and 9254V (Fe-1.39Si-0.60Cr-0.16V-0.53C). After various heat-treatments and mechanical tests, the microstructures were analyzed by optical microscopy(OM), transmission electron microscopy(TEM)and scanning electron microscopy(SEM).The correlation between mechanical properties and its microstructures was established. Test results show that the optimal heat-treatment condition of these spring steels is austenitizing at 850℃for thirty minutes,oil-quench and then tempering at 400℃ for one hour. Its hard-ness can between HRC 50~52,the optimal hardness for use in spring steels. At this treating condition, the master tempering parameter M=T(12+logt) for 9254,9254LV and 9254V are8200~8600,8300~8500 and 8100~8300 respectively. When the specimen was tempered at 350℃, tempered martensite embrittlement (TME) occured due to the decom- position of interlath retained austenite into cementite. Vanadium is a powerful grain-refiner as well as an effective softening-resister as the specimens were quenched from 850℃ and tempered at 500~650℃.