【摘要】：A508-3鋼是一種低碳低合金鋼,其大鍛件作為核電站壓力容器而廣泛應(yīng)用,在熱處理過程中因體積巨大,不同部位的傳熱速率不同,從而產(chǎn)生熱應(yīng)力和相變應(yīng)力,所以涉及溫度、應(yīng)力、相變?nèi)叩慕换プ饔?以往的研究大都忽略了應(yīng)力這一參數(shù),針對這一實際現(xiàn)象,本文選取A508-3鋼作為研究對象,研究不同冷卻速度下應(yīng)力對相變動力學(xué)及顯微組織的影響,以期不僅在理論上解釋相變機制,而且為實際工程應(yīng)用提供參考,主要試驗研究和結(jié)論如下:利用高溫相變儀對A508-3鋼非等溫相變過程開展試驗,獲得連續(xù)加熱和冷卻膨脹曲線,確定各組織轉(zhuǎn)變點,繪制出連續(xù)冷卻轉(zhuǎn)變曲線,并分別對先共析鐵素體、珠光體、貝氏體及馬氏體相變開展研究,研究發(fā)現(xiàn),A508-3鋼的先共析鐵素體相變開始點與冷速之間的關(guān)系能夠很好地滿足Scheil疊加法處理的蘭格模型;所得珠光體和貝氏體體積分數(shù)與時間之間滿足指數(shù)關(guān)系模型;在高冷卻速度下的馬氏體轉(zhuǎn)變滿足變溫馬氏體相變動力學(xué)方程;依據(jù)CCT曲線,珠光體和貝氏體的臨界冷速均低于0.1℃/s;馬氏體的臨界冷速介于2℃/s和5℃/s之間。利用Gleeble-1500D熱力模擬試驗機對壓應(yīng)力作用下連續(xù)冷卻過程中的組織轉(zhuǎn)變展開研究,結(jié)果表明:連續(xù)冷卻過程中,單軸壓應(yīng)力增加了先共析鐵素體的冷速范圍,促進了先共析鐵素體和珠光體相變的進程,且壓應(yīng)力越大,促進作用越明顯;壓應(yīng)力減緩了貝氏體相變速率,促進形核,抑制長大,且壓應(yīng)力值越大,抑制作用越明顯;隨壓應(yīng)力增加,珠光體的顯微形貌變化不明顯;粒狀貝氏體中呈線性沿一定方向規(guī)則排布的M/A島長徑比下降,數(shù)量也減少,彌散分布的顆粒狀M/A島尺寸減小,數(shù)量增加。利用Gleeble-1500D熱力模擬試驗機對拉應(yīng)力作用下連續(xù)冷卻過程中貝氏體相變展開研究,結(jié)果表明:連續(xù)冷卻過程中,單軸拉應(yīng)力促進冷速為0.5℃/s的貝氏體轉(zhuǎn)變,對冷速為1~2℃/s下的貝氏體相變起抑制作用,貝氏體形貌受拉應(yīng)力的影響與壓應(yīng)力作用下保持基本一致。
[Abstract]:A508-3 steel is a kind of low carbon low alloy steel. Its large forgings are widely used as pressure vessels in nuclear power plants. Because of their huge volume and different heat transfer rates in different parts during heat treatment, they produce thermal stress and phase transformation stress, so they involve temperature. The interaction of stress and phase transformation has been neglected in previous studies. In view of this practical phenomenon, A508-3 steel is chosen as the object of study in this paper. The effect of stress on phase transformation kinetics and microstructure at different cooling rates is studied in order not only to explain the mechanism of phase transition theoretically, but also to provide a reference for practical engineering application. The main experimental results and conclusions are as follows: the non-isothermal phase transformation process of A508-3 steel was tested by high temperature phase transformation instrument, the continuous heating and cooling expansion curves were obtained, the transformation points of each microstructure were determined, and the continuous cooling transformation curves were drawn. The transformation of proeutectoid ferrite, pearlite, bainite and martensite were studied respectively. It was found that the relationship between the starting point of preeutectoid ferrite transformation and the cooling rate of A508-3 steel could well meet the Lange model treated by Scheil superposition method. The obtained volume fraction of pearlite and bainite satisfies the exponential model of time, the martensite transformation at high cooling rate satisfies the dynamic equation of martensite transformation at variable temperature, and according to the CCT curve, The critical cooling rate of pearlite and bainite is lower than 0.1 鈩，

本文編號：2148163