本文選題：SA508-3鋼 + 顯微組織��； 參考：《沈陽理工大學(xué)》2017年碩士論文

【摘要】：SA508-3鋼作為一種低碳低合金鋼以較高的強度和良好的低溫沖擊韌性等優(yōu)點,廣泛應(yīng)用于核電壓力容器中。核電壓力容器是核電機組的關(guān)鍵部件,決定了核電站的安全性和服役年限。壓力容器鍛件質(zhì)量對反應(yīng)堆安全運行起著重要的作用。隨著反應(yīng)堆壓力容器壁厚的增加,壁厚中心部位常出現(xiàn)低溫沖擊韌性偏低且波動的現(xiàn)象,嚴(yán)重降低了核電壓力容器服役安全性。針對此問題,本研究系統(tǒng)分析了核電壓力容器鍛件沿壁厚方向的顯微組織變化規(guī)律和SA508-3鋼連續(xù)冷卻過程的相變特征,通過在實驗室使用不同的冷卻方式模擬實際鍛件壁厚不同位置的冷卻條件,對比分析實驗室不同冷卻條件下得到試樣與實際鍛件不同位置試樣的顯微組織和力學(xué)性能,提出了影響鍛件壁厚中心位置低溫沖擊韌性的原因,在此基礎(chǔ)上設(shè)計了在不依賴于提升淬火過程冷卻速率的前提下,優(yōu)化鍛件壁厚中心位置粒狀貝氏體中馬氏體-奧氏體島(M-A島)的熱處理新工藝,提高了材料的低溫沖擊韌性。本論文的主要研究內(nèi)容和結(jié)論如下:1.厚大斷面SA508-3鋼鍛件實際解剖分析從純凈度、顯微組織、力學(xué)性能三個方面對實際鍛件進行了解剖分析,結(jié)果表明,在鍛件化學(xué)成分、氣體含量、夾雜物控制良好的情況下,鍛件外壁的強度和沖擊韌性良好,組織為馬氏體和下貝氏體;而鍛件心部低溫沖擊韌性較差,其粒狀貝氏體組織是低溫沖擊韌性較差且出現(xiàn)波動的主要原因。2.SA508-3鋼相變特征研究利用Thermo-Calc熱力學(xué)軟件計算了SA508-3鋼的平衡相圖,使用熱膨脹儀、顯微硬度計測試了SA508-3鋼平衡相變點和連續(xù)冷卻轉(zhuǎn)變曲線,利用金相顯微鏡、掃描電鏡對不同冷速下的SA508-3鋼微觀組織進行表征。結(jié)果表明,SA508-3鋼平衡態(tài)析出相主要有KSI碳化物、合金滲碳體、MC_SHP、M7C3等;連續(xù)冷卻過程中,當(dāng)冷速在0.35~20℃/s時,組織以貝氏體為主,并隨冷速的增加,先后出現(xiàn)粒狀貝氏體、上貝氏體和下貝氏體;在實驗室使用不同冷卻方式模擬實際鍛件壁厚不同位置的冷卻條件,通過顯微組織對比分析,選擇實驗室水冷模擬實際錐形筒體的表面狀態(tài),砂冷模擬實際錐形筒體的心部狀態(tài)。發(fā)現(xiàn)砂冷試樣的平均沖擊功顯著低于水冷試樣,隨著溫度的降低,兩者的差距變大;砂冷試樣的顯微組織為粒狀貝氏體,主要由貝氏體鐵素體基體和呈不規(guī)則塊狀或細條狀的M-A島組成,水冷試樣的顯微組織為馬氏體+下貝氏體。3.SA508-3鋼亞溫淬火+回火熱處理工藝研究研究了亞溫淬火對SA508-3鋼相變規(guī)律的影響,與正常奧氏體化的CCT曲線相比,800℃亞溫淬火提高淬透性。當(dāng)冷速在0.35~15℃/s時,組織以貝氏體為主,并隨冷速的增加,先后出現(xiàn)粒狀貝氏體、上貝氏體和下貝氏體。分別研究了不同亞溫淬火奧氏體化溫度和不同回火溫度下采用實驗室砂冷模擬實際鍛件心部時的組織和性能,發(fā)現(xiàn)800℃亞溫淬火+650℃回火后砂冷試樣粒狀貝氏體中M-A島尺寸和數(shù)量明顯減少,顯著提高了材料的低溫沖擊韌性。
[Abstract]:As a low carbon low alloy steel, SA508-3 steel is widely used in nuclear power pressure vessel as a kind of low carbon low alloy steel, which has the advantages of high strength and good low temperature impact toughness. The nuclear power pressure vessel is the key part of the nuclear power plant. It determines the safety and service life of the nuclear power plant. The quality of the forgings of the pressure vessel plays an important role in the safe operation of the reactor. With the increase of the wall thickness of the reactor pressure vessel, the low and fluctuating phenomenon of the low temperature impact toughness at the center of the wall is often appeared, which seriously reduces the service safety of the nuclear pressure vessel. In this study, the change law of the explicit microstructure along the wall thickness of the nuclear Pressure Vessel Forgings and the continuous cooling of the SA508-3 steel are systematically analyzed. By using different cooling modes in the laboratory to simulate the cooling conditions of the different positions of the wall thickness of the actual forgings in the laboratory, the microstructure and mechanical properties of the specimens at different positions in different cooling conditions in the laboratory are compared and analyzed, and the original impact toughness of the center position of the forgings is proposed. On the basis of this, a new process of heat treatment for Martensitic austenite Island (M-A Island) in granular bainite, which is not dependent on the cooling rate of the hardening process, is designed to improve the low temperature impact toughness of the material. The main contents and conclusions of this paper are as follows: 1. thick section SA508-3 steel forgings The actual anatomical analysis of the actual forgings is analyzed from three aspects of purity, microstructure and mechanical properties. The results show that the strength and impact toughness of the outer wall of forgings are good under the condition of the chemical composition, gas content and inclusion control of the forgings. The microstructure is martensite and lower bainite, and the low temperature impact toughness of the forgings is low. Poor, its granular bainite structure is the main cause of low temperature impact toughness and the main cause of fluctuation of.2.SA508-3 steel. Study on the equilibrium phase diagram of SA508-3 steel using Thermo-Calc thermodynamic software, using thermo dilatometer and microhardness tester to test the equilibrium phase change point and continuous cooling transition curve of SA508-3 steel, using Jin Xiangxian. Microscopes and scanning electron microscopy (SEM) are used to characterize the microstructure of SA508-3 steel at different cooling rates. The results show that the main precipitates in the equilibrium state of SA508-3 steel are KSI carbides, alloy carburized bodies, MC_SHP, M7C3 and so on. In the continuous cooling process, when the cooling rate is 0.35~20 C /s, the microstructure is mainly bainite, and the granular bainite appears successively with the increase of cooling speed. Bainite and lower bainite are used in the laboratory to simulate the cooling conditions of the different positions of the wall thickness of the actual forgings in the laboratory. Through the comparison and analysis of the microstructure, the surface state of the actual cone cylinder is simulated by the water cooling in the laboratory, and the sand cold is used to simulate the heart state of the actual cone cylinder. With the decrease of temperature, the gap becomes larger with the temperature decreasing, and the microstructure of the sand cold specimen is granular bainite, mainly composed of bainite ferrite matrix and the irregular lump or fine strip M-A island. The microstructure of the water cooled sample is studied by the martensitic + subbainite.3.SA508-3 steel and the tempering and tempering heat treatment process. The effect of subtemperature quenching on the phase transformation of SA508-3 steel was compared with the CCT curve of normal austenitizing. At 800 C, the temperature quenching increased the hardenability. When the cooling rate was at 0.35~15 /s, the microstructure was bainite, the upper bainite and the lower bainite with the increase of cooling speed. The austenitizing of different subtemperature quenching was studied respectively. The microstructure and properties of the core in the actual forging are simulated at the temperature and the tempering temperature at different tempering temperatures. It is found that the size and quantity of the M-A island in the granular bainite of the sand cold sample at 800 C after tempering at +650 C is obviously reduced and the low temperature impact toughness of the material is greatly improved.
【學(xué)位授予單位】：沈陽理工大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TL351.6;TG142.1

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