本文選題：2205雙相不銹鋼　切入點：激光復(fù)合焊接　出處：《江蘇科技大學(xué)》2017年碩士論文

【摘要】：2205雙相不銹鋼是一種具有優(yōu)良性能的不銹鋼,其強(qiáng)度高、耐蝕性好,被廣泛應(yīng)用在使用條件苛刻的環(huán)境中。然而材料在經(jīng)過焊接后,焊接接頭的組織與母材會產(chǎn)生很大的差異,因此研究接頭組織與耐蝕性的關(guān)系具有重要的意義。本課題通過調(diào)節(jié)激光與熔化極惰性氣體保護(hù)焊(MIG)的焊接參數(shù),雙熱源同時作用復(fù)合焊接2205雙相不銹鋼,得到不同組織的焊接接頭,并對焊接接頭進(jìn)行力學(xué)性能和耐蝕性能測試。通過對焊接接頭進(jìn)行金相觀察發(fā)現(xiàn):焊縫組織中,在厚度方向上,奧氏體相逐漸減少;并且在焊縫中下部相界和鐵素體基體以及熱影響區(qū)會析出有害相;此外,熱輸入的增加會適當(dāng)提高焊縫中奧氏體的含量,但是接頭中奧氏體整體含量一般會低于50%。拉伸測試結(jié)果表明,焊接接頭的強(qiáng)度較母材略有下降,同時對斷口進(jìn)行分析發(fā)現(xiàn),在焊縫中,部分鐵素體相受晶內(nèi)析出相的影響發(fā)生準(zhǔn)解理斷裂。硬度測試結(jié)果表明:焊縫金屬硬度有所降低,熱影響區(qū)硬度稍微增加;出現(xiàn)這種情況是因為在焊縫中,焊絲會填入一定量的Ni,從而降低了硬度,同時接頭中的N由于高溫作用會有一定的燒損,降低固溶強(qiáng)化作用;在熱影響區(qū)(HAZ)由于晶粒的長大,導(dǎo)致硬度增加。動電位掃描測試結(jié)果表明,焊接接頭的耐蝕性比母材低,而接頭耐蝕性薄弱區(qū)域在熱影響區(qū)位置;此外,焊接接頭的點蝕電位與母材相近而保護(hù)電位明顯低于母材,這說明接頭點蝕再鈍化能力較差。臨界點蝕溫度和動電位掃描測試結(jié)果表明,焊接接頭的臨界點溫度比母材要低,并且接頭中組織均勻性越差,臨界點蝕溫度越低。通過掃描電鏡(SEM)觀察發(fā)現(xiàn):接頭中點蝕發(fā)生的部位在各區(qū)域有明顯差別,在沒有析出相的部位,點蝕主要發(fā)生在奧氏體相上,并且會向鐵素體相中擴(kuò)展;在含有大量析出相的部位,點蝕主要在析出相部位產(chǎn)生。不同溫度下的Mott-Schottky(M-S)測試結(jié)果表明,焊接接頭在3.5%NaCl溶液中表面鈍化膜完整性與母材相近;在不同的外加電位下,鈍化膜會表現(xiàn)出不同的半導(dǎo)體特性;同時,在不同焊接接頭的相似部位,奧氏體含量越高,鈍化膜中載流子濃度越大,點蝕越易發(fā)生。此外,適當(dāng)?shù)臏囟瓤梢源龠M(jìn)鈍化膜的生長,提高耐蝕性,并且溫度的增加對鈍化膜內(nèi)載流子濃度沒有影響,但是會提高鈍化膜的平帶電位,使得耐點蝕性能下降。
[Abstract]:2205 duplex stainless steel is a kind of stainless steel with excellent properties, its strength is high, corrosion resistance is good, and it is widely used in harsh environment.However, after welding, the microstructure and base metal of welded joints will be very different, so it is of great significance to study the relationship between microstructure and corrosion resistance of welded joints.In this paper, by adjusting the welding parameters of laser and MIG, the double heat source is used to weld 2205 duplex stainless steel at the same time. The welded joints with different structures are obtained, and the mechanical properties and corrosion resistance of the welded joints are tested.Through metallographic observation of welded joints, it is found that the austenitic phase decreases gradually in the thickness direction of the weld, and the harmful phase is precipitated in the middle and lower part of the weld, the ferrite matrix and the heat-affected zone.The increase of heat input will increase the content of austenite in weld properly, but the total content of austenite in joint is generally lower than 50.The tensile test results show that the strength of the welded joint is slightly lower than that of the base metal. At the same time, by analyzing the fracture surface, it is found that the quasi-cleavage fracture occurs in some ferrite phases affected by the precipitation phase in the weld.The hardness test results show that the hardness of the weld metal decreases and the hardness of the HAZ increases slightly, which is due to the fact that the wire will fill in a certain amount of Niin in the weld, thus reducing the hardness.At the same time, the N in the joints will be burned due to the effect of high temperature, and the solution strengthening will be reduced, and the hardness will increase due to the growth of the grains in the HAZ.The results of potentiodynamic scanning showed that the corrosion resistance of the welded joint was lower than that of the base metal, while the weak zone of the joint corrosion resistance was located in the heat-affected zone, in addition, the pitting corrosion potential of the welded joint was close to that of the base metal and the protective potential was obviously lower than that of the base metal.This indicates that the pitting and repassivation ability of the joints is poor.The results of critical pitting temperature and potentiodynamic scanning show that the critical point temperature of welded joint is lower than that of base metal, and the worse the microstructure uniformity is, the lower the critical pitting temperature is.Scanning electron microscopy (SEM) shows that the pitting sites in the joints are different in different regions. In the regions without precipitated phases, the pitting corrosion occurs mainly in austenitic phase and extends to ferrite phase.The pitting corrosion occurs mainly in the precipitated phase where there is a large amount of precipitated phase.The results of Mott-Schottkyy M-S test at different temperatures show that the integrity of the passivation film on the surface of the welded joint in 3.5%NaCl solution is similar to that of the base metal, and that the passivation film exhibits different semiconductor properties at different applied potentials.The higher the austenite content is, the higher the carrier concentration in the passivation film is, and the pitting is easy to occur.In addition, the proper temperature can promote the growth of passivation film and improve the corrosion resistance, and the increase of temperature has no effect on the carrier concentration in the passivated film, but it will increase the flat band potential of the passivated film and decrease the pitting corrosion resistance.
【學(xué)位授予單位】：江蘇科技大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2017
【分類號】：TG407

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