碳含量對鐵-8.8鋁-30錳-4鉻-碳合金顯微結構與孔蝕之影響

Abstract

本實驗利用極化及浸泡測試來研究碳含量在 0.06 到 1.58 wt.%的鐵-8.8鋁-30錳-4鉻-碳合金，於淬火狀態下對孔蝕行為的影響。經由 TEM、SEM、EDS和AES的分析，可以將此不同碳含量的合金分為三種，分別為麻田散鐵型、(α+γ) 雙相型和沃斯田鐵型(γ) 鐵鋁錳鉻碳合金。 在麻田散鐵型合金中，由於 18R 麻田散鐵的形成導致應力場產生，因此孔蝕容易發生在麻田散鐵和肥粒鐵基材之間。隨著碳含量由 0.06 增加到 0.61 wt.%，沃斯田鐵相逐漸增加，但是由於 18R 麻田散鐵仍存在於肥粒鐵基地中，因此孔蝕電位無法大幅地提升。此外，當 18R 麻田散鐵在肥粒鐵基地中消失時，很明顯地可以增加抗孔蝕的能力，而且發生孔蝕的地方則由 α / γ 晶粒邊界取而代之。接著增加碳含量會造成沃斯田鐵基地中有細微的 κ' 相產生，由於此 κ' 相與基地間具有較低的介面能，所以對於孔蝕並沒有太大的影響。然而殘留的硫化錳和氧化鋁的夾渣物，對於具抗孔蝕佳的沃斯田鐵相有較大的傷害影響。

The pitting corrosion behaviors of the as-quenched Fe-8.8Al-30Mn -4Cr-C alloys with various carbon contents in the range from 0.06 to 1.58 wt.% were studied by potentiodynamic polarization and immersion tests. Through TEM, SEM, EDS and AES analyses, the alloys examined can be classified into three main groups, namely martensitic, (α+γ) duplex and austenitic(γ) alloys. In martensitic alloys, great strain-field accompanying by the formation of the 18R martensite results in favorable sites for the pitting nucleation between the martensite and the ferrite matrix. The volume fraction of the austenite phase was increased with increasing the carbon content ranging from 0.06 to 0.61 wt%. However, there was no obvious rise in the critical pitting potential while the 18R martensite was still present. Besides, the pitting resistance of the duplex alloy could be noticeably improved as a result of the absence of the 18R martensite. In the meanwhile, the α/γ grain boundaries substituted the initial pitting sites for the 18R martensite in the duplex alloys. When the carbon content was high enough to form the fine κ'-carbides within the austenite matrix, the lower coherent interfacial and strain energies would have no detrimental effect on the pitting corrosion resistance. The retained inclusions of manganese sulfide and aluminum oxide, however, were responsible for the deterioration in the pitting resistance of the austenite phase.