回火型含銅低碳麻田散鐵鋼之抗氫脆研究

Abstract

這份工作目標為發展可以使用於海上結構材之抗氫脆低碳麻田散鐵鋼。麻田散鐵鋼焠火回火製程產生之奈米析出物具備二次硬化效果，並且奈米析出物具備作為氫陷阱的潛力。這份工作始於銅顆粒之在麻田散鐵鋼中的吸氫能力與抗氫脆的表現。使用掃描式顯微鏡,穿透式顯微鏡與原子探針分析檢查微結構，利用電化學方式充氫與自主設計之熱脫分析儀進行氫含量的量測，並使用慢速率拉伸研究鋼鐵抗氫脆的能力。研究中發現回火產生之銅顆粒具備吸氫的能力，使用克森爵方法訂出其氫陷阱活化能為30-35千焦/莫耳，慢速率拉伸試驗結果顯示具備銅顆粒吸出之鋼種具備抗氫脆的能力，並以此驗證了引入氫陷阱可使材料抗氫脆的概念。延伸此概念研究同時具備兩種析出物的鋼鐵材料。發現使用階段性析出的材料，銅顆粒為主要的氫陷阱，其較大的碳化物在常溫下並不具備吸氫能力，因此吸氫效果有限，而銅顆粒與合金碳化物共析出鋼中發現銅顆粒與碳化物間之局部晶格扭曲可以使材料具備強大的吸氫能力，達1.5 重量百萬分比。因此將共析出概念引入產線有利之直接焠火與回火製程，並發展具業界生產潛力之共析出直接焠火回火低碳麻田散鐵鋼。

This work aims to develop offshore low carbon martensitic steel with hydrogen embrittlement resistance. Nano-precipitation, that trigger secondary hardening effect in quenched-and-tempered (Q T) martensitic steel, act as hydrogen trapping sites in steels that deters hydrogen diffusion. The first work aims to characterize hydrogen desorption capability and resistance to hydrogen embrittlement in copper contained Q T martensitic steels. Microstructure was investigated by scanning electron microscope, transmission electron microscope, and atom probe tomography. Hydrogen was electrochemically charged and hydrogen desorption was measured by the house-constructed thermal desorption analyzer. Resistance to hydrogen embrittlement was finally tested via the notched slow strain rate tensile test (SSRT). It’s found that copper precipitation can enhance the hydrogen trapping in Q T steel. The activation energy of ε-copper as hydrogen trapping site was estimated as 30-35 kJ mol-1 by Kissinger formula and Choo Lee method. In SSRT tests, ε-copper contained steel shows better resistance to hydrogen embrittlement. The result validates ε-copper can act as benign hydrogen trapping site that deters hydrogen embrittlement in steels. By inheriting the idea that nano-precipitation can act as benign hydrogen trapping site, steels with two kinds of nano-precipitation was manufactured and introduced by designed heat treatment. In sequence precipitation, oval TiC carbide was the legacy of austenization process, while ε-copper was introduced by tempering. It is found that ε-copper was the major hydrogen trapping site and oval TiC does not show strong hydrogen trapping capability under room temperature. On the other hand, in co-precipitation heat treatment, ε-copper and TiC carbide was both introduced by tempering. Co-precipitated copper precipitate and platelet TiC carbides shows a strong hydrogen trapping capability with a series of hydrogen trapping activation energy. The excess hydrogen content was purposed to be introduced by size limitation by each precipitates, and lattice imperfection by tetragonality between precipitates. The idea of co-precipitation was accepted and adopted into modern industrialized manufacturing process by implementing direct quenching and tempering process with proper alloying composition. This work finally developed industrial favored co-precipitated DQ T martensitic steel with hydrogen embrittlement resistance.