The influence of niobium-additive and heat treatment on the hydrogen embrittlement of ultra-high strength boron steel
Date Issued
2014
Date
2014
Author(s)
Yu, Chen-Ping
Abstract
15B30 steel, an ultra-high strength boron steel, has 2000MPa ultra-high tensile strength in martensitic state. Due to its high strength property, 15B30 steel is best used in structural steel and automotive plate. However, ultra-high strength steel is prone to hydrogen embrittlement in practical usage, which will lead to severe unpredictable fracture. The aims of this research are to build up an evaluation method of hydrogen embrittlement and find the effects niobium-additive and heat treatment on the hydrogen embrittlement of 15B30 steel. After chemical hydrogen charging, the as-quenched 15B30 exhibits the worst resistance of hydrogen embrittlement. Its percentage loss of UTS after hydrogen charging is about 70%, and the fractography is intergranular fracture. Through tempering processes in different temperatures, it is found that 200℃ tempering processing is effective to resist the hydrogen embrittlemet. This feature is mainly ascribed to the ε carbide precipitates in the martensite lath, which can provide additional hydrogen trapping site. The cementite will precipitate along martensite lath interface and prior austenite grain boundary during the tempering at 300℃ and 400℃. This type of hydrogen trapping sites lead to a weaker resistance to hydrogen embrittlement. Niobium-added 15B30, namely the 15B30Nb steel, can have the refined grains due to the existence of (Nb,Ti)(CN) and NbC precipitates. These refined grains could improve noticeably the hydrogen embrittlement resistance in the tempering state. Constant load test shows that the hydrogen pre-charged 15B30Nb steel could sustain a longer fracture time, which also reveals that the refined grains have better hydrogen embrittlement resistance.
Subjects
氫脆
超高強度鋼
回火麻田散鐵
碳化鈮
Type
thesis
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