Study on the Hydrogen Embrittlement of Aermet100 Using Hydrogen Permeation and SSRT Techniques
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INTRODUCTION
AERMET100 steel, which is widely used in the aerospace industry, is a type of secondary hardened ultra-high-strength steel with a high Ni and Co content. In 1993, Ayer and Machmeier first reported on the microstructure of Aermet100 steel,[1] which contained a quenched and tempered martensite lath matrix with a high dislocation density and coherent nano-scale M2C carbides. This increases the strength of the steel to as high as 1750 MPa and results in a plane strain fracture toughness greater than 120 MPa m1/2.[2] Hydrogen always has deleterious consequences on the mechanical properties of high-strength steel, a process commonly referred to as hydrogen embrittlement (HE). HE susceptibility usually increases with the strength of the steel.[3] Compared to some other ultra-high-strength steels such as 300M, Aermet100 already exhibits higher resistance to HE due to the austenite layer formed at martensite lath boundaries.[4] D. Figueroa’s study indicated that trans-granular cleavage usually initiates from many sites around the circumference and propagates YABO HU is with the Corrosion and Protection Center, Key Laboratory for Corrosion and Protection (MOE), University of Science and Technology Beijing, Beijing 100083, China and also with the Petrochina Pipeline Company, Langfang 065099, China. CHAOFANG DONG, HONG LUO, KUI XIAO, and XIAOGANG LI are with the Corrosion and Protection Center, Key Laboratory for Corrosion and Protection (MOE), University of Science and Technology Beijing. Contact e-mail: [email protected] PING ZHONG is with the Beijing Institute of Aeronautical Materials, Beijing 100095, China. Manuscript submitted May 17, 2016.
METALLURGICAL AND MATERIALS TRANSACTIONS A
into the center of Aermet100 steel with electroplated cadmium during slow strain rate tensile tests (SSRT).[4] Thomas demonstrated that even for only 1 ppm diffusible hydrogen in weight, Aermet100 is susceptible to severe HE at a threshold stress intensity as low as 20 MPa m1/2.[5] In our previous study, pitting corrosion of Aermet100 was obvious in chloride-containing environments, which may act as origination sites for crack initiation.[6] Even passively, hydrogen uptake from a corrosive environment can be enhanced by rupture of the film or corrosion products.[7] To improve Aermet100’s corrosion resistance, electroplated cadmium, zinc, or aluminum is always used to provide another source of hydrogen uptake. HE of Aermet100 can be divided into two types: HE caused by hydrogen absorbed during the coating deposition process, which is referred to as direct embrittlement[8] or internal HE (IHE),[9] and HE caused by hydrogen generated and absorbed during the corrosion process as part of the service life, which is referred to as re-embrittlement[8] or hydrogen environment embrittlement (HEE).[9] To reduce IHE, a baking treatment after electroplating is always used.[9] As for HEE, Aermet100 may have a sacrificial coating defect that acts as a cathode when exposed to a corrosive environment. Hydrogen generated at the surface can be transp
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