TEM Microstructures of Phase Transformations and Precipitation Hardening in Copper Containing Steel

• Main Authors: Chien-Nan Hsiao, 蕭健男
• Other Authors: Jer-Ren Yang
• Format: Others
• Language: en_US
• Published: 2000
• Online Access: http://ndltd.ncl.edu.tw/handle/05237396592892717316

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 89 === During the last-three decade, copper-containing steels have been successfully developed and used for several engineering fields. These kinds of steels are found to offer advantages of precipitation hardness behavior. Precipitation of copper in the Fe-Cu binary system has been a subject of numerous studies. However, there has been no detailed microstructural examination of phase decomposition after solution treatment and aging. In order to understand the influence of matrix microstructures on age-hardening behavior in copper-containing steel, the phase transformation of austenite (during a variety of continuous cooling treatments) and the consequent precipitation reactions of copper particles (during isothermal aging) were investigated by dilatometry, optical metallography, hardness measurement, transmission electron microscopy, and field-emission-gun transmission electron microscopy (FEG-TEM). It is found that at a wide range of cooling rates (about from 30 to 0.3℃/s) after austenitization at 900℃ for 15 min the steel gives a mixture of martensite and bainite in NAK 80 steel. Three various pre-treated specimens (obtained from continuous cooling at 120, 5 and 1℃/s respectively) have been investigated for copper age-hardening response. It is shown that the general level of peak hardness for the fully martensitic specimen is smallest, when compared with that for the other two specimens which separately contain a mixture of nearly equal volume fractions of martensite and bainite. The findings in this work show that tempering martensite during aging drastically hinders the hardening of copper precipitates. Precipitation behaviors and aging reactions of the commercial 17-4 PH (precipitation hardening) stainless steel were investigated. In the solution-treated specimen, the microstructure consists chiefly of lath martensite with significant amounts of micro-twins. Aging at 480℃ for 1 h (H900 treatment) leads to a peak aging condition. The lath martensite matrix still exhibited a very high density of dislocations. There were few strain strings caused by copper precipitates in the dislocation of martensite.; however, over aging at 620℃ for 4h (H1150 treatment) brings about reversed austenite and the precipitation of incoherent face-centered cubic copper-rich particles that have a Kurdjumov-Sachs orientation relationship with the matrix. After aging at 620℃ for 8h, the reversed austenite has a Nishiyama-Wasserman orientation relationship with the matrix. The twinning reversed austenite obtained from the specimen aged at 620℃ for 16 h. It shows that the block-shaped and band-like austenite colonies still contain finely dispersed copper particles.