Effect Of Ni Content On Microstructure And Properties Of Cu Precipitation Steel

Low-Carbon Cu precipitation strengthened steel with a good combination in strength, toughness and weldability has been used widely in the different applications such as ship structure, offshore platform and pipeline for low temperature service, etc. Cu precipitation strengthened steels with different Ni content were heated at 900℃ and aged at different temperature. The effect of Ni content on structure and property of Cu precipitation strengthened steel were studied, and the mechanism of Ni content on Cu precipitation strengthened steel was dissussed through the mechanical proterties testing, the optical microscope, SEM and TEM observation and analysising. The research focus on the influence of Ni content on continuous cooling transformation and the copper-rich phase.The microstructure transformation and properties variations of experimental steel with varies of Ni content during continuous cooling were investigated with Formastor transformation tester, and the influence of Ni content on the Continuous cooling transformation characteristics of Cu precipitation strengthened steels were analyzed as well. The result indicated:With the ascent of Ni content in steel, the critical transformation temperature of the experimental steel was descended. In the condition of same cooling rate, bainite was the only observed microstructure of low content experimental steel, but as the Ni content increasing, the microstructure consisted of martensite and bainite, and only martensite formed the microstructure of experimental steel in a certain high Ni content. Furthermore, with the Ni content ascending, the cooling rate of critical transformation from bainite to martensite was lowered, which also meant cooling curve shifting to the right, and martensite transformation temperature was reduced.While the Cu precipitation strengthened steels aged at 550℃, the strength of which increased a lot, and the ductile-brittle transition temperature decreased by the Ni content increasing. The result showed that the tensile strength rised from 780MPa to 919MPa, the ductile-brittle transition temperature decreased from -70℃ to -84℃. It is proved that the size of packet of microstructure has significant influence on toughness of steel. The result shows that the size of packet decreased by 16.31μm to 10.57μm with the Ni content increasing. The coarser packet cause fall in the resistance of crack propagation of steel and thereby lead to the decrease of ductile-brittle transition temperature.Variation of Ni content had a significant influence on the aging curve of experimental steel, the increasing Ni content could lower the peak temperature of experimental steel. During the period of over aging, as the heating temperature approaching to Ac1, a local austenization occurred in the experimental steel of over 1.97% Ni content, and in the process of cooling later on, a massive amount of martensite-austenite constituent were formed. While the more Ni content added to the steel, the greater extent of austenization could occur, which caused the increase of tensile strength in this range of temperature.While the Ni content increased, the particle size and the distribution of the rich copper changed a lot. The number of copper-rich phase increased and the particle size decreased with the increasing of the Ni content. The particles of the rich copper are small and densely distributed in the steel with higher Ni content, while in the lower Ni content steel the rich copper particles are relatively large and distribute sparsely. Large numbers of copper-rich phases which are tiny and distributing dispersedly,are precipitated in steel after aging. They take strong pinning action on dislocation, and the smaller and dispersiver of the particles, the stronger of the pinning actin on dislocation. It was infavor of the precipitation of copper-rich phase in the process of aging with the improving of Ni content, which makes strength increased.