| LF2 alloy is a nickel saving age-hardening-type Fe-Ni base alloy with excellent hot-gas corrosion resistance and high temperature strength. Its high temperature performance is equivalent to Nimonic 80A and Inconel 751 alloy which are nickel base alloy and widely used in valve. Because the Ni content of LF2 alloy is reduced from 70% to 30%, and it has a significant reduction in costs, LF2 alloy has been used extensively for high-load exhaust valve of diesel engine. In order to study the effect of Ti content on Microstructure and mechanical properties of the alloy, provide a theoretical basis for the practical production, six ovens of LF2 alloys containing 1.67-2.59wt.% Ti was contrastively investigated in this article.First of all, using the software of Thermo-Calc calculated the change of precipitation phase under the equilibrium state in the different Ti content alloy, which served as the reference for the evolution of precipitation in above-mentioned nonequilibrium state. The microstructure and mechanical properties of LF2 alloy with different Ti content were investigated, using scanning electron microscopy, transmission electron microscopy, micro phase analysis and mechanical property test etc.The results showed that:the main existing phases in LF2 alloy are γ’phase、MC phase、Laves phase and σ phase. γ’phase is nearly spherical precipitated in the grain, with a comprehensive analysis of the amount, shape and size of all the precipitations, the γ’phase contribute most to the high temperature mechanical properties of the alloy, it is the main strengthening phase in the alloy; Laves phase precipitated in fibrous and bulk in the grain boundary, a small amount distributed in the grain. With the increase of Ti content, the number of y’phase increased; the number of Laves phase increased, changed from fibers into short rod; the strength of the alloy increased gradually, plasticity gradually declined, high temperature strength increased gradually.In order to determine the best heat treatment process, solid solution temperature selected(950℃,970℃,990℃,1020℃,1050℃ and 1070℃)×1hWQ, aging temperature selected(680℃,720℃,760℃ and 800℃)×10hAC, aging time selected 5h,10h,15h and 20h. Tensile and impact test at room temperature, and microstructure observation, analysis the influence of different heat treatment process on the microstructure and mechanical properties. The results showed that, when solid solution temperature is 990℃, aging temperature is 760℃ and aging time is 10h, the alloy can get the best performance.In this study, the alloys were long time aged up to 2000 hours at 700℃, which was equivalent to 200～250 thousand kilometers vehicle driving.In the long aging process at 700℃, with the increasing of Ti content, the strength of the alloy increased gradually during long time aging, plastic declined gradually; the content of element γ’ phase, Laves phase and σ phase increased gradually, but the mass fraction of MC phase changed little, y’phase is still the main strengthening phase in the alloy.When the aging time was in the range of 0-500h, the yield strength of LF2 alloy increase rapidly, mainly due to y’phase precipitated amountly in the number, play a very significant role in strengthening the alloy; in the range of 500-1500h, the yield strength of the alloy rising slowly due to part of γ’ particles grow up, result in strengthening effect weakening; when the aging time is 1500h, no more small granulated y’phase precipitated, y’phase particles grow up further, the segregation grew up of grain boundary carbides and a large number of slender needle σ phase precipitated, lead in the strength of LF2 alloy decrease. |
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