Investigation On Mechanism And Control Methods Of Impact Energy Fluctuation Of Low Alloy Steels Used For Heavy Wall Forgings

Heavy wall forgings are widely used in heavy machinery,electricity,metallurgy,petroleum and atomic energy field.Low alloy steel heavy wall forgings account for more than 80%of these forgings,such as wind power main shaft,petrochemical hydrogenation reactor,marine crankshaft and nuclear pressure vessel,etc.These heavy wall forgings are often used as load-bearing and/or protective components in these equipments.Therefore,they are subjected to complex loading under harsh working environment.However,with the thickness increasing,the Charpy impact energy of the material at the central parts of low alloy heavy wall forgings always suffers low values or severe fluctuation.This phenomenon not only lowers the rate of qualified products,but also induces a serious threat to the operation of heavy equipment.Therefore,how to improve the toughness and its stability of the low alloy heavy forgings is a critical problem for the heavy manufacturing industry.In this study,through the systematical analyses on some low alloy steel heavy wall forgings produced by manufacturer,the essential factors influencing the values and their stability of Charpy impact energy of the low alloy steels are discussed.And then the mechanisms behind are revealed.Finally,some optimization on chemical composition and heat treatment techololgy are proposed to improve the values and their stability of Charpy impact energy of the low alloy steels.The main contents and results are drawn as follows:(1)The reason why the values of Charpy impact energy are low and/or severe fluctuation in low-alloy steels,such as like 2.25Cr-1 Mo-0.25V steel and SA508-3 steel,used for heavy wall forgings is studied.The results show that the low values and/or severe fluctuation of the Charpy impact energy are related to granular bainite.Accordingly,we design a model low alloy steel with granular bainite microstructure.The mechanisms behind the low values or severe fluctuation of the Charpy impact energy of low alloy steel are investigated by combining three-point bend testing and finite element simulation.It's found that the blocky M-A islands or their decomposition products after tempering are the key factor influencing the values and their stability of Chary impact energy.Accordingly,some methods are proposed to solve this problem:decreasing size and number of the blocky M-A islands in as-quenched microstruture by rare earth(RE)micro-alloying and constrcturing ferrite film,and modifying the decomposion products of M-A islands in tempered microstruture by optimizing tempering process.(2)The effects of RE addition on transformation,microstructure and mechanical properties of 2.25Cr-1 Mo-0.25 V steels are studied systematically.The results show that the addition of RE to 2.25Cr-1 Mo-0.25V steels decreases the start temperature of bainitic transformation,results in the formation of more lath bainitic ferrite having smaller effective grain sizes and the reduction of carbon-rich M-A islands in the normalized micro structures of the steels.When the steel contains 0.012 wt.%RE,an excellent combination of tensile strengths,Charpy impact upper shelf energy(USE)and ductile-to-brittle transition temperature(DBTT)could be obtained.However,the addition of RE in excess of 0.012 wt.%leads to a substantial increase in the volume fraction and density of large-sized inclusions,which are extremely detrimental to the impact properties of the steel.(3)A new heat treatment process(NHTP)for improving the impact toughness of model low alloy steel with granular bainite is proposed.That is,a small quantity of dispersed nanoscale ferrite films are constructed by adopting a proper intercritical heat treatment.The study indicates that the NHTP can improve the impact toughness of model low alloy steel dramatically without degradation of tensile properties.This due to the nanoscale ferrite films can localize the carbon diffusion during granular bainitic transformation,and thus suppress the formation of C-rich blocky M-A islands.Accordingly,the specific NHTPs applied to 2.25Cr-1Mo-0,25V and SA508-3 steels are also researched and proposed for solving the blockage problem of the impact toughness of heavy wall forgings.(4)The characteristic of blocky M-A(martensite(M)and/or retained austenite(RA))decomposition and its effects on mechanical properties of 2.25Cr-1Mo-0.25V steel are investigated.The results show that RA in the blocky M-A usually has more negative influence than M on the impact toughness of the steel after high-temperature(700?)tempering.This is because the RA is decomposed into a cluster of coarse M23C6 carbides and ferrite during tempering at 700?.These coarse M23C6 carbides decorate the boundary of the carbide cluster,which deteriorates the impact toughness of the steel.Accordingly,the size and distribution of M23C6 carbides within the carbide cluster can be modified by introducing a proper pre-tempering.The experimental results show that 455? is the optimal pre-tempering temperature to improve Charpy impact toughness of the steel.The microstructure observation reveals that during the 455? pre-tempering,the RA decomposes completely into fine bainite consisting of fine bainitic plates and high density of M3C carbides,which provide abundant nucleation sites for M23C6 carbides inside carbide clusters during subsequent 700? tempering,and thus avoid the formation of coarse M23C6 carbides along cluster boundaries.