The Effect Of Normalizing Cooling Rate On The Structure And Mechanical Properties Of 2.25Cr-1Mo Heat-resistant Steel

With the development of the economy,China's demand for electricity is increasing in recent years.Nuclear power,as a new type of green energy,has received widespread attention in various countries.In recent years,2.25Cr-1Mo heat-resistant steel has been used to manufacture high-temperature components in the fourth-generation nuclear power plant,which are vigorously developed at home and abroad.The components with large differences in size and thickness including forgings and tubes for reactor internals and heat exchangers.The components have a service temperature of 500~530? and a life requirement of not less than 20 years.The service characteristics of high temperature and long life of nuclear power equipment require 2.25Cr-1Mo steel to have good heat strength and structural stability during service.For large forgings,it is difficult to obtain a uniform microstructure at the center and surface during heat treatment.It is an important basis for evaluating service safety and formulating heat treatment process whether there are great differences in the performance of different microstructures during service.In this paper,four kinds of microstructures of austenitized 2.25Cr-1Mo test steel were simulated by controlling four cooling rates(air-cooling,30 ?/min,5 ?/min and 1 ?/min)to simulate the corresponding cooling states of different parts in the wall thickness direction during normalizing cooling process of large forgings.Aging tests of 2000 h and creep rupture tests at 530? were carried out for four kinds of microstructures test steels.The mechanical properties tests such as stretching and impact,and OM,SEM,TEM and other microscopic analysis methods were used to study the effects of different microstructures on the long-term aging stability and creep rupture performance of 2.25Cr-1Mo steel.Concluded as follow:(1)As the normalizing cooling rate slows down,the content of pro-eutectoid ferrite in 2.25Cr-1Mo steel increases gradually,? phase transits from bainite lath with high dislocation density to massive ferritewith low dislocation density.(2)The microstructure of 2.25Cr-1Mo steel with good strengthtoughness matching is ferrite(< 1%)+bainite,and ferrite(< 36.7%)+bainite with good creep rupture strength.Normalizing cooling rate should be controlled during heat treatment of large forgings to ensure that the cooling rate at the center of thick wall is higher than 5 ?/min,and the ferrite content in the microstructure is less than 36.7%.(3)There is high content of large ferrite in slow cooling microstructure,low strength of ferrite and fast coarsening and aggregation rate of interfacial carbides along grain boundaries are the main reasons for reducing the strength and toughness of 2.25Cr-1Mo steel during service.