Precipitations And Dislocation Creep In Martensitic Heat-resistant Steels

Energy is the fundamental backbone for the development of any society and economy.In order to achieve the goal of sustainable development,humans should try each and every effort to use the natural resource and energy effectively and efficiently.In our country,fossil fuels are the main source for the generation of electricity.In order to meet the ever-increasing demand for transfer efficiency,temperature and pressure of the steam also rose dramatically in the past few decades.However,for increasing the transfer efficiency in the same manner,the creep behavior of the structural material has become a major concern.Under this circumstance,this research gives an insight into the creep behavior in martensitic heat resistant steels.The aim is to find the critical point during creep mechanism in the martensitic steel,which would not only limit the creep behavior but will also help us to develop anadvancd heat reasistant steel for the future.Within the scope of knowledge,we all know that the creep behavior in steels depends on the evolution of their microstructure.Precipitates in the steel matrix can impede the movement of dislocations in order to improve the stability of the microstructure.Different precipitates vary in volume fraction,size,number density and stability at high temperatures.This research is based on two types of nitride strengthened steels namely NS1 and NS2,and four types of carbon-nitride strengthened steels namely CNS,SIMP1,SIMP2,P92 with a motivation to distinguish the functions of various types of precipitates in the steel matrix.In the research,the microstructures of samples in "High Temperature and Stress" and "No Stress Aging" conditions were investigated and compared.By comparision,we discussed the creep mechanism in the martensitic steel.Fundamentals of the electrochemical theory were applied to study and determine the thermodynamic and dynamic behavior of the secondary phases during precipitation.Then,Scanning electron microscopy,transmission electron microscopy,confocal laser scanning microscopy techniques were used to observe the morphology,distribution and size of them.The results showed that the MX phase and M23C6 phase shortly precipitated into the martensitic matrix at the beginning of tempering.1500 seconds later,the precipitation reaction got completed and MX phase was found to be randomly distributed in the matrix.It appeared like disc or sphere shaped structures in three dimensions,with an average size of around 20 nanometers.M23C6 phase was found to be uniformaly distributed along the primary austenite grain boundaries or lath boundaries.They looked like rods-shaped structures in the matrix,with average size of more than 100 nanometers.Among the six experimental steels,SIMP1 showed the maximum volume of precipitates with a volume fraction of 4.3%in the matrix and with a number density of 3.79×1020k/m3.While NS1 showed the least volume of precipitates,the volume fraction and number density being 0.34%and 2.46×1020/m3,respectively.Results showed that during high temperature aging,unstable precipitates transformed into stable ones and led to an increase in the total volume of precipitates.It was also observed that the Laves phase precipitated sooner or later during the aging process.The volume of Laves showed an inverse dependecy on aging temperature,the lower the aging temperature,the higher the volume of Laves,and vice-versa.After the determination of various entities like types,volume,morphology and the number density of precipitates in the six experimental steels,characterization analysis was carried out,and microstructures of creep and aging samples were further observed.During the aging experiment,it was observed that NS1 and NS2 steels containing small size nitrides recrystallized soon While the steel with larger M23C6 precipitates having a higher microstructural stability recrystallized much later.This is due to the fact that the large M23C6 precipitates can resist dislocation climbing during creep and hinder the evolution from lath to subgrain.These precipitates can also restrict the emergence of subgrains so that recrystallization couldn't occur.It was obvious that the more the volume fraction of precipitates,i.e.,the bigger its size,the larger the number density and hence the more stable microstructure during aging.During the creep experiment,the steel containing large size M23C6 precipitates such as CNS,SIMP1,P92 showed poor creep microstructural stability.On the contrary,the steel which showed a poor aging behavior such as NS1 steel showed better creep property.The differences in the microstructures evolving from "with-stress experiment" and "without-stress experiment" shed light on the creep mechanism of the martensitic steel.Later,a series of calculations were made to reveal the mechanism of creep and a model was established.During the creep test,the dislocation density in martensitic matrix continuously decreased.This behavior illustrated that the propagation rate of dislocations under stress was lower than their annihilation.This behavior supported the fact that propagation rate of dislocations is a crucial process in controlling the creep rate.By calculations,it was found that the large size M23C6 precipitates acted as a linkage to Frank-Read source mechanism.During the creep test,M23C6 precipitates generated dislocations by Frank-Read mechanism unceasingly until the dislocations climbed over them.Since the propagation of dislocations is the key property in controlling the creep rate,the more the generated dislocations the faster the creep rate.That is the main reason why steels containing M23C6 precipitates usually show a poor creep behavior.Creep behavior of martensitic steels with high dislocation densities mainly depends on the generation of dislocations.On the contrary,creep behavior of materials with low dislocation densities depends mainly on the annihilation of dislocations.Our research helped in getting a further understanding about the creep mechanism of martensitic heat resistant steel.