Study On Aging And Life Of High Temperature Fastening Bolt Material For Steam Turbine

The high-temperature fastening bolt,as the connecting part of the upper and lower flanges of the cylinder,plays a very important role in the safe operation of the ultra-high parameter steam turbine unit.This article through studies 2Cr11Mo1Ni WVNb N bolt material related parameters of the best heat treatment process and its fatigue life damage under creep interaction,find out the different quenching temperature and tempering temperature on the microstructure and mechanical properties of bolt material,and the influence of the residual life of bolts for the bolt material heat treatment process and life management put forward the scientific advice,for improving the efficiency of the steam turbine unit,the choice of the reasonable bolt heat treatment process,bolts prolong service life,has the important practical significance.In this paper,2Cr11Mo1Ni WVNBN bolt material is taken as the research object to explore the effects of different heat treatment processes on the structure and properties of the bolt material,the equivalent stress of the bolt under the actual working conditions and its remaining life under the interaction of fatigue and creep,so as to ensure the safe service of the bolt.OM,SEM,EDS,XRD,this paper material organization analysis,tensile test,impact test and hardness test of mechanics performance test method,the finite element software ABAQUS and the theoretical analysis methods of Miner linear cumulative damage rule,mainly around the single hardening quenching temperature on the material the original austenite grain size and hardness,gradient hardening fixed temperature tempering,and the influence of the fixed temperature quenching gradient,tempering on the microstructure and mechanical properties of bolt material,the influence of bolt under service condition of equivalent stress and residual life.The main research results are as follows:(1)When quenching separately,between 820?and 1070?,with the increase of quenching temperature,the austenite grains of each sample will gradually increase and grow up to the state of homogenization,and its room temperature structure is also more and more quenched martensite.After quenching at 1120?,part of the precipitated phase melts into austenite grains and causes them to grow abnormally.The Brinell hardness of the sample increases first and then decreases,which is due to the continuous growth of the original austenite grain and the coarsening of martensite beam in the grain caused by the abnormal growth.The optimal quenching temperature of the bolt material is determined to be 1070?.(2)When quenching at 670??1120?and tempering at 670?,the tensile strength and Brinell hardness of the sample will increase first and then decrease with the increase of quenching temperature,and the elongation and impact energy fluctuate within a certain range;When quenching at 1070?and tempering between 570?and 770?,the tensile strength and Brinell hardness of the sample will decrease with the increase of tempering temperature,while the elongation and impact energy will increase slowly and reach the best value at 670?.After the tempering of each sample,the tempered martensite structure with small batten bundles is obtained.There are M₂₃C₆ type carbide and MX equal precipitates between each martensite batten bundles.After long-term aging,the bolts can precipitate Laves phase and Z phase,and the optimal heat treatment process of bolt materials is determined as 1070?quenching+670?tempering.(3)Bolts in service will mainly bear bolt pre-tightening stress caused by cold tightening and hot tightening,thermal stress caused by heat transfer of flange,steam stress of steam turbine unit and bending stress caused by bolt installation process.The equivalent stress of the bolt under ideal service condition is 485.3 MPa by means of relevant stress calculation method.Finite element method is used to simulate the pre-tightening and heating process of the bolt.The stress of the middle rod of the bolt is 463.5MPa,and the stress concentration occurs at the first screw thread of the bolt,and the maximum stress reaches 780.4 MPa.Under the rule of Miner linear cumulative damage,bolt in one load cycle was calculated by the interaction of fatigue and creep life of damage for D=0.11821,thus proposed the bolt aging index?aging and the corresponding rating standard.