Stress Corrosion Cracking And Electrochemical Behavior During Slow Strain Rate Test Process Of TC4 Alloy Weld Treated By Induction Heat Treatment

As a structural and functional material,titanium alloy has been widly used due to the excellent physical and the chemical properties.Due to the limitations of the production technology and the requirement of the service environment,welding technology has become a improtant processing method in the application fields of titanium alloy,but the residual stress with high level and the corrosion sensitive microstructure which were generated in the welding process can lead to the stress corrosion cracking of titanium alloy weld in some corrosive mediums.In order to solve the problem,the residual stress and the microstructure of the TC4 alloy weld were adjusted by the local induction heat treatment,and the stress corrosion cracking behavior of the weld was improved under the different corrosive environment,and the effect of the induction heat treatment on the stress corrosion cracking susceptibility and the electrochemical behavior during the slow strain rate test process was researched in this study.According to the electromagnetic properties,the shape-dimension and the heat treatment characteristics of the TC4 alloy weld,the local induction heat treatment device with the form of reciprocating motion was manufactured.As the induction heat treatment temperatures were lower than 700 ?,the residual stresses in the welds gradully decreased with the increase of the induction heat treatment temperature and the reciprocating time,whcih were almost impervious to the induction heat treatment temperature and the reciprocating time when the induction heat treatment temperatures were higher than 910 ?.As the induction heat treatment temperature was 700 ? and the reciprocating time was 15,the residual stress in the weld was decreased from 317 MPa to 47 MPa and reduced by 85.3%,which was effectively released and comparable to those in the welds which were treated by the induction heat treatment with the heat treatment temperatures which were in the range of 910~970 ?.The induction heat treatment with the temperatures which were lower than ?700 had al most no influence on the microstructures of the welds which were acicular ?? martensite phases,and when the induction heat treatment temperatures were higher than 910 ?,the acicular ?? martensite pha ses in the welds decomposed into the lathy ? phases and the transformed ? phases,the content of the lathy ? phases decreased with the increase of the induction heat treatment temperature and the reciprocating time,which implied the increasing content of the transformed ? phases.With the increase of Cl-content in the corrosive medium and the anode polarization potential,the stress corrosion cracking susceptibility increased during the slow strain rate test,which decreased with the increase of strain rate and the cathodic polarization potential.Through the induction heat treatment,the stress corrosion cracking resistance of the weld during the slow strain rate test under the different corrosive environment was improved,and the mechanical properties and stress corrosion cracking susceptibility of the weld respectively increased and decreased with increasing induction heat treatment temperature.During the slow strain rate test process,the charge transfer resistance and the electrochemical noise resistance of the welds gradully decreased,and the absolute differences of the skewness and the kurtosis of the current noise signals and the signal energy intensity of the welds gradully increased,which showed the corrosion resistances of the welds gradully decreased under the slow strain rate test condition.As the heat treatment temperature increased,the charge transfer resistance and the electrochemical noise resistance of the welds gradully increased,and the absolute differences of the skewness and the kurtosis of the current noise signals and the signal energy intensity of the welds gradully decreased,which showed the corrosion resistances of the welds was effectively improved after the induction heat treatment,and the higher heat treatment temperature leaded to the higher corrosion resistance.For the welds in the the as-welded and the heat treated condition with the induction heat treatment temperatures of 700 ?,910 ?,970 ?,the characteristic frequency of the current noise signal was below 0.25 Hz in the segment of 0~4 h,0~8 h,0~10 h and 0~14 h,respectively,which was below 0.13 Hz in the segment of 6 h,10~12 h,12~14 h and 16~18 h.The characteristic frequency of the current noise signal showed that the occurrence times of the localized corrosion delayed after the induction heat treatment,the higher heat treatment temperatures leaded to the later occurrence times of the localized corrosion and the longer times for the initiation periods of the stress corrosion cracking and the fracture of the welds during the slow strain rate test process.