Microstructural Characterization and Damage Detection in Steels with Linear and Nonlinear Ultrasonic Test

The objective of this research is to study the potential of using nonlinear ultrasonic testing (NLUT) to detect damage early due to mechanical deformation or creep, as well as using it as a mean to assess microstructure changes. A comparison is also undertaken between the sensitivity of the NLUT (using nonlinearity parameter and acoustoelastic constant) and linear ultrasonic testing (LUT) technique (using wave velocity), for each damage type and microstructure assessment. This investigation consists of three different parts. In the first part of the investigation, assessment of the mechanical damage in a single-phase metal, Al-1100, was conducted to associate the strain directly with the NLUT and LUT parameters. In the second part, both NDE techniques were correlated with changes in the microstructure of the alloy after heat treatments; an A572 steel was intercritically annealed at different temperatures to generate different volume fractions of ferrite and martensite. In the third part of this study AISI 410 stainless steel specimens were submitted to different levels of creep; such damage includes both mechanical straining and microstructure changes due to exposures to different creep times and total strains. The results showed that the NLUT has the potential to detect the most minute changes in the microstructure with a sensitivity about 150 times more effective than LUT. However, using the LUT methods can still be beneficial in mapping the localized damage especially with the immersion techniques such as Scanning Acoustic Microscope. In the case of mechanical damage, the NLUT showed a continuous increase or decrease depending on the damage. For the case of creep damage assessment more work is needed to interpret the results due to the complexity of this type of damage due to mechanical and microstructural changes that occur simultaneously. There needs to be caution when interpreting the results since several factors are influential, particularly the initial condition of the component before service.