Attempts are being made to develop NDT techniques such as ultrasonics, magnetic, optical and eddy current techniques as advanced sensors for on-line sensing of microstructure and properties during processing of materials to facilitate automated material processing. As a first step towards developing an on- line sensor for studying the dynamic microstructural changes during processing of austenitic stainless steels, we have carried out ultrasonic velocity measurements for

• studying the microstructural changes during post deformation annealing,
• sensing the state of microstructure in the hot deformed samples, and © determination of grain size in austenitic stainless steels.

Study of the annealing behaviour of Ti-modified austenitic stainless steel: Ultrasonic velocity measurements were carried out at different frequencies on the cold worked and annealed samples. The materials was given 20% cold work and annealed in the temperature range of 923-1123 K for times ranging from 0.5 - 1000 hrs. At 923 K ultrasonic velocity slightly increased from that of cold worked condition and remained almost constant for annealing upto 1000 hrs. Microstructural observations indicated the absence of recrystallization at 923 K. Thus, the small change in velocity as a function of annealing time at 923 K is attributed to recovery. At 1023 K the change in velocity as a function of annealing time exhibited peaks and troughs for annealing upto 100 hrs and decreased continuously with further increase in the annealing time. Even after 1000 hrs. of annealing the velocity values have not reached saturation. Microstructural observations showed retardation of recrystallization upto 100 hrs of annealing and only partial recrystallization of the sample annealed for 1000 hrs. At 1073 K and 1123 K, the change in velocity as a function of annealing time showed three distinct stages. In the first stage the change in velocity with annealing time increased from that of cold worked condition. The second stage exhibited a decrease in the velocity with increasing annealing time followed by saturation in the third stage. Microstructural observations distinguished the three stages as recovery, recrystallization and completion of recrystallization. Thus the velocity measurements could follow the progress of annealing starting from recovery, onset of recrystallization, progress of recrystallization and completion of recrystallization.

Sensing the state of microstructure of the hot deformed samples of AISI 304 stainless steel: Microstructure evolved during the compression testing of AISI 304 stainless steel samples were characterised by ultrasonic velocity measurements. The compression tests were carried out in the temperature range of 923 - 1523 K at strain rates of 10^-2 to 10²s^-1. The velocity measurements carried out after the tests could sense the recovered, recrystallized and coarse grained microstructure distinctly in the deformed samples. This suggest that the state of microstructure of the hot deformed samples can be identified using velocity measurements.

Determination of grainsize in AISI type 316 stainless steel: A master plot of ultrasonic velocity as a function of grainsize has been generated for grainsizes in the range of 30-170 µm. Using this master plot grainsizes were determined from ultrasonic velocity measurements for different samples. The estimated grainsizes compared very well with the grain sizes measured using optical metallography.

To achieve the desired mechanical properties in the products made of austenitic stainless steels it is important to control the evolution of grain size during processing. The grain size evolution during processing is influenced by the annealing processes such as recovery, recrystallization and grain growth. The results from our studies suggest that ultrasonic velocity measurements can distinctly sense the occurrence of microstructural changes such recovery, recrystallization and grain growth in austenitic stainless steels and also measure grain size.

The above studies based on post processing measurement bring out the potentials of ultrasonic velocity measurements as a sensor for direct and dynamic sensing of microstructure during processing of austenitic stainless steels.