·Table of Contents
·Materials Characterization and testing
Study of Cold Worked Austenitic Stainless Steel - Using LCR Waves
Physics Department, Anna University, Chennai - 600 025, India
P. Palanichamy, T. Jayakumar, P. Kalyanasundaram and Baldev raj
Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research,Kalpakkam - 603 102, India
Ultrasonic nondestructive testing and evaluation plays a major role in the present day condition monitoring and life assessment of components and structures in various industries, during pre-service and in-service inspections. While bulk waves are useful for characterising the volumetric properties, surface waves are used for evaluating the surface and sub-surface properties of the materials. In the recent past, Bray and Junghans have successfully demonstrated the potential applications of critically refracted longitudinal (LCR) ultrasonic waves. LCR waves travel just below the surface of the material and have the properties of both surface and bulk waves. LCR waves are found to have several advantages over conventional Rayleigh surface waves and bulk waves for specific applications. In the present work, perspex wedges have been designed to generate and detect LCR waves with probes of frequency 2 MHz. One wedge-probe assembly for generating and another for detecting LCR waves have been used in this study. The transmitter-receiver probe assembly thus fabricated has been used for studying austenitic stainless steel (AISI type 304) specimens cold worked in the range 0 to 50%. Precise transit time measurements and velocity calculations of LCR waves were made in different specimens. The results were compared with the results of earlier measurements made with bulk waves. It is observed that LCR wave velocity measurements have practical advantage for assessment of the percentage cold working in austenitic stainless steel.
In nuclear and chemical industries, austenitic stainless steels are preferred due to their excellent corrosion resistance and high temperature properties. Mechanical working of metals below the recrystallisation temperature to give them different shapes, sizes and strength is known as cold working. The cold working results in increase in strength and hardness. Depending on the particular type of application, different percentages of cold working at different strain rates are employed. For example, for nuclear fuel cladding tubes in Fast Breeder Reactors austenitic stainless steel subjected to 20% cold working is used to obtain resistance to radiation damage. Chemical composition (wt%) of the stainless steel used is S/0.002, P/0.03, C/0.08, Cr/18.0, Si/1.0, Mn/2.0, Ni/10.5 and the balance is Fe. The percentage change in the velocities (longitudinal and shear) with cold work and attenuation coefficients at different cold worked percentages in AISI type 304 stainless steel specimens have been reported by Jayakumar et al. While the bulk waves are useful for characterising the volume properties, surface waves are used for evaluating the surface and sub-surface properties of the materials. Recently, Bray and Junghans have demonstrated the potential applications of critically refracted longitudinal (LCR) ultrasonic waves. These waves travel just below the surface of the material and have the properties of both surface waves and bulk waves. These waves have several advantages over conventional Rayleigh surface waves and bulk waves for specific applications. LCR waves are more sensitive to stress fields in a finite thickness of test pieces. LCR waves have been successfully employed for assessment of grain size in AISI type 316 stainless steel. In the present work, perspex wedges have been designed to generate and detect LCR waves with probes of frequency 2 MHz. One wedge probe assembly for generating and another for detecting purpose are used. The transmitter-receiver probe assembly thus fabricated is used for transmit time from which the LCR wave velocity measurements are made.
LCR WEDGE PREPARATION
To generate LCR waves in the cold worked test samples, perspex wedges (clear acrylic, polymethyl methacrylate) are used. To facilitate this, initially the longitudinal ultrasonic wave velocities are measured both in perspex and steel medium. The angle of incidence of ultrasonic beam at perspex/steel surface necessary for producing LCR waves in the specimens is estimated as sine inverse of the ratio of longitudinal ultrasonic velocity in perspex to that in steel specimens. This angle has been estimated to be 27.23°
in the present study. Proper tapering, at this angle is given on a rectangular parallelopiped shaped perspex bar at one end and a normal beam probe is fixed there. This generates LCR waves in the test pieces. A generalised software has also been developed for fabricating generator/receiver probes depending on the availability of normal beam probes of different crystal dimensions and for different applications. Two commercially available piezoelectric ceramic normal beam longitudinal wave probes (Krautkramer MB2S-N 2 MHz 10 mm crystal diameter) have been used to fabricate the probe assembly. Both the transmitter wedge and receiver wedge are of same dimensions, in the present work, and the dimensions of the wedges are shown in Fig.1.
Fig 1: Geometry of perspex wedge for transmitter / receiver probe assembly
(All dimensions are in mm)
In this study, six plates of AISI type 304 stainless steel with 20 mm thick each were cut from plate stock and solution annealed. Keeping one as reference plate (zero % cold work), the others were cold worked for 10%, 20%, 30%, 40% and 50% thickness at room temperature. These plates were cut to the dimensions 120 mm x 40 mm.
MEASUREMENT OF LCR WAVE VELOCITY
To estimate the velocity of LCR waves in cold worked plates, the transit time of the LCR waves has to be measured for its passage through a known distance through the specimen. This has been achieved in two steps. In the first step, the transmitter and receiver probe assemblies were kept in contact with each other, that is with the shortest distance possible between them (Fig.2). Using Tektronix TDS 524A oscilloscope with 500 MHz sampling rate and Ultrasonic Transducer Analyzer (UTA-3, Aerotech Laboratories, USA), the transit time (t1) of LCR waves between the two probes has been noted. In the second step, a delay of known thickness (16 mm) was introduced between the two probe assemblies as shown in Fig.3. Again the transit time (t2) of the LCR waves between the two probes has been measured. The transducer assembly is kept in such a way that the velocity of the LCR waves propagating in the rolling direction is measured. Now the difference between the two measured transit times (D
t = t2 - t1) is taken as the transit time for travelling through a thickness (D
x), equal to the thickness of the delay, of the sample under study. Hence the velocity of LCR waves is estimated by dividing the thickness of the delay (D
x) by the transit time difference (D
t). The variation in percentage change in velocity of LCR waves with increase in cold work has been shown in Fig.4.
Fig 2: Transit time measurement without delay(T - Transmitter ; R - Receiver)
Fig 3: Transmit time measurement with delay
RESULTS AND DISCUSSION
Figure 4 shows the percentage change in LCR wave velocity with percentage cold work. It is observed in the present work that, as the sample is gradually cold worked starting from solution annealed condition, the LCR wave velocity in the sample decreases from 0% to 10% cold working. During 10% to 30% cold working the LCR wave velocity has a marginal variation. As the cold working is further increased from 30% to 50% the LCR wave velocity gradually decreases.
Fig 4: Percentage change in velocity of LCR wave with cold work in
AISI type 304 stainless steel
The above results are also compared with bulk wave velocity measurements carried out on a similar cold rolled plates as reported earlier by Jayakumar et al. Among all the three measured longitudinal velocities in rolling, transverse and normal directions (V11, V22 and V33), the longitudinal velocity measured in rolling direction (V11) has the maximum variations (3.2 %) with cold work upto 50% and has a decreasing trend with increase in cold work. The change in the velocity of LCR waves is very much similar as that of the percentage change in longitudinal bulk wave velocity in rolling direction (V11). However, the percentage change for the LCR waves is much higher after 10% cold work. The overall variation in velocity of LCR wave could be attributed not only due to the influence of metallographic texture due to cold work but also due to the changes in the micro and submicrostructural variations as reported by Palanichamy et al.
It is also interesting to note that LCR wave velocity measurements cold be used as a tool for measuring the degree of rolling under practical situations. This is because though the variation in bulk wave velocity (V11) with percentage cold work in appreciable, the measurements could not be realised in practical situations and it requires cutting a specimen from the cold rolled plates. Whereas LCR wave velocity has the advantage of being measurable by keeping the transducer assembly on the rolling plane and the waves are made to propagate in the surface and subsurface of the plates - along the rolling direction.
SUMMARY AND CONCLUSION
In the present work, a probe assembly has been fabricated to generate and receive LCR waves in AISI type 304 stainless steel plates. Using the assembly, accurate velocity measurements have been made on different percentage cold worked stainless steel plates. The variation in LCR wave velocity with percentage cold work thus measured has been compared with the earlier results made with bulk wave. LCR wave velocity measurements have practical advantage that the LCR wave velocity can be used under practical situations to assess the percentage cold work.
The inspiration for the present work originates from the pioneering work done by Don E.Bray, a former Professor of Texas A&M University, USA. One of the authors, B.N.Sankar expresses his gratefulness to him for introducing the LCR method to him. He is also thankful to Dr.Baldev Raj, Director, MCRG, Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam for the encouragement and support rendered to him. Authors also thank Dr.P.Rodriguez, Director, IGCAR and Dr.C.Mohana Doss, Head of the Department of Physics, Anna University for constant encouragement and support.
- Bray, D.E. and Junghans, P., "Application of the LCR ultrasonic technique for evaluation of post-weld heat treatment in steel plates", NDT&E International, 28(4), pp.235-242, 1995.
- Jayakumar, T., Mukhopadhyay, C.K., Kasi Viswanathan, K.V., and Baldev Raj, "Acoustic and magnetic methods for characterisation of microstructures and tensile deformation in AISI type 304 stainless steel", Trans. Indian Inst. Mat., 51(6), pp.485-509, 1998.
- Bray, D.E., Tang, W., and Grewal, D.S., "Ultrasonic stress evaluation in a compressor rotor", J. of Test and Eval., 25(5), pp.503-509, 1997.
- Sankar, B.N., Palanichamy, P., Rajkumar, K.V., and Jayakumar, T., "Critically refracted longitudinal waves-Generation/Detection/Application", Proceedings of International Conference and Exhibition on Ultrasonics (ICEU '99), National Physical Laboratory, New Delhi, India, Vol.2, pp.391-394, 1999.
- Palanichamy, P., Vaidehi Ganesan and Kalyanasundaram, P., "Ultrasonic NDE of cold worked austenitic stainless steel microstructures", J. of the Acoustical Society of India, XXVII (1-4), pp.253-258, 1999.