ULTRASONIC AND METALLOGRAPHIC STUDIES ON AISI 4140 STEEL EXPOSED TO HYDROGEN AT HIGH PRESSURE AND TEMPERATURE. Abstract.

Transcription

1 Session T1C-5 ULTRASONIC AND METALLOGRAPHIC STUDIES ON AISI 4140 STEEL EXPOSED TO HYDROGEN AT HIGH PRESSURE AND TEMPERATURE Malavika Oruganti and Malur Srinivasan Department of Mechanical Engineering Lamar University Beaumont, TX Abstract This paper deals with an investigation conducted to study the effects of hydrogen exposure at high temperature and pressure on the behavior of AISI 4140 steel. Piezoelectric ultrasonic technique was primarily used to evaluate surface longitudinal wave velocity and defect geometry variations, as related to time after exposure to hydrogen at high temperature and pressure. Critically refracted longitudinal wave technique was used for the former and pulseecho technique for the latter. Optical microscopy and scanning electron microscopy were used to correlate the ultrasonic results with the microstructure of the steel and to provide better insight into the steel behavior. The results of the investigation indicate that frequency analysis of the defect echo determined using the pulse-echo technique at regular intervals of time, appears to be a promising tool for monitoring defect growth induced by high temperature and high pressure hydrogen-related attack. Introduction Hot hydrogen attack is of particular concern in chemical and petrochemical industries that are susceptible to hydrogen diffusion into steel vessels at partial pressure exceeding 30 bar at temperatures exceeding 300 ºC. Under such conditions, the pressure vessel may lose much of its ductility and strength suddenly [1], often leading to catastrophic failure. This is caused by the nucleation, growth and merging of methane bubbles along grain boundaries to form fissures. HHA is an internal decarburization of the steel following a reaction of the type: M 3 C + 2H 2 = 3M + CH 4, where M, C, H, CH 4 refer to metal, carbon, hydrogen and methane respectively. The pressure of methane formed by this reaction causes cavity nucleation and growth along the grain boundaries. The methane pressure decreases with decreasing partial pressure of hydrogen and also with increase in temperature [2]. While HHA would be primary cause of failure at high pressures, the combination of lower pressure and high temperature may lead to association of creep with HHA. It stands to reason therefore that there is a need to predict and monitor the formation and growth of cracks induced by HHA. Also, it would be useful to detect any surface stress changes that may occur during HHA, as such a test would be useful in examining large pressure vessels. Some previous studies on HHA have focused upon HHA in heat affected zone in weldment [3], development of a better grade of steel to resist

2 HHA [4] and use of critically refracted longitudinal wave (L CR ) technique to evaluate load damage, where the possibility of using the technique to detect HHA has been discussed [5]. Nondestructive evaluation using ultrasonic wave propagation has long been used successfully to detect defects. A form of this technique is known as Critically Refracted Longitudinal Wave Technique (L CR ). Srinivasan, in association with Bray and graduate students [6-17] has successfully used this technique to evaluate the surface residual stress and microstructure in alloys. Also, Bray and his associates [18] have successfully used frequency analysis of pulseecho waves to determine the location and details of internal features in beef.. With this as the background, Srinivasan and his graduate student (the authors of this paper) conducted an investigation to determine the L CR velocity variation in a AISI 4140 steel sample subjected to hydrogen exposure at high temperature and pressure, enough to qualify for HHA. Also, they planned and conducted frequency analysis along the thickness of the sample using pulse-echo technique to correlate central frequency with the shape and location of HHA defects identified using scanning electron microscopy. The results of this investigation are presented and discussed in this paper. Experimental Details. Test Samples 150 mm long block was cut from 25 mm thick and 75 mm wide AISI 4140 steel (nominal composition (wt.%): C %, Si %, Mn %, Cr %, Mo %, bal-fe), and held at 900 C for 4 hr in a muffle furnace and cooled in the furnace to room temperature, to minimize texture. The block was then cut into two equal length (75 mm each) pieces. The surfaces were then lightly ground to remove scale. One block was then subjected to hydrogen at a pressure of 4 MPa (580 psi) for 48 hr at a temperature of 400 ºC ( 750 ºF) for 48 hr [19] in an autoclave shown in Fig. 1. The other block was retained in the authors laboratory in a desiccator and used as the control block in the ultrasonic experiments. Fig.1: Autoclave [19]

3 LCR Velocity Measurement. Each block was then placed in a water bath in succession and LCR velocity was measured daily for about 50 days. The set up is shown in Fig. 2. The send and receive probes were both of 15 MHz nominal frequency. LeCroy 9310M Oscilloscope, 5052 Parametric Pulser-Receiver and Parametric Pre-amplifier were used in the ultrasonic circuit. Velocity was measured as detailed in references [3-14]. Briefly, in this technique, critically refracted ultrasonic longitudinal wave is generated by inserting a send probe in a clear acrylic wedge that is placed in contact with the surface of the specimen. The ultrasonic wave travels under the surface of the specimen and it is collected by a receive probe that is placed on another clear acrylic wedge that is a oriented in the opposite direction of the first wedge. A high-resolution oscilloscope used for the purpose of display and analysis then receives the signals. LCR velocity is then determined as the distance between the entrance and exit points of the longitudinal wave divided by the travel time between the main bang and the first echo of the longitudinal ultrasonic wave. Fig. 2: LCR Velocity Measurement Setup Pulse-echo Waveform Determination The experimental set up for pulse-echo waveform determination is shown in Fig. 3. A 1 MHz probe was used both as the send and receive probe and the frequency of a prominent defect echo seen in the through-thickness ultrasonic signals was analyzed.

4 Fig. 3. Normal Incidence Pulse-echo Waveform Determination Metallography Samples were cut from the defect region (after correlating with the peak frequency as detailed in results and discussion) and subjected to scanning microscope examination to observe the defect details. Vertical section of one edge of the hydrogen-exposed block was ground, polished and etched with 2% solution of nitric acid in alcohol to observe decarburization details. Results and Discussion In Fig. 4 and Fig. 5 are shown the L CR velocity at the top surface of the hydrogen-exposed block with standard deviation and standard error respectively. The former is an index of the variability of the data points while the latter represents the variability of the mean values. It is clear that from both figures that the L CR velocity values have good repeatability, as both standard deviation and standard error are small and that the curves drawn show the true trend in the L CR velocity variation with respect to time. There is a significant reduction in velocity after 45 days indicating that some stress-reducing activity at the surface has occurred at this time. Since the L CR velocity did not show much variation even after nearly a month, it was decided to use frequency analysis also in addition to L CR measurements. The first frequency analysis was performed on the 29 th day after starting the L CR measurements and not surprisingly, the central frequency showed a peak at the same time L CR velocity showed a significant drop. The results of the frequency analysis are shown in Fig. 6 and Fig.7, which again indicate that the true trend in variation of central frequency with time is shown. In Table 1, Table 2 and Table 3 are shown the frequency analysis data on days 15, 16 and 17 showing the significant rise in the mean central frequency on day 16, which corresponds to day 45 of the L CR measurement.

5 Fig. 4: L CR Velocity (m/s) at Top Surface Showing Standard Deviation Fig. 5: L CR Velocity (m/s) at Top Surface Showing Standard Error

6 Fig. 6: Frequency Analysis Showing Standard Deviation Fig. 7: Frequency Analysis Showing Standard Error

7 Table 1. Frequency Analysis on Day 15 S.No. f hi (MHz) f lo (MHz) f pk (MHz) f cent (MHz) Mean(MHz) Standard Deviation(MHz) Standard Error(MHz) Table 2. Frequency Analysis on Day 16 S.No. f hi (MHz) f lo (MHz) f pk (MHz) f cent (MHz) Mean(MHz) Standard Deviation(MHz) Standard Error(MHz)

8 Table 3. Frequency Analysis on Day 17 S.No. f hi (MHz) f lo (MHz) f pk (MHz) f cent (MHz) Mean(MHz) Standard Deviation(MHz) Standard Error(MHz) It may be noted that the mean central frequency rose from 3.63 on day 15 to 6.24 on day 16 but dropped to 3.85 on day 17 indicating the occurrence of critical activity on day 16, which corresponds to 45th day of the L CR measurement when also a critical activity (probably the same) occurred. The control block velocity and frequency values were unaltered from the initial values. The ultrasonic experiments were stopped a little after the occurrence of the critical events and the test block was sectioned horizontally along the plane corresponding to the defect, as estimated from the defect echo in the pulse-echo signals. This plane was examined in a scanning electron microscope and the result was as shown in Fig. 8. It is seen that there is a crack at the grain boundary with somewhat smoothened edges. It is very likely that this crack grew significantly on the 16 th day of the frequency analysis (45 th day of the L CR velocity measurement) but some event like creep [2] may have occurred thereafter to blunt the crack, resulting in slowing of crack growth. One possibility is that repetition of such events will cause total failure. If ultrasonic frequency analysis indicates recurrence of sharp rises in frequency then the pressure vessel should be taken out of service. The drop in L CR velocity during the growth of the crack may be related to stress relaxation spreading to the surface and thus, could be a useful guide for the onset of failure in the interior.

9 Fig. 8: SEM Photograph Showing Grain Boundary Crack in the Defect Echo Plane In Figure 9 is shown a photomicrograph of a vertical section. The decarburization seen at the corner confirms that significant HHA has occurred in the block. The decarburized zone is essentially ferritic, as compared to the adjacent zones, which consist of both ferrite and pearlite. (The gray zone at the left and top edge of the photograph is the mounting material for the metallographic sample) Fig. 9: Optical Photomicrograph Showing Decarburization in the Surface Edges.

10 Concluding Remarks The present investigation indicates that piezoelectric ultrasonic techniques are very useful tools to study the effects of exposure of hydrogen exposure at high temperature and pressure on the behavior of AISI 4140 steel. The simplicity and nondestructive nature of the ultrasonic technique allow monitoring of the effects of hydrogen exposure at high temperature and pressure, continuously with respect to time, which is hard to achieve with other techniques. Optical and scanning electron microscopy provide valuable correlating information of the hydrogen-related effects and also actual shape of defects, but can only be used to study the initial and end of the event features, as they require destruction of the sample. X-ray techniques can provide information on the variation of the shape of the defects as a function of time but are expensive and require protection from radiation. A suggestion for future work would be to include X-ray examination whenever ultrasonic signals show unusual trends. Acknowledgement The authors would like to sincerely thank Dr. Don E.Bray for his constant support and advice and Mr. A. Adeleke of Honeywell corrosion Solutions for conducting the autoclave experiments. The help of Mr. Dan Rutman of Lamar University in doing SEM work is gratefully acknowledged. The optical microscopy was done with the facility at Gerdau Ameristeel Inc. References [1]. Shewmon, P.G., Hydrogen attack of pressure vessel steels, Materials Science and Technology, Institute of Metals, London, 1, 2-11, (1985) [2] Saugerud, O.T., D.E. Moore and G.R. Odette, A model-based approach to evaluating the probability and criticality of hot hydrogen attack in C-1/2Mo equipment, PVP Vol.239/MPC, 33, Serviceability of petroleum, process and power equipment, ASME, 1-5 (1992) [3] Parthasarathy, T.A., and P.G. Shewmon, Hydrogen attack behavior of the heat affected zone of a 2.25 Cr-1Mo steel weldment, Metallurgical Transactions A. 18A, (1987) [4] George, T., E.R. Parker, and R.O. Ritchie., Susceptibility of hydrogen attack of a thick-section 3Cr-1Mo-1Ni pressure vessel steel-role of cooling rate, Materials Science and Technology, 1, (1985) [5] Bray, D.E., Evaluation of load damage in steel using the L CR ultrasonic technique, ASME Pressure Vessel and Piping Division (Publication) PVP, 1, (2008) [6] Srinivasan, M.N., D.E. Bray, P. Junghans, and A. Alagarsami, "Critically Refracted Longitudinal Wave Technique, a New Tool for Residual Stress Measurement in Castings, Transactions of the American Foundrymen's Society, 99, (1991) [7]. Chundu, S.N., M.N. Srinivasan, and D.E.Bray, "Residual Stress Measurement in Ductile Cast Iron Using L CR Technique," ASME Journal of Pressure Vessels and Piping, 216, 49-54, (1991). [8] Chundu, S.N., M.N. Srinivasan and D.E. Bray, "Residual Stress Measurement in Ductile Cast Iron Using L CR Technique," ASME Journal of Pressure Vessel and Piping, 216, (1991). [9] Srinivasan, M.N., S.N. Chundu, D.E. Bray and A. Alagarsamy, "Detection of Stress in Ductile Cast Iron Using L CR Technique", Transactions of the American Foundrymen's Society, 100, (1992)

11 [10]. Srinivasan, M.N., S.N. Chundu, D.E. Bray and A. Alagarsamy, "Residual Stress Measurement in Continuous Cast Ductile Bars Using L CR Technique," Journal of Testing and Evaluation, ASTM, 20, 5, (1992). [11] Pathak, N., D.E. Bray and M.N. Srinivasan, Detection of Stress in a Turbine Disk Using L CR Technique", ASME Journal of Pressure Vessel and Piping, PVP,. 239 (MPC Vol. 33), D. Bagnoli, M. Prager and D.M. Schlader (editors), 1-3, (1992). [12] Bray, D.E., and M.N. Srinivasan, The L CR Ultrasonic Technique for Stress Measurement and Materials Characterization", Proceedings of the 1993 International Chemical and Petroleum Industry Inspection Technology III, The American Society for Nondestructive Testing, Inc., Columbus, Ohio, , (1993). [13] Bray, D.E., N. Pathak and M.N. Srinivasan, Residual Stress Distribution in the Rim of a Steam Turbine Disk Using L CR Ultrasonic Technique, Proceedings, Seventh International Symposium on Nondestructive Characterization of Materials, Czech Technical University, Czech Republic, June 19-22, (1995) [14] Bray, D.E., R. Pfluger and M.N. Srinivasan, Evaluation of Residual Stress Gradients in Ductile Iron using The Critically Refracted Longitudinal Wave Technique, Proceedings of the International Chemical and Petroleum Industry Inspection Technology Topical IV Conference, Houston, Texas, , (June 1995) [15] Bray, D.E., N. Pathak and M.N. Srinivasan, Residual Stress Mapping in a Steam Turbine Disk Using the L CR Ultrasonic Technique, Materials Evaluation, 54, , (1996) [16] Srinivasan, M.N., and D.E. Bray, Near-surface and Through-thickness Residual Stress Evaluation in Ductile Iron Using Critically Refracted Longitudinal Waves, Proceedings of the ASME-ASIA 97 International Conference, Singapore, ASME Publication No. 97-AA-97, (Sept. 1997). [17] Srinivasan, M.N., and K. Ramakrishnan, Evaluation of Residual Stress in Gray Iron Using Critically Refracted Longitudinal (L CR ) Ultrasonic Waves, Proceedings of the Sixth International Conference on Residual Stress, Oxford, United Kingdom, IOM Communications, , (2000). [18] Park, B., A.D. Whittaker, R.K. Miller and D.E. Bray, Measuring intermuscular fat in beef with ultrasonic frequency analysis, Journal of Animal Science. 72, , (1994) [19] Adeleke, A., Personal Communication, Honeywell Corrosion Solutions, Houston, (June 2010).