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Effect of residual stresses and metallographic stability on the over all performance of integral diaphragm material
Materials Science and Engineering A 517 (2009) 85–90
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of residual stresses and metallographic stability on the over all performance of integral diaphragm material Geetha Sen a , P. Sampathkumaran b,∗ , D.S. Nadig c , R. Manjunatha c , S. Seetharamu b a b c
Liquid Propulsion System Centre, ISRO, Bangalore 560 008, India Materials Technology Division, Central Power Research Institute, Bangalore 560 080, Karnadaka, India Centre for Cryogenic Technology, Indian Institute of Science, Bangalore 560 012, India
a r t i c l e
i n f o
Article history: Received 11 July 2008 Received in revised form 13 March 2009 Accepted 8 April 2009 Keywords: Residual stress X-ray diffraction Realization process Microstructure Retained austanite
a b s t r a c t The integral diaphragm pressure transducer consists of a diaphragm machined from precipitation hardened martensitic (APX4) steel. Its performance is quite significant as it depends upon various factors such as mechanical properties including induced residual stress levels, metallurgical and physical parameters due to different stages of processing involved. Hence, the measurement and analysis of residual stress becomes very important from the point of in-service assessment of a component. In the present work, the stress measurements have been done using the X-ray diffraction (XRD) technique, which is a non-destructive test (NDT). This method is more reliable and widely used compared to the other NDT techniques. The metallurgical aspects have been studied by adopting the conventional metallographic practices including examination of microstructure using light microscope. The dimensional measurements have been carried out using dimensional gauge. The results of the present investigation reveals that the diaphragm material after undergoing series of realization processes has yielded good amount of retained austenite in it. Also, the presence of higher compressive stresses induced in the transducer results in non-linearity, zero shift and dimensional instability. The problem of higher retained austenite content and higher compressive stress have been overcome by adopting a new realization process involving machining and cold and hot stabilization soak which has brought down the retained austenite content to about 5–6% and acceptable level of compressive stress in the range −100 to −150 MPa with fine tempered martensitic phase structure and good dimensional stability. The new realization process seems to be quite effective in terms of controlling retained austenite content, residual stress, metallurgical phase as well as dimensional stability and this may result in minimum zero shift of the diaphragm system. © 2009 Published by Elsevier B.V.
1. Introduction The performance evaluation of launch vehicle materials/ components such as propellant storage tanks, cooling water tanks, etc. is gaining importance keeping in view of their criticality and the low margin of safety provided to such components [1]. Further, an in-depth analysis reveals possible solutions depending upon the damage tolerance capacity of such components/materials. However, knowledge of residual stress levels in an in-service component will help the metallurgist, designers and mechanical engineers to take proper precautions during design and processing stages for improved reliability and better performance in subsequent applications.
∗ Corresponding author. Tel.: +91 080 23600399; fax: +91 080 23600399. E-mail address: [email protected] (P. Sampathkumaran). 0921-5093/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.msea.2009.04.066
The role of pressure transducer sensor used in space application is considered important from the point of sensing the propellant pressure. It incorporates an integral diaphragm made out of precipitation-hardened stainless steel. The effect of induced residual stresses in the integral diaphragm transducer system on the performance is significant. Integral diaphragm pressure transducer system (Fig. 1) is class of pressure pickup gadget wherein the sensing element is a machined diaphragm. The pressure is applied from one side of the diaphragm and strain induced is measured by the strain gauges mounted on the other face. The strain is subsequently converted into a measurable voltage output using a Wheatstone bridge circuit. This type of transducers is used in space program for propellant gauging system and other areas of precision pressure measurements. As this method suffers from the point of reliability of the data, one has to resort to NDE method. Also the precision and long term performance of the transducer system is critical.
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G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
relieved slowly over a period of time and this leads to zero shift or drift depending on the location of strain induced. The hysterisis of diaphragm will be high if the residual stress is high. This will lead to higher deviation of the calibrated curve with respect to the best fit curve. 2. Experimental The experimental programme consists of studying the diaphragm material at every stage of the realization process currently followed in respect of residual stress, retained austenite (RA) content, dimensional stability as well as microstructure. Similarly the work programme has been repeated for the new or the modified process evolved followed by interpretation of the data. 2.1. Diaphragm fabrication process in force
Fig. 1. Schematic of integral diaphragm pressure transducer.
This paper addresses the key issues relating the zero shift of the transducer system on account of the realization process currently practiced which results in higher compressive stresses obtained by NDE method as well as metallographic and dimensional instability. The performance of the integral diaphragm material has now been improved by way of controlling the stress level, dimensional variations as well as the metallurgical phases due to the adoption of the new realization process. 1.1. Effect of residual stress on the transducer performance The presence of high level of residual stress results in the zero errors in the transducers. Zero errors are generally termed as zero shift, zero drift, non-linearity plus hysterisis. Zero shift error is the error due to change in calibrated zero value. Zero drift is change in calibrated zero value in a random manner over the entire span of measurement. Non-linearity plus hysterisis is the deviation with respect to the calibrated value from the best fit curve. If the level of residual stress in diaphragm is high, the effect on the transducer performance is that the accumulated stress will get
The integral diaphragm material is made up of APX4, a precipitation hardened steel produced from the raw material available in the form of a rod. This is further rolled to the desired dimensions. The current practice of realization process involving the various stages of fabrication process is shown in Fig. 2. These precipitationhardened steels are generally solution annealed and free from hardening agent [2]. On cooling, a martensitic phase is formed due to the process of diffusion-less phase transformation by extreme limited movement of elemental and carbon atoms into positions nearly approaching equilibrium state at lower temperature [3,4]. A tempering treatment results in strengthening the phase due to the precipitation of secondary phases in the alloy system [5,6]. The present realization process practiced has resulted in zero shifts there by the performance of the transducer system was affected. Hence, a new process shown in Fig. 3 had to be developed in order to improve the performance of the diaphragm. In this process, a stage-machining concept has been introduced by adopting intermittent stress relieving treatments to control the level of stresses. This is followed by cold and hot stabilization processes, which is a modified treatment compared to the existing process being followed as well as the one reported in the literature. By way of this process, a stable phase and surface free of microcracks by limiting the stabilization temperature to −80 ◦ C during the cold stabilization process using a cold chamber has been achieved. 2.2. Methods of measurement There are several destructive and non-destructive techniques presently available for the residual stress measurements [7]. The destructive methods cannot be applied on finished components.
Fig. 2. Realization process currently practiced.
G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
87
Fig. 3. Modified realization process.
Fig. 4. Schematic of X-ray diffraction phenomenon.
Therefore the non-destructive techniques (NDT) are preferred for stress measurements. Some of them include (i) ultrasonic, (ii) Xray diffraction (XRD), (iii) Acoustic Barkhausen Noise (ABN), (iv) Magnetic Barkhausen Noise (MBN) and (v) neutron diffraction. The X-ray diffraction (XRD) technique involving Bragg’s law is used for residual stress measurements [8]. In the X-ray diffraction method, the X-rays are applied on to a material and during the process some of them get absorbed by atoms, while the others are sent back in all the directions from the irradiated area known as scattering of X-rays (Fig. 4). The scattered X-rays of a chromium target from the irradiated atoms/crystals are reinforced in a specific direction of an extremely narrow range and propagated. The phenomenon is called as diffraction. The condition of diffraction is defined by the equation called Bragg’s law as n = 2d sin , where d is the inter-atomic spacing, is the Bragg’s angle and is the wave length of characteristics of X-ray radiation. The Bragg angle 2 determined by way of arrangement of atoms in a material is the angle between the incident and diffracted ray as shown in Fig. 4. When a material is subjected to any type of fabrication process, elastic strains between crystallographic planes (i.e., d-spacing) are produced in the individual crystal lattice through the measurement of inter-atomic spacing and hence it is proportional to the stress indirectly. When the material is in tension, the dspacing increases and, when it is under compression the d-spacing decreases. A series of measurements made at different X-ray beam approach angles ( ) are used to fully characterize the d-spacing. The slope of the least squares fit on the graph of the 2 versus sin2 is used to calculate the stress using the following equation: = K
K=
∂2 ∂ sin2
−1 2[E/(1 + )] cot (/180)
where K is the stress constant factor, E is the Young’s modulus of elasticity, is the Bragg’s angle and is the poission’s ratio of the
angle is the angle material, is the stress factor in MPa and between plane and surface normal. The value of slope ∂2/∂ sin2 has been obtained in the present experiments by the method of least squares. The magnitude of the small incremental change in the interplanar distance is a measure of residual stress and the direction of the shift gives the nature of the stress (tensile/compressive) prevailed. Generally, the tensile stress of higher level is detrimental leading to catastrophic failure, on the other hand compressive stress of medium range increases the fatigue strength and there by the service life gets enhanced. Austenite (␥) is a solid solution of iron having a face centered cubic (FCC) structure, consisting of carbon, possibly nickel, chromium or manganese. Steel with low carbon content produced by melting and casting upon cooling yields austenite. Below 723 ◦ C, it generally gets decomposed into ferrite, a solid solution of carbon in iron having body centered cubic (BCC) structure plus iron carbides. Quenching results in rapid cooling and as a result, austenite gets transformed to martensite (␣) with some amount of retained austenite present in the matrix. There are several methods to determine the austenite content such as quantitative microscopy, X ray diffraction technique, etc. The former one does not give accurate results and hence the later one viz., the X-ray diffraction method is preferred as it gives reasonably accurate results. The austenite contents were measured by the XRD technique on the integral diaphragm steel samples subjected to various realization processes using chromium as target operating at 30 kV, 10 mA with 1◦ divergence slit and receiving slits respectively. The diffraction patterns of ␣ and ␥ phases were recorded in a data acquisition system and the austenite content was determined using the Table 1 Chemical composition of diaphragm material in wt.%. C
Si
Mn
Cr
Ni
Mo
S
P
0.08
0.95
1.10
16.2
4.10
1.20
0.029
0.041
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G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
Table 2 Data on the existing realization process practiced. Sl. no.
Material identification
Range in stress (MPa)
Range in austenite (%)
Range in hardness (HRC)
1. 2. 3.
Raw material (RM) in the form of rods Machined material (MM) without intermediate stress relieving Machined, hot and cold stabilized material (MHCM)
− 2 to + 26 −264 to −268 −316 to −333
10–12 10–11 8–9
33–34 33–34 34–35
formula: 100 %␥ = 1 + 0.3A␣ /A␥ where A␣ is the intensity corresponding to area under the curve of ␣ phase and A␥ is the intensity corresponding to the area under the curve of ␥ phase respectively. A dimensional gauge has been used to measure the dimensional stability in terms of flatness of the integral diaphragm material. 3. Results and discussion The chemical composition of the diaphragm material analyzed is shown in Table 1. Table 2 gives the residual stress, retained austenite (RA) content and hardness value pertaining to various stages of currently practiced realization process. The dimensional stability data measured are given in Table 3. Table 4 displays the residual stress, RA content (%) and hardness at every stage of the new process evolved. Figs. 5–7 show the microstructures of the diaphragm material pertaining to raw material (RM), machined material (MM) and machined, hot and cold stabilized material (MHCM) in respect of the present process. Figs. 8–10 are the light microscopic features of first stage machining (FSM), second stage machining (SSM), machined material after cold and hot stabilization (MMCH) and finally cold and hot stabilized material with thermal cycling (CHTM) stages of the new process evolved.
Now considering the existing process, the results projected in Tables 2 and 3 are discussed. Initially the APX raw material (RM) is in stress free state as seen from Table 2. The microstructure corresponding to this is acicular coarse martensite (Fig. 5) structure of lamellar type with the RA in the range 10–12%. The hardness measured is in the range 33–34 HRC. The moment RM undergoes machining process without stress relieving operation (MM); the nature of the stress becomes more compressive in the range −264 to −268 MPa. But there is hardly any difference in the hardness value, and there is a slight decrease in the RA content. The reason for inducing higher compressive stress is due to the machining as it allows the sub-surface lattice to shrink. The microstructural features noticed for this condition is coarse martensitic structure as shown in Fig. 6. Subsequent to the machining process, the APX material undergoes hot and cold stabilization treatment (Table 2). In the cold stabilization process the material is dipped in LN2 till the boiling stops. It is known that martensitic steels invariably have higher level of RA and to control the RA % level, sub-zero treatment in LN2 has been reported [9] to yield better properties. The influence of this treatment has been assessed based on the residual stress, RA, hardness measurements and the same is displayed in Table 2. The stress level now shows (−316 to −333 MPa) much higher than the one seen during the machine process. The stress level of about
Table 3 Results of dimensional stability measurements. sl. no.
01 02 03 04 05 06 07 08 09 10
Flatness 1.65 ± 0.01 mm for 0–300 bar
Remarks
After machining (A)
After hot and cold stabilization (B)
Variation ub fkatbess (B − A) in microns
4 3 21 7 17 28 15 25 26 2
11 15 18 2 21 42 22 7 24 26
+7 +12 −3 −5 +4 +14 +7 −18 −2 +24
Fig. 5. Microstructure of RM diaphragm material (200×).
Fig. 6. Microstructure of MM diaphragm material (200×).
Fig. 7. Microstructure of MHCM diaphragm material (200×).
G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
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Table 4 Data pertaining to modified realization process evolved. Sl. no.
Material identification
Range in stress (MPa)
Range in austenite (%)
Range in hardness (HRC)
1. 2. 3. 4.
First stage material machining (FSM) Second stage material machining after stress relieving (SSM) Machined material after cold and hot stabilization (MMCH) Cold and hot stabilized material with thermal cycling (CHTM)
−240 to −250 −150 to −160 −170 to −190 −140 to −150
10–11 10–11 6–7 5–6
34–35 34–35 35–36 36–37
−150 MPa is considered acceptable since at this level, the hysterias and non-linearity all bear minimum. The RA % has come down from 10–11 to 8–9, but it is only marginally different. The hardness has increased from 33–34 to 34–35 HRC. The microstructure shows martensitic structure but it is still not that fine. At the end of the process, one looks for better stability of the transducer but the dimensional stability results (Table 3) indicate that these seems to be variation in the flatness in the range −18 to +24 micrometer. Further the residual stress and RA values are also are on the higher side. The microstructure also reveals martensite with coarse needles and does not show the indication on tempered martensite. Due to these reasons, the integral diaphragm material has experienced instability of the system resulting in zero shifts. Thus the process on the whole does not seem to be acceptable from the point of structural integrity of the diaphragm system. Hence, a new and modified process has been evolved as detailed in Fig. 3. The data pertaining to this are discussed below. The FSM of the new process of fabrication has induced compressive stress level in the range −240 to −250 MPa, the RA and hardness value measured shows 10–11% and 34–35 HRC
respectively (Table 4). The corresponding microstructure exhibits martensitic structure. Subsequent to this process, stress reliving treatment followed by second stage machining has been given to the diaphragm material. The results indicate that the stress level has decreased in the range −150 to −160 MPa, but both RA and hardness remain unchanged. Due to the stress relieving treatment the stress would have come down to bear minimum, but because of the machining process, it would have introduced the stress which has resulted in the range −150 to −160. Although the induced stresses are in the acceptable range, the RA is on the higher side and the microstructure shows tempered martensite. In order to bring down the RA level, cold and hot stabilization treatment of the material was undertaken followed by machining. This results in slight increase in the stress value in the range −170 to −190. But the RA content has come down from 10–11% to 6–7% and hardness has shown improvement by one HRC. As expected the RA has come down by 60% in view of the fact that the material has been subjected to cold and hot stabilization process resulting in conversion of austanite to martensite was cryo-treated in LN2 for conversion of austenite to martensite. Hence the hardness also shows an increase in trend from 34–35 to 36–37. The microstructure pertaining this is fine tempered martensite. In the present work also the same concept has been tried resulting in substantial reduction in the RA level. In order to further reduce the level of residual stress to an acceptable level, the material was subjected to thermal cycling in the range of −70 to 150 ◦ C. This process revealed that the residual stress decreased from −170 to −190 MPa to −140 to −150 MPa, the RA has decreased to 5–6% and the hardness increased to 36–37 HRC, respectively, due to thermal cycling which can otherwise be called as tempering treatment. The material subjected to the modified realization process was evaluated for the dimensional stability measurements, which shows in the range ±5 m, a value considered promising for the transducer to perform better. A substantial reduction in the RA in the range 3–4% from the present level of 5–6% may ideal for a long term stability of the pressure transducer with minimal drift and thus contributing to better performance. This needs to be addressed in future.
Fig. 8. Microstructure of FSM diaphragm material (200×).
Fig. 9. Microstructure of SSM diaphragm material (200×).
Fig. 10. Microstructure of CHTM diaphragm material (200×).
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In summary the work reports that the modified realization process followed during fabrication of the integral diaphragm material is expectedly shown better performance in terms of acceptance level of residual stress, lower level of RA and better dimensional stability when compared to the process that is being currently followed. 4. Conclusions The following inferences are drawn from the above investigation:
Acknowledgements The authors wish to acknowledge the management of LPSC, ISRO and CPRI for according permission to publish this paper. The authors also acknowledge the relentless technical support rendered by Mr. B.S. Natarajan, Manager - TPAT, Dr. M.M. Nayak and the staff of special process facility. The whole hearted support given by Mr. D. Mollaiah of IISc, Mr. R.K. Kumar, M. Janadhana and Mr. J. Shankar of CPRI in connection with mechanical and metallurgical studies is gratefully acknowledged. References
• The level of residual stress and retained austenite in the diaphragm material influences the performance and long term stability of the transducer. • The realization processes that is being followed shows higher compressive stresses as well as retained austenite content, which results in zero shift of the integral diaphragm transducer system. • The instability of the diaphragm system is attributed to higher dimensional variation and also coarse martensitic structure observed. • The study on the new realization process adopted reveals acceptable level of compressive stress (∼150 MPa) and RA level in the range 5–6%. • The dimensional measurements of the new process show tempered martensitic structure with minimum dimensional variation. • Control of austenite content below 3% would be ideally suited for promoting better performance and stability of the diaphragm transducer system. This forms the future work programme.
[1] P. Ramesh Narayanan, A.K. Jah, V. Diwakar, T. Lakshmanan, Proceedings of the National Work Shop on Residual Stress Measurements: Techniques and Experiences, VSSC, Thiruvanathapuram, 2001. [2] ASM Metals Hand Book, Metallography and Microstructures—Wrought Stainless Steel, vol. 9, ASM International, Materials Park, OH, 1995. [3] S.H. Avner, Introduction to Physical Metallurgy, New York City Community College, USA, 1997. [4] D.A. Porter, K.E. Esterling, Phase Transformation of Metal and Alloys, Department of Engineering Materials, International Student Edition, University of Lulea, USA, 2000. [5] ASM Metals Hand Book, Heat Treating, vol. 4, 9th ed., ASM International, Materials Park, OH, 1995. [6] ASM Metals Hand Book, Properties and Selection: Stainless Steels tool Materials and Special Purpose Metals Corrosion Resistant Materials, vol. 3, 9th ed., ASM International, Materials Park, OH, 1980. [7] P.J. Withers, H.K.D.H. BhadeshiaJah, Material Science and Technology 17 (April) (2001). [8] B.D. Cullity, Elements of X ray diffraction, Addison Wesley Publishing Co. Inc., 1978. [9] S. Zhirafar, A. Rezaeian, M. Pugh, Effect of cryogenic treatment on the mechanical properties of 4340 steel, Journal of Materials Processing Technology 186 (1–3) (2007) 203–298.
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of residual stresses and metallographic stability on the over all performance of integral diaphragm material Geetha Sen a , P. Sampathkumaran b,∗ , D.S. Nadig c , R. Manjunatha c , S. Seetharamu b a b c
Liquid Propulsion System Centre, ISRO, Bangalore 560 008, India Materials Technology Division, Central Power Research Institute, Bangalore 560 080, Karnadaka, India Centre for Cryogenic Technology, Indian Institute of Science, Bangalore 560 012, India
a r t i c l e
i n f o
Article history: Received 11 July 2008 Received in revised form 13 March 2009 Accepted 8 April 2009 Keywords: Residual stress X-ray diffraction Realization process Microstructure Retained austanite
a b s t r a c t The integral diaphragm pressure transducer consists of a diaphragm machined from precipitation hardened martensitic (APX4) steel. Its performance is quite significant as it depends upon various factors such as mechanical properties including induced residual stress levels, metallurgical and physical parameters due to different stages of processing involved. Hence, the measurement and analysis of residual stress becomes very important from the point of in-service assessment of a component. In the present work, the stress measurements have been done using the X-ray diffraction (XRD) technique, which is a non-destructive test (NDT). This method is more reliable and widely used compared to the other NDT techniques. The metallurgical aspects have been studied by adopting the conventional metallographic practices including examination of microstructure using light microscope. The dimensional measurements have been carried out using dimensional gauge. The results of the present investigation reveals that the diaphragm material after undergoing series of realization processes has yielded good amount of retained austenite in it. Also, the presence of higher compressive stresses induced in the transducer results in non-linearity, zero shift and dimensional instability. The problem of higher retained austenite content and higher compressive stress have been overcome by adopting a new realization process involving machining and cold and hot stabilization soak which has brought down the retained austenite content to about 5–6% and acceptable level of compressive stress in the range −100 to −150 MPa with fine tempered martensitic phase structure and good dimensional stability. The new realization process seems to be quite effective in terms of controlling retained austenite content, residual stress, metallurgical phase as well as dimensional stability and this may result in minimum zero shift of the diaphragm system. © 2009 Published by Elsevier B.V.
1. Introduction The performance evaluation of launch vehicle materials/ components such as propellant storage tanks, cooling water tanks, etc. is gaining importance keeping in view of their criticality and the low margin of safety provided to such components [1]. Further, an in-depth analysis reveals possible solutions depending upon the damage tolerance capacity of such components/materials. However, knowledge of residual stress levels in an in-service component will help the metallurgist, designers and mechanical engineers to take proper precautions during design and processing stages for improved reliability and better performance in subsequent applications.
∗ Corresponding author. Tel.: +91 080 23600399; fax: +91 080 23600399. E-mail address: [email protected] (P. Sampathkumaran). 0921-5093/$ – see front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.msea.2009.04.066
The role of pressure transducer sensor used in space application is considered important from the point of sensing the propellant pressure. It incorporates an integral diaphragm made out of precipitation-hardened stainless steel. The effect of induced residual stresses in the integral diaphragm transducer system on the performance is significant. Integral diaphragm pressure transducer system (Fig. 1) is class of pressure pickup gadget wherein the sensing element is a machined diaphragm. The pressure is applied from one side of the diaphragm and strain induced is measured by the strain gauges mounted on the other face. The strain is subsequently converted into a measurable voltage output using a Wheatstone bridge circuit. This type of transducers is used in space program for propellant gauging system and other areas of precision pressure measurements. As this method suffers from the point of reliability of the data, one has to resort to NDE method. Also the precision and long term performance of the transducer system is critical.
86
G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
relieved slowly over a period of time and this leads to zero shift or drift depending on the location of strain induced. The hysterisis of diaphragm will be high if the residual stress is high. This will lead to higher deviation of the calibrated curve with respect to the best fit curve. 2. Experimental The experimental programme consists of studying the diaphragm material at every stage of the realization process currently followed in respect of residual stress, retained austenite (RA) content, dimensional stability as well as microstructure. Similarly the work programme has been repeated for the new or the modified process evolved followed by interpretation of the data. 2.1. Diaphragm fabrication process in force
Fig. 1. Schematic of integral diaphragm pressure transducer.
This paper addresses the key issues relating the zero shift of the transducer system on account of the realization process currently practiced which results in higher compressive stresses obtained by NDE method as well as metallographic and dimensional instability. The performance of the integral diaphragm material has now been improved by way of controlling the stress level, dimensional variations as well as the metallurgical phases due to the adoption of the new realization process. 1.1. Effect of residual stress on the transducer performance The presence of high level of residual stress results in the zero errors in the transducers. Zero errors are generally termed as zero shift, zero drift, non-linearity plus hysterisis. Zero shift error is the error due to change in calibrated zero value. Zero drift is change in calibrated zero value in a random manner over the entire span of measurement. Non-linearity plus hysterisis is the deviation with respect to the calibrated value from the best fit curve. If the level of residual stress in diaphragm is high, the effect on the transducer performance is that the accumulated stress will get
The integral diaphragm material is made up of APX4, a precipitation hardened steel produced from the raw material available in the form of a rod. This is further rolled to the desired dimensions. The current practice of realization process involving the various stages of fabrication process is shown in Fig. 2. These precipitationhardened steels are generally solution annealed and free from hardening agent [2]. On cooling, a martensitic phase is formed due to the process of diffusion-less phase transformation by extreme limited movement of elemental and carbon atoms into positions nearly approaching equilibrium state at lower temperature [3,4]. A tempering treatment results in strengthening the phase due to the precipitation of secondary phases in the alloy system [5,6]. The present realization process practiced has resulted in zero shifts there by the performance of the transducer system was affected. Hence, a new process shown in Fig. 3 had to be developed in order to improve the performance of the diaphragm. In this process, a stage-machining concept has been introduced by adopting intermittent stress relieving treatments to control the level of stresses. This is followed by cold and hot stabilization processes, which is a modified treatment compared to the existing process being followed as well as the one reported in the literature. By way of this process, a stable phase and surface free of microcracks by limiting the stabilization temperature to −80 ◦ C during the cold stabilization process using a cold chamber has been achieved. 2.2. Methods of measurement There are several destructive and non-destructive techniques presently available for the residual stress measurements [7]. The destructive methods cannot be applied on finished components.
Fig. 2. Realization process currently practiced.
G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
87
Fig. 3. Modified realization process.
Fig. 4. Schematic of X-ray diffraction phenomenon.
Therefore the non-destructive techniques (NDT) are preferred for stress measurements. Some of them include (i) ultrasonic, (ii) Xray diffraction (XRD), (iii) Acoustic Barkhausen Noise (ABN), (iv) Magnetic Barkhausen Noise (MBN) and (v) neutron diffraction. The X-ray diffraction (XRD) technique involving Bragg’s law is used for residual stress measurements [8]. In the X-ray diffraction method, the X-rays are applied on to a material and during the process some of them get absorbed by atoms, while the others are sent back in all the directions from the irradiated area known as scattering of X-rays (Fig. 4). The scattered X-rays of a chromium target from the irradiated atoms/crystals are reinforced in a specific direction of an extremely narrow range and propagated. The phenomenon is called as diffraction. The condition of diffraction is defined by the equation called Bragg’s law as n = 2d sin , where d is the inter-atomic spacing, is the Bragg’s angle and is the wave length of characteristics of X-ray radiation. The Bragg angle 2 determined by way of arrangement of atoms in a material is the angle between the incident and diffracted ray as shown in Fig. 4. When a material is subjected to any type of fabrication process, elastic strains between crystallographic planes (i.e., d-spacing) are produced in the individual crystal lattice through the measurement of inter-atomic spacing and hence it is proportional to the stress indirectly. When the material is in tension, the dspacing increases and, when it is under compression the d-spacing decreases. A series of measurements made at different X-ray beam approach angles ( ) are used to fully characterize the d-spacing. The slope of the least squares fit on the graph of the 2 versus sin2 is used to calculate the stress using the following equation: = K
K=
∂2 ∂ sin2
−1 2[E/(1 + )] cot (/180)
where K is the stress constant factor, E is the Young’s modulus of elasticity, is the Bragg’s angle and is the poission’s ratio of the
angle is the angle material, is the stress factor in MPa and between plane and surface normal. The value of slope ∂2/∂ sin2 has been obtained in the present experiments by the method of least squares. The magnitude of the small incremental change in the interplanar distance is a measure of residual stress and the direction of the shift gives the nature of the stress (tensile/compressive) prevailed. Generally, the tensile stress of higher level is detrimental leading to catastrophic failure, on the other hand compressive stress of medium range increases the fatigue strength and there by the service life gets enhanced. Austenite (␥) is a solid solution of iron having a face centered cubic (FCC) structure, consisting of carbon, possibly nickel, chromium or manganese. Steel with low carbon content produced by melting and casting upon cooling yields austenite. Below 723 ◦ C, it generally gets decomposed into ferrite, a solid solution of carbon in iron having body centered cubic (BCC) structure plus iron carbides. Quenching results in rapid cooling and as a result, austenite gets transformed to martensite (␣) with some amount of retained austenite present in the matrix. There are several methods to determine the austenite content such as quantitative microscopy, X ray diffraction technique, etc. The former one does not give accurate results and hence the later one viz., the X-ray diffraction method is preferred as it gives reasonably accurate results. The austenite contents were measured by the XRD technique on the integral diaphragm steel samples subjected to various realization processes using chromium as target operating at 30 kV, 10 mA with 1◦ divergence slit and receiving slits respectively. The diffraction patterns of ␣ and ␥ phases were recorded in a data acquisition system and the austenite content was determined using the Table 1 Chemical composition of diaphragm material in wt.%. C
Si
Mn
Cr
Ni
Mo
S
P
0.08
0.95
1.10
16.2
4.10
1.20
0.029
0.041
88
G. Sen et al. / Materials Science and Engineering A 517 (2009) 85–90
Table 2 Data on the existing realization process practiced. Sl. no.
Material identification
Range in stress (MPa)
Range in austenite (%)
Range in hardness (HRC)
1. 2. 3.
Raw material (RM) in the form of rods Machined material (MM) without intermediate stress relieving Machined, hot and cold stabilized material (MHCM)
− 2 to + 26 −264 to −268 −316 to −333
10–12 10–11 8–9
33–34 33–34 34–35
formula: 100 %␥ = 1 + 0.3A␣ /A␥ where A␣ is the intensity corresponding to area under the curve of ␣ phase and A␥ is the intensity corresponding to the area under the curve of ␥ phase respectively. A dimensional gauge has been used to measure the dimensional stability in terms of flatness of the integral diaphragm material. 3. Results and discussion The chemical composition of the diaphragm material analyzed is shown in Table 1. Table 2 gives the residual stress, retained austenite (RA) content and hardness value pertaining to various stages of currently practiced realization process. The dimensional stability data measured are given in Table 3. Table 4 displays the residual stress, RA content (%) and hardness at every stage of the new process evolved. Figs. 5–7 show the microstructures of the diaphragm material pertaining to raw material (RM), machined material (MM) and machined, hot and cold stabilized material (MHCM) in respect of the present process. Figs. 8–10 are the light microscopic features of first stage machining (FSM), second stage machining (SSM), machined material after cold and hot stabilization (MMCH) and finally cold and hot stabilized material with thermal cycling (CHTM) stages of the new process evolved.
Now considering the existing process, the results projected in Tables 2 and 3 are discussed. Initially the APX raw material (RM) is in stress free state as seen from Table 2. The microstructure corresponding to this is acicular coarse martensite (Fig. 5) structure of lamellar type with the RA in the range 10–12%. The hardness measured is in the range 33–34 HRC. The moment RM undergoes machining process without stress relieving operation (MM); the nature of the stress becomes more compressive in the range −264 to −268 MPa. But there is hardly any difference in the hardness value, and there is a slight decrease in the RA content. The reason for inducing higher compressive stress is due to the machining as it allows the sub-surface lattice to shrink. The microstructural features noticed for this condition is coarse martensitic structure as shown in Fig. 6. Subsequent to the machining process, the APX material undergoes hot and cold stabilization treatment (Table 2). In the cold stabilization process the material is dipped in LN2 till the boiling stops. It is known that martensitic steels invariably have higher level of RA and to control the RA % level, sub-zero treatment in LN2 has been reported [9] to yield better properties. The influence of this treatment has been assessed based on the residual stress, RA, hardness measurements and the same is displayed in Table 2. The stress level now shows (−316 to −333 MPa) much higher than the one seen during the machine process. The stress level of about
Table 3 Results of dimensional stability measurements. sl. no.
01 02 03 04 05 06 07 08 09 10
Flatness 1.65 ± 0.01 mm for 0–300 bar
Remarks
After machining (A)
After hot and cold stabilization (B)
Variation ub fkatbess (B − A) in microns
4 3 21 7 17 28 15 25 26 2
11 15 18 2 21 42 22 7 24 26
+7 +12 −3 −5 +4 +14 +7 −18 −2 +24
Fig. 5. Microstructure of RM diaphragm material (200×).
Fig. 6. Microstructure of MM diaphragm material (200×).
Fig. 7. Microstructure of MHCM diaphragm material (200×).
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Table 4 Data pertaining to modified realization process evolved. Sl. no.
Material identification
Range in stress (MPa)
Range in austenite (%)
Range in hardness (HRC)
1. 2. 3. 4.
First stage material machining (FSM) Second stage material machining after stress relieving (SSM) Machined material after cold and hot stabilization (MMCH) Cold and hot stabilized material with thermal cycling (CHTM)
−240 to −250 −150 to −160 −170 to −190 −140 to −150
10–11 10–11 6–7 5–6
34–35 34–35 35–36 36–37
−150 MPa is considered acceptable since at this level, the hysterias and non-linearity all bear minimum. The RA % has come down from 10–11 to 8–9, but it is only marginally different. The hardness has increased from 33–34 to 34–35 HRC. The microstructure shows martensitic structure but it is still not that fine. At the end of the process, one looks for better stability of the transducer but the dimensional stability results (Table 3) indicate that these seems to be variation in the flatness in the range −18 to +24 micrometer. Further the residual stress and RA values are also are on the higher side. The microstructure also reveals martensite with coarse needles and does not show the indication on tempered martensite. Due to these reasons, the integral diaphragm material has experienced instability of the system resulting in zero shifts. Thus the process on the whole does not seem to be acceptable from the point of structural integrity of the diaphragm system. Hence, a new and modified process has been evolved as detailed in Fig. 3. The data pertaining to this are discussed below. The FSM of the new process of fabrication has induced compressive stress level in the range −240 to −250 MPa, the RA and hardness value measured shows 10–11% and 34–35 HRC
respectively (Table 4). The corresponding microstructure exhibits martensitic structure. Subsequent to this process, stress reliving treatment followed by second stage machining has been given to the diaphragm material. The results indicate that the stress level has decreased in the range −150 to −160 MPa, but both RA and hardness remain unchanged. Due to the stress relieving treatment the stress would have come down to bear minimum, but because of the machining process, it would have introduced the stress which has resulted in the range −150 to −160. Although the induced stresses are in the acceptable range, the RA is on the higher side and the microstructure shows tempered martensite. In order to bring down the RA level, cold and hot stabilization treatment of the material was undertaken followed by machining. This results in slight increase in the stress value in the range −170 to −190. But the RA content has come down from 10–11% to 6–7% and hardness has shown improvement by one HRC. As expected the RA has come down by 60% in view of the fact that the material has been subjected to cold and hot stabilization process resulting in conversion of austanite to martensite was cryo-treated in LN2 for conversion of austenite to martensite. Hence the hardness also shows an increase in trend from 34–35 to 36–37. The microstructure pertaining this is fine tempered martensite. In the present work also the same concept has been tried resulting in substantial reduction in the RA level. In order to further reduce the level of residual stress to an acceptable level, the material was subjected to thermal cycling in the range of −70 to 150 ◦ C. This process revealed that the residual stress decreased from −170 to −190 MPa to −140 to −150 MPa, the RA has decreased to 5–6% and the hardness increased to 36–37 HRC, respectively, due to thermal cycling which can otherwise be called as tempering treatment. The material subjected to the modified realization process was evaluated for the dimensional stability measurements, which shows in the range ±5 m, a value considered promising for the transducer to perform better. A substantial reduction in the RA in the range 3–4% from the present level of 5–6% may ideal for a long term stability of the pressure transducer with minimal drift and thus contributing to better performance. This needs to be addressed in future.
Fig. 8. Microstructure of FSM diaphragm material (200×).
Fig. 9. Microstructure of SSM diaphragm material (200×).
Fig. 10. Microstructure of CHTM diaphragm material (200×).
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In summary the work reports that the modified realization process followed during fabrication of the integral diaphragm material is expectedly shown better performance in terms of acceptance level of residual stress, lower level of RA and better dimensional stability when compared to the process that is being currently followed. 4. Conclusions The following inferences are drawn from the above investigation:
Acknowledgements The authors wish to acknowledge the management of LPSC, ISRO and CPRI for according permission to publish this paper. The authors also acknowledge the relentless technical support rendered by Mr. B.S. Natarajan, Manager - TPAT, Dr. M.M. Nayak and the staff of special process facility. The whole hearted support given by Mr. D. Mollaiah of IISc, Mr. R.K. Kumar, M. Janadhana and Mr. J. Shankar of CPRI in connection with mechanical and metallurgical studies is gratefully acknowledged. References
• The level of residual stress and retained austenite in the diaphragm material influences the performance and long term stability of the transducer. • The realization processes that is being followed shows higher compressive stresses as well as retained austenite content, which results in zero shift of the integral diaphragm transducer system. • The instability of the diaphragm system is attributed to higher dimensional variation and also coarse martensitic structure observed. • The study on the new realization process adopted reveals acceptable level of compressive stress (∼150 MPa) and RA level in the range 5–6%. • The dimensional measurements of the new process show tempered martensitic structure with minimum dimensional variation. • Control of austenite content below 3% would be ideally suited for promoting better performance and stability of the diaphragm transducer system. This forms the future work programme.
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