Optimization of a TRIP Steel for Adiabatic Fragment Protection

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ScienceDirect Materials Today: Proceedings 2S (2015) S639 – S642

International Conference on Martensitic Transformations, ICOMAT-2014

Optimization of a TRIP steel for adiabatic fragment protection N.J. Wengrenovicha,*, G. B. Olsona a

Department of Material Science and Engineering, Northwestern University, 2220 Campus Drive, Evaston, IL 60208-3108, USA

Abstract Current analysis into the property requirements of materials designed for fragment protection has led to the need for high shear localization resistance to prevent failure from plugging. Transformation induced plasticity (TRIP) is known to significantly delay plastic instability in both tension and shear stress states, thus extending the uniform shear deformation which delays the onset of localization. TRIP-180, a novel computationally designed, fully austenitic TRIP steel, is investigated to explore optimized austenite stability, quantified by the Msσ temperature, under adiabatic shear conditions. It is compared to HSLA-100, a low-alloy martensitic steel, where by a significant improvement in performance is seen. © 2014 The Authors. Published by Elsevier Ltd. © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations (http://creativecommons.org/licenses/by-nc-nd/4.0/). 2014. This an open access under the BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection andisPeer-review underarticle responsibility of CC the chairs of the International Conference on Martensitic Transformations 2014. Keywords: TRIP steels; martensitic transformation; high strength steels; adiabatic loading; plugging failure; high strain rate

1. Introduction Blast and fragment protection for military and civilian applications require high strength materials with superior flow stability in both tension and shear. This flow stability in tension gives rise to high uniform ductility for absorbing energy from the main pressure wave from an explosion. In shear, this flow stability extends uniform deformation and leads to the delay of the instability caused by microvoid formation, growth, and coalescence [1] which occurs during fragment impact. Without this delay of the shear instability, high strength steels are seen to prematurely fail during ballistic fragment impact from the plugging phenomenon [2]. Using the principles of predictive materials science and materials design, a novel fully austenitic TRIP steel, TRIP-120, was developed

* Corresponding author. Tel.: +0-312-380-9027 . E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi:10.1016/j.matpr.2015.07.365

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N.J. Wengrenovich and G.B. Olson / Materials Today: Proceedings 2S (2015) S639 – S642

through the doctoral work of Sadhukhan [3] and optimized under tensile loading conditions by the doctoral work of Feinberg [4]. This optimized steel is now coined TRIP-180 to reflect the higher tensile strength. The objective of this research is to garner insight into the mechanical behavior of TRIP-180 under adiabatic shear conditions and compare it to the mechanical behavior of HSLA-100, a conventional martensitic steel. In a step to investigate the performance of TRIP-180 for fragment penetration resistance, hat shaped specimens have been fabricated for testing in a split Hopkinson pressure bar setup as this test has been proven enlightening in studying high strain rate behavior by Peirs et al. [5]. The test setup is designed to obtain strain rates on the order of 104 per second, placing the test in the adiabatic regime. As only one composition of TRIP steel is investigated, emphasis is placed on determining the optimum austenite stability for penetration resistance at the current strength level. The knowledge gained from this study will allow for greater accuracy in future designs by determining the performance level and property requirements needed for blast and fragment resistance. 2. Experimental approach 2.1. Methods The split Hopkinson pressure bar experimental technique is used to achieve high strain rate loading of specimens. For this experiment, an axisymmetric hat shaped specimen with dimensions seen in Fig. 1 is placed between two Hopkinson bars. A striker bar, launched by compressed gas, impacts the input bar generating an incident compressive wave that travels down the bar and into the specimen. The compressive wave interacts with the specimen and is partly transmitted into the output bar and is partly reflected back into the input bar. Strain gauges located on the input and output bars record the incident, transmitted, and reflected waves which allow the calculation of stress and strain using one-dimensional elastic-wave propagation theory [6]. The setup in this analysis uses steel bars (E ≈ 212 GPa) with a 25 mm diameter. Seven hat shaped specimens were fabricated to the dimensions shown in Fig. 1, six out of TRIP-180 and one out of HSLA-100. All specimens were tested to failure at room temperature with strain rates on average of 22,000 per second. After failure, a small piece of the gauge section, as outlined in red in Fig. 1, was machined out for martensite fraction calculations by magnetic analysis for the TRIP-180 specimens. The specimens were suspended in a nonmagnetic gel and placed SQUID magnetometer. Their weights varied from 1 mg to 3 mg to prevent saturating the magnetometer. The specimens were cycled through -40,000 to 40,000 Gauss while recording the magnetization. The saturation magnetization was then calculated for each specimen by extrapolating the magnetization to infinite magnetic field. From this the fraction of each phase was calculated by knowledge of the specific saturation magnetization of the austenite and martensite phases. The specific saturation magnetization of the phases was calculated using the mart5 database in the ThermoCalc software package.

Fig. 1. Geometry of the hat type specimens with the gauge section enlarged (mm).

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N.J. Wengrenovich and G.B. Olson / Materials Today: Proceedings 2S (2015) S639 – S642

2.2. Materials The primary material used in this study was TRIP-180, a fully austenitic TRIP steel designed to obtain high strength and uniform ductility. The composition and processing was designed through the doctoral work of Sadhukhan [3] as TRIP-120, and its processing was optimized through the addition of warm-working to eliminate a grain boundary cellular reaction and extend fracture ductility through the doctoral work of Feinberg [4] as seen in the work of Latourte et al. [7]. TRIP-180 achieves a strength level of 1241 MPa (180 ksi) by the generation of dislocations through a warm-working process and nanoscale precipitation of the metastable Ni3(Al,Ti) γ’ phase. The coherent γ’ phase inhibits dislocation shearing by forming anti-phase boundaries with the austenite phase [8]. In this study, tempering time is controlled from 15 minutes to 6.25 hours at 700°C in Ar to vary austenite stability. HSLA-100 provides a reference material to show improvement in performance. HSLA-100, currently used in manufacturing Navy warships hulls, is a martensitic steel treated to an overaged, 690 MPa (100 ksi) strength level. 3. Results and discussion Table 1 shows a summary of the results of the dynamic test. During the final test of TRIP-180 (Sample 1), a strain gauge delaminated upon impact of the striker bar and no data was collected during the test. The Msσ(sh) temperatures for each of the TRIP-180 specimens were calculated using a series of calibrated parametric models [4] with outputs from the PrecipiCalc and ThermoCalc software packages as inputs for the Olson-Cohen model [9]. The performance product is the product of the strength and plastic strain as is taken as a first order measure of penetration resistance. The rate parameter is the quotient of martensite fraction and plastic strain. Most quantities in Table 1, with the exception of shear strength and performance product, correlate with either increasing or decreasing austenite stability. The amount of transformed martensite and the rate parameter generally Table 1. Summary of results for the seven hat shaped samples. Tempering Time at 700°C (hr)

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

Sample 6

HSLA-100

6.25

5.25

4.67

2.5

0.83

0.25

N/A

T - Msσ(sh) (°C)

-8

-1

21

39

58

128

N/A

Shear Strength (MPa)

--

1080

1150

1290

1420

1080

970

Plastic Shear Strain (mm/mm)

--

0.0308

0.0673

0.106

0.144

0.161

0.0903

Performance Product (MPa)

--

33.2

77.2

138

205

173

87.7

Transformed Martensite (%)

6.0

4.1

5.4

3.6

2.5

1.6

N/A

Rate Parameter

--

1.33

0.803

0.337

0.172

0.101

N/A

Fig. 2. (a) Martensite fraction and rate parameter versus T - Msσ(sh) temperature; (b) Performance product of TRIP-180 versus T - Msσ(sh) temperature as compared to HSLA-100 (solid gray line).

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N.J. Wengrenovich and G.B. Olson / Materials Today: Proceedings 2S (2015) S639 – S642

decrease with increasing austenite stability. In TRIP-180, tempering leads to rapid growth and coarsening of γ’ precipitates [4] in the supersaturated austenite matrix. As the Ni3(Al,Ti) γ’ particles continue to coarsen in the austenite matrix, Ni is reduced in the austenite matrix which decreases the austenite stability (decreasing T - Msσ(sh)) and leads to more transformed martensite per unit strain as indicated by the rate parameter in Fig. 2(a). On the other hand, the amount of plastic strain increases with increasing austenite stability due to lower strength levels from less tempering. This fits with what is expected from the classical trend of sacrificing strength for ductility. With increased tempering time, the γ’ precipitates coarsen, which leads to higher strength [4]. When evaluating the performance product, there is a peak at T - Msσ(sh) temperature of 58°C as seen in Fig. 2(b) where TRIP-180 is compared to the performance of HSLA-100. At this stability there is enough transformation plasticity to extend the region of uniform plastic deformation at an adequate strength level. This optimum level of stability aligns well with what Young has found for a TRIP steel in isothermal tension [10] where fracture ductility peaks at approximately 60°C above Msσ. This shift in optimum austenite stability above Msσ could be the necessary shift into the strain-induced transformation regime where the stress required for transformation is approximately equal to or just below the stress required for necking. This could delay the onset of necking more than at Msσ where the stress required for transformation equals the yield stress. It should also be noted that it appears to be a broad optimum which is beneficial for industrial applications. TRIP-180 at its optimal austenite stability shows a 1.5 times higher strength, a 1.6 times greater amount of plastic strain, and a 2.3 higher performance product level. 4. Conclusions The performance of a novel, fully austenitic TRIP steel designed for blast resistance, TRIP-180, was determined under adiabatic shear conditions on a split Hopkinson pressure bar at strain rates on the order of 104 per second and was compared to the performance of HSLA-100. There was a broad optimum of the austenite stability seen at 60°C above the Msσ temperature, which proves similar to the behavior as seen under isothermal tensile conditions as determined by Young [10]. At the optimum austenite stability, TRIP-180 exhibits a higher shear stress before instability, more plastic strain, and thus a higher performance product than HSLA-100 by factors of 1.5, 1.6, and 2.3, respectively. The behavior of TRIP-180 under these conditions indicates there is sufficient transformation plasticity to alter and delay the onset of shear localization behavior even with small quantities of austenite transforming (1.4% - 6%) due to adiabatic heating of the steel. This quantification of adiabatic shear localization resistance and austenite stability define parameters for the future design of adiabatic TRIP steels for fragment protection. Acknowledgements This research was carried out under financial support from the Office of Naval Research under awards number N00014-12-1-0455. The authors would like to acknowledge the help of Joe Schaefer and Prof. Isaac Daniel for their guidance on the split Hopkinson pressure bar tests and Dr. Oleksandr Chernyashevskyy for his work with the SQUID magnetometer for the martensite fraction measurements. The authors would also like to thank Dr. Xian Jie Zhang at the Naval Surface Warfare Center Carderock Division for providing the HSLA-100 steel. References [1] J. Cowie, M. Azrin, G. Olson, Metallurgical Transactions A, 20 (1989) 143-153. [2] J. Mescall, H. Rogers, Sagamore Army Materials Research Conference (1989) 287-313. [3] P. Sadhukhan, Computational Design and Analysis of High Strength Austenitic TRIP Steels for Blast Protection Applications, PhD thesis, Northwestern University, 2008. [4] Z. Feinberg, Design and Optimization of an Austenitic TRIP Steel for Blast and Fragment Protection, PhD thesis, Northwestern University (2012). [5] J. Peirs, P. Verleysen, J. Degrieck, F. Coghe, Int. J. Impact Eng. 37 (2010) 703-714. [6] H. Kolsky, P. Phys. Soc. Lond. B 62 (1949) 676-700. [7] F. Latourte, X. Wei, Z. Feinberg, A. de Vaucorbeil, P. Tran, G. Olson, H. Espinosa, Int. J. Solids Struct. 49 (2012) 1573-1587. [8] S. Asgari, J. Mater. Process. Technol. 118 (2001) 246–250. [9] G. Olson, M. Cohen, Metall. Trans. A 7 (1976) 1897-1904. [10] C. Young, Transformation Toughening in PhosphoCarbide Strengthened Austenitic Steels, PhD thesis, MIT, 1988.