Development of Maraging Steel with Retained Austenite in Martensite Matrix

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Materials Today: Proceedings 2S ( 2015 ) S711 – S714

International Conference on Martensitic Transformations, ICOMAT-2014 Development of maraging steel with retained austenite in martensite matrix

M.K. El-Fawkhry*, M. Eissa, A. Fathy, T. Mattar

Central Metallurgical R&D Institute (CMRDI), P. O. Box 87 Helwan, Cairo, Egypt

Abstract

This research considers as a trial for solving the debate on the role of retained austenite in improving the mechanical properties and design’s requirements of maraging steel. Four compositions of Maraging steel have been selected in this study, and the role of retained austenite in improving the mechanical properties of maraging steel was evaluated. The results showed a significant effect of retained austenite fraction on the mechanical properties of the developed steel in particular ductility, tensile strength, and strain hardening property. Therefore, the retained austenite content has a positive effect in the term of material designing. © 2014 The Authors. Published by Elsevier Ltd. ©Selection 2015 The and Authors. Peer-review Published under by responsibilityElsevier Ltd. of the chairs of the International Conference on Martensitic Transformations Selection2014. This and is Peer-reviewan open access under article responsibility under the of CC the BYchairs-NC of- NDthe Internationallicense (http://creativecommons.org/licenses/by Conference on Martensitic Transformations-nc-nd/3.0/ 2014.).

Keywords: Maraging steel; Retained austenite; TRIP effect; Strain-hardening property; strengthening precipitations

1. Introduction

Maraging steels are considered as the most applicable alloy in the high strength design applications. Its high strength is commonly accompanied with reasonable ductility and toughness. It was well established that the odd mechanical properties of maraging steel are entirely dependent on its microstructure components. High nickel content is the main responsible about forming of lath martensite after solution annealing. However, this steel is delivered as annealed, but the users must apply an ageing regime in order to obtain the optimum mechanical properties. Ageing process is required for precipitation of super strength intermetallic compounds like Ni3Mo, Ni3Ti

* Corresponding author. Tel.: +201152529685; fax: +225010643. E-mail address: [email protected]

2214-7853 © 2015 The Authors. Published by Elsevier Ltd. Selection and Peer-review under responsibility of the chairs of the International Conference on Martensitic Transformations 2014. doi: 10.1016/j.matpr.2015.07.381 712 M.K. El-Fawkhry et al. / Materials Today: Proceedings 2S ( 2015 ) S711 – S714

and laves. These precipitates have a lower misfit factor with the martensite matrix. Accordingly, they act as coherent strengthening particles that multiply the ultimate and yield strength as a result of interaction with dislocations [1]. Thereby, the final structure of maraging steel is Lath martensite, intermetallic compounds, laves, and retained austenite or reverted austenite. Retained austenite may be existed in the matrix through solution annealing, or prolonged ageing of high nickel alloys, called reverted austenite [2]. As growing the comprehension of the TRIP effect of alloy containing retained austenite, there is a great debate on the possibility of improving the mechanical properties of the maraging steel containing retained austenite by TRIP effect. The small ultimate to yield ratio considers as the main challenge faced the material designer of maraging steel [3]. Many speculations have been proposed on the role of TRIP effect on enhancing this ratio. On the other hand, it was well established that the Nickel content over 11% promote the formation of reverted austenite in the ageing regime [4]. However, there is not clear data on the retained austenite content in the low nickel maraging steel. Then, the gist of this research is tracking the retained austenite, and its effect on the mechanical properties of low Ni, Ti-containing maraging steel.

2. Experimental method

Four heats were conducted in open air induction furnace; samples were collected through melting regime and analyzed by Emission Spectrophotometer as shown in Table 1. The melt was poured in metal mold with φ = 100 mm, h = 120 mm by uphill teeming casting technique. The Dilatation was employed to determine the phases start and finishing temperature. Forging was applied at 1200-1000 ºC to produce square cross section rods 15*15 mm2. Samples were collected for solution annealing and ageing treatment at different temperatures. XRD- Cu Kα was applied to detect the phases of the solution annealed samples. Image analyzer software attached to optical microscope was used to detect the volume fraction of the retained austenite of the solution annealed samples with specific etchants. SEM was utilized for observing the formation of retained austenite and the other phases with nano- size. Tensile test was performed to evaluate the effect of retained austenite on the mechanical properties, and strain hardening exponent.

Table 1. The chemical composition of four maraging steel. Grade Chemical Composition, wt.% C Mn Si Cr Ni Mo S P Ti Al M2 0.0413 0.0909 0.3 4.62 12.9 3.19 0.0282 0.0168 0.276 0.0797 M22 0.062 0.0908 0.138 0.004 12.08 3.28 0.0141 0.0278 0.465 0.16 M231 0.0297 0.073 0.035 0.004 11.47 3.11 0.0113 0.0189 0.623 0.102 M241 0.039 0.106 0.299 0.004 11.03 3 0.0124 0.0271 1.36 0.298

3. Results and discussion

It was well proven that the retained austenite tends to be more stabilized as decreasing Ms Temperature. All alloying element except Al and Co tend to stabilize the retained austenite with decreasing the Ms Temperature. Fig. 1 shows that Titanium has not a significant effect on the martensite start and finishing temperature, as in M22, M241, M231 while Cr tends to stabilize austenite through shifting the Ms Temperature. This observation may be agree with previous works studied the effect of Cr on the formation of retained austenite in high Chromium steel [5]. Thermocalc program was used in speculating the change of phases during solidification as a function of chemical composition by TCER database. Fig. 2 depicts that retained austenite is normally formed during solidification in the four alloyed steel with different percentage according to the percentage of Ti, and Cr. On the other hand laves compound are often formed beyond 500oC [6]. XRD observation depicts that Cr might have acted as a stabilizer of the retained austenite as shown in Fig. 3, but Ti has a significant power in forming laves Fe2 (Mo, Ti). The volume fraction of austenite was calculated by comparison method between the austenite and martensite peaks as shown in Table 2. M.K. El-Fawkhry et al. / Materials Today: Proceedings 2S ( 2015 ) S711 – S714 713

It is well known the difficulty of observing the retained austenite in the main bcc matrix by XRD, due to the vast interference between γ (111) peak and α/ (110) peak [7]. Then, it was recommended to use optical observation technique to record the volume fraction of retained austenite in the bcc matrix by using specific etchants. Fry’s reagent was used to darken the austenite matrix. On the other side, solution of Beraha reagent used after etching with Picric acid to enhance the contrast between Dark area (martensite) and bright area (austenite) [8]. It is clear from table 2 that the retained austenite is proportional to the Chromium content. In addition, SEM declares that the retained austenite grows in the grain boundaries of the fine martensite, the hardening precipitates is mainly Ni3 (Mo, Ti) and HCP laves Fe2 (Mo, Ti) as shown in Fig. 4.

Fig. 1. The dilatation of four maraging steels. Fig. 2. Solidification diagram of the four margaing steels.

Fig. 3. XRD of the four Maraging steels. Fig. 4. SEM of Maraging steel after solution treatment.

Table 2. The volume fraction of retained austenite by using different techniques. Steel grade XRD Fry’s reagent Sod. bisulphate M2 10 4.4 6.65 M22 9 5.33 7.76 M231 10 3.5 9.2 M241 5 4.05 4.3 714 M.K. El-Fawkhry et al. / Materials Today: Proceedings 2S ( 2015 ) S711 – S714

Obviously, retained austenite leads to reduce the strength of maraging steel in one hand with enhancing the ductility as observed in as annealed M22, M231, respectively as Fig. 5. On contrary, retained austenite improves the ultimate to yield ratio as observed in M2, M22, and M231, which considers as a crucial turning point in designing purposes of maraging steel. In addition to those observations, it can be referred that the strength of high Ti- containing steel M241performed better in strength curve than high Cr-containing steel; M2, which can be interpreted by the formation of HCP phase (laves) Fe2 (Mo, Ti) is increased with Ti content, as well as the great affinity of Ti in forming nitrides and carbides, which in role raise up the strength of the material [9]. The strain-hardening exponent has been calculated through using Hollomon relationship (S=K ern). Figure 6 depicts that the high strain hardening exponent is being as a function of retained austenite fraction as in M22, M231, and M2 respectively.

Fig. 5. True stress-strain curve of the four as annealed alloys. Fig. 6. Strain hardening exponent of the four as annealed alloys.

4. Conclusions

Ti- containing maraging steel performed its typical strength with reducing the Ni content from 18% to 12%. Cr promotes the retained austenite formation in the maraging steel through reducing the Ms temperature. Almost the strengthening precipitates formed during solution treatment of Ti-containing maraging steel is mainly titanium-containing precipitates Fe2 (Mo, Ti), Ni3 (Mo, Ti). Retained austenite fraction enhances the ultimate strength to yield strength ratio. The strain hardening property is dependent on the retained austenite fraction of the four investigated steel, as well as the intermetallic precipitates.

References

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