A dilatometric study of the phase transformations in 300 and 350 maraging steels during continuous heating rates

A dilatometric study of the phase transformations in 300 and 350 maraging steels during continuous heating rates

Leandro Gomes de CarvalhoI, ^*; Margareth Spangler Andrade^II; Ronald Lesley Plaut^I; Fabrício Mendes Souza^I; Angelo Fernando Padilha^I

^IDepartamento de Engenharia Metalúrgica e de Materiais, Escola Politécnica, Universidade de São Paulo - USP, Av. Professor Mello de Moraes, 2463, CEP 05508-030, São Paulo, SP, Brazil

^IIFundação Centro Tecnológico de Minas Gerais, Setor de Tecnologia Metalúrgica, Av. José Cândido da Silveira, 2000, CEP 31170-000, Belo Horizonte, MG, Brazil

ABSTRACT

The influences of the chemical composition and heating rate have been studied in 300 and 350 maraging steels using dilatometry. For these tests, heating was carried out with heating rates of 1, 10 and 28 °C/s. The results have shown that the precipitation mechanism for both materials in the studied range is by lattice diffusion. Furthermore, Co and Ti contents influence strongly the precipitation. The lattice diffusion mechanism in the martensite reversion is influenced by Ni and Co contents and heating rate. For small heating rates ( ~1 °C/s) this mechanism prevails in the 300 maraging steel while for the 350 maraging steel has a minor importance. The mechanism of martensite reversion for 350 maraging steel in the studied range is mainly by shear mechanism. For higher heating rates (~28 ºC/s) the shear mechanism prevails in both maraging steels.

Keywords: maraging steels, precipitation, martensite reversion, phase transformations

1. Introduction

Maraging steels are low carbon martensitic steels that can be hardened by precipitation of intermetallic phases^1,2. These steels are used in industrial applications that demand high strength steels, such as in aerospace and nuclear technologies^3,4. Commercial maraging steels are iron-nickel-cobalt alloys with molybdenum, titanium and aluminum additions².

These steels are commonly subjected to a two stage thermal treatment aiming to achieve a high strength level: a solution annealing at 820 °C, followed by aging at 480 °C. Martensitic microstructure of maraging steels is characterized by a supersaturated matrix of alloying elements containing high density of crystalline defects, mainly dislocations. This microstructure is formed during quenching from the austenitic field⁵.

On heat treating, precipitate nucleation occurs predominantly on dislocations. Furthermore, high dislocation density accelerates the growing of theses precipitates during aging because solute diffusion by pipe diffusion has a lower activation energy than by lattice diffusion⁶. Studies show that precipitate formation of Ni₃X (X=Ti, Mo) occurs during aging, and the formation of a more stable phase Fe-Mo (Fe₂Mo or Fe₇Mo₆) demands higher exposure times^1,6. Additionally, austenite nucleation for longer aging times can result both in precipitate dissolution Ni₃X (X=Ti, Mo) and in Fe-Mo precipitate formation. This causes nickel enrichment in the matrix, stabilizing the austenite and decreasing the initial temperature of the martensite to austenite transformation or reversion^7-12. It is noteworthy that both diffusion and shear mechanisms can occur simultaneously during this transformation^8,13. Previous studies¹³ have shown that these mechanisms can depend on the heating rates. Dilatometry has been largely used for studies of phase transformations on several types of steels¹⁴, such as low-carbon steels¹⁵ and maraging steels^13,16. In this work the dilatometric technique was used to study the influence of chemical composition on precipitation and the martensite reversion phenomena, as well as to evaluate the activation energies of these phase transformations during the heating stage.

2. Method and experimental procedure

Samples were prepared from three maraging bars with diameters of 98, 139, 140 mm respectively for the 300, 350, 350 maraging steels, supplied in the solution annealed condition. Disks of 10 mm have been cut from these bars. Chemical composition (as per certificate) of specimens of each bar were provided, and reproduced in Table 1.

Table 1. Chemical composition (% wt) of the studied bars.

Samples having 2 mm diameter and 12 mm length were machined in the radial direction from these disks. Heating and cooling cycles were performed in the quenching Adamel-Lhomargy LK 02 dilatometer of the CETEC-MG Research Institution. For these tests, heating was carried out under low vacuum, approximately 10^-1 mbar with heating rates of 1, 10 and 28 °C/s. Length increase (∆L) and temperature (T) changes were measured during heating for each heating rate.

3. Results and discussion

Figure 1 shows a typical dilatometric curve for a complete heating and cooling cycle (rate 1 °C/s), inflections indicating the phase transformation temperatures.

Figure 1.
Dilatometric curve of heating and cooling for bar B, heating rate =1 °C/s.

During the heating and cooling thermal cycling, three phase transformations in the maraging steels may be observed: precipitation, martensite reversion and martensitic transformation. In this work, tests have been performed only for the phase transformations occurring during continuous heating, analyzing precipitation and martensite reversion.

It is well known that for the dilatometric technique that the maximum transformation rate can be used to evaluate the activation energy^17,18. These activation energies were determined through the linear thermal expansion coefficient α_L^17,18. This coefficient is obtained by the slope of dilatometric curve.

Figures 2, 3 and 4 present the variation of the linear expansion coefficient as a function of temperature.

Figure 2.
Linear expansion coefficient between 400 and 900 °C (for bar A) for different heating rates

Figure 3.
Linear expansion coefficient between 400 and 900 °C (for bar B) for different heating rates.

Figure 4.
Linear expansion coefficient between 400 and 900 °C (for bar C) for different heating rates.

According to Habiby et al.¹⁹ and Kapoor et al.²¹, the solute in the matrix during heating leads to precipitation in the maraging steels causing a lattice contraction and changes of specimen macroscopic dimensions. Figures 2, 3 and 4 show the temperatures where the maximum rate of phase transformation for precipitation and martensite reversion occurred. It must be pointed out, in Figure 2, 3 and 4, that for higher heating rates the time available for precipitation decreases leading to a smaller specimen contraction. This is due to the smaller quantity of precipitates formed during heating for higher heating rates²¹.

For samples of bar A (maraging 300) a smaller contraction during precipitation could be observed when compared to samples of bar B and C (maraging 350). This is linked to bar A has a smaller quantity of Ti and Co is in solid solution in the martensite, hence forming a smaller amount of precipitates^5,20.

Kapoor et al.¹³ have shown that the reversion mechanisms occur by shear or by diffusion and that these depend on the heating rates. The present research also showed that this reversion occur in two steps. For slower heating rates, there is a predominance of diffusion in the first step, while in a second step shear predominates¹³. On the other hand, for higher heating rates, reversion occurs as a unique step with predominance of the shear mechanism¹³. Furthermore, Peters has shown that nickel accelerates the formation of austenite, while cobalt slows it down⁹. Again Figures 2, 3 and 4 show that for bar C shear prevails while for bars A and B the two steps may be observed (see arrows for the 2^nd step of martensite reversion). Further, it may be observed that this splitting (into the two steps) occurs for 1 °C/s and 10 °C/s for bar A and at 1 °C/s for bar B. This is linked to higher Ni and smaller Co contents of bar A (300 maraging), increasing the influence of diffusion on the reversion reaction for smaller heating rates, if compared to the bars B and C (350 maraging).

Viswanathan et al.¹⁶ have shown that activation energies of these transformations, which were evaluated through the Kissinger method¹⁷, can be written in a simplified form by:

Where, Φ is the heating rate (°C/s), E is activation energy (kJ/mol), R the gas constant (J/mol.K), T_M is the temperature (K) for which there is a maximum phase transformation rate and C is an arbitrary constant. This equation is applicable only for a maximum transformation rate (measured by the maximum sample contraction rate)¹⁷.

Figure 5 shows the linear regression obtained for the evaluation of the activation energies using the Kissinger method and Table 2 presents the activation energies (E) obtained through the slope of this linear regression. In that Table the obtained values of E are compared with the results from other authors^16,21,22.

Table 2. Comparison of the activation energies for precipitation and martensite reversion for different maraging steels.

Figure 5.
Method for the evaluation of the E values: (a) precipitation and (b) martensite reversion (P_M is the temperature for which there is a maximum precipitation rate and A_M is the temperature for which there is a maximum martensite reversion rate, see Figure 4).

Vasudevan et al. reported that initial precipitation occurs on dislocations, followed by a growing step of the precipitates by pipe diffusion⁶. Table 2 also shows that there is some discrepancy in the obtained activation energy values of this research from those reported by Viswanathan et al.¹⁶. The values diverge possibly due to different range of heating rates (Φ), because Viswanathan used heating rates in the range of 10 to 40 °C/min ( 0.17 to 0.67 °C/s). Furthermore, they suggested that for smaller heating rates (see Table 2), the redistribution of solute reaches a sufficient level in order to allow precipitation on dislocations by pipe diffusion (due to lower activation energies). Guo et al.²² and Kapoor and Batra²¹ have been observed that for higher heating rates, solute diffusion would be occurring by lattice diffusion (due to higher activation energies). In addition, from Table 2 it may be observed that the estimated activation energies for lattice diffusion are very close to those of the activation energies for lattice diffusion of the Ti (272 kJ/mol) and Mo (238 kJ/mol) in ferrite (similar to martensite)^16,22.

Table 2 also indicates that there is discrepancy in the values of martensite reversion activation energy as a function of the heating rate. For smaller heating rates, the lattice diffusion mechanism is predominant because the activation energies (224 and 342 kJ/mol) are close to those of nickel (246 kJ/mol) and molybdenum (238 kJ/mol) (for which the mechanism is diffusion in ferrite)^16,22. For higher heating rates Kapoor and Batra¹⁹ suggest that for Φ < 2 C°/s the value of E is 423 kJ/mol and for Φ > 2 C°/s the value of E is 828 kJ/mol. This observation has shown that activation energy is larger due to the shear mechanism that occurs in a more expressive way.

This work analyzed similar materials and pointed out that the shearing mechanism occurs also with larger intensity for higher heating rates (by comparing E in the range of 562 and 646 kJ/mol).

4. Conclusions

From the above observations it may be concluded that:

• The precipitation mechanism for both materials in the studied range is by lattice diffusion Furthermore, Co and Ti contents influence strongly this transformation;

• The lattice diffusion mechanism in the martensite reversion is influenced by Ni and Co contents and heating rate. For small heating rates ( ~1 °C/s) this mechanism prevails in the 300 maraging steel while for the 350 maraging steel has a minor importance;

• The mechanism of martensite reversion for 350 maraging steel in the studied range is mainly by shear mechanism. For higher heating rates (~28 °C/s) the shear mechanism prevails in both maraging steels.

Acknowledgements

The authors are grateful to EPUSP and CETEC-MG for the provided experimental facilities, CTMSP, CNPq and FAPEMIG for the financial support and to Nilton J. L. Oliveira for the dilatometric testing.

Received: October 2, 2012

Revised: December 11, 2012

• 1. Rao MN. Progress in understanding the metallurgy of 18% nickel maraging steels. International Journal of Materials Research. 2006; 97(11):1594-1607.
• 2. Garrison Junior WM. Martensitic non-stainless steels: high strength and high alloy. In: Buschow KHJ, Cahn RW, Flemings MC, Ilschner B, Kramer EJ and Mahajan S, editors. Encyclopedia of Materials: Science and Technology. London: Pergamon; 2008. v. 6, p. 5197-5203.
• 3. Avadhani GS. Optimization of process parameters for manufacturing of rocket casings: A study using processing maps. Journal of Materials Engineering and Performance, 2003; 12(6):609-622. http://dx.doi.org/10.1361/105994903322692394
• 4. Hamaker JC and Bayer AM. Applications des aciers maraging. Cobalt 1968; 24:3-12.
• 5. Schmidt M and Rohrbach K. Heat treatment of maraging steels. In: Metals Handbook. 10th ed. Materials Park: ASM International; 1990. v. 1, p. 219-228.
• 6. Vasudevan VK, Kim SJ and Wayman CM. Precipitation reactions and strengthening behavior in 18 wt pct nickel maraging steels. Metallurgical Transactions A 1990; 21(10):2655-2668. http://dx.doi.org/10.1007/BF02646061
• 7. Ahmed M, Nasim I and Husain SW. Influence of nickel and molybdenum on the phase stability and mechanical properties of maraging steels. Journal of Materials Engineering and Performance 1994; 3(2):248-254. http://dx.doi.org/10.1007/BF02645850
• 8. Li X and Yin Z. Reverted austenite during aging in 18Ni (350) maraging steel. Materials Letters 1995; 24(4):239-242. http://dx.doi.org/10.1016/0167-577X(95)00109-3
• 9. Peters DT. A Study of austenite reversion during aging of maraging steels. Transactions of ASM 1968; 61:62-74.
• 10. Tavares SSM, Abreu HFG, Maria Neto J, Da Silva MR and Popa I. A thermomagnetic study of the martensite-austenite transition in the maraging 350 steel. Journal of Alloys and Compounds. 2003; 358:153-156. http://dx.doi.org/10.1016/S0925-8388(03)00335-9
• 11. Tavares SSM, Da Silva MR, Maria Neto J, Pardal JM and Cindra Fonseca MP. Magnetic properties of a Ni-Co-Mo-Ti maraging 350 steel. Journal of Alloys and Compounds. 2004; 373:304-311. http://dx.doi.org/10.1016/j.jallcom.2003.11.009
• 12. Viswanathan UK, Dey G and Sethumandhavan V. Effects of austenite reversion during overageing on the mechanical properties of 18Ni (350) maraging steel. Materials Science and Engineering 2005; 398:367-372. http://dx.doi.org/10.1016/j.msea.2005.03.074
• 13. Kapoor R, Kumar L and Batra IS. A dilatometric study of the continuous heating transformations in 18wt. % Ni maraging steel of grade 350. Materials Science and Engineering A 2003; 352:318-324. http://dx.doi.org/10.1016/S0921-5093(02)00934-6
• 14. Andrés CG, Caballero FG, Capdevilla C and Álvarez LF. Application of dilatometric analysis to the study of solid-solid phase transformations in steels. Materials Characterization 2002; 48(1):101-111. http://dx.doi.org/10.1016/S1044-5803(02)00259-0
• 15. Oliveira FLG, Andrade MS and Cota AB. Kinetics of austenite formation during continuous heating in low carbon steel. Materials Characterization 2007; 58(3):256-261. http://dx.doi.org/10.1016/j.matchar.2006.04.027
• 16. Viswanathan UK, Kutty TRG and Ganguly C. Dilatometric technique for evaluation of the kinetics of solid-state transformation of maraging steel. Metallurgical Transactions A 1993; 24(12):2653-2656. http://dx.doi.org/10.1007/BF02659489
• 17. Kissinger HM. Reaction kinetics in differential thermal analysis. Analytical Chemistry 1957; 29(11):1702-1706. http://dx.doi.org/10.1021/ac60131a045
• 18. Mittemeijer EJ. Analysis of the kinetics of phase transformations. Journal of Materials Science 1992; 27(15):3977-3987. http://dx.doi.org/10.1007/BF01105093
• 19. Habiby F, Siddiqui TN, Hussain H, Ul Haq A and Khan AQ. Lattice changes in the martensitic phase due to ageing in 18 wt% nickel maraging steel grade 350. Journal of Materials Science1996; 31(2):305-309. http://dx.doi.org/10.1007/BF01139144
• 20. Decker RF, Eash JT and Goldman AJ. 18% Nickel maraging steel. Transactions of ASM 1962; 55:58-76.
• 21. Kapoor R and Batra IS. On the α' to γ transformation in maraging (grade 350), PH 13-8 Mo and 17-4 PH steels. Materials Science and Engineering A 2004; 371:324-334. http://dx.doi.org/10.1016/j.msea.2003.12.023
• 22. Guo Z, Sha W and Li D. Quantification of phase transformation kinetics of 18 wt.% Ni C250 maraging steel. Materials Science and Engineering A 2004; 373:10-20. http://dx.doi.org/10.1016/j.msea.2004.01.040

Publication Dates

• Publication in this collection
  10 May 2013
• Date of issue
  Aug 2013

History

• Received
  02 Oct 2012
• Reviewed
  11 Dec 2012