Effect of tungsten on tensile properties and flow behaviour of RAFM steel

Journal of Nuclear Materials 433 (2013) 412–418

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Effect of tungsten on tensile properties and flow behaviour of RAFM steel J. Vanaja a,⇑, K. Laha a, M. Nandagopal a, Shiju Sam b, M.D. Mathew a, T. Jayakumar a, E. Rajendra Kumar b a b

Metallurgy and Materials Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India Institute for Plasma Research, Gandhinagar 382 428, Gujarat, India

a r t i c l e

i n f o

Article history: Received 31 May 2012 Accepted 25 October 2012 Available online 5 November 2012

a b s t r a c t Effect of tungsten in the range of 1–2 wt.% on tensile properties and flow behaviour of 9Cr–W–Ta–V Reduced Activation Ferritic–Martensitic (RAFM) steel has been investigated. The tungsten in the investigated range was found to have only minor effect on the tensile properties of the steel over the temperature range of 300–873 K and at a strain rate of 3  103 s1. The tensile flow behaviour of the RAFM steels was adequately described by the Voce’s constitutive equation. The tensile strength of the steels were predicted well from the parameters of the Voce’s constitutive equation. The Voce’s strain hardening parameter ‘nv’ was found to be quite sensitive to the tungsten content and predicted the onset of dislocation climbing process at relatively higher testing temperature with the increase in tungsten content. The equivalence between tensile and creep deformations and the influence of tungsten have been discussed. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction In the last three decades, focused efforts have been laid in the development of Reduced Activation Ferritic/Martensitic (RAFM) steels which are considered as potential candidates in the fusion reactor material programmes for the fabrication of blanket module [1,2]. The know-how gained from the use of conventional austenitic stainless steel in nuclear fission power plants led to the finding of their drawbacks such as helium embrittlement and void swelling [3] which result in severe handling problem of the irradiated materials. However, these drawbacks are being dealt with by the development of ferritic steels considered to be deployed in fast reactor as in-core components. This kind of steels possesses lower thermal expansion coefficient and higher thermal conductivity in addition to their excellent resistance to irradiation induced void swelling than the austenitic steels. In order to minimize the environmental impact of the irradiated steel after the service lifetime of blanket module in fusion reactor and thereby increasing the environmental attractiveness of fusion power, a need arose for the development of low activation ferritic steels having relatively less residual induced radioactivity. This has been made possible by selectively replacing elements like Mo and Nb in the ferritic steel, which would transmute into high-energy radiation emitters with long half life on neutron irradiation, with elements like W and Ta respectively and by keeping strict control on the impurity

⇑ Corresponding author. Address: Creep Studies Section, Mechanical Metallurgy Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India. Tel.: +91 44 27480500x21210. E-mail address: [email protected] (J. Vanaja). 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.10.040

level of the steel. This has led to the development of Reduced Activation Ferritic/Martensitic (RAFM) steel. Research and development work was undertaken in India at Indira Gandhi Centre for Atomic Research, Kalpakkam to develop an India-specific RAFM steel by studying the effects of specific alloying elements (W and Ta) to obtain better combination of mechanical strength and toughness. Towards this effort, RAFM steel having tungsten content in the range of 1–2 wt.% and tantalum content in the range of 0.06–0.14 wt.% have been studied. In this paper, the effects of tungsten on tensile properties and tensile flow behaviour of RAFM steel with 0.06Ta have been described. Efforts have also been made to correlate the tensile flow and creep deformation behaviour of the steels.

2. Experimental Four heats of RAFM steels each weighing around 100 kg have been produced. Ingots were produced by vacuum induction melting followed by vacuum arc remelting processes. The heats are described as 1W–0.06Ta, 1W–0.06Ta (A), 1.4W–0.06Ta and 2W–0.06Ta based on their tungsten content. The chemical compositions of the steels produced by the four heats are given in Table 1. The ingots were hot-forged and rolled into 12 mm thick plates. The steel plates were subjected to the final heat treatment consisting of normalizing at 1253 K for 30 min and air cooled followed by tempering at 1036 K for 90 min and cooled in air to room temperature. The microstructure of the specimens has been analysed using scanning electron microscope. Button-head cylindrical specimens having gauge length of 20 mm and a gauge diameter of 4 mm were machined from the

J. Vanaja et al. / Journal of Nuclear Materials 433 (2013) 412–418 Table 1 Chemical composition (wt.%) of the melted RAFM steels. Alloy/elements

1W–0.06Ta

1W–0.06Ta (A)

1.4W–0.06Ta

2W–0.06Ta

Cr C Mn V W Ta N O P S B Ti Nb Mo Ni Cu Al Si Co As + Sn + Sb + Zr

9.04 0.08 0.55 0.22 1.00 0.06 0.0226 0.0057 0.002 0.002 0.0005 <0.005 0.001 0.001 0.005 0.001 0.004 0.09 0.004 <0.03

9.07 0.093 0.56 0.22 1.01 0.06 0.02 0.0046 0.002 0.002 0.0005 <0.005 0.001 0.002 0.005 <0.001 0.004 0.09 <0.002 <0.03

9.03 0.126 0.56 0.24 1.39 0.06 0.03 0.002 <0.002 <0.001 <0.0005 <0.005 <0.001 <0.002 0.005 0.002 0.0035 0.06 0.005 <0.004

8.99 0.12 0.65 0.24 2.06 0.06 0.02 0.00241 <0.002 0.0014 <0.0005 <0.005 <0.001 <0.002 0.004 0.002 0.002 0.06 <0.005 <0.0025

heat-treated plates. Gauge length of the specimen was parallel to the rolling direction of the plate. Tensile tests were carried out in a screw driven tensile testing machine fitted with high temperature furnace and digital data acquisition system. Tensile tests were conducted in air at a nominal strain rate of 3  103 s1at temperatures ranging from 300 to 873 K with an interval of 50 K. The temperature during the test was controlled within ±1 K. The cross head displacement of the tensile machine was taken as the specimen extension. The initial elastic portion (contributed by the sample, machine frame and load–train assembly) was subtracted from the total true strain for the calculation of true plastic strain. The true stress–true plastic strain data were fitted to Voce constitutive equation of tensile flow using Levenberg–Marquardt (L–M) algorithm with unknown constants as adjustable parameters. The validity of the fit was judged by the low value of chi

413

square (v2), which is the sum of the squares of the deviations of calculated stress values from the experimental data and by least error band in determining the constants of the constitutive equation. Tensile tests at 823 K at strain rates of 3  102 s1 and creep tests at 823 K over a stress range of 160–260 MPa were also carried out on the steels to estimate saturation stress and the steady state creep rate respectively, with an intention of correlating tensile deformation with creep deformation.

3. Results and discussion 3.1. Microstructure The SEM micrograph of the as-received normalized and tempered RAFM steels are shown in Fig. 1. The as-received RAFM steels had a typical tempered martensitic structure. The prior austenite grain size of the steels were estimated to be in the range 17 ± 3 lm. The increase in tungsten content had no appreciable effect on the prior austenitic grain size of the steels. Details of the microstructure investigated by transmission electron microscope along with the selected area electron diffraction (SAED) combined with EDS (Energy Dispersive Spectroscopy) analysis of the precipitates of the steel 1W–0.06Ta have been reported earlier [4]. The martensitic laths were decorated with carbides. The carbides were chromium and tungsten rich M23C6 type of carbides. The fine intralath precipitates were of MX type with an enrichment of Ta and V. Similar microstructural features in RAFM steels was reported by Jayaram and Klueh [5]. The decrease in lath size and M23C6 carbide with increase in tungsten content had been reported by Abe [6]. The steel was normalized at 1253 K against the commonly used normalizing temperature of 1323 K or higher in conventional Cr– Mo–V power plant steels. This was carried out to refine the prior austenite grain size of the steel for better toughness. Undissolved primary Ta carbo-nitrides in the steel during normalization restrict the growth of austenitic grain. Similar normalizing heat treatment had been adopted for Eurofer’97 [7].

Fig. 1. SEM micrographs of the normalised and tempered steels showing the tempered martensitic structure: (a) 9Cr–1W–0.06Ta, (b) 9Cr–1.4W–0.06Ta and (c) 9Cr–2W– 0.06Ta.

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3.2. Tensile properties Engineering stress–strain curves of the materials tested at a strain rate of 3  103 s1 showing the effects of tungsten on tensile properties at room temperature (300 K) and 823 K are shown in Fig. 2a and b. Similar variations of stress and strain were also observed at all other test temperatures. The variations revealed that the materials exhibited monotonic stress–strain curves over the entire temperature range with no serrated flow in the intermediate temperature range. The strain to failure (ef) was found to reduce gradually with increase in temperature from 300 to 673 K, beyond which it increased. The influence of tungsten on the yield stress and ultimate tensile strength of the steel at various test temperatures is shown in Fig. 3. The tensile strength of all the three steels decreased with the increase in test temperature. Increase in tungsten from 1 to 2 wt.% could not impart much variation in tensile strength of the RAFM steel. The tensile ductility of the steel was found not to change systematically and appreciably with the tungsten over the temperature range investigated Figs. 4a and 4b. The effect of tungsten on tensile properties of 9Cr–W–Ta–V RAFM steel having tempered martensitic microstructure has been studied by Klueh et al. [8]. They have reported no significant effect of tungsten in the range of 2–3 wt.% on the tensile strength of the steel. Similar results of no appreciable effect of tungsten in the range 1–2 wt.% has been reported by Miyahara et al. [9]. The effect of tungsten on the tensile strength of yttrium oxide dispersion strengthened (9CrODS) steel has been studied by Narita et al. [10]. Tungsten is considered to impart tensile strength to the steel both by solid solution strengthening from misfit strain and from residual a-ferrite. The residual a-ferrite content in the steel had been reported to depend on tungsten and oxygen contents.

Fig. 3. Effect of tungsten on yield stress and ultimate tensile strength of RAFM steels.

(a) Fig. 4a. Effect of tungsten on elongation % of RAFM steels.

(b)

Fig. 4b. Effect of tungsten on reduction in area % of RAFM steels.

Fig. 2. Tensile curves of the RAFM steel: (a) 300 K and (b) 823 K at a strain rate of 3  103s1.

By increasing oxygen content (0.147 wt.%) in the 2 wt.% tungsten 9CrODS steel, they could achieve fully martensitic structure in the steel free from residual a-ferrite, as in 9CrODS steel having 0.04 wt.% tungsten. The increase in yield strength in the ODS steel having 2 wt.% of tungsten due to solid solution strengthening effect alone has been estimated to be about 18 MPa at 973 K. They concluded that the contribution of solid solution strengthening by 2 wt.% tungsten to the tensile strength of the ODS steel was

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relatively small as compared to those imparted by other microstructural features of the martensitic ODS steel. The ferritic–martensitic RAFM steel derives its mechanical strength from the solid solution of tungsten and from the complex microstructure consisting of a high dislocation density, dislocation cell structure and subgrain boundaries decorated with M23C6 carbides and fine coherent intragranular tantalum and vanadium carbonitride precipitates of the MX type [4–6,8,11]. In the present investigation on the effect of tungsten on tensile strength (Fig. 3), the contribution of solid solution strengthening to the steel from tungsten over the range 1–2 wt.% is appeared to be relatively less than those from the martensitic substructure and the intragranular (V,Ta)CN precipitates which is within the experimental scatter band of around ± 20 MPa. It is to be noted that the 1W–0.06Ta steel has lower carbon content than the other two steels having higher tungsten content. In spite of the appreciable difference in carbon content, the tensile strength of the steels are almost similar. No appreciable effect of carbon content in the range of 0.05–0.15 wt.% on creep rupture strength of a 9Cr–3W–3Co–0.2V–0.05Nb–0.05N steel has been reported by Abe [6]. Precipitation hardening in this material is primarily from MX type of carbo-nitrides. In these steels, vanadium and tantalum forms MX carbides which requires about 0.06 wt.% of equivalent carbon (C + 67 N). In the present study the steels have sufficient carbon to form MX type of carbo-nitrides and the excess amount of carbon then goes to the formation of M23C6 type of carbides. This implies that the variation in carbon in different steels would not play appreciable role in the strengthening mechanisms of the complex steels. Tensile strength on two heats of the 9Cr–1W–0.06Ta steel with carbon contents of 0.08 and 0.093 wt.% respectively did not show any appreciable change.

To understand the tensile flow behaviour of the steel with respect to varying tungsten content, the true stress–true plastic strain curves were analysed using Voce’s constitutive equation of plastic deformation. The applicability of this equation to various ferritic steels including the RAFM steel has been demonstrated by several investigators over wide ranges of temperatures and strain rates [12–14]. The Voce’s constitutive equation of plastic deformation overcomes the negativity of power function of Hollomon’s and Ludwik’s constitutive equations which suggest that all the materials become indefinitely strong after severe deformation. The Voce’s constitutive equation predicts an upper limit to the degree of strain hardening attainable on deformation of a material [15,16]. This constitutive equation is applicable to the materials which show the tendency to attain saturation of stress at large strains. The variations of true stress with true–strain of the materials show the near saturation behaviour of true stress with strain (Fig. 5a and b). The Voce’s strain hardening equation is represented as [15,16]:

ð1Þ

where rs is the saturation stress, ri and ei are true stress and true plastic strain at the onset of plastic deformation respectively, and ec is a constant. For condition ei = 0, Eq. (1) reduces to

r ¼ rs  ðrs  ri Þ expðnv eÞ

(a)

(b)

Fig. 5. Variation of true stress and true plastic strain of the RAFM steels at: (a) 300 K and (b) 823 K.

3.3. Tensile flow behaviour

r ¼ rs  ðrs  ri Þ exp½ðe  ei Þ=ec 

415

ð2Þ

with three constants rs, ri and nv = 1/ec. The true stress and the true-strain data were fitted to the Voce’s constitutive equation using Levenberg–Marquardt least square method with unknown constants as free parameters. The best fit is indicated by a low value of chi square (v2) (which is the sum of the squares of the deviations of calculated stress values from the experimental data) and by least error band in determining the constants of the constitutive equation. Table 2 shows the

estimated values of the parameters in the Voce constitutive equations with the values of v2 and error band for the RAFM steels. Fig. 5a and b depict experimental re data along with fitted flow relationships based on Voce’s constitutive equation at 300 K and 823 K respectively. Figs. 6–8 respectively show the variation of the Voce’s constitutive equation parameters rs, ri and nv with temperature for the steels having different tungsten contents. Both the initial stress (ri) and saturation stress (rs) decrease continuously with increasing temperature, but no appreciable and systematic effect of tungsten was found as observed in tensile strength (Fig. 3). The decrease in rs with temperature implies that saturation in stress reaches at lower stresses as recovery starts early with increase in temperature. The stress saturation with deformation, as interpreted by Mecking and Kocks [17], is controlled by the competition between production and annihilation (rearrangement) of dislocations until instability or necking sets-in in the specimen during tensile deformation. For this reason, the saturation stress is also known as the steady state stress. Saturation stress attained in the constant strain rate tensile test and the steady state strain rate attained in the creep test are both due to similar underlying mechanisms resulting from the saturated state of dislocation substructures [18]. Weertman [19] has suggested and attempted to demonstrate that the steady state strain rate is related to the applied stress in a constant stress creep test by the same function as the strain rate is related to the saturated stress in a constant strain rate tensile test. The steady state creep rate (?s) with applied stress (ra) in the creep test can be described by a power law relation known as Norton’s law:

es ¼ A1 rna where A1 and n are constants.

ð3Þ

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Table 2 Estimated values of the parameters in the Voce constitutive equations with the values of v2 and error band (3  103 s1 strain rate) of the RAFM steels. 9Cr–1W–0.06Ta 2

v

9Cr–1.4W–0.06Ta 6.86697 0.99731

R2

9Cr–2W–0.06Ta

4.40853 0.99769

7.10263 0.99783

300 K

rs rI nv

v2

Value 748.56

Error

555.53

+1.08

rs rI

62.31

+0.92

nv

+0.78

0.31712 0.99815

2

R

Value 697.50

Error

529.02

+0.86

rs rI

58.59

+0.795

nv

+0.63

0.82436 0.99691

Value 752.92

Error +0.8418

533.35

+0.968

46.005

+0.5881

0.5212 0.99745

823 K

rs rI nv

Value 432.32

Error

365.28

+1.07

rs rI

+8.23

nv

381.9

+0.21

Value 418.94

Fig. 6. Effect of tungsten in the variation of initial stress (ri) in Voce’s constitutive relation with temperature.

Error

347.73

+1.099

rs rI

262.55

+7.226

nv

+0.38

Value 403.985 343.978 219.446

Error +0.324 +0.74 +5.414

Fig. 8. Effect of tungsten in the variation of nv in Voce’s constitutive relation with temperature.

Fig. 7. Effect of tungsten in the variation of saturation stress (rs) in Voce’s constitutive relation with temperature.

Similarly, the relationship between saturated or steady state stress rs and applied strain rate ðea Þ in a tensile test can be described as:

ea ¼ A2 rns

ð4Þ

 are constants. where A2 and n Application of Eqs. (3) and (4) to the saturated/steady state tensile and creep deformation behaviours at 823 K of the RAFM steels

Fig. 9. Equivalence between saturation stress at a given strain rate in a tensile test and steady state creep rate at constant stress in a creep test depicting the effect of tungsten.

having different tungsten content are shown in Fig. 9. Equivalence ´ are observed for all the steels havbetween A1 and A2 and n and n ing different levels of tungsten content. This confirms that irrespective of the nature of deformation route, the deformation mechanism remains same. Fig. 9 shows the effect of tungsten on

J. Vanaja et al. / Journal of Nuclear Materials 433 (2013) 412–418

attaining the steady state or saturation state for both tensile and creep tests. Even though tungsten has no appreciable effect on the strength related parameters (rs and ri) of Voce’s constitutive relation, tungsten content has appreciable effect on the strain hardening parameter ‘‘nv’’ especially at higher test temperatures (Fig. 8). The variation of nv with temperature shows two slope behaviour (Fig. 8). Initially it decreases gradually up to around 700 K followed by rapid decrease. This rapid decrease in the nv values with increasing temperature indicates acceleration of recovery. The change in slope implies the change in deformation mechanism. It has been suggested that when cross slip dominates, the nv is associated with relatively high values, whereas a change in controlling mechanism from cross-slip to climb and sub-boundary migration at higher temperatures result in very low nv values [20]. This trend is observed in all the steels having different tungsten contents. The temperature range in which the slope change occurs is found to increase with the increase in tungsten content. This indicates that the recovery of the dislocations is delayed with the increase in tungsten content. The recovery is the result of annihilation of excess dislocations by dislocation climb and interaction with the carbides and other dislocations. The rate determining process in the case of dislocation climb is considered mainly to be self-diffusion and in the present case, iron plays the main role. It is evident from the literature that the rate of Fe diffusion is influenced by the addition of tungsten as it decreases the diffusion coefficient of iron. Chromium has almost no effect on the diffusivity of iron below 10 wt.% of Cr. This suggests that the addition of tungsten to 9Cr steel retards the recovery process, by decreasing the self-diffusion rate of iron [21]. Stiffer fall in steady state creep rate with stress in RAFM steel having higher tungsten content (Fig. 9) is due to lower recovery rate of dislocation substructure during creep/tensile deformation. The increase in creep deformation resistance with the increase in tungsten content has been reported by Abe and Nakazawa [22]. Tungsten addition refines the martensitic lath structure and decreases the coarsening of chromium rich M23C6 carbides. The M23C6 carbide stabilizes the martensitic laths and its stability decreases the rate of recovery during creep exposure, which led to decrease in steady state creep rate with tungsten content (Fig. 9). 3.4. Relation between Voce’s parameters and tensile strength Applicability of any theory to the mechanical properties of metals and alloys can be confirmed by the accurate prediction and explanation of its mechanical behaviour with respect to

417

Fig. 11. Comparison between yield stress and Voce’s parameter ri of the steels.

temperature, stress or strain rate. Mishra et al. [23] have derived an expression, based on Voce’s constitutive Eq. (2), to calculate ultimate tensile strength by invoking the Consideré criterion of instability at the onset of necking. On differentiating Eq. (2) and substituting exp (nve) with (rs  rs)/(rs  ri), we get:

@r ¼ nv ðrs  rÞ @e

ð5Þ

Now invoking Consideré criterion ð@@re ¼ rÞ by substituting @@re and r with ruv (ultimate tensile strength based on Voce’s constitutive equation), we get:

ruv ¼ nv rs =ð1  nv Þ

ð6Þ

Fig. 10 compares the experimentally obtained true ultimate strength, ru (true stress corresponding to maximum engineering stress) with that obtained from Voce’s constitutive equation parameters (ruv) for all the steels. It is observed that ru is linearly related to ruv by ru = 5.89 + 1.016 suv with a correlation coefficient of 0.9994. The Voce’s constitutive parameter ri, is the stress at the start of the plastic strain and so it can be correlated with the yield stress of the material. Fig. 11 compares the initial stress ri with the yield stress of the steels for all the tests carried out at different strain rates and temperatures. It is observed that the yield stress (Y.S.) is linearly related to initial stress with a correlation coefficient of 0.9605 as Y.S. = 0.932 si + 23.75. 4. Conclusion Following conclusions have been drawn based on the comparative studies of tensile properties and tensile flow behaviour of the RAFM steels having different tungsten contents:

Fig. 10. Comparison between the experimentally determined true ultimate tensile stress and the value determined based on Voce’s equation of the steels.

1. The increase in tungsten content of the 9CrWVTa RAFM steel from 1 to 2 wt.% did not significantly change the tensile strength and ductility. 2. Tensile flow behaviour of the steels having different tungsten content has been adequately described by Voce constitutive equation. 3. Good correlations have been observed between Voce’s constitutive equation parameters with tensile strengths of the RAFM steel. 4. Correspondence between tensile deformation and creep deformation of the steels has been found to obey in these RAFM steels.

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Acknowledgements The authors thank Mr. S.C. Chetal, Director, Indira Gandhi Centre for Atomic Research, Kalpakkam, India for his keen interest in this work. Research support from Institute for Plasma Research, Gandhinagar, India is gratefully acknowledged. References [1] [2] [3] [4]

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