Carbides and mechanical properties in a Fe–Cr–Ni–Mo high-strengthsteel with different V contents

Tao Wen, Xiaofeng Hu, Yuanyuan Song, Desheng Yan, Lijian Rongn

Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, 110016, China

a r t i c l e i n f o

Article history:Received 16 April 2013Received in revised form23 August 2013Accepted 7 September 2013Available online 16 September 2013

Keywords:High-strength steelVanadiumCarbidesMechanical properties

a b s t r a c t

Fe–Cr–Ni–Mo high-strength steels with four different V contents (0%, 0.03%, 0.08% and 0.14%) wereprepared in this paper and carbides evolution was investigated by means of three-dimensional atomprobe (3DAP) and transmission electron microscopy (TEM). The results indicated that there appearedMC,M2C and M6C types of carbides after addition of V in the steels while it was M23C6 in 0.0%V steel (M is anycombination of Cr, Mo, Mn, Fe or V), and the size of carbides decreased gradually with improving Vcontent. The mechanical properties significantly depended on the content of V. The strength andelongation increased gradually with increase in the V content, meanwhile the impact toughnessdecreased gradually. The excellent combination of mechanical properties can be obtained in the steelwith about 0.03%—V content.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent decades, a large number of researchers have devotedthemselves to the development of high-strength steels [1–8].Compared to the conventional steels, high-strength steels havemany advantages, such as high strength, well ductility andexcellent weldability [2,3]. Their excellent combination of strengthand impact toughness especially offers the capacity to fulfill theneeds in special fields, such as water turbine in hydroelectricpower station, pressure vessel, shipbuilding section etc [4–6].Generally speaking, high-strength steels are Fe–Cr–Ni–Mo serieswith the alloying elements (Cr, Mo, Ni and so on) in a reasonablerange. Apart from this, it is necessary to mention the V elementbecause its minor addition will play a decisive role in the remark-able mechanical properties of high-strength steels by precipitatingthe carbides [7,8]. Many researchers have studied the effect of Velement on carbides and mechanical properties of high-strengthsteels [9–11]. Michaud [9,12] found that there mainly appearedV-riched MC carbides in 5% Cr martensitic high-strength steelcontaining 0.84%V, which can effectively prevent the coarsening ofprior austenite grain during austenitizing and have obvious effectson precipitation strengthening. Takashi [10] studied the effect of Von precipitation behavior and mechanical properties of high Crsteel and found that M2C carbides are formed in the steel with

0.06%V and they transformed into MC when V content is increasedto 0.12%, which also affect the mechanical properties significantly.Zhang [11] proposed the influence of alloying elements onprecipitation by using the theory of phase equilibrium thermo-dynamics, and found that the precipitation of M6C carbidescontaining V element is conducive to impede the emergence ofM7C3 precipitation which is detrimental largely to impacttoughness.

However, the effect of V on the microstructure in high-strengthsteels has not been studied systematically, particularly on thevariation of carbides (morphology, type and chemical composi-tions) in the steel with different V contents. In this paper, fourFe–Cr–Ni–Mo high-strength steels with different V contents areprepared to investigate the influence of V content on carbides andthe mechanical properties in high-strength steel. Three-dimensional atom probe (3DAP) and transmission electron micro-scope (TEM) were used to observe the morphology and detect thechemical compositions of carbides. The influence mechanism of Von carbides and the mechanical properties is proposed, which maybe accumulated as theoretical basis for future studies.

2. Experimental procedure

Four Fe–Cr–Ni–Mo steels with different V contents weredesigned in this study, named A1, A2, A3 and A4, respectively.The chemical compositions (wt%) are 3.8Ni, 1.6Cr, 0.6Mo, 0.6Mn,0.3Si, 0.3C, 0.008S, 0.007P, balanced by Fe, and the nominalcontents of V are shown in Table 1. The 25 kg cast ingots wereprepared by vacuum induction melting. The as-cast steels were

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n Correspondence to: Division of Materials for Special Environments, Institute ofMetal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016,China. Tel.: þ86 24 23971979; fax: þ86 24 23978883.

E-mail address: ljrong@imr.ac.cn (L. Rong).

Materials Science & Engineering A 588 (2013) 201–207

hot forged to 35 mm plates and then hot rolled to 12 mm inthickness.

The heat treatment process used in this paper is optimized,which offers better combination of strength and impact toughness[13], and the detailed parameters are listed in Table 2. After theheat treatment, the 12 mm sheets were machined into standardtensile and Charpy V-notch impact samples. In particular, V-notchoriented the rolling direction for the Charpy impact samples.Tensile tests were performed at room temperature on a AG-100KNG, and Charpy impact tests were performed at �50 1C on aRKP 450.

The specimens prepared for TEM observation were cut from12 mm heat treated sheets, which were firstly grinded to 50 μm inthickness and then were punched to 3 mmwafers in diameter. Thewafers were electropolished with a twin-jet electropolisher usinga solution of 10% perchloric acid in ethanol at �20 1C and at 10 V.The microstructures were examined by Tecnai G2 20 TEM oper-ated at 120 kV.

The 3DAP apparatus used in this article is a type of position-sensitive probe (POSAP) [14] to yield the original 3D position ofdetected C, Cr, Mo and V atoms etc. The atom probe specimenswere first cut into small rods (0.7 mm�0.7 mm�21 mm) fromtempered materials, and then mechanically polished to pins withthe cross sectional area of 0.5 mm�0.5 mm [15]. The electropol-ishing procedures contain two stages [16]. Firstly a mixed solutionof 25% perchloric acid in acetic acid was used to form a neck on thepins; and secondly the necks were electrolytically thinned until itwas separated into two parts in a solution of 2% perchloric acid inbutyl cellosolve. Thus, two tips whose radius was less than 100 nmwere prepared. Field ion microscopy (FIM) was used employing(2.0–2.5)�10�3 Pa of neon as the imaging gas at 60 K. Atom probeanalysis was performed at 70 K with the vacuum pressure of1�10–8 Pa and a pulse fraction of 20%, as well as a pulse repetitionrate of 5000 Hz. All the 3DAP data analysis was implementedusing the Posap software offered with the instrument.

3. Results

3.1. Microstructure

3.1.1. TEM investigationsFig. 1 shows the morphology of carbides in the four tempered

steels. Bright-field TEM micrographs show that a large amount ofcarbides precipitate in all experimental steels, which are mainly instrip-like and spherical shapes. It is obvious that the amount ofstrip-like carbides is more than that of spherical carbides. It can beseen that the carbides prefer to appear along grain boundaries andprior martensite lath interfaces, relative to the interior of α-ironlaths (ferrite). It can also be noticed that the amount of carbideschanged indistinctly with the increase of V content.

Fig. 2 displays the morphology and the corresponding selectedarea electron diffraction (SAED) pattern of carbides in the 0.0%V,0.03%V and 0.14%V steels. When there is no V element in the steel,

Table 1The vanadium content of experimental steels (wt%).

Sample A1 A2 A3 A4

Vanadium content 0.00 0.03 0.08 0.14

Table 2The heat treatment conditions for experimental steels.

Process Temperature (K) Time (min) Cooling method

Normalizing 1133 60 Air cooling (AC)Quenching 1133 40 Oil quenching (OQ)Tempering 883 120 Water cooling (WC)

Fig. 1. The TEM morphology of four experimental steels with different V contents after tempering. (a) 0.0%V, (b) 0.03%V, (c) 0.08%V, and (d) 0.14%V.

T. Wen et al. / Materials Science & Engineering A 588 (2013) 201–207202

the carbides for both strip-like and spherical are mainly M23C6

(1a–1c of Fig. 2). And it can be seen that two shapes ofMC carbidesform when V content is increased to 0.14% (3a–3c of Fig. 2).Nevertheless, when 0.03%V content is added to the steel, thereappear M2C and M6C types of carbides which correspond to thestrip-like and spherical precipitation respectively (2a–2c of Fig. 2).Similar to the situation in the steel with 0.03%V, there also appearM2C and M6C types of carbides after the addition of 0.08%V in thesteel. It can be found that the average dimension of carbidesgradually decreases with increasing V content (Figs. 1 and 2). Thelengths for strip-like carbides are 195, 172, 157 and 120 nmrespectively for A1, A2, A3 and A4 steels, while the diameters ofspherical carbides are 55, 47, 35 and 20 nm respectively. Inciden-tally, the length of strip-like carbides and diameter of sphericalcarbides are the average of at least five determinations. It isextremely difficult to determine the compositions of carbides byTEM-XEDS (X-ray energy dispersive spectrometry), owing to theirsize in nano-scale and the strong magnetism of the matrix.

3.1.2. 3DAP investigationsFig. 3 displays the distribution of individual atom for different

elements in the tempered A1 steel without V element. It can beseen that the solute elements (C, Cr, Mn and Mo) segregate in thesame position, while the atoms of other elements (Fe, Ni, Si)distribute uniformly in the matrix (Fig. 3a). In order to reduce theinterference caused by the deviation from randomness of thespatial distributions of all elements and to gain concentrationprofiles, a box with volume of 5 nm�5 nm�20 nm is selected(Fig. 3b). The result shows that the distribution of all elements isthe same as Fig. 3a. Therefore, it can be concluded that C, Cr, Moand Mn have formed alloying carbides together in the A1 steel.

Similarly, Fig. 4 shows the distribution of individual atom forthe different elements in the tempered A3 steel with 0.08%Vcontent. There is an interesting change for atom distributioncompared with the steel without V. Taking the strip-like carbidesfor example, it can be seen that Cr and Mn atoms seemingly nolonger segregate obviously, but V atoms segregate in the same

Fig. 2. The TEM morphology and lattice structure of steels with 0.0%V, 0.03%V and 0.14%V content after tempering. (1) 0.0%V, (2) 0.03%V, (3) 0.14%V, (a) bright field image,(b) and (c) diffraction patterns of strip-like and spherical carbides respectively.

T. Wen et al. / Materials Science & Engineering A 588 (2013) 201–207 203

place with C and Mo atoms (Fig. 4a). A subvolume is also selectedand the result reveals that the distribution of Mn atoms as well asthe atoms of Fe, Ni and Si is fairly random. However, Cr atomssegregate together with C, Mo and V in the same location (Fig. 4b).

Therefore, it is clear that alloying carbides in A3 are mainlycomposed of C, Cr, Mo and V atoms.

Fig. 5 also displays the distribution of individual atom for thedifferent elements in the tempered A4 steel with 0.14%V content.

Fig. 3. The 3DAP morphology in A1 steel without vanadium element. (a) Atom maps for different elements, (b) 3D-reconstructed atom maps from selected subvolume, and(c) corresponding concentration profiles in the subvolume.

Fig. 4. The 3DAP morphology in A3 steel with 0.08% vanadium content. (a) Atom maps for various elements, (b) 3D-reconstructed atom maps from selected subvolume, and(c) corresponding concentration profiles in the subvolume.

T. Wen et al. / Materials Science & Engineering A 588 (2013) 201–207204

There appears a characteristic phenomenon which is drasticallydistinct from Figs. 3 and 4. It can be clearly seen that the atomicdistribution of carbide-formers includes two kinds of situations:one is that C atoms segregate together with Cr, Mo and V atomswhen carbide has a smaller size, and the other one is that C atomssegregate along with Mn atoms besides Cr, Mo and V atoms whencarbide has a greater size (Fig. 5a). And the former is namedcarbide-1 while it is carbide-2 for the latter. A subvolume isselected as before and the result reveals that the atomic distribu-tion of carbide-formers is inconsistent partially with Fig. 5a(Fig. 5b). It can be clearly seen that Fe atoms also segregatetogether with C atoms in the carbide-2. Therefore, it is clear thatthere appear two kinds of alloying carbides with different compo-sitions in 3DAP map of tempered A4 steel. Carbide-1 is mainlycomposed of C, Mo, V and minor Cr atoms, and carbide-2 is mainlycomposed of C, Cr, Mn, Fe and minor Mo, V atoms.

Figs. 3c–5c show the concentration profile in the representativesubvolume as shown in Figs. 3b–5b, respectively. The carbon-enriched region of the atom maps are all regarded as the region ofcarbide and the carbon-depleted region indicates similarly theferrite phase.

3.2. Mechanical properties

Fig. 6 shows tensile, yield strength and elongation at roomtemperature for the four tempered steels with different V con-tents. It can be seen that tensile strength and yield strength of theexperimental steels increase obviously with increasing V content.

Particularly, it is worth noting that 0.03% addition of V induced arapid increase of strength, and the increasing trend of strengthdecreases gradually with further increase in the V content. Fig. 6also demonstrates the elongation profile at room temperature forthe experimental steels with different V contents. It can be seenthat the overall trend of elongation is incremental with improvingV content, although it is almost the same after addition of 0.03% or0.08% V in the steel.

Fig. 5. The 3DAP morphology in A4 steel with 0.14% vanadium content. (a) Atom maps for various elements, (b) 3D-reconstructed atom maps from selected subvolume, and(c) corresponding concentration profiles in the subvolume.

Fig. 6. The variation of strength and elongation by V content at room temperature.

T. Wen et al. / Materials Science & Engineering A 588 (2013) 201–207 205

Fig. 7 displays V depended impact absorbing at �50 1C. It canbe seen that the trend of impact absorbing energy is exactlyreverse to strength. Similarly, it can be noticed that the downwardtrend of impact absorbing energy also reduces gradually as the Vcontent is increased.

4. Discussion

4.1. The effect of V on the carbides evolution during tempering

Generally, the phase transformation resistance is composed ofinterfacial energy and elastic strain energy, and the latter is closelyrelated to the geometrical shape of precipitations [17]. It isreported that the elastic strain energy of strip-like carbides islower than that of the spherical ones , while it is moderate tointerfacial energy for strip-like carbides [17]. In addition, it isreported that the sequence ofM2C nucleation is earlier than that ofM6C [18,19]. It seems to be reasonable that M2C nucleation iseasier than that of M6C. Therefore, the amount of strip-likecarbides is greatly more than that of spherical carbides on thewhole in all experimental steels. It is found that grain boundaryand interface can both offer phase transformation driving force byreleasing interfacial energy and then decreasing nucleation energy[17]. As a result, the precipitations in this high-strength steelemerge more easily in the grain boundaries and prior martensitelath interfaces than in the matrix (ferrite).

It is mentioned previously that V plays an important role in theprocess of forming carbides. It has been found that the amount ofcarbides would increase greatly if V content is improved from0.47% to 0.84% [9]. However, the representative TEM micrographsof four experimental steels in this paper demonstrate that theamount of carbides does not change obviously, when V content isincreased from 0.0% to 0.14% (Fig. 1). The reason may be that Velement varies weakly among the four experimental steels. There-fore, it cannot be seen that the amount of carbides increasesdistinctly along with improving V content.

In all steels used in this paper, V is the strongest carbide-forming element, followed by Mo, Cr, and Mn atoms in sequence[11]. During formation of carbides, there exists a complex compe-titive relationship among these elements. Vanadium atoms willpreferentially combine with carbon atoms to form carbides once Velement is added into experimental steels. It is well known thatMC type is the most stable for V-carbides. If V content is sufficientin the steel, V atoms will force almost all C atoms to diffusetowards themselves owing to their powerful combining capacitywith C atoms and form riched-V MC carbides finally. If V content is

less, the amount of V atoms is not enough to form stable riched-VMC type of carbides within a limited time, then the nearby Cr andMo atoms will diffuse into carbides, thereby MC, M2C and M6Ctypes of carbides appear. Therefore, there appearMC,M2C andM6Ctypes of carbides after the addition of V in the steels (Fig. 2). Cr-riched M23C6 carbides with minority Mo and Mn mainly appearwhen there is no V in the steel (Fig. 2). The reason may be that Moand Cr atoms actually have similar combining capacity with Catoms and Cr atoms form M23C6 type rather easily if there is no Vin the steel.

It is worth noting that the dimension of carbides decreasesgradually when V content is increased from 0.0% to 0.14%(Figs. 1 and 2). Vanadium seemingly has a resistance whichimpedes carbides coursing. It has been reported that the additionof V could reduce the diffusivity of carbon atoms by formingstrong carbide in the steel [20]. Vanadium, as a trace element inthe four experimental steels, may simultaneously reduce thediffusivity of C, Cr, Mo and Mn atoms. Therefore, the number ofC, Cr, Mo and Mn atoms that enter into alloying carbide is ratherlittle, which results in a smaller size of carbides. Moreover, thisinfluence of V on these alloying elements will be reinforcedactually when V content is increased, so the dimension of carbidesdecreases gradually with increasing V content.

It is obvious that Fe atoms are absent in the smaller carbide-1,while they really exist in the greater carbide-2 (Fig. 5). Accordingto the alloying element-to-carbon ratio in Fig. 5c, we can deducethat the carbide types are M2C and M23C6 respectively whichcorresponds to carbide-1 and carbide-2. Similarly, it is inferredthat the strip-like carbide is M2C in Fig. 4. Though the distributionof Fe atoms is fairly random in Fig. 3, they really participatein forming alloying carbide. If Fe atoms are also taken into accountin the process of calculating the alloying element-to-carbon ratio,it will be exactly consistent with M23C6 in Fig. 3. In addition, it isreported that M23C6 is really the Fe–Cr-rich carbide [21]. It isnoteworthy that the MC type of carbides is not detected by 3DAPin Fig. 5, which may be because the space detected by 3DAP israther small. In addition, it can be deduced reasonably that thereexists minor M23C6 type of carbides in every steel with V element.

It is also noteworthy that there are scarcely any Mn atomswhen V concentration is higher in the alloying carbides(Figs. 4 and 5). It seems that there is a so-called repulsive forceinteraction between the V and Mn atoms. Vanadium is a strongcarbide-former, which preferentially combines with C atoms, andblock the entrance by which Mn atoms cannot access the alloyingcarbides. Therefore, there are hardly any Mn atoms in M2C, M6Cand MC types of alloying carbides.

4.2. The effect of V on mechanical properties

The strength, impact properties and elongation significantlydepend on the content of V (Figs. 6 and 7). The variation of Vcontent results in great modification in the dimension, type andchemical compositions of carbides , which have significant effecton the mechanical properties of the experimental steels.

The dimension of carbides gradually decreases with increasingV content, which is conducive to both strength and impacttoughness [17]. It has been found that prior austenite grain wasrefined and the sizes of grain are 26 μm, 20 μm, 16 μm and 10 μmrespectively for A1, A2, A3 and A4 steels, which is also favorable forthe strength and impact toughness. And generally, if the type ofcarbides is more stable, then the strengthening effect is better,whereas the impact toughness is much worse at the same time[22,23]. In addition, the compositions of carbides also affect themechanical properties greatly. In this paper, V-riched MC has thestrongest strengthening effect, followed by M2C and M6C contain-ing V element, and M23C6 without V element is last [22].

Fig. 7. The variation of impact absorbing energy by V content at �50 1C.

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Comparing with the steel without V, the type, dimension andchemical compositions of carbides all vary after addition of 0.03%Vin the steel, and the variations are all conducive to strength, whichcause the fastest increase in strength during the process. When Vcontent is increased from 0.03% to 0.08%, only the dimension andchemical compositions of carbides vary, therefore the increasingtrend of strength reduces at this stage. Although the type, dimen-sion and chemical compositions of carbides all change, theinfluence of them on strength may decrease when V content isincreased from 0.08% to 0.14%. Therefore, the increasing trend ofstrength reduces further in the process. Similarly, the downwardtrend of impact toughness also drops gradually with increasing Vcontent.

The effect of carbides on elongation is quite complex. Ingeneral, the trend of elongation is exactly opposite to strength.Nevertheless, the elongation increases overall with increasing Vcontent in this paper, which is the same as strength. The reasonmay be that the type of carbides tends to stable together with thedecrease of carbides in size are both useful to elongation. Particu-larly, the former may play a predominant role. Therefore, theelongation varies hardly owing to appearance of the same typecarbides when V content is 0.03% or 0.08% in the steel.

5. Conclusion

In this paper, four experimental high-strength steels withdifferent V contents were prepared. 3DAP and TEM were used toinvestigate the morphology, lattice type and compositions ofcarbides. The following conclusions were obtained:

(1) After addition of different V contents in the steel, the carbidesvary significantly. The competitive relationship between V andother alloying elements (Cr, Mo, Mn) results in the appearanceof M2C and M6C carbides after the addition of 0.03% or 0.08%Vin the steels, MC carbides appear as the V content is furtherincreased, while M23C6 mainly appear when there is no V inthe steel (M is any combination of Cr, Mo, Mn, Fe or V). And thesize of carbides generally decreases with increasing V content.

(2) The strength, impact properties and elongation significantlydepend on the content of V whose variation causes themodification of carbides. The increasing of V content is con-ducive to both strength and elongation greatly but has a

detrimental effect on impact toughness. Excellent combinationof mechanical properties can be obtained in the steel withabout 0.03%-V content.

Acknowledgment

This work was financially supported by the Major Researchplan of the National Natural Science Foundation of China (Grantno. 91226204).

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