Microstructure and mechanical properties of Cr14 ultra-high-strength steel at different tempering temperatures around 773 K

Materials Science & Engineering A 698 (2017) 152–161

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Microstructure and mechanical properties of Cr14 ultra-high-strength steel at different tempering temperatures around 773 K

MARK

⁎

Yangpeng Zhanga,b, Dongping Zhana, , Xiwei Qia, Zhouhua Jianga, Huishu Zhangc a b c

School of Metallurgy, Northeastern University, Shenyang 110819, China School of Materials Science and Engineering, Northeastern University, Shenyang 110819, China Metallurgical Engineering College, Liaoning Institute of Science and Technology, Benxi 117004, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Ultra-high-strength steel Tempering temperature Inclusions Microstructure Mechanical properties

This study investigated the effect of tempering temperature on the microstructure and mechanical properties of Cr14 ultra-high-strength steel. The steel was first normalized at 1223 K for 1 h, cold treated at 200 K for 1 h, and then tempered at five different temperatures between 723 K and 833 K for 4 h. The microstructures of the Cr14 steel were characterized by X-ray diffraction, optical microscopy, field emission scanning electron microscopy, and transmission electron microscopy. The tensile strength, impact toughness, and fracture toughness properties of Cr14 steel at different tempering temperatures were evaluated. Results show that inclusions in the steel mainly comprise MgO–Al2O3–SiO2 and a (Mo, W)C precipitated phase. With increasing tempering temperature, carbides became increasingly segregated and exhibited a chain-like distribution. In addition, the tensile strength increased and the yield strength, elongation, impact toughness, and fracture toughness decreased with increasing tempering temperature. For tempering temperatures in the range 753–773 K, the volume fraction of austenite decreased rapidly from 20% to approximately 12% while the mechanical properties changed rapidly as well.

1. Introduction Ultra-high-strength steel (UHSS) is a key structural material for aerospace technology and has become widely used after decades of development [1–10]. Several types of UHSS have been developed: 1) low-alloy UHSS, with alloy amounts of < 5%, such as 300 M alloy steel [2,5]; 2) maraging UHSS, such as 18Ni maraging steel [11]; 3) secondary-hardening UHSS, which usually has high Co and Ni mass fractions. A typical example of this last steel type is Aermet 100, which was originally developed by Carpenter Technology Corporation to meet the demanding specifications for the landing gear of U.S. Navy's carrierbased jet aircraft [1,4,6,7,12]. Our study is based on Cr14 steel, a new type of laboratory-prepared secondary-hardening UHSS, which has a higher Mo content compared to Aermet 100. During the tempering process, a large number of carbide particles precipitate in secondary-hardening UHSS, producing changes in the steel microstructure. Thus, the tempering process plays an important role in the manufacture process of secondary-hardening UHSS, and therefore, many studies have been conducted related to this topic [1,6,7,12–14]. Martensite formation and carbide precipitation are generally believed to have a strengthening effect in UHSS [1,6,7,14].

⁎

Corresponding author. E-mail address: [email protected] (D. Zhan).

http://dx.doi.org/10.1016/j.msea.2017.05.060 Received 28 February 2017; Received in revised form 28 April 2017; Accepted 16 May 2017 Available online 17 May 2017 0921-5093/ © 2017 Published by Elsevier B.V.

Most notably, the strength of Aermet 100 steel is very sensitive to the tempering temperature, especially in the vicinity of 755 K, because of the formation of M2C carbide [1,14]. In Aermet 100 steel, Cr2C forms as an early precipitation phase, which eventually changes to the Mo2C phase [7]. In addition, over-aging during the tempering process results in coarsening of the precipitates and produces fringes in M2C carbides [1,6], which reduces the steel strength. However, retained and reverted austenites are mainly responsible for toughening steel [14]. The reverted austenite forms at lathmartensite boundaries, which probably reduces local stress concentrations during deformation and produces greater plasticity. However, austenite is ductile and soft, which may prevent rapid propagation of cracks, resulting in increased fracture toughness [1]. Double tempering is therefore considered to be the optimal tempering process [12]. Previous studies on secondary-hardening UHSS mainly deal with Aermet 100 steel, with some dealing with A1410 steel [13], and most of these studies focus on the tempering process. Cr14 steel is a new secondary-hardening UHSS prepared in a laboratory process. Compared with Aermet 100 steel, Cr14 has a higher Mo content and can contain a large amount of precipitated carbide, which usually results in increased strength in UHSS. However, an appropriate tempering process must be

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test method,” the specimens were processed into V-notch Charpy impact specimens (notch depth=2 mm) with dimensions of 10 mm×10 mm×55 mm. The tensile properties of the specimens were measured using an electronic universal testing machine controlled by a SANS-CMT5105-type microcomputer. Tensile properties were measured at room temperature with a crosshead speed of 1 mm min−1 and an extensometer calibration interval of 25 mm. Impact tests were performed on the JBW-500 screen of an impact test machine. Specimens were also taken to evaluate the fracture toughness for each tempering temperature. Fracture toughness tests were performed using standard compact tension specimens with a size of 37.5 mm×36 mm ×15 mm. Fatigue cracks in the notches were first produced using an MTS810 fatigue testing machine. Then, the prepared specimens were stretched to the point of fracture, with the condition that the specified rate of increase of the stress intensity factor was within the range 0.55–2.75 MPa m0.5/s. To better observe microstructures using the optical microscope, the specimen was etched with a solution of 1.5 g CuCl233 ml HCl: 33 ml H2O at room temperature. Field emission scanning electron microscopy was used to observe the fracture morphologies of the specimens after being stretched and impacted. A JEM-2100F transmission electron microscope (TEM) was used to observe the specimen microstructures after chemically thinning the specimens in a dual submerged jet polisher using a solution of 5% perchloric acid and 95% acetic acid. To study the influence of different tempering temperatures on the microstructures of the experimental steel, the volume fraction of austenite after tempering was quantitatively analyzed by X-ray diffraction (XRD). XRD analysis was performed on an X′Pert Pro X-ray diffractometer using Cu radiation.

Table 1 Chemical components (in wt%) of the Cr14 steel investigated. C

Si

Cr

Co

Mo

P

S

O

Mg

0.11

0.21

14.5

12.1

4.69

< 0.005

0.0049

0.012

0.0006

used to enhance the mechanical properties, or coarsening of precipitates resulting from an inappropriate tempering process can produce simultaneous decreases in strength and toughness. Different tempering temperatures affect the microstructure and mechanical properties of Cr14 UHSS; this relationship is investigated in this paper. 2. Experimental procedures 2.1. Material preparation The Cr14 steel under investigation was melted at 1873 K with a 50 kg vacuum induction furnace. The chemical components of the steel are represented in Table 1. During the smelting process, a small amount of magnesium alloy was added to deoxidize the melt. The casting ingot (160 mm diameter) was forged into a 40 mm billet after heating at 1323 K for 1 h at a forging temperature between 1323 and 1223 K. To prevent cracking, the ingot was given a stress-reducing treatment of heating at 933 K for 20 h after forging. The ingot was then cut into different sizes for various types of characterization. 2.2. Heat treatment process To select the optimal temperature, quenching experiments were performed by heating samples at 1123 K, 1223 K, 1323 K, and 1423 K for one hour, followed by oil quenching. Cr14 is a type of martensite secondary-hardening steel, and as the Ms point is very low, the microstructure after quenching contains a large amount of retained austenite [15]. This requires the steel to undergo a deep cryogenic treatment (200 K, placed in a mixture of dry ice and alcohol for 1 h) to promote the transformation of austenite to martensite [1,4]. After cryogenic treatment, tempering temperatures of 723 K, 753 K, 773 K, 793 K, 813 K, and 833 K were selected. Because a large number of secondary phases precipitate in the vicinity of 755 K, which contribute to precipitation strengthening, temperatures around 755 K are usually considered to have the strongest influence on the mechanical properties of UHSS [1,4,16]. The specimens were held at temperature for 4 h and air-cooled during the tempering process.

3. Results and discussion 3.1. Inclusions Inclusions have an important influence on the mechanical properties of UHSS [13,18,19]. Table 2 shows the inclusion properties for Cr14 steel. The average inclusion size is 1.57 µm, with more than 50% of the inclusions under 1.5 µm. The number of inclusions per mm2 is 115. The main types of inclusions are MgO–Al2O3–SiO2 and MgO–Al2O3–SiO2–MnS, as shown in Figs. 1 and 2, and most are spherical in shape. In the early stage of Cr14 steel smelting, inclusions in molten steel mainly comprise Al2O3, which originates from both the pure iron and crucible. After the alloying element Si is added to the molten bath, a part of the Si will react with oxygen to generate SiO2. The subsequent addition of Mg is expected to produce the following modification reaction [20] because the reducibility of Mg is stronger than that of Si and Al:

2.3. Test methods A metallographic specimen named “E1” was cut with dimensions of 10 mm×10 mm×10 mm to perform an analysis of inclusions. A ZEISS optical microscope with IPP6.0 software was used to determine the number and size of inclusions. The morphology and components of the inclusions were examined using an X-550 scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). After the tempering process, specimens with a diameter of 5 mm and a gauge length of 25 mm were cut from along their axes for conducting tensile testing of metallic materials at room temperature. According to the GB/T 229-2007 [17] “metallic Charpy notch impact

2[Mg]+SiO2=[Si]+2MgO

(1)

3[Mg]+Al2O3=2[Al]+3MgO

(2)

Table 2 Size distribution of inclusions in the steel ingot. Distribution/%

E1

0.5–1.0 µm

1.0–1.5 µm

1.5–2.0 µm

2.0–3.0 µm

3.0–5.0 µm

17.78

33.48

24.30

21.78

2.67

153

Number of inclusions in per area/mm−2

Average diameter/µm

115

1.57

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Fig. 1. Typical morphology and components of a MgO–Al2O3–SiO2 inclusion.

Elem ment O Mgg All Sii S Mnn

WT% 48.485 4.061 30.589 3.537 4.605 8.723

AT% 63.671 3.511 23.818 2.646 3.018 3.336

Fig. 2. Typical morphology and components of a MgO–Al2O3–SiO2–MnS inclusion.

Fig. 3. Microstructures formed under different quenching temperatures observed at 500× magnification.

3.2. Microstructure 3.2.1. Effect of quenching temperature on microstructure The microstructures formed after different quenching temperatures are shown in Fig. 3. Grain size increased with increasing temperature, and the grain size at 1323 K is significantly larger than that at 1223 K, which is consistent with the results of the work by Zhang [21]. This is because the Ac3 temperture of 1306 K (calculated using Thermo-calc software) lies between 1323 K and 1223 K. Cr14 steel requires full austenitic quenching, and a temperature of 1323 K produces a fully austenitic experimental steel with a moderate grain size. Therefore, 1323 K is the optimal quenching temperature for Cr14 steel. 3.2.2. Effect of tempering temperature on microstructure The purpose of tempering after cold treatment is to obtain a stable tempered martensite and induce the retained austenite in the steel. During the tempering process, secondary phases are precipitated and the material properties improve [22,23]. Fig. 4 shows the austenite

Fig. 4. The volume fraction of austenite formed under different tempering temperatures.

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Fig. 5. Microstructures formed under two different tempering temperatures (753 K and 813 K).

Fig. 6. TEM images showing martensite formed at 753 K and 813 K: (a) bright field image of the 753 K specimen; (b) selected-area diffraction pattern of the 753 K specimen; (c) bright field image of the 813 K specimen; (d) selected area diffraction pattern of the 813 K specimen.

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Fig. 7. TEM images of two different microstructures in the 813 K specimen. Table 3 Results of mechanical properties tests. Temperture/K

Yield strength/MPa

Tensile strength/MPa

yield ratio/1

Elongation/%

Reduction in area/%

Inpact energy/J

Fracture toughness/MPa m0.5

723 753 773 793 813 833

1240 1200 1080 1070 965 1090

1620 1690 1800 1870 1890 1850

0.765 0.710 0.600 0.572 0.511 0.589

24.00 22.40 15.52 15.20 14.40 14.96

58.45 60.80 49.29 51.71 51.00 50.18

65 59 41 36 47 33

106 101 91.7 85 77.5 79.7

energy and dislocation density in the retained austenite is very high. When the quenched high-alloy steel is tempered, retained austenite will decompose into martensite during the temper cooling [15]. However, to reduce the lattice distortion, the enrichment of C and N at dislocations will promote the formation of Cottrell atmosphere. The Cottrell atmosphere acts as strong obstacles to the dislocation motion, which results in the limitation of martensitic transformation. But when the tempering temperature is high enough, C and N could release and the Cottrell atmosphere will reduce significantly, causing drastically decomposition of retained austenite [24,25]. Therefore, the retained austenite decomposition temperature in the experimental steel was

volume fraction in the steel determined by XRD after different tempering temperatures (723 K, 753 K, 773 K, 793 K, 813 K, and 833 K). The total austenite volume fraction contains the retained austenite formed after quenching and the reverted austenite formed after tempering. Using low temperatures for tempering produces less reverted austenite; thus, the main form of austenite is retained austenite. During hightemperature tempering processes, retained austenite is unstable and converts to martensite [15,23] so that the main austenite phase is reverted austenite. As seen in Fig. 4, the austenite volume fraction decreases gradually with increasing tempering temperature, and the proportion drops quickly below 773 K to 753 K. it is because the stored 156

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Fig. 5 shows the microstructures in Cr14 steel after tempering at 753 K and 813 K where the main phase is martensite. The 753 K specimen contains bulk retained austenite. Fig. 6 is a TEM image demonstrating that the microstructures are dominated by martensite in the two specimens, which is also clearly indicated by the selected area diffraction pattern. From Fig. 6, it can be seen that the martensite lath widths in the 813 K specimen are slightly larger compared to the 753 K specimen. This is because with the increase of tempering temperature, the recovery effect is strengthened. By moving, merging and recombination of the dislocations, the small angle grain boundaries between the martensite laths will disappear and the adjacent laths will merge together. Fig. 7 shows ferrite and martensite microstructures in the 813 K specimen. The lower part of this image shows a slab of martensite, while the upper part shows densely precipitated particles of ferrite, which can be more clearly seen in the magnified right-hand image. With increasing tempering temperature, a large amount of precipitation occurs in the carbide, which results in a reduction in carbon content in a part of the martensite and conversion to ferrite [1,6,23,27],and a decrease in the dislocation density. However, due to the low tempering temperature of 753 K, there is almost no precipitates in the image except several points we marked with circle. This finding is consistent with the results above.

Fig. 8. Tensile strength data under different tempering temperatures.

3.3. Mechanical properties Table 3 displays the mechanical property data obtained from tensile tests, impact experiments, and fracture toughness tests. 3.3.1. Tensile testing Tensile strength tests were performed in 6 groups according to different tempering temperatures. The results of tensile testing are shown in Table 3. Steel strengthening results mainly from the presence of martensite and second-phase particles. The martensite lath width is related to the strengthening effect, where smaller martensite laths produce a higher yield strength. The second-phase particles hinder the movement of dislocations in the steel, thereby increasing the steel strength. However, if the second-phase particles are too large and many in number, they not only cease to strengthen the steel but can also harm the performance of the steel. In addition to microscopic and secondphase particles, other factors influencing the strength are the degree of cleanliness and the grain size [13,18,19]. As seen from Fig. 8, with increasing temperature, tensile strength first increases and then decreases, yield strength first decreases and then increases, and both values reach an extreme value at 813 K. Although the yield strength and tensile strength are general positively correlated, the results of this experiment show a negative correlation. This is the combined result of the retained austenite transformation, martensite coarsening, the precipitation of carbide, and the transformation of lath martensite. The grain size has a strong influence on yield strength; however, the second-phase particles and retained austenite have a strong influence on the tensile strength [28–30]. When tempering at low temperatures, the volume fraction of austenite is higher and the number of second-phase particles is low; however, the martensite is narrow, resulting in low tensile strength and high yield strength. When the temperature increased, the yield strength decreased but tensile strength increased, the negative correlation results are caused by following aspects. Firstly, the retained austenite will transform to martensite when the temperature is high, which has a positive influence on both tensile strength and yield strength. Secondly, as the carbon atoms moving faster at higher temperature, a large number of precipitated phases precipitate in the mastensite and results in transformation of martensite to ferrite, which leads to the decrease of yield strength. Besides，these precipitated phases could improve the strain hardening capacity of the specimen, thereby increasing tensile strength. Thirdly, high-temperature tempering drastically reduced dislocation

Fig. 9. Impact energy values under different tempering temperatures.

Fig. 10. The relation between impact energy and volume of austenite.

between 753 K and 773 K and was very sensitive to the tempering temperature. The reverted austenite content is controlled by two factors: the amount of austenite transformation and its stability in the temper cooling process [26]. With the increase of tempering temperature, the transformated austenite will increase but its stability will decrease, so, in general, the reverted austenite firstly increases and then decreases after temper-cooling. When the temperature was lower than 753 K, the retained austenite amount was almost unchanged, but the reverted austenite content increased, resulting in the austenite proportion increased from 723 K to 753 K. 157

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Fig. 11. Topography of the impact fracture surface.

Fig. 12. Small particles in the dimples of 723 K and 753 K specimens (High magnification of the red frame area of Fig. 11). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

density and grain boundary energy [31], in addition, high temperature also leads to the coarsening of matensite lath and decrease of grain boundary area. All these changes will lead to the decrease of yield strength. In addition to the above three points, the internal stress produced in quenching process decreased with the increase of tempering temperature, which will lead to the decrease of yield strength. In our experiment, when the tempering temperature is higher than 753 K, the dislocation density decreases rapidly (as shown in Fig. 7) the lath size becomes large, which decreases the yield ratio rapidly as the martensite lath size has an important influence on the yield ratio [28,32]. The tempering-sensitive temperature range for tensile testing is also between 753 and 773 K, as shown in Fig. 8. 3.3.2. Impact toughness Impact toughness reflects the resistance of metal materials to external shock loads. Steel with a poor toughness has a low impact toughness value [8,33]. Fig. 9 shows the impact toughness values of the specimens. Impact toughness decreases with increasing tempering temperature, which is consistent with the change in the volume fraction of austenite in steel. The relationship between the volume fraction of

Fig. 13. Fracture toughness of Cr14 steel under different tempering temperature.

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(a)

(b)

(c)

(d)

Al-Mg-Si-O

Si-Al-Mn-Ti-O

(e)

(f)

(Mo, W)C

(Mo, W)C

Fig. 14. Results of fracture analysis by SEM and EDS: (a) image of the 753 K specimen at 200× magnification; (b) image of the 813 K specimen at 200× magnification; (c) image of inclusions in the 753 K specimen at 2000× magnification; (d) image of inclusions in the 813 K specimen at 2000× magnification; (e) SEM image of carbides in the 753 K specimen at 20,000× magnification; (f) SEM image of carbides in the 813 K specimen at 20,000× magnification.

3.3.3. Fracture toughness Fracture toughness represents the performance of a material in resisting crack propagation [34]. Fig. 13 shows the fracture toughness results for the test steel. With increasing tempering temperature, fracture toughness displays a decreasing trend, which is caused by a decrease in the amount of austenite. However, this trend is also due to an increase in tempering temperature, which results in martensite coarsening and extensive precipitation and aggregation [1,34,35]. Fracture analysis was performed on the 753 K and 813 K specimens. Fig. 14(a) and (b) show that the 753 K fracture surface is smooth, with a fine undulating texture, and the 813 K specimen is rough, with many convex features and a large number of internal cleavage fracture surfaces. From the fracture surface (Fig. 14(c) and (d)), it can be seen that the 753 K specimen contains many dimples with small particles,

austenite and impact toughness (Fig. 10) also confirms that these values are positively correlated: the higher the volume fraction of austenite, the higher the impact toughness. Fracture surface analysis by SEM (as shown in Fig. 11) indicates that at temperatures between 723 K and 753 K, the fracture surface containes many dimples, and small particles can be seen in the dimples (as shown in Fig. 12), suggesting that the fracture is a ductile fracture caused by coalescence of microvoids. There are also particles in the dimples of 773–833 K specimens, however, as shown in Fig. 11, the dimples area of these specimens decreased with the increase of tempering temperature, but the ratio of cleavage planes increases. i.e., the fracture mechanism of specimens is diverted from dimple ductile fracture to the manner of intercrystalline fracture with partial dimple fracture, and the change at 753–773 K is clear. 159

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Fig. 15. Images demonstrating precipitated phase occurrence in chains and cracks: (a) cracks in the 753 K specimen; (b) precipitated phase exhibiting chain shapes in the 753 K specimen; (c) cracks in the 813 K specimen; (d) precipitated phase with in chain shapes in the 813 K specimen.

mately 12%. 2. In the tempering temperature range of 723–833 K, tensile strength increases with increasing temperature, while the yield strength, elongation, impact toughness, and fracture toughness decrease. 3. In the studied specimens, the larger second-phase particles comprise MgO–Al2O3–SiO2 inclusions with smaller precipitated phases of (Mo, W)C. Precipitates are easily segregated, with the segregation phenomenon becoming more pronounced with increasing tempering temperature, exhibiting a chain-like distribution. The segregation has a strong influence on the fracture toughness.

indicating that the ductile fracture formed by the coalescence of microvoids and that the specimen has good fracture toughness. The fracture of 813 K specimen is mixed with tear ridges and dimples, which indicates a quasi-cleavage fracture characteristic and that the fracture toughness is lower than that for 753 K specimens. The secondphase particles analyzed by EDS were found to contain both inclusions and precipitates. These inclusions mainly comprise O–Mg–Al–Si and are large in size, approximately 2–5 µm, as shown in Fig. 14(c) and (d). This result is similar to that of the previously analyzed inclusions. As shown in Fig. 14(e) and (f), the precipitated phase in the two specimens is (W, Mo)C, and the precipitated phase in the 813 K specimen is clearly aggregated, with an average size (approximately 500 nm) larger than that of the 753 K specimen (approximately 300 nm). In addition, the precipitated phase in the 813 K specimen is much more abundant than that in the 753 K specimen. Aggregation of the precipitated phases does not strengthen the steel but reduces both the strength and toughness of steel. Precipitated phases occurring in chain shapes were observed in both the 753 K and 813 K specimens, as shown in Fig. 15(b) and (d). At the beginning of fracture toughness tests, microcracks form first in the precipitates and matrix boundaries, and cracks rapidly growth along the orientatin direction of the precipitated phases. Lastly, cracks appeared in the shape of gullies, as shown in Fig. 15(a), Fig. 15(c), and by the arrows in Fig. 14(b).

Acknowledgements The authors would like to thank National Natural Science Foundation of China (51574063), Fundamental Research Funds for the Central Universities (N150204012 and N152306001) and Program for Liaoning Excellent Talents in University (LJQ2015056). The authors would like to thank Enago (www.enago.cn) for the English language review. References [1] X.H. Shi, W.D. Zeng, Q.Y. Zhao, W.W. Peng, C. Kang, Study on the microstructure and mechanical properties of Aermet 100 steel at the tempering temperature around 482 °C, J. Alloy. Compd. 679 (2016) 184–190. [2] A.H. Meysami, R. Ghasemzadeh, S.H. Seyedein, M.R. Aboutalebi, An investigation on the microstructure and mechanical properties of direct-quenched and tempered AISI 4140 steel, Mater. Des. 31 (2010) 1570–1575. [3] M.H. Khani Sanij, S.S. Ghasemi Banadkouki, A.R. Mashreghi, M. Moshrefifar, The effect of single and double quenching and tempering heat treatments on the microstructure and mechanical properties of AISI 4140 steel, Mater. Des. 42 (2012) 339–346. [4] C. Hao, L. Dong, H.B. Tang, S.Q. Zhang, X.Z. Ran, H.M. Wang, Effect of hot isostatic pressing on fatigue properties of laser melting deposited aermet100 steel, J. Iron

4. Conclusions 1. Temperatures in the range 753–773 K are the most influential for Cr14 steel during the tempering process, and the steel structure and performance change rapidly in this range. The volume fraction of austenite at 753–773 K decreases rapidly from 20% to approxi160

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[22]

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[31]

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