Effect of Deep Cryogenic Processing on Tensile ... - Springer Link

International Journal of Steel Structures September 2014, Vol 14, No 3, 571-578 DOI 10.1007/s13296-014-3014-9

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Effect of Deep Cryogenic Processing on Tensile Toughness of 45WCrV7 Steel Seyed Ebrahim Vahdat* Department of Engineering, Ayatollah Amoli Branch, Islamic Azad University, Amol, Iran

Abstract Toughness is an important property for being used in steels in engineering applications. In this research, tensile toughness of 45WCrV7 steel was measured and calculated in 10 different processing conditions. The results of tensile test showed that two samples had maximum tensile toughness. Microstructural studies demonstrated that the required condition for high tensile toughness was simultaneous increase in two microstructural factors named content and population density of the secondary carbides because a matrix which was poor of carbon and alloying elements was softened and thus increased total tensile toughness. Keywords: tensile toughness, SEM, STEM, XRD

1. Introduction Suitable tool steel properties include high strength, wear resistance and hardness in addition to suitable toughness for enduring mechanical impact loadings. On the other hand, hardness and toughness have an opposing relationship so that any increase in strength or wear resistance of a tool steel occurs at the expense of reduced toughness. For example, DCP improves wear resistance of tool steels; but, for cold work Cr8Mo2SiV and Cr12MoV tool steel specimens, researchers (Li et al., 2010) reported that toughness was reduced to one-third by DCP compared to the standard specimens. Also, researchers (Das and Ray 2012) reported that, fracture toughness of DCP specimens was slightly lower to almost seven percent when compared to that of conventionally treated AISI D2 tool steel. In addition, for cold work die steel SDC99, researchers (Li et al., 2013) stated that toughness decreased by DCP compared with the conventional processing. In contrast, researchers (Chi et al., 2010) reported that, when quenching at lower austenitizing temperature, higher toughness could be obtained for Cr8-type cold work die steel by DCP compared with the conventional processing. Also, Note.-Discussion open until February 1, 2015. This manuscript for this paper was submitted for review and possible publication on February 21, 2014; approved on June 3, 2014. © KSSC and Springer 2014 *Corresponding author Tel: +98-911-121-4008 E-mail: [email protected]

researchers (Koneshlou et al., 2011) demonstrated that tempering after and/or before of DCP improved 20% of impact toughness of H13 hot work tool steel with respect to conventional processing. In addition, researchers (Vahdat et al., 2013; 2014) reported 12-35% improvement in tensile toughness for 45WCrV7 cold work die steel by DCP compared with the conventional processing. Therefore, the results of researchers (Li et al., 2010; Das and Ray 2012; Li et al., 2013) were in opposition to other researchers (Chi et al., 2010; Koneshlou et al., 2011; Vahdat et al., 2013) on toughness enhancement and there is no any clear reason for that. The goal of this work was to clear how microstructure parameters effect on tensile toughness of 45WCrV7 medium carbon-low alloy tool steel.

2. Material and Methods Table 1 presents chemical analysis of 45WCrV7 steel used in this study. The temperature-time history for the DCP treated specimens is depicted in Fig. 1(a). The DCP work followed a processing outlined in Fig. 1(b) using a programmable cryogenic processor (Farhani and Niaki, 2011; Farhani et al., 2012). The specimens had to be machined to the required sizes prior to their processing. Preparation of the specimens was carried out using a CNC machine. SEM TESCAN MIRA II machine (SEM, 2011) used at Department of Mechanical Engineering, Iranian Research Organization for Science and Technology, Tehran, Iran. The SEM specimens were of 15 mm length and 12 mm

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Table 1. Chemical analysis of the 45WCrV7 steel Content

C

Mn

Cr

S

V

P

W

Si

Fe

Wt%

0.48

0.34

1.12

0.02

0.02

0.06

1.57

1.00

Rest

Figure 1(c). Standard specimen at room temperature according to BS EN 10002-1.

Figure 1(a). Cryogenic processing cycle.

diameter. To calculate each phase size and content, at least five SEM images were taken from five different scanned regions using a ×104 magnifications. The reported values were average ones. Size and content of SC and PC were measured using phase analysis software, OLYSIA m3. It was calibrated for images with 2048×1536 pixels. JEOL ARM 200 atomic resolution microscope operated at 200 kV AppFive “Topspin” precession diffraction analysis machine (STEM, 2013) used for finding very fine retained austenite. STEM machine used at LeRoy Eyring Center for Solid State Science, John Cowley Center for

HREM, Arizona State University, USA. Theoretical patterns calculated for the phases listed in the ASTM E975-13 spec (E975, 2013). The precipitation of SC initiates during cryogenic soaking itself, rather it takes place during tempering processing. SCs grow during tempering. Carbide which is not dissolved after austenitization processing is termed as PC. PCs can precipitate during solidification. The specimens were given codes for easy identification during and after the experiment. The coding procedure described in Table 2. (a) Water quenched specimens: The first two digits of the specimen code for a specimen indicate the soaking time (in hours) at −196oC. The last digit indicates the tempering time (in hours) at 200oC. Therefore, specimen code 361, indicates a specimen: soaked at −196oC for 36 hours and tempered at 200oC for 1 hour. (b) Water quenched and tempered specimens (no DCT): Specimen 002 has been tempered at 200oC for 2 hours, but no DCT has been performed.

Figure 1(b). Cryogenic processing flow chart.

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Effect of Deep Cryogenic Processing on Tensile Toughness of 45WCrV7 Steel

Table 2. Codes for processing of the 45WCrV7 tool steel Specimen code

Cryogenic procedure

002 241 242 243 361 362 363 481 482 483

Quenching procedure

Cryogenic Soaking soaking time temperature (hours)

Cooling -

-

Tempering temperature

Tempering time (hours)

Final cooling

200±5oC (rate~3oC/min)

2 1 2 3 1 2 3 1 2 3

Cooled in room to ambient temperature

24

heated to 900±5oC (rate~ 5.5oC/min)

soaked at 900oC for 60±5 min

-150±4oC (rate~ 1.3oC/min)

quenched in water

-196oC

36

48

Mechanical testing machine used at Razi lab center, Tehran, Iran. The uniaxial force-displacement tensile test (F-∆L) was carried out on a standard specimen, which was 5 mm in diameter and 25 mm parallel length, at room temperature according to BS EN 10002-1. Specimen dimensions illustrated at Fig. 1(c). The strain rate was 0.00166 S−1. Tensile toughness was calculated as shown in Eq. (1) (Dieter, 2000) where UT is tensile toughness (MJ), σUTS is ultimate tensile strength (MPa) and ef is fracture strain in unit volume (m3). The results listed in Table 3. UT=2/3×σUTS×ef

(1)

3. Results and Discussion The austenite to martensite transformation (Ms) started at 274.63oC and finished (Mf) at 59.63oC. Ms and Mf values were obtained using Payson and Savage’ Eqs. (2) and (3) (Vermeulen et al., 1996), which are valid for 45WCrV7 tool steel according to chemical composition.

Ms (oC)=498.9−(316.7×C%)−(33.3×Mn%) −(27.8×Cr%)−(16.7×Ni%)−11.1×(Si+Mo+W)

(2)

Mf(oC) = Ms(oC)−215

(3)

o

Therefore, at −196 C, the retained austenite is completely transformed into martensite. This confirms the results from Cu Kα radiation diffraction phase analysis (XRD) Philips Electronic Instruments, Inc. at 40 kV. XRD analysis machine used at Faculty of Metallurgy and Materials Engineering, Tehran University, Tehran, Iran. As shown in Fig. 2 for the 002, 243 and 483 specimens, the retained austenite was not observed. After tempering of specimens, there would be no martensite and almost would be turned to ferrite and carbides. While; Fig. 3(a) shows definite signs of very fine retained austenite (0.23 vol%) by using precession diffraction analysis of 97000 electron diffraction patterns. The analysis region was ≈850000 nm2 and the retained austenite surface (Fig. 3(a)) was almost 2000 nm2. Therefore the content of retained austenite was ≈2000 nm2/≈850000 nm2×100=0.23 vol%. In Fig. 2, intensity of peak 1 (≈320) in specimen 002

Table 3. Results of tensile test Specimen code

Yield strength (MPa)

UTS (MPa)

Elongation (%)

Bulk Hardness (RC)

Tensile Toughness (MJ)±0.65 UT=2×σuts×ef/3

002 241 242 243 361 362 363 481 482 483

2006±50 2007±15 2019±23 1869±49 1990±50 1945±50 2003±50 1972±44 1943±50 1996±18

2229±65 2279±21 2265±31 2137±53 2268±65 2201±65 2245±65 2244±64 2206±65 2249±28

5.50±1.50 4.75±0.75 2.00±1.00 6.00±1.50 7.00±1.50 5.00±1.50 5.00±1.50 6.20±0.30 7.50±1.50 6.20±0.80

56.50±0.60 56.00±0.30 56.00±0.50 57.90±0.50 54.60±0.50 56.00±0.50 54.50±0.50 56.30±0.60 56.00±0.40 55.00±0.50

81.70 72.20 30.20 85.50 105.90 73.40 74.80 92.80 110.30 93.00

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Figure 2(a). Diffraction of Cu Kα ray after processing for 002 specimen.

Figure 2(b). Diffraction of Cu Kα ray after processing for 243 specimen.

Figure 2(c). Diffraction of Cu Kα ray after processing for 483 specimen.

which was water quenched and tempered at 200oC much higher than intensity of peak 1 (≈235) in specimen 243 and 483 which was water quenched and then deep cryogenic treated and after that tempered at 200oC. On the other hand, intensity of peak 3 in specimens 002 and 243 was equal to 70; i.e. content of tempered martensite (ferrite, M7C3 and M23C6 carbides) was equal in these two specimens. In contrast, intensity of peak 3 in specimen 002 (≈70) was less than that of peak 3 in specimen 483 (≈80). This issue indicated higher content of tempered martensite (ferrite, M7C3 and M23C6 carbides) in the 483 specimen compared to the 002 specimen. Peak 4 belonged to

tempered martensite (ferrite and M7C3 carbides). Intensity of this peak (≈18 to ≈20) was almost equal in all the three specimens. In other words, content of tempered martensite (ferrite and M7C3 carbides) was almost the same in all the three specimens. Figure 3(a) shows very fine blade shape (max 130 nm in length and max 10 nm in width) of retained austenite. As shown in Fig. 3(b), averages of Vf, size and PD of the PC were approximately constant at about 0.4±0.2 V%, 0.62±0.1 µm and 62500±2500 mm−2, respectively, which was because specification of PC depended on the austenitization processing parameters which were kept

Effect of Deep Cryogenic Processing on Tensile Toughness of 45WCrV7 Steel

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Figure 3(a). Very fine blade shape of retained austenite (green) for specimen of 483.

Figure 3(b). Size, PD and Vf of PC, tempering time trends.

constant (Fig. 1(b)) in this research. Figure 3(c) shows spherical shape carbides with maximum diameter of 1 micron. Also, SCs are dispersed as seen in this low magnification micrograph by using STEM. Figure 4 shows that average Vf of the SC increased from 2.18 to 12.87 V% and the maximum PD of the SC occurred in specimens with 36 hours of soaking time (specimen 361). Accordingly, specimens with 1 hour of tempering time (specimens 241, 361 and 481) showed maximum PD of the SC. The average size of the SC increased from 0.22 to 0.46 µm (≈twice). As shown in Fig. 4, Vf of the SCs in specimens 002 (1.8%) was smaller than (≈3 to 7 times) those in specimens 361 and 482 (4.6% and 12.6%, respectively); also, PD of the SCs in specimens 002 (160000 mm−2) was smaller than (≈3 to 5 times) that in specimens 361 and 482 (894000 and 650000 mm−2, respectively). As demonstrated in Fig. 5, the specimens which had high PD and high Vf of SCs had high tensile toughness as well (the specimens 361 and 482). But, its reverse form was not correct; i.e., (the specimens 362 and 363). Therefore in these specimens, high PD and high Vf of SCs were two effective parameters for tensile toughness enhancement. Figure 5 presents about 23-26% improvement in tensile toughness for specimens 361 and 482 compared to specimen 002. This improvement in properties was contributed by debonding toughening mechanism of SC (Das and Ray

2012; Vahdat et al., 2013). But debonding mechanism of spherical SC particles was negligible in tensile toughness increase (from 0.003 to 0.007%) (Shen et al., 2000; Chawla and Allison, 2001; Chawla, 2005; Vahdat et al., 2014). According to Fig. 4, with increasing time of tempering or soaking, the amount of SC constantly increased. Thus, the matrix metal around the carbide had poor carbon and alloying elements. Accordingly, the matrix which was poor of carbon and alloying elements could have an effective role in increasing tensile toughness. The matrix was poor of carbon and alloying elements because: 100 g of the studied steel was almost 12.66 cm3 in volume; in this volume, there were about 1.1 g chrome atom (or 0.021 mol chrome atom) and 1.57 g tungsten atom (or 0.009 mol tungsten atom) because: 100 g/(ρalloy =7.9 g/cm3)≈12.66 cm3 1.12 g/(MCr =52 g/mol)≈0.021 mol Cr 1.57 g/(MW =183.85 g/mol)≈0.009 mol W Content of PC particles is much less than that of SC particles; therefore, it is neglected in calculations (totally 2%). For specimen 361, PD of SC particles is 894000 with size of 0.3 micron. Size of each SC cell is 1.09 nm. Thus, the number of SC cells is equal to 1.239×1020 per 12.66 cm3 because:

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Figure 3(c). Morphology and size of SCs (grey) for specimen of 483.

Figure 4. Size, PD and Vf of SC, soaking time trends.

PD≈ 894000mm−2 = 894000 mm−1 = 894000 ×10 cm−1 =( 894000 ×10)3cm−3 =0.84×1012 cm−3 Number of particles in 12.66 cm3=0.84×1012 ×12.66 =10.73×1012 Number of M23C6 cells for 0.3 µ Diameter in 12.66 cm3 =(4π×0.153/3×10−18)/(1.093×10−27)×10.73×1012=1.2×1020 Accordingly, for specimen 361, maximum sum of the

number of chrome and tungsten atoms which participate in the formation of SC particles is almost equal to 0.27×1022 because: Maximum Number of W and Cr atoms in M23C6 ≈23×1.2×1020 Maximum Number of W and Cr atoms in M23C6 ≈0.27×1022

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Figure 5. Tensile toughness of specimens, PD and Vf of SCs, soaking time trends.

While total number of tungsten and chrome atoms in 12.66 cm3 of the studied alloy (i.e. 100 g) is equal to 1.87×1022 because: Total Number of W and Cr atoms =0.021 mol+0.009 mol=0.03 mol≈1.87×1022 Therefore, in these samples, almost 14.73% of the tungsten and chrome atoms are consumed for the formation of carbide and the rest is placed in the matrix (as well as 3.08% for Carbon atoms). Thus, the matrix becomes soft and total tensile toughness is generally improved. It was calculated for other specimens and illustrated in Fig. 6. In Fig. 6, with increasing deep cryogenic or tempering times, the amount of tungsten and chromium atoms that contribute to the formation of SC is higher. Also, in Fig. 4, with increasing tempering or deep cryogenic times, the amount of SC is steadily increasing. As a result, the amount of tungsten and chromium atoms that contribute to the formation of SC is greater. Figure 6 shows the matrix of specimen of 483 is softer than the matrix of specimen of 482 however the tensile toughness of specimen of 483 is less than the tensile toughness of specimen of 482 (Fig. 5). While; Fig. 4 shows PD of SC of specimen of 482 was higher than specimen of 483. Therefore sub-micron SCs dispersed in the soft matrix is responsible for this improvement.

4. Conclusions In this research, ten sets of 45WCrV7 steel specimens were deep cryogenic processed at −196oC for 24, 36 and 48 hours and were tempered at 200oC for 1, 2 and 3 hours. Tensile toughness increased 23-26% for 361 and 482 specimens compared to specimen of 002. This improvement in properties was contributed by softening of matrix. This soft matrix strengthens by high PD of submicron sized carbides.

Acronyms Deep Cryogenic Processing: DCP PC: Primary Carbide PD: Population Density SC: Secondary Carbide SEM: Scanning Electron Microscopy STEM: Scanning Transmission Electron Microscopy Vf: Volume Fraction XRD: X-ray diffraction

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Number of ( W + Cr ) in carbide- × 100 ------------------------------------------------------------------------Number of ( W + Cr ) in alloy

Figure 6. The number of W and Cr atoms contributed for carbide formation.

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