S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL 385–391 MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL MIKROSTRUKTURA IN RAST UTRUJENOSTNE RAZPOKE V POBOLJ[ANEM 23CrNiMoV JEKLU Shuaishuai Zhu 1,2 , Baosen Zhang 1,2 , Xiangyang Mao 1,2 , Zhixin Ba 1,2 , Yuming Dai 1,2* , Zhangzhong Wang 1,2 1 School of Materials Science and Engineering, Nanjing Institute of Technology, Nanjing 21167, China 2 Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing 211167, China Prejem rokopisa – received: 2019-09-05; sprejem za objavo – accepted for publication: 2019-12-23 doi:10.17222/mit.2019.210 The microstructure, mechanical properties and fatigue-crack growth of the heavy-forging steel 23CrNiMoV with quenching and tempering were systematically studied. Microstructural characterization involved an optical microscope, field-emission scanning electron microscope and a transmission electron microscope. The fatigue-crack growth (FCG) behavior of the steel tempered at 600 °C was investigated at various stress ratios. The microstructure of the 23CrNiMoV tempered at 600 °C was composed of tempered martensite and considerable M7C3-type acicular carbides and M23C6-type spheroidal carbides, which contributed to the excellent strengthening/toughness match of the steel. The high ultimate tensile strength (UTS), yield strength (YS), elongation and impact energy of the steel tempered at 600 °C were 1155 MPa, 1091 MPa, 15.4 % and 102.9 J, respectively. The FCG threshold ( Kth) of the steel decreased with the increasing R. The fatigue fracture was composed of tearing ridges and secondary cracks. Besides, the secondary cracks were easily caused by the spheroidal precipitates during cyclic deformation surrounding the fatigue crack tip, which may enhance the FCG rate. Keywords: 23CrNiMoV steel, microstructure, precipitates, fatigue-crack growth Avtorji ~lanka so sistemati~no {tudirali mikrostrukturo, mehanske lastnosti in rast utrujenostne razpoke s kovanjem mo~no deformiranega in pobolj{anega (kaljenega in popu{~enega) jekla 23CrNiMoV. Mikrostrukturno karakterizacijo jekla so izvedli z opti~nim (LM), vrsti~nim elektronskim mikroskopom na emisijo polja (FE/SEM) in z presevnim elektronskim mikroskopom (TEM). Rast utrujenostne razpoke (FCG; angl.: fatigue crack growth) so opazovali v jeklu, popu{~enem pri 600 °C, in pri razli~nih napetostnih razmerjih R. Mikrostruktura pobolj{anega jekla 23CrNiMoV je bila sestavljena iz popu{~enega martenzita, znatno koli~ino igli~astih karbidov tipa M7C3 in kroglastih karbidov tipa M23C6, ki prispevajo k odli~nemu utrjevanju jekla in pove~anju njegove `ilavosti. Jeklo popu{~eno pri 600 °C je imelo natezno trdnost (UTS) 1155 MPa, mejo te~enja (YS) 1091 MPa, raztezek 15,4 % in udarno `ilavost 102,9 J. Mejna napetost rasti utrujenostne razpoke ( Kth) je pri preiskovanem jeklu padala z nara{~ajo~im napetostnim razmerjem. Na prelomu jekla nastalega zaradi njegovega utrujanja so bile vidne brazde in sekundarne razpoke. Poleg tega so krogli~ni izlo~ki pospe{evali nastanek sekundarnih razpok med cikli~no deformacijo v okolici konice razpoke, kar lahko {e dodatno pospe{uje hitrost rasti utrujenostne razpoke. Klju~ne besede: 23CrNiMoV jeklo, mikrostruktura, izlo~ki, rast utrujenostne razpoke 1 INTRODUCTION The low- and medium-carbon CrNiMoV steels are widely used as heavy-forging materials, such as for the brake discs and axles on high-speed trains, low-pressure rotors and pressure vessels, 1–4 because of the good formability, high hardenability, ultra-high strength and fatigue strength, high creep resistance and heat-shock resistance. Some of the reasons for this are that the alloyed elements (Cr, Ni, Mo, V) promote the precipi- tation of alloy carbides, which mainly precipitated on the austenite/ferrite interface or in supersaturated austenite and ferrite during the thermo-mechanical treatments and heat treatments. 5–7 Many studies have focused on the evolution of carbides during the process of hot forging or quenching and tempering. The precipitation behavior of the carbides can be controlled by thermo-mechanical processes. 8,9 The alloy carbides of the interphase precipi- tation are arranged periodically with a low density of dislocations, and the average diameters of the carbides are a few nanometers to several tens of nanometers. 10,11 The research of Ebrahimi et al. 12 on 26NiCrMoV steel indicated that the dynamic precipitation of carbide part- icles could effectively postpone dynamic recrystalli- zation, and the dynamic precipitation occurred at tempe- ratures below 1000 °C and strain rates of 0.001–1 s –1 . Wang et al 13 reported that hot-deformation promotes the diffusion of the C in micro-alloyed steels and the for- mation of coarse (Ti, Mo)C precipitates. On the other hand, the nano-carbides should be precipitated during the high-temperature tempering of martensite or bainite. Those carbides are finely dispersed in the martensitic lath or bainitic ferrites with a high density of dislocations. 14–16 Moreover, many researches show that the dispersed nano-carbides in alloyed steels bring high strength and excellent plasticity. Li et al. 17 studied the effect of microstructure evolution on the Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 385 UDK 67.017:621.7.019.1:669.14.018.298 ISSN 1580-2949 Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 54(3)385(2020) *Corresponding author's e-mail: y.m.dai@hotmail.com (Yuming Dai) strength and impact toughness of G18CrMo2-6 steel. The study found that the precipitation and refinement of MC, M 3 C increase the strength and toughness, but the precipitation and coarsening of M 23 C 6 at the ferrite grain boundaries decreases the impact toughness sharply. The yield ratio of heat-treated Mn-Ni-V steels became high (from 0.7 to 0.9) compared to unalloyed steel on account of the strengthening of vanadium precipitates during the quench and temper process. 18 Meanwhile, many inves- tigators have paid more attention to the fatigue behaviour of the precipitation strengthening alloy. Both the fatigue strength and fatigue strength ratio of the micro-alloyed steel increase with the fine precipitation of MC car- bides. 19 The appropriate controlling of the carbides pre- cipitation is a promising way to enhance the fatigue per- formance. 20 The fatigue-crack propagation rate is signifi- cantly influenced by the matrix precipitate characteristics due to a smaller cyclic plastic zone of the alloy. 21,22 The aim of the present paper is to study the precipi- tation strengthening of the heavy-forging steel 23NiCrMoV with quenching and tempering. The micro- structure and mechanical properties of the 23NiCrMoV steel with different tempering temperatures were evaluated. Meanwhile, the fatigue-crack propagations of the 23NiCrMoV steel with high strength and toughness were investigated. 2 EXPERIMENTAL PART The material investigated was a 23CrNiMoV steel for high-speed trains’ wheel-mounted brake discs forged at 1200 °C. The chemical composition of the steel is listed in Table 1. The samples were cut out with a size of 100 mm × 50 mm × 30 mm from the hot-forged brake disc. The preparatory heat treatment process was nor- malised at 900 °C + isothermal annealed at 600 °C, which was used to eliminate the coarse and mixed crystals structure produced by the forging process. The samples had a mixed microstructure of ferrite and pearlite with a grain size of approximately 25–35 μm after the preparatory heat treatment. The pre-treated samples were austenitized at 900 °C for 1 h followed by a cooling in iced water. Then, those samples were tem- pered at 550 °C, 600 °C and 650 °C for 2 h. The heat- treatment processes were carried out in a box resistance furnace. Heat-treated samples were mechanically polished using standard metallographic procedures and etched with 4 % nital for examination of the micro- structure. The microstructure of the specimen was observed by optical microscopy (OM) and scanning elec- tron microscopy (SEM). The transmission electron microscope (TEM) was used to analyse the morphology and structure of the precipitations. Mechanical properties were determined through hardness, tensile and impact tests at room temperature. Tensile specimens were machined to a length of 90 mm and a diameter of 6 mm according to ASTM: E8 specification. The tensile tests were conducted at room temperature using a Sans CMT5105 materials testing system with a cross-head speed of 2 mm min –1 . The Charpy V-notched impact tests were performed on a JBW-300H impact-test machine according to the ASTM E23 standard, using (10 × 10 × 55) mm specimens. The fatigue-crack growth (FCG) tests of the 23NiCrMoV steel were conducted in an Amsler HFP5100 fatigue test machine with the SENB3 specimens according to the ISO 12108 standard. Figure 1 shows the adopted dimen- sions for the SENB3 specimens. The load-shedding technique was used to obtain the K th value by de- creasing the load steps at selected crack-size intervals using the following equation C K K a = ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ ⋅ ⎛ ⎝ ⎜ ⎞ ⎠ ⎟ 1d d (1) where C is the normalized K-gradient and a is the crack length. During K-decreasing process, the C value was controlled between –0.05 mm –1 and –0.15 mm –1 . The fatigue-crack length was automatically measured using a CCD camera and an image-processing system. 3 RESULTS AND DISCUSSION 3.1 Microstructures The microstructures corresponding to the different tempering temperatures of the 23CrNiMoV steels are shown in Figure 2. The microstructures of the tempered steels consist of tempered martensite. The tempered martensites have a significant influence on the strength and toughness of the CrNiMoV steels. Figure 2a indi- cates that the microstructure consists of nearly all martensitic lath after tempering at 550 °C. Figure 2b and Figure 2c show that the size of the tempered martensitic lath decreased with the tempering temperature for the decomposition of the carbides. The fine martensitic should promote the increase of the toughness 23 . Figure 3 shows the SEM micrograph of the as-quenched 23CrNiMoV steels tempered at 550 °C, 600 °C and 650 °C for 2h. The microstructures of the S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL 386 Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 Table 1: Chemical composition of 23CrNiMoV alloyed steel (mass fraction, w/%) C Si Mn Cr Ni Mo V Cu Al Fe 0.21 0.442 1.09 0.914 1.03 0.645 0.07 0.082 0.029 Bal. Figure 1: Schematic diagram of the SENB3 specimen dimensions (mm) tempered martensites consist of lath ferrites and precipi- tated carbides. The carbide precipitates have a plate-like structure at 550 °C, and the precipitates are mainly acicular carbides and spheroidal carbides at 600 °C and 650 °C. The evolution of the carbide morphology corres- ponds to the change of tempered martensite morphology shown in Figure 2. The acicular carbides precipitated mainly in the martensite lath, the spheroidal carbides precipitated mainly at the inter-plate boundaries and the prior austenite grain boundaries. According to the com- parison among Figure 3a to 3c, the acicular carbides mainly precipitated during the tempering of 600 °C. The acicular carbides could transform into the spheroidal carbides as the tempering temperature increased to 650 °C, which was in line with the values reported previously for similar CrNiMoV alloyed steels. 24,25 The size and amounts of the precipitates increased with the increasing tempering temperature. The polygon ferrites were observed in Figure 3c, which indicated that more martensite laths were decomposed with tempering at 650 °C. Figure 4 shows the TEM micrograph and the corresponding selected-area electron-diffraction (SAED) pattern of the steel tempered at 600 °C. As shown in Figue 4, the acicular carbides dispersed and precipitated in the matrix with the size of 35 nm × 200 nm. Those acicular carbides enhanced the strength and toughness S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 387 Figure 3: SEM micrograph of the as-quenched 23CrNiMoV steels tempered at: a) 550 °C, b) 600 °C, c) 650 °C Figure 2: OM micrograph of the as-quenched 23CrNiMoV steels after tempered at: a) 550 °C, b) 600 °C, c) 650 °C due to the precipitation strengthening, the spheroidiza- tion and refinement of the carbides during the tempering process. The chains of the precipitation consist of spheroidal carbides precipitated at the phase and grain boundary with a size of about 150 nm. Those precipi- tation chains reduced the interface bonding strength which were harmful to the ductility and toughness. The SAED patterns (the inserts in Figure 4) indicate that the acicular precipitates were M 7 C 3 -type carbides, and the spheroidal precipitates were M 23 C 6 -type carbides. 18 In addition, the dislocation density in the martensite lath region was high, which should have a beneficial effect on the strength. 3.2 Mechanical Properties Figure 5 shows the typical engineering stress-strain curves of the specimens tested at room temperature. The curves indicate that the ultimate tensile strength (UTS), yield strength (YS) of the steel decreased with the increasing tempering temperatures. The measured me- chanical properties including yield strength, tensile strength, total elongation, impact energy and hardness of steels are summarized in Table 2. The high UTS, YS and hardness of the steel tempered at 550 °C were 1251 MPa, 1176 MPa and 376.7 HV, respectively. The UTS, YS and hardness of the steel tempering at 600 °C were only 7.6 %, 7.2 % and 7.0 % lower than that of the steel tem- pered at 550 °C. Furthermore, the elongation and impact energy of the steel tempered at 600 °C were 15.4 % and 102.9 J, which were 7.7 % and 67.0 % higher than that of the steel temperied at 550 °C. Figure 6 is the Charpy impact test fracture surface of the steel tempered at 550 °C, 600 °C and 650 °C. Figure 6a indicates that the steel tempered at 550 °C exhibits representative cleavage crack, and a very small plastic region has been found in this region. Figure 6b shows that the steel tempered at 600 °C displays the feature of the cleavage contained S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL 388 Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 Figure 5: Experimental engineering stress-strain curves after different tempering temperatures Figure 4: TEM micrograph of the as-quenched 23CrNiMoV steel after tempering at 600 °C Figure 6: Charpy impact test fracture surface of the steel tempered at: a) 550 °C, b) 600 °C, c) 650 °C some toughness region and equiaxed dimples (shown with arrows). Figure 6c indicates that the fracture of the steel tempered at 650 °C has a significant plastic zone and a large number of equiaxed dimples. The high strength and toughness of the steel tempered at 600 °C were caused by the mixed microstructure of tempered martensites and the secondary hardening of the acicular M 7 C 3 -type carbides. 26,27 For the recovery and decom- position of the martensies and the growth of the carbides tempering at 650 °C, the tensile strength (UTS and YS) and hardness of the steel were relatively low, but, the ductility and toughness were high. 28 Table 2: Determined mechanical properties at different tempering temperatures Tempering temperature 550 °C 600 °C 650 °C UTS / MPa 1251 1155 924 YS / MPa 1176 1091 844 Elongation / % 14.3 15.4 19.1 Impact energy / J 61.6 102.9 172.8 Hardness / HV 377 350 294 3.3 Fatigue-Crack Propagation Behavior Figure 7 shows the curves of the fatigue-crack growth (FCG) rate versus the applied stress-intensity factor range (da/dN- K curves) of the as-quenched 23CrNiMoV steel tempered at 600 °C at stress ratio R = 0.1, 0.25 and 0.5. The fatigue crack growth threshold ( K th ) decreased with the increase of R. The way to determine the threshold was using a straight line fitted to a minimum of five log (da/dN) versus log ( K) data pairs between 10 –6 mm/cycle and 10 –7 mm/cycle. The K th can be calculated by the Paris-Erdogan equation 29 : d d a N CK n = 1 1 Δ (2) where C 1 and n 1 are the intercept and slope of the fitting line, respectively. The value of K corresponding to a crack growth rate equal to 10 –7 mm/cycle is defined as K th . According to equation (2), the K th of the speci- mens tempered at 600 °C with R = 0.1, 0.25 and 0.5 were 11.65 MPa m 1/2 , 10.49 MPa m 1/2 , 4.19 MPa m 1/2 , respectively, which are shown in Table 3. Table 3: The K th under different stress ratios RC 1 n 1 K th (MPa·m 1/2 ) 0.1 1.17E-17 9.31 11.65 0.25 1.82E-19 11.5 10.49 0.5 2.21E-9 2.65 4.19 Fracture-surface morphologies of the FCG at the stage of stable expanding region for the steel tempered at 600 °C are presented in Figure 8, which indicated that the fractures were mainly composed of tearing ridges and secondary cracks. The arrows denoted the FCG direction. Figure 8b, 8d and 8f are high magnification images of the circle line areas of Figure 8a, 8c and 8e, respectively. The high-magnification images show that the fatigue striations were perpendicular to the localized propagation direction of the fatigue crack. Meanwhile, the continuous distribution of fatigue striations were interrupted by some secondary cracks. The cracks may cause the acceleration of fatigue-crack growth rates during the process of coalescing with the main crack. 30,31 It is noteworthy that some nanoparticles (marked by the white arrows) were presented near the secondary cracks. As a brittle phase, the spherical precipitates promoted the cracking of the matrix under cyclic loading during the fatigue-crack growth. The stress ratio had a signifi- cant effect on the slip behavior and fracture mode around the crack tip. Compared with the low multiple fatigue fractures, the size of the tearing edge increased with the R ratios, the secondary crack density decreased with R ratios. Figure 8b, 8d and 8f illustrate that the striation separation was decreased with the R ratios. This was consistent with the values reported previously for slip behaviour around the crack tip 32,33 . 4 CONCLUSIONS In the present study, the microstructure, mechanical properties and fatigue crack growth of the heavy-forging steel 23CrNiMoV with quenching and tempering were investigated. The main conclusions can be summarized as follows: • The microstructures of the quenched 23CrNiMoV steel tempered at 550–650 °C were tempered martensite. During the process of tempering, the M 7 C 3 -type acicular carbides precipitated mainly in the martensite lath, the M 23 C 6 -type spheroidal carbides precipitated mainly at the inter-plate boundaries and the prior austenite grain boundaries, and some spherical precipitates change into acicular precipitates with the increase of the tempering temperature. S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 389 Figure 7: Fatigue-crack growth rate for the as-quenched steel tem- pered at 600 °C with R = 0.1, 0.25, 0.5 • The high density of the dislocation in the tempered martensite region and the M 7 C 3 -type acicular car- bides in the martensite lath significantly enhanced the strength and toughness of the 23CrNiMoV steel. The UTS, YS, elongation and impact energy of the steel tempering at 600 °C were 1155 MPa, 1091 MPa, 15.4 % and 102.9 J, respectively. • The FCG threshold ( K th ) of the 23CrNiMoV steel tempered at 600 °C with stress ratio R = 0.1, 0.25 and 0.5 were 11.65 MPa m 1/2 , 10.49 MPa m 1/2 , 4.19 MPa m 1/2 , respectively. The fractures were mainly composed of tearing ridges and secondary cracks. The secondary cracks were easily produced around the spherical carbides, but coalesced with the main crack during the FCG. Acknowledgment The authors kindly acknowledge the joint support by the National Nature Science Foundation of China (No. 51775259), the Opening Project of Jiangsu Key Labo- ratory of Advanced Structural Materials and Application Technology (No. ASMA201704), Science Foundations of Nanjing Institute of Technology (No. QKJ201702). S. ZHU et al.: MICROSTRUCTURE AND FATIGUE-CRACK GROWTH OF QUENCHED AND TEMPERED 23CrNiMoV STEEL 390 Materiali in tehnologije / Materials and technology 54 (2020) 3, 385–391 Figure 8: SEM micrograph of fatigue-fracture surfaces at the stable expanding region with the K of 21.0–21.5 MPa m 1/2 for the as-quenched steel tempered at 600 °C with different R, a,b) 0.1, c,d) 0.25, e,f) 0.5 5 REFERENCES 1 N. Harada, M. Takuma, M. Tsujikawa, K. 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