2017 Volume 57 Issue 3 Pages 558-563
A novel low-carbon high-silicon martensitic steel, 22SiMnCrNiMo, whose comprehensive mechanical properties approach the 00Ni18Co9Mo4Ti maraging steel, was investigated. Microstructure characterizations revealed that the steel was composed by lath martensite, retained austenite films and ε-carbides after different heat treatment processes. And the 22SiMnCrNiMo steel obtained optimal mechanical properties of a yield strength of 1261 MPa, a tensile strength of 1548 MPa, an impact toughness of 120 J/cm2, a fracture toughness (KIC) of 94.8 MPa·m1/2 and a threshold value (ΔKth) of 7.1 MPa·m1/2 when the steel was austenitised at 900°C for 1 h followed by water quenched and then tempered at 320°C for 1 h. The 22SiMnCrNiMo steel exhibited excellent mechanical properties similar to those of the 00Ni18Co9Mo4Ti maraging steel as well as high resistance to fatigue crack initiation and growth.
Conventional ultra-high strength steels such as maraging steel, AISI4340 steel and AF1410 steel, are expensive because of high alloy content.1,2,3) The maraging steel exhibits excellent strength and toughness because of high content of Ni (18%) and Co (8.4%). A mountain of intermetallic compounds, Ni3Mo, Ni3Ti, and Ni3Al are dispersed in high density dislocations matrix and then results in the high strength of maraging steel.4,5) Moreover, the precipitation of these intermetallic compounds notably reduces the solid solubility of these alloy element in matrix, such as Ni, Ti, Mo and Al, which inevitably decreases the solution strengthening effect of these alloy element and increases the toughness of matrix of maraging steel. Medium-carbon low-alloy martensitic steel AISI4340 contains 1.8% Ni and 0.7% Cr. The strength of this type steel is comparable with that of the maraging steel, but this steel has a lower toughness.6) AF1410 steel has an ultra-high strength and a high fracture toughness because of high content of Co (14%) and Ni (10%). High content of Ni alloying element reduces the Ac1 temperature of AF1410 steel, which facilitates the formation of reverted austenite during the aging process. The reverted austenite can effectively prevent crack initiation and crack propagation growth.2,7)
Low-alloy martensitic steel has been extensively used not only for its low cost, but high strength and toughness.8,9) In recent years, new types of steel with high strength and toughness similar to maraging steel have been developed. Rao et al. designed a kind of steel with a high strength and toughness using Cr, C, Mn and Ni elements.7) Tomita et al. studied a bainite/martensite dual phase steel with high strength and toughness using C, Ni, Cr and Mo elements.10) Fang et al. fabricated a 1500 MPa grade 0.2C-2Mn-1Si-0.5Cr air cooled carbide-free bainite/martensite steel.11,12) Wang et al. developed a 35MnSiCrNiAlMo bainitic steel whose mechanical properties approached those of 140-maraging steel by adding a certain amount of Al and combinations of Mn, Si, Cr and Ni elements.13)
A novel low-carbon high-silicon martensitic steel, 22SiMnCrNiMo, was designed by using high Si and high Mn. The comprehensive mechanical properties of the novel low-carbon high-silicon martensitic steel are comparable with those of the 00Ni18Co9Mo4Ti maraging steel. The two steels can be applied in many industrial fields, such as bearing, gear and railway systems (including rails and crossings), the fatigue damage is inevitable in the application. Thus, it is necessary to understand the fatigue properties of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel. It has been recognized that grain size significantly affect the fatigue crack growth (FCG) behavior.14,15) However, the effects of grain size on crack threshold value in various materials are different. For high manganese steel,14) the FCG threshold increased with the increasing grain size, but for the Cu alloy the threshold value decreased with increasing grain size.15) The effective grain size is not quite clear in lath martensitic steels because of its complicated microstructure. Li et al.16) revealed that decreasing the packet size and block size of the lath martensitic steels, the cracks growth would be slowed because of the increased encounter frequency of cracks with substructure boundaries. In this paper, the fatigue crack initiation and growth behaviors of the two steels were analysed. Furthermore, high strength and toughness mechanisms of the 22SiMnCrNiMo low-carbon high-silicon martensitic steel were analysed.
A novel 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel were used in the experiment, and their chemical compositions are listed in Table 1. The 22SiMnCrNiMo steel was water quenched after being austenitised for 1 h at 900°C and tempered for 1, 3 and 5 h at 320°C and for 1 h at 350 and 380°C. The 00Ni18Co9Mo4Ti maraging steel was water quenched after solution treatment for 0.5 h at 860°C and aged for 4 h at 480°C.
| Test steels | C | Mn | Si | Cr | Co | Ti | Mo | Ni | Al |
|---|---|---|---|---|---|---|---|---|---|
| 22SiMnCrNiMo | 0.21 | 1.1 | 1.8 | 0.7 | – | – | 0.19 | 0.14 | 0.006 |
| 00Ni18Co9Mo4Ti | <0.001 | <0.0 | 0 | – | 8.4 | 0.18 | 4.32 | 17.50 | 0.094 |
Tensile testing was carried out on a MTS material testing machine at RT with an initial strain rate of 2×10−3 s−1. Tensile samples with a gauge of 25 mm and a gauge diameter of 5 mm were fabricated and three samples were tested for each process. A TH501 Digital Rockwell Hardness Tester was used to test the hardness. 10 data points were measured on each sample and the average values were taken. The impact toughness with a U-notch width of 2 mm was measured using a 300 J Charpy testing machine. The sample size was 10 mm × 10 mm × 55 mm, and three samples were tested for each process.
The FCG test was carried out on a MTS material testing machine in accordance with the Chinese standard GB/T 4161-2007 at room temperature under the control of stress intensity factor range (ΔK). The length (L), width (W) and thickness (B) of the specimen were 210, 40 and 20 mm, respectively. The ratio of the span (S) to the width is 4:1. The notch size was 4 mm in width and 0.45 W in depth. The surfaces of the specimens were ground in successive stages by abrasive papers from 150 to 2000 grit to obtain smooth surfaces and then polished mechanically using the diamond suspension of size 2.5 μm to obtain a mirror finish. Positive stress ratio of R=0.1 and a constant frequency of 15 Hz were used. Prior to the FCG test, a fatigue pre-crack of 1.3 mm length was produced from the notch according to the standard GB/T 4161-2007. The tests for producing pre-cracks were controlled by the MTS-made commercial software. At the beginning, decreasing K tests were performed, with ΔK reduced gradually until the FCG rate was 10−7 mm/cycle. Increasing K tests were continued and ΔK increased progressively to obtain crack growth behaviour in the Paris regime. During the FCG test, the applied load was reduced or increased in steps by no larger than 10%. At each step, the increment of crack growth was set as 4 to 6 times larger than the assumed plasticity zone size at the previous load level.
The microstructures of the two types of steel after different heat treatments were determined by optical microscopy (OM) and transmission electron microscopy (TEM, JEM-2010) analyses. Nital (4%) was used for etching the microstructures of the 22SiMnCrNiMo steel. The chemical reagent for etching the microstructures of the 00Ni18Co9Mo4Ti maraging steel is 1 g of picric acid + 5 ml of HCl + 100 ml of C2H5OH. The microstructures were examined using TEM operating at 200 kV. Foils for the TEM analysis were cut into 0.6 mm thickness, ground to 30 μm by SiC abrasive paper and then thinned to perforation on a TenuPol-5 twin-jet unit with an electrolyte consisting of 7% perchloric acid and 93% glacial acetic acid. X-ray experiments were conducted using a D/max-2500/PC X-ray diffractometer to analyse the phase composition and relative contents in the sample. The Cu-Kα wavelength λ is 0.15406 nm. During the step scanning, the step width is 0.02°, step time is 1.5 s and the scanning range is 30° to 120° with unfiltered Cu-Kα.
Figure 1 shows Engineering stress–strain curves of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel. And the mechanical properties of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel after different heat treatment processes are summarized in Table 2. It can be seen that optimal mechanical properties of the 22SiMnCrNiMo steel, with a yield strength of 1261 MPa, a tensile strength of 1548 MPa and an impact toughness of 120 J/cm2, were obtained when the steel was austenitised at 900°C for 1 h followed by water quenched and then tempered at 320°C for 1 h. For the 22SiMnCrNiMo steel, with increasing the isothermal temperature from 320 to 380°C, the tensile strength decreased slightly, yield strength increased slightly and impact toughness decreased by 21%, and with increasing the isothermal time from 1 to 5 h, the tensile strength decreased slightly, the yield strength increased slightly and the impact toughness decreased by 17%. The mechanical properties of the 00Ni18Co9Mo4Ti maraging steel, with a yield strength of 1426 MPa, a tensile strength of 1496 MPa and an impact toughness of 105 J/cm2, was obtained. The tensile strength of the 00Ni18Co9Mo4Ti maraging steel was lower than that of the 22SiMnCrNiMo steel, and the yield strength was of the 00Ni18Co9Mo4Ti maraging steel 16% higher than the 22SiMnCrNiMo steel. The impact toughness of the 00Ni18Co9Mo4Ti maraging steel was 12.6% lower than that of the 22SiMnCrNiMo steel. The hardness of the 00Ni18Co9Mo4Ti maraging steel was 3.4 HRC lower than that of the 22SiMnCrNiMo steel. It can be concluded that the 22SiMnCrNiMo steel shows a better mechanical properties than the the 00Ni18Co9Mo4Ti maraging steel.
Engineering stress–strain curves of 22SiMnCrNiMo steel and 00Ni18Co9Mo4Ti maraging steel. (Online version in color.)
| Steels | Heat treatment processes (°C × h) | Hd (HRC) | σs (MPa) | σb (MPa) | Ψ (%) | δ (%) | aKU (J/cm2) |
|---|---|---|---|---|---|---|---|
| 22SiMnCrNiMo | 320 × 1 | 48.3 | 1261 | 1548 | 60.5 | 13.2 | 120.2 |
| 320 × 3 | 47.6 | 1261 | 1535 | 57.8 | 13.8 | 106.4 | |
| 320 × 5 | 48.3 | 1263 | 1525 | 59.6 | 14.7 | 99.5 | |
| 350 × 1 | 48.6 | 1245 | 1528 | 60.3 | 13.4 | 117.1 | |
| 380 × 1 | 46.1 | 1313 | 1519 | 60.1 | 11.3 | 95.2 | |
| 00Ni18Co9Mo4Ti | 860 × 0.5, 480 × 4 | 44.9 | 1426 | 1496 | 64.5 | 11.6 | 105.0 |
Figure 2 shows the optical images of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel after different heat treatment processes. One can see that the microstructures of the 22SiMnCrNiMo steel after different heat treatment processes were tempering martensite. Figures 2(b) and 2(c) show that substructure in the 22SiMnCrNiMo steel was more coarsened as compared with Fig. 2(a). The microstructure of the 00Ni18Co9Mo4Ti maraging steel showed in Fig. 2(d) was lath martensite.
Metallographic micrographs of 22SiMnCrNiMo steel austenitised at 900°C followed by water quenched and then tempered at (a) 320°C for 1 h, (b) 320°C for 5 h and (c) 380°C for 1 h; Metallographic micrograph of 00Ni18Co9Mo4Ti maraging steel after heat treatment process (d).
Figure 3 shows the typical TEM micrographs of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel after different heat treatment processes. The TEM micrographs of the 22SiMnCrNiMo steel reveal that the steel was composed by lath martensite, retained austenite films and ε-carbides. High dislocation density existed in lath martensite, lath martensite was alternate arrangement with retained austenite films and ε-carbides were dispersed in the lath martensitic matrix. Figure 3(a) shows the TEM micrograph of the 22SiMnCrNiMo steel is shown when the steel was austenitised at 900°C followed by water quenched and then tempered at 320°C for 1 h. The width of a single lath martensite was about 200 nm, a certain amount of retained austenite films existed in lath interface and a small amount of ε-carbides was dispersed wthin lath martensite. In this state, optimal mechanical properties of the 22SiMnCrNiMo steel were obtained. With extending tempering time, the single lath martensite width of the 22SiMnCrNiMo steel is increased to 500 nm, partial retained austenite films was decomposed to disappearing, the amount of the ε-carbides increased and ε-carbides were coarsened as shown in Fig. 3(b). With increasing tempering temperature, the single lath martensite width of the 22SiMnCrNiMo steel is increased to 500–700 nm and a large number of coarsen ε-carbides were dispersed in the lath martensite as shown in Fig. 3(c). Figure 3(d) shows the typical TEM micrograph of the 00Ni18Co9Mo4Ti maraging steel. The width of single lath martensite is about 250 nm. High dislocation density existed within the lath martensite and a large number of nanoscale spherical, rod-like or needle precipitated phase Ni3(Mo, Ti) dispersed uniformly within the lath martensite.
TEM micrographs of 22SiMnCrNiMo steel austenitised at 900°C followed by water quenched and then tempered at (a) 320°C for 1 h, (b) 320°C for 5 h and (c) 380°C for 1 h; TEM micrograph of 00Ni18Co9Mo4Ti maraging steel after heat treatment process (d).
Figures 4(a) and 4(b) show the bright and dark fields of ε-carbides precipitated in the 22SiMnCrNiMo steel austenitised at 900°C followed by water quenched and then tempered at 320°C for 1 h. The ε-carbides display as short flakes in the martensitic matrix and longitudinal dimension is about 100 nm. The carbide type was confirmed in SAED patterns and was indexed in Fig. 4(c). The carbide is hexagonal close-packed lattice (hcp) ε-carbides and is not orthorhombic structure Fe3C. It is beneficial to the promotion of the toughness of the 22SiMnCrNiMo steel.
Morphologies of ε-carbides precipitated in 22SiMnCrNiMo steel austenitised at 900°C followed by water quenched and then tempered at 320°C for 1 h (a) bright field; (b) dark field; (c) SAED pattern and index.
Figure 5 shows XRD patterns of the 22SiMnCrNiMo steel after different heat treatment processes. Only bcc (α) and fcc (γ) phases are present in the spectrum and no carbide diffraction peaks can be observed. The amount of ε-carbides is few, therefore, carbide diffraction peaks cannot detected. FCC diffraction peaks are very weak and the retained austenite content of the 22SiMnCrNiMo steel after different heat treatment processes was little. The calculations show that the content of the retained austenite is below 1%. High dislocation density is produced in the process of martensitic transformation.17) It is well known that the dislocation density in lath martensite was between 1014–1015 m−2.18) Therefore, the dislocation density should be one of the key factors to be evaluated in researching the mechanical properties of martensitic steel.19,20) The microstrains in crystal of the 22SiMnCrNiMo steel were deduced through the X-ray analysis. Dislocation density in lath martensite can be calculated using the Williamson Hall formula, ρ=14.4ε2/b2 21,22) (ρ is dislocation density, ε is microstrain and b is Burgers vector), and the dislocation density is displayed in Table 3. This method over-estimates the dislocation density,20) but the results can be used to compare the dislocation density under different heat treatment processes. In Table 3, the dislocation density of the 22SiMnCrNiMo steel after different heat treatment processes has the same order of magnitudes. The 22SiMnCrNiMo steel had the highest dislocation density when the steel was tempered at 320°C for 1 h. With an isothermal time of 1 h, dislocation density decreased with increasing tempering temperature. With an isothermal temperature of 320°C, dislocation density is also gradually reduced with increasing tempering time.
XRD patterns of samples treated by different heat treatment processes in 22SiMnCrNiMo steel. (Online version in color.)
| Heat treatment processes (°C × h) | 320°C×1 h | 320°C×3 h | 320°C×5 h | 350°C×1 h | 380°C×1 h |
| Dislocation density | 2.91×1015 | 2.68×1015 | 2.62×1015 | 2.82×1015 | 2.59×1015 |
The FCG rate, da/dN, versus the applied stress intensity factor, ΔK, obtained for the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel are plotted in Fig. 6. It can be seen that the da/dN values decrease rapidly with decreasing ΔK in the near threshold regime (Fig. 6) and the FCG threshold values of the 00Ni18Co9Mo4Ti maraging steel are higher than those of the 22SiMnCrNiMo steel. In the Paris regime, the relationship of ΔK and da/dN is approximately linear in double logarithmic coordinates. The FCG rate of the 22SiMnCrNiMo steel is slightly higher than that of the 00Ni18Co9Mo4Ti maraging steel with the same ΔK. The FCG behaviours are different in the 22SiMnCrNiMo steel under different heat treatment processes. The FCG rate is the lowest when the 22SiMnCrNiMo steel was tempered at 320°C for 1 h. The conventional Paris model, da/dN=C·(ΔK)m is applied to describe the FCG behaviour of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel, where C and m are constants that vary among materials. Fracture toughness (KIC), threshold value (ΔKth), C and m values of the 22SiMnCrNiMo steel and the 00Ni18Co9Mo4Ti maraging steel are shown in Table 4. The m value is obtained according to the intercept and slope of the Paris curve. The m value represents the steep degree of da/dN-ΔK curves in the Paris regime. The smaller the m value, the better the ability is to resist crack growth in steels. As seen in Table 4, the fracture toughness of the 00Ni18Co9Mo4Ti maraging steel is higher than that of the 22SiMnCrNiMo steel. And the 00Ni18Co9Mo4Ti maraging steel has the smallest m value and a higher FCG threshold value. Therefore, the fracture toughness and FCG behaviour of 00Ni18Co9Mo4Ti maraging steel are superior to those of the 22SiMnCrNiMo steel.
Fatigue crack growth rate versus stress intensity factor of 22SiMnCrNiMo steel and 00Ni18Co9Mo4Ti maraging steel. (Online version in color.)
| Steels | Heat treatment processes (°C×h) | KIC (MPa·m1/2) | ΔKth (MPa·m1/2) | C | m |
|---|---|---|---|---|---|
| 22SiMnCrNiMo | 320×1 | 94.8 | 7.1 | 3.7×10−9 | 3.16 |
| 350×1 | 107.6 | 7.0 | 2.2×10−9 | 3.35 | |
| 380×1 | 94.4 | 7.1 | 2.1×10−9 | 3.65 | |
| 00Ni18Co9Mo4Ti | 860×0.5, 480×4 | 117.7 | 9.8 | 7.2×10−9 | 2.90 |
The excellent comprehensive mechanical properties of the 22SiMnCrNiMo steel are related to its chemical compositions and microstructures. From the aspects of chemical compositions, the carbon content of the 22SiMnCrNiMo steel is about 0.2%. The low carbon content not only benefits the steel to obtain ultra-high strength by the solid solution strengthening of carbon atoms but also avoid appearing twin substructure. This is beneficial to the improvement of toughness and reduction of quenching crack.7) The Si content in the 22SiMnCrNiMo steel is about 1.8%. High Si content promotes the rise of the temperature range of martensite temper brittleness.23) Therefore, the 22SiMnCrNiMo steel can be tempered at high temperatures (350°C) for long time. During tempering, the carbon element was redistributed between martensite and retained austenite films resulting in the amount of solid solution carbon atoms decreased in martensitic matrix. Moreover, the lattice distortion of martensite is adjusted and internal stress is reduced. Si, Mn, Ni and Mo elements in the 22SiMnCrNiMo steel has certain interstitial solution strengthening effects. Compared with the solid solution strengthening of carbon atoms, the interstitial solution strengthening has less harmful effects to the toughness, which is in favour of the toughness improvement of steels. Mn, Cr, Ni and microalloying elements are in the 22SiMnCrNiMo steel, which cooperated with Si atoms, thereby the retained austenite films is filled with C, Mn, Cr, Ni and microalloying elements in the lath martensite phase boundary. Therefore, the retain austenite films is stable enough. From the aspects of microstructures, the 22SiMnCrNiMo steel microstructures under different heat treatment processes are composed of lath martensite, retained austenite films and ε-carbides. High dislocation density in the lath martensite is the foundation of the 22SiMnCrNiMo steel with excellent strength and toughness.9,24) ε-carbides dispersed in the lath martensitic matrix further enhance the strength of the 22SiMnCrNiMo steel. High carbon retained austenite films between the laths are beneficial to prevent crack initiation and alleviate the extension of cracks.2,7)
The substructure changes in the 22SiMnCrNiMo steel after different heat treatment processes are lath martensite width increased, dislocation density decreased, decomposition of retained austenite, the amount of the ε-carbides increased and ε-carbides were coarsened.25) During tempering, the dislocation density of the 22SiMnCrNiMo steel decreased because of continuous dislocation movement, rearrangement or disappearance. Therefore, tensile strength of the 22SiMnCrNiMo steel decreases with increasing tempering temperature and time. The carbon atoms occur segregation in dislocation accumulation places so that fine dispersed carbide can easily precipitate in the tangled dislocation with increasing tempering temperature and time. These carbides have significant pinning effects on dislocations, resulting in the amount of movable dislocations decrease. Therefore, a larger stress is needed to start the dislocation source, leading to yield strength increase with increasing tempering temperature and time. Phase transition from ε-carbides to Fe3C is not discovered during tempering, because of the higher Si content in the 22SiMnCrNiMo steel suppresses the phase transition and increases the stability of the ε-carbides. However, ε-carbides were coarsened with increasing tempering temperature and time, which resulted in impact toughness decreased of the 22SiMnCrNiMo steel. The 22SiMnCrNiMo steel had the best mechanical properties after being tempered at 320°C for 1 h, which is the result of the joint action of high density dislocations in the lath martensite, retained austenite films and fine ε-carbides were dispersed in lath martensite.
The martensitic substructure type, carbides amount and size, retained austenite morphology, distribution and amount has obvious effects on fracture toughness. The twin substructure in steels is the most important factor that reduces fracture toughness of the martensitic steel. Phase transformation twins are significant obstacles for dislocations movement,26,27) and they often become the core of the crack initiation. Therefore, avoiding the formation of phase transformation twins and reducing the number of twins are important. It means that the martensitic steel fracture toughness is improved. Carbide spacing is another important factor that affects the fracture toughness of materials. Fine dispersion carbides make the fracture toughness increase and increase the base metal cleavage cracking stress.28) A certain critical size of the carbide leads to cleavage initiation and growth. Retained austenite films in laths can promote crack passivation, bifurcation and diversion.2,7) Therefore, the fracture toughness of lath martensite would be improved.
After different heat treatment processes, the microstructures of the 22SiMnCrNiMo steel are composed of lath martensite, retained austenite films and ε-carbides. High dislocation density existed in lath martensite, lath martensite alternately arranged with retained austenite films and ε-carbides were dispersed in the lath martensite. It is the basic reason that the 22SiMnCrNiMo steel has higher fracture toughness. The 22SiMnCrNiMo steel had the highest dislocation density, a certain amount of retained austenite films existed in lath martensite, ε-carbides were displayed as short flakes in the martensitic matrix with longitudinal dimension about 100 nm when the steel tempered at 320°C for 1 h. With increasing tempering temperature, dislocation density in lath martensite decreased, partial retained austenite films decomposed and the amount of the ε-carbides increased and ε-carbides were coarsened. Therefore, the fracture toughness of the 22SiMnCrNiMo steel decreases with increasing tempering temperature.
Superior FCG behaviour of the 22SiMnCrNiMo steel is closely associated with its substructure. The synthetic factors of the solid solution strengthening caused by a small amount of carbon atoms dissolved in the octahedral gap, solid solution strengthening caused by Si, Mn and other alloy elements and precipitation strengthening caused by ε-carbides resulted in the high strength of lath martensitic matrix. The high strength of the 22SiMnCrNiMo steel prevents effectively the local plastic region deformation. The crack is hard to initiate so the crack initiation cycle increased. Meanwhile, higher micro-plastic presents in low carbon martensitic steel. Therefore, plastic deformation capacity is higher before the fracture, and the plastic deformations in local regions are smaller under the same stress. From the aspects of crack growth, the plastic deformation causes crack-tip stress redistribution, and stress peaks are reduced. Therefore, crack growth is prevented. In addition, the micro-plastic is related to retained austenite films in the lath martensite phase boundary. Retained austenite induces phase transformation under the reaction of alternating stress, which plays an important role in preventing the fatigue crack initiation and growth.
The packet size and block width were affect significantly the FCG behavior in martensitic steel. The packet size and block width are positively correlated to prior austenite grain size16,29). The prior austenite grain size of the 22SiMnCrNiMo steel tempered at different temperature was the same. However, the FCG rate was higher under the same ΔK when the samples were austenitised at 900°C for 1 h followed by water quenched and then tempered at 380°C for 1 h. It suggests that the substructure played an important role on the FCG behavior. After different heat treatment processes, the width of a single lath martensite of the 22SiMnCrNiMo steel was different. The lath martensite width increased with increasing tempering temperature and time, the number of ε-carbides increased and ε-carbides coarsened. Therefore, the FCG rate in Paris regime increased with increasing tempering temperature in the 22SiMnCrNiMo steel.
(1) The 22SiMnCrNiMo steel had optimal mechanical properties after being austenitised at 900°C for 1 h followed by water quenched and then tempered at 320°C for 1 h. It showed that yield strength reached 1261 MPa, tensile strength reached 1548 MPa and impact toughness was 12.6% higher than that of the 00Ni18Co9Mo4Ti maraging steel.
(2) The microstructure of the 22SiMnCrNiMo steel after different heat treatment processes were composed by lath martensite, retained austenite films and ε-carbides. High dislocation density existed in lath martensite, lath martensite was alternate arrangement with retained austenite films and ε-carbides were displayed as a short flake in the martensitic matrix with longitudinal dimension about 100 nm when the steel tempered at 320°C for 1 h.
(3) The 22SiMnCrNiMo steel had the lowest FCG rate in Paris regime when the steel tempered at 320°C for 1 h. The fracture toughness and FCG behaviour of the 22SiMnCrNiMo steel are lower than those of the 00Ni18Co9Mo4Ti maraging steel. However, these properties are very close. Therefore, the 22SiMnCrNiMo steel has a superior fracture toughness and resistance to fatigue crack initiation and growth.
This work was supported by the Natural Science Foundation of China (No. 51471146).
