Effect of microstructural characteristics on the impact fracture behavior of cryogenic 9Ni steel

The percentage of shear fracture largely determines the service performance of 9Ni steel used in low-temperature pressure vessels. Through elemental analysis, microstructural characterization, and mechanical property tests, this study investigates why the percentage of shear fractures is low in 9Ni steel and clarifies the mechanism by which the microstructural characteristics influence the low-temperature impact behavior of 9Ni steel. It was found that cleavage fracture zones, formed when segregation bands appear in the microstructure, decrease the percentage of shear fractures at the impact fracture surface. Specifically, as the segregation area increases from 0.9% to 7.1%, the shear-fracture percentage in 9Ni steel sharply decreases from 100% to 65%, accompanied by a deterioration in low-temperature toughness. The segregation zone is enriched in austenite-forming elements (Ni, C, Mn), leading to a tempered martensite microstructure with a lath shape. The small number of high-angle grain boundaries and low interface bonding strength cannot effectively prevent crack initiation and propagation, resulting in brittle cleavage fracture. In contrast, the non-segregated zone is tempered sorbite with a uniform structure, several high-angle grain boundaries, and a high interface bonding strength. These features hinder crack initiation and propagation. Furthermore, the shear-fracture zone generated in the non-segregated zone exhibits ductile fracture characteristics.


Introduction
Owing to its high comprehensive service performance and excellent low-temperature toughness, 9Ni steel is widely used in low-temperature and harsh environments.One major use of 9Ni steel is the storage and transportation of liquefied natural gas [1][2][3].9Ni steel is an irreplaceable metal material in the liquefied natural gas sector due to its excellent thermomechanical properties and cryogenic toughness [4][5][6].Ni-based cryogenic steels are commonly prepared through quenching and tempering (QT) treatments [7,8].An appropriate heat treatment ensures excellent low-temperature toughness of 9Ni steel.After QT treatment, the main microstructures of 9Ni steel are tempered martensite and retained austenite, but after quenching, lamellarizing and tempering (QLT) treatment, the microstructure is tempered martensite, ferrite and reversed austenite [9][10][11].A deep understanding of the martensite, retained austenite, and grain boundary characteristics would enable microstructural control of Ni-based cryogenic steels.
Control and optimization of the microstructures and properties of Ni-based cryogenic steels have been widely reported.Zhang et al [12] studied the high-temperature thermal deformation behavior of 9Ni steel.They related the deformation resistance to the deformation degree, deformation temperature, and strain rate of the steel.Kinney et al [13] investigated the microstructure of lath martensite in quenched 9Ni steel and clarified the 'block-and-packet' structure of lath martensite in low-carbon steel.Chen et al [14] studied the microstructure evolution and mechanical properties of low-Ni cryogenic steel treated by a thermo-mechanical control process (TMPC) with ultra-fast cooling and lamellarizing/tempering.He pointed out that dispersed and stably retained austenite, along with fine grains, are the premise of excellent mechanical properties.Chen et al [15], who studied the influence of process routes on the microstructure and mechanical properties of 6.8Ni cryogenic steel, reported excellent low-temperature toughness after ultrafast cooling, intercritical quenching, and tempering treatment.Jang et al [16] correlated the microstructural change with the fracture characteristics in the coarsegrained heat affected zones of 9% Ni steel.Their results explained the mechanism by which the content, morphology, and size of martensite-austenite constituents influence the low-temperature toughness.
Research on Ni-containing cryogenic steel has mainly focused on refining the microstructure, understanding the role of retained/reversed austenite, and optimizing the QLT processing parameters [17][18][19][20].In contrast, the percentage of shear fractures (PSF) among the low-temperature impact fractures has been rarely investigated.Therefore, the influential factors of PSF in 9Ni steel and the relationship between PSF and the microstructural characteristics require systematic investigation.Along with the impact absorption energy, the PSF is an important indicator of the low-temperature toughness of cryogenic steel [21,22].The presence and stability of the PSF directly determine the safety and reliability of 9Ni steel in service.During actual industrial production processes, the PSF of 9Ni steel is often unstable and steel plates with <75% PSF are disqualified.To resolve the above problems, this study presents the microstructure, mechanical properties, elemental composition, fracture observations, and crack growth of 9Ni steel with different PSFs.The aims are to reveal the fracture mechanism of 9Ni steel and to clarify the influences of element segregation and microstructural characteristics on the fracture behavior of 9Ni steel under low-temperature impacts.

Material preparation
The steel sample was a commercial steel plate produced by an iron and steel company.The chemical composition of the sample is shown in table 1.The continuously cast slab of 9Ni steel was hot-rolled into a 20mm-thick plate using TMCP technology.As shown in figure 1, the initial temperatures of the rough-rolling and finish-rolling processes were 1150 °C and 900 °C, respectively.After finish-rolling, the sample was water-cooled from its final finish-rolling temperature (850 °C) to the ambient temperature.The hot-rolled steel plate was quenched and tempered by an off-line heat treatment.The steel plate was first heated to 820 °C for 60 min and then water-quenched to room temperature.The quenched steel plate was reheated to 590 °C for 120 min, followed by air cooling to room temperature, finally obtaining a cryogenic steel plate for liquefied natural gas storage tanks.

Microstructural characterization and mechanical property tests
The microstructural characteristics of the 9Ni steel were observed under an optical microscope (OM) (Olympus BX53M) and a field emission scanning electron microscope (FE-SEM) (Zeiss Ultra 55).The impact fracture morphology was observed under a SEM (Zeiss Ultra 55).The elemental distributions on the microstructures of different samples were observed using an electron probe micro-analyzer (EPMA) (JEOL-8530).The microstructures of the 9Ni steel were characterized using electron back-scattered diffraction (EBSD) (Symmetry S3 detector).Prior to EBDS analysis, the samples were mechanically polished and then electrolytically polished in perchloric acid/alcohol solution.The EBSD results were post-processed using AZtecCrystal software, obtaining high-quality image maps, grain boundary images, inverse pole figure (IPF) maps, and kernel average misorientation (KAM) maps.Tensile tests and Charpy V-notch impact tests were carried out on a tensile testing machine (WDW-300) and a drop weight impact tester (Instron Dynatup 9250), respectively.The rod-shaped tensile specimen had a gauge length of 50 mm and a diameter of 5 mm.The impact samples were sized 10 mm in the normal direction, 10 mm in the rolling direction, and 55 mm in the transverse direction.Prior to impact tests, the samples were uniformly cooled to −196 °C in liquid nitrogen and quickly placed on the impact testing machine.After the low-temperature impact test, the fracture morphologies (cleavage and shear-fracture zones) were observed under SEM and the PSF value was calculated as where A 0 (mm 2 ) is the original total area of impact fractures and A c (mm 2 ) is the area of the cleavage fracture zone in the impact fractures.appeared in the center of the fracture and originated from the V-notch, while other areas belonged to the shear fracture zone.The PSF of the impact fracture increased with reducing area of the cleavage fracture, indicating an improvement in the low-temperature toughness.When the cleavage fracture zone disappeared, the PSF of the impact fracture reached 100% (figure 2(c)) and the low-temperature toughness was maximized.Figure 3 shows the micro-morphologies of the cleavage and shear-fracture zones.The cleavage fracture zone comprised cleavage planes and cleavage steps, along with some tearing ridges.In contrast, the shear-fracture zone comprised dimples with different sizes and depths.Obvious plastic deformation between the dimples affirmed a microvoid-coalescence fracture mode [23][24][25].

Results and discussion
Table 2 gives the mechanical properties of the 9Ni steels with different PSFs.The yield and tensile strengths and total elongations of the samples showed no obvious dependence on PSF, indicating that PSF is not directly related to strength or plasticity.However, the PSF apparently influenced the low-temperature impact energy of the samples.The impact energy at −196 °C was only 157 J in the 60% PSF sample, increasing to 205 J in the 100% PSF sample.Therefore, increasing the PSF substantially improved the low-temperature toughness of 9Ni steel.The mechanical test results indicated that segregation bands in the microstructure directly affect the lowtemperature impact energy and PSF of 9Ni steel.

Microstructure
Figures 4 and 5 display OM and SEM images, respectively, of the 9Ni steel samples with different PSFs.The main microstructure was tempered sorbite and the steel plates presented a slight banded structure.In the OM images,   the banded structure occupied 7.1% of the area of the PSF 60% sample and 3.8% of the area of the 75% PSF sample.The banded area was smallest (0.9%) in the PSF 100% sample.These changes indicate that increasing the PSF gradually diminishes the banded structure.As shown in figure 4, the microstructure became more  uniform and the carbide more dispersed in samples with higher PSF.In addition, the lath shape was more obvious and the carbides more compact in the banded microstructure than in other areas.Figure 6 exhibits the elemental distribution maps of the banded structures in samples with different PSFs.The elements in the banded structures of all samples were obviously segregated.The most segregated element was Ni, followed by C. The Mn and Si elements were weakly segregated and P showed no evidence of segregation.Comparing the elemental distributions in the banded structures of samples with different PSFs, the strongest elemental segregation was observed at 60% PSF.The elemental segregation degree of the banded structure gradually decreased with increasing PSF and became negligible at 100% PSF.Therefore, the banded (segregated) structures were formed through segregation and enrichment of the alloying elements (especially Ni).In turn, these structures affected the microstructural characteristics of the tested steel.
Figure 7 shows the EBSD-derived microstructural characteristics of the segregated bands (panels a, c, e, g, i) and the non-segregated zones (panels b, d, f, h, j) in the tested 9Ni steel.In the segregation band, the microstructure was insufficiently tempered and was characterized by many small-sized substructures and a low content of high-angle grain boundaries.Moreover, the samples containing segregation bands were microstructurally heterogeneous, indicating notably different grain sizes in the segregated bands and nonsegregated zones.The grains in the segregated bands were small and the alloy elements were largely enriched, enhancing the tempering stability and facilitating the formation of tempered martensite.In contrast, the nonsegregated zones favored the formation of tempered sorbite [26,27].In the non-segregated zones, the microstructure, prior austenitic grain size, and martensitic lath width were uniform, the proportion of highangle grain boundaries was high, and the grains were anisotropic.Therefore, during the impact fracture process, crack initiation and propagation were more suppressed in the non-segregated zones than in the segregated bands.The IPF maps showed no obviously preferred orientations in the segregated and non-segregated zones, and the same prior austenitic grains exhibited similar Miller indices.The KAM values were noticeably higher in the segregation zones than in the non-segregated zones and their distributions indicated high residual stress, subgrain structure, and storage energy in the segregation bands [28,29].These findings indicate ineffective suppression of crack initiation and deflection in the segregation bands under impact; consequently, the PSFs and low-temperature toughness are reduced in these zones.

Role of microstructure characteristics on ultra-cryogenic toughness
As evidenced from the microstructure characteristics and fracture morphologies, the PSF gradually decreased with increasing degree of elemental segregation in the steel samples, indicating that the segregated band dominantly affects the impact fracture mechanism.The enrichment of Ni, Mn and C alloy elements prevented sufficient tempering in the segregation zone.Therefore, the microstructure after tempering was tempered martensite with a low proportion of high-angle grain boundaries, which weakly resisted crack growth during impact fracture [30,31].In addition, as many of the alloy elements were widely separated in the segregation band, the binding strength was lowered at the grain and lath boundaries, facilitating crack propagation [32].The 60% PSF samples were most severely segregated and exhibited the lowest impact energy.In contrast, the nonsegmented area was fully tempered and its main microstructure was tempered sorbite with uniformly distributed high-angle grain boundaries.The fully tempered sorbite structure exhibited complete stress release and low dislocation density.The composition and microstructure were also uniform in the non-segregated zones.During the impact fracture process, the non-segmented area effectively released its stress concentration at the crack tip, promoted crack deflection, and suppressed crack propagation [33][34][35].Therefore, the nonsegregated sample exhibited an ultra-high low-temperature impact energy and 100% PSF.

Conclusions
This study characterized and analyzed the microstructures and properties of 9Ni steel with different PSFs using a range of techniques (OM, SEM, EPMA, EBSD, tensile testing, and low-temperature impact testing).The influences of microstructure characteristics on the low-temperature fracture mechanism were elucidated.The main conclusions are summarized below.
(1) Under low-temperature impacts, 9Ni steel exhibited two types of fracture characteristics, namely, brittle cleavage fracture with cleavage planes and ductile shear fracture with dimples.Increasing the PSF decreased the cleavage fracture area of the impacted fracture surface and improved the low-temperature impact toughness of the sample.
(2) The microstructures of 9Ni steel samples with lower PSFs exhibited obvious segregation bands.Ni was more segregated than other elements in the alloy.Such element segregation enhanced the tempering stability in the segregation bands, leading to a dominant microstructure of tempered martensite in lath form.In contrast, the microstructure of the non-segregation zone was homogeneously tempered sorbite.
(3) The segregation bands disrupted the homogeneity of the 9Ni steel microstructure, increased the tempered martensite content and KAM values, and decreased the number of high-angle grain boundaries.Crack initiation and propagation were ineffectively suppressed during the impact fracture process, thereby reducing the PSF and the low-temperature impact energy.As the segregation area decreased from 7.1% to 0.9%, the PSF of the 9Ni steel plate increased from 65% to 100% and the impact energy increased from 157 J to 205 J, demonstrating excellent low-temperature toughness in the non-segmented zone.

3. 1 .
Mechanical propertiesFigures2 and 3show the macro-and micro-morphologies, respectively, of fractured 9Ni steel plates with different PSFs after low-temperature impact tests.As shown in the macro-morphology image (figure2), the impact fracture primarily comprised cleavage and shear-fracture zones.The cleavage fracture zone generally

Table 1 .
Chemical composition of the tested 9Ni steel (mass fraction %).Rolling and heat treatment process parameters of the 9Ni steel.

Table 2 .
Mechanical properties of the tested 9Ni steel with different PSF.