Spherical Cementite Research Articles

Microstructural evolution and corresponding mechanical properties in nanostructured bainite under reheating conditions

ABSTRACT This study investigates the microstructural variations and corresponding changes in mechanical properties of super bainite achieved at 300°C, followed by reheating to temperature range of 400-650°C keeping for 1 h. Results indicated that, reheating the super bainite up to 400°C did not affect its microstructure and mechanical properties, maintaining a yield strength of 1125 ± 15 MPa, ultimate tensile strength of 1514 ± 20 MPa, elongation of 29.1 ± 2%, and impact energy of 18 ± 2 J. However, reheating to 500°C resulted in the highest values of yield and ultimate tensile strengths, in combination with the lowest amounts of elongation and impact toughness. Different mechanical properties were attributed to microstructural changes including cementite precipitation, austenite decomposition to pearlite, cementite spheroidization, and complete decomposition of microstructure to spherical cementite particles within a ferritic matrix. Big austenite microblocks were more susceptible to decomposition to pearlite, despite the austenite films that were thermally more stable.

Achieving balanced mechanical properties in Fe–0.16C plain low-carbon steel by trimodal microstructure and fine cementite

In the present study, for the first time, a trimodal microstructure (including coarse, fine, and ultrafine ferrite grains) was created in plain low-carbon steel by simple thermomechanical processing including intercritical annealing (at 770 °C, 800 °C, and 830 °C), 75% cold rolling, and finally heat treatment at 550 °C. The results showed that martensite with two different morphologies consisting of lath (low-carbon) and plate (high-carbon) were created in the microstructures of 800-75% and 830-75% sheets due to the non-homogeneous distribution of carbon atoms as a result of not using the homogenization treatment. After the final heat treatment, the samples exhibited trimodal grain size distribution of ferrite including coarse grains (larger than 5 micrometers), fine grains (between 1 and 5 micrometers), and ultrafine grains (UFGs) (smaller than 1 micrometer). The formation of ultrafine grains was ascribed to the presence of martensite with lath morphology in the 800-75% and 830-75% samples. The size of cementite particles in the 770-75%-550 sample was larger than the other two samples (800-75%-550 and 830-75%-550) owing to the presence of martensite phase only with plate morphology in 770-75%-550 steel. Unlike the 800-75%-550 sheet, the entire microstructure of the 770-75%-550 and 830-75%-550 samples had not undergone complete recrystallization and several deformed ferrite grains were still visible in the microstructures. The strength and hardness of the heat-treated sheets was larger than that of the as-received steel due to the presence of fine spherical cementite particles and fine ferrite grains in the microstructure of the heat-treated samples. Based on the obtained results, the 800-75%-550 sample revealed the best combination of strength-ductility-toughness due to trimodal microstructure along with the formation of fine spherical cementite particles. The failure mode of all heat-treated samples was ductile and there were both fine and coarse dimples in the fracture surfaces.

Effect of Spherical θ Precipitation in 1.5 GPa Grade Tempered Martensitic Steel on the Occurrence of Delayed Fracture

The objective of this study is to clarify the effect of spherical cementite (θ) precipitation on the occurrence of delayed fracture in 1.5 GPa grade tempered martensitic steels. Constant load tests were performed with a cathodically charged specimen. A 1GPa-load was applied to the specimen, and cathodic charging was performed in 3% NaCl + 3 g/L NH4SCN solution. The specimen of steel without spherical θ did not fracture at the current density of 5 A·m−2 or even by increasing to 50 A·m−2. On the other hand, the specimen of steel with spherical θ fractured after 0.2 h at 5 A·m−2. The strain around the spherical θ after 30%-rolling observed by transmission electron backscatter diffraction showed that the local deformation around the spherical θ was larger than that in the whole measurement field by 3.05 × 1014 m−2 in terms of geometrically necessary dislocation density. In the hydrogen desorption curve by thermal desorption analysis, steel with spherical θ after 30%-rolling showed a larger hydrogen desorption peak around 250 °C than steel without spherical θ. The value of the activation energy of the 250 °C-peak was 109.2 kJ·mol−1. From these results, the 250 °C-peak is inferred to be hydrogen at the disordered interface of θ/tempered martensite. Transmission electron microscopy observation showed cracks and voids on the spherical θ near the delayed fracture surface. These results indicate that the precipitation of spherical θ facilitates the occurrence of delayed fracture. Cracks appear to form around spherical θ.

An investigation on the formation and hardening mechanisms of white etching layer in hypoeutectoid rail surface

The stress distribution in a hypoeutectoid U71Mn rail surface due to the wheel-rail contact was simulated via finite element, and the microstructure and hardening of the after-service rail surface were comparatively studied with heat treated (holding at 850°C for 30min and cooling at 15°C/s) samples under atmospheric pressure (AP-850) and under hydrostatic pressure of 1GPa (HP-850), respectively. The rail surface experiences 3-dimensional stress-state of 1GPa and large shear stress. A white etching layer (WEL) was formed, showing martensite-type microstructure with spherical cementite particles and a hardness of 1101HV. The phase composition and hardening show great resemblance to those of the HP-850 sample, but differ significantly from those of AP-850 sample. High pressure martensitic transformation is proposed to interpret the formation of WEL and the hardening is analyzed according to the structural cause.

The effect of microstructure on the dynamic shock response of 1045 steel

The effect of microstructure on the shock response of 1045 steel is investigated via plate impact experiments and postmortem characterization. Three unique microstructures are explored: ferrite-pearlite, martensite, and ferrite with spheroidal cementite (i.e. spheroidized). Two spall recovery experiments, at approximate peak pressures of 3.2 and 3.5 GPa, are conducted to assess the Hugoniot elastic limit (HEL), spall strength, and damage morphology of the various microstructures. The ferrite-pearlite and martensite microstructures exhibit continuous yielding at both quasi-static and dynamic rates, while the spheroidized condition displays discontinuous yielding. Discontinuous yielding of the spheroidized microstructure is attributed to a combined low initial dislocation density coupled with a low dislocation nucleation rate. The spall strength of ferrite-pearlite is consistently lower than the spheroidized microstructure, attributed to elongated cementite that is more susceptible to cracking than more spherical cementite precipitates. Despite a high density of boundaries, martensite exhibits the highest spall strength. A large percentage of the boundaries within the martensite microstructure are found to be low energy (i.e. Σ3 or low angle), and are thus less susceptible to spall damage. Overall, the high spall strength of martensite is likely linked to traditional strengthening mechanisms that limit dislocation motion.

High-Power Diode Laser Surface Transformation Hardening of Ferrous Alloys

A high-power direct diode laser (HPDDL) having a rectangular beam with a top-hat intensity distribution was used to produce surface-hardened layers on a ferrous alloy. The thermal conditions in the hardened zone were estimated by using numerical simulations and infrared (IR) thermography and then referred to the thickness and microstructure of the hardened layers. The microstructural characteristics of the hardened layers were investigated using optical, scanning electron and transmission electron microscopy together with X-ray diffraction. It was found that the major factor that controls the thickness of the hardened layer is laser power density, which determines the optimal range of the traverse speed, and in consequence the temperature distribution in the hardened zone. The increase in the cooling rate led to the suppression of the martensitic transformation and a decrease in the hardened layer hardness. The precipitation of the nanometric plate-like and spherical cementite was observed throughout the hardened layer.

Study on the hydrogen-induced delayed fracture behavior of Q-P980 and MS980

In this paper, the hydrogen diffusion behavior and hydrogen induced delayed fracture (HIDF) of Q-P980 (Q-P: Quenching and Partitioning) and MS980 (MS: Martensitic steel) steels were investigated using hydrogen penetration, slow strain rate tensile (SSRT) tests, thermal desorption spectroscopy (TDS) tests, fracture analysis, and microstructural examination in this paper. The austenite in Q-P980 is massive retained-austenite (RA) with low stability. The TRIP (Transformation Induced Plasticity) effect will occur in the process of strain and change into high carbon martensite. HIDF is caused by a substantial amount of surplus hydrogen being enriched at the border and flaws. The fracture has a broad cleavage surface and is a typical quasi-cleavage fracture. MS980 has been sufficiently tempered, resulting in a substantial quantity of distributed spherical cementite (150nm) precipitating around the lath martensite. This size and form of cementite may successfully trap hydrogen while maintaining the material’s mechanical characteristics. And tempering can effectively reduce the local stress level of steel, so MS980 has a very low HE susceptibility. HIDF is related to local stress and hydrogen accumulation. We suppose that Z is a constant and Z C is a critical value which associated to σ and C H (the local stress and local hydrogen concentration), rising as σ and C H rises. The atomic bonds at the crack tip, lattice position and the phase interface will fracture when Z C reaches a particular value Z. Tempering to minimize local stress and carbide precipitation to capture hydrogen are two strategies for reducing hydrogen embrittlement (HE) susceptibility, particularly for dislocation strengthened steel. Microalloying elements can generate precipitates that function as hydrogen traps and obstruct the HELP (Hydrogen Enhanced Localized Plasticity) process, lowering local stress and hydrogen accumulation.

Cementite decomposition in 100Cr6 bearing steel during high-pressure torsion: Influence of precipitate composition, size, morphology and matrix hardness

Premature failure of rail and bearing steels by White-Etching-Cracks leads to severe economic losses. This failure mechanism is associated with microstructure decomposition via local severe plastic deformation. The decomposition of cementite plays a key role. Due to the high hardness of this phase, it is the most difficult obstacle to overcome in the decaying microstructure. Understanding the mechanisms of carbide decomposition is essential for designing damage-resistant steels for industrial applications.We investigate cementite decomposition in the bearing steel 100Cr6 (AISI 52100) upon exposure to high-pressure torsion (maximum shear strain, Ƴmax = 50.2). Following-up on our earlier work on cementite decomposition in hardened 100Cr6 steel (Qin et al., Act. Mater. 2020 [1]), we now apply a modified heat treatment to generate a soft-annealed microstructure where spherical and lamellar cementite precipitates are embedded in a ferritic matrix. These two precipitate types differ in morphology (spherical vs. lamellar), size (spherical: 100–1000 nm diameter, lamellar: 40–100 nm thickness) and composition (Cr and Mn partitioning). We unravel the correlation between cementite type and its resistance to decomposition using multi-scale chemical and structural characterization techniques.Upon high-pressure torsion, the spherical cementite precipitates did not decompose, but the larger spherical precipitates (≥ 1 μm) deformed. In contrast, the lamellar cementite precipitates underwent thinning followed by decomposition and dissolution. Moreover, the decomposition behavior of cementite precipitates is affected by the type of matrix microstructure. We conclude that the cementite size and morphology, as well as the matrix mechanical properties are the predominating factors influencing the decomposition behavior of cementite. The compositional effects of Cr and Mn on cementite stability calculated by complementary density functional theory (DFT) calculations are minor in the current scenario.

Microstructure engineering for superior wear and impact toughness strength of hypereutectoid powder metallurgy steel

ABSTRACT In this study, powder metallurgical steel samples containing 1.5% C by weight were produced by sintering in a vacuum atmosphere at 1200°C. The impact test and dry sliding wear test specimens were prepared in accordance with ISO 5754 and ASTM G-99 standards, respectively. The wear and impact toughness properties of spherical cementite in the bainitic matrix (SCBM) microstructured specimens produced with different heat treatment routes were compared with pearlite plus primary cementite, fully tempered martensitic, fully bainitic and spherical cementite microstructured specimens in ferritic matrix (SCFM). Microstructural characterisations of all specimens were performed by SEM, EBSD and XRD analyses methods. Compared to fully tempered martensitic and fully bainitic microstructured specimens with similar hardness, both the dry sliding wear and the impact toughness properties of the specimens having SCBM microstructures were improved with the increased austempering time after spheroidisation treatment.

Improving the impact toughness properties of high carbon powder metallurgy steels with novel spherical cementite in the bainitic matrix (SCBM) microstructures

Spherical cementite in the bainitic matrix (SCBM) microstructures in the high carbon powder metal steels having 1.5 wt% C were achieved in specimens with the spherical cementite in the ferritic matrix microstructures by isothermal annealing in a salt bath at 350 °C after annealing at 750 °C. The toughness properties of the specimens with SCBM microstructures were compared with pearlitic plus primary cementite, fully martensitic plus tempering, and the spherical cementite particles in the ferritic matrix microstructures. The toughness properties of specimens having SCBM microstructures have been improved by 300% and 80% compared to specimens having other microstructures.

Improvement of wear resistance in ferrite-pearlite railway wheel steel via ferrite strengthening and cementite spheroidization

It is important and necessary to improve the wear resistance of ER8 railway wheel steel without changing its hardness. In this paper, ferrite strengthening and cementite spheroidization were adopted to achieve the better wear performance in the improved wheel steel, i.e., HiSi steel. Firstly, alloying elements including Si, Mn and Ni were added to achieve ferrite strengthening. Besides, high-temperature tempering was applied to the HiSi steel to break the lamellar cementite into plates and spheroids. Subsequently, ER8 and HiSi steels were subjected to sliding wear test against to U71MnG steel. Finally, wear performance and relevant mechanisms were discussed in detail. Compared to the ER8 steel with lamellar cementite, the HiSi steel showed lower UTS of 846 MPa but higher elongation of 23.12% and Charpy absorbed energy of 37 J due to the spherical cementite. Moreover, less wear losses of the single pin and tribological pairs were also achieved in the HiSi steel. Adhesive wear and surface cracking were the dominant wear damage for both wheel pins. HiSi pin displayed more adhesive film, less and shallower surface cracks, thicker deformation layer and would suffer lower shear stress and normal stress. The better wear resistance of the HiSi steel can be attributed to the ferrite strengthening, less proeutectoid ferrite and granulate pearlite simultaneously.

Designing spherical cementite in bainitic matrix (SCBM) microstructures in high carbon powder metal steels to improve dry sliding wear resistance

Spherical cementite particles in the bainitic matrix in high carbon steel processed by powder metallurgy (PM) method were produced and compared with separately sintered, fully bainitic and spherical cementite in the ferritic matrix microstructures for their microstructure-hardness-dry sliding wear properties. The primary cementite plus dense lamellar pearlite structures were achieved in the sintered specimens. The spherical cementites in the ferritic matrix (SCFM) were obtained by over-tempering of the martensite phase produced by quenching after the sintering process. To produce spherical cementite in the bainitic (SCBM) microstructure, SCFM specimens were partially austenitised at 735 °C over Ac1 eutectoid temperature for 3 min and austempered at 300 °C for 1–2 h. Dry sliding wear resistance behavior of the specimens were examined under the constant load of 10 N at sliding distance of 1500 m. The dry sliding wear resistances of the SCBM specimens were much better than the microstructures produced by conventional heat treatments.

Effect of cementite spheroidization on improving corrosion resistance of pearlitic steel under simulated bottom plate environment of cargo oil tank

The influence of cementite spheroidization on the corrosion resistance of a pearlitic steel used as cargo oil tank under the simulated bottom plate environment was investigated by employing microstructural characterizations, electrochemical measurements and gravimetric tests. The results indicate that for the as-forged pearlitic steel with the gradual accumulation of lamellar cementite on the surfaces, the galvanic effect between the anodic ferrite and the cathodic cementite is enhanced during corrosion evolution. This leads to intense corrosion acceleration. However, after cementite spheroidization, the discrete spherical cementite is easy to separate from the matrix when the surrounding ferrite matrix is dissolved. The area ratio of cathodic to anodic keeps at a stable value, and both the anodic dissolution of iron and cathodic HER are retarded by cementite spheroidization. As a result, the galvanic acceleration effect is restrained and the corrosion resistance of the pearlitic steel is improved significantly.

Achievement of high strength-ductility combination in railway wheel steel with thin pearlite and spherical cementite via composition and undercooling design

To achieve high strength and ductility simultaneously, an improved railway wheel steel with high content of Si, i.e., HiSi steel was designed. For comparison, traditional wheel steel of ER8 was also machined from a commercial wheel rim. Then HiSi and ER8 steels were subjected to complete austenitizing followed by water quenching and spray cooling. The stop-cooling temperature during water quenching was about 580, 550 and 520 °C to achieve various degrees of undercooling. Specimens cooled to 520 °C were defined as ER8-520 and HiSi-520. Microstructure characteristics in terms of volume fraction proeutectoid ferrite (VFPF), interlamellar spacing of pearlite (ISP), cementite morphology were examined and mechanical properties including Brinell hardness, tensile properties, Charpy impact properties were characterized. The results indicated that spherical cementite of about 120 nm and submicron plate cementite were achieved simultaneously in HiSi-520 steel. ISP and VFPF in HiSi-520 were 120 nm and 8.9%, respectively. HiSi-520 steel exhibited higher hardness, tensile strength, Charpy impact energy and similar elongation compared to ER8-520 steel. Nanoscale spherical cementite, submicron plate cementite, thin ISP and some proeutectoid ferrite as well as solid solution strengthening from Si and Ni led to the excellent mechanical properties in HiSi-520 steel simultaneously.

Ultra-low-carbon steel spheroidization and torsion

Spheroidizing annealing and torsion testing of 0.027 wt% carbon steel rod were conducted to evaluate spheroidization kinetic behavior at 943 K (670 °C) under deformed and non-deformed states. Kinetic curves were also predicted using the Johnson–Mehl–Avrami–Kolmogorov equation, and the results agree well with the experimental ones. After spheroidization was performed twice, the spherical cementite and precipitated carbides became smaller and the distribution was more uniform. Comparison of materials subjected to single and double spheroidizing annealing indicated a difference in grain size. Torsion performance was considerably improved under double spheroidization, especially the maximum torque with slight variations.

Extending the boundaries of mechanical properties of Ti-Nb low-carbon steel via combination of ultrafast cooling and deformation during austenite-to-ferrite transformation

We underscore here a novel approach to extend the boundaries of mechanical properties of Ti-Nb low-carbon steel via combination of ultrafast cooling and deformation during austenite-to-ferrite transformation. The proposed approach yields a refined microstructure and high density nano-sized precipitates, with consequent increase in strength. Steels subjected to ultra-fast cooling during austenite-to-ferrite transformation led to 145 MPa increase in yield strength, while the small deformation after ultra-fast cooling process led to increase in strength of 275 MPa. The ultra-fast cooling refined the ferrite and pearlite constituents and enabled uniform dispersion, while the deformation after ultra-fast cooling promoted precipitation and broke the lamellar pearlite to spherical cementite and long thin strips of FexC. The contribution of nano-sized precipitates to yield strength was estimated to be ~247.9 MPa and ~358.3 MPa for ultrafast cooling and deformation plus ultrafast cooling processes. The nano precipitates carbides were identified to be (Ti, Nb)C and had a NaCl-type crystal structure, and obeyed the Baker-Nutting orientation relationship with the ferrite matrix.

Molecular dynamics simulations of point defect production in cementite and Cr23C6 inclusions in α-iron: Effects of recoil energy and temperature

The number of point defects formed in spherical cementite and Cr23C6 inclusions embedded into ferrite (α-iron) has been studied and compared against cascades in pure versions of these materials (only ferrite, Fe3C, or Cr23C6 in a cell). Recoil energies between 100 eV and 3 keV and temperatures between 400 K and 1000 K were used. The overall tendency is that the number of point defects — such as antisites, vacancy and interstitials — increases with recoil energy and temperature. The radial distributions of defects indicate that the interface between inclusions and the host tend to amplify and restrict the defect formation to the inclusions themselves, when compared to cascades in pure ferrite and pure carbide cells.

Improvement of Mechanical Properties of Spheroidized 1045 Steel by Induction Heat Treatment

The effects of induction heat treatment on the formation of carbide particles and mechanical properties of spheroidized 1045 steel were investigated by means of microstructural analysis and tensile testing. The induction spheroidization accelerated the formation of spherical cementite particles and effectively softened the steel. The volume fraction of cementite was found to be a key factor that affected the mechanical properties of spheroidized steels. Further tests showed that sequential spheroidization by induction and furnace heat treatments enhanced elongation within a short spheroidization time, resulting in better mechanical properties. This was due to the higher volume fraction of spherical cementite particles that had less diffusion time for particle coarsening.

Influence of morphology and structural size on the fracture behavior of a nanostructured pearlitic steel

Subjecting pearlitic steels to severe plastic deformation is known to transform the original colony structure to a nanostructured pearlite with the microstructural constituents aligned parallel to the deformation direction. Besides a huge increase in strength due to an enormous reduction of the interlamellar distance, other mechanical properties such as fracture toughness are deteriorated especially when the crack propagation is parallel to the shear deformed structure. Post-deformation heat-treatments up to 600°C were applied to modify the oriented nanostructured pearlite after applying high pressure torsion to a micro-duplex structure of ultrafine grained ferrite with spherical cementite dispersoids. Already moderate annealing led to a pronounced increase of the fracture toughness accompanied only by a slight drop in strength. Nevertheless, at those low annealing temperatures the deformation structure was partially present which crucially influenced the crack propagation behavior and thereby the mechanical anisotropy. By raising the annealing temperature it was possible to produce a fully spheroidized microstructure and in further consequence mechanical isotropy was achieved. The decrease in strength due to microstructural coarsening is balanced by a remarkable gain in fracture toughness.

Improvement of Hole-Expansion Property for Medium Carbon Steels by Ultra Fast Cooling After Hot Strip Rolling

The improvement of hole-expansion properties for medium carbon steels by ultra fast cooling (UFC) after hot strip rolling was investigated. It was found that finely dispersed spherical cementite could be formed after ultra fast cooling, coiling and annealing treatment. Tensile strength of the steel after annealing was measured to be about 440 MPa. During hole-expansion test, cracks were observed in the edge region around the punched hole because necking or cracking took place when tangential elongation exceeded the forming limit. Cracks were mainly formed by the coalescence of micro-voids. Fine and homogeneous microstructure comprised of ferrite and spheroidized cementite could increase elongation values of the tested sheets by suppressing the combination of the adjacent micro-voids, resulting in the improved hole-expansion property.