Phase Transformation Strengthening Research Articles

Influence of ultrasonic surface rolling on tensile properties of high carbon low alloy quenching-partitioning-tempering steel

The effect of ultrasonic surface rolling process on tensile properties of a novel Fe-0.6C-2.0Al-1.5Mn-0.04 Nb quenching-partitioning-tempering steel was investigated. Optical microscope, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy were brought out to characterize the microstructure. Compared with quenching-partitioning-tempering steel, the strength and plasticity of the ultrasonic rolling specimen were increased simultaneously, reaching 1329.1 MPa and 23.6%, respectively, and the product of strength and elongation reached the range of the third generation Advanced High Strength Steels. The plasticity improvement mechanism of the USRP1specimen is mainly stemmed from two aspects: First, obvious strain gradients during the tensile process was found in the USRP specimens, which not only avoids the strain localization, but also generates a biaxial stress state. The geometrically necessary dislocations induced by the strain gradient and the dislocations induced by the biaxial stress state promoted extra work hardening, thus ensuring high plasticity of the material. Second, there was a moderate content and small size of twinned martensite in the USRP1 specimen. The improvement in yield strength of the USRP specimens primarily results from grain refinement strengthening, dislocation strengthening, phase transformation strengthening, and back stress strengthening. Among them, phase transformation strengthening contributed the most.

The Characteristics of Refractory Steel and Its Development and Application in Petroleum and Other Fields

Because the strength of structural steel decreases sharply at high temperature, it must be sprayed with fire-resistant coating or fire-resistant materials for composite use. The strength of refractory steel can be reduced by no more than 1/3 of the room temperature at high temperature, and maintain good high temperature strength. This article mainly summarizes the development history of refracto-ry steel in China and other countries, analyzes the material selection basis and strengthening mecha-nism of refractory steel, and finds that by combining Mo element solid solution strengthening, phase transformation strengthening, and Nb, the strength guarantee of refractory steel at high temper-atures and V element precipitation strengthening are obtained. Designers can calculate the service temperature of steel structure through the initial stress and design strength of components to select refractory steel. Finally, the application status of refractory steel and its application prospect in the field of oil rescue are summarized and prospected.

Ultra-high strength and high toughness 24CrNiMo alloy steel fabricated by laser powder bed fusion and subsequent quenching

A nearly fully dense grade 24CrNiMo alloy steel with superior comprehensive performance was fabricated by laser powder bed fusion (LPBF) under optimum laser parameters and subsequent quenching treatment was applied. The effect of quenching temperatures on microstructure evolution and mechanical properties of the LPBF 24CrNiMo parts were first systematically characterized and analyzed. The results demonstrated that the microstructure of LPBF 24CrNiMo steel (submicron grain size of 2.99 μm on average) was mainly constituted by block and stripe granular bainite containing twin substructure and θ-Fe3C carbides with no macroscopic segregation and anisotropy. After quenching, with the increase of quenching temperature from 800 to 1000 °C, the microstructure gradually changed from the acicular ferrite to martensite, finally to tempered martensite. The fine grains, the numerous twins were retained and the nanoscale carbides η-Fe2C were precipitated after quenching at 850 °C, simultaneously obtaining ultra-high mechanical properties including the hardness, yield strength, ultimate tensile strength and elongation were 446 HV and 1380 MPa, 1517 MPa, 13.8 %, respectively as well as the surface scratch resistance improved about 25 %. As quenching temperatures varied to 1000 °C, the mechanical properties lowered due to the coarsening and uneven distribution of the grain and θ-Fe3C carbides as well as the decrease in the number of twins. The improved mechanical properties via quenching treatment was attributed to phase transformation strengthening, grain refinement strengthening and precipitate strengthening.

Design and performance of nitrogen-alloyed iron-based coating for enhancing galling resistance by plasma transferred arc welding

Iron-based alloys have been widely used as high-temperature galling resistant coatings. In this study, we designed and prepared a galling-resistant coating using plasma transferred arc welding with cored wires of varying nitrogen content. Our approach was based on the concept that nitrogen alloying can lead to the formation of hard precipitates and promote strain-induced phase transformations. The coating was primarily composed of austenite, and as the nitrogen content increased, the grain size decreased and the amount of nitride precipitation increased. The threshold galling stress of this cobalt-free coating was found to reach 80% of that of a laser-clad Stellite 21 coating at 300 °C. Following the galling test, the hardness of the worn area increased and a phase transformation from austenite to martensite occurred beneath the worn surface. The improvement in galling resistance under dry heavy load sliding conditions can be attributed to the synergistic effect resulting from multiple strengthening mechanisms, including fine-grain strengthening, second phase strengthening and phase transformation strengthening.

Effects of Cr and Ni on the microstructure and corrosion resistance of high-strength low alloy steel

The effects of Cr and Ni on the microstructure and corrosion resistance of high-strength low alloy steel (HSLAs) were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction, electrochemical tests, X-ray photoelectron spectroscopy and Raman spectroscopy. The results showed that the grain size and microstructure of the steel were optimized due to the increase of crystallinity and Cr3C2 relative content by adding Cr and Ni. The tensile strength was enhanced because of the refined crystalline strengthening, phase transformation strengthening and solution strengthening. Electrochemical tests displayed that the self-corrosion current density of steel and the total anode current density as well as the number of total anodes of the WBE decreased significantly by adding Cr and Ni during immersion in 3.5 wt% NaCl solutions for 24 h. During the corrosion process, the composition and phase structure of the rust layers changed for the existence of Cr and Ni in the Cr–Ni steel, and the formation of NiFe2O4 and FeCr2O4 promoted the increase of the Fe2+/Fe3+ ratio and the relative content of α-FeOOH in the rust layers enhancing the compactness and protectiveness of the corrosion products. Besides, the microstructure of the steel containing Cr and Ni had significant effects on the mechanical strength and corrosion resistance.

Effect of Tungsten Addition on Continuous Cooling Transformation and Precipitation Behavior of a High Titanium Microalloyed Steel

Effects of tungsten addition on the continuous cooling transformation (CCT) characteristics and precipitation behavior of a high titanium microalloyed steel were investigated by dilatometry, optical microscopy, scanning and transmission electron microscopy, and hardness measurements. The results showed that the ranges of transformation products were moved to the right side of the CCT diagram when the 0.4% W was added. Accordingly, the following observations were made: (i) the ferrite phase transformation was shifted to the side of lower cooling rates and reduced temperatures; (ii) the bainite phase transformation region ran throughout the whole cooling rate range studied. Addition of W had a positive effect on the particle size refinement and number density increase of the precipitates. At the low cooling rates, in the range of <14 °C/s, W addition shifted the precipitation hardening peak to the low cooling rate side as the ferrite transformation induced stronger precipitation strengthening than the bainite one. Furthermore, the effect of W addition on phase transformation strengthening was obvious (increase in hardness: ~40Hv) at the high cooling rate range, over 14 °C/s.

Microstructural evolution and mechanical properties of a novel biomedical Ti–6Zr–4Fe alloy during solution and aging treatment

Microstructural evolution and its effect on microhardness and Young's modulus of Ti–6Zr–4Fe alloy solution and aging treated at different temperatures were studied. The α phase transformed into β phase completely at 860 °C, meanwhile, the athermal ω phase was found at 860 °C and it resulted in a higher Young's modulus. The ω phase content increased from 250 °C to 400 °C, and the content decreased with further increase of aging temperature, there were only α phase and β phase at 600 °C. The phase transformation sequence was β → ω + β → ω + α + β → α + β during aging treatment. The microhardness increased from 315 HV to 492 HV due to the element solution strengthening and phase transformation strengthening. The peak microhardness of Ti–6Zr–4Fe alloy aged at 400 °C reached 652 HV. The formation of ω phase resulted in the increase of microhardness and Young's modulus in Ti–6Zr–4Fe alloy.

Influence of Additives on Microstructure and Mechanical Properties of Alumina Ceramics

Alumina is one of the most commonly used and researched structural ceramic because of its excellent properties. However, its intrinsic brittleness is the fatal drawback, which hinders it from wider applications. How to improve its fracture toughness as well as the bending strength is always challenging for material researchers. In this paper, alumina matrix composites were fabricated by hot-pressing, in which some additives, including zirconia, alumina platelets, and MXene, were incorporated. The influence of the introduced additives on their microstructure and mechanical properties was investigated. Compare with the monolithic alumina, both bending strength and fracture toughness of all samples were improved greatly. Incorporation of zirconia was beneficial to the mechanical properties due to the phase-transformation strengthening and toughening mechanism. While alumina platelets resulted in high fracture toughness because of the self-toughening of elongated grains. The synergistic effect of alumina platelets and MXene enormously improved the fracture toughness from 2.9 ± 0.3 MPa·m1/2 for monolithic alumina to 7.5 ± 0.4 MPa·m1/2 for the composite, which was increased by 159%. This work will provide useful references for the fabrication of high-strength and high-toughness alumina ceramics by introducing additives properly.

Utilizing local phase transformation strengthening for nickel-base superalloys

Almost 75 years of research has been devoted to producing superalloys capable of higher operating temperatures in jet turbine engines, and there is an ongoing need to increase operating temperature further. Here, a new disk Nickel-base superalloy is designed to take advantage of strengthening atomic-scale dynamic complexions. This local phase transformation strengthening provides the alloy with a three times improvement in creep strength over similar disk superalloys and comparable strength to a single crystal blade alloy at 760 °C. Ultra-high-resolution chemical mapping reveals that the improvement in creep strength is a result of atomic-scale η (D024) and χ (D019) formation along superlattice stacking faults. To understand these results, the energy differences between the L12 and competing D024 and D019 stacking fault structures and their dependence on composition are computed by density functional theory. This study can help guide researchers to further optimize local phase transformation strengthening mechanisms for alloy development.

Isothermal transformation and precipitation behaviors of titanium microalloyed steels

The microstructure transformation and precipitation behavior of nano-carbides in Ti microalloyed steel during isothermal process were studied by a compression test on a Gleeble 3800 thermomechanical simulator and analyzed by optical microscopy, transmission electron microscopy and other methods. The results show that γ → α phase transformation and TiC precipitation take place in Ti microalloyed steel during the isothermal process, and time–temperature–transformation curve and precipitation–time–temperature (PTT) curve are all of “C”-type. During the isothermal process, the interphase precipitation of TiC mostly occurs at the period of the phase transformation, and the random precipitation of TiC mostly occurs on the ferrite after the phase transformation. The increment in yield strength at the initial stage of isothermal transformation mainly comes from phase transformation strengthening. With the increase in isothermal time, the precipitation hardening effect becomes more important for nucleation and growth of titanium carbides and eventually reaches the maximum value at the precipitation finished point of the PTT curve.

Effects of Finish Rolling Temperature and Deformation on Microstructural Evolution and Precipitation Behavior of Non‐quenched and Tempered Carbide‐Free Bainitic Steel

Double compression tests are carried out to investigate the microstructural evolution and precipitation behavior of Nb–Ti microalloyed carbide‐free bainitic steel under different finish rolling conditions. With decreasing finish rolling temperature and increasing finish rolling deformation, the area fraction of lath bainite (LB) and martensite (M) was increased, the width of bainitic ferrite lath was decreased, the dislocation density and the number fraction of high‐angle grain boundaries are raised. Meanwhile, the larger deformation and lower temperature reduce the amount of retained austenite (RA) and refine the precipitated particles, which benefit the performance further. Hence, the phase transformation strengthening, refinement strengthening, dislocation strengthening, and precipitation strengthening synergistically contribute to the increase in hardness with increasing deformation and decreasing temperature.

Tailoring phase transformation strengthening and plasticity of nanostructured high entropy alloys.

Metastable high entropy alloys (HEAs) have attracted extensive attention due to their excellent combination of high strength and great plasticity. In this work, we utilize constraining effects to tailor phase transformation strengthening and plasticity of nanostructured HEA ([double bond, length as m-dash]FeCoCrNi) thin films prepared by the magnetron sputtering technique via HEA/Cu and HEA/Ni nanotwinned nanolaminates (NTNLs). It is uncovered that the HEA/Cu NTNLs without phase transformation exhibit the fashion of "smaller is stronger" and HEA layers become more favorable to detwin at a smaller layer thickness (h). By contrast, the HEA/Ni NTNLs manifest an ultra-high peak hardness plateau accompanied by the FCC-to-HCP phase transformation in HEA layers at large h≥ 25 nm, whereas they manifest size-dependent hardness when detwinning occurs at smaller h. This unusual plastic deformation behavior of HEA/X (X = Cu, Ni) NTNLs was rationalized by partial-based mechanisms. These findings open a new avenue to achieve superior mechanical properties of HEAs particularly at the nanoscale.

Phase Transformation Strengthening of Hot Extruded Inconel 625 under High-temperature Load Environment

Microstructure evolution and properties of hot-extruded Inconel 625 alloy were investigated at different creep temperatures, aging time and strain rates. The experimental results indicate that the Inconel 625 alloy exhibits an excellent creep resistance at 700 °C and below. When the creep temperature rises to 750 °C, the creep resistance falls drastically due to the failure of phase transformation strengthening and the precipitation of a large amount of δ phase and σ phase at the grain boundary. The special temperature-sensitive characteristics of Inconel 625 alloy play a very important role in its fracture. When the strain rate is 8.33×10−3 s−1, the strength of the specimen is higher than that of other parameters attributed to the effect of phase transformation strengthening. With the increase of Ni3 (Al, Ti), the phase transformation strengthening inhibits thickening of the stacking faults into twins and improves the overall mechanical properties of the alloy. With the increase of the aging time, the granular Cr-rich M23C6 carbides continue to precipitate at the grain boundary, which hinders the movement of the dislocations and obviously increases the strength of the samples. Especially, the yield strength increases several times.

Controlling of reheated quenching temperature of 1000 MPa grade steel plate for hydropower station

AbstractThe strength and toughness of 1000 MPa grade steel plate for hydropower station treated by different reheated quenching temperatures were investigated. With the increasing of reheated quenching temperature, the yield strength and tensile strength increase sharply, whereas the value of impact toughness decreases slowly. The lath martensite with high density dislocations enhances dislocation strengthening. On the contrary, the acicular or block ferrite (soft phase) produced by intercritical quenching reduces the phase transformation strengthening. Moreover, the ferrite has a low solubility of interstitial carbon due to the body‐centered‐cubic lattice structure. The bar‐shaped precipitates occur during the isothermal holding at the intercritical temperature and it will reduce the precipitation strengthening. The ferrite phase and high misorientation boundaries are the main factors that contribute to the toughening of the experimental steel. The lower the reheated quenching temperature is, the higher proportion of ferrite and high misorientation boundaries becomes. Considering the requirements for mechanical properties of 1000 MPa grade steel plate for hydropower station, the optimal temperature of reheated quenching is ∼920 °C.

Exceptional phase-transformation strengthening of ferrous medium-entropy alloys at cryogenic temperatures

High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe60Co15Ni15Cr10 (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.

Phase evolution and mechanical properties of coarse-grained heat affected zone of a Cu-free high strength low alloy hull structure steel

Weldability of a Cu-free high strength low alloy (HSLA) hull structure steel is studied. Strengthening contributions of this steel are analyzed and phase continuous cooling transformation in simulated coarse-grained heat affected zone (CGHAZ) is investigated. Indeed, strengthening contributions of simulated CGHAZs under heat input 50 and 20 kJ/cm, respectively, are estimated. It shows this Cu-free HSLA hull structure steel is dominated at grain refinement strengthening and supplemented by precipitation strengthening and phase transformation strengthening, while the simulated CGHAZ under heat input 50 or 20 kJ/cm is dominated at phase transformation strengthening and supplemented by solid solution strengthening and grain refinement strengthening. The drawn simulated heat affected zone continuous cooling transformation (SH-CCT) diagram shows this Cu-free HSLA hull structure steel can obtain main lath-like microstructures in a wide cooling rate range with low phase start temperature. Mechanical property tests of simulated CGHAZ samples indicate high strengths and good low temperature impact toughness. All results exhibit this Cu-free HSLA hull structure steel has satisfactory weldability, which suggests its wide potential applications.

MICROSTRUCTURE AND SINTERING MECHANISM OF SiC CERAMICS REINFORCED WITH NANOSIZED ZrO2

Silicon carbide-based (SiC) ceramics has attracted quite broad attention due to their excellent mechanical, chemical and thermal properties. However, their widespread industrial application is hindered by difficulties in sintering and poor fracture toughness of sintered bodies. In this work, we present an alternative way to produce SiC-based ceramics with improved microstructure and mechanical properties. We incorporated ZrO 2 nanofibres into the ceramic matrix to achieve a combined reinforcing effect of partially stabilized zirconia, namely fibre and phase transformation strengthening. For comparison, we also prepared silicon carbide ceramics containing yttria stabilized zirconia (YSZ) particles. SiC-based green bodies containing 5 wt%, 10 wt% and 15 wt% ZrO 2 nanofibres and particles, respectively, were subjected to spark plasma sintering (SPS) at relatively low (1700°C) temperatures with high heating and cooling rates. The effects of nanofibres on mechanical properties were studied by determining the Vickers hardness and Young’s modulus of sintered ceramics from instrumented indentation tests. The microstructural patterns were investigated, as well.

Processing and Evaluation of Ferritic-Bainitic Multi-phase Steel

Production of bainite-ferrite multi-phase steels on industrial CSP scale provides approach to improve mechanical properties along with large cost reduction opportunity. In terms of hot-rolling conditions, chemical composition, and microstructure; characterizing ferritic-bainitic steels and their difference from ferritic-pearlitic steels regarding properties and cost becomes of a great importance to understand the optimum conditions for production. This work is aiming to improve mechanical properties of steels by producing bainite-ferrite steel through industrial scale runs on the 6 stands hot strip mill (HSM) with at EZDK steel using low alloy Al-killed steel. The effect of hot rolling coiling temperature and alloy content on final properties was studied. In addition to characterizing differences between ferrite-bainite steels and conventional ferrite-pearlite steels. Ferrite-bainite steel has been successfully produced in different conditions, showing up to 27% (~100 MPa) improvement in yield strength, 8% (42 MPa) in tensile strength, 13% in total elongation, and 11% (27 J) in impact toughness. Vanadium microalloying has proven to have almost no effect on mechanical properties of ferrite-bainite steels upon its rising from 0.045% to 0.063% which is not the case in ferrite-pearlite steels. Ferrite-bainite steel offers a great margin for alloy cost saving compared to ultra-low carbon microalloyed steels (up to 56%) through replacing chemical strengthening with phase transformation strengthening. We expect this new approach to significantly reduce the cost of production without sacrificing the quality.

Effect of quenching on the matrix microstructure of SiCf/Ti–6Al–4V composites

Heat treatment can change the matrix microstructure of metal based composites, which leads to the change of mechanical properties of the composites. In the present work, the variation of microstructure and hardness of the matrix of SiCf/Ti–6Al–4V composites prepared by foil-fiber-foil (FFF) method were investigated after quenching from 950, 980 and 990 °C. The results indicated that the β transus temperature of the matrix was close to 990 °C, which is slightly higher than that of the Ti–6Al–4V alloy, which is between 950 and 970 °C. This increase was attributed to the diffusion effect of carbon atoms. Some carbide particles were precipitated at the prior β grain boundaries when full martensite was obtained. Transmission electron microscopy examination observed three α′ variants that obeyed the Burgers orientation relationship with the β phase in the sample quenched from 950 °C. The hardness of the matrix increased with the increase in quenching temperature, and the highest hardness of 420 HV was obtained after quenching from 990 °C, which is much higher than that of the Ti–6Al–4V alloy. The high hardness was resulted from phase transformation strengthening by martensite, solid solution strengthening of carbon atoms and the dispersion strengthening of TiC precipitates.

Phase transformation strengthening of high-temperature superalloys.

Decades of research has been focused on improving the high-temperature properties of nickel-based superalloys, an essential class of materials used in the hot section of jet turbine engines, allowing increased engine efficiency and reduced CO2 emissions. Here we introduce a new ‘phase-transformation strengthening' mechanism that resists high-temperature creep deformation in nickel-based superalloys, where specific alloying elements inhibit the deleterious deformation mode of nanotwinning at temperatures above 700 °C. Ultra-high-resolution structure and composition analysis via scanning transmission electron microscopy, combined with density functional theory calculations, reveals that a superalloy with higher concentrations of the elements titanium, tantalum and niobium encourage a shear-induced solid-state transformation from the γ′ to η phase along stacking faults in γ′ precipitates, which would normally be the precursors of deformation twins. This nanoscale η phase creates a low-energy structure that inhibits thickening of stacking faults into twins, leading to significant improvement in creep properties.