Microalloying Elements Research Articles

Tensile strength improvement of martensitic ODS steels with Zr and Hf additions

Aimed at evaluating the effectiveness of Zr and Hf additions on oxide particle refinement and tensile strength improvement of transformable oxide dispersion strengthened (ODS) steels, 9Cr–Zr and 9Cr–Hf ODS martensitic steels were fabricated by mechanical milling, spark plasma sintering and tempering heat treatment at 800 °C. The as-sintered 9Cr–Zr steel consisted of martensite and certain amount of transformed ferrite with M23C6 (M = Fe, Cr) precipites within ferrite interior. With respect to the as-sintered 9Cr–Hf steel, the microstructure consisted of martensite and small amount of residual ferrite. The residual ferrite is characteristics of higher number density of oxide nanoparticles than those in martensite. The added Zr or Hf cannot transform all Y2O3 into Y–Zr–O or Y-Hf-O complex oxides. Except for Y2O3, ZrO2 and Y4Zr3O12 were identified in 9Cr–Zr steel, and YxHfyOz and Y2H2O7 were found in 9Cr–Hf steel. The orientation relationship and interface structure between Y–Zr–O/Y-Hf-O and the matrix were studied in detail, as well as the size distributions of oxide nanoparticles in both as-sintered and tempered conditions. Tempering is mandatory to restore ductility of martensitic steels. The tensile strength of tempered 9Cr–Hf steel is superior to most reported results, while the tempered 9Cr–Zr steel exhibits a good combination of tensile strength and ductility. The ferrite phase in both tempered 9Cr–Zr and 9Cr–Hf steels has negligible effects on ductility degradation. The inferior strength of high-angle boundaries to tempered martensite itself is suggested to be responsible for limited ductility of steels. Compared to Al and Ti constituents, Zr and Hf can be considered as potential microalloying elements to produce ODS steels with favorable mechanical performance.

Experimental Analysis on Grain Growth Kinetics of SS316L Austenitic Stainless Steel

This paper presents an investigation of austenitic stainless steel grain growth kinetics of SS316L under different heating temperature ranges and holding times. The main variables such as apparent activation energy (Q), rate constant (K), and kinetic exponent (n) were analyzed to understand the grain growth kinetics in austenitic stainless steel. The empirical procedure was developed leading to the obtainment of variables that could define the grain growth kinetics based on different temperature ranges. The heat treatment process was isothermally held using quenching and deformations dilatometer at a temperature ranging from 900 °C to 1200 °C and holding times between 30 s to 240 s. The kinetic rates were estimated using an empirical equation. Based on the observations obtained by using optical microscopy. The result shows that the grain size can be predicted at a lower temperature than 1200 °C. However, the grains show irregular growth at a recrystallization temperature of 1200 °C which leads to a difficult estimation of grain size. It was observed that the variation in values of n and K are associated with the precipitation of the different micro-alloyed elements presented in the stainless steel SS316L. It can be also concluded that the texture plays an important role in the resulting kinetics change.

Effects of Ti/Mo and Ti/Cu ratio on precipitation behavior of Ti-bearing steel: findings from experiments and critical patent analysis

A dual phase structure consisting of ferrite and martensite has been achieved in Ti-bearing steels microalloyed with Mo or Cu, respectively, which were treated under different austenitizing and hot deformation conditions to investigate the effects of second microalloying elements and thermomechanical processes on the microstructural evolution and mechanical properties. The results showed the largest microhardness values, 343 HV0.1 for Ti–Mo steel and 307 HV0.1 for Ti–Cu steel, in microalloyed steels austenitized at 1200 °C with 30% deformation strain. Furthermore, critical patent analysis revealed that higher tensile strength is achieved in a Ti–Mo steel having a larger Ti/Mo atomic ratio, but no such relationship between tensile strength and Ti/Cu atomic ratio applies to Ti–Cu steel. This finding can be ascribed to the different metallurgical effects of Mo and Cu in the Ti-bearing steels. For example, the addition of Mo in a Ti-bearing steel leads to tiny carbides due to the decrease in coarsening via the synergistic effect of Ti and Mo. On the other hand, the separate precipitation of TiC and Cu in the Ti-bearing steels is consistent with the results of patent analysis. That is, the tensile strength of these steels is not associated with the Ti/Cu atomic ratio.

Effect of final cooling temperature on the microstructure and mechanical properties of high-strength anti-seismic rebar

Rebar is an important material in the major structural engineering, and its fine structure has a very important effect on the performance of the rebar. In this work, the Gleeble-3800 thermal simulator was used to simulate and control the final cooling temperature process to explore the effect of the precipitation behavior of the microalloying elements on the microstructure and mechanical properties of the rebar. The electron backscatter diffraction (EBSD), high-resolution transmission electron microscope (TEM), and universal tensile testing machine were used to characterize the microstructural transformation and mechanical properties of high-strength anti-seismic rebar. The results shows that under the conditions of different final cooling temperatures, the microstructure of the rebar were mainly composed of ferrite and pearlite. When the final cooling temperature decreased from 750 °C to 650 °C, the ferrite grain size decreased from 0.01237 mm to 0.00678 mm and the pearlite lamellar spacing decreased from 0.226 μm to 0.114 μm. The EBSD results found that the most of ferrite grains with larger misorientation angle (20° ∼ 60°) formed by the different austenite grains. The TEM results found that the main precipitates were (Nb, Ti, V) C, which precipitated on the ferrite matrix, and the shapes were oval, and the average particle sizes were about 20 ∼ 30 nm. When the final cooling temperature was 650 °C, the tensile strength and yield strength of the rebar reached 712.94 MPa and 562.97 MPa, respectively, and strength yield ratio was 1.27. With the decreases in the final cooling temperature, the tensile strength and yield strength of the rebar gradually increased.

Enhanced Thermal Stability of the Low‐Carbon Ultrafine Grain Steel with Nanoprecipitates

Niobium (Nb), a microalloy element, is purposefully added to improve the thermal stability of ferrite grains in a low‐carbon ultrafine grain (UFG) steel. Results manifest the excellent thermal stability in the Nb‐UFG steel with 0.028 wt% Nb addition by providing favorable kinetics and thermodynamic stabilization effects. The almost unchanged grain size and mechanical properties of Nb‐UFG steels annealed for 45 and 180 min at 500 °C are simultaneously obtained by the heterogeneous ferrite grains, NbC precipitates, and geometrically necessary dislocations (GNDs) mainly formed during tensile tests. The imperceptible change in size and volume fraction of NbC particles of the two Nb‐UFG steels annealed at different time are attributed to the inherent thermal stability of NbC and main precipitation pattern of NbC particles from the α phase.

The relation between mechanical properties and microstructural evolution induced by Sc microalloying in Al-20Zn-3Cu alloy

We demonstrate the effect of Sc microalloying on the mechanical properties of Al-20Zn-3Cu-xSc (x = 0, 0.1, 0.3, 0.5 wt%) alloys. Trace amounts of Sc addition (<0.5 wt%) simultaneously improve the mechanical strength and the ductility of the Al-20Zn-3Cu-xSc alloys. Among the developed alloys, the Al-20Zn-3Cu-0.3Sc alloy shows the highest tensile strength of 363 MPa with an elongation of 6.8%. Comprehensive microstructural investigation reveals that the Sc microalloying element induces macroscopic (~μm scale) and microscopic (~nm scale) structural changes. Macroscopically, the Al grain size and large particles typically formed in the grain boundaries are significantly reduced. Microscopically, the formation of nanoprecipitates is facilitated by the addition of trace amounts of Sc, resulted in generation of local strain field. Quantitative symmetry investigation then demonstrates that the formation of nanoprecipitates severely break the local symmetry of Al, which affects the mechanical properties of Al-20Zn-3Cu-xSc alloys.

Partitioning of the High Strength Steel Contained Ti Microalloy Element at the Temperatures Above and Below Martensitic Transformation Start Temperature (Ms)

Partitioning of the High Strength Steel Contained Ti Microalloy Element at the Temperatures Above and Below Martensitic Transformation Start Temperature (Ms)

Chemical heterogeneous structure and internal record of deformation behavior in Cu-Fe-Zr metallic glasses

Although chemical structural heterogeneities were induced into metallic glasses (MGs) to improve their ductility, the nature of the influence of the heterogeneous structure on their deformation behavior is still unclear. Here, by using microalloying elements to tailor the microstructure in Cu-Fe-Zr MGs, the formation of heterogeneous structure and the correlation between the heterogeneous structure and the deformation behavior are discussed. The results show that a honeycomb-like heterogeneous structure is formed in the (Cu0.5Fe0.5)40Zr59X1 (XNb and Ta) alloys. From a viewpoint of dynamic, the nature of the microalloying effect on the microstructure has been analysed. The deformation behavior of (Cu0.5Fe0.5)40Zr59X1 MGs with a high population of nanoscale glassy particles (NGPs) is investigated by nanoindentation and bending test. These glassy particles play an interesting role in the internal record of the local structure evolution. It was found that NGPs contiguous to shear bands (SBs) coarsen by Ostwald ripening. However, NGPs within SB would dissolve due to direct mechanical shearing and mixing. It indicates that the volume of shear transformation zone decreases obviously in the heterogeneous MGs due to the high fraction of interfaces. This work is of significance in understanding the relationship between structure and property of MGs.

Influence of Sc Microalloying on the Microstructure of Al5083 Alloy and Its Strengthening Effect

In this study, we investigate the influence of Sc microalloying on the microstructure of the Al5083 alloy. Trace amounts of Sc addition drastically improve the mechanical properties of the Al5083 alloy from 216 MPa to 233 MPa. Macroscopically, the addition of Sc significantly reduces the grain size of Al by approximately 50%. Additionally, a microstructural investigation reveals that the Sc microalloying element induces fine Al3Sc nanoprecipitates in the Al matrix. The formation of Al3Sc nanoprecipitates results in a pinning effect on the dislocations, leading to accumulated dislocations. Compared to a Sc-free Al5083 alloy specimen, the number density of dislocations in the Sc-added Al5083 alloy significantly increases after hot rolling, enhancing the tensile properties. We reveal that the improved mechanical properties of Al5083 with Sc microalloying originate from the grain refinement and the formation of fine Al3Sc nanoprecipitates.

Microstructure and Mechanical Properties of High Strength Low Alloy Steel Q550D

Drilling equipment is the key of oil and gas drilling development. Its manufacturing quality and service performance have important influence on oil and gas drilling development safety. The application of high strength grade steel plays an important role in improving drilling equipment manufacturing level and service performance. In this paper, the chemical composition, microstructure, tensile properties, impact properties and hardness of TMCP Q550D and Q-T Q550D high-strength low-alloy steel were tested and compared, and the application feasibility for drilling equipment manufacture was analyzed comprehensively. The experimental results show that the mechanical properties of Q550D by two different methods were obviously higher than the requirement of national standard. Q550D steel had excellent plasticity and toughness, which meets the requirement of drilling equipment manufacture. The main difference between different steel lied in their chemical composition and micro-structure. Carbon content of TMCP Q550D steel plate was lower than that of QT Q550D, and TMCP Q550 was mainly depend on TMCP technology and micro-alloy elements, whose micro-structure was mainly granular bainite. Q-T Q550D was mainly depend on Q-T technology, and its microstructure was tempered sorbite with obvious banded structure and slightly low toughness.

Effect of Carbonitride Precipitates on the Corrosion Resistance of Low-Alloy Steels under Operating Conditions of Oil-Field Pipelines

An investigation into the corrosion resistance of steels with various contents of carbon and microalloying elements was carried out. It was shown that the presence of a large amount of nanosized (2–3 nm and less) precipitates of the interphase type, particularly niobium carbonitride and vanadium carbonitride, leads to a decrease in the corrosion resistance of hot-rolled sheet products. It was found that, after heat treatment of rolled products at 710 °C, the corrosion resistance of the metal is improved. One of the reasons for this is a decrease in the amount of interphase precipitates, which negatively affect the corrosion resistance of steel, while particles formed in austenite and ferrite do not have such an effect. To ensure high corrosion resistance of steels for oil-field pipelines, microalloying with niobium instead of vanadium is advisable, as well as heat treatment at temperatures above 710 °C.

Effect of Microalloyed Elements M (M = Ce, Ti, V, and Nb) on Mechanical Properties and Electronic Structures of γ‐Fe: Insights from a First‐Principles Study

Herein, the first‐principles calculation is used to study the solid solution behavior of microalloyed elements M (M = Ce, Ti, V, and Nb) in steel, and the effects of M on the mechanical properties and electronic structures of the doped systems are analyzed. The calculated results of the solvation energy and formation enthalpy show that Ce, Ti, V, and Nb can be solubilized in γ‐Fe. The elastic modulus calculated results show that M doped reduces the incompressibility and rigidity and of the doped system, but the toughness and machinability are improved. A combination of Bader charge and differential charge density analysis shows that the metallic bond strength of the Fe–M system is weaker than that of the pure Fe system, which is the main reason for the decrease in incompressibility and rigidity of the doped system; the higher electron cloud density of the doped system is the main reason for its toughness increase.

Tailoring grain sizes of the biodegradable iron-based alloys by pre-additive manufacturing microalloying

We demonstrated the design of pre-additive manufacturing microalloying elements in tuning the microstructure of iron (Fe)-based alloys for their tunable mechanical properties. We tailored the microalloying stoichiometry of the feedstock to control the grain sizes of the metallic alloy systems. Two specific microalloying stoichiometries were reported, namely biodegradable iron powder with 99.5% purity (BDFe) and that with 98.5% (BDFe-Mo). Compared with the BDFe, the BDFe-Mo powder was found to have lower coefficient of thermal expansion (CTE) value and better oxidation resistance during consecutive heating and cooling cycles. The selective laser melting (SLM)-built BDFe-Mo exhibited high ultimate tensile strength (UTS) of 1200 MPa and fair elongation of 13.5%, while the SLM-built BDFe alloy revealed a much lower UTS of 495 MPa and a relatively better elongation of 17.5%, indicating the strength enhancement compared with the other biodegradable systems. Such an enhanced mechanical behavior in the BDFe-Mo was assigned to the dominant mechanism of ferrite grain refinement coupled with precipitate strengthening. Our findings suggest the tunability of outstanding strength-ductility combination by tailoring the pre-additive manufacturing microalloying elements with their proper concentrations.

Effect of micro alloying of steel with niobium on the mechanical properties of heat-strengthened rebar

One of the most effective micro-alloying elements in the production of class 500 fittings is vanadium. However, the production technology is multi-stage and expensive, so the use of ferrovanadium makes the production of vanadium-containing fittings for mass use unprofitable. In this regard, it is necessary to reduce the cost of finished products by replacing ferrovanadium with ferroniobium while preserving all the mechanical, physical and operational characteristics of the metal.Objective: to determine the effect of the quantitative content of niobium in steel in the production of hot-rolled rebar S-500WC in coils at the mill 370/150 of OJSC “BSW – Management Company of the Holding “BMK” in accordance with the requirements of the Israeli standard SI 4466-3:2013 at various temperature and speed parameters of rolled products. The normalized mechanical properties of s-500WC reinforcing steel of all profiles (No. 8–16) in accordance with THE requirements of si 4466-3:2013 are: yield strength – 500–650 MPa, ductility – 1.15–1.35, elongation of – at least 11 % and total elongation at break of – at least 7.5 %.

Correlation of structure parameters and performance characteristics of alloy steels for shipbuilding

The article presents the results of a study of the relationship between strength and performance (temperatures of ductile-brittle transition Tdb and zero plasticity NDT, critical opening at the crack tip CTOD at a test temperature of —40°C) on the structure parameters of thick plate products made of low-carbon low-alloy steels with different contents of basic alloying and microalloying elements.

Influence of Nb as Microalloying Element on the Recovery and Recrystallization of Fe–25Mn–12Cr–C–N Twinning‐Induced Plasticity Steels

The influence of Nb on the microstructure during annealing at 950, 1000, and 1100 °C is analyzed in two types of twinning‐induced plasticity (TWIP) steels, Fe–25Mn–12Cr–C–N (TWIP‐0) and Fe–25Mn–12Cr–C–N–Nb (TWIP‐Nb). The addition of Nb as a microalloying element affects various phenomena taking place during annealing, namely, recrystallization, grain coarsening, and recovery processes. Microstructural characterization is conducted via light microscopy, scanning electron microscopy, and electron back scattering diffraction (EBSD). Recovery takes place after annealing at 950 °C, where remaining deformation and grain nucleation can be seen. Microstructural analyses indicate that the location of the recrystallization nuclei in the recovered structure is associated with the local chemical segregation of Mn and Cr, which leads to differences in the driving force for the martensitic transformation at microscale, and therefore local deformation mechanisms. The presence of Nb as a microalloying element decelerates recovery and recrystallization kinetics. At 1100 °C/10 min, both steels exhibit complete recrystallization; moreover, abnormal grain growth starts.

Understanding the beneficial microalloying elements in Fe-based bulk metallic glasses via clusters

Glass forming ability (GFA) is one of the key factors hindering the application of bulk metallic glasses (BMGs). Microalloying has shown beneficial effects on GFA. However, the mechanism is still unclear. A new guidance via clusters was applied to understand and quickly choose beneficial microalloying elements for enhancing GFA which might help build a foundation for further quantitative analysis of minor alloying via clusters. Under the guidance, a series of beneficial microalloying elements could be quickly discovered, verified and understood. The results show that the beneficial microalloying elements for enhancing GFA should be chosen under the condition of bringing in new types of topologically efficiently packed clusters.

Energetic segregation of B, C, N, O at the γ-TiAl/α2-Ti3Al interface via DFT approach

The effects of small atom impurities (B, C, N, O) segregation at the γ-TiAl/α2-Ti3Al interface were studied employing density functional theory (DFT) method. Energetic and bonding properties of the most stable interstitial configuration in the TiAl(111)/Ti3Al(0001) interface with and without segregated solutes were investigated.Results found that the octahedral interstices are energetically the preferential location for interstitial impurities at the TiAl/Ti3Al interface. Based on the data of interfacial energy, alloying with B and C atoms can destabilize the γ/α2 interface, while N and O atoms are beneficial for the stability by reducing the interface energy compared to the pure γ/α2 one. The most expected reductions are found for O impurity that makes the best stability. Besides, the calculation results of cleavage energy indicate that the segregation of B, N, and O will strengthen the γ/α2 interface, while the presence of C can maximally improve the ductility. Furthermore, charge density difference was computed to analyze interfacial atomic bonding behaviors. The mechanical properties are expected to be greatly improved in TiAl alloys reinforced wear-resistance coating and other functional devices after adding micro-alloying elements.

Comparison of microstructure and heat treatment distortion of gear steels with and without Nb addition

In order to research the effect of microalloying Nb, the microstructure and heat treatment distortion of case carburizing steels with and without Nb addition are compared. Results show that a uniform and fine prior austenite grain size was obtained for the steel with the addition of 0.03 wt.% Nb even after being carburized at 980 °C for 37 h, resulting in a very deep hardened layer of about 4 mm. Nb is an effective microalloying element to hinder austenite grain growth of gears during high-temperature carburizing. Theoretical calculation and experimental observation of NbC precipitates indicate that fine NbC precipitates have not evidently dissolved at 980 °C, and thus they can act as grain refiners due to pinning effect. Heat treatment distortion of Nb-added steel is much lower than that of the steel without Nb addition. It may be contributed to its fine and uniform grain size, which presumably influences stress during martensitic transformation.

Toughness Property Control by Nb and Mo Additions in High-Strength Quenched and Tempered Boron Steels

The synergetic effect on hardenability by combining boron with other microalloying elements (such as Nb, Mo and Nb + Mo) is widely known for high-strength medium carbon steels produced by direct quenching and subsequent tempering treatment. The improvement of mechanical properties could be reached through optimization of different mechanisms, such as solid solution hardening, unit size refinement, strain hardening, fine precipitation hardening and the effect of carbon in solid solution. The current study proposes a procedure for evaluating the contribution of different microstructural aspects on Charpy impact toughness. First, the effect that austenite conditioning has on low-temperature transformation unit sizes and microstructural homogeneity was analysed for the different microalloying element combinations. A detailed crystallographic characterization of the tempered martensite was carried out using electron backscattered diffraction (EBSD) in order to quantify the effect of unit size refinement and dislocation density. The impact of heterogeneity and presence of carbides was also evaluated. The existing equations for impact transition temperature (ITT50%) predictions were extended from ferrite-pearlite and bainitic microstructures to tempered martensite microstructures. The results show that microstructural refinement is most beneficial to strength and toughness while unit size heterogeneity has a particularly negative effect on ductile-to-brittle transition behaviour. By properly balancing alloy concept and processing, steel having a yield strength above 900 MPa and low impact transition temperature could be obtained by direct quenching and tempering.