Interphase Precipitation Research Articles

Cu Precipitation Behaviors and Microscopic Mechanical Characteristics of a Novel Ultra-Low Carbon Steel.

We studied the effect of Cu addition on the hardness of ultra-low carbon steels heat treated with different cooling rates using thermal simulation techniques. The microstructural evolution, Cu precipitation behaviors, variations of Vickers hardness and nano-hardness are comparatively studied for Cu-free and Cu-bearing steels. The microstructure transforms from ferritic structure to ferritic + bainitic structure as a function of cooling rate for the two steels. Interphase precipitation occurs in association with the formation of ferritic structure at slower cooling rates of 0.05 and 0.2 °C/s. Coarsening of Cu precipitates occurs at 0.05 °C/s, leading to lower precipitation strengthening. As the cooling rate increases to 0.2 °C/s, the interphase and dispersive precipitation strengthening effects are increased by 63.9 and 50.0 MPa, respectively. Cu precipitation is partially constrained at cooling rate of 5 °C/s, resulting in poor nano-hardness and Young’s Modulus. In comparison with Cu-free steel, the peak Vickers hardness, nano-hardness and Young’s Modulus are increased by 56 HV, 0.61 GPa and 55.5 GPa at a cooling rate of 0.2 °C/s, respectively. These values are apparently higher than those of Cu-free steel, indicating that Cu addition in steels can effectively strengthen the matrix.

Evolution of microstructure and room temperature tensile properties in aluminum‐bearing high strength steel processed by ultrafast cooling

AbstractAn aluminum‐bearing high strength steel with relatively high precipitation hardening was fabricated by lowering coiling temperature to 627 °C. The influence of coiling temperature on microstructure evolution and precipitation behavior was comprehensively investigated by means of scanning electron microscopy, electron back‐scatter diffraction and transmission electron microscopy. Both yield strength and tensile strength can be increased by around 100 MPa through decreasing coiling temperature to 627 °C, whereas there is nearly no deterioration in ductility. In addition, The grain boundary precipitation of (Fe, Cr, Mn)xCy‐type carbides can be effectively suppressed by lowering coiling temperature. Regardless of coiling temperature, both interphase precipitation consisting of curved and planar shape and random precipitation can be observed. Moreover, the sheet spacing can be refined from 29 nm–47 nm to 18 nm–27 nm by lowering coiling temperature from 721 °C to 627 °C. This small sheet spacing provides a greater precipitation hardening. Compared with a coiling temperature of 721 °C, the precipitation hardening can be increased by around 100 MPa for a coiling temperature of 627 °C.

Interphase boundary segregation and precipitate coarsening resistance in ternary alloys: An analytic phase-field model describing chemical effects

Many experimental and first principles studies on precipitation hardening alloys show that segregation of elemental species to the matrix-precipitate interphase boundary (IB) reduces the boundary’s energy. This segregation mechanism can thermally stabilize the microstructure against precipitate coarsening processes and allow for higher operating temperatures in structural applications. In this paper, we develop a phase-field modeling framework to describe IB solute segregation in ternary alloys. The interfacial thermodynamics is effectively described by defining an IB phase with a characteristic free energy-concentration dependence. Equilibrium for the IB phase is established via the parallel tangent plane construction, analogous to classical treatments for segregation to free surfaces and grain boundaries. Analytic steady-steady solutions elucidating the dependence of IB properties on bulk phase composition, temperature and model parameters are derived for a one-dimensional system. Analytic relations for the classical thermodynamic quantities–IB energy and relative solute excess–are derived and the Gibbs adsorption equation is shown to hold; therefore, predictions of the model can be compared with experiments and atomistic simulations. An application of the model is demonstrated for Zn segregation to Mg/Mg2Sn using representative IB parameters. A two-particle coarsening simulation of IB segregation is performed: the result demonstrates enhanced coarsening resistance of the ternary alloy relative to the binary alloy.

The effect of molybdenum on interphase precipitation at 700 °C in a strip-cast low-carbon niobium steel

The effect of Mo on the clustering and interphase precipitation occurring during phase transformation has been investigated in a strip-cast low-carbon steel containing Nb. In this work, isothermal coiling treatments were performed at 700 °C for various times to form interphase precipitates. Transmission electron microscopy (TEM), small-angle neutron scattering (SANS) and atom probe tomography (APT) results show that the addition of Mo increases the number density and volume fraction of clusters and interphase precipitates but has a negligible effect on their size, and overall provides for greater strengthening in the Mo-containing alloy. APT analysis shows that the interphase precipitates are composed of Nb, C and N in both alloys. The segregation of Mo is also observed in the interphase precipitates in the Mo-containing alloy, which is believed to relieve the lattice misfit between the precipitate and the matrix. It is proposed that the reduced lattice misfit allows for precipitate nucleation to occur more easily in the Mo-containing alloy.

Isothermal Precipitation Behavior of Vanadium Carbonitride in V‐, N‐Added Low‐Carbon Steel

Herein, the isothermal precipitation behaviors of V(C,N) in ferrite are studied. At 700 °C, the dominant precipitation behavior of V(C,N) is interphase precipitation (IP), whereas at 660 and 620 °C, random precipitation (RP) is dominant. Both IP and RP show the Nishiyama–Wassermann orientation relationship (OR) with ferrite, unlike the common OR between precipitation and ferrite. The size of most precipitated particles varies in the ranges of 5–8 nm at 660 °C and 3–5 nm at 620 °C. The average values of hardness of ferrite grain formed at 700, 660, and 620 °C are 195.1, 237.6, and 278.6 HV, respectively.

Effects of Various Factors on Interphase Precipitation of Nano-Sized Alloy Carbides in Low-Carbon Steels

Effects of Various Factors on Interphase Precipitation of Nano-Sized Alloy Carbides in Low-Carbon Steels

Nanoindentation response of interphase precipitation in a Nb microalloyed steel

ABSTRACT In this work, dedicated isothermal transformation studies were performed in a Fe–C–Mn–Nb alloy to obtain two types of precipitate morphology embedded in a similar ferritic matrix. Nano-hardness of individual ferrite grain was measured by using an in situ nano-mechanical tester. The peak of nano-hardness distribution from interphase and dispersed precipitate strengthened ferrite corresponds to the nano-hardness range of 2.6–2.8 and 2.5–2.6 GPa, respectively. A wider spread of nano-hardness from interphase precipitate strengthened ferrite was observed and rationalised by considering the variation in inter-particle spacing on slip planes. The ‘pop-ins’ on the load-depth curve was not able to be correlated with the morphology of a precipitate.

Effect of the Final Rolling Temperature on the Precipitation Behavior and Toughening Mechanism of Nanoparticles in Ferritic Steel

We have analyzed the effect of the final rolling temperature on the morphology, mechanical properties, carbide nanoparticle size, and dislocation density of Ti-Mo ferritic steel. The microstructure was mainly composed of equiaxed ferrite, ultrafine ferrite, and polygonal deformed ferrite, whose three proportions were affected significantly by the final rolling temperature. The change trend of the mechanical strength was in line with the dislocation density under different finishing rolling temperatures, whereas the ferrite grain size change trend was the opposite. There were various types of nanoparticle precipitation in the test steels: interphase precipitation, dispersion, dislocation precipitation, and deformation-induced precipitation. The optimum mechanical properties were obtained at a final rolling temperature of 900 °C. Quantitative analysis showed that the effect of the final rolling temperature on the yield strength was mainly because of dislocation and precipitation strengthening. The toughening mechanism is attributed to the fine grain strengthening of nanoparticles in austenite stage and the precipitation strengthening of ferrite stage.

Effect of Deformation on Precipitation Behaviors of Carbide in Ti-Nb-Mo Steel

The precipitation behaviors of (Ti, Nb, Mo)C in specimens of Ti-Nb-Mo steel hold at 700°C under no deformation condition and deformation condition with engineering strain 0.4 imposed in austenite region have been studied. The results indicate: under no deformation condition, the rows of interphase precipitation show straight and/or curve morphology, and the precipitation behaviors include interphase precipitation, random precipitation and fibrous precipitation; under deformation condition, the rows of interphase precipitation are straight, and the precipitation behaviors include interphase precipitation and strain-induced precipitation. Furthermore, the dispersion of precipitate particles formed under deformation condition is more uniform than those formed under no deformation condition.

Weak influence of ferrite growth rate and strong influence of driving force on dispersion of VC interphase precipitation in low carbon steels

Dispersion of nano-sized alloy carbides formed by interphase precipitation at migrating ferrite/austenite interphase boundaries is expected to be affected by local conditions at the boundaries, including ferrite growth rate and driving force for precipitation. The effects of these two factors were systematically investigated by changing alloy composition and transformation temperature. Quantitative analyses by using three-dimensional atom probe reveal that VC interphase precipitation becomes higher in number density and smaller in size at lower transformation temperature or with higher V content. The dispersion of VC is also slightly refined by lower Mn or higher Si content, but remains almost unchanged by increasing the C content. Such variations show good correlations with the driving force for its precipitation that finer VC precipitates can be obtained by enlarging the driving force. In contrast, the influence of the ferrite growth rate on the dispersion in most cases is quite small in the range of this study, which can be explained by considering nucleation kinetics of VC at migrating ferrite/austenite interface.

The Microstructure Evolution and Precipitation Behavior within HSLA Steels with Different M/C Atomic Ratios

The microstructure and hardening behavior of HSLA steels with different microalloying element/carbon (M/C) atomic ratios, namely high M/C steel (H-M/C) and low M/C steel (L-M/C), were investigated in the present study. A dual precipitation morphology of nanosized carbides, namely interphase precipitation carbides and random precipitation carbides, was found at the same ferrite grain of both kind of steels, and these precipitates are believed to provide the main strengthening ability within ferrite. Of the two steels with different M/C atomic ratios, the ferrite matrix of H-M/C steel showed a rising trend in micro-hardness with increasing isothermal holding times from 5 min to 60 min at 650°C to 700°C. This higher strengthening ability occurring within ferrite grains after holding at longer times can be ascribed to the higher austenite decomposition rate occurring in the H-M/C atomic ratio steel. Therefore, the hardening mechanisms in both kinds of steel are also discussed in the present study.

Synergistic effects of carbon content and Ti/Mo ratio on precipitation behavior of HSLA steel: Insights from experiment and critical patent analysis

A one-step heat treatment comprising isothermal holding in the two-phase region for different time periods and subsequent quenching contributed to the development of a ferrite and martensite dual-phase structure in TiMo microalloyed steels with different carbon and microalloying element contents. Irrespective of the Ti/Mo atomic ratio, a dual precipitate morphology, namely interphase precipitation and random precipitation, was found in the ferrite matrix. Furthermore, under the fixed carbon content, harder ferrite grains were obtained in the steel with a high [Ti]/[Mo] atomic ratio. This could be explained by the fact that a large volume fraction of tiny-sized carbides formed within the ferrite matrix when the [Ti]/[Mo] atomic ratio was increased. In addition, several microalloyed steels with different [Ti]/[Mo] atomic ratios listed in highly-cited patents were also retrieved and analyzed to support the above deduction.

Interphase Precipitation and Application to Practical Steels

Interphase precipitation is a phenomenon in which precipitates are generated on interphase boundaries during phase transformation, creating carbides having a form like bands or fibers in micro-alloyed steels. In isothermal transformation, a lower transformation temperature reduces the carbide diameter and band spacing, and a higher cooling rate reduces them in the continuous cooling process.

Roles of transformation interfaces in the design of advanced high strength steels

Alloy partitioning and segregation or precipitation at transformation interfaces have been important in designing the mechanical properties of final products. Some representative examples in high-strength low-alloy steels are briefly discussed, with an emphasis on the transformation interface. For achieving better strengthening-ductility balance in TRIP-aided low alloy steels, it is essential to control carbon enrichment in austenite during ferrite and bainite transformations and the stability of retained austenite. Recent studies on alloy partitioning including carbon enrichment into austenite, provides a deeper insight into transformation mechanisms. We show that energy dissipation during interface migration and strain energy accumulation are important factors to control carbon enrichment into untransformed austenite during the ferrite/bainite transformation. As a counterpart of TRIP steels, interphase precipitation of alloy carbides during ferrite transformation leads to high yield strength and good elongation in low-carbon ferritic steels. A key for further strengthening is clarification of the conditions for refining the carbide dispersion. It is shown that incoherent ferrite/austenite interfaces are superior nucleation sites for carbides.

The effect of strain on interphase precipitation characteristics in a Ti-Mo steel

In the current study, the effect of deformation in the non-recrystallization region on the characteristics of interphase precipitation was extensively studied in a Ti-Mo steel. The present observation revealed a novel mechanism affecting the interphase precipitation characteristics during an austenite-to-ferrite transformation, as a result of the thermomechanical processing. The hot deformation primarily led to the fragmentation of an austenite grain interior through the formation of elongated dislocation walls, locally deviating the orientation within a grain. Relatively coarse strain-induced precipitates were initially formed on the dislocation walls upon cooling. During the isothermal austenite to ferrite transformation, the interphase precipitation took place on the advancing austenite-ferrite interface front. This was similar to the strain-free condition, though the alignment of planar rows was abruptly altered while encountering the microband walls due to local orientation change at either side of a given microband. In addition, the deformation reduced the planar spacing of interphase precipitates up to a given strain (i.e., ∼0.3), beyond which it became nearly constant. This was due to the saturation of dislocation density within austenite above a certain strain and/or the consumption of alloying elements through the strain-induced precipitation formed on the microbands prior to the austenite to ferrite transformation.

A Gibbs Energy Balance Model for Growth Via Diffusional Growth-Ledges

Growth ledges are commonly observed on interphase boundaries during diffusional phase transformations and are of great importance for understanding inter-sheet spacing of interphase precipitates. A simple model based on Gibbs Energy Balance (GEB) for describing growth kinetics via diffusional growth-ledges of height λ is presented for the case of ferrite growth into austenite. The model is validated against the case of austenite to ferrite transformation involving interphase precipitation in a V, Mn, Si alloyed HSLA steel where, λ is assumed to be equal to the inter-sheet spacing of interphase carbide precipitates. The presented model provides a computationally efficient and versatile method for predicting the ledge height, λ, and the growth kinetics of ferrite from initial nucleation through to final soft impingement considering the evolution of solute drag at growth ledge risers. It is suggested that the intrinsic mobility of growth ledge risers is: M_{m}^{aR} = 0.58(exp\frac{-140x10^{3}{RT})mmol.J^{-1}s^{-1}, with R the gas constant and T the absolute temperature in K.

The Influence of Isothermal Holding Times on Microstructural Evolution of X90 Linepipe Steel

Microstructural phase of linepipe steels depends on different isothermal conditions. Thermal cycling testing of X90 linepipe steel was conducted using a thermomechanical simulator. The results have shown that with increased holding time, the microstructure constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. There was the amount of martensite in the microstructure for isothermal holding times of 5 and 10 s at 700oC. The effects of the martensite strengthening were weak. The changes of the hardness curve are decided by microstructural phase and the precipitation behavior. The interphase precipitation seems to begin for isothermal holding times greater than 30 s. It has a peak value for isothermal holding time of 600 s. Some fcc (Ti, Nb) (N, C) particles which belong to the MX-type precipitates were obtained at this condition. It is a major microstructural contributor to the hardness. The beneficial effects of the precipitates decreased considerably due to coarsening of the precipitates with prolonged holding, which results in the hardness quickly dropping. As a result, the hardness exhibited a low value after isothermal holding for 3600 s because the extent of precipitation strengthening was lessened and a lot of polygonal ferrites were formed.

Microstructural features and precipitation behavior of Ti, Nb and V microalloyed steel during isothermal processing

Thermal simulations of Ti, Nb and V microalloyed steel were conducted using a thermomechanical simulator, and the microstructural evolution and the precipitation behavior during isothermal processing were analyzed. The results show that with increasing holding time, the microstructural constituents change from the martensite and bainitic ferrites to granular bainite and polygonal ferrite. The maximum hardness is obtained for the specimen after isothermal holding for 5 s due to the martensite strengthening effect. The hardness of the specimen decreases after isothermal holding for 10 s, because the strengthening contribution of fine dispersed precipitates becomes weaker. The hardness values of the specimens increase and then remain high after isothermal holding for 60 and 600 s. This is attributed to the contribution of the interphase precipitation hardening to the hardness of the studied steel. The precipitates in the specimen are coarsened after isothermal holding for 3600 s, even though the coarsening is not remarkable. These precipitates are fcc (Ti, Nb)(N, C) particles and belong to the MX-type precipitates. The beneficial effects of precipitation strengthening are lost. The hardness decreases to a minimum hardness value due to the presence of large amounts of polygonal ferrite after isothermal holding for 3600 s. Relatively coarse precipitates are the primary origin of the hardness decrease.

Atom Probe Compositional Analysis of Interphase Precipitated Nano-Sized Alloy Carbide in Multiple Microalloyed Low-Carbon Steels.

The composition of nano-sized alloy carbides formed by interphase precipitation in V-Nb and V-Ti multiple microalloyed low-carbon steels is analyzed by using three-dimensional atom probe. Carbide-forming alloying elements including V, Nb, and Ti, are simultaneously precipitated from the early stage of isothermal treatment, whose atoms are uniformly distributed in the carbide particles, even after prolonged holding. Cluster analysis by the maximum separation method, with parameters optimized using different methods, is carried out to extract alloy carbides from matrix. The composition of alloy carbides evaluated by site fraction of substitutional carbide-forming alloying elements indicates that at the early stage of their formation, Nb and Ti are more strongly enriched than V.

The effect of molybdenum on interphase precipitation and microstructures in microalloyed steels containing titanium and vanadium

Despite much research into steels strengthened through interphase precitation, there remains much that is not clear, such as the role of a range of elements, particularly Mo, in the interphase precipitation process. Four steels were manufactured with identical composition, but with variations in Ti, V, Mo and N content to investigate the effect of composition on interphase precipitation. Alloys were rapidly cooled from the single austenite phase field and isothermally transformed at 630 °C and 650 °C for 90min. The addition of Mo was found to significantly reduce the austenite to ferrite transformation kinetics, particularly for the V steel. Interphase precipitation was observed in all alloys at both transformation temperatures. For the Ti bearing steel, the two types of precipitate were observed throughout the sample, namely TiC (finer) and Ti2C (coarser), while for the V bearing steels, VC (finer) and V4C3 (coarser) were observed. Where Mo was present in the alloy, it was found dissolved in all carbide types. The (Ti,Mo)C and (V,Mo)C formed by classical planer interphase precipitation (PIP) while the (Ti,Mo)2C and (V,Mo)4C3, that had a much wider row spacing, formed through curved interphase precipitation (CIP). Each adopted one variant of the Baker-Nutting orientation relationship. The Ti-microalloyed steels exhibited the smallest precipitates of all the steels, which were approximately the same size irrespective of whether Mo was present in the alloy and irrespective of the transformation temperature. However, the addition of Mo to the V bearing steels resulted in a significant increase in precipitate volume fraction and a reduction in precipitate size. The mechanisms of interphase precipitation leading to the coincident production of two different precipitate types is considered and the role of Mo on the interphase precipitation process is discussed. The resultant effect on strength is considered.