The effect of the precipitation of coherent and incoherent precipitates on the ductility and toughness of high-strength steel

The effect of the coexistence of coherent and incoherent precipitates, such as M₂C and NiAl, on the ductility and plane strain fracture toughness of 5 wt pct Ni-2 wt pct Al-based high-strength steels was studied. In order to disperse coherent and incoherent precipitates, the heat treatments were carried out as follows: (a) austenitizing at 1373 K, (b) tempering at 1023 or 923 K for dispersing the incoherent precipitates of M₂C and NiAl, and then (c) aging at 843 K for 2.4 ks to disperse the coherent precipitate of NiAl into the matrix, which contains incoherent precipitates, such as M₂C and NiAl. The results were obtained as follows: (a) when the strengthening precipitates consist of coherent ones, such as M₂C and/or NiAl, the ductility and toughness are extremely low, and (b) when the strengthening precipitates consist of coherent and incoherent precipitates, such as M₂C and NiAl, the ductility and fracture toughness significantly increase with no loss in strength. It is shown that the coexistence of coherent and incoherent precipitates increases homogeneous deformation, thus preventing local strain concentration and early cleavage cracking. Accordingly, the actions of coherent precipitates in strengthening the matrix and of incoherent precipitates in promoting homogeneous deformation can be expected to increase both the strength and toughness of the material.     

Optimization of mechanical properties of high-carbon pearlitic steels with Si and V additions

Systematic research has been undertaken on the effects of single and combined additions of vanadium and silicon on the mechanical properties of pearlitic steels being developed for wire rod production. Mechanical test results demonstrate that the alloy additions are beneficial to the mechanical properties of the steels, especially the tensile strength. Silicon strengthens pearlite mainly by solid-solution strengthening of the ferrite phase. Vanadium increases the strength of pearlite mainly by precipitation strengthening of the pearlitic ferrite. When added separately, these elements produce relatively greater strengthening at higher transformation temperatures. When added in combination the behavior is different, and substantial strength increments are produced at all transformation temperatures studied (550 °C to 650 °C). The addition of silicon and vanadium to very-high-carbon steels (>0.8 wt pct C) also suppresses the formation of a network of continuous grain-boundary cementite, so that these hypereutectoid materials have high strength coupled with adequate ductility for cold drawing. A wire-drawing trial showed that total drawing reductions in area of 90 pct could be obtained, leading to final tensile strengths of up to 2540 MPa in 3.3-mm-diameter wires.     

Microstructure and mechanical properties of a 2000 MPa grade co-free maraging steel

The precipitation kinetics in the aging temperature range of 713 to 813 K in a 2000 MPa grade Co-free maraging steel (Fe-18.9 pct Ni-4.1 pct Mo-1.9 pct Ti, mass pct) has been studied. Study on microstructure and mechanical properties showed that a great deal of Ni₃Ti and a type of unknown spheroidal precipitates both with average diameter of 2 to 3 nm are formed in the early aging stage at 713 K, which results in a high strength and a relatively low fracture toughness. Ni₃Ti precipitates grow into needle or rod shape and become the main precipitation as the aging time is prolonged. Strength increases and fracture toughness (K _IC ) decreases with growth of the precipitates. The ultra-high strength of the maraging steel subjected to long-time aging at 713 K is attributed to the high resistance to coarsening of the precipitates. The strengthening in the underaged condition at 713 K is a combination of dislocations cutting through precipitates and the Orowan mechanisms. Aged at 813 K, the size of Ni₃Ti precipitates is seriously nonuniform at the early stage and a small amount of interlath reverted austenite is formed. Thereafter, Ni₃Ti precipitates coarsen sharply accompanied with the embrittlement. Intralath reverted austenite appears subsequently. In the later stage of aging, the coarsened Ni₃Ti precipitates dissolve into the striplike intralath reverted austenite that is disorderly embedded in the matrix. All of these result in a low strength and low fracture toughness under overaging condition. Analysis shows that the formation of reverted austenite contains the diffusion and Kudjumov-Sachs (K-S) and Nishiyama-Wassermann (N-W) shear mechanisms.     

Effect of phosphorus on the mechanical properties of hot-rolled 0.1c–1.0mn steel strip

The stress-strain behavior of simulated hot-rolled strip of a series of O.1C-l.OMn steels containing up to 0.2 pct phosphorus was determined at temperatures between -196 and 300° (-321 and 572∮F). The strengthening effect of the phosphorus additions did not depend on the simulated coiling temperature,i.e., quenching to 482 or 704° (900 or 1300∮F), and the magnitude of strengthening-28, 52, and 90 MPa (4, 7.5, and 13 ksi) for 0.05, 0.1 and 0.2 pct phosphorus, respectively-was similar to that observed previously in Fe and in normalized 0.1C-1.0Mn steel. Comparison of the present results with those obtained previously shows that phosphorus strengthens by solid-solution hardening, and its strengthening effect is additive to other strengthening mechanisms. Simulating a coiling temperature of 482° as compared with 704° resulted in an increase in strength of about 55 MPa (8 ksi) and a reduction in the notch-impact transition temperature of 34° (61∮F). Although the notch-impact transition temperature increased with phosphorus additions regardless of simulated coiling temperature, the steel quenched to 482° and containing 0.05 pct phosphorus had a lower transition temperature than the base steel quenched to 704° even though the yield and tensile strengths were about 90 MPa (13 ksi) greater.     

Effect of Boron on the Strength and Toughness of Direct-Quenched Low-Carbon Niobium Bearing Ultra-High-Strength Martensitic Steel

The effect of boron on the microstructures and mechanical properties of laboratory-control-rolled and direct-quenched 6-mm-thick steels containing 0.08 wt pct C and 0.02 wt pct Nb were studied. The boron contents were 24 ppm and a residual amount of 4 ppm. Two different finish rolling temperatures (FRTs) of 1093 K and 1193 K (820 °C and 920 °C) were used in the hot rolling trials to obtain different levels of pancaked austenite prior to DQ. Continuous cooling transformation (CCT) diagrams were constructed to reveal the effect of boron on the transformation behavior of these steels. Microstructural characterization was carried out using various microscopy techniques, such as light optical microscopy (LOM) and scanning electron microscopy-electron backscatter diffraction (SEM-EBSD). The resultant microstructures after hot rolling were mixtures of autotempered martensite and lower bainite (LB), having yield strengths in the range 918 to 1067 MPa with total elongations to fracture higher than 10 pct. The lower FRT of 1093 K (820 °C) produced better combinations of strength and toughness as a consequence of a higher degree of pancaking in the austenite. Removal of boron lowered the 34 J/cm² Charpy-V impact toughness transition temperature from 206 K to 158 K (?67 °C to ?115 °C) when the finishing rolling temperature of 1093 K (820 °C) was used without any loss in the strength values compared to the boron-bearing steel. This was due to the finer and more uniform grain structure in the boron-free steel. Contrary to expectations, the difference was not caused by the formation of borocarbide precipitates, as verified by transmission electron microscopy (TEM) investigations, but through the grain coarsening effect of boron.     

Effects of some carbide stabilizing elements on creep-rupture strength and microstructural changes of 18-10 austenitic steel

The creep rupture test has been carried out for 18Cr-10Ni-0.1 wt pct C stainless steels bearing individually Ti, Nb(Cb), and V, followed by the microstructural study. The highest value of 700°C-10⁴ h rupture strength in a titanium and niobium series (the steel containing various amounts of titanium and niobium, respectively) has been obtained at Ti/C and Nb/C atomic ratio of 0.8 and 0.2 to 0.4, respectively. On the other hand, in a vanadium series, the creep rupture strength of the steel showed its maximum at V/C atomic ratio of about unity in the testing at the temperature of 700° and 800°C, but at 600°C, the strength increases monotonically with vanadium content up to 1.53 wt pct. Such high strength in the steels con-taining proper amount of Ti, Nb, and V is related mainly with the fine distribution of M₂₃C₆ precipitates which is caused by the acceleration of nucleation due to the foregoing precipi-tation of a MC type carbide within the austenite grains. And it has been deduced that the solid solution strengthening effect of the vanadium contributes also to the remarkable in-crease in the rupture strength of the vanadium steel at 600°C.     

Laboratory simulation of seamless tube piercing and rolling using dynamic recrystallization schedules

A seamless tube mill rolling process was simulated with the aim of designing alloys and schedules for the production of as-hot-rolled yield strength levels which are currently attained only by quenching and tempering. Tests were performed on four candidate Ti-V-N steels with two levels each of carbon (0.10 and 0.18 wt pct) and vanadium (0.10 and 0.16 wt pct). Two different types of schedule were used: (1) a high-temperature schedule, corresponding to the production of large tube diameters, which utilizes the principles of recrystallization-controlled rolling (RCR) and (2) a second (for small tubes), which relies ondynamic recrystallization-controlled rolling (DRCR). The DRCR schedule, together with the higher cooling rate achievable on the smaller tube sizes, produces a much finer ferrite grain size than the RCR schedule. However, this is largely offset by increased strengthening in the RCR structurevia precipitation hardening. As-hot-rolled yield strengths in the range of 483 to 715 MPa (70 to 104 ksi) can be achieved using either type of schedule.     

Effects of B,C, and Zr on the structure and properties of a P/M nickel base superalloy

The boron and carbon levels of a P/M nickel base superalloy were systematically varied in order to determine the mechanisms by which these elements strengthen the alloy, and their optimum concentration. Carbon levels were reduced to 20 ppm while the boron level was varied from 0.02 to 0.10 wt pct. Carbon levels of 0.002 and 0.05 wt pct were also studied, while maintaining a boron concentration of 0.02 wt pct. Zirconium levels were maintained at 0.06 wt pct. The resulting alloys were subjected to identical heat treatments and examinedvia SEM, TEM, and STEM microscopy. The alloys were also subjected to tensile, creep, stress-rupture, and fatigue crack growth tests. Results show that both carbon and boron have a strong influence on the formation of grain boundary precipitates, as expected. Carbon was present as the MC and M₂₃C₆ type carbides, while boron combined to form an intergranular M₃B₂ boride. Boron and zirconium were observed to be critical to the alloys' mechanical properties, although boron levels above the solubility limit resulted in no further improvement or debit in strength. Carbon additions resulted in no improvement in properties, indicating the feasibility of a carbon-free P/M superalloy. The role of the minor element additions is discussed in terms of both microstructural features and related strengthening mechanisms.     

Production of ultra-high strength wire rod steels by vanadium microalloying

Ultra-high strength high-carbon wire rod steels have been produced using vanadium-microalloying technique instead of the conventional expensive and environment polluting lead patenting treatment. The strength increment attained in the hot rolled steels due to vanadium additions is maintained in the cold drawn wire. By using this technique, high tensile strength levels of 1550-1600 N/mm² were attained either by cold drawing of 0.17% V microalloyed high-carbon steel to 45-47% reduction or by cold drawing of 0.20% V microalloyed high-carbon steel to 25-30% reduction. An equation has been developed to predict the tensile strength from the chemical composition, cooling rate and reduction of area due to cold drawing. A combination of vanadium microalloying and accelerated cooling resulted in additional strength increment due to refining of microstructure and increasing the precipitation strengthening component. Inspite of the decrease in the amount of vanadium precipitates due to the increase in cooling rate, it is suggested that an increase in precipitation strengthening due to refining of these precipitates by accelerated cooling more than offsets the loss of precipitation strengthening due to decreasing the precipitates fraction.     

Age hardening and mechanical properties of a 2400 MPa grade cobalt-free maraging steel

The age hardening kinetics in the temperature range of 713 to 813 K of a 2400 MPa grade cobalt-free maraging steel (Fe-(18.8 ∼ 19.1) pct Ni-(4.4 ∼ 5.4) pct Mo-2.6 pct Ti, wt pct) has been studied. Study of microstructure and mechanical properties showed that a high number of Ni₃Ti and Fe₂(Mo,Ti) precipitates were formed during the ageing process, which resulted in high strength and relatively low fracture toughness. Ni₃Ti was the main precipitation phase. Fractography has shown ductile failure of tensile and fracture toughness specimens. Thermodynamic calculations showed that the equilibrium phases are Ni₃Ti, Fe₂(Mo,Ti), ferrite, and austenite.     

Structure and properties of rapidly solidified 7075 P/M aluminum alloy modified with nickel and zirconium

Aluminum alloy 7075 was modified by additions of 1.1 wt pct nickel and 0.8 wt pct zirconium, rapidly solidified by ultrasonic gas atomization, canned, cold compacted, hot extruded, and evaluated in terms of structure and properties. Significant improvements in tensile strength (627 MPa YS and 680 MPa UTS) and crack growth rates were realized, along with a decrease in fracture toughness (23.7 MPa√m) while maintaining ductility (10 pct elong.) as compared to nominal I/M 7075 behavior. The stress for 10⁷ cycles fatigue life was greater than 275 MPa, which represents a 73 pct increase over that of I/M 7075. A variety of experiments was performed to evaluate effects on strength, ductility, and on structure. The variables were: powder size distribution, extrusion ratio, extrusion profile, different size fractions from the same lot of powder, and different locations of test bars in the several extrusions. Tensile properties, toughness, and fatigue properties were not importantly influenced by the location of test bars in the cross section or length of rectangular extruded bars. A comparison of mechanical properties from extruded bars prepared from ?53 μm powdersvs 53 to 250 μm powders showed a small loss of ductility and fatigue stress for 10⁷ cycles for the fine powder product. Higher extrusion ratios were beneficial for mechanical properties.     

Effects of Alloying Elements on Creep Properties of Mg-Gd-Zr Alloys

The minimum creep rate and microstructures of aged samples of Mg-Gd-Zr alloys, with and without alloying additions of Zn and/or Y, have been investigated in the present work. The creep tests were performed at 523 K (250 °C) and under 80 to 120 MPa, and the microstructures before and after creep tests were characterized using scanning electron microscopy, transmission electron microscopy, and the high-angle annular dark-field imaging technique. It is found that dislocation creep predominates in the steady-state creep stage for all alloys. The Mg-2.5Gd-0.1Zr (at. pct) alloy, strengthened by the β′ precipitates, has minimum creep rates in the range 1.0 × 10^?8 to 3.8 × 10^?8 s^?1 under 80 to 120 MPa. The addition of 1.0 at. pct Zn to the Mg-2.5Gd-0.1Zr alloy reduces the 0.2 pct proof strength and increases the minimum creep rate, resulting from the formation of γ′ basal plates at the expense of β′ precipitates. The replacement of 1.0 at. pct Gd by Y in the Mg-2.5Gd-1.0Zn-0.1Zr alloy leads to a substantial reduction in the minimum creep rate, even though it does not cause much change to the 0.2 pct proof strength. The reduced minimum creep rate is attributed to a much lower diffusivity of Y atoms than Gd in the solid magnesium matrix. An increase in the Gd content from Mg-1.5Gd-1.0Y-1.0Zn-0.1Zr to Mg-2.5Gd-1.0Y-1.0Zn-0.1Zr leads to a denser distribution of precipitates, a higher 0.2 pct proof strength, and a further reduction in the minimum creep rate.     

Structure and mechanical properties of unidirectionally solidified Fe-Cr-C and Fe-Cr-X-C alloys

In this work four different microstructures were obtained by unidirectional solidification of Fe-Cr-C eutectic alloys. Conditions for zone coupled growth were determined in alloys containing approximately 30 wt pct chromium. Furthermore, mechanical testing indicated that the maximum strength was exhibited by Fe-30Cr-C alloys with cerium or titanium additions. These alloys had the largest volume fraction of eutectic fibers and their ultimate tensile strength was of the order of 3250 MPa. Correlations between the rate of crystal growth(u) and fiber spacing (λ) or tensile strength(Rm) were found and an expression of the typeRm = Aλ^-b2 was obtained whereb ₂ varied between 0.283 and 0.685. Finally, manganese or chromium (35 wt pct Cr) additions did not lead to appreciable improvements in composite strength for this alloy system.     

Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels

Austenite reversion in martensitic steels is known to improve fracture toughness. This research focuses on characterizing mechanical properties and the microstructure of low-carbon, high-nickel steels containing 4.5 and 10 wt pct Ni after a QLT-type austenite reversion heat treatment: first, martensite is formed by quenching (Q) from a temperature in the single-phase austenite field, then austenite is precipitated by annealing in the upper part of the intercritical region in a lamellarization step (L), followed by a tempering (T) step at lower temperatures. For the 10 wt pct Ni steel, the tensile strength after the QLT heat treatment is 910 MPa (132 ksi) at 293 K (20 °C), and the Charpy V-notch impact toughness is 144 J (106 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). For the 4.5 wt pct Ni steel, the tensile strength is 731 MPa (106 ksi) at 293 K (20 °C) and the impact toughness is 209 J (154 ft-lb) at 188.8 K (?84.4 °C, ?120 °F). Light optical microscopy, scanning electron and transmission electron microscopies, synchrotron X-ray diffraction, and local-electrode atom-probe tomography (APT) are utilized to determine the morphologies, volume fractions, and local chemical compositions of the precipitated phases with sub-nanometer spatial resolution. The austenite lamellae are up to 200 nm in thickness, and up to several micrometers in length. In addition to the expected partitioning of Ni to austenite, APT reveals a substantial segregation of Ni at the austenite/martensite interface with concentration maxima of 10 and 23 wt pct Ni for the austenite lamellae in the 4.5 and 10 wt pct Ni steels, respectively. Copper-rich and M₂C-type metal carbide precipitates were detected both at the austenite/martensite interface and within the bulk of the austenite lamellae. Thermodynamic phase stability, equilibrium compositions, and volume fractions are discussed in the context of Thermo-Calc calculations.     

Thermal processing of ferritic 5Mn steel for toughness at cryogenic temperatures

It is shown that a thermal treatment which combines grain refinement with an intercritical temper (the 2BT treatment) may be used to achieve a promising combination of strength and toughness in a nickel-free ferritic steel of nominal composition Fe-5Mn-0.2Mo-0.04C at temperatures as low as -196 °C. The properties achieved are attributed to a symbiotic influence between the grain refinement treatment and the introduction of thermally stable retained austenite during intercritical tempering, a conclusion supported by a comparison of the results to those obtained with simpler heat treatments. The influence of carbon, manganese, and nickel additions to the base compositions are studied. An increase in carbon content above 0.04 wt pct causes a deterioration in toughness, as does an increase in manganese to 8 wt pct. An addition of 1 to 3 wt pct nickel is beneficial giving an increase in alloy strength at -196 °C without loss of toughness. Formerly Visiting Scientist, Lawrence Berkeley Laboratory.     

Strengthening Mechanisms and Their Relative Contributions to the Yield Strength of Microalloyed Steels

Microalloyed steels are used widely in oil and gas pipelines. They are a class of high-strength, low-carbon steels that contain small additions (in amounts less than 0.1?wt pct) of Nb,?Ti, and/or V. The steels may contain other alloying elements, such as Mo, in amounts exceeding 0.1?wt pct. Precipitation in these steels can be controlled through thermomechanical-controlled processing, leading to precipitates with sizes that range from several microns to a few nanometers. Microalloyed steels have good strength, good toughness, and excellent weldability, which are attributed in part to the presence of the nanosized carbide and carbonitride precipitates. Because of their fine sizes, wide particle size distribution, and low volume fractions, conventional microscopic methods are not satisfactory for quantifying these precipitates. Matrix dissolution is a promising alternative to extract the precipitates for quantification. Relatively large volumes of material can be analyzed so that statistically significant quantities of precipitates of different sizes are collected. In this article, the microstructure features of a series of microalloyed steels (X70, X80, and X100) as well as a Grade 100 steel are characterized using optical microscopy (OM) and scanning electron microscopy (SEM). A chemical dissolution technique is used to extract the precipitates from the steels. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) are combined to analyze the chemical composition of these precipitates. Rietveld refinement of the XRD patterns is used to quantify fully the relative amounts of these precipitates. The size distribution of the nanosized precipitates is quantified using dark-field imaging (DF) in the TEM. The effects of microalloying content, finish rolling temperature (FRT), and coiling temperature (CT)/interrupted cooling temperature (ICT) on the grain size and the amount of nanoprecipitation are discussed. Individual strengthening contributions from grain size effects, solid-solution strengthening, and precipitation strengthening are quantified to understand fully the strengthening mechanisms for these steels.     

Cu-Precipitation Strengthening in Ultrahigh-Strength Carburizing Steels

Ultrahigh hardness levels greater than 700 VHN can be obtained in secondary hardening carburizing steels but depend on costly Co alloying additions to maximize hardness achieved through M₂C-type carbide precipitation strengthening. This study aims to incorporate nanometer-scale bcc Cu precipitates to both provide strength as well as catalyze M₂C nucleation in the absence of or with reduced Co. Cu additions of 1.0 and 3.7?wt pct were investigated, using a series of mechanistic models coupled with thermodynamic computational tools to derive final compositions. Thirty-pound experimental heats were cast of each designed alloy, samples of which were carburized and tempered to determine their hardness response. Characterization revealed the successful incorporation of Cu alloying additions into this family of steels, demonstrating a secondary hardening response even in the absence of Co. Matrix strength levels were close to those predicted by design models; however, all four alloys demonstrated a hardness deficit of approximately 200 VHN at the carburized surface, suggesting recalibration of the M₂C precipitation strengthening model may be required in these alloys.     

The effect of two-step aging on the quench sensitivity of an Al-5 Pct Zn-2 Pct Mg alloy with and without 0.1 Pct Cr

The effect of two-step aging on the quench sensitivity of an Al-5 pct Zn-2 pct Mg alloy with and without 0.1 pct Cr has been studied. Results show that the quench sensitivity effect can be eliminated in thin samples of these alloys by two-step aging if the slow cooling during quenching does not allow the precipitation process to proceed too far. Lack of achievement of full strength in the aged condition due to a slow quench rate can be attributed to 1) loss of vacancies during quenching and 2) formation of incoherent-type precipitates during quenching. The trend to lower strength due to the loss of vacancies can be reversed by two-step aging; however, if incoherent-type precipitates form, some strength potential of the alloy is permanently lost. A 0.1 pct Cr addition increases the quench sensitivity effect by accelerating incoherent-type precipitation during quenching. These incoherent precipitates, which appear in the form of bands within the grains and in the grain boundaries, lead to an increase in ductility. Formerly Research Assistant, M.I.T., Cambridge, Mass.     

Influence of thermomechanical treatments on the microstructure and mechanical properties of HSLA-100 steel plates

The influence of thermomechanical treatment (TMT), i.e., controlled rolling and direct quenching, as a function of rolling temperature and deformation on the microstructure and mechanical properties of HSLA-100 steel have been studied. The optical microstructure of the direct quenched (DQ) and tempered steel rooled at lower temperatures (800 °C and 900 °C) showed elongated and deformed grains, whereas complete equiaxed grains were visible after rolling at 1000 °C. The transmission electron microscope (TEM) microstructure of the 800 °C rooled DQ steel showed shorter, irregular, and closer martensite laths with extremely fine Cu and Nb(C,N) precipitates after tempering at 450 °C. The precipitates coarsened somewhat after tempering at 650 °C; the degree of coarsening was, however, less compared to that of the reheat-quenched (RQ) and tempered steel, indicating that the DQ steel was slightly more resistant to tempering. Similar to the RQ steel, at a 450 °C tempering condition, the DQ steel exhibited peak strength with extremely poor impact toughness. After tempering at 650 °C, the toughness of the DQ steel improved significantly, but at the expense of its strength. In general, the strength of the DQ and tempered steel was good and comparable to that of the RQ and tempered steel, although, its impact toughness was marginally less than the latter. The optimum combination of strength and toughness in the DQ steels was achieved after 900 °C rolling with 50 pct deformation, followed by direct quenching and tempering at 650 °C (yield strength (YS)=903 MPa, ultimate tensile strength (UTS)=928 MPa, and Charpy V-notch (CVN) strength=143 J at −85 °C).