The boron hardenability effect in thermomechanically processed,direct-quenched 0.2 Pct C steels

The segregation and precipitation of boron have been studied in thermomechanically processed 0.2C-0.6Mn-0.5Mo steels containing nominally 0, 10, 20, 50, and 100 ppm B. These steels were hot-rolled in the laboratory (in simulation of production multipass rolling), and their transformation behavior during subsequent water quenching was examined for different finish-rolling temperatures (980 °C and 870 °C) and quenching temperatures (730 °C to 950 °C). The results showed that in general, a “free” boron content of 10 to 20 ppm (which is similar to the levels used for conventional quenched-and-tempered steels) will provide a boron hardenability increment similar to that for conventional quenched-and-tempered steels. The delay time prior to quenching (over the range of 10 to 100 seconds) did not have a significant effect on hardenability except in the steels containing 50 or more ppm B. In these higher B steels, precipitation of borocarbides occurred along austenite grain boundaries with a resultant decrease in hardenability.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel-II. Fine scale segregation behaviour

A combination of TEM, FIM, AP and IAP has been used to study boron grain boundary segregation in austenitic stainless steels of the types 316L (with 40 ppm or with <1 ppm boron) and “Mo-free 316L” (23 ppm boron). High resolution segregation profiles were determined for cooling rates from 0.29 to 530°C/s for three starting temperatures: 800, 1075 and 1250°C. Boron grain boundary segregation was found after all heat treatments. The segregation behaviour was mainly of the nonequilibrium type after cooling from 1075 or 1250°C whereas equilibrium segregation dominated after rapid cooling from 800°C. The influence of the relative grain orientation on the amount of non-equilibrium segregation was small for general boundaries. However, no segregation was detected at coherent twin boundaries. The binding energy of boron to austenite grain boundaries was estimated at 0.65 ± 0.04 eV for both types of steels. The influence of the composition and boron content of the steels on the segregation behaviour is discussed and the experimental techniques used are presented.     

The non-equilibrium segregation of boron during the recrystalization of Nb-treated HSLA steels

The effect of boron segregation on recrystallization was investigated in two Nb and two Nb-free steels. The results indicate that the addition of B to a Nb steel retards the recrystallization of austenite after high temperature deformation to a significant degree. By contrast, the effect of B when added alone is not of commercial importance. At deformation temperatures above 950°C, the influence of boron is mainly due to grain boundary segregation and solute drag; below 900°C, the retardation results from the precipitation of Nb carbonitride. During isothermal holding, there are two types of non-equilibrium segregation on boundaries. Strain-induced segregation occurs on the original boundaries; it increases and attains a maximum rapidly, then decreases and disappears after 60 s of holding at 1000°C. A second type of non-equilibrium segregation occurs on newly formed boundaries during recrystallization. It develops quickly as the new boundaries are formed and begin to move, and can persist until grain impingement occurs and the motion is halted. The enhanced effect of B in the presence of Nb is attributed to the formation of (Nb, B) complexes, which appear to increase the solute drag force and decrease the boundary velocity.     

Nb(C,N) Precipitation and Austenite Recrystallization

The influence of boron addition, amount of deformation, and solution heat-treatment temperature on the precipitation and recrystallization behaviors of a family of high-strength low-alloy (HSLA) steels was studied. A stress relaxation technique was employed to detect the occurrence of austenite recrystallization and to determine the precipitation start(P _s) and finish(P _f) times. After preheating to 1100 °C or 1200 °C for 30 minutes, the specimens were cooled to test temperatures between 800 °C and 1000 °C. They were subsequently deformed to true strains of 5 or 25 pct and subjected to stress relaxation. The advent of recrystallization produced a sharp increase in relaxation rate, while the occurrence of carbonitride precipitation led to the appear- ance of a stress plateau. The results indicate that the presence of boron (1) accelerates carbo- nitride precipitation and (2) retards austenite recrystallization when present in combination with Nb. The precipitation-time-temperature (PTT) diagrams determined in this investigation are C-shaped for both the B-free as well as the B-modified Nb steels. These data were analyzed in terms of the classical theory of nucleation, on the basis of which it is demonstrated that the acceleration of the nucleation kinetics of precipitation can be attributed to the segregation of boron and of boron-vacancy complexes to dislocations and grain boundaries, as well as to the faster diffusion of Nb in the presence of boron.     

Grain boundary segregation of boron in INCONEL 718

The segregation behavior of boron at grain boundaries in two INCONEL 718⁺ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe−Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954°C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843°C, 954°C, or 1066°C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Overview no. 63 Non-equilibrium grain boundary segregation of boron in austenitic stainless steel—IV. Precipitation behaviour and distribution of elements at grain boundaries

The distribution of elements and the precipitation behaviour at grain boundaries have been studied in boron containing AISI 316L and “Mo-free AISI 316L” type austenitic stainless steels. A combination of microanalytical techniques was used to study the boundary regions after cooling at 0.29–530°C/s from 800, 1075 or 1250°C. Tetragonal M₂B, M₅B₃ and M₃B₂, all rich in Fe, Cr and Mo, precipitated in the “high B” (40 ppm) AISI 316L steel whereas orthorhombic M₂B, rich in Cr and Fe, was found in the “Mo-free steel” with 23 ppm B. In the “high B steel” a thin (<2 nm), continuous layer, containing B, Cr, Mo and Fe and having a stoichiometry of typically M₉B, formed at boundaries after cooling at intermediate cooling rates. For both types of steels a boundary zone was found, after all heat treatments, with a composition differing significantly from the bulk composition. The differences were most marked after cooling at intermediate cooling rates. In both types of steel boundary depletion of Cr and enrichment of B and C occurred. It was found that non-equilibrium grain boundary segregation of boron can affect the precipitation behaviour by making the boundary composition enter a new phase field. “Non-equilibrium phases” might also form. The synergistic effect of B and Mo on the boundary composition and precipitation behaviour, and the observed indications of C non-equilibrium segregation are discussed.     

Segregation of phosphorus and boron in austenite in Fe–10%Mn–P–B-alloys

Equilibrium segregation of phosphorus at the grain boundaries of austenite has been studied in Fe–10%Mn–P–B-alloys. The samples were equilibrated at temperatures of 750–1100°C and analysed after rapid quenching using Auger-electron spectroscopy. The results show that boron markedly reduces segregation of phosphorus in austenite. Boron was also found to be segregated at the prior austenite grain boundaries. The intergranular boron concentration increases slightly with a rising austenitising temperature, but does not show any dependence on boron or phosphorus content of the alloy. The results can be explained by assuming segregation equilibria and mutual displacement between B and P in austenite, a value of ?97 kJ/mole for the free energy of boron segregation and ?47 kJ/mole for phosphorus segregation.     

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.     

The effect of phosphorus content on grain boundary cementite formation in AISI 52100 steel

Intergranular fracture surfaces of high phosphorus (0.023 wt pct P) and low phosphorus (0.009 wt pct P) AISI 52100 steels were investigated by Auger Electron Spectroscopy (AES). Cementite, identified by composition and Auger peak shape, was found to form on austenite boundaries in specimens oil quenched from 960 °C to room temperature as well as in specimens quenched from 960 °C and isothermally held at temperatures between A_cm and A₁. Phosphorus segregates to austenite boundaries during austenitizing and accelerates cementite formation on the austenite boundaries. Concentration profiles obtained by AES during ion sputtering showed that phosphorus may be incorporated in the first-formed cementite and concentrates at cementite/matrix interfaces in later stages of cementite growth. The amount of interphase P segregation in the later stages is proportional to bulk alloy P concentration in accord with McLean’s theory of grain boundary segregation in dilute alloys and appears to approach equilibrium at high reaction temperatures (785 °C). At lower reaction temperatures (740 °C), the interphase segregation is lower than expected, a result that may be attributed to reduced diffusivity of P at the lower reaction temperature.     

High temperature ductility loss in carbon-manganese and niobium-treated steels

The causes of embrittlement in several plain carbon-manganese and niobium-treated steels between 800 and 1200 °C have been investigated. Tensile ductility was measured as a function of temperature and strain rate. Percent elongation and reduction in area were used to characterize the temperature dependence and severity of the ductility loss. The size, distribution, and composition of grain boundary precipitates were measured on extraction replicas. Grain boundary segregation was measured by AES on samples that were deformed at 900 °C before being fractured under ultra-high vacuum at room temperature. Segregation of impurity residual elements and grain boundary precipitation are the primary factors responsible for the observed ductility loss. The embrittlement results in a low ductility fracture which is largely intergranular through the austenite grain boundaries. Segregation of Cu, Sn, and Sb was found on the fracture surfaces of the embrittled samples. High temperature deformation was necessary to produce segregation as no segregation was detected in undeformed samples. Grain boundary precipitation, particularly AIN but also Nb (C,N), contributed to the embrittlement when there was a relatively fine distribution of precipitates along the austenite grain boundaries. The most severe ductility loss occurred when grain boundary precipitation combined with Cu, Sn, and Sb segregation. Formerly Graduate Student, Lehigh University     

Effect of Boron on the Hot Ductility Behavior of a Low Carbon Advanced Ultra-High Strength Steel (A-UHSS)

This research work studied the effect of boron additions (14, 33, 82, 126, and 214 ppm) on the hot ductility behavior of a low carbon advanced ultra-high strength steel. For this purpose, specimens were subjected to a hot tensile test at different temperatures [923 K, 973 K, 1023 K, 1073 K, 1173 K, and 1273 K (650 °C, 700 °C, 750 °C, 800 °C, 900 °C, and 1000 °C)] under a constant true strain rate of 10^?3 s^?1. The reduction of area (RA) of the tested samples until fracture was taken as a measure of the hot ductility. In general, results revealed a marked improvement in hot ductility from 82 ppm B when the stoichiometric composition for BN (0.8:1) was exceeded. By comparing the ductility curve of the steel with the highest boron content (B5, 214 ppm B) and the curve for the steel without boron (B0), the increase of hot ductility in terms of RA is over 100 pct. In contrast, the typical recovery of hot ductility at temperatures below the Ar₃, where large amounts of normal transformation ferrite usually form in the structure, was not observed in these steels. On the other hand, the fracture surfaces indicated that the fracture mode tends to be more ductile as the boron content increases. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism, which in turn causes hot ductility loss. In general, results are discussed in terms of boron segregation and precipitation on austenitic grain boundaries during cooling from the austenitic range and subsequent plastic deformation.     

The effect of niobium on the hardenability of microalloyed austenite

The powerful effect that varying the extent of niobium-carbide dissolution has on the “hardenability” of microalloyed austenite is demonstrated using dilatometric measurement of the critical cooling rate required to from microstructures containing >95 Pct martensite. The results can be rationalized on the hypothesis that the hardenability of austenite is enhanced by niobium in solid solution, possibly by its segregation to austenite grain boundaries, but is decreased by precipitation of niobium-carbide particles. This effect appears analogous to that of boron in steels and is found to be independent of variations in the austenite grain size.     

Long-time isothermal temper embrittlement in Ni-Cr-Mo-V steels

Four commercial purity Ni-Cr-Mo-V steels of closely comparable bulk chemistry and grain size, but tempered to various strength levels, were embrittled by exposure at 600°, 750°, and 850°F for times up to 35,000 hr. Maximum temper embrittlement occurred at 850°F in all steels. Severe cases of embrittlement resulted in a marked decrease in tensile ductility and an intergranular tensile fracture. Auger electron emission analysis showed that P, Sn, Ni, and Cr were segregated at prior austenite boundaries in the steels exposed to 750° and 850°F. Increased segregation of phorphorus and tin was always accompanied by increased segregation of nickel and chromium. The severity of grain boundary segregation increased with increasing values of fracture transition temperature. Despite comparable bulk chemistry and grain size, the degree of segregation was different in different steels. Under exposure conditions causing severe embrittlement, the FATT values displayed a strong dependence on the strength level of the steel. In a giyen steel, while the composition and morphology of carbides at austenite boundaries were the same as in the matrix, the density and size of carbides were much higher at the austenite boundaries. The preference of these boundaries as fracture sites would seem to arise from two considerations, namely, a high degree of impurity and alloy element segregation and the fact that the density and size of carbides at these boundaries is higher than that in the matrix. On educational leave from Westinghouse Research Laboratories, Pittsburgh, Pa.     

Grain Boundary Segregation Behavior of Boron in Low-Alloy Steel

The boron concentration profiles around prior austenite grain boundaries in Fe-0.05C-0.5Mo-0.001B (mass pct) are examined using aberration-corrected STEM-EELS. In order to obtain the precise distribution of boron around the boundaries, tilt series measurements with thin specimens (<30 nm) are performed and the EEL spectra are analyzed by principal component analysis (PCA) and multivariate curve resolution (MCR). The boron concentration profile changes with the cooling rate from the solid solution temperature. The concentration at grain boundaries is maximized at a medium rate (30 °C/s), where the concentration reaches 8 at. pct, and it decreases at a larger (250 °C/s) or smaller (5 °C/s) rate. On the other hand, the boron distribution becomes wider as the cooling rate becomes smaller. The current results suggest that the boron segregation in the alloy is formed by the “non-equilibrium segregation mechanism.”     

Effects of second-phase particles on coarsening of austenite in 0.15 Pct carbon steels

The effects of second-phase particles formed by the addition of vanadium, nitrogen, and aluminum on the austenite grain coarsening behavior of 0.15 pct carbon steels were studied. The oxidation and etching technique has been adopted to reveal the prior austenite grain boundaries. The specimens were austenitized at intervals of 50°C within the range of 900°C to 1150°C under high vacuum (<10⁻⁴ torr) for half an hour, toward the end of which they were oxidized for about one minute by introducing oxygen at about 250 mm Hg to reveal the grain boundaries, and then quenched into iced water. The variation of prior austenite grain size with temperature in these steels indicates that vanadium carbonitride, V(C, N), is much more effective in austenite grain refinement than vanadium carbide, VC, at all temperatures. The effect of vanadium carbonitride in austenite grain refinement is more or less the same as that of aluminum nitride. AlN, at temepratures below 1000°C, but this effect of vanadium carbonitride in austenite grain refinement decreases with increasing temperature. Above 1000°C, aluminum nitride is a much better grain refiner than vanadium carbonitride. The presence of the V (C, N) and AlN particles in the same steel causes moderate grain growth of austenite. MD. Mohar Ali Bepari, formerly with the Department of Metallurgy, The University of Sheffield, Sheffield, England, is Associate Professor of Department of Metallurgical Engineering, Bangladesh University of Engineering and Technology, Dhaka, Bangladesh.     

Carbonitride precipitation in niobium/vanadium microalloyed steels

A detailed study of carbonitride precipitation in niobium/vanadium microalloyed steels is presented. A thermodynamic model is developed to predict the austenite/carbonitride equilibrium in the Fe-Nb-V-C-N system, using published solubility data and the Hillert/Staffansson model for stoichiometric phases. The model can be used to estimate equilibrium austenite and carbonitride compositions, and the amounts of each phase, as a function of steel composition and temperature. The model also provides a method to estimate the carbonitride solution temperatures for different steel compositions. Actual carbonitride precipitation behavior in austenite is then examined in two experimental 0.03Nb steels containing 0.05V and 0.20V, respectively. Samples were solution treated, rolled at 954 °C (20 pct or 50 pct), held isothermally for times up to 10,000 seconds at 843 °C, 954 °C, or 1066 °C, and brine quenched. The process of carbonitride precipitation in deformed austenite is followed by analytical electron microscopy (AEM) of carbon extraction replicas. Precipitates are. observed at prior-austenite grain boundaries, and also within the grains (presumably at substructure introduced by the rolling deformation). Analysis of the grain-boundary and matrix precipitate compositions by AEM indicates that the grain-boundary precipitates are consistently richer in vanadium than the matrix precipitates, although compositional trends with holding time and temperature are similar for the two types of precipitates. The compositions of both the grain-boundary and matrix precipitates are not significantly influenced by the rolling reduction or the holding time at temperature. As predicted by the thermodynamic model, the precipitates become more vanadium-rich as the vanadium level in the steel is increased and as the temperature is reduced. The agreement between the measured and predicted precipitate compositions is quite good for the grain-boundary precipitates, although the matrix precipitates are consistently more niobium-rich than predicted by the model.     

Phosphorus segregation in austenite in Fe–P–C,Fe–P–B and Fe–P–C–B alloys

The equilibrium grain boundary segregation of phosphorus was investigated in Fe–P–C, Fe–P–B and Fe–P–C–B alloys after austenitising at temperatures ranging from 825–1100 °C. The grain boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. Phosphorus, carbon and boron segregate to the austenite grain boundaries. The segregation of P in austenite occurs mainly in equilibrium, but some additional segregation takes place during quenching. Boron and, in a lesser degree, carbon were found to decrease the grain boundary concentration of phosphorus. The results can be explained by assuming equilibrium segregation and mutual displacement of these elements in austenite.     

Transformation during the isothermal deformation of low-carbon Nb-B steels

The transformation behavior during isothermal deformation of four steels containing different microalloying additions was investigated by means of the “strain-rate change” technique. The flow curves obtained at temperatures ranging from 620 °C to 850 °C, and the associated microstructures, indicate that the transformation in the Mo-Nb-B and Mo-B steels is of the austenite-to-bainite type. Here, dramatic increases in flow stress are observed at lower temperatures. By contrast, the transformation in the Nb-15B and Nb-64B steels is basically of the austenite-to-ferrite type; in these two grades, the flow stress increases observed are attributable to strengthening by NbC precipitation. Large intergranular and intragranular Fe₂₃(C,B)₆ particles were found in the Nb-64B steel samples deformed to ɛ=0.1 after holding for 60 seconds at 800°C. These large precipitates are considered to be responsible for accelerating the transformation in the Nb-64B steel by reducing the concentration of boron atoms available for boundary segregation and by acting as nucleation sites for the formation of polygonal ferrite. The flow curves of the Mo-Nb-B steel exhibit distinct serrations, indicating that a displacive mechanism is involved in the γ-to-B transformation.     

Continuous cooling transformation temperatures determined by compression tests in low carbon bainitic grades

The transformation behaviors of six steels containing microalloying additions of B, Nb, and Mo were investigated under continuous cooling conditions. Continuous cooling compression (CCC) tests were employed to study the effects of chemical composition (mainly, Nb, Mo, and B) and deformation parameters (reheat temperature, prestrain, and holding time) on the transformation temperatures (A _r3 and B _s). It was found that for the Mo−Nb−B, Mo−B, and B steels, the transformation temperatures are relatively stable, and vary in a range of about 20°C when the reheat temperature is changed from 900°C to 1200°C. Both the stress-temperature curves and the associated microstructures show that transformation in the Mo−Nb−B steel is basically of the γ-to-B type; i.e., the resulting microstructure is low carbon bainite. By contrast, for the Nb−B steels, the transformation temperatures vary significantly when the reheat temperature is changed. The concentration of boron in solution strongly affects the transformation behavior of this type of steel. In the Nb−48B steel, the latter is of the γ-to-B type, while in grades with either higher (Nb−64B) or lower (Nb−15B) boron concentrations, it is mainly of the γ-to-α type. Large Fe₂₃(C,B)₆ particles, which were found at low reheat temperatures and long holding times, are considered to be responsible for raising the transformation temperatures. T.M. MACCAGNO, formerly with the Department of Metallurgical Engineering, McGill University