Poor Hot Ductility Research Articles

Effect of Vanadium and Strain Rate on Hot Ductility of Low-Carbon Microalloyed Steels

Hot tensile tests were conducted in this study to investigate the effect of strain rate (10−3 and 10 s−1) and vanadium content (0.029 and 0.047 wt.%) on the hot ductility of low-carbon microalloyed steels. The results indicate that a hot ductility trough appears at a low strain rate (10−3 s−1) because of the sufficient time for ferrite transformation and the growth of second particles, but it disappears at a high strain rate (10 s−1). The hot ductility is improved with the increase in strain rate at 700 °C or higher temperatures. In addition, with the increase in vanadium content, the large amounts of precipitate and increased ferrite transformation result in poor hot ductility of steels fractured at a low temperature range (600~900 °C). However, when the steel is fractured at a high temperature range (1000~1200 °C), more vanadium in the solid solution in the austenite inhibits the growth of parental austenite grains and results in grain refinement strengthening, slightly improving the hot ductility.

Role of carbides on hot deformation behavior and dynamic recrystallization of hard-deformed superalloy U720Li

Cracks are easy to occur during cogging of U720Li ingots due to the high deformation resistance and poor hot ductility. In this paper, the low-carbon alloy (0.014 wt% C) and high-carbon alloy (0.071 wt% C) ingots were fully homogenized. The effect of carbides on hot deformation behavior and dynamic recrystallization (DRX) of as-homogenized U720Li alloy was investigated with aim of providing some guidance for improving hot working performance of such hard-deformed superalloys. It has been found that the blocky MC carbide is the major form of carbon in both alloys. Compared with low-carbon alloy, the size, number density and area fraction of MC carbides in high-carbon alloy are much greater, but those of γ′ are similar. The existence of more MC carbides leads to finer initial grains in the high-carbon alloy. During compression deformation of both alloys at 1060 °C, intergranular cracking occurred preferentially along initial grain boundaries (GBs), but cracking of the high-carbon alloy occurred at larger strains than that of low-carbon alloy. Before the occurrence of cracking, the yield and flow stress of high-carbon alloy were obviously lower than those of low-carbon alloy. An interesting phenomenon was discovered that the DRX always occurred preferentially around the MC carbides. Based on this phenomenon and electron backscatter diffraction analysis, the MC carbides were assumed to act as nucleation sites during initiation of DRX by a particle stimulated dynamic recrystallization mechanism. In addition, initial GBs are also preferential sites for DRX initiation. The greater size and number density of MC carbides and finer initial grains significantly increased the stored energy and nucleation sites, which distinctly promoted DRX in the high-carbon alloy. The faster DRX is the major contributor in reducing deformation resistance and cracking of the high-carbon alloy.

Influence of Aluminium on Castability of V-Microalloyed Steels

The influence of aluminium content on the hot ductility behaviour in V-N steels was investigated. Cylindrical specimens were subjected to thermal cycles and strain rates approximating those experienced by a conventional slab surface during continuous casting. The resulting microstructures were examined using light optical and electron microscopy and correlated with measured reduction-in-area (RA) values, calculated precipitate chemistries and volume fractions, as well as the flow stress behaviour. It was found that removal of aluminium significantly improved the hot ductility. However, increasing the total [V][N] product in Al-free steels reduces RA. Poor hot ductility is caused by low austenite grain boundary mobility characterized by high work hardening rates. The fracture mode in brittle specimens is intergranular along thin ferrite films. AlN appears to inhibit austenite grain boundary mobility in V-high N steels when the cooling rate and strain rate are both very slow as experienced during unbending. SEM analysis of fracture surfaces revealed the presence of MnS-AlN particles in microvoids. TEM-EDAX spectra showed that the larger particles observed in the Al-containing steels are mostly a constitution of duplex/triplex grain boundary precipitates, i.e., MnS-AlN and V(C,N). Conversely, good ductility in austenite is associated with high grain boundary mobility that produces fine, recrystallised grains and subsequent dimple fracture after plastic tensile stress.

Gleeble-Simulated and Semi-Industrial Studies on the Microstructure Evolution of Fe-Co-Cr-Mo-W-V-C Alloy during Hot Deformation.

Fe-Co-Cr-Mo-W-V-C alloy is one of the most important materials for manufacturing drills, dies, and other cutting tools owing to its excellent hardness. However, it is prone to cracking due to its poor hot ductility during continuous hot working processes. In this investigation, the microstructure characteristics and carbide transformations of the alloy in as-cast and wrought states are studied, respectively. Microstructural observation and first-principles calculation were conducted on the research of types and mechanical properties of carbides. The results reveal that carbides in as-cast Fe-Co-Cr-Mo-W-V-C alloy are mainly Mo2C, VC, and Cr-rich carbides (Cr7C3 and Cr23C6). The carbides in wrought Fe-Co-Cr-Mo-W-V-C alloy consist of Fe2Mo4C, VC, Cr7C3, and a small amount of retained Mo2C. For these carbides, Cr7C3 presents the maximum bulk modulus and B/G values of 316.6 GPa and 2.48, indicating Cr7C3 has the strongest ability to resist the external force and crack initiation. VC presents the maximum shear modulus and Yong’s modulus values of 187.3 GPa and 465.3 GPa, which means VC can be considered as a potential hard material. Hot isothermal compression tests were performed using a Gleeble-3500 device to simulate the flow behavior of the alloy during hot deformation. As-cast specimens were uniaxially compressed to a 70% height reduction over the temperature range of 1323–1423 K and strain rates of 0.05–1 s−1. A constitutive equation was established to characterize the relationship of peak true stress, strain rate, and deformation temperature of the alloy. The calculated results were in a good agreement with the experimental data. In order to study the texture evolution, the microstructures of the deformed specimens were observed, and an optimal deformation temperature was selected. Using the laboratorial optimal temperature (1373 K) in forging of an industrial billet resulted in uniform grains, with the largest size of 17 µm, surrounded by homogenous spherical carbides.

Effects of B on the Hot Ductility of Fe-36Ni Invar Alloy

Abstract This work conducted systematic studies on the effect of B on the hot ductility behavior of Fe-36Ni alloy over the temperature range of 900–1,200 °C by use of Gleeble-3500 thermal simulator, Thermo-Calc software, transmission electron microscopy and secondary ion mass spectroscopy. The influencing factors and mechanisms are also discussed in the present work. Results show that all the values of area reduction of the investigated alloy samples are below 60 % in the temperature range of 900–1,000 °C, indicating the poor hot ductility of the investigated alloys in this temperature range. When the grain boundary sliding occurs during the hot tensile processes, the fine secondary phase particles at grain boundaries prevent the occurrence of dynamic recrystallization and promote the nucleation and propagation of cracking simultaneously, resulting in the poor hot ductility of the investigated alloys in this temperature range. In the B bearing alloy, the segregation of B atoms around austenite grain boundaries promotes the solute dragging effects at grain boundaries and strongly inhibits the occurrence of dynamic recrystallization, which increases the brittle temperature to 1,000 °C. When the temperature exceeds 1,050 °C, the occurrence of dynamic recrystallization improves the hot ductility significantly. However, the coarsening of recrystallized grains and the formation of inter dendritic cracks decrease the hot ductility of the alloy gradually when the temperature increases from 1,100 °C to 1,200 °C.

The Effect of Boron Addition on Precipitation and Hot Ductility of 1.5Mn-0.1Nb-Ti Carbon Steels in As-Cast Condition

Twelve experimental steels with a base composition 1.5wt% Mn, 0.01 wt% V and 0.1 wt% Nb and varying C (0.05, 010 and 0.20 wt%), Ti (20 – 260 ppm) and B (0 – 100 ppm) contents have been systematically examined to quantify the effects of composition on precipitation behavio-ur and hot ductility during simulated continuous casting conditions. Nb-rich precipitates were present in the alloys with 0.10 wt-% C and 0.20 wt-% C. Alloys with 0.05, 010 and 0.20wt% C contained 50 – 100 nm size Ti-Nb carbonitrides. Boron was bound in 20 – 100 nm size boronitrides located in prior austenite grain boundaries. A Gleeble 3800 was used to study hot ductility and strain induced precipitation processes in the alloys. Alloys without B and Ti additions exhibited poor hot ductility at 850°C and 950°C, whereas the 0.05 wt-% C and 0.10 wt-% C alloys showed improved hot ductility (reduction in area 40-50%) by the addition of either >50 ppm B or 250 ppm Ti. The 0.2 wt-% C alloys showed no improvement from B or Ti additions. Examination of fracture surfaces of hot ductility specimens showed that boronitrides were located at prior austenite grain boundaries in alloys containing 80 – 100 ppm of B. Compression-relaxation tests showed that alloying with boron caused a noticeable decrease of the start temperature of strain-induced precipitation in the alloys.

Effect of Mg content on the hot ductility of wrought Fe-36Ni alloy with Ti addition

Austenitic alloys are prone to cracking as a result of their poor hot ductility during continuous casting and hot working processes. In this study, hot tensile tests were performed to study the effect of Mg content on the hot ductility of Fe-36Ni alloy over the temperature range of 700–1200°C at a strain rate of 10−2s−1. The effects of the Mg content on the particle (i.e., inclusion and precipitate) characteristics and the growth of austenite grains were also investigated. The results indicated that the number density of the particles increased significantly while the mean diameter decreased with increasing Mg content in the experimental alloys. The growth of austenite grains was effectively inhibited by pinning particles in the Mg-treated alloys during solution treatment. Both the pinning particle and austenite grain size markedly affected the hot ductility behaviors of the alloys. The hot ductility curves of the studied alloys had different trends in the intermediate temperature range and were controlled by the competition between dynamic recrystallization (DRX) and grain boundary sliding (GBS). Compared with the alloys without Mg addition, an appropriate number of pinning particles in the alloy with 49ppm Mg addition resulted in a smaller grain size during the solution treatment, which significantly improved the hot ductility by accelerating DRX during the subsequent hot tensile process. However, an excessive number of pinning particles at grain boundaries in the alloy with 60ppm Mg addition inhibited DRX and promoted GBS during the hot tensile process, which were detrimental to the hot ductility. In contrast, the alloys exhibited similar hot ductility troughs at higher temperatures, indicating that Mg has only a slight effect on the hot ductility at these temperatures.

Effects of Non-metallic Inclusions on Hot Ductility of High Manganese TWIP Steels Containing Different Aluminum Contents

The characteristics of inclusions in Fe-16Mn-xAl-0.6C (x = 0.002, 0.033, 0.54, 2.10 mass pct) steels have been investigated and their effects on hot ductility of the high manganese TWIP steels have been discussed. Ductility is very poor in the steel containing 0.54 mass pct aluminum, which is lower than 20 pct in the temperature range of 873 K to 1473 K (600 °C to 1200 °C). For the steels containing 0.002 and 2.10 mass pct aluminum, ductility is higher than 40 pct in the same temperature range. The hot ductility of steel containing 0.033 mass pct aluminum is higher than 30 pct throughout the temperature range under examination. With increasing aluminum content, the main inclusions in the steels change along the route of MnO/(MnO + MnS) → MnS/(Al2O3 + MnS) → AlN/(Al2O3 + MnS)/(MgAl2O4 + MnS) → AlN. The thermodynamic results of inclusion types calculated with FactSage software are in agreement with the experimental observation results. The inclusions in the steels containing 0.002 mass pct aluminum do not deteriorate the hot ductility. MnS inclusions whose average size, number density, and volume ratio are 1.12 μm, 15.62 mm−2, and 2.51 × 10−6 in the steel containing 0.033 mass pct aluminum reduce the ductility. In the steel containing 0.54 mass pct aluminum, AlN inclusions whose average size, number density, and volume ratio are 0.878 μm, 16.28 mm−2 and 2.82 × 10−6 can precipitate at the austenite grain boundaries, prevent dynamic recrystallization and deteriorate the hot ductility. On the contrary, in the steel containing 2.10 mass pct aluminum, the average size, number density and volume ratio of AlN inclusions change to 2.418 μm, 35.95 mm−2, and 2.55 × 10−5. They precipitate in the matrix, which do not inhibit dynamic recrystallization and thereby do not lead to poor hot ductility.

Hot Ductility Behavior of Boron Containing Microalloyed Steels with Varying Manganese Contents

The hot ductility is measured for six different steel grades with different microalloying elements and with varying manganese contents using the hot tensile test machine with melting/solidification unit at the Department of Ferrous Metallurgy RWTH Aachen University. To identify the influence of manganese on hot ductility, tests are performed with varying the manganese content from 0.7 to 18.2 wt pct, a high manganese steel. Additionally, the effect of different cooling and strain rates is analyzed by changing the particular rate for selected samples in the minima. To investigate and detect the cause of cracking during testing, the fracture surfaces in the ductility minima are considered with scanning electron microscope–energy dispersive X-ray spectroscopy. Thermodynamic modeling is conducted on basis of the commercial software ThermoCalc©. A sharp decrease of the hot ductility is recognizable at 1398 K (1125 °C), at only 0.7 wt pct manganese because of the low manganese to sulfur ratio. The grades with a Mn content up to 1.9 wt pct show a good ductility with minimal ductility loss. In comparison, the steel grade with 18.2 wt pct has a poor hot ductility. Because of the formation of complex precipitates, where several alloying elements are involved, the influence of boron on hot ductility is not fully clarified. By increasing the cooling rate, the reduction of area values are shifted to smaller values. For high test temperatures, these measured values are decreased for lower strain rates. Thereby, an early drop of the ductility is noticeable for the high temperatures around 1373 K (1100 °C).

Effect of Rare Earth Yttrium on the Hot Ductility of Fe-36Ni Invar Alloy

AbstractThe hot ductility of Fe-36Ni invar alloy doped with and without yttrium was investigated using a Gleeble-3800 thermal-mechanical simulator over the temperature range 850–1050 °C and the improvement mechanism of the hot ductility was analysed with a combination of SEM, EDS and OM. The results showed that Fe-36Ni invar alloy had a poor hot ductility below 1050 °C, which was mainly attributed to the presence of the grain boundary sliding and weak grain boundaries. The addition of 0.048% yttrium had a substantial improvement in the hot ductility of Fe-36Ni invar alloy over the whole testing temperature range especially at 950–1000 °C. At 850–900 °C, the improvement of the hot ductility was mainly associated with the grain boundary strengthening and the restriction of the grain boundary sliding because the addition of yttrium could reduce the segregation of sulfur at grain boundaries and refine the grain structure. At 950–1000 °C, the hot ductility was highly improved, which was owed to the acceleration and occurrence of dynamic recrystallization as a result of the refinement of the grain structure by addition of yttrium.

The Mechanism of Hot Ductility Loss and Recovery in Nb-Bearing Low Alloy Steels

In low alloy steels containing Nb, the poor hot ductility is basically due to the austenite grain boundary segregation of sulfur and the additional matrix strengthening of Nb(C,N) precipitates, both of which decrease the equicohesive temperature. The recovery of hot ductility is therefore attributed to not only the clean grain boundaries that the segregated sulfur is scavenged through the MnS reaction but also the matrix softening by the coarse Nb(C,N) precipitates.

Hot ductility of eco-friendly low carbon resulphurised free cutting steel with bismuth

The hot ductility of low carbon resulphurised free cutting steel with different bismuth contents was studied using a Gleeble-1500 thermal–mechanical simulator, and the hot brittleness mechanism was analysed with a combination of SEM and energy dispersive X-ray spectroscopy. The results showed that the steels with bismuth contents of 0·12 and 0·17 wt-% both had a good hot ductility (RA, >70%) at 1100–1200°C, which illustrated that the steels might be suitable for the rolling provided the finish rolling temperature does not go <1100°C. However, the hot ductility of the steels deteriorated with the increase in bismuth content at 950–1100°C because of the presence of the low melting point liquid bismuth films at austenite grain boundaries and the delay of the dynamic recrystallisation by Bi additions. At 850–900°C, the steels all had very poor hot ductility, which was mainly attributed to the presence of the ferrite film and the unrecrystallised austenite as well as the presence of the low melting point phases pure Bi and Fe rich (Fe,Mn)S.

Influence of B and Ti on hot ductility of high Al and high Al, Nb containing TWIP steels

The addition of ∼0·002%B and ∼0·04%Ti as microalloying additions to improve the poor hot ductility and high risk of cracking on continuous casting of high Al containing twinning induced plasticity (TWIP) steels has been examined. Tensile specimens were either cast in situ or heated to 1250°C before cooling at 60 K min−1 to test temperatures in the range 700–1100°C and strained to failure at 3×10−3 s−1. For tensile specimens reheated to 1250°C, the presence of B with sufficient Ti to combine with all the N improved ductility over the temperature range of 700–950°C, the reduction in area (RA) values being >40%. For the higher strength more complex high Al, TWIP steels having Nb present, there was no improvement in ductility with a similar B and Ti addition, when the average cooling rate after melting to the test temperature was 60 K min−1. Reducing the cooling rate to 12 K min−1 resulted in the RA values being close to the minimum required to avoid transverse cracking throughout the temperature range 800–1000°C. Using these additions of B and Ti, transverse cracking was found not to be a problem when continuously casting these high Al containing TWIP steels.

Interrupted and Isothermal Solidification Studies of Low and Medium Carbon Steels

Low and medium carbon steels experience multiple phase transformations during solidification and subsequent cooling. The sequence, extent, and nature of the different transformations have a significant bearing on the microstructural evolution that occurs in the steel. The change in microstructure with temperature is very important, since it may influence the hot ductility of the steel during casting and/or rolling and the subsequent response of the material to thermoprocessing. The aim of this investigation was to gain a better understanding of the development of the as-cast structure in low and medium carbon steels. Of particular interest is the origin of the large austenite grains frequently associated with poor hot ductility. Interrupted and isothermal solidification experiments were therefore conducted to study the nonequilibrium and near-equilibrium structures which form at different stages of the freezing process. The results of the investigation established delta-ferrite as the primary solidifying phase in low carbon steels. Austenite forms as the secondary phase by nucleation at the solidification (delta-ferrite) boundaries. While excessive austenite grain coarsening is suppressed by the coexistence of the second phases delta-ferrite or liquid, this suppression occurs over only a limited temperature range, just below the peritectic temperature. Subsequent cooling leads to very large austenite grains, ranging up to 5 mm in diameter, in steels of low carbon content.

Effect of precipitation on hot ductility of niobium and aluminium microalloyed steels

The precipitation reactions occurring in C–Mn–Al and C–Mn–Nb steels before and after hot deformation have been examined and their influence upon hot ductility is discussed. Precipitation has been studied at 850°C, when ductility is poor, and also at 1100°C, when the ductility is good. Rapid intergranular precipitation occurred at 1100°C, but the precipitation present before deformation did not prove to be detrimental to ductility and grain boundary mobility at this temperature. Although only a limited amount of precipitation occurred at 850°C before deformation, intergranular precipitation continued during deformation resulting in embrittlement of the steels. At this temperature, strain induced transgranular precipitation of Nb(CN) occurred in the C–Mn–Nb steel and this is thought to be a major cause of poor hot ductility in this steel. By holding the steels for 15 min at 800–850°C before reheating to 1100°C, the area fraction of intergranular precipitation at 1100°C was increased. This produced a decrease in ductility at this temperature in the C–Mn–Al steel but had a less marked effect in the C–Mn–Nb steel.MST/107

Metallurgical Factors Influencing Hot Ductility of Austenitic Steel Piping at Weld Heat-Affected Zone Temperatures

The variable hot ductility of wholly austenitic Type 347 steel was found to be associated with the variable temperature of formation of a liquid phase (liquation temperature) in the microstructure of the steel. In wholly austenitic materials a low liquation temperature leads to poor hot ductility. The liquation temperature of the steel was determined to be a function of its columbium, carbon, and nitrogen contents. When ferrite is present in the steel, poor hot ductility can also occur as a result of the crack sensitivity of the two-phased structure.