Improvement Of Hot Ductility Research Articles

Diffusionless α′ to γ transformation and its effect on hot ductility in a high-nickel steel

The role of diffusionless α′ to γ transformation on the microstructure evolution and hot ductility of high-nickel steel is investigated. In-situ Auger electron spectroscopy fracture test reveals the correlation of ductility loss and sulfur segregation at the grain boundary. The α′ to γ transformation start temperature is measured by heating the initial α′-martensite at various rates. The transformation temperature remains constant when the heating rate is higher than 15 °C/s. Despite the α′ to γ phase transformation, the diffusionlessly transformed specimen shows a similar γ grain size with the initial prior γ grain size. The γ grain size decreases dramatically after the continuous annealing of the transformed specimen. The temperature-dependent recrystallization behavior and corresponding hot-ductility are evaluated. The improved hot ductility is achieved by active recrystallization at 900–1100 °C. Due to the late recrystallization behavior, the improvement of hot ductility is limited after annealing at 800 °C. The origin of hot ductility loss and enhancement depending on the thermal cycles is discussed by sulfur segregation at the grain boundary and recrystallization behavior after diffusionless transformation.

Hot ductility improvement of copper-arsenic steel by rare earth cerium

The effect of Ce on hot ductility of Cu-As bearing steel in the temperature ranging from 700 to 1100 °C was investigated using Gleeble-3800 thermal-mechanical simulator. The hot ductility evaluation and fracture morphology observation showed that the hot ductility was obviously improved as Ce content increased from 0 to 0.0290 wt%. Detailed, the upper limit temperature of the ductility trough was decreased from 1057 to 765 °C, and the temperature corresponding to the minimum reduction of area was decreased from 850 to 750 °C. The fracture morphology was overall transformed from the dominantly intergraular brittle failure to the ductile failure characterized by the large and deep dimples. Analysis of metallographic, inclusions and grain boundary elements suggested that increasing Ce content inhibited the proeutectoid ferrite formation, which enhanced hot ductility of the steel bearing Cu and As. Moreover, the competitive grain boundary segregation of the solute Ce and As in steel with 0.0022 wt% Ce without arsenious rare earth inclusions decreased the quantity of As distributed at grain boundaries and thereby improved the hot ductility. When the Ce content reached 0.0290 wt%, a large number of arsenious rare earth inclusions were formed, and the grain boundary segregation of As was further decreased As, which was more beneficial to hot ductility than that with the lower Ce content in austenite single phase region.

A comparative study of hot ductility of unadded and Ce-added SA508-4N RPV steels

The hot ductility of the SA508-4N RPV steel samples, unadded and added with 0.015 wt.% Ce, has been investigated in the range 750–1000 °C. The study is carried out by using a Gleeble machine and various evaluation techniques. Between 750 and 1000 °C, the steel is located in the single-phase austenite region. In this region, the unadded steel presents a hot ductility trough between 750 and 900 °C and the reduction of area (RA) at the trough bottom is rather small, being just ~50% at 850 °C. However, the hot ductility of the steel can be substantially enhanced by a trace addition of Ce, so that the hot ductility trough is eliminated for the Ce-added steel. In the testing temperature range, the RA values of the Ce-added steel are all higher than ~75%. Microanalysis demonstrates that grain boundary segregation of Ce plays an important role in the improvement of hot ductility by enhancing the intergranular cohesion and thus suppressing the boundary sliding. Furthermore, the initial temperature of dynamic recrystallization occurrence can be lowered to ~850 °C from ~900 °C by the Ce addition. This implies that Ce can also facilitate the occurrence of dynamic recrystallization of SA508-4N RPV steel, thereby raising its hot ductility performance.

Abnormal influence of impurity element phosphorus on the hot ductility of SA508Gr.4N reactor pressure vessel steel

The effect of phosphorous on the hot-ductility of SA508Gr.4N reactor pressure vessel (RPV) steel has been studied. The investigation is done with the use of a Gleeble machine in conjunction with optical metallography, scanning electron microscopy, and transmission electron microscopy. It is observed that both the undoped and P-doped steels exhibit hot-ductility troughs between 750 and 900 °C. However, the trough for the P-doped steel is much shallower than that for the undoped steel. At around 850 °C, the reduction of area (RA) of the undoped steel is quite low (~50%), in comparison to the P-doped steel (~75%). Since there is no grain-boundary precipitation of ferrite and carbides in both undoped and P-doped samples over this temperature range, it can be inferred that the enhancement in hot-ductility for the P-doped steel is independent of these two factors. Grain-boundary microanalysis shows the presence of grain-boundary segregation of P in the tested P-doped samples, implying that the P-induced hot-ductility improvement of this RPV steel may arise from the suppression of grain-boundary sliding by P grain-boundary segregation. Undoubtedly, the present effect of P is unusual and the underlying mechanism for this effect has been briefly discussed. It needs to be underlined that a minor addition of P can play a positive role in the hot-ductility improvement of SA508Gr.4N RPV steel.

Improvement of Hot Ductility in a Low‐Carbon Steel by the Application of the Precooling

The effect of precooling on hot ductility of a low‐carbon steel is investigated. It shows a wide reduction of ductility in the temperature range of 700–900 °C with a general hot tensile test. The hot ductility is increased by applying precooling before the hot‐tensile test. The increase in the reduction of area (RA) reaches 40–50% when the steel is precooled to 500 °C. Active precipitation of (Mn, Cu)‐sulfide is observed during the precooling. The increased RA is attributed to the active sulfide formation, which decreases sulfur segregation at grain boundaries.

Grain refinement technology for duplex stainless steel using rapid cooling immediately before hot working

For the purpose of grain refinement and improvement of the hot formability of duplex stainless steel, a new concept of the thermomechanical processing method using the austenite phase transformation from the δ ferrite phase in the hot working temperature range was proposed. The supercooled δ ferrite phase is generated by rapid cooling immediately before hot working, and crystal grains are refined by the austenite transformation induced by hot working immediately after rapid cooling. Furthermore, since the new thermomechanical processing method is characterized by a large amount of soft δ ferrite phase during hot working, reduction of hot flow stress and improvement of hot ductility are possible in addition to grain refinement. The material used here was 25 % Cr duplex stainless steel, and the temperature history was controlled by combining induction heating and nitrogen gas cooling. It was confirmed that the δ ferrite phase was obtained as the parent phase by initial heating at 1320 °C, and a supercooled δ ferrite phase was generated by rapid cooling at 30.0 °C/s from the initial heating temperature to the hot working temperature of 1000 ℃. When uniaxial compression of 60 % at strain rates of 0.1, 1.0 and 10.0 s −1 was applied to the supercooled δ ferrite phase after cooling to 1000 ℃, nucleation of austenite grains was induced by the degree of supercooling and compressive strain, and as a result, the microstructure was successfully refined by strain-induced fine austenite grains. The grain diameter of the austenite grains obtained by strain-induced transformation was several μ m. The flow stress under thermomechanical processing, which succeeded in microstructure refinement, was much lower than that under the condition simulating the conventional process. The hot ductility investigated in the uniaxial tensile test was also greatly improved under thermomechanical processing. Finally, the results of a detailed crystal orientation analysis revealed that the nucleated fine austenite grains obtained by strain-induced transformation have a K–S relationship with the supercooled δ -ferrite parent phase.

Effect of minor rare earth cerium addition on the hot ductility of a reactor pressure vessel steel

The hot ductility of the specimens for a reactor pressure vessel steel (SA508-III), undoped and doped with minor rare earth Ce, is studied through hot tensile tests by means of a Gleeble machine in conjunction with scanning electron microscopy, optical microscopy, and field emission gun scanning transmission electron microscopy. Both the undoped and Ce-doped specimens are heated at 1300 °C and then cooled down to different test temperatures between 650 and 1050 °C, followed by tensile deformation up to fracture. As demonstrated by the results, for the undoped steel, there is a wide and deep hot ductility trough and the bottom of the trough is flat and wide (700–850 °C), being only about 60% in reduction of area (RA). However, for the Ce-doped steel, the hot ductility trough becomes considerably narrower and shallower. In addition, the flat and wide bottom of the trough disappears. Overall, the Ce-doped specimens exhibit a much higher hot ductility than the undoped ones, being over 70%RA at temperatures above 800 °C. Therefore, the minor addition of rare earth Ce can considerably enhance the hot ductility of the steel. It may be expected that this Ce-doped steel will have a high performance in hot working. As shown by microanalysis, the segregation of Ce to austenite grain boundaries occurs, which could play an important role in the enhancement of ductility in the high-temperature single austenite range (above 700 °C). Moreover, the Ce-induced grain refinement may also contribute to the hot ductility improvement.

Effect of compound addition of Nb-B on hot ductility of Cr-Mo alloy steel

The hot tensile tests were conducted in this study to investigate the combined effect of Nb and B on hot ductility of 25CrMo alloy steel in temperature range of 700–1100°C with a strain rate of 0.5s−1. Besides, the influences of DRX grains growth behavior, ferrite transformation, precipitates and B non-equilibrium segregation on hot ductility were also investigated by use of OM, FE-SEM, TEM, AES and SIMS. The results indicated that the amount of Nb(C,N) particles decreased with increasing deformation temperatures. Moreover, the precipitation of BN on Nb(C,N) particles, which precipitated in matrix abundantly rather than at grain boundaries, reduced stress acting on grain boundaries, and the coarsening BN particles precipitating on Nb(C,N) reduced the pining effect of fine Nb(C,N) particles on dislocations. Besides Nb(C,N) and BN particles, there were M2B, M23C6 and M7C3 precipitates in γ+α two-phase region. The mechanism for hot ductility improvement by compound addition of Nb-B in single phase austenite region was that although the DRX was retarded, the DRX grains were refined obviously by adding Nb and B. Meanwhile, the B atoms segregated rapidly to grain boundaries occupying the voids and enhancing grain boundary cohesion. Besides, the BN particles, precipitating on Nb(C,N) particles in matrix rather than at grain boundaries, reduced the adverse effects on hot ductility. The improvement of hot ductility in γ+α two-phase region was attributed to the restraint of ferrite transformation by the addition of B and the deformation induced precipitation of carbides, which consumed strengthening alloy elements in austenite.

Hot Ductility Improvement in Continuous Casting by Predeformation Treatment

Predeformation treatment is known to be one of effective methods for preventing slab surface cracks in continuous casting.

Improvement of hot ductility of C-Mn Steel containing arsenic by rare earth Ce

The effect of different Ce content on the hot ductility of C-Mn steel containing arsenic was investigated at the temperature ranging from 700 to 1100 °C conducting Gleebel-1500 thermal-mechanical simulator. The reduction of area (RA%) was used to evaluate the hot ductility. The 0.16 mass% As widened the ductility trough range and especially, decreased the RA value at 850–950 °C. Conversely, adding Ce in the steel could remedy the arsenic-induced hot ductility deterioration. Moreover, with the increase of Ce content from 0 to 0.035 mass%, the RA value at 800–950 °C significantly increased, compared to that of the arsenic steel. When the content of Ce reached 0.027–0.035 mass%, the RA value at 800–850 °C was even higher than that of steel without As. Besides, the corresponding fracture morphology was changed from intergranular feature to ductile and/or interdendritic feature. Grain refinement by Ce addition, the formation of arsenious rare earth inclusions and grain boundary segregation of Ce were considered to improve the hot ductility of the steel containing As.

Causes of High Temperature Ductility Trough of Microalloyed Steels

Influence of continuous casting parameters and hot rolling regimes on the hot ductility in microalloyed pipeline steels and structural steels has been investigated using Gleeble 3800 system. Dependence of the hot ductility on temperature for different thermomechanical treatment conditions has been studied by physical simulation. The high temperature ductility drop occurs close to the phase and magnetic transition temperature for all the studied schedules. The elastic modulus at this temperature abnormally increases. The main reason of the ductility decrease is the elastic modulus increase. The structural factors (e.g. non-metallic inclusions on initial grain boundaries, interstitial impurities, grain size) lead to the additional decrease of the ductility. Increase of the grain boundary lengths and steel refining lead to the hot ductility improvement. The ductility trough disappears after the deformation of metal in austenitic region to decrease the austenite grain size significantly.

Effect of microalloying additions on the hot ductility of cast FeNi36

Hot tensile tests were carried out on FeNi36 (Invar alloy) over the temperature range of 850–1150°C and at strain rates of 0.001–1s−1. The hot ductility improved by increasing temperature and strain rate. The improvement of hot ductility at high temperatures was attributed to the influence of dynamic recrystallization. On the other hand, increasing strain rate could reduce the time for the propagation of voids and therefore postpone final fracture. The influences of microalloying additions from Al, Zr, Ti and Mg on the hot ductility were investigated by similar hot tensile tests. The results showed that the addition of Al as a microalloying element could harm the hot ductility. It was associated with the increase of the stacking fault energy of the base metal, thereby decreasing the tendency for dynamic recrystallization. In addition, the aluminum oxide particles could act as the preferential sites for cracking. The addition of Zr degraded the hot ductility. It was attributed to increasing the stacking fault energy of the base metal and thereby decreasing the tendency for dynamic recrystallization. The hot ductility was improved by the addition of Ti. The positive effect of Ti was attributed to its effect on tie-up of S in TiS compounds. Magnesium was found as the most effective microalloying addition which could remarkably improve the hot ductility of the base metal. The influence of Mg on reacting with S thereby avoiding the grain boundary weakness was found responsible for the remarkable improvement of hot ductility. By adding 0.003wt.% Mg the amount of S in the material decreased from 0.0022 to 0.0011wt.%.

Hot ductility behavior of near-alpha titanium alloy IMI834

Abstract The hot ductility of rolled IMI834 titanium alloy has been studied by conducting tensile tests with a strain rate of 0.1 s−1 and temperature range of 750–1 100 °C to obtain the optimum hot working conditions. The alloy showed minimum hot ductility in the lower alpha–beta region in the temperature range 750–950 °C. Further microstructural characterizations showed improvement in hot ductility by increasing temperature, which was attributed to reduction of volume fraction of high strength alpha phase. The best hot ductility was observed at 1 000 °C, i. e. in the upper alpha–beta region. The better hot ductility at higher temperature could be related to the increase in the volume fraction of beta phase and the occurrence of dynamic restoration phenomena. The second decline in hot ductility appeared at higher temperatures in the beta region and was attributed to the high stacking fault energy and self-diffusion of beta phase leading to limitation of dynamic recrystallization.

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.

Influence of Isothermal Treatment on MnS and Hot Ductility in Low Carbon, Low Mn Steels

Hot ductility tests were used to determine the hot-cracking susceptibility of two low-carbon, low Mn/S ratio steels and compared with a higher-carbon plain C-Mn steel and a low C, high Mn/S ratio steel. Specimens were solution treated at 1623 K (1350 °C) or in situ melted before cooling at 100 K/min to various testing temperatures and strained at 7.5 × 10−4 s−1, using a Gleeble 3500 Thermomechanical Simulator. The low C, low Mn/S steels showed embrittlement from 1073 K to 1323 K (800 °C to 1050 °C) because of precipitation of MnS at the austenite grain boundaries combined with large grain size. Isothermal holding for 10 minutes at 1273 K (1000 °C) coarsened the MnS leading to significant improvement in hot ductility. The higher-carbon plain C-Mn steel only displayed a narrow trough less than the Ae3 temperature because of intergranular failure occurring along thin films of ferrite at prior austenite boundaries. The low C, high Mn/S steel had improved ductility for solution treatment conditions over that of in situ melt conditions because of the grain-refining influence of Ti. The higher Mn/S ratio steel yielded significantly better ductility than the low Mn/S ratio steels. The low hot ductility of the two low Mn/S grades was in disagreement with commercial findings where no cracking susceptibility has been reported. This discrepancy was due to the oversimplification of the thermal history of the hot ductility testing in comparison with commercial production leading to a marked difference in precipitation behavior, whereas laboratory conditions promoted fine sulfide precipitation along the austenite grain boundaries and hence, low ductility.

Phosphorus-induced hot ductility enhancement of 1Cr–0.5Mo low alloy steel

The effect of phosphorus on the hot ductility of 1Cr–0.5Mo low alloy steel is investigated over a temperature range of 700–1050°C using a Gleeble thermomechanical simulator. The steel samples undoped and doped with phosphorus are heated at 1300°C for 3min and then cooled at a rate of 5Ks−1 down to different test temperatures, followed by tensile deformation until fracture. The results show that the hot ductility of the steel, evaluated by the reduction in area, can be substantially enhanced by the addition of phosphorus, especially in the range 750–950°C. Nanohardness measurements and microscopy indicate that, in the austenite–ferrite regions, the hot ductility improvement is mainly caused by the phosphorus-induced solid solution hardening of ferrite layers formed along austenite grain boundaries, and over 900°C, by the phosphorus-promoted dynamic recrystallization of austenite.

Hot ductility behavior of a low carbon advanced high strength steel (AHSS) microalloyed with boron

The current study analyses the influence of boron addition on the hot ductility of a low carbon advanced high strength NiCrVCu steel. For this purpose hot tensile tests were carried out at different temperatures (650, 750, 800, 900 and 1000 °C) at a constant true strain rate of 0.001 s −1. Experimental results showed a substantial improvement in hot ductility for the low carbon advanced high strength steel when microalloyed with boron compared with that without boron addition. Nevertheless, both steels showed poor ductility when tested at the lowest temperatures (650, 750 and 800 °C), and such behavior is associated to the precipitation of vanadium carbides/nitrides and inclusions, particularly MnS and CuS particles. The fracture mode of the low carbon advanced high strength steel microalloyed with boron seems to be more ductile than the steel without boron addition. Furthermore, the fracture surfaces of specimens tested at temperatures showing the highest ductility (900 and 1000 °C) indicate that the fracture mode is a result of ductile failure, while in the region of poor ductility the fracture mode is of the ductile–brittle type failure. It was shown that precipitates and/or inclusions coupled with voids play a meaningful role on the crack nucleation mechanism which in turn causes a hot ductility loss. Likewise, dynamic recrystallization (DRX) which always results in restoration of ductility only occurs in the range from 900 to 1000 °C. Results are discussed in terms of boron segregation towards austenitic grain boundaries and second phase particles precipitation during plastic deformation and cooling.

Hot ductility of a Fe–Ni–Co alloy in cast and wrought conditions

The hot ductility of Fe–29Ni–17Co alloy was studied in both cast and wrought conditions by hot tensile tests over temperature range of 900–1250 °C and at strain rates of 0.001–1 s −1. Over the studied temperature range, the wrought alloy represented higher elongation and reduction in area as compared to the cast alloy. Dynamic recrystallization was found responsible for the higher hot ductility of the wrought alloy and the improvement of hot ductility of the cast alloy at high temperatures. At temperature range of 1000–1150 °C the wrought alloy exhibited a hot ductility drop while a similar trough was not observed in case of the cast alloy. It was also found that at temperatures of 1150–1250 °C the best hot ductility is achieved in both cases of cast and wrought alloy. The experimental data of flow stress were constitutively analyzed and the apparent activation energy of deformation was estimated to be 344 kJ/mol.

Effect of Thermal Cycle and Nitrogen Content on the Hot Ductility of Boron-bearing Steel

Hot ductility of Boron (B)-bearing steel has been examined in view of slab corner cracking problem. Addition of B to the low carbon steel reduced its hot ductility under a thermal cycle in which samples were cooled directly to the test temperature before straining. The change in hot ductility of B-bearing steel with deformation temperature showed one trough in the temperature range of 800–1000°C, which covered the lower temperature region of austenite single phase (region (I)), and near the austenite/ferrite transformation temperature (Ae3) (region (II)). An abrupt temperature decrease and reheating before straining heavily deteriorated the hot ductility of B-bearing steel in the region (I). In all steels, the strain concentration in the film-like ferrite primarily reduced hot ductility in region (II) regardless of the addition of B and the thermal cycles before straining. The ductility reduction of B-bearing steel is caused by the distribution and amount of BN precipitation, which is determined by the thermal cycles and the N content. Increase in the N content remarkably reduced hot ductility of B-bearing steel in region (I), where the behavior of BN precipitates controlled hot ductility. The results shows that the improvement of hot ductility in B-bearing steel can be attained by decreasing the N content and by avoiding an abrupt temperature decrease in the secondary cooling stage of the slab after solidification.

Hot Ductility of Sulfur-containing Low Manganese Mild Steels at High Strain Rate

The hot ductility of low manganese mild steels was investigated at a high strain rate of 22 s−1 to simulate a roughing mill in the hot rolling process for various contents of manganese and sulfur. With all steels, ductility loss occurred in a low temperature range in the austenite region after reheating at 1573 K. A decrease in the manganese content and an increase in the sulfur content deteriorated hot ductility. A low reheating temperature and a long holding time before tensile deformation improved hot ductility. Tensile strength decreased at reduction-in-area below 40% at a constant temperature, and had no relationship with chemical composition. The brittle fracture surfaces were intergranular fractured ones and were observed at reduction-in-area below 40%. (Fe,Mn)S precipitates existed in small dimples on the brittle fracture surfaces. The quantities of the precipitated sulfides were calculated by Thermo-Calc, and increased with increasing manganese and sulfur contents. An increase in the manganese content promoted coarse (Fe,Mn)S at the same temperature. The sizes of the sulfides were also correlated with thermal diffusion at various temperatures and holding times. Hot ductility was understood by the quantity and size of the precipitated sulfides at high reheating temperatures. Embrittlement was accelerated by an increase in the quantity of precipitated sulfides and a decrease in the precipitate size. At low reheating temperatures, improvement of hot ductility was recognized as a result of a decrease in solved sulfur in the reheating process.