Ductility Trough Research Articles

Fracture analysis and embrittlement phenomena of machined brass components

Fracture analysis and metallographic characterization is performed in selected machinable brass components, for use in hydraulic installations. Evidence of brittle fracture was mainly indicated by the presence of intergranular failure and limited plasticity areas. Multiple cracking was observed at the most highly stressed areas, such as the crests of the threads, the cross section transitions and thinnest wall locations. Fractographic analysis performed using Field Emission SEM, suggested the occurrence of embrittling phenomena, probably attributed to overheating and subsequent Pb-phase grain boundary segregation. Metallographic cross section investigation rather supported the failure hypothesis, implying that hot shortness is the plausible cause of ductility trough and brittle failure.

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.

Hot workability of cast and wrought Ni–42Cu alloy through hot tensile and compression tests

In order to analyze the flow behavior and workability of Ni–42Cu in cast and wrought conditions, hot deformation tests were performed at temperatures and strain rates within the ranges of 900–1150 °C and 0.001–1 s−1, respectively. Tensile tests showed a “hot ductility trough” at 950 °C for both alloys. The drop in hot ductility was more considerable in the cast alloy because of the sluggish dynamic recrystallization. The hot ductility drop and grain boundary cracking, particularly in the cast alloy, were attributed to the segregation of detrimental atoms to the boundaries. It was shown that the hot ductility of the wrought alloy could be improved with increasing strain rate. It was associated with increasing the fraction of dynamic recrystallization at higher strain rates. This finding corroborated the change in the mechanism of dynamic recrystallization with strain rate. The strain rate sensitivity and instability parameters calculated for the wrought alloy showed that the material is prone to strain localization at low temperatures, i.e., 950–1050 °C, and high strain rates of 0.1 and 1 s−1. Based on the tensile and compression tests, the best temperature range for a desirable hot workability was introduced as 1050–1150 °C.

Hot ductility trough elimination through single cycle of intense cooling and reheating for microalloyed steel casting

Surface transverse cracking, especially corner cracking, is prone to generate in continuously cast slabs of microalloyed steels. The method of surface structure control (SSC) was supposed to the best way to avoid the detrimental defects. However, the mechanism of improving hot ductility by SSC and the specific parameters to control the process are still unclear for the reasonable adoption in production. In the present work, the impact of cooling rate, holding temperature and holding time on austenite decomposition, and the austenite grain size before and after intense cooling were investigated by thermal simulation method. With the increase of cooling rate, it is observed that the phase is transformed from austenite → grain boundary film-like alltromorph ferrite → Widmanstätten ferrite plates (or intragranular ferrite plates) → bainite+martensite. Mostly important, the film-like ferrite can be eliminated through intense cooling and the following reheating, but the austenite grain size is not observed to be refined through the single γ → α → γ cycle. Even though, the reduction of area (RA) is improved drastically to over 70% in the third ductility trough, whereas the RA value is just < 40% for the conventional cooling. The reason can be deduced by eliminating grain boundary film-like ferrite, and carbonitrides were precipitated inside the grain, which simultaneously act as the nucleus of intragranular ferrite. Thus, the principle of SSC process was established, and based on heat transfer calculation, water flow adjustment was put forward to implement the intense secondary cooling, which is a meaningful exploration for reducing transverse cracking.

Effects of temperature and strain rate on the tensile behaviors of SIMP steel in static lead bismuth eutectic

In order to assess the susceptibility of candidate structural materials to liquid metal embrittlement, this work investigated the tensile behaviors of ferritic-martensitic steel in static lead bismuth eutectic (LBE). The tensile tests were carried out in static lead bismuth eutectic under different temperatures and strain rates. Pronounced liquid metal embrittlement phenomenon is observed between 200 °C and 450 °C. Total elongation is reduced greatly due to the liquid metal embrittlement in LBE environment. The range of ductility trough is larger under slow strain rate tensile (SSRT) test.

Effect of Rare-Earth Cerium on Impurity Tin-Induced Hot Ductility Deterioration of SA508-III Reactor Pressure Vessel Steel

The samples of SA508-III reactor pressure vessel steel, undoped, doped with Sn, and doped with Sn + Ce, are heated at 1300 °C and then cooled down to different temperatures between 650 and 1050 °C, followed by tensile deformation until fracture using a Gleeble machine. The steel hot ductility evaluated by the reduction of area (RA) is determined at each test temperature. The tested samples are characterized using scanning electron microscopy, optical microscopy, and field emission gun scanning transmission electron microscopy. The steel exhibits a hot ductility trough and Sn substantially deteriorates the hot ductility of the steel by deepening and widening the ductility trough. However, Ce can well suppress this Sn-induced hot ductility deterioration. Grain boundary segregation of Sn or Sn + Ce occurs in the Sn-doped or Sn + Ce-doped samples. The segregation of Sn is primarily responsible for the hot ductility deterioration as it weakens grain boundary, thereby enhancing grain boundary sliding and deteriorating hot ductility. The segregation of Ce is mainly responsible for the suppression of the deterioration as it strengthens grain boundary, thereby restraining grain boundary sliding and suppressing Sn-induced hot ductility deterioration.

Hot Ductility Characterization of Sanicro-28 Super-Austenitic Stainless Steel

The hot ductility behavior of a super-austenitic stainless steel has been studied using tensile testing method in the temperature range from 1073 K to 1373 K (800 °C to 1100 °C) under the strain rates of 0.1, 0.01, and 0.001 s−1. The hot compression tests were also performed at the same deformation condition to identify the activated restoration mechanisms. At lower temperatures [i.e., 1073 K and 1173 K (800 °C and 900 °C)], the serration of initial grain boundaries confirms the occurrence of dynamic recovery as the predominant restoration process. However, in the course of applied deformation, the initial microstructure is recrystallized at higher temperatures [i.e., 1273 K and 1373 K (1000 °C and 1100 °C)]. In this respect, annealing the twin boundaries could well stimulate the recrystallization kinetic through initiation new annealing twins on prior annealing twin boundaries. The hot tensile results show that there is a general trend of increasing ductility by temperature. However, two regions of ductility drop are recognized at 1273 K and 1373 K (1000°C)/0.1s−1 and (1100°C)/0.01s−1. The ductility variations at different conditions of temperature and strain rate are discussed in terms of simultaneous activation of grain boundary sliding and restoration processes. The observed ductility troughs are attributed to the occurrence of grain boundary sliding and the resulting R-type and W-type cracks. The occurrence of dynamic recrystallization is also considered as the main factor increasing the ductility at higher temperatures. The enhanced ductility is primarily originated from the post-uniform elongation behavior, which is directly associated with the strain rate sensitivity of the experimental material.

Influence of V–N Microalloying on the High-Temperature Mechanical Behavior and Net Crack Defect of High Strength Weathering Steel

Abstract The influence of V–N microalloying on the high-temperature mechanical behavior of high strength weathering steel is discussed through thermomechanical simulation experiment. The difference of tensile strength caused by variation of [%V][%N] appears after proeutectoid phase change, and the higher level of [%V][%N] is, the stronger the tensile strength tends to be. The ductility trough apparently becomes deeper and wider with the increase of [%V][%N]. When the level of [%V][%N] reaches to 1.7 × 10−3, high strength weathering steel shows almost similar reduction of area to 0.03% Nb-containing steel in the temperature range of 800–900℃, however, the ductility trough at the low-temperature stage is wider than that of Nb-containing steel. Moreover, the net crack defect of bloom is optimized through the stable control of N content in low range under the precondition of high strength weathering steel with sufficient strength.

Hot Ductility Behavior of a Peritectic Steel during Continuous Casting

Hot ductility properties of a peritectic steel for welded gas cylinders during continuous casting were studied by performing hot tensile tests at certain temperatures ranging from 1200 to 700 °C for some cooling rates by using Gleeble-3500 thermo-mechanical test and simulation machine in this study. The effects of cooling rate and strain rate on hot ductility were investigated and continuous casting process map (time-temperature-ductility) were plotted for this material. Reduction of area (RA) decreases and cracking susceptibility increases during cooling from solidification between certain temperatures depending on the cooling rate. Although the temperatures which fracture behavior change upon cooling during continuous casting may vary for different materials, it was found that the type of fracture was ductile at 1100 and 1050 °C; semi-ductile at 1000 °C, and brittle at 800 °C for the steel P245NB. There is a ductility trough between 1000 and 725 °C. The ductility trough gets slightly narrower as the cooling rate decreases.

Mechanism of hot ductility loss in C–Mn steels based on nonequilibrium grain boundary segregation of impurities

Abstract

Effect of Temperature Reversion on Hot Ductility and Flow Stress–Strain Curves of C-Mn Continuously Cast Steels

The influence of temperature reversion in secondary cooling and its reversion rate on hot ductility and flow stress–strain curve of C-Mn steel has been investigated. Tensile specimens were cooled at various regimes. One cooling regime involved cooling at a constant rate of 100 °C min−1 to the test temperature, while the others involved temperature reversion processes at three different reversion rates before deformation. After hot tensile test, the evolution of mechanical properties of steel was analyzed at various scales by means of microstructure observation, ab initio prediction, and thermodynamic calculation. Results indicated that the temperature reversion in secondary cooling led to hot ductility trough occurring at higher temperature with greater depth. With increasing temperature reversion rate, the low temperature end of ductility trough extended toward lower temperature, leading to wider hot ductility trough with slightly reducing depth. Microstructure examinations indicated that the intergranular fracture related to the thin film-like ferrite and (Fe,Mn)S particles did not changed with varying cooling regimes; however, the Widmanstatten ferrite surrounding austenite grains resulted from the temperature reversion process seriously deteriorated the ductility. In addition, after the temperature reversion in secondary cooling, the peak stress on the flow curve slightly declined and the peak of strain to peak stress occurred at higher temperature. With increasing temperature reversion rate, the strain to peak stress slightly increased, while the peak stress showed little variation. The evolution of plastic modulus and strain to peak stress of austenite with varying temperature was in line with the theoretical prediction on Fe.

Hot ductility behavior of an extruded 7075 aluminum alloy

The ductility behavior of an extruded 7075 aluminum alloy was studied using the tensile testing method in a wide range of temperatures (100–450°C) and strain rates (10−1–10–3s−1). The mechanical behavior of the experimental alloy at high temperatures was also studied through hot compression testing method to explain the related deformation mechanisms. The results indicated that the ductility was monotonically increased by increasing deformation temperature. This was justified considering the fact that the volume fractions of the second phase particles might decrease at higher temperature thereby their detrimental effects on cavity nucleation and growth would be decreased. However, two regions were found where the general trend changed and the ductility decreased. The observed ductility drops at 350°C/0.01, 0.001s−1 were attributed to the dynamic precipitation of Fe-rich secondary phases during deformation. The second ductility trough was observed at 450°C/0.1s−1 and attributed to the occurrence of grain boundary sliding phenomena, which would facilitate the development of W-type or R-type cracks thereby reducing the ductility.

Influence of Different Levels of Cu Equivalent on the Hot Ductility of 20CrMnTi Steel

AbstractThe hot ductility of 20CrMnTi steel with different levels of Cu equivalent was investigated. The results show that Sn and Cu in 20CrMnTi steel are detrimental to its hot ductility. Sn was found to segregate to the boundaries tested by EPMA, moreover, Cu was not found to segregate to boundaries, however, the fracture morphology was examined with SEM and showed many small and shallow dimples on the fracture of steels with large Cu equivalent (&gt;0.15) and fine copper sulfide was found from carbon extraction replicas using TEM. The adverse effect of large Cu equivalent (&gt;0.15) on the hot ductility was due to Sn segregation and fine copper sulfide in the steel as well as their retarding the occurrence of dynamic recrystallization (DRX). The proeutectoid ferrite film precipitating along the austenite grain boundary causes the ductility trough of the five examined steels. Moreover, in this case, the level of Cu equivalent should be controlled below 0.15, which would not deteriorate the hot ductility significantly.

Hot Ductility and Deformation Behavior of C-Mn/Nb-Microalloyed Steel Related to Cracking During Continuous Casting

Hot ductility studies have been performed on C-Mn and C-Mn-Nb steels with an approach to simulate the effect of cooling conditions experienced by steel in secondary cooling zone during continuous casting. Thermal oscillations prior to tensile straining deteriorate hot ductility of steel by deepening and widening the hot ductility trough. C-Mn steels are found to exhibit ductility troughs in three distinct zones whereas C-Mn-Nb steel shows drop in ductility only at low temperature in the vicinity of ferrite transformation temperatures. Start of ferrite transformation in steels causes yield ratio to increase while work hardening rates and strength coefficient decrease with decrease in test temperature in presence of thermal oscillation prior to tensile testing. Inhibition of recrystallization due to build-up of AlN particles along with the presence of MnS particles in structure and low work hardening rates causes embrittlement of steel in austenitic range. Alloying elements enhancing work hardening rates in austenitic range can be promoted to improve hot ductility. The presence of low melting phase saturated with impurities along the austenitic grain boundaries causes intergranular fracture at high temperature in C-Mn steels.

Hot Deformation Behavior of Incoloy 901 Through Hot Tensile Testing

Hot deformation tests were conducted on cast and wrought Incoloy 901 produced by electro-slag remelting at temperatures of 950-1150 °C and strain rates of 0.01-1 s−1. Both materials showed acceptable hot workability in the studied range of temperature and strain rates. However, better workability of the wrought material was associated with easier dynamic recrystallization compared to in the cast material. A complete dynamic recrystallization in the wrought material was observed at temperatures above 1100 °C. On the other hand, in the cast material that was characterized by a coarse grain structure, dynamic recrystallization occurred partially and was attributed to the low density of grain boundaries. It was recognized that in order to avoid the risk of premature fracture, hot processing of the studied material should be conducted in the temperature range of 1000-1100 °C. At high temperatures, over 1100 °C, both materials suffered from a lack of acceptable hot workability. Rather, the wrought material showed a slight hot ductility trough around 1050 °C that could be attributed to the segregation of detrimental solute atoms such as S and P to the grain boundaries. The grain boundary decohesion was controlling the fracture mechanism of the wrought material. Due to the absence of extensive DRX in the cast material, the incipient melting, void formation, and decohesion of precipitates were found responsible for the final fracture.

Effect of Tin on Hot Ductility and High-temperature Oxidation Behavior of 20CrMnTi Steel

AbstractThe hot ductility and high-temperature oxidation behavior of 20CrMnTi steel with 0.02% Cu and x% Sn (0.004 ⩽ x ⩽ 0.049) were investigated. The results show that tin has no significant effect on tensile strength of sample with less than 0.049% Sn. The critical temperature where the hot ductility reduces dramatically rises with the increase of tin content while the hot ductility decreases with its increase. The average tin content at austenite grain boundaries (GB) and substrate is 0.108% and 0.045% respectively in the specimen containing 0.049% tin quenched after heated to 1223 K and held for 600 s. Sn-segregation at the GB deteriorates the hot ductility. There is no direct relationship between the cause of the ductility trough and tin. However, Sn-segregation at the GB causes it to deepen a lot. The tin content should be controlled below 0.021%, which would not deteriorate the hot ductility significantly. There is no tin-enrichment at the scale/substrate interface when tin content is less than 0.049%. Moreover, although Sn is enriched under the steel surface, any liquid Sn-enrichment wasn't observed at the oxide/steel interface even in as high as 0.45% Sn-bearing steel with 0.02% Cu.

Unified mechanism of intergranular embrittlement based on non-equilibrium grain boundary segregation

The review is aimed at presenting a unified approach in understanding the mechanism of non-equilibrium grain boundary segregation, which can satisfactorily describe the three types of intergranular embrittlement, namely, reverse temper embrittlement of steels, intergranular corrosion embrittlement of stainless steels and intermediate temperature embrittlement of metals and alloys. The review starts with a broad perspective of non-equilibrium grain boundary segregation, including thermally induced non-equilibrium grain boundary segregation and stress induced non-equilibrium grain-boundary segregation. Next, it focuses on the recent progress made in the non-equilibrium grain boundary segregation, including (1) critical time, (2) segregation peak temperature, (3) segregation peak temperature movement for thermally induced and stress induced non-equilibrium grain boundary segregation, and (4) the effect of temperature difference on thermally-induced non-equilibrium grain boundary segregation. Next, the attention is focused on the grain boundary coverage of elements and intergranular embrittlement phenomena. Three types of intergranular embrittlement is analysed in terms of (1) the ductility healing effect induced by the critical time, (2) embrittlement peak or ductility trough induced by the segregation peak temperature, (3) embrittlement peak or ductility trough movement induced by the segregation peak temperature movement and (4) widening and deepening of ductility trough induced by differences in temperature. These experimental phenomena concerning the three types of intergranular embrittlement are consistent with the models of thermally induced and stress induced non-equilibrium grain boundary segregations of impurities, instead of precipitation or equilibrium grain boundary segregation. Towards the end, we visit the subject of grain boundary segregation and associated embrittlement process from the viewpoint of fracture resistance and briefly discuss different perspectives that are of practical significance.

Research of 50Mn2V continuous casting slab transverse cracking during its retarded cooling process

In this paper, the reason of 50Mn2V transverse cracking is studied by optical metallographic, SEM and in situ analysis system. Results show that the pearlite is surrounded by the net-like pro-eutectoid ferrite which locates at the former austenite grain boundary, the hydrogen content (ca. 2×10−4wt%) exceeds the critical solubility in room temperature structure, divers types and sizes inclusions (10–100μm), plenty of fissures (2–50μm) and classical cleavage fracture surface are obviously observed, meanwhile, a great degree of elements segregation (P, S, V, Ni, Cr correspond to 1.757, 1.674, 1.729, 1.475, 1.195, respectively) and a poorly 0.9283 integral density exist in this blank, high temperature thermoplastic experiment shows the ductility trough existing until the dynamic re-crystallization temperature range. From the above, internal gas pressure should be the dominant factor that leads the transverse embrittlement, while internal stresses, element segregation and defects play an ancillary role.

Hot ductility behavior of V-N and V-Nb microalloyed steels

The hot ductility of V-N and V-Nb microalloyed steels was investigated on a Gleeble-1500 thermomechanical simulator, and the results were compared with those of V and Nb microalloyed steels. A ductility trough is found in both the steels in the temperature range of 700 to 1050°C. Compared to the V steel, the V-N steel has a wider and deeper ductility trough with the increase of N content, due to the increased precipitation of V(C, N) in the steel. Above 930°C, when 0.047wt% V is added to the 0.028wt% Nb-containing steel, the ductility becomes worse, owing to the rise of the onset dynamic recrystallization temperature. However, the ductility gets better at 800 to 930°C because of the coarsening of precipitates in austenite. With the improvement in ductility, the fracture mechanism is changed from intergranular to high ductile fracture in the temperature range of 800 to 1050°C.

Hot working behavior of Fe–29Ni–17Co analyzed by mechanical testing and processing map

The hot workability of Fe–29Ni–17Co was analyzed using hot compression tests over temperature range of 900–1200°C and at strain rates of 0.001s−1 to 10s−1. In order to analyze the hot ductility behavior of the material, hot tensile tests were performed at the same temperature range and strain rate of 0.1s−1. The flow curves at temperatures higher than 1000°C were typical of dynamic recrystallization, while at lower temperatures work hardening was more dominant. The strain rate sensitivity parameter increased considerably with increasing temperature and the corresponding influence of strain rate was found negligible. The power dissipation and the instability maps of the studied material were developed based on the dynamic material model. From the developed processing map of the material, better workability at temperatures beyond 1000°C was elicited. Hot tensile tests reported a hot ductility trough over the range of 1100–1150°C. It contributed to a slight decrease in the reduction in area and strain to fracture. Despite the influence of hot ductility trough, mechanical testing and microstructural observations emphasized the better workability of the studied material at temperature range of 1000–1200°C and the studied strain rates.