Temperature Ductility Research Articles

Experimental studies and calculation of crack propagation at the nil ductility temperature of shipbuilding steel

Resistance to crack propagation in low-alloyed steels is verified by the experimental evaluation of ductile-to-brittle transition temperatures. Though, correlations of test results obtained according to used methods with the minimum design temperature of marine structures should be substantiated. The paper suggests a fracture mechanics based formula for the required nil ductility temperature (NDT). An original FEM simulation method with uniform mesh size is applied.

Determination of the ductile-to-brittle transition temperature of NIOMOL 490 K steel welded joints

This paper is dedicated to the ductile–brittle transition behaviour of the microalloyed structural steel NIOMOL 490 K. This steel grade is used for welded pressure vessels subjected to dynamic loads and operating at sub-zero temperatures. Therefore, it must have an acceptable toughness. Due to its importance for the safety assessment of pressure vessels, a characterization of this steel was carried out using the Charpy V-notch impact test in the temperature range between −60 °C and + 60 °C. The notches were located in parent material, heat affected zone and weld metal. In this paper, the tensile strength properties at ambient temperature and the nil ductility temperature in the temperature range from −60 °C to + 60 °C are presented.

42CrMo4 steel flow behavior characterization for high temperature closed dies hot forging in automotive components applications

The application of new forming processes as the high temperature hot forging in closed dies in an industrial environment still requires further investigation due to the lack of flow stress data at these temperatures. To determine the flow behavior of the 42CrMo4 steel at high temperatures hot compression tests have been carried out in a Gleeble® 3800 thermomechanical tester for a temperature range that covers the material behavior from the hot forging until the Nil Ductility Temperature (1250 °C-1375 °C) and for three different orders of magnitudes for the strain rates (0.1 s−1, 1 s−1 and 10 s−1). Then, the Hansel-Spittel model, widely used in automotive commercial software as FORGE®, has been employed to obtain the adequate constants of the constitutive equation for high temperatures.Finally, the newly obtained flow behavior model has been validated by comparison between experimental and simulated compression tests and by the process simulation of a commercial automotive component comparing the results of the simulation with the already made experimental tests in a laboratory cellule of the new technology.Hence, this paper shows the procedure for the determination and the obtention of a new constitutive model for the 42CrMo4 steel flow stress characterization at a temperature range between 1250 °C–1375 °C. This will contribute in the knowledge of material flow stress behavior models at high temperatures and will allow the prediction or simulation of high temperature hot forging in closed dies processes, enhancing the possibility of the application of these technologies from an industrial point of view.

Hot Ductility Evolution Mechanism of Titanium‐Bearing Microalloyed Steels

Titanium‐bearing (Ti‐bearing) microalloyed steels have high strength and toughness by grain refinement effect of carbonitride precipitates. However, they can induce surface cracks of continuous casting slab when the Ti alloyed content is high. A microalloyed steel with Ti content (0.10–0.15 wt%) is carried out by thermalmechanical simulator over 600–1350 °C to analyze hot ductility evolution mechanism. Fracture surface morphology, phase transition, and behavior of precipitates of the tensile samples are investigated by experimental detection and thermodynamic calculation. The ductility–temperature curves show that the third brittle temperature range is 600–890 °C, which is mainly attributed to the thin proeutectoid ferrite film and precipitated titanium carbonitride particles, widening the embrittlement temperature ranges through of steel. In addition, the tensile samples at 890–1350 °C have good hot ductility, indicating the dynamic recrystallization of deformed austenite can trigger grain boundaries migration away from cracks and avoid the side effect of the Ti (C,N) particles on hot ductility. The first brittle temperature range of 1350 °C‐melting point is mainly ascribed to the partial melting of the grain boundaries with element segregation of sulfur and phosphorus, and microporosity loose among dendrites.

The role of the strengthening phases on the HAZ liquation cracking in a cast Ni-based superalloy used in industrial gas turbines

This work presents the influence of microstructural constituents on liquation crack formation in the cast Ni-based superalloy, René 108. The investigation was divided into three parts: characterisation of the material's microstructure in pre-weld condition, hot ductility studies and analysis of liquation cracking induced by the gas tungsten arc welding process. Using advanced electron microscopy techniques it is shown that the base material in pre-weld condition is characterised by a complex microstructure. The phases identified in René 108 include γ matrix, γ' precipitates, MC and M23C6 carbides, and M5B3 borides. Based on Gleeble testing, it was found that René 108 is characterised by high strength at elevated temperatures with a maximum of 1107 MPa at 975 °C. As a result of constitutional liquation, the superalloy’s strength and ductility were significantly reduced. The nil strength temperature was equal to 1292 °C, while the nil ductility temperature was 1225 °C. The low ductility recovery rate (32.1), ratio of ductility recovery (36.2) and hot cracking factor (Rf = 0.05) values confirmed the low weldability of Renѐ 108. In the heat-affected zone (HAZ) induced by welding, constitutional liquation of mainly γ' precipitates, with a contribution of M23C6 carbides and M5B3 borides, was observed. The thin non-equilibrium liquid film, which formed along high-angle grain boundaries, led to crack initiation and their further propagation during cooling. The eutectic γ–γ' re-solidification products are visible on the crack edges.

Tuning strength-ductility combination of AISI 1008 steel through large strain single-roll drive rolling and intercritical annealing

In this study, the influence of large-strain single-roll drive rolling and intercritical annealing on the microstructure evolution and mechanical behavior of AISI 1008 steel was investigated. The microstructures of all fabricated dual-phase (DP) steels exhibited equiaxed and homogeneously distributed fine ferrite grains and martensite islands. Compared to the as-received sample, the hardness of the produced dual-phase steels increased by up to 100%. As the temperature of intercritical annealing treatment increased, the hardness of the dual-phase samples increased from 233.2 HV to 258.4 HV. All fabricated DP steels showed much higher strength and toughness than the as-received sample. Overall, with increasing the temperature of intercritical annealing, the strength, ductility, and toughness of DP steels were improved due to increasing the martensite fraction. Also, by increasing the temperature of intercritical annealing from 770 to 860°C, the work hardening rate of the DP sample was enhanced. The hardness and strength of dual-phase steels had a good linear correlation to the fraction of martensite. The failure mechanism of the 80% rolled sample was mainly brittle while all DP samples exhibited a perfect ductile fracture characterized by many homogeneous deep dimples without any cleavage facets.

Susceptibility of High-Manganese Steel to High-Temperature Cracking.

Tests were carried out on two high-Mn steels: 27Mn-4Si-2Al-Nb with Nb microaddition and 24Mn-3Si-1.5Al-Nb-Ti with Nb and Ti microadditions. High-manganese austenitic steels, due to their good strength and plastic properties belong to the AHSS (Advanced High-Strength Steel) group and are used in the automotive industry. The main difficulties faced during the casting of the steel and hot working are hot cracks, which can appear in the surface of the ingot. Cracks on the edges of the sheet after hot rolling are the reason for cutting the edges of the sheet and increasing production costs and material losses. The main reason for the formation of hot cracks is the decrease in metal ductility in the high-temperature brittleness range (HTBR). The width of the HTBR depends on mechanical properties and microstructural factors, i.e., non-metallic inclusions or intermetallic phases at austenite grain boundaries. In this paper, a hot tensile test was performed. The research was performed on the GLEEBLE 3800 thermomechanical simulator. This test allows us to determine the width of the high-temperature brittleness range (HTBR), the Nil Strength Temperature (NST), the Nil Ductility Temperature (NDT), and the Ductility Recovery Temperature (DRT). Hot ductility was determined from the value of the reduction in area R(A). The obtained results make it possible to determine the temperature of the beginning of hot working from the tested high-Mn steels. Fractographic research enabled us to define mechanisms of hot cracking. It was found that hot cracks form as a result of disruptions in the liquid film on crystals' boundaries.

Study of Crack Sensitivity of Peritectic Steels

By comprehensively considering both the high temperature mechanical properties and peritectic transformation during peritectic steel solidification, the strain εCth is proposed to evaluate the crack sensitivity of peritectic steels produced in the brittle temperature range in the present work. The zero ductility temperature (ZDT) and the zero strength temperature (ZST) of Fe–C–0.32Si–1.6Mn–0.01P–0.015S steel under nonequilibrium conditions by taking the effect of the peritectic transformation on the solute segregation into account were calculated by the CK microsegregation model (Clyne–Kurz model) and were compared with the measured data. The comparison results show that this model can well simulate the nonequilibrium solidification process of peritectic steel. Then, based on the calculation of the CK microsegregation model, the strain during the peritectic phase transformation in the brittle temperature range (ZDT < TB < LIT) was calculated under nonequilibrium conditions. The results show that the calculated strain is in good agreement with the actual statistical longitudinal crack data indicating that the strain can therefore be used to predict the crack sensitivity of peritectic steels effectively.

Microstructure and Mechanical Properties of the Ductile Al\u2013Ti\u2013Mo\u2013Nb\u2013V Refractory High Entropy Alloys

A number of non-equimolar refractory high entropy alloys (RF HEAs) from the Al–Ti–Mo–Nb–V system are synthesized, with the selected compositions aimed to balance the conflicting requirements of the low-temperature ductility and high-temperature corrosion protection. Based on the thermodynamic modeling and experimental results, all the obtained alloys are characterized by the single-phase B2 structure with V acting as the main phase stabilizer. The microstructure and mechanical properties appear to be controlled mainly by the Al content, which is especially visible on the example of hardness, with a maximum value of 545 HV for Al20Ti5Mo25Nb25V25 composition. For the selected Al20Ti5Mo25Nb25V25 and Al10Ti30Mo20Nb20V20 alloys, the measured stress–strain curves indicate the highly coveted, ductile room temperature behavior, with the values of ultimate strain measured under compression mode being 9.17 and 9.00 pct, respectively, and compressive fracture strain of 13.38 and 13.25 pct, respectively. The obtained results suggest that it is possible to include Al as a vital component of refractory HEAs without compromising their low-temperature ductility. The next intended step will be the characterization of the high-temperature corrosion behavior in order to investigate the potential selective oxidation capabilities of such materials.

Temperature and strain-rate dependence of the elevated temperature ductility of Inconel 718 prepared by selective laser melting

The elevated temperature tensile properties of SLM IN718 were measured as a function of strain rate and test temperature to better understand the time-dependent and thermal aspects of environmental sensitivity. SLM and wrought materials were solution treated, aged, and tested over the range of 550–750 °C and 10−3 to 10−6 s−1. The SLM material tested across all conditions had inferior strength and ductility compared to wrought material of the same heat treatment because of environmental sensitivity in the form of dynamic embrittlement. SLM samples show evidence of brittle intergranular fracture, crack growth, and oxidized NbC particles on the fracture surface, which all contribute to the observed poor ductility. These features, and subsequent embrittlement, are intensified with decreasing strain rate and increasing temperature. Higher strain rates and lower temperatures are shown to improve the ductility in SLM IN718 but despite this recovery it remains susceptible to environmental attack even in the extreme cases of the current study. The prevalence of NbC at grain boundaries in the SLM material is the principal source of dynamic embrittlement and environmental sensitivity.

Environment dependence of KIc of glass

The role of temperature (T) and moisture on fracture toughness (KIc) of glasses, as measured by the single-edge precracked beam method (SEPB), is investigated. The brittle to ductile temperature, shown by an increase of KIc, is ~Tg. At T below 0.9Tg, the decrease of the bond strength with T results in the decrease of both the fracture surface energy and Young's modulus, so that KIc decreases with T. From above Tg to below 1.1Tg, the work of fracture increases because of dissipative processes, including confined viscoplasticity and blunting at the crack front. From 1.1Tg, viscous flow extends to the whole specimen and becomes more uniform, resulting in the macroscopic deformation of the specimen, eventually preventing against fracture. The results on humidity sensitive glasses show that both precracking and final fracture steps need to be performed in inert environment to meet the intrinsic KIc value, unless the test is fast enough.

Analysis of the static and dynamic properties of wear-resistant Hardox 600 steel in the context of its application in working elements

Abstract The article discusses the static and dynamic properties of high-strength, boron-containing Hardox 600 steel that is resistant to abrasive wear, both in its delivery state and after normalization. Since the available published material in the literature does not have any real mechanical indicators of the abovementioned steel, a static tension test was carried out at an ambient temperature. The steel’s tensile strength, yield strength, Young’s modulus, elongation and reduction of area were determined from the test. The Charpy impact test at temperatures of −40 °C, −20 °C, 0 °C, and +20 °C and fractographic analysis were performed to determine the transition temperature of ductility to brittleness. In dynamic load conditions, the assigned values of impact energy do not always truly determine the material behavior. Thus, the aim of the fractography was to provide precision when determining the behavior. A significant difference in the impact energy of the tested steel with respect to its heat treatment and ductile-brittle transition temperature was observed and determined based on the impact test result, as well as the nature of the fracture. On the basis of the determined structural and strength characteristics, an analysis of the possibility of application of Hardox 600 steel on selected elements of working machines was performed.

Hot crack susceptibility of cast Mg97Y2Zn1

AbstractFormation of hot cracks in alloys is a critical issue for their production and processing. In this study, the susceptibility of the cast Mg97Y2Zn1 alloy containing long period stacking ordered (LPSO) structures to hot cracking was investigated using tensile tests in a Gleeble testing machine, and described in terms of the brittle temperature range (BTR), which was concluded from the zero strength temperature (ZST), zero ductility temperature, and ductility recovery temperature (DRT). The zero ductility and the DRTs were obtained directly from the mechanical tests, verified using fracture surface analysis in a scanning electron microscope, and correlated with the solidification range that was determined using differential scanning calorimetry. The extent of the BTR was 31 K, the width of the solidification range 100 K. The ZST coincided with the dissolution temperature of the LPSO structures, which means that the hot cracking susceptibility of the cast Mg97Y2Zn1 alloy is dominated by the dissolution of LPSO structures.

Influence of Hot Isostatic Pressing on the Hot Ductility of Cast Alloy 718: The Effect of Niobium and Minor Elements on the Liquation Mechanism

The influence of two hot isostatic pressing (HIP) treatments on liquation behavior was investigated and compared with regard to the extent of heat-affected zone liquation cracking in cast Alloy 718. The extent of liquation was seen to increase after HIP treatment at 1190 °C due to solute changes caused by the homogenization of Nb, which contributed to extensive grain boundary melting. The HIP treatment at 1120 °C exhibited lower liquation with contributions from particle liquation of the Laves phase and constitutional liquation of NbC carbides. This was also reflected in a lower ductility recovery temperature, with slower recovery for the former due to the extensive liquation. Interestingly, the nil ductility temperatures were both below the predicted equilibrium solidus of the alloy, which suggests that the ductility drop is related to liquation caused by solute segregation at the grain boundaries.

Heat Treatments to Develop High-Strength Ferritic Ductile Iron

The influence of grain refinement on the mechanical properties of ferritic ductile iron was investigated. Alloys containing nominally 2%, 3%, and 4% silicon were produced for this study. Various heat treatments were employed to produce a fine-grained ferritic structure. Hardness, tensile, and Charpy impact properties were determined. The three alloys were austempered at 385 °C (725 °F). The austempered materials were then annealed between 540 and 732 °C (1000–1350 °F) to produce a fine-grained, 100% ferrite matrix. Intercritical heat treatments were also performed to produce a fine-grained acicular microstructure. Samples were both air-cooled and oil-quenched from the intercritical temperature and subsequently annealed to ferritize the material and enhance toughness. All three alloys were also tested in the fully annealed ferritic condition. All approaches to refine ferrite grain size were successful; the heat-treated alloys developed strengths comparable to the fully annealed SSSF 4%Si alloy. Based on Charpy impact testing, the grain-refined alloys displayed nil ductility temperatures below room temperature, whereas that of the 4% Si alloy was above room temperature.

Effect of FCC Slurry on Performance of SBS Modified Bitumen

Fluid catalytic cracking (FCC) slurry is a by-product of petrochemical industry rich in aromatics. In this paper, the effect of FCC slurry on the physical properties of SBS modified bitumen was investigated by softening point, penetration, ductility and low temperature flexibility test. The influence of FCC slurry on the compatibility of SBS modified bitumen was evaluated by fluorescence microscope and segregation test. The results show that FCC slurry improves the softening point, penetration, ductility and low temperature flexibility of SBS modified bitumen. Fluorescence microscope showed that FCC slurry promotes the dispersion of SBS in bitumen, which is conducive to the formation of network crosslinking structure of SBS modified bitumen. The segregation test indicated that the compatibility of SBS with bitumen was improved with the content of FCC slurry increasing.

Comparison of cracking sensitivity between high-permeability 1.5 wt% Si steel and 6.5 wt% Si steel in mushy zone

Twin-roll thin strip casting process (SC) has been utilized in laboratory to fabricate typical high-permeability electrical steel (HPES) such as 1.5 wt% Si and 6.5 wt% Si steel with ideal crystallographic orientation and excellent permeability after a series of heat treatments. Whereas, there exists a problem associated to the cracking sensitivity in high temperature mushy zone, which further restricts the industrial application of SC to produce HPES. Mechanical behavior of 1.5 wt% Si and 6.5 wt% Si steels in high temperature mushy zone were comparatively studied by Gleeble-3800 physical simulation system under strain rate 3 s−1, systematically. The high temperature brittle region composed of zero strength temperature (ZST) and zero ductility temperature (ZDT) in the mushy zone were quantitatively determined, and the critical strain threshold associated to hot crack initiation in the brittle region were further evaluated following empirical model proposed by Y Won. Results show that the tensile strength and fracture ductility in the mushy zone are both closely associated with the varying volume fraction of network-shaped structure during solidification, and are decreased with decreasing solid fraction concerning studied 1.5 wt% Si and 6.5 wt% Si steels. The high temperature brittle region corresponding to the studied 1.5 wt% Si and 6.5 wt% Si steels in mushy zone are separately confirmed to be 1459 ~ 1465℃ and 1390 ~ 1427℃, and the thresholds of critical strain for hot cracks initiation in high temperature brittle region are calculated to be ~ 1.18% and ~ 0.24%, respectively. Considering from perspective of width of high temperature brittle region and critical strain threshold for hot crack initiation, the cracking sensitivity of 6.5 wt% Si steel in the mushy zone is more serious compared to 1.5 wt% Si steel.

High-Temperature Mechanical Properties of 4.5%Al δ-TRIP Steel

The high-temperature mechanical properties of a 4.5% Al-containing δ-transformation-induced plasticity (TRIP) steel were studied by using the Gleeble 3500 thermomechanical simulator. The zero ductility temperature (ZDT) and the zero strength temperature (ZST) were measured, and the brittle zones were divided. The phase transformation zone was determined by differential scanning calorimetry (DSC). The temperature of the phase transformation and the proportion of the phase were calculated by the Thermo-Calc software. The ZDT and the ZST of the 4.5% Al-containing δ-TRIP steel are 1355 and 1405 °C, respectively. The first brittle zone and the third brittle zone of the steel are 1300–1350 °C and 800–975 °C, respectively. The reason for the embrittlement of the third brittle zone of the 4.5% Al-containing δ-TRIP steel is that the α-ferrite formed at the austenite grain boundary causes the sample to crack along the grain boundary under stress. The ductility of the 4.5% Al-containing δ-TRIP steel decreases first and then increases with the increase of the α-ferrite. When the proportion of the α-ferrite reaches 37%, the reduction of area (RA) of the 4.5% Al-containing δ-TRIP steel is reduced to 44%. The 4.5% Al-containing δ-TRIP steel has good resistance to the high-temperature cracking.

Scientific and Technological Bases for Developing Cold-Resistant Steel with a Guaranteed Yield Strength of 315–750 MPa for Arctic Conditions: Part 2. Manufacturing Technology, Structure, Properties, and Serviceability of Sheet Products

Abstract—The paper presents results of the industrial implementation of hot plastic deformation and thermal treatment schemes for the production of rolled sheet products based on cold-resistant steel with yield strength not lower that 315–750 MPa for the Arctic. On the basis of the studies performed, a set of scientific and technological techniques has been developed for various technological processes (thermomechanical treatment followed by accelerated cooling, quenching from the rolling heating, and separate furnace heating with high-temperature tempering). The developed complex method provides the formation of a structure having an acceptable heterogeneity and anisotropy level corresponding to different morphological and crystallographic parameters throughout the entire thickness of rolled products up to 100 mm based on low-alloy steels with a yield strength not lower than 315–460 MPa and up to 60 mm based on economically alloyed steels with a yield strength not lower than 500–750 MPa. The structure of sheet products is presented that provides guaranteed characteristics of strength, cold resistance (impact energy KV at a testing temperature ranging from –60 to –80°C, critical ductile-to-brittle transition temperature Tkb, and nil ductility temperature NDT), and crack resistance according to a CTOD criterion in a low-temperature range to meet the requirements of the RMRS “Rules for the Classification and Construction of Ships” for steel with the Arc40 index.

Scientific and Technological Bases for Developing Cold-Resistant Steel with a Guaranteed Yield Strength of 315–750 MPa for Arctic Conditions. Part 1: Alloying Principles and Requirements for Sheet Product Structure

Abstract—The results obtained upon choosing rational alloying and microalloying for cold-resistant steels with a guaranteed yield strength of 315–750 MPa on the basis of established interrelations between phase transformations, structure, mechanical properties, serviceability parameters, and the content of main alloying elements are presented. Quantitative requirements for various structural parameters and their maximum permissible difference throughout sheet product thickness up to 100 mm have been developed, depending on the strength category and manufacturing technology (thermomechanical treatment with accelerated cooling, quenching from separate furnace heating or rolling heating with high temperature tempering) to provide guaranteed characteristics of strength, cold resistance (impact energy KV at a testing temperature from –60 to –80°С, critical ductile-to-brittle transition temperature Тkb, and nil ductility temperature NDT), and crack resistance according to the criterion of critical crack tip opening displacement (CTOD).