Mechanical properties of sinter-hardened Cr–Si–Ni–Mo based steel foam

Highly porous sinter-hardenable Cr–Si–Ni–Mo based steel foam for automotive applications was produced by space holder method. Steel powders were mixed with binder (polyvinylalcohol) and space holder (carbamide), and compacted. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were then sintered at temperatures between 1100 °C and 1250 °C for sintering times of 15, 30 and 45 min. In addition, the steel foams were sinter-hardened to enhance mechanical properties. Sinter-hardening combines sintering and heat treatment in one step by increasing the post-sintering cooling rate. This reduces the cost of operation and makes powder metallurgy more competitive. Effects of sinter-hardening process parameters on compressive strength, Young’s modulus, hardness and energy absorption of the steel foams were investigated.     

Effect of heat treatment on mechanical properties of low alloy steel foams

Effect of heat treatment on compressive properties of low alloy steel foams (Fe–1.75 Ni–1.5 Cu–0.5 Mo–0.6 C) having porosities in the range of 47.4–71.5% with irregular pore shape, produced by the space holder-water leaching technique in powder metallurgy, was investigated. Low alloy steel powders were mixed with different amounts of space holder (carbamide), and then compacted at 200 MPa. Carbamide in the green compacts was removed by water leaching at room temperature. The green specimens were sintered at 1200 °C for 60 min in hydrogen atmosphere. Sintered compacts were heat treated by austenitizing at 850 °C for 30 min and then quenched at 70 °C in oil and tempered at 210 °C for 60 min. In this porosity range, compressive yield strengths of as-sintered and heat treated specimens were 28–122 MPa and 18–168 MPa, respectively. The resultant Young’s moduli of the as-sintered and heat treated specimens were 0.68–3.12 GPa and 0.47–3.47 GPa, respectively. The heat treatment enhanced the Young’s modulus and compressive yield strength of the foams having porosities in the range of 47.4–62.3%, as a consequence of matrix strengthening. However, the compressive yield stress and Young’s modulus of the heat treated foam having 71.5% porosity were lower than that of the as-sintered foam’s, as a result of cracks in the structure. The results were discussed in light of the structural findings.     

High temperature compression properties of open-cell Ni–20Cr foams produced by impregnation

The mechanical properties of open-cell Ni–20Cr foams were studied by uniaxial compression tests, which were performed at a strain rate of 10^− 3 s^− 1 at ambient temperature, 250 °C, 400 °C and 550 °C respectively. Then, the effect of temperature on the mechanical properties of these foams was analyzed. It was found that both the compressive strength and energy absorbed by Ni–20Cr foam at elevated temperatures became lower than those at room temperature. The compressive strength changed with temperatures in a form of parabola, which satisfied the model derived from Gibson–Ashby model combined with Kocks' kinetic model. A multi-parameter phenomenological elasto-plastic constitutive model was modified to take the temperature effect into consideration. The relationships between model parameters and temperature were determined. The fitting stress–strain curves of the model were in good agreement with the experimental results.     

Stainless steel foams made through powder metallurgy route using NH4HCO3 as space holder

Stainless steel (316) foams of varying porosities have been made through powder metallurgy route using NH₄HCO₃ as a space holder. Green compacts of stainless steel powder with NH₄HCO₃ were sintered at two different temperatures: 1100 °C and 1200 °C. At higher sintering temperatures, neighboring stainless steel powders fused together to form polycrystalline grain structure with iron–chromium intermetallic phases segregated along the grain boundaries. Whereas, the fusion of neighboring stainless steel powders was limited around the particle–particle contact only when the green compacts were sintered at 1100 °C, which resulted in a larger amount of microporosities in the cell wall. These foams exhibited strain hardening behavior in the plateau region under compressive loading. The yield stress and the flow stress (at lower strain levels) of foams, sintered at 1100 °C were higher. But, the reverse is true for the flow stress at higher strain levels. The exponents and the coefficients of the power law relationships varied with sintering temperature and strain levels.     

Effect of strain rate on the dynamic compressive mechanical behaviors of rock material subjected to high temperatures

Strain rate is not only an important measure to characterize the deformation property, but also an important parameter to analyze the dynamic mechanical properties of rock materials. In this paper, by using the SHPB test system improved with high temperature device, the dynamic compressive tests of sandstone at seven temperatures in the range of room temperature to 1000 °C and five impact velocities in the range of 11.0–15.0 m/s were conducted. Investigations were carried out on the influences of strain rate on dynamic compressive mechanical behaviors of sandstone. The results of the study indicate that the enhancement effects of strain rates on dynamic compressive strength, peak strain, energy absorption ratio of sandstone under high temperatures still exist. However, the increase ratios of dynamic compressive strength, peak strain, and energy absorption ratio of rock under high temperature compared to room temperature have no obvious strain rate effects. The temperatures at which the strain rates affect dynamic compressive strength and peak strain most, are 800, and 1000 °C, respectively. The temperatures at which the strain rates affect dynamic compressive strength and peak strain weakest, are 1000 °C, and room temperature, respectively. At 200 and 800 °C, the strain rate effect on energy absorption ratio are most significant, while at 1000 °C, it is weakest. There are no obvious strain rate effects on elastic modulus and increase ratio of elastic modulus under high temperatures. According to test results, the relationship formula of strain rate with high temperature and impact load was derived by internalizing fitting parameters. Compared with the strain rate effect at room temperature condition, essential differences have occurred in the strain rate effect of rock material under the influence of high temperature.     

Preparation and properties of porous Ti–10Mo alloy by selective laser sintering

In this study, porous Ti–10Mo alloy was prepared from a mixture of titanium, molybdenum and epoxy resin powders by selective laser sintering preforming, debinding and sintering at 1200 °C under a pure argon atmosphere. The influence of sintering process on the porous, microstructural and mechanical properties of the porous alloy was discussed. The results indicate that the pore characteristic parameters and mechanical properties mainly depend on the holding time at 1200 °C, except that the maximum strain keeps at about 45%. The matrix microstructure is dominated by α phase with a small quantity of β phase at room temperature. As the holding time lengthens from 2 to 6 h, the average pore size and the porosity decrease from 180 to 50 μm and from 70 to 40%, respectively. Meanwhile, the Young's modulus and the compressive yield strength increase in the ranges of 10–20 GPa and 180–260 MPa, respectively. Both the porous structure and the mechanical properties of the porous Ti–10Mo alloy can be adjusted to match with those of natural bone.     

Phase,microstructure and properties evolution of fine-grained W–Mo–Ni–Fe alloy during spark plasma sintering

Fine-grained tungsten (W) heavy alloy containing molybdenum (Mo) with W particle sizes of less than 5 μm were fabricated by spark plasma sintering (SPS) pre-milling W–2Mo–7Ni–3Fe powder at a lower temperature of 1000–1250 °C. Phase, microstructure and mechanical properties evolution of W–Mo–Ni–Fe alloy during spark plasma sintering were studied in detail. As increasing sintering temperature, the hardness of the alloy decreased rapidly. However, bending strength of the alloy demonstrated a fall–rise–fall trend, and the maximum strength was obtained at 1150 °C. The W–2Mo–7Ni–3Fe alloy microstructure was composed of white W-grain, gray W-rich structure, black γ-(Ni, Fe, W, Mo) binding phase, and deep-gray W-rich structure. The intergranular fracture along the W/W grain boundary is the main fracture modes of W–2Mo–7Ni–3Fe alloy.     

Experimental study on the dynamic compressive mechanical properties of concrete at elevated temperature

Strain rate effect and temperature effect are two important factors affecting the mechanical behavior of concrete. Each of them has been studied for several years. However, the two factors usually work together in the engineering practice. It is necessary to understand the mechanical responses of concrete under high strain rate and elevated temperature. A self-designed high temperature SHPB apparatus was used to study the dynamic compressive mechanical properties of concrete at elevated temperature. The results show that the dynamic compressive strength and specific energy absorption of concrete increase with strain rate at all temperatures. The elastic modulus decreases obviously with strain rate at room temperature and stabilizes at a level with slightly decrease at elevated temperature. The dynamic compressive strength of concrete at 400 °C increases by nearly 14% compared to the room temperature. However, it decreases at 200 °C, 600 °C and 800 °C with the decrease ratio of 20%, 16% and 48%, respectively. The dynamic elastic modulus decreases largely subjected to elevated temperature. The specific energy absorption at 200 °C, 400 °C and 600 °C is higher than room temperature and decreases to be lower than room temperature at 800 °C. Formulas are established under the consideration of mutual effect of strain rate and temperature.     

An investigation into the mechanical behavior of a new transformation-twinning induced plasticity steel

In recent years, the transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels have been the focus of great attention thanks to their excellent tensile strength-ductility combination. Accordingly the mechanical behavior of an advanced microalloyed TRIP–TWIP steel, the compression tests were conducted from 25 to 1000 °C. This experimental steel shows a high compressive strength of 1280 MPa with the yield strength of 385 MPa as well as an outstanding strain hardening rate of about 14,000 MPa at the 25 °C. In addition the results indicate that the plastic deformation in the range of 25–150 °C is controlled by both the strain-induced martensite formation and mechanical twinning. However the mechanical twinning has been speculated as the only deformation mechanism in the temperature range of 150–1000 °C. This as well has led to an outstanding grain refinement via grain partitioning. The occurrence of mechanical twinning at such high temperatures is a novel observation in this grade of TRIP–TWIP high manganese steels.     

Tensile properties of hot rolled Mg–3Sn–1Ca alloy sheets at elevated temperatures

Mechanical behavior of hot rolled Mg–3Sn–1Ca (TX31) magnesium alloy sheets were studied in the temperature range 25–350 °C. The microstructure of the alloy consisted of the eutectic structure of α-Mg + Mg₂Sn and a dispersion of needle-like CaMgSn. The highest room-temperature ductility of 18% was obtained by hot rolling of the cast slabs at 440 °C, followed by annealing at 420 °C. The high temperature tensile deformation of the material was characterized by a decrease in work hardening exponent (n) and an increase in strain rate sensitivity index (m). These variations resulted in respective drops of proof stress and tensile strength from 126.5 MPa and 220 MPa at room temperature to 23.5 MPa and 29 MPa at 350 °C. This was in contrast to the ductility of the alloy which increased from 18% at room temperature to 56% at 350 °C. The observed variations in strength and ductility were ascribed to the activity of non-basal slip systems and dynamic recovery at high temperatures. The TX31 alloy showed lower strength than AZ31 magnesium alloy at low temperatures, while it exhibited superior strength at temperatures higher than 200 °C, mainly due to the presence of thermally stable CaMgSn particles.     

Hot workability of 00Cr13Ni5Mo2 supermartensitic stainless steel

The hot workability of 00Cr13Ni5Mo2 supermartensitic stainless steel was investigated by hot compression and hot tension tests conducted over the temperature range of 950–1200 °C and strain rates varying between 0.1 and 50 s^?1. The processing map technique was applied on the basis of dynamic materials model and Prasad instability criterion. Microstructure evolutions, Zener–Hollomon parameter as well as hot tensile ductility were examined. The results show that, as for the hot working of 00Cr13Ni5Mo2 supermartensitic stainless steel in the industrial production, the large strain deformation should be carried out in the temperature range 1140–1200 °C and strain rate range 0.1–50 s^?1, where the corresponding Zener–Hollomon parameters exhibit low values. Moreover, when deformed under high strain rate range (above 15 s^?1), the deformation temperature can be reduced reasonably.     

Novel Cu-bearing economical 21Cr duplex stainless steels

A new family of 21Cr–2 Ni–1.0Mo–0.2 N–xCu (x = 0.5, 1.0, 1.5) economical duplex stainless steels have been developed by examining the effect of Cu on the microstructure and properties of solution-treated specimens. The results have shown that these alloys have a balanced ferrite–austenite duplex structure. The ferrite content increases with the solution treatment temperature, but decreases with an increase in Cu. Some precipitates such as sigma phase, ε-Cu and Cr₂N were found when solution-treated at 780 °C for 30 min. The yield strength, tensile strength and fracture elongation values of experimental alloys solution-treated at 1020 °C for 30 min were about 540 MPa, 1000 MPa, and 35%, respectively. The pitting corrosion potentials of the solution-treated alloys were all above 500 mV in 1 mol/L NaCl solution at room temperature and the pitting corrosions always occur in ferrite phase. The mechanical properties and corrosion resistance of the designed alloys with lower production cost are better than those of AISI 316L austenitic stainless steel.     

Elevated temperature low cycle fatigue of grey cast iron used for automotive brake discs

This paper evaluates the fatigue life properties of low carbon grey cast iron (EN-GJL-250), which is widely used for automotive brake discs. Although several authors have examined mechanical and fatigue properties at room temperatures, there has been a lack of such data regarding brake discs operating temperatures. The tension, compression and low cycle fatigue properties were examined at room temperature (RT) and at brake discs’ working temperatures: 500 °C, 600 °C and 700 °C. The microstructure of the material was documented and analysed. Tensile stress–strain curves, cyclic hardening/softening curves, stress–strain hysteresis loops, and fatigue life curves were obtained for all the above-mentioned temperatures. It was concluded, that Young’s modulus is comparable with both tension and compression, but yield its strength and ultimate strength are approximately twice as great in compression than in tension. All the mechanical properties remained quite stable until 500 °C, where at 700 °C all deteriorated drastically. During fatigue testing, the samples endured at 500 °C on average at around 50% of cycles at room temperature. Similar to other materials’ properties, the cycles to failure have dropped significantly at 700 °C.     

Fracture characteristics of thermally exposed 9Cr–1Mo steel after tensile and impact testing at room temperature

The paper deals with the fracture analysis of thermally exposed 9Cr–1Mo ferritic steel after tensile and impact testing at room temperature. The temperatures of the thermal expositions were 580, 620, and 650 °C, respectively. The duration of annealing experiments varied from 500 to 5000 h. The influence of thermal expositions on Vickers hardness as well as tensile properties was found to be negligible. On the other hand, remarkable effects of the annealing on room temperature impact toughness were observed. Fracture behaviour of the 9Cr–1Mo steel is strongly affected by the presence of precipitates of secondary phases. Fracture surfaces of tensile samples indicate mainly transgranular dimple fracture mechanism. By contrast, the fracture mode of the samples after impact testing is more complex. It shows both – ductile dimple tearing as well as inter-lath decohesion.     

The liquid phase sintering of molybdenum with Ni and Cu additions

To increase the sintered density of Mo compacts, liquid phase sintering was employed by adding sintering aids, Ni and Cu. With 1.5 wt.% Ni sintered at 1370°C, the sintered density of Mo increased from 86.0 to 97.5%. When one-third of the Ni was replaced by Cu, the density was improved from 82.1 to 99.1% when sintered at 1300°C. Although copper has little solubility in molybdenum, and vice versa, it aids nickel in enhancing the sintering of molybdenum compact during heating in the solid state and during the liquid phase sintering. The dilatometer run indicated that the liquid formation temperature was lowered by 90°C. Better wetting on molybdenum particles was also observed. However, the Mo–Ni–Cu compact was still brittle due to the presence of brittle Mo–Ni compounds at the grain boundaries.     

Structural characteristics and elevated temperature mechanical properties of AJ62 Mg alloy

Structure and mechanical properties of the novel casting AJ62 (Mg–6Al–2Sr) alloy developed for elevated temperature applications were studied. The AJ62 alloy was compared to commercial casting AZ91 (Mg–9Al–1Zn) and WE43 (Mg–4Y–3RE) alloys. The structure was examined by scanning electron microscopy, x-ray diffraction and energy dispersive spectrometry. Mechanical properties were characterized by Viskers hardness measurements in the as-cast state and after a long-term heat treatment at 250 °C/150 hours. Compressive mechanical tests were also carried out both at room and elevated temperatures. Compressive creep tests were conducted at a temperature of 250 °C and compressive stresses of 60, 100 and 140 MPa. The structure of the AJ62 alloy consisted of primary α-Mg dendrites and interdendritic nework of the Al₄Sr and massive Al₃Mg₁₃Sr phases. By increasing the cooling rate during solidification from 10 and 120 K/s the average dendrite arm thickness decreased from 18 to 5 μm and the total volume fraction of the interdendritic phases from 20% to 30%. Both factors slightly increased hardness and compressive strength. The room temperature compressive strength and hardness of the alloy solidified at 30 K/s were 298 MPa and 50 HV 5, i.e. similar to those of the as-cast WE43 alloy and lower than those of the AZ91 alloy. At 250 °C the compressive strength of the AJ62 alloy decreased by 50 MPa, whereas those of the AZ91 and WE43 alloys by 100 and 20 MPa, respectively. The creep rate of the AJ62 alloy was higher than that of the WE43 alloy, but significantly lower in comparison with the AZ91 alloy. Different thermal stabilities of the alloys were discussed and related to structural changes during elevated temperature expositions.     

A new 1.9 GPa maraging stainless steel strengthened by multiple precipitating species

A new ultra-high strength maraging stainless steel with composition of 13Cr–13Co–4.5Ni–3.5Mo–0.5Ti (at.%) has been developed. It was demonstrated that the ultimate tensile strength of the steel could reach 1.9 GPa with reasonable ductility. This breakthrough was achieved by a combined strengthening of three different species of precipitates. The evolution of precipitates with respect of size, morphology and chemical composition during aging at 500 °C was characterized using atom probe tomography (APT) and transmission electron microscopy (TEM). The precipitates were identified to be η-phase Ni₃(Ti, Al) phase, Mo-rich R′ phase and Cr-rich α′ phase, developing out of the precursor clusters, Ni–Ti–Al-rich cluster, Mo-rich cluster and Cr-rich cluster, separately. The segregation of Mo and Cr atoms at the precipitate/matrix interfaces was detected and is considered to impede the coarsening of η-phase. Based on the characterizations, the precipitation process of these phases and their effect on mechanical properties were analyzed.     

The ambient and high temperature deformation behavior of Al–Si–Cu–Mg alloy with minor Ti,Zr, Ni additions

The principal aim of the present work was to investigate the effects of minor additions of nickel and zirconium on the strength of cast aluminum alloy 354 at ambient and high temperatures. Tensile properties of the as-cast and heat-treated alloys were determined at room temperature and at high temperatures (190 °C, 250 °C, 350 °C). The results show that Zr reacts only with Ti, Si and Al. From the quality index charts constructed for these alloys, the quality index attains minimum and maximum values of 259 MPa and 459 MPa, in the as-cast and solution-treated conditions; also, maximum and minimum values of yield strength are observed at 345 MPa and 80 MPa, respectively, within the series of aging treatments applied. A decrease in tensile properties of ∼10% with the addition of 0.4 wt.% nickel is attributed to a nickel–copper reaction. The reduction in mechanical properties due to addition of different elements is attributed principally to the increase in the percentage of intermetallic phase particles formed during solidification; such particles act as stress concentrators, decreasing the alloy ductility. Tensile test results at ambient temperatures show a slight increase (∼10%) in alloys with Zr and Zr/Ni additions, particularly at aging temperatures above 240 °C. Additions of Zr and Zr + Ni increase the high temperature tensile properties, in particular for the alloy containing 0.2 wt.% Zr + 0.2 wt.% Ni, which exhibits an increase of more than 30% in the tensile properties at 300 °C compared with the base 354 alloy.     

Low cycle fatigue behavior of Cr–Mo–V low alloy steel used for railway brake discs

The cyclic stress–strain response and the low cycle fatigue (LCF) behavior of Cr–Mo–V low alloy steel which was used for forged railway brake discs was studied. Tensile strength and LCF properties were examined over a range from room temperature (RT) to 600 °C using specimens cut from circumferential direction of a forged disk. The fully reversed strain-controlled LCF tests were conducted at a constant total strain rate with different axial strain amplitude levels. The cyclic strain–stress relationships and the strain–life relationships were obtained through the test results, and related LCF parameters of the steel were calculated. The studied steel exhibits cyclic softening behavior and behaves Masing type, especially at higher strain amplitudes. At higher than 600 °C, carbide particles aggregated and a decarburized layer developed near the specimen surface. Micro voids distribute within the depth of 50 μm from the specimen surface could coalesce with fatigue cracks. Multiple crack initiation sites were observed on the fracture surface. The oxide film that generated at 600 °C covered the fatigue striations and accelerated the crack propagation. Final fracture area with bigger and deeper dimples showed better ductility at higher temperature. The investigated LCF behavior can provide reference for brake disc life assessment and fracture mechanisms analysis.     

High-temperature corrosion behavior of Ni–50Cr coating deposited by high velocity oxygen–fuel technique on low alloy ferritic steel

Ni–50Cr coatings were deposited using the HVOF technique on low alloy ferritic steel (2.25Cr–1Mo) substrates to improve their performance in high temperature steam environments. Different thermal spray parameters were studied in order to optimize the corrosion resistance of the coatings. High temperature thermal tests at 650 °C in different CO₂ atmospheres (air with 0, 15 and 25 vol.% CO₂) and thermal cycling tests in air at 550 °C and 650 °C were conducted to study the effectiveness of the coating protection system. The uncoated specimens were severely corroded but no oxidation in the coated substrates was detected. A reduction of 10 times in terms of weight change per area unit in the coated steel was obtained after 360 h of testing respect to that of the uncoated steel.