Effect of Thermomechanical Treatment on Functional Properties of Biodegradable Fe-30Mn-5Si Shape Memory Alloy

The structure, martensitic γ ↔ ε transformation temperatures, Young’s modulus, mechanical properties, and electrochemical behavior of Fe-30Mn-5Si (wt pct) biodegradable shape memory alloy subjected to various thermomechanical treatments (TMT) comprising hot rolling or cold rolling with post-deformation annealing were characterized by optical microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, differential scanning calorimetry, tensile testing, open circuit potential, and polarization curves measurements in Hanks’ solution, as compared to reference heat treatment. The optimum combination of mechanical properties (low Young’s modulus, high tensile strength, and appropriate ductility) for biomechanical compatibility was obtained after TMT with hot rolling at 600 and 800 °C due to the formation of favorable dynamically polygonized and recrystallized structures and decrease in the γ↔ε transformation starting temperature down to the human body temperature. The TMT did not show a significant effect on the corrosion rate as compared to the appropriate corrosion rate after the reference heat treatment. It is concluded that the TMT with hot rolling at 600 or 800 °C, which provides an optimum combination of the required corrosion rate in the simulation body fluid with high biomechanical compatibility, can be considered a promising treatment of Fe-30Mn-5Si biodegradable alloy for bone implants.

     

Precipitates in As-Cast and Heat-Treated ASTM F75 Co-Cr-Mo-C Alloys Containing Si and/or Mn

The effect of the addition of Si or Mn to ASTM F75 Co-28Cr-6Mo-0.25C alloys on precipitate formation as well as dissolution during solution treatment was investigated. Three alloys—Co-28Cr-6Mo-0.25C-1Si (1Si), Co-28Cr-6Mo-0.25C-1Mn (1Mn), and Co-28Cr-6Mo-0.25C-1Si-1Mn (1Si1Mn)—were heat treated from 1448 K to 1548 K (1175 °C to 1275 °C) for a holding time of up to 43.2 ks. In the case of the as-cast 1Si and 1Si1Mn alloys, the precipitates were M₂₃C₆-type carbide, η phase (M₆C-M₁₂C–type carbide), and π phase (M₂T₃X-type carbide with a β-Mn structure), while in the case of the as-cast 1Mn alloy, M₂₃C₆-type carbide and η phase were detected. The 1Si and 1Si1Mn alloys required longer heat-treatment times for complete precipitate dissolution than did the 1Mn alloys. During the solution treatment, blocky dense M₂₃C₆-type carbide was observed in all the alloys over the temperature range of 1448 K to 1498 K (1175 °C to 1225 °C). At the heat-treatment temperature of 1523 K (1250 °C), starlike precipitates with stripe patterns—comprising M₂₃C₆-type carbide and metallic face-centered-cubic (fcc) γ phase—were detected in the 1Si and 1Si1Mn alloys. A π phase was observed in the 1Si and 1Si1Mn alloys heat treated at 1523 K and 1548 K (1250 °C and 1275 °C) and in the 1Mn alloy heat treated at 1548 K (1275 °C); its morphology was starlike-dense. The addition of Si appeared to promote the formation of the π phase in Co-28Cr-6Mo-0.25C alloys at 1523 K and 1548 K (1250 °C and 1275 °C). Thus, the addition of Si and Mn affects the phase and morphology of the carbide precipitates in biomedical Co-Cr-Mo alloys.     

Microstructural aspects of the dissolution and melting of Al2Cu phase in Al-Si alloys during solution heat treatment

The dissolution and melting of Al₂Cu phase in solution heat-treated samples of unmodified Al-Si 319.2 alloy solidified at ≈10 °C were studied using optical microscopy, image analysis, electron probe microanalysis (EPMA), and differential scanning calorimetry (DSC). The solution heat treat-ment was carried out in the temperature range 480 °C to 545 °C for solution times of up to 24 hours. Of the two forms of Al₂Cu found to exist,i.e., blocky and eutectic-like, the latter type is more pronounced in the unmodified alloy (at ≈10 °C) and was observed either as separate eutectic pockets or precipitated on preexisting Si particles, β-iron phase needles, or the blocky Al₂Cu phase. Dissolution of the (Al + Al₂Cu) eutectic takes place at temperatures close to 480 °C through frag-mentation of the phase and its dissolution into the surrounding Al matrix. The dissolution is seen to accelerate with increasing solution temperature (505 °C to 515 °C). The ultimate tensile strength (UTS) and fracture elongation (EL) show a linear increase when plotted against the amount of dissolved copper in the matrix, whereas the yield strength (YS) is not affected by the dissolution of the Al₂Cu phase. Melting of the copper phase is observed at 540 °C solution temperature; the molten copper-phase particles transform to a shiny, structureless phase upon quenching. Coarsening of the copper eutectic can occur prior to melting and give rise to massive eutectic regions of (Al + Al₂Cu). Unlike the eutectic, fragments of the blocky Al₂Cu phase are still observed in the matrix, even after 24 hours at 540 °C.     

Examination of the Influence of Heat Treatment on the Properties of AI‐Si Alloys

In this paper the influence of heat treatment on the structural and mechanical properties of Al‐Si alloys was investigated. Silicon content in the examined alloys was in the range 11 to 14%, the contents of the other alloying elements were in the standard range [1] but all alloys were modified with strontium. The regime of the applied heat treatment was quenching (520°C/6h – cooling in water) + aging (205°C/7h –air cooling). The examinations were carried out at room temperature as well as at 250°C and 300°C. The obtained results showed a positive influence of the applied heat treatment on the mechanical properties of the examined alloys. The improvement of the mechanical properties can be considered as a consequence of a redistribution and change of morphology of the phases present in the structure of the alloys.     

On the silicon spheroidization in Al–Si alloys

It is commonly known that eutectic silicon spheroidizes during a T6 solution heat treatment in Al–Si cast alloys. This spheroidization of the brittle silicon is the main reason for the good fracture elongation values in peak hardened T6 condition. As the silicon spheroidization is known as a side effect of common T6 treatment, there is little known about the influence of sole silicon spheroidization on the mechanical properties. The following paper presents fundamental aspects of the spheroidization process of eutectic silicon in Al–Si cast alloys. It is theoretically as well as experimentally found that the disintegration and spheroidization of well modified eutectic silicon is finished within minutes of exposure to temperatures above 500 °C. The application of this new silicon spheroidization treatment (SST) to thixoformed components results in outstanding fracture elongation values (up to 18%) at good yield strength level (∼230 MPa). FEM simulations of the short high temperature heat treatment confirm a short process time for successful SST even for components with notably variable wall thicknesses.     

Effect of heat treatment on the microstructure,tensile properties,and fracture behavior of permanent mold Al-10 wt pct Si-0.6 wt pct Mg/SiC/10p composite castings

The present study was undertaken to investigate the effect of solution treatment (in the temperature range 520 °C to 550 °C) and artificial aging (in the temperature range 140 °C to 180 °C) on the variation in the microstructure, tensile properties, and fracture mechanisms of Al-10 wt pct Si-0.6 wt pct Mg/SiC/10_p composite castings. In the as-cast condition, the SiC particles are observed to act as nucleation sites for the eutectic Si particles. Increasing the solution temperature results in faster homogenization of the microstructure. Effect of solution temperature on tensile properties is evident only during the first 4 hours, after which hardly any difference is observed on increasing the solution temperature from 520 °C to 550 °C. The tensile properties vary significantly with aging time and temperature, with typical yield strength (YS), ultimate tensile strength (UTS), and percent elongation (EL) values of ∼300 MPa, ∼330 MPa, and ∼1.4 pct in the underaged condition, ∼330 MPa, ∼360 MPa, and ∼0.65 pct in the peakaged condition, and ∼323 MPa, ∼330 MPa, and ∼0.8 pct in the overaged condition. Prolonged solution treatment at 550 °C for 24 hours results in a slight improvement in the ductility of the aged test bars. The fracture surfaces exhibit a dimple morphology and cleavage of the SiC particles, the extent of SiC cracking increasing with increasing tensile strength and reaching a maximum in the overaged condition. Microvoids act as nucleation sites for the formation of secondary cracks that promote severe cracking of the SiC particles. A detailed discussion of the fracture mechanism is given.     

Fluidized bed heat treatment of cast Al-Si-Cu-Mg alloys

The effects of fluidized bed heat treatment on the microstructural and mechanical properties of Al-Si-Cu-Mg cast alloys, namely, 354 and 319, were studied. The heating rate in fluidized beds (FBs) is greatervis-à-vis conventional electrical resistance furnaces (CFs). The high heating rate in FBs increases the kinetics of metallurgical phenomena such as Si fragmentation and spherodization during solution heat treatment, as well as the precipitation rate of phases such as Al₅Cu₂Mg₈Si₆ and Al₂Cu during aging. It is observed that the dissolution rate of phases such as Mg₂Si and Al₅Cu₂Mg₈Si₆ takes place very rapidly. The solution heat treatment of 319 alloy using FB results in complete dissolution of Mg₂Si and Al₅Cu₂Mg₈Si₆ particles within 45 minutes. However, for phases such as Al₂Cu and Ferich intermetallics, the dissolution rate is relatively slow. Even on prolonged solution heat treatment for 6 hours, these phases do not dissolve completely. It is observed that incomplete dissolution of the Al₂Cu phase does not significantly affect tensile properties of T4-treated alloys. The optimum solution heat-treatment time in FB for both 354 and 319 alloys is 45 minutes at 527 °C and 493 °C, respectively. Thermal analysis shows an exothermic peak owing to recrystallization and coarsening of eutectic grains during solution heat treatment. The high heating rate in FB causes this transformation to take place at a lower temperature than in CF. It is observed that the nucleation rate of Al₅Cu₂Mg₈Si₆ during aging in FB is greater than using CF. Thermal analysis of samples during the ramp-up stage while aging using FB did not show any phase transformation, while those using CF show two endothermic transformations, which are most likely due to the dissolution of GP zones or the co-cluster of solutes. Aging at 200 °C results in a greater number density of precipitates than those at 240 °C. The tensile strength of samples aged at 200 °C is greater than those aged at 240 °C, because the amount of precipitates formed at 200 °C is greater than that at 240 °C. The total heat-treatment time for T6 temper is less than 2 hours in FBs, which is a significant reduction in heat-treatment time, as well as energy consumption.     

Aging Effects on Heat Treatment Response and Mechanical Properties of Al-(1 to 13 pct)Si-Mg Cast Alloys

The effects of solution treatment time and Si content and morphology on microstructures and mechanical properties of heat-treated Al-Si-Mg cast alloys were investigated systematically. Five alloys, with Si levels ranging from 1 to 13 pct, were tested in as-cast, T4, and T61 conditions. The eutectic Si was both unmodified and Sr-modified. Results show that the microstructures are affected significantly by alloy composition, eutectic Si morphology, and solution treatment time. Si content has significant effects on ultimate tensile strength (UTS), yield strength (YS), and elongation as well as a strong influence on solution treatment response. In T61 treatment with different solutionizing times, UTS and YS reach their maximum values in ~1 hour of solutionizing followed by a decrease, then a slight increase, and finally, a plateau close to the maximum level. Elongation of alloys with a high Si content, 7 pct and 13 pct, increases rapidly at solutionizing times of 1 to 2 hours then varies in a wide range, showing improvements in the 4 to 10 hours range. The data indicate that a solution treatment time of ~1 hour is sufficient to achieve maximum strength. The changes in mechanical properties were correlated to changes in microstructure evolution—Mg-Si precipitation, Si particle fragmentation, and microstructure homogenization. Empirical models uniquely relating Si content to UTS and YS are given for T61 heat-treated alloys.     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

Embrittlement of a continuously cooled Fe-25 Cr alloy

A study has been carried out on the effects of isothermal heat treatment at 475 and 550‡C and of continuous cooling at different rates from 850°C on the brittleness (as assessed by the ductile-brittle impact transition temperature) of a vacuum melted Fe-25 Cr alloy. The ductile-brittle transition temperature was found to be the lowest for the water quenched condition and highest for the furnace cooled condition and for material aged at 475‡C for long times (~500 h). An increase of brittleness with decreased cooling rate in the continuously cooled samples is attributed to the formation of more continuous and larger amounts of chromium nitrides and carbonitrides at the grain boundaries. Very little or no body centered cubic chromium-rich phase (alpha prime), associated with 475°C embrittlement, was observed. On aging at 550°C, the increased brittleness with time is also accounted for by the formation of grain boundary nitrides and carbonitrides. Although a similar effect takes place in the alloy heat treated at 475°C, the precipitation of alpha prime after long aging times enhances the brittleness. The tendency towards a more brittle condition with aging treatment and slower cooling rate is explained in terms of the Cottrell theory for brittle fracture.     

Effect of Heat Treatment on Microstructure and Mechanical Properties of API X90 Pipeline Steel Welded Joints

In this work, X90 pipeline steel was welded by shielded metal arc welding with root and submerged arc welding with calk and cap. After that, the joint was tempered at 550 °C and 600 °C. Subsequently, the microstructure and mechanical properties were investigated by optical microscope, scanning electron microscope (SEM) and Vickers hardness tester. The fracture surface of welded joint was analyzed by SEM. The experiments show that: After high-temperature tempering treatment, the Widmanstatten structure disappears in coarse-grained region. The number of lath martensite in the original structure of the welded joint decreases, and the quantity of granular bainite increases. The acicular ferrite disappears gradually and the polygonal ferrite grains increase, and the M/A island components decompose into fine grains with the increase in tempering temperature. The mechanical properties of the joints also change and the hardness rises because of the disappearance of Widmanstatten structure improves after heat treatment, and yield strength and tensile strength of the specimens have the lowest values after tempering at 550 °C. After tempering at 600 °C, the hardness decreases and strength–ductility reaches 11,571 MPa%. The tensile fracture of 600 °C sample presents delamination and ductile fracture. Through comparative analysis, it can be concluded that the plasticity and toughness of multilayer welded joints after tempering at 600 °C is improved, and they have better comprehensive mechanical properties.

     

Mechanical properties of a 29 Pct Cr-4 Pct Mo-2 Pct Ni ferritic stainless steel

The mechanical properties of a new ferritic stainless steel consisting essentially of 29 pct Cr, 4 pct Mo, 2 pct Ni (29-4-2) have been evaluated. The mechanical properties of the alloy are dependent on the thermomechanical processing and the final heat treatment conditionsi.e., both annealing temperature and cooling rate from the anneal. The alloy has excellent toughness, ductility and strength at room temperature when fast cooled from elevated temperatures. Slow cooling from elevated temperatures results in a degradation of impact resistance and an increase in strength. The alloy is subject to two major forms of embrittlement. One form results from the precipitation of intermetallic compounds in the temperature range 704°C (1300°F) to 954°C (1750°F) while the other results from the classical phenomenon called 475°C (885°F) embrittlement in the temperature range 399°C (750°F) to 510°C (950°F). Degradation of room temperature impact resistance occurs faster after the high temperature type of embrittlement and failure is characterized by an intergranular fracture mode. Embrittlement after exposure to 475°C (885°F) results in a slower degradation in toughness and results in failure by a transgranular cleavage mode. Impact resistance and tensile ductility are also decreased by exposure to 593°C (1100°F); however, to a lesser degree than 475°C (885°F) or 760°C (1400°F) exposure. The alloy deforms by slip or twinning depending on the metallurgical condition of the material. Deformation by twinning rather than slip is not manifested by a reduction in either toughness or ductility. Exposure to 482°C (900°F) promotes deformation by twinning whereas exposure to 760°C (1400°F) does not.     

Effects of Thermalrate Treatment Technique on Microstructure and Mechanical Properties of Hypoeutectic Al–Si Alloys

In this study, effects of thermalrate treatment (TRT) technique on microstructure and mechanical properties of hypoeutectic Al–Si alloys with addition of Ti were studied. The superheating temperatures of the melt were ascertained based on the DSC result. The results show that when the alloy castings in sand mold were treated with TRT technique at the superheating temperature of 930 °C, α-Al changes into smaller equiaxial crystals from coarse dendrites, and hardness of the alloy increases by 12.7 %, compared to that of the alloy treated with conventional casting technique. In addition, the supercooling increases to 8.5 °C and the characteristic temperatures of eutectic solidification are all the lowest with TRT technique at the superheating temperature of 930 °C. As holding time increases at the pouring temperature of 730 °C in TRT at the superheating temperature of 930 °C, the effects on microstructure and mechanical properties of the alloy casting in sand mold decrease. TRT technique plays a limited role in the alloy casting in permanent mold.     

A Study of Microstructure and Performance of Tempered Fe‐V‐W‐Mo Alloy

Influences of tempering temperature, holding time and tempering times on the microstructure and performance of Fe‐5%V‐5%W‐5%Mo‐5%Cr‐3%Nb‐2%Co(Fe‐V‐W‐Mo) were investigated by means of metallography, optical microscopy, hardness measurements, impact tester and pin abrasion tester. The results show that the hardness of Fe‐V‐W‐Mo alloy remains constant when tempered below 350°C. The hardness decreases gradually as the tempering temperature increase until around 475°C and then it increases again to a peak at 525°C. The hardness of Fe‐V‐W‐Mo alloy reaches nearly the highest value after the first tempering and decreases after triple‐tempering. The toughness of Fe‐V‐W‐Mo alloy increases until the tempering temperature reaches 475°C and then decreases until the temperature reaches 525°C. However, it increases again when tempering is beyond that temperature. The excellent wear resistance can be obtained by tempering at 500‐550°C.     

Influence of heat treatments on microstructure,mechanical properties,and corrosion resistance of weld alloy 625

The effects of heat treatments of the industrial type (eight-hour hold times at temperatures between 600 °C and 1000 °C) on the structural, mechanical, and corrosion resistance characteristics of weld alloy 625 have been studied. During the heat treatment, the mean concentration ratios of Nb, Mo, Si, Cr, Ni, and Fe elements between the interdendritic spaces and dendrite cores show little evolution up to 850 °C. Beyond that temperature, this ratio approximates 1, and the composition heterogeneity has practically disappeared at 1000 °C. An eight-hour heat treatment at temperatures between 650 °C and 750 °C results in increased mechanical strength values and reduced ductility and impact strength linked to the precipitation of body-centered tetragonal metastable intermetallic γ″ Ni₃Nb phase in the interdendritic spaces. An eight-hour treatment in the temperature range between 750 °C and 950 °C has catastrophic effects on all mechanical characteristics in relation with the precipitation, in the interdendritic spaces, of the stable orthorhombic intermetallic δ Ni₃(Nb, Mo, Cr, Fe, Ti) phase. At 1000 °C, the ductility and impact strength are restored. However, the higher the heat treatment temperature, the weaker the mechanical strength. Heat treatments have no effect on the pitting resistance of weld alloy 625 in sea water. The comparison of the results of this study on weld alloy 625 with those previously obtained on forged metal 625 shows that heat treatments below 650 °C and above 1000 °C are the sole treatments to avoid embrittlement and impairment of the corrosion resistance characteristics of alloy 625.     

Effect of Bath Temperature on Cooling Performance of Molten Eutectic NaNO3-KNO3 Quench Medium for Martempering of Steels

Martempering is an industrial heat treatment process that requires a quench bath that can operate without undergoing degradation in the temperature range of 423 K to 873 K (150 °C to 600 °C). The quench bath is expected to cool the steel part from the austenizing temperature to quench bath temperature rapidly and uniformly. Molten eutectic NaNO₃-KNO₃ mixture has been widely used in industry to martemper steel parts. In the present work, the effect of quench bath temperature on the cooling performance of a molten eutectic NaNO₃-KNO₃ mixture has been studied. An Inconel ASTM D-6200 probe was heated to 1133 K (860 °C) and subsequently quenched in the quench bath maintained at different temperatures. Spatially dependent transient heat flux at the metal–quenchant interface for each bath temperature was calculated using inverse heat conduction technique. Heat transfer occurred only in two stages, namely, nucleate boiling and convective cooling. The mean peak heat flux (q _max) decreased with increase in quench bath temperature, whereas the mean surface temperature corresponding to q _max and mean surface temperature at the start of convective cooling stage increased with increase in quench bath temperature. The variation in normalized cooling parameter t ₈₅ along the length of the probe increased with increase in quench bath temperature.

     

Dissolution of iron intermetallics in Al-Si Alloys through nonequilibrium heat treatment

Conventional heat treatment techniques in Al-Si alloys to achieve optimum mechanical properties are limited to precipitation strengthening processes due to the presence of second-phase particles and spheroidization of silicon particles. The iron intermetallic compounds present in the microstructure of these alloys are reported to be stable, and they do not dissolve during conventional (equilibrium) heat treatments. The dissolution behavior of iron intermetallics on nonequilibrium heat treatment has been investigated by means of microstructure and mechanical property studies. The dissolution of iron intermetallics improves with increasing solution temperature. The addition of manganese to the alloy hinders the dissolution of iron intermetallics. Nonequilibrium heat treatment increases the strength properties of high iron alloys until a critical solution temperature is exceeded. Above this temperature, a large amount of liquid phase is formed as a result of interdendritic and grain boundary melting. The optimum solution treatment temperature for Al-6Si-3.5Cu-0.3Mg-lFe alloys is found to be between 515 °C and 520 °C.     

Third Generation 0.3C-4.0Mn Advanced High Strength Steels Through a Dual Stabilization Heat Treatment: Austenite Stabilization Through Paraequilibrium Carbon Partitioning

In excess of 30 vol. pct austenite can be retained in 0.3C-4.0Mn steels subjected to a dual stabilization heat treatment (DSHT) schedule—a five stage precisely controlled cooling schedule that is a variant of the quench and partition process. The temperature of the second quench (stage III) in the DSHT process plays an essential role in the retained austenite contents produced at carbon-partitioning temperatures of 723 K or 748 K (450° C or 475 °C) (stage IV). A thermodynamic model successfully predicted the retained austenite contents in heat-treated steels, particularly for a completely austenitized material. The microstructure and mechanical behavior of two heat-treated steels with similar levels of retained austenite (~30 vol. pct) were studied. Optimum properties—tensile strengths up to 1650 MPa and ~20 pct total elongation—were observed in a steel containing 0.3C-4.0Mn-2.1Si, 1.5 Al, and 0.5 Cr.     

Mechanical properties of new low‐nickel cobalt‐free maraging steels

To study the mechanical properties of newly developed low‐nickel cobalt‐free maraging steels, six laboratory series with varied Ti, Mo, and Cr mass contents (0.003‐ 1.65, 0.0074‐ 5 and 0.004‐ 5 %) were studied. The study showed that increasing Ti and Mo contents improve the tensile strength and yield strength without affecting ductility markedly. The chromium alloyed grades M2, M6, M8 and M12 exhibit properties comparable to those of T 250 grade. To investigate the effects of heat treatment temperature on the mechanical properties, the solution treatment was carried out at temperatures varying between 820 and 1100 °C and holding times of 15 min at different cooling rates. The ageing temperature varied from 400 to 500 °C at ageing times of 60, 120 and 240 min, respectively. The ageing response of hardness as a function of temperature at different soaking conditions was determined to optimise the heat treatment conditions. The mechanical properties of the different steels after solution treatment and ageing at optimum temperature and time were determined.