Effect of Annealing on Properties of Carbonaceous Materials. Part II: Porosity and Pore Geometry

The pore structure of carbonaceous materials was studied using image analysis. The effect of annealing on the porosity and pore geometry of cokes, chars, and pyrolyzed coals (laboratory chars) was examined in the temperature range of 973 K to 1773 K (700 °C to 1500 °C). The porosity of chars and pyrolyzed coals significantly increased during annealing at temperatures below 1373 K (1100 °C) due to volatile matter release. Further increasing of the annealing temperature from 1373 K to 1773 K (1100 °C to 1500 °C) caused marginal porosity evolution. The porosity of cokes was not affected by annealing at temperatures below 1573 K (1300 °C) and slightly increased in the temperature range 1573 to 1773 K (1300 °C to 1500 °C). The increase in the porosity of chars and pyrolyzed coals during annealing at temperatures 1373 K to 1773 K (1100 °C to 1500 °C), and cokes at 1573 K to 1773 K (1300 °C to 1500 °C), was a result of reactions with oxides of their mineral phases. Annealing had a marginal effect on the pore shape (Feret ratio) of carbonaceous materials, but enlarged the pore size of chars and pyrolyzed coals and decreased their pore density.     

Low temperature impact properties of phosphorus and sulfur doped and sensitized type 304 stainless steel

The low temperature impact strength of four sulfur and four phosphorus doped and sensitized Type 304 stainless steels, based on a commercial heat that had been vacuum remelted to include the dopants, has been determined and compared to that of an ultra low sulfur and the undoped heat. The impact energies and the fracture surface morphologies vary with sensitizing time, temperature, and dopant. The impact energy is reduced up to 50 pct for the sulfur doped heats which fractured transgranularly, and the impact energies which varied with dopant addition and increasing time and temperature were reduced by up to 10-fold for the phosphorus doped heats which had a mixed fracture surface morphology. A minimum in impact energy was observed at 750 °C and sensitization times as low as 1500 seconds. As the dopant level increased, the reduction in impact energy extended to a broader temperature range centered about 750 °C. The impact strength was generally regained at heat treatmentsT > 750 °C andt > 1500 seconds depending on the dopant level.     

The recrystallization of commercially pure and doped tungsten wire drawn to high strain

The recrystallization processes in both undoped and doped tungsten wire after drawing to a true strain of 7.7 were examined by light microscopy and transmission electron microscopy. High angle grain boundary migration commenced at approximately the same temperature in both materials, but proceeded much more rapidly in the undoped wire, where the absence of a potassium bubble dispersion allowed a coarser, more equiaxed grain structure to form. No change from the (110) deformation texture was observed in either case. Recrystallization in the undoped wire was dominated at lower temperatures (1100 to 1200°C) by the growth of large grains into a much finer structure. As the annealing temperature was increased, this process was replaced by a general grain coarsening which eventually produced a relatively equiaxed recrystallized grain structure. It appeared probable that it was the second phase dispersion inhibition alone that prevented similar structural changes in the doped wire. This paper is based on a presentation made at a symposium on “Recovery Recrystallization and Grain Growth in Materials” held at the Chicago meeting of The Metallurgical Society of AIME, October 1977, under the sponsorship of the Physical Metallurgy Committee.     

Density changes accompanying the annealing of wrought thoriated tungsten wire

Density measurements were made on 0.762 mm W-l wt pct ThO₂ wires which had been annealed at temperatures between 1000°C and 2000°C for times up to 120 min. Selected specimens were examined using optical and scanning electron microscopy. The results of this study indicate that the low, as-worked wire density is a result of large cracks associated with the ThO₂ particles. Time-temperature annealing regimes, significantly below those necessary for recrystallization of the wire, result in partial healing of the cracks. This crack healing process can be followed qualitatively by observing longitudinal fracture surfaces in the scanning electron microscope and quantitatively by changes in the density of the wire. The apparent activation energy for the crack healing process was found to be 57.8 kcal/mole. The results suggest that crack closure is a result of both shear and grain boundary diffusion transport processes. It is concluded that the presence of cracks can influence the morphology of the recrystallized grains.     

Effect of Annealing on Properties of Carbonaceous Materials. Part III: Macro and Microstrengths

Carbonaceous materials including cokes, chars, and pyrolyzed coals were annealed at temperatures ranging from 973 K to 1773 K (700 °C to 1500 °C) in an inert atmosphere. Macro and microstrengths of original and annealed carbonaceous materials were characterized by the tensile strength and fracture toughness. Fracture toughness was determined for inert maceral-derived component (IMDC) and reactive maceral-derived component (RMDC) using ultramicro indentation. Experimental data obtained by tensile tests were processed using the Weibull statistical method to find “inherent” strength. Tensile strength of chars and coals was significantly increased by annealing at temperatures ranging from 973 K to 1373 K (700 °C to 1100 °C); further increase in annealing temperature to 1773 K (1500 °C) increased their tensile strength only slightly. Tensile strength of cokes decreased with the increasing annealing temperature; the major effect was observed in the temperature range from 1573 K to 1773 K (1300 °C to 1500 °C). Fracture toughness of chars and coals was enhanced significantly by heat treatment at temperatures ranging from 973 K to 1373 K (700 °C to 1100 °C) as a result of pyrolysis, while that of cokes increased slightly by heat treatment. Fracture toughness of IMDC was higher than RMDC. Macrostrength of carbonaceous materials was strongly affected by their porosity and microstrength. The effect of pore geometry on macrostrength was marginal. Decreasing the porosity was more effective compared with increasing the microstrength in improving the macrostrength of carbonaceous materials.     

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten arc welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075 °C, 1121 °C, 1200 °C, and 1300 °C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075 °C and 1121 °C for 24 hours, and at 1200 °C and 1300 °C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a 〈001〉 orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a 〈101〉 orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.     

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten are welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075°C, 1121°C, 1200°C, and 1300°C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075°C and 1121°C for 24 hours, and at 1200°C and 1300°C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a <001> orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a <101> orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.     

The effect of solution annealing on the microstructural behavior of alloy 22 welds

Multipass gas tungsten are welds of alloy 22 were subjected to solution annealing durations of 20 minutes, 24 hours, 72 hours, and 1 week at temperatures of 1075°C, 1121°C, 1200°C, and 1300°C. The specimens were studied in cross section by secondary electron microscopy to determine the effect of solution annealing on tetrahedrally close-packed (TCP) precipitate stability. Electron backscatter diffraction mapping was also performed on all of the specimens to determine the recrystallization behavior of the welds. It was found that complete TCP precipitate dissolution occurs after solution annealing at 1075°C and 1121°C for 24 hours, and at 1200°C and 1300°C for durations of 20 minutes. Regions of most rapid recrystallization were correlated to the regions of lowest solute content, highest plastic strain, and highest residual stresses. Texture analysis indicated that while the columnar dendrites originally present in the weld grew with a <001> orientation in the transverse direction (approximately opposite the heat flow direction), the recrystallized grains adopt a <101> orientation in the transverse direction when recrystallization and TCP phase dissolution occur simultaneously.     

Thermal embrittlement of 18 Ni(350) maraging steel

The embrittlement of as-solutionized 18 Ni(350) Maraging steel was monitored as a function of heat treatment variables by means of Charpy impact tests. The processing parameters of interest were annealing temperatures in the range of 1900° to 2400°F, intermediate holding temperatures in the range of 1300° to 1800°F, and the quenching rate. The changes in fracture mode with heat treatment were characterized by replica and scanning electron microscopy. The severity of thermal embrittlement increases with decreasing cooling rate from the annealing treatment upon direct quenching to room temperature. Intermediate isothermal holding, particularly at 1500° to 1600°F, further accentuates the embrittlement. A large grain size is beneficial to the toughness when rapid direct quenches from the annealing range are imposed but is detrimental upon air cooling or intermediate holding. The major loss in toughness may be associated with the diffusion of interstitial impurity atoms (C+N) to the austenite grain boundaries during cooling or intermediate isothermal holding below 2000°F. An advanced stage of the embrittlement is characterized by the discrete precipitation of Ti(C,N) platelets on these boundaries. Thermal embrittlement is accompanied by change in fracture mode from transgranular dimpled rupture to intergranular quasi-cleavage.     

Degradation of mechanical properties after 100° to 250 °c storage of 25 μm Al-1 pct Si microelectronic interconnect wire

Degradation of mechanical properties of 25 μm Al-1 pct Si wire stored at 100° to 250 °C in an air atmosphere has been investigated utilizing mechanical, structural, and kinetic approaches. Forty pct of the strength of wires stored at 100 °C and 90 pct of the strength of wires stored at 250 °C disappeared within the first twenty-four (24) hours. Elongation measurements showed that the wire can be embrittled at temperatures as low as 200 °C within twenty-four (24) hours, and elongation can decrease to less than 1 pct within this time at 200 °C. Scanning electron micrographs of electro-polished wire revealed particles distributed throughout the wire which increase in size as a function of annealing time and temperature. A kinetic analysis showed that the particle coarsening was controlled by a 97 ±34 kJ/mole activation energy process. These observations are consistent with earlier findings^1,2,3 that silicon coarsens in aluminum with an activation energy of 118 ±8 kJ/mole. We therefore attribute the degradation of mechanical properties to the coarsening of silicon in aluminum wire. It has not been previously shown that this process proceeds during air storage and in the 100° to 250 °C temperature ranee.     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced «110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the «110» drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

The recrystallization of heavily-drawn

The recrystallization of 0.18 mm doped tungsten wire, swaged and drawn to a true strain of 7.7 at temperatures of <0.47T _m, was investigated by light microscopy and by transmission electron microscopy. The as-drawn structure of the wire consisted of greatly elongated, ribbon-shaped grains which had a pronounced ?110” fiber texture. These contained well-developed, elongated cells with few transverse boundaries. The onset of sub-boundary and grain boundary migration, together with the formation of potassium-containing bubbles in rows oriented parallel to the drawing direction, was observed after annealing at 800°C for 1 h. At higher temperatures, the spacing and the misorientation of the longitudinal subboundaries increased, and new transverse subboundaries were formed. Both subboundary and grain boundary migration were strongly inhibited by the bubble rows, as well as by the uniformity of the deformation and the well-developed texture. At 2100°C these mechanisms produced a fine-grained, partially recrystallized structure (1.2 μm average longitudinal boundary spacing) without change in the deformation texture. At 2150°C and above, large grains of high aspect ratio, which also retained the ?110? drawing texture, were formed by exaggerated grain growth. This process was initiated by a very small population of grains which had acquired the necessary size advantage during the growth of the fine-grained structure.     

Effect of carbon addition on the strength and creep resistance of FeAl alloys

We investigated the effect of carbon content (0.05, 0.12, and 0.2 wt pct C) and heat-treatment temperature (1100°C and 1300°C for 2 hours and air cooled) on the tensile and the creep properties of Fe-24 wt pct Al alloy. The increase of carbon content increased the yield strength without affecting the tensile ductility of the alloys. Carbon content appears to be beneficial in suppressing the hydrogen embrittlement at the grain boundary, because the fracture mode changes from predominantly intergranular failure in a low carbon (0.05 wt pct C) alloy to a predominantly transgranular cleavage failure in a high carbon (0.2 wt pct C) alloy. With the increase of carbon content, the anomalous yield strength peak shifted to a higher temperature possibly due to the interaction between carbon and vacanies. Significant improvements were noted in the tensile and the creep properties of medium (0.12 wt pct C) and high carbon (0.2 wt pct C) alloys after heat treating at 1300°C. The improvements in the tensile and the creep properties were attributed to the synergetic effect of retained vacancies and fine carbide precipitates present in the alloys after 1300°C heat treatment. However, the improved strength and creep properties associated with 1300 °C heat treatment were lost when the heat-treated alloys were further subjected to a vacancy removal annealing. Our results suggest that the retained vacancies present in the FeAl alloys after high-temperature heat treatment and air cooling are effective in improving the creep resistance at 700°C, and yield strength up to 800°C. The creep resistance of the present high carbon FeAl alloy is comparable to or better than several grades commercial heat-resistant Fe-based and Ni-based alloys. The work was carried out when the authors were with Chrysalis Technologies Inc., Richmond, VA. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Transverse creep and stress-rupture of borsic-aluminum composites and borsic-aluminum composites containing stainless steel and titanium

The transverse creep and stress rupture behavior of a number of Borsic®-aluminum composites was investigated at temperatures from 200° to 400°C. The cpmposites studied consisted of nominally 50 vol pct Borsic fiber and included matrices of 6061, 2024, 2219, and 5052 aluminum alloys. The effect of heat treatment was studied in the heat-treatable alloys. Where transverse composite behavior differed from matrix alloy behavior, the difference was found to be due primarily to a change in fracture mode at higher matrix strength levels from matrix failure to one which involves longitudinal fiber splitting. Of the four basic matrix alloys tested, the best creep resistance was obtained with the 2024 matrix. Additional improvement of transverse creep and stress rupture resistance was realized by incorporating transverse reinforcements such as SAP alloy foil, titanium alloy foil, and 0.002 in. stainless steel wire in the composites. These reinforcements made possible good transverse properties at 400°C with density increases of ≤15 pct. The two best additions were 21 pctβ III titanium foil and 6 pct AFC-77 stainless steel wire. A transverse fracture mode incorporating longitudinal fiber splitting was documented and characterized, and its effect on composite behavior determined. The use of nonsplitting fibers such as 5.6 mil B and 5.7 mil Borsic in preventing this fracture mode was investigated.     

The fatigue and fracture resistance of a Nb-Cr-Ti-Al alloy

The microstructure, fatigue, and fracture behaviors of a cast and heat-treated Nb-Cr-Ti-Al alloy were investigated. The microstructure of the cast alloy was manipulated by annealing at a temperature ranging from 500 °C to 1500 °C for 1 to 24 hours. The heat treatment produced Cr₂Nb precipitates along grain boundaries in all cases except in the 500 °C heat-treated material. Fracture toughness tests indicated low fracture resistance in both the as-cast and heat-treated materials. Fatigue crack growth tests performed on the 500 °C heat-treated material also indicated a low fatigue crack growth resistance. Direct observations of the near-tip region revealed a cleavage-dominated fracture process, in accordance with fractographic evidence. The fracture behavior of the Nb-Cr-Ti-Al alloy was compared to that of other Nb-Cr-Ti alloys. In addition, theoretical calculations of both the unstable stacking energy (USE) and Peierls-Nabarro (P-N) barrier energy are used to elucidate the role of Al additions in cleavage fracture of the Nb-Cr-Ti-Al alloy. The results indicate that an Al alloying addition increases the USE, which, in turn, prevents the emission of dislocations, promotes the nucleation and propagation of cleavage cracks from the crack tip, and leads to a reduction in the fracture toughness.     

The solubility of nitrogen in liquid manganese

The solubility of nitrogen in liquid manganese, in equilibrium with gas containing varying amounts of nitrogen, has been determined at 1300, 1400, and 1500°C. At one atm pressure of nitrogen, the solubility is 2.6 wt pct at 1300°C and decreases to 1.7 wt pct at 1500°C. Dissolution of nitrogen in manganese does not follow Sievert’s law. The activity coefficient of nitrogen in manganese increases with increasing nitrogen concentration. At one atm nitrogen pressure, the value of the activity coefficient, relative to the infinitely dilute solution, is 1.55 between 1300 and 1500°C.     

Grain boundary segregation of boron in INCONEL 718

The segregation behavior of boron at grain boundaries in two INCONEL 718⁺ based alloys with different B concentrations was studied. The alloys, one containing 11 ppm of B and the other 43 ppm, were homogenized at 1200 °C for 2 hours followed by water quenching and air cooling. A strong segregation of boron at grain boundaries was observed using secondary ion mass spectrometry after the heat treatment in both the alloys. The segregation was found mainly to be of nonequilibrium type. The homogenized samples were also annealed at 1050 °C for various lengths of time. During annealing, boride particles were observed to first form at grain boundaries and then to dissolve on continued annealing at 1050 °C. The mechanisms of segregation and desegregation of B are discussed.     

Microstructure,Mechanical and Abrasive Wear Behavior of 8.0 wt pct Cr White Iron Subjected to Continuous and Cyclic Annealing Treatment

Continuous annealing treatment (austenitization for 4 hours followed by furnace cooling) and cyclic annealing treatment (four cycles of austenitization, each of 0.66 hours duration followed by forced air cooling) of 8.0 wt pct Cr white iron samples are undertaken at 1173 K, 1223 K, 1273 K, 1323 K, and 1373 K (900 °C, 950 °C, 1000 °C, 1050 °C, and 1100 °C) as steps of destabilizing the as-cast structure. Continuous annealing results in precipitation of secondary carbides on a matrix containing mainly pearlite, while cyclic annealing treatment causes similar precipitation of secondary carbides on a matrix containing martensite plus retained austenite. On continuous annealing, the hardness falls below the as-cast value (HV 556), while after cyclic annealing treatment there is about 70 pct increase in hardness, i.e., up to HV 960. Decrease in hardness with increasing annealing temperature is quite common after both heat treatments. The as-cast notched impact toughness (4.0 J) is nearly doubled by increasing to 7.0 J after both continuous and cyclic annealing treatment at 1173 K and 1223 K (900 °C and 950 °C). Cyclic annealing treatment gives rise to a maximum notched impact toughness of 10.0 J at 1373 K (1100 °C). Abrasive wear resistance after continuous annealing treatment degrades exhibiting wear loss greater than that of the as-cast alloy. In contrast, samples with cyclic annealing treatment show reasonably good wear resistance, thereby superseding the wear performance of Ni-Hard IV.

     

The influence of heat treatment on the structure and properties of a near-α titanium alloy

The microstructure and tensile properties of a near-α titanium alloy, IMI-829 (Ti-6.1 wt pct Al-3.2 wt pct Zr-3.3 wt pct Sn-0.5 wt pct Mo-1 wt pct Nb-0.32 wt pct Si) have been studied after solutionizing (and no subsequent aging) at two different temperatures separately, one above the β transus (1050 °C) and another below the β transus (975 °C) followed by various cooling rates (furnace, air, oil, or water). While 1050 °C treatment resulted in coarse Widmanstätten structures on furnace or air cooling, fine Widmanstätten structure on oil quenching and martensitic structure on water quenching, 975 °C treatment produced duplex microstructures consisting of equiaxed alpha and partially transformed beta phases. Transmission electron microscopy studies revealed the morphology, size, and distribution of the α, β, and martensite phases and also the presence of small ellipsoidal suicide particles and an interface phase with fcc structure at almost all α-β interfaces. The oil quenched structure from 1050 °C has been found to be a mixture of fine Widmanstätten α coexisting with martensite laths and retained beta at the lath boundaries. Silicides with hcp structure of about 0.4 μm size were observed in specimens solution treated at 975 °C. The interface phase is seen in all slowly-cooled specimens. The YS and UTS are superior for 975 °C treatment compared to 1050 °C treatment after water quenching or oil quenching. The tensile ductility values are superior for any cooling rate after 975 °C solution treatment as compared to 1050 °C solution treatment. The specimens failed in tension diagonally by shear after 1050 °C treatment and by cup and cone fracture after 975 °C treatment. In all cases fracture has taken place by microvoid coalescence and in most cases, along the α-β boundaries.     

Hot Dip Galvanizing Simulation of Interstitial Free Steel by Liquid Zinc Spin Coater: Influence of Dew Point on Surface Chemistry and Wettability

In order to establish the relationship between surface chemistry and wettability as a function of dew point, an attempt has been made to simulate the hot‐dip galvanizing process with an ‘in‐house’ built Liquid Zinc Spin Coater. Interstitial free (IF) steel was annealed at 820°C in N₂‐5%H₂ gas atmospheres with dew points of ‐79°C, ‐29°C and 0°C, respectively. The wettability tests were conducted at 470°C at low dew point of ‐79°C. Surface analyses prior to wetting were carried out by using X‐ray photoelectron spectroscopy (XPS) and Field Emission Scanning Electron Microscopy (FE‐SEM). As expected, external oxidation of Al was observed only at the low dew point. With increasing dew point the oxidation of Cr and Si becomes internal. The formation of manganese silicates was observed at all dew points. While sulphur was detected on the specimen surface after all annealing conditions, the segregation of P starts to be significant at dew point 0°C by forming Mn‐phosphates. Despite the surface oxides, specimens annealed at all dew points are in the wetting regime by liquid zinc. Investigations on the steel/zinc interface of IF steel by using the liquid zinc spin coater were successful.