Overaging Treatment Research Articles

Influence of high temperature exposure on the mechanical behavior and microstructure of 17-4 PH stainless steel

The mechanical behavior and microstructural evolution of 17-4 PH stainless steels in three conditions, i.e. unaged (Condition A), peak-aged (H900) and overaged (H1150), exposed at temperatures ranging from 200 to 700°C were investigated. The high-temperature yield strength of each condition decreased with an increase in temperature from 200 to 400°C except for Condition A at 400°C with a longer hold time where a precipitation-hardening effect occurred. At temperatures from 500–700°C, the decrease in after-exposure hardness of Condition A and H900 at longer exposure times was caused by a coarsening effect of copper-rich precipitates. A Similar microstructural change was also responsible for the hardness of H1150 exposed at 700°C decreasing with increasing exposure time. Scanning electron microscopy (SEM) observations indicated that the matrix structures of Condition A and H900, when exposed at 600°C and above, exhibited lamellar recrystallized α-ferrite in the tempered martensite and the size and quantity of these lamellar ferrite phases increased with exposure time. X-ray diffraction (XRD) analyses showed that the reverted austenite phase in H1150 that formed during the over-aging treatment was stable and hardly affected by deformation at temperatures of 200–400°C.

Effect of overaging and particle size on tensile deformation and fracture of particle-reinforced aluminum matrix composites

The effect of reinforcement particle size and overaging treatment on the tensile behavior and fracture morphology of a 2080/SiC/20p composite was investigated. Tensile behavior was profoundly influenced by particle size and matrix strength. The composite strength increased with a decrease in particle size, while overaging greatly reduced the strength of the composite, independent of particle size. Almost all particles on the fracture plane were fractured, and the amount of particle fracture in the composites was insensitive to overaging and particle size, due to the excellent bonding between SiC particles and the Al matrix. Fractography showed that void nucleation in the matrix of peak-aged composites took place primarily at very fine SiC particles, which were much smaller than the average SiC particle size. Subsequent failure took place by the tearing topography surface (TTS) mechanism. In the overaged composite, composites failed by a more conventional void nucleation and growth process, where void nucleation took place at coarsened S precipitate particles, resulting in smaller and more elongated voids.

Cyclic overaging heat treatment for ductility and weldability improvement of nickel based superalloys

A new cyclic overaging heat treatment for ductility and weldability improvement of advanced nickel based superalloys has been investigated. The heat treatment consists of intermittent heating cycles during the cooling stage to check the nucleation of fine γ' in the material during the overaging. Using Rene 80 alloy as a model material, the effects of intermittent heating rate and duration were studied and optimised. Quantitative metallography was carried out to compare the microstructures produced by the proposed heat treatment with those produced by other conventional pre-weld heat treatments. Tensile tests and welding tests both showed that the proposed cyclic over-aging heat treatment is effective in improving the tensile ductility and weldability of Rene 80 alloy.

Texture analysis of the transition from slip to grain boundary sliding in a discontinuously recrystallized superplastic aluminum alloy

Texture and microtexture measurements were correlated with mechanical property data for a superplastic 5083 aluminum alloy. Prior processing had included an overaging treatment followed by severe rolling deformation and the as-received material was annealed prior to mechanical testing. Discontinuous recrystallization by particle-stimulated nucleation during the annealing accounts for a predominantly random texture, although a weak {100}<0 vw> component was present, as well as a random grain boundary disorientation angle distribution. During elevated temperature deformation under dislocation-creep-controlled conditions, a distinct <111> fiber component and a relatively weak {100}<001> cube orientation, which are mutually compatible during uniaxial tensile extension, became apparent in the texture. Also, low-angle boundaries became evident in the disorientation distribution. In contrast, the random texture component and the randomness of the disorientation distribution became more evident when the material was deformed under conditions of grain boundary sliding control of deformation. A transition from dislocation creep to grain boundary sliding observed in the microtexture measurements of this work may be predicted by treating constitutive equations for dislocation creep and grain boundary sliding in an additive manner.

Correlation of the Aging Characteristics and Deformation Behavior of A357 Alloy

The mechanical properties, microstructures, and deformation behavior for a cast Al-7Si-Mg casting alloy A357 under various aging conditions have been investigated in the present paper. It is shown that the combination of strength properties and elongation varies with aging parameters. Aging at 165 °C gives the optimum balance of strength and ductility. Transmission electron microscopy (TEM) observation shows that, in the underaged conditions in which Guinier-Preston (GP) zones are formed, the tensile deformation involves dislocations, which cut through the GP zones. Planar slip bands can be seen in the deformed substructures, and the slip bands coarsen with increasing aging temperature or time. In the overaged conditions in which β phases precipitate, the dislocations bypass precipitates and distribute uniformly, except some dislocation tangles around coarser precipitates. When the interaction mode between dislocations and particles changes from shearing to bypassing, the deformation homogeneity greatly increases for the castings with fine β precipitates, which may explain the significant secondary elongation behavior. Based on these studies, a step overaging treatment has been used to improve the overall tensile properties.

Influence of Overaging Treatment on Localized Corrosion of Al 6056

Abstract The influence of T78 overaging treatment on the corrosion behavior of Al 6056 (UNS A96056) in 1 M sodium chloride (NaCl) solution was investigated. The overaged alloy presented the same localized corrosion mechanisms as Al 6056-T6. However, Al 6056-T78 coarse intermetallic Al-Mg-Si-containing particles were found to be more reactive than those in the Al 6056-T6 alloy and were nucleation sites for pits. Pitting and intergranular corrosion were dependent upon each other; intergranular corrosion nucleated on pit walls. A quantitative study showed that T78 overaging of Al 6056 offered a better resistance to intergranular corrosion compared to T6 peak-aging treatment. The overaged alloy will be able to replace the traditional Al 2024 (UNS A92024) because of the efficiency of this overaging treatment

Corrosion fatigue behavior of 7050 aluminum alloys in different tempers

Corrosion fatigue (CF) experiments, including both high-cycle axial fatigue ( S– N curve) and fatigue crack growth (FCG), have been performed on 7050 aluminum alloys in a 3.5 wt% NaCl solution as a function of aging treatment. The results of these environmental tests were compared with those obtained in laboratory air to characterize the effect of aging treatment on CF susceptibility. Fatigue resistance in both peak aged (T6) and overaged (T73) tempers was dramatically reduced by the aqueous chloride environment. The FCG rates for T73 condition were lower than the counterparts for T6 condition in both air and saline solution. 7050-T73 alloy exhibited longer fatigue lives in air but shorter ones in the corrosive environment as compared to the T6 temper. This may be attributed to the formation of more extensive and larger corrosion pits acting as crack nuclei to facilitate crack initiation, in the T73 tempered condition. Comparison of CF and stress corrosion cracking (SCC) results reveals that overaging treatments used to improve grain boundary characteristics and increase the intergranular SCC resistance might not guarantee an equivalent improvement in the resistance to transgranular CF cracking.

Effects of Heat Treatment and Reinforcement Size on Reinforcement Fracture during Tension Testing of a SiCp Discontinuously Reinforced Aluminum Alloy

The effects of heat-treatment, matrix microstructure, and reinforcement size on the evolution of damage, in the form of SiCp cracking, during uniaxial tension testing of an aluminum-alloy based composite have been determined. A powder metallurgy Al-Zn-Mg-Cu alloy reinforced with 15 vol pct of either 5 or 13 μm average size SiCp was heat treated to solution annealed (SA), underaged (UA), and overaged (OA) conditions. The SA treatment exhibited lower yield strength and higher ductility for both particulate sizes compared to the UA and OA conditions. The evolution of damage, in the form of SiCp fracture, was monitored quantitatively using metallography and changes in modulus on sequentially strained specimens. It is shown that the evolution of SiCp fracture is very dependent on particulate size, matrix aging condition, and the details of the matrix-reinforcement interfacial regions. SiCp fracture was exhibited by the UA and OA treatment over a range of strains, while a preference for failure near the SiCp/matrix interfaces and in the matrix was exhibited in the OA material. While thepercentage of cracked SiCp at each global strain typically was equal or somewhat lower in the material reinforced with 5 μm average size SiCp, theabsolute number of cracked SiCp was always higher at each global stress and strain in the material containing 5 μm average size SiCp, for each heat treatment. Damage(e.g., voids) in the matrix and near the SiCp/matrix interfaces was additionally observed, although its extent was highly matrix and particle-size dependent. It was always observed that increases in stress (and strain) produced a larger amount of fractured SiCp. However, neither a global stress-based nor a global strain-based model was sufficient in converging the amount of SiCp fractured for all heat treatments and particle sizes tested.

ＲＲＡ処理した７０５０アルミニウム合金の応力腐食割れ

Strength and SCC resistance of RRA-treated 7050 aluminum alloy were investigated, compared with the alloy conven-tional-treated. Over-aging treatments such as T76 and T73 were effective for the improvement of SCC resistance, but a certain loss in strength was unavoidable. The RRA-treatment for 7050 alloy improved the SCC resistance up to the level compared to that of T73. Far from decreasing, the strength of RRA-treated alloy increased slightly, compared with T6. The strength obtained in T6 treatment and the SCC resistance in T73, therefore, can completely coexist in the case of RRA-treated alloy. This special effectiveness for the 7050 alloy was considered to result from homogeneity in the precipitation structure of 7050 alloy, probably induced by the zirconium addition in 7050 alloy.

Effect of cooling rate in overageing and other thermomechanical process parameters on grain refinement in an AA7475 aluminium alloy

Fine-grained AA7475 aluminium alloy sheets were produced in this study by a thermomechanical treatment involving solution anneal, overageing, rolling and recrystallization steps. It has been found that the cooling rate after the intermediate overageing treatment should be fast to obtain the finest grain size. The fast cooling rate ensured the presence of relatively large particles of MgZn2 and some supersaturation prior to cold rolling. Generally, the final grain structure was heterogeneous, with bands of fine grains lying parallel to the rolling direction. In material rapidly cooled after overageing, bands of fine grains were also observed in the transverse direction and these bands were associated with shear bands formed during rolling. The fine-grained AA7475 alloy sheets with an average grain size of about 9 μm showed large tensile elongations of about 800% when deformed at 516 °C and with an initial strain rate of 5×10−4s−1.

Determination of nitrogen and carbon interstitially dissolved in continuously‐annealed steel sheets by means of a fully automatic torsion pendulum

A fully automatic torsion pendulum of the Collette type was built to test continuously‐annealed steels in the thickness range between 0.15 and 0.4 mm for their contents of dissolved nitrogen and carbon. The influence of composition and production conditions on unalloyed steels which had been annealed commercially and in the laboratory were investigated. It was found that, with an increase in the total carbon and aluminium contents, the percentages of the dissolved nitrogen and carbon decrease, whereas higher nitrogen contents resulted in an increase of the percentages dissolved. A higher coiling temperature of the hot band leads to a decrease in the nitrogen dissolved due to the formation of AIN, but to an increase of the carbon dissolved. As these effects almost compensate each other, the coiling temperature is not a suitable means to influence the aging potential. During the continuous annealing process, temperatures of more than 750°C result in a decrease of the nitrogen dissolved, which is completely precipitated as AIN leaving the carbon content nearly unaffected. No major differences were observed, when the material was annealed either with or without a subsequent overaging treatment. However, rapid cooling to low temperatures definitely increased the percentages of elements dissolved.

Surface Characteristics of Continuously Annealed Steel Sheet in Overaged Condition

The overaging treatment of a continuous annealing line was simulated in the laboratory. The chemical state of the surface of low carbon steel sheet was determined by using ESCA and SEM. The surfaces of a conventional, low alloyed deep drawing quality (DDQ) steel and a dual phase (DP) steel alloyed with Mn and Si were characterised in simulated and production line conditions. The influence of the surface state of the steels on the formation of surface phosphates was investigated. During overaging in the temperature range 200–400° C in a protective H2 + N2 atmosphere iron oxide is reduced to metallic iron. The alloying elements manganese and silicon are oxidised. Silicon especially is markedly enriched on the surface, in the form of pure Si02, due to its high diffusibility. The manganese and silicon oxides form particles on the surface during the overaging treatment. The results indicate that iron oxide is dissolved in the particles. Sodium and calcium are also present in their oxide states on the surfaces of the production line sheets; they originate in the rinsing waters used. Temper rolling reduces the amount of surface oxide particles to about half the prerolling value. It is suggested that this is due to the stripping of particles from the surface and the pressing of particles into the steel matrix. The phosphating experiments show that the steel sheets treated in the simulation furnace and taken from the production line form surface phosphates readily. The phosphate crystals are small, uniform, and cover the entire surface.

Microstructure and mechanical properties of a cobalt-free tungsten-bearing maraging steel

A new high strength, high toughness tungsten-bearing maraging steel without cobalt and molybdenum has been developed. The composition of the new maraging steel (W-250) is Fe-19wt.%Ni-4.5wt.%W-1.2wt.%Ti-0.1wt%Al. The alloy is strengthened by orthorhombic Ni 3W and hexagonal η-Ni 3Ti precipitates in a low carbon b.c.c. lath martensitic matrix. The alloy exhibited a yield strength of 1780 MPa (257 klbf in −2) and a Charpy impact energy of 25 J at room temperature, which were the same as obtained for conventional 18 wt.% Ni (grade 250) maraging steel. The overaging treatment (at 650°C for 3 h, followed by air cooling) produced a volume fraction of retained austenite of about 13% in martensitic matrix. The yield strength of the overaged condition was lower than that of the solution-treated condition. The retained austenite formed during the overaging treatment did not offer any advantage in impact energy over the fully martensitic structure of the solution-treated condition.

７０５０系合金押出材の破壊じん性に及ぼす鉄，シリコンの影響

Effect of (Fe+Si) content on the fracture toughness of extruded 7050 alloys was investigated. A reducing of (Fe+Si) content increased the macroscopic fracture toughness of the alloy. An over-aging treatment also did it. These effects may be appreciated by a good interrelation between (Fe+Si) content and inclusions acting as crack sites. However, reducing (Fe+Si) content tended to lead to formation of localized brittle fracture under a lower stress, especially for T651 materials. Facility in forming localized brittle fracture was related to development of a double fiber texture with and , and the higher development, the more facility. Accordingly, in extrusions such as of 7050 alloy, it may be worthwhile to examine microscopically the details of fracture features as well as the macroscopic fracture toughness. A fractographical examination and an AE monitoring suited for this purpose.

７０５０合金押出材の応力腐食割れに及ぼす合金純度の影響

Stress corrosion resistance of 7050 extrusions with four levels of purity was investigated in the extruding direction. In the case of T651, the most superior resistance was found in the alloy with 0.24% (Fe+Si), in this experiment, and increasing or decreasing purity resulted in a lowering of resistance. Marked improvement of resistance was obtained by the over-aging treatment (T751) for each alloys, although a similar purity effect was also detected. A lowering of resistance with decreasing purity seems to be due to the increase of amounts of inclusions acting as crack sites. On the other hand, a reducing of resistance with increasing purity appears to be owing to development of the double fiber texture with and which can cause localized brittle fracture at a lower load. To select the optimum purity and/or to avoid severe development of the double fiber texture are preferable to improve resistance of extrusions in the extruding direction.

Ａｌ‐Ｌｉ系合金の靭性に及ぼす中間加工熱処理の影響

RI-ITMT method was applied to improve the toughness of Al-Li system alloy. This method was found to be effective for the improvement of the toughness of Al-Li system alloy. The significantly finer grains were obtained by RI-ITMT method. However, the greatest toughness was not necessarily obtained in the finest grain size. The improvement in toughness was achieved in the specimens in which the second phase particles precipitated during overaging treatment in thermomechanical treatment process had remained after the final solid solution and aging treatment. Therefore, the improvement in toughness was achieved by preventing the stress concentration to the grain boundary, caused by the large planner slip, by the introduction of the second phase particles in addition to grain refinement. In addition to the Al3Zr precipitate, Cu-rich precipitate also formed duplex phase with δ' precipitate. The heat treatment in Ar atmosphere was more effective in preventing the phenomenon of Li loss. However, it was impossible to prevent Li loss completely. Under the dynamic loading condition toughness increased due to the shear-lip associated with Li lost layer.

７０７５系アルミニウム合金の応力腐食割れ

The stress corrosion resistance of commercial 7075, 7175 and 7475 alloys plates 60mm in thickness was investigated. The resistance of these specimens strongly depends on the orientation of specimen, T-L specimens have the resistance highly superior to that of S-L specimens. The overaging treatment (T7) is extremely effective for improvement of the resistance of these alloys. The residual stress removing treatment (TX51) has a considerable effect to improve the resistance of S-L specimens, but has little effect to do the resistance of T-L specimens. S-L specimens show intergranular stress corrosion cracking, while T-L specimens show ductile fracture with many laminated cracks on intergranular stress corrosion cracks. Stress corrosion cracks are attended by a hydrogen embrittled zone pre-existing ahead of the crack. Nucleation and propagation of the crack take place under an assistance of hydrogen embrittlement.

Effects of Prior Cold Rolling and Post Temper Rolling on the Properties of Continuously Annealed Low Carbon Dual Phase Steel

The effects of two processing variables, prior cold reduction and temper rolling, on the mechanical properties of a continuously annealed 0.05 pct C low alloy dual phase steel were studied. It was found that cold rolling affected the strength of the dual phase steel through its influence on the recrystallized ferrite grain size. The correlation between yield strength and mean free path in ferrite was shown to follow a Hall-Petch type relationship. Similarly, temper rolling also had a pronounced effect on the yield strength of the steel. The effect was strongly dependent upon the previous overaging treatment; the lower the overaging temperature, the stronger the effect. Basically, the effect of temper rolling on dual phase steel was not much different from that on mild steel and HSLA in terms of the change in yield strength and yield point elongation. Another interesting observation was the influence of quenching rate on the strength of the dual phase steel. A slower quenching rate in the heavier gauge dual phase steel caused pearlite rims to form around martensite colonies and thus decreased the strength of the steel.

Heat transfer analysis of roller quench system in continuous annealing line.

The Water-Quench System developed by NKK has a advantage of producing high strength cold rolled steel sheet with low cost.However, when it is used for the production of drawing quality (DQ) cold rolled steel sheet, the process can not prevent the excess cooling. Therefore, additional energy cost is necessary to raise the strip temperature to the temperature required for the overaging treatment. To overcome this disadvantage, NKK has newly developed the Roll Quench (RQ) process which does not produce this reheating energy cost.The RQ process consists of cooling the steel strip by contact with internally water-cooled rolls. For the development of RQ process, various basic heat transfer data were requested for the design of system. Contact thermal conductance between the surface of strip and roll were investigated. Thermal behaviors of roll shell were analyzed through theoretical simulation. Mathematical model of the RQ process was established, which is useful for both the design and operation of RQ process.

Processing of phosphorus-alloyed lovv-carbon steel sheet by continuous annealing

Low-carbon steel containing 0·07 wt-% phosphorus has been heat treated in the laboratory in a manner designed to simulate an industrial continuous-annealing process. The resulting structures have been examined by optical metallography and X-ray diffraction, while mechanical properties were determined from tensile tests. Process variables investigated were: (i) hot-band ‘coiling’ temperature, (ii) cold-rolling reduction, (iii) time and temperature of annealing, and (iv) the quenching temperature of the sheet after annealing. A standard overaging treatment was given in all cases. Strength levels of the steel sheets were rather insensitive to processing treatments, but increased somewhat with lower coiling and annealing temperatures as a result of grain refinement. Both ductility and plastic anisotropy were favoured by a higher coiling temperature, and an optimum combination of properties was foundfor the 67% cold-rolled sheet annealed for 80 s at 750°C. With heavier cold reductions, the optimum annealing temperature was ∼800°C. The quenching temperature has a significant effect on ductility, because it controls the dispersion of carbide particles and the amount of carbon remaining in solid solution in the matrix. A good combination of properties could be achieved by appropriate control of these processing variables.