Full Heat Treatment Research Articles

Effect of Ti on Microstructural Evolution of Re containing Single Crystal Superallys

The influence of titanium on the microstructural evolution of the rhenium containing second generation single crystal superalloys has been investigated by changing titanium content only. The results showed that with the increase of titanium, the percentage of γ/γ’ eutectic increased, initial melting temperature lowered, and the size of γ’ phase of the alloys after full heat treatment increased. Three-dimensional atom probe (3DAP) analyses showed that the addition of titanium increased the rhenium distribution ratio in γ and γ’ phase as well as the misfit of γ and γ’ phase, and hence to promote the γ’ phase rafting and growing up. The experimental results also showed that the amount of TCP (μ) phase precipitates increased gradually with the increase of the content of titanium during prolonged aging at 1093°C.

Evolution of microstructure and mechanical properties of Ti modified superalloy Nimonic 80A

Nickel based superalloy Nimonic 80A modified with various Ti contents has been developed. Microstructure evolutions were investigated by optical microscope, X-ray diffraction, scanning electron microscope and transmission electron microscope in relation to the room temperature tensile properties and stress-rupture properties at 750 °C/310 MPa. After full heat treatment, the precipitate changed from γ′ phase to γ′ + η phases, the volume fraction of γ′ phase, lattice misfit of γ′/γ and room temperature tensile strength increased with the increase of Ti content. With the increase of lattice misfit, the orderly {1 1 1} atomic planes in the γ′/γ interface changed to slight distorted atomic planes and edge dislocations with Burgers vector of a/3 〈1 1 1〉 were identified. The stress-rupture life first increased and then decreased with the increase of Ti content, and the decrease was primarily due to the precipitate of intermetallic η phase at grain boundaries. After stress-rupture tests, the precipitate of Cr23C6 carbide exhibited an orientation relationship with the γ′ and γ phases, the spherical morphology of γ′ phase transformed to cubic shape for the alloy with longest stress-rupture life and the morphology transformation process of γ′ phase was schematically suggested.

Strengthening mechanisms of carbon in modified nickel-based superalloy Nimonic 80A

Nickel-based superalloy Nimonic 80A with various carbon contents has been developed. Various strengthening mechanisms of carbon were analyzed by optical microscope, X-ray diffraction, scanning electron microscope equipped with energy dispersive spectroscopy and transmission electron microscope. Detailed microstructural analysis revealed that with the increase of carbon content from 0.01% to 0.10%, both the lattice parameters of γ′ and γ phases decreased, and the misfit δ between the coherent γ′ and γ phases increased slightly from 0.289% to 0.317% after full heat treatment at 1070 °C/8 h, A.C. + 700 °C/16 h, A.C. The diameter of spherical γ′ phase decreased from 22.24 nm of 0.01%C alloy to less than 15.76 nm of 0.06%C and 0.10%C alloys. Cr 23C 6 carbide precipitated at the grain boundary in the alloys with carbon content higher than 0.06%, and when carbon content increased to 0.10%, Cr 23C 6 carbide had an orientation relationship with the γ matrix: [ 00 1 ¯ ] Cr 23C 6 [ 00 1 ¯ ] γ matrix and ( 100 ) C r 23 C 6 (1 0 0) γ matrix, which can enforce the grain boundary strength. The growth of room temperature tensile strength with the increase of carbon content was primarily due to the coherent strain strengthening and the precipitate strengthening of Cr 23C 6 carbide. After stress-rupture test at 750 °C/310 MPa, the misfit δ increased from 0.182% to 0.237% with the increase of carbon content, but the values were lower than those after full heat treatment. The precipitate of blocky Cr 23C 6 in 0.10%C alloy tended to become spherical and dislocation lines were found in the γ matrix channels between γ′ phases, the combination of the dislocation strengthening and precipitate strengthening of Cr 23C 6 carbide at grain boundary was beneficial to the stress-rupture properties.

Alnico thin films with high coercivities up to 6.9 kOe

The spinodal decomposition has been investigated in Alnico thin films prepared by sputtering. The as-made bcc thin films were heat treated both by the complex heat treatment of bulk Alnico magnets and by a simple annealing at different temperatures in the range of 600-900 °C. The simple heat treatment gave a coercivity of 2.6 kOe at 600°C and 6.9 kOe at 800°C. The maximum coercivity observed is approximately ten times larger than the bulk Alnico V value. Electron diffraction patterns can be mostly indexed to a new fcc phase with a=7.79 Å. The traditional Alnico heat treatment gave a similar coercivity (6.7 kOe after full heat treatment) and similar diffraction patterns. The coercivity was found to depend strongly on the film thickness. In samples with thickness above 100 nm, the coercivity declined dramatically and at 150 nm it was only 500 Oe. These findings suggest structural transformations in films which are drastically different from the spinodal decomposition observed in bulk Alnico.

Effect of Solidification Parameters on the Microstructure and Creep Property of a Single Crystal Ni-base Superalloy

A single crystal Ni-base superalloy was processed with withdrawal rates between 2 and 7 mm/min. The as-cast microstructures, heat treatment response and creep property have been characterized as a function of the withdrawal rate. As expected, the primary and secondary dendrite arm spacing decreased with increasing withdrawal rate; microsegregation degree and porosity distribution were also varied with different withdrawal rates. The withdrawal rate of 2 mm/min resulted in a noticeable residual microsegregation even after full heat treatment. The samples solidified at 7 mm/min exhibited a high density of cast porosities, and this led to a dramatical decline of the creep strain. 4 or 6 mm/min appeared to be the optimum withdrawal rate in the present study, which resulted in a uniform microstructure and an optimum density of cast porosity.

Effects of prior damage on the creep failure behaviour of similar and dissimilar welded CrMoV main steam pipes incorporating a partial repair

Creep damage finite element analyses, with the inclusion of “prior damage”, were performed for partially-repaired circumferential welds in a thick-walled, main steam, CrMoV pipe. The repair consists of aged parent material, weld metal and one HAZ region being partially excavated and replaced by new weld metal. The pipe welds were subjected to realistic internal pressure and uniform axial loading, the magnitude of the latter being up to that allowed by design codes. The material properties used are related to those of a CrMoV weldment at 640 °C. It is assumed that a full post-weld heat treatment has been carried out and that the effects of welding induced residual stresses reduce to negligible levels. The results obtained are used to examine the subsequent performance for “similar” and “dissimilar” welds with a range of “repair times” (defined as prior damage levels), magnitudes of axial (system) load, etc. From these results, the failure behaviour of this particular partial repair case was evaluated and discussed.

React-Wind-Sinter Processing of High Superconductor Fraction Bi$_2$Sr$_2$CaCu$_{2}$O$_{\rm x}$/AgMg Round Wire

Bi2Sr2CaCu2Ox (Bi2212) conductor technology has advanced significantly but the development of magnets is still hampered by difficulties associated with the partial-melt process (for wind&react magnets) and strain limitations (for react& wind magnets). To avoid these problems, the React-Wind-Sinter (RWS) approach has been proposed. Here we report on experiments that investigate three split processes that are based on the conventional partial-melt process within the RWS concept. The partial-melt process was interrupted at T1, T1 - 10degC and TS. After cooling to room temperature, the conductor is bent to a series of diameters (40 mm-100 mm), replicating magnet construction. The heat treatment process is then resumed on the bent samples from the split point and the heat treatment completed. The critical current is measured at 4.2 K in self-field using the four-probe method and the microstructure and phase composition of the Bi2212/AgMg wire are examined with scanning electron microscopy. For the split processes, the critical current after full heat treatment is as high as those from conventionally processed short samples, and in at least one case it is increased by 40% relative to conventional processing. These results show that a split process is a promising approach to improved Bi2212 conductors and magnets, and more broadly shows that conventional Bi2212 partial-melt processing is far from optimized.

Optimisation of the Strength of Aluminium Foam Sandwich (AFS) Panels by Different Heat Treatments

Aluminium foam sandwich panels (AFS) made of a low-density aluminium alloy AlSi6Cu6 foam core and two dense 6082 alloy face sheets were fabricated, after which the panels were subjected to two different heat treatments. First, the AFS panels were aged to increase their strength without further solution heat treatment and fast quenching, a process which resembles a T5 treatment. Second, to define a reference point the face sheets of AFS samples were cut off the foam and subjected to a full T6 treatment. Hardness profiles were measured across the thickness of the face sheets after the two different treatments and the microstructure was investigated. The main conclusion is that mechanical performance of AFS panels can be considerably increased by heat treatment without full solution heat treatment (T5), but without reaching the level of a full T6 treatment. The potential use of an easy to apply T5 treatment is an important cost reducing factor.

Application of Bismuth for Solidification Structure Refinement and Properties Enhancement in As-cast High-speed Steels

In order to exhibit good all-round performance the impact toughness enhancement of as-cast high-speed steels is obligatorily needed. In general, different methods are used commercially to achieve cast structure refinement and, as a consequence, their properties are improved. Introduction into the melt of inoculant particles or surface-active additions is among most beneficial. However, the effect of modifying additions in as-cast high-speed steels has been studied insufficiently. In fact, a restricted number of modifiers is used for structure and properties improvement in the as-cast high-speed steels compared to the common cast alloys. In the present work several kinds of alloys including tungsten-molybdenum high-speed steels of M2 and T30 types and low-alloy tungsten-free 1.1C-5Mo-1.7V high-speed steel were melted to investigate the effect of bismuth on their structures and properties.It has been found that additions of bismuth produce a very fine cast structure and affect the shape of the matrix grains and the morphology of the eutectic carbides as well as the redistribution of the main alloying elements of high-speed steels between solid solution and eutectic carbides. The microstructural changes, induced by bismuth during solidification, are explained by the surface activity of bismuth, which segregates to liquid/solid interface, significantly blocking dendrite growth in the direction along certain crystallographic planes. It has been shown that during eutectic solidification, carbides are also exposed to the barrier effect of bismuth being surrounded in the melt by this element. The main metallographic features of the modified cast structure alongside with the purifying effect produced by bismuth are retained after full heat treatment affecting the final mechanical properties of as-cast high-speed steels. As a result, bismuth significantly increases impact toughness and wear resistance of as-cast high-speed steels but decreases their red hardness.

Creep properties and failure assessment of new and fully repaired P91 pipe welds at 923 K

A creep testing programme was conducted at 923 K to identify the properties for new and repaired welds fabricated in a 9CrMoNbV grade P91 steel using a 9CrMoNbV weld metal. The material data obtained were assessed and used to determine the material creep and damage constitutive equations for the parent material (PM), weld metal (WM) and heat-affected zone (HAZ) materials in the P91 welds. Steady state creep and continuum damage finite element analyses were performed to assess the failure behaviour of the circumferential welds in the pipes, under two internal pressures and a range of end load levels. A full post-weld heat treatment is assumed to have been carried out, and any effects of welding-induced residual stresses were neglected. The results obtained have illustrated the main features of life reduction of a P91 pipe owing to the presence of welds, an additional life reduction due to repair and the more limited life reduction due to geometry difference, for the particular set of material properties used.

Tribological (wear) properties of aluminum–silicon eutectic base alloy under dry sliding condition

Low expansion, low density and high resistance to corrosion at ambient temperature make the aluminum–silicon alloys very suitable for wear resistance components in the automotive industry. In this study, wear behaviors of both the as-cast and heat-treated Al–Si alloy have been investigated. The cylindrical shaped wear testing specimens were prepared from cast ingot and some of the specimens were then heat-treated according to ASTM standard. Wear experiments on both samples were conducted with a pin-on-disk type wear testing apparatus. The test variables were the rotational speed, input weight, and time/sliding distance. The extent of wear damage was estimated by means of weight loss technique. The full heat-treatment has shown a great influence on the wear properties of the aluminum–silicon piston alloy. In general, the increase in wear with an increase in speed, input load and sliding distance was observed for both as-cast and heat-treated specimens.

Life prediction of repaired welds in a pressurised CrMoV pipe with incorporation of initial damage

Creep damage FE modelling was performed for fully and partially repaired, thick-walled, circumferential pipe weldments, in which initial damage was incorporated into the calculations to take account of the material degradation of the aged materials. The pipe welds were subjected to a realistic internal pressure and axial loading, the latter of which is allowed to vary within the range allowed by design codes. The material properties used are related to a CrMoV weldment at 640 °C. The initial damage distribution was numerically determined using an established procedure. A full post weld heat treatment is assumed to be carried out and the effects of welding induced residual stresses were neglected. The results obtained cover a number of initial damage levels, magnitudes of axial load, and repair excavation depth. On this basis, the sensitivities of the failure life of the repaired welds to these important factors can be evaluated. It was found that both the peak initial damage and the total life are very sensitive to the repair time, particularly when system load is high. The effect of the repair depth for depth: thickness ratios ≥0.5 is generally small for these loadings. There could be a significant benefit if the initial damage in the HAZ of the repair weld, which could be relatively high when the repair time is relatively large, could be reduced by repair welding or by post weld heat treatment.

Magnetic domain structure in SmCo 2 : 17 permanent magnets

The magnetic domain structure and properties of sintered Sm(Co/sub bal/Cu/sub 0.07/Fe/sub 0.22/Zr/sub 0.04/)/sub 7.4/ have been investigated, at different aging stages and applied fields. A low coercivity (0.2-2 kOe) developed during isothermal aging at 800/spl deg/C for 16 h, while a maximum coercivity of 31 kOe was obtained after subsequent slow cooling down to 400/spl deg/C (fully heat treatmented sample). Although the microstructural morphology remains unchanged for aging over 3 h, the observed magnetic domain structures are different. Flat or less wavy domain walls are seen in the magnets subjected only to the isothermal aging. The domain walls are wavy with a zigzag shape following the cell boundaries in the samples subjected to a full heat treatment. The evolution of domain structure with applied fields is consistent with the change of magnetization along the initial curve. This paper shows direct evidence of domain wall pinning in Sm/sub 2/Co/sub 17/-type magnets, where the degree of domain wall waviness is strongly related to their magnetic hardening.

Effects of Full Annealing Heat Treatment on Long-term Creep Strength of 2.25Cr-1Mo Steel Welded Joint

Long-term creep strength of Cr-Mo steel with fully annealed ferrite and pearlite microstructure is higher than those of the steels with martensite, tempered martensite and bainite microstructures. In this study, influence of post weld heat treatment at the temperature of austenite single phase region(full annealing PWHT) on long-term creep strength at 550°C has been investigated on 2.25Cr-1Mo steel, in comparison with that of normal post weld heat treatment at the temperature below A C1 (standard PWHT). Creep rupture strength of the welded joint subjected to standard PWHT is lower than those of base metal and weld metal in the short-term. However, such differences in creep rupture strength disappear after long-term creep exposure for about 50,000h at 550°C. On the other hand, creep strength of the welded joint subjected to full annealing PWHT is obviously higher than those of the welded joint subjected to standard PWHT, base metal and weld metal in the long-term condition. It may be also expected to reduce a probability of mechanical damage such as Type IV cracking by the full annealing PWHT, since inhomogeneous microstructures in weld metal and heat affected zone have been essentially extinguished. Full annealing at the temperature of austenite single phase region has been proposed as a new post weld heat treatment condition available to obtain high creep strength and to improve a reliability of welded joint for low alloy Cr-Mo steels.

Analytical Electron Microscopy of a Magnesium Alloy Containing Neodymium

Microstructure and phase composition studies of QE-22 magnesium based alloy with 2–3% Ag, 1.8–2.5% Nd and min. 0.4% Zr, before and after full heat treatment have been carried out using various electron microscopy techniques like EDX mapping, line scans and TEM with SAED. An extrapolation technique enabled to eliminate the influence of Mg-matrix on the quantitative analysis results and allowed to estimate the composition of the second phase which was mostly (Mg, Ag)12Nd type in the as cast alloy and (Mg, Ag)12Nd and Mg12Nd2Ag equilibrium precipitates after the precipitation process.

Effects of silicon addition and process conditions upon α-phase sintering, sinter hardening, and mechanical properties of Fe-1.5Mo powder

Alpha phase sintering, sinter hardening, and mechanical properties of prealloyed Fe-1·5Mo base powder with and without additions of elemental Si, ferrosilicon, and carbon under various process conditions have been investigated. Liquid paraffin, as a new lubricating agent, was found to be useful in reducing segregation, interparticle and die wall frictions, as well as reducing ejection forces and die and tool wear. It was found that addition of Si to the base powder enhanced the sintering process by stabilisation of the α-phase and formation of two kinds of liquid phase at ~1045 and ~1180°C, corresponding to the solidus and liquidus temperatures, respectively. This addition increased the tensile strength of the as sintered Fe-1·5Mo from 174 to 445MPa owing to massive solid solution strengthening effect of Si. An optimum sinter hardenable alloy, of composition Fe-1·5Mo + 3Si + 1·2C, provided a high sintered density of 7·55g cm-3, tensile and bend strengths of7 64 and 1405MPa, respectively, with 2·5% elongation, after sintering at 1250°C for 1h under hydrogen or vacuum using moderate cooling rates of ≤20K min-1. Faster cooling rates caused brittleness and very low UTS for the high carbon steel. Full heat treatment improved the UTS by ~200MPa which was useful only for the high carbon steel with high cooling rates ≥30K min-1. Depending on the cooling rate, the as sintered microstructures consisted of mainly fine or coarse pearlite, bainite, martensite, and some retained austenite with hardness in the range 250-720HV10. Some proeutectoid grain boundary cementites were also present in the as sintered high carbon steel. This work, therefore, has shown that high densities with acceptable microstructures and good mechanical properties are achievable with single stage compaction and single sintering operations by using the optimum process conditions and alloying composition without the need for a post-sintering heat treatment.

Corrosion resistance and properties of pump pistons coated with hard materials

Low expansion aluminium–silicon eutectic alloys are cast to produce most of the automotive pistons. The structure and properties of these alloys are very much dependent on the cooling rate, composition, modification and heat treatment operations. In this study, locally available automotive ‘scrap pistons’ were used as basic raw materials and a natural gas fired crucible furnace was used for melting purpose. The wear behaviour of both as-cast and heat treated specimens were studied under dry sliding conditions at room temperature using a pin-on-disc type wear testing apparatus. The extent of wear damage and the type of wear were investigated by means of weight loss measurement and optical microscopy techniques. The full heat treatment showed a great influence on the wear properties of the aluminium–silicon piston alloy as it reduced the wear rate of the specimens. The exceptional high tensile strength and hardness were attributed to the heat treatment condition with decrease in ductility. Significant changes in structure were also observed to occur specially in the primary and eutectic silicon phases. Some of the results of this study can be recommended for tribological use of this alloy in manufacturing automobile spare parts.

Study on wear properties of aluminium–silicon piston alloy

Low expansion aluminium–silicon eutectic alloys are cast to produce most of the automotive pistons. The structure and properties of these alloys are very much dependent on the cooling rate, composition, modification and heat treatment operations. In this study, locally available automotive ‘scrap pistons’ were used as basic raw materials and a natural gas fired crucible furnace was used for melting purpose. The wear behaviour of both as-cast and heat treated specimens were studied under dry sliding conditions at room temperature using a pin-on-disc type wear testing apparatus. The extent of wear damage and the type of wear were investigated by means of weight loss measurement and optical microscopy techniques. The full heat treatment showed a great influence on the wear properties of the aluminium–silicon piston alloy as it reduced the wear rate of the specimens. The exceptional high tensile strength and hardness were attributed to the heat treatment condition with decrease in ductility. Significant changes in structure were also observed to occur specially in the primary and eutectic silicon phases. Some of the results of this study can be recommended for tribological use of this alloy in manufacturing automobile spare parts.

Effect of process variables on structure and properties of aluminium–silicon piston alloy

The majority of automotive pistons are cast in one of the groups of low expansion Al–Si eutectic alloys. The structure and mechanical properties of these alloys are highly dependent upon cooling rate, composition, modification and the heat-treatment operations. The effect of these variables were investigated in this study, using locally available automotive `scrap pistons' as basic raw materials. Strontium master alloy was used as the source of the modifier and a natural-gas-fired crucible furnace was used for melting purposes. The microstructures of the alloy, both in the modified and unmodified conditions, were studied in this investigation. Properties such as the ultimate tensile strength (UTS), percentage elongation, hardness and percentage porosity for both rates of cooling have been determined, both with and without the addition of strontium. Significant changes in structures were observed to occur, especially in the primary and eutectic silicon phases. Full heat treatment has also shown a great influence on the structure and properties of the aluminium–silicon piston alloy. The residual stress in the modified alloy is fully relieved due to thermal treatment. Finally, attempts were made to correlate the observed structures and properties obtained during the experiment.

Metallurgical investigation of laser welds in wrought Waspaloy

Wrought Waspaloy was autogenously welded by means of Nd-YAG pulsed and CO2 continuous wave lasers and the microstructures of laser welds investigated using scanning and transmission electron microscopy. The fusion zones of as welded specimens contained fine γ′ and isolated MC carbides. The hardness profile of laser welds in wrought Waspaloy peaked at the fusion zone and heat affected zone (HAZ) boundaries, owing to fine γ′ precipitation and MC carbides in the fusion zone, and γ′ and M23C6 precipitates at the HAZ boundaries. After a full post-weld heat treatment, the hardness of the bead can be recovered to around 400 IIV but the hardness peaks at the HAZ boundaries became higher relative to the fusion zone and base material after aging at 450°C for 20 h. The microstructures in the fusion zone of post-weld heat treated welds consist of M23C6, MC, and γ′ which are quite uniform in size. The low cycle fatigue (LCF) strength of laser welds decreased to 400 MPa compared with 450 MPa for the base material. Fatigue cracks are initiated at the boundaries of the fusion zone and grow into the HAZ or along the boundary of the fusion zone. Fractography of LCF specimens showed ductile transgranular fracture.