Solution-annealed Condition Research Articles

Multimodal size distributions of γ′ precipitates during continuous cooling of UDIMET 720 Li

The technological properties of many nickel-based superalloys originate in the morphology and distribution of Ni 3(Al, Ti) particles (γ′ precipitates). Starting from the solution-annealed condition, the distribution and the morphology of γ′ precipitates are investigated experimentally during continuous cooling of the nickel-base superalloy UDIMET 720 Li. Characterization of the precipitates is carried out by scanning electron microscopy and energy-filtered transmission electron microscopy investigations. Depending on cooling rate, monomodal, bimodal and even trimodal size distributions are observed. The experimental observations of the size distributions are confirmed by numerical simulations of the γ′ precipitation kinetics. The theoretical background for occurrence of multimodal size distributions during continuous cooling is discussed.

Characterisation of microstructure and its effect on the strength and toughness of 17-4PH stainless steel

The influence of microstructure on the strength and toughness of 17-4 Precipitation-Hardened (PH) Stainless Steel (SS) was studied as a function of duration of ageing at 783 K. Lath martensite is formed in this steel in its solution-annealed condition. X-ray diffraction studies detected the formation of a copper-rich phase after ageing for 0.25 h, with the amount of the copper-rich precipitates increasing with the duration of ageing. The coarsening of the copper-rich precipitates on ageing for 2 h led to a sharp decrease in hardness, which was attributable to the transformation of the coherent copper-rich body centred cubic (b.c.c.) phase to the incoherent copper rich face centred cubic (f.c.c.) phase. Further ageing for 4 h led to a dip in hardness and strength and an increase in toughness. The dip in hardness upon long-term ageing could be attributed to the formation of coarse copper-rich precipitates, which also led to an improvement in the toughness of the material.

Investigation on precipitation phenomena of Ni–22Cr–12Co–9Mo alloy aged and crept at high temperature

The Ni–22Cr–12Co–9Mo alloy (UNS alloy N06617, also known as alloy 617), of relatively common use at high temperature, is also one of the candidate materials for the most ambitious EU and US projects for future generations of power plants. Its microstructure in the solution-annealed condition and after long-term creep tests at 700 and 800 °C was characterized by means of light optical, scanning electron and transmission electron microscopy. Differential scanning calorimetry (DSC) analyses were also performed to check the usefulness of this technique to detect microstructural modification in specimens exposed to high temperature. Microstructural features were correlated to the hardness evolution in a chronological range of several thousand hours. The major microstructural features were identified to be the presence of inter- and/or intragranular particles ((Cr,Mo,Fe) 23C 6 and other types of carbides, δ-Ni 3Mo and γ′-Ni 3(Al,Ti)) that clearly played an important role in the mechanical behaviour of the alloy. The microstructural features of the investigated alloy changed at different exposure temperatures (and times) with respect to the as-received condition. The observed microstructural instability of the alloy suggested that creep data obtained at the highest temperature should be very carefully considered in extrapolating the creep strength of the alloy at about 700 °C, since they should lead to a substantial underestimation of the creep life.

Corrosion of 2205 duplex stainless steel in chloride medium containing sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) play significant role in the corrosion of stainless steels exposed to marine and soil environment. Sulfate reduction by bacterial species results in the production of H 2S, which can significantly influence the anodic and cathodic processes and ultimately enhances the corrosion of materials. In the present study, 2205 type duplex stainless steel (DSS) coupons in solution-annealed condition were exposed to chloride medium containing SRB species, Desulfovibrio desulfuricans, for 40 days and examined by microscopy. Etching of the duplex structure, pitting as well as crevice attack were noticed. Similar results were obtained by microscopic studies with coupons exposed for 14 days in medium containing SRB followed by anodic polarization. SEM studies, of crevices observed after anodic polarization, indicated that the attack was initiated at the grain boundaries and slowly encroached into austenite grains. The initiation of attack was also evident from AFM studies of coupon exposed under freely corroding conditions for 7 days. ESCA studies reveal that under anaerobic conditions of SRB growth sulfidation of passive film occurs. The modified passive film found to depolarize the cathodic reactions. The nature and mechanism of SRB attack on DSS and possibility of detection at early stages were discussed.

Cracking Susceptibility of Alternative Composite Tube Alloys in a Molten Sodium Sulfide Hydrate-Sodium Hydroxide Salt Mixture

Abstract The cracking of composite tube systems based on Type 304L stainless steel (SS) (UNS S30403) in the lower-furnace section of kraft recovery boilers continues to be a serious problem. One remedial approach used by pulp mills involves replacing the conventional system based on Type 304L SS with alternative composite tube systems based on Alloys 825 (UNS N08825) and 625 (UNS N06625). U-bend stress corrosion cracking (SCC) tests were conducted on these alloys in a sodium sulfide hydrate-sodium hydroxide (Na2S·xH2ONaOH) molten salt mixture at 180°C (simulated wash-water environment) to help rationalize the observed service performance and to help assist with the selection of the more resistant composite tube system. Attention was given to studying relative SCC resistance of those three alloys in relevant metallurgical conditions, which include mill-annealed, sensitized, cold-worked, and solution-annealed conditions. It was concluded that Type 304L SS is susceptible to cracking even with an ideal micros...

Mechanical and Physical Properties of Titanium-12Molybdenum-6Zirconium-2Iron Beta Titanium Alloy

Abstract Howmedica developed the Titanium-12Molybdenum-6Zirconium-2Iron beta titanium alloy (also known as TMZF® beta titanium alloy) in the late 1980s. Two patents were issued for this high strength, low modulus, ductile, biocompatible, titanium base alloy [1,2]. The mechanical properties for wrought machining and forging bar in ASTM F 1813 show higher tensile and yield strengths than Ti-6Al-4V ELI (ASTM F 136), the most widely used titanium grade for medical and surgical implant applications. According to the ASTM standard, the TMZF alloy also has improved ductility over Ti-6Al-4V ELI, which is to be expected when comparing beta and alpha+beta titanium alloys. TMZF alloy offers higher tensile strength and greater flexibility than Ti-6Al-4V alloy. Data are presented for semi-finished TMZF mill product forms in the solution-annealed condition. The influence of oxygen as an interstitial strengthening element is shown for Ti-12Mo-6Zr-2Fe and compared with common titanium alloys used in or being considered for use in the medical device industry.

Development of an electrochemical reactivation test procedure for detecting microstructural heterogeneity in Ni-Cr-Mo-W alloy welds

Alloy-22 (UNS N06022), a Ni-Cr-Mo-W based alloy shows excellent corrosion resistance in both oxidizing and reducing environments and is a candidate material for fabrication of outer wall of nuclear waste containers [1–3]. This alloy is considered, as readily weldable [3–6]. In solution-annealed condition, wrought Alloy22 reveals a single-phase austenite structure without any grain boundary secondary phase precipitates [1, 2]. Topologically close packed (TCP) phases such as μ, σ , and P, which are stable at higher temperatures, were observed to be retained at room temperature as terminal solidification constituents during solidification of weld metal [6]. As the chemical composition of these TCP phases is rich in Mo and W, presence of these phases could result in localized depletion of alloying elements in the matrix and make the alloy more susceptible to corrosive attack. Moreover, the weld microstructure revealed segregation of Mo and W at the interdendritic regions [3, 4]. Multipass welding and post weld heat treatment could increase the quantity of secondary phase precipitation. As the corrosion and toughness properties are affected by the presence of TCP phases and segregation of alloying elements [3], it is important to quantify these microstructural changes in comparison with that of solution annealed condition. A rapid nondestructive testing technique such as electrochemical potentiokinetic reactivation (EPR) test could be an appropriate tool for such microstructural evaluation, especially in-situ service conditions. EPR test has been successfully applied for quantifying the degree of sensitization in austenitic and duplex stainless steels [7, 8]. There is no standard EPR test procedure available for detecting intergranular corrosion susceptibility of NiCr-Mo alloys. Recently the present authors reported a test procedure to detect Cr depletion in thermally aged Alloy-22 wrought materials using an electrolyte of 1 M H2SO4 + 0.5 M NaCl + 0.01 M KSCN [9]. However, this test solution could not reactivate the weld metal, as the Cr profile was not affected by the solidification structure and the TCP phases [4]. In this short communication, development of a testing procedure, which could detect the presence of Mo and W depleted regions in Alloy-22 welds, is reported. 38 mm thick Alloy-22 (UNS N06022) plates (chemical composition 0.002% C, 0.14% Mn, 0.009% P, 0.001% S, 0.04% Si, 59.2% Ni, 20.62% Cr, 13.91% Mo, 0.01% Co, 0.01% Cu, 2.68% W, 2.8% Fe and

Nanograin size, high-nitrogen austenitic stainless steels

Abstract A new class of high-strength steels has been developed by combining solid solution hardening and grain boundary hardening in high-nitrogen austenitic stainless steels. Average grain diameters from 50 to 250 nm can be achieved, and yield strength levels of more than 2000 MPa in the solution-annealed condition have been measured. The effect of nitrogen solid solution hardening and grain boundary hardening is systematically explored.

The effect of solution heat treatment on a single-crystal Ni-based superalloy

Understanding of the solution heat treatment response of a second generation, single-crystal nickel-based superalloy, CMSX-4, is necessary before the process can be shortened in an effort to save money. The current solution heat treatment used for CMSX-4 involves high temperatures for long durations and can be quite expensive. This investigation helps to characterize the heat-treatment process for this alloy. The achieved microstructure is described for both the as-cast and fully solution-annealed conditions. In addition, an alternate, less expensive heat treatment was selected for its lower temperatures and shorter overall duration. Microstructures and differential thermal analysis are presented for all three conditions.

Corrosion behavior of austenitic alloy 690 under anodic and cathodic potentials

The corrosion behavior of austenitic alloy 690 in a solution-annealed condition has been evaluated with the application of anodic as well as cathodic potentials in an acidic chloride solution at room temperature (RT). In a 0.5M H2SO4 + 0.5M NaCl solution, the alloy displayed active-passive pitting behavior with the application of an anodic potential. Surface films, formed at the onset and later stage of the passive region, were characterized using X-ray photoelectron spectroscopy (XPS). The XPS revealed that the surface film formed at the onset of passivity (+ 100 mV SCE) consisted of Cr(OH)3, without any Fe+3/Fe+2. The presence of nickel in the film was found in a transition state of Ni+2 and Ni0. The passive film formed at the higher anodic potential (+ 700 mV SCE) consisted of Cr2O3 without any Fe+3/Fe+2 or even Ni+2/Ni0. Microscopic studies of alloy 690 after anodic polarization in an acidic chloride solution revealed pitting, which was found to be initiated at large, faceted TiN-type inclusions. The susceptibility of the alloy to hydrogen embrittlement has been investigated by conducting cathodic charging of the tensile samples in a 0.5M H2SO4 solution at RT and by subsequent tensile testing of the charged samples in air at a strain rate of 1.3 × 10−4 s−1 up to fracture. An indication toward hydrogen-induced ductility loss was noticed for the samples of the alloy, which is believed to be attributable to a hydrogen-enhanced microvoid growth process. Since the microvoid growth process occurs at the last stage of fracture, the effect of hydrogen on the ductility of the alloy is little.

Substructural recovery in a cold worked Ti-modified austenitic stainless steel during high temperature low cycle fatigue

Total axial strain controlled fatigue tests have been conducted in air at 823 and 923 K to ascertain the influence of cold work on LCF behaviour of a 15Cr–15Ni, Ti modified austenitic stainless steel, designated as Alloy D9. The LCF behaviour of the 20% cold worked alloy was compared with that of solution annealed alloy. A symmetrical triangular waveform at a constant strain rate of 3×10 −3 s −1 was employed for all the tests performed over the strain amplitude in the range of ±0.25 to ±1.00%. The cyclic stress response varied as a complex function of microstructure, temperature and strain amplitude. The cyclic stress response of solution-annealed alloy was generally characterized by initial hardening to the maximum stress followed by a regime of nearly stable peak stresses. The degree of initial hardening was higher at 823 K due to dynamic strain ageing effects. The cold worked alloy displayed a gradual softening prior to the attainment of saturation stress response stage. The plastic strain fatigue resistance of the cold worked alloy was inferior compared to that in solution-annealed condition at both 823 and 923 K. At low strain amplitudes, the cold worked alloy exhibited better total strain fatigue resistance at 923 K. The observed variations in fatigue life and cyclic stress response behaviour have been explained on the basis of crack initiation, development of substructure, precipitation behaviour and evolving changes in plastic strain during cycling.

Grain-boundary chemistry and intergranular corrosion in alloy 825

Alloy 825, a former candidate material for radioactive high-level waste containers, was investigated to assess its thermal stability and the time-temperature conditions for sensitization. Alloy specimens with a carbon content of 0.01 wt pct in the mill-annealed (MA) and solution-annealed (SA) conditions were studied after thermal exposure to temperatures ranging from 600 °C to 800 °C for periods of up to 1000 hours. Sensitization was evaluated by using corrosion tests that were correlated to grainboundary chemistry analyses. Sensitized microstructures were found to contain M23C6-type carbides and a chromium-depleted region in the vicinity of the grain boundaries. Thermal aging at 700 °C for 100 hours resulted in the highest sensitization. While heat treatment at 640 °C showed a progressive development of sensitization with time, healing was found to occur after aging at 800 °C for 100 hours. The degree of sensitization, quantified by an equivalent chromium-depleted-zone size, correlates well with the corrosion rate in the nitric acid test. Thermodynamic models were used to calculate the interfacial chromium concentration, chromium depletion profile, and the depleted-zone width. Comparisons between experimental measurements and model calculations indicate that reliable prediction depends on the selection of key model parameters.

The effect of carbide precipitation on the hydrogen-enhanced fracture behavior of alloy 690

Alloy 690 is susceptible to hydrogen embrittlement where hydrogen reduces the ductility and causes the fracture morphology to change to predominantly intergranular. The role of carbide precipitation in the embrittlement behavior is not well defined. The objective of this work is to understand the effect of intergranular carbide precipitation on the hydrogen embrittlement of alloy 690. The work reported herein used tensile and compact-tension specimens in both the solution-annealed condition (minimal grain-boundary carbide precipitation) and in the solution-annealed condition followed by an aging treatment to precipitate grain-boundary carbides. By performing the mechanical tests on materials in both uncharged and hydrogen-charged conditions, it was possible to evaluate the degree of embrittlement as a function of the carbide precipitation. It is shown that the embrittlement due to hydrogen increased as the material was aged to allow grain-boundary carbide precipitation. It is proposed that the increase in embrittlement was caused by increased hydrogen at the carbide/matrix interface due to the trapping and increased stresses at the precipitate interface, which developed from strain incompatibility of the precipitate with the matrix. It is further shown that increasing the hydrostatic stress increased the tendency for intergranular fracture, as is consistent with other nickelbase alloys.

Tensile behavior of irradiated manganese-stabilized stainless steels

Seven experimental high-manganese reduced-activation austenitic stainless steels and type 316 stainless steel in the solution-annealed and 20% cold-worked condition were irradiated in the fast flux test facility up to ca. 44 dpa at 420, 520 and 600°C. Tensile tests were conducted at the irradiation temperature. The experimental alloys included an Fe20Mn12Cr0.25C (in wt%) base composition and this composition with combinations of Ti, W, V, P and B. In the solution-annealed condition, the steels hardened at all three temperatures, but hardening occurred at only 420°C in the 20% cold-worked condition. Hardening caused a decrease in ductility. The steel with a combination of all the alloying elements showed the greatest resistance to irradiation-induced hardening and loss of ductility.

Effect of strain rate on the high-temperature low-cycle fatigue properties of a nimonic PE-16 superalloy

Strain-rate effects on the low-cycle fatigue (LCF) behavior of a NIMONIC PE-16 superalloy have been evaluated in the temperature range of 523 to 923 K. Total-strain-controlled fatigue tests were per-formed at a strain amplitude of +/-0.6 pct on samples possessing two different prior microstructures: microstructure A, in the solution-annealed condition (free of gamma' and carbides); and microstructure B, in a double-aged condition with gamma' of 18-nm diameter and M23C6 carbides. The cyclic stress response behavior of the alloy was found to depend on the prior microstructure, testing temperature, and strain rate. A softening regime was found to be associated with shearing of ordered gamma' that were either formed during testing or present in the prior microstructure. Various manifestations of dynamic strain aging (DSA) included negative strain rate-stress response, serrations on the stress-strain hysteresis loops, and increased work-hardening rate. The calculated activation energy matched well with that for self-diffusion of Al and Ti in the matrix. Fatigue life increased with an increase in strain rate from 3 x 10(-5) to 3 x 10(-3) s-1, but decreased with further increases in strain rate. At 723 and 823 K and low strain rates, DSA influenced the deformation and fracture behavior of the alloy. Dynamic strain aging increased the strain localization in planar slip bands, and impingement of these bands caused internal grain-boundary cracks and reduced fatigue life. However, at 923 K and low strain rates, fatigue crack initiation and propagation were accelerated by high-temperature oxidation, and the reduced fatigue life was attributed to oxidation-fatigue interaction. Fatigue life was maximum at the intermediate strain rates, where strain localization was lower. Strain localization as a function of strain rate and temperature was quantified by optical and scanning electron microscopy and correlated with fatigue life.

Microstructural response of titanium-modified austenitic stainless steels to neutron exposure of 70 dpa in FFTF/MOTA

JPCA, a titanium-modified austenitic stainless steel, in solution-annealed or cold-worked condition and a compositionally modified JPCA in solution-annealed condition were examined by transmission electron microscopy following irradiation in FFTF/MOTA to an exposure level of up to about 70 dpa at 390 to 600°C. At lower temperatures, all the materials developed qualitatively similar cavity-, dislocation- and precipitate-microstructures. The lower-temperature swelling peak, which appeared at near 410°C, was more efficiently suppressed by phosphorus addition than cold-working. Irradiation at or above 520°C produced substantially large swelling in solution-annealed JPCA. The cavities contributed to this higher-temperature swelling developed in association with M 6C-type precipitates. Neither cavities other than very small helium bubbles nor massive particles of M 6C-type precipitates were observed in cold-worked and phosphorus-modified materials, in which MC-type precipitates developed at very high concentration. The effect of pre-irradiation microstructure and compositional modification on the behavior of these precipitates is discussed.

Stress corrosion cracking of AISI 304 and 316 stainless steels in lithiated water at 350°C

The stress corrosion cracking susceptibility of AISI Types 304 and 316 SS tubes with different heat treatments were investigated in lithiated water at 350°C. A comparison of the behaviour of these steels in the form of tubes with those made of sheets under stress corrosion cracking (SCC tests was discussed. The results showed that the sensitized conditions of both types exhibited the lowest SCC susceptibility compared to the solution-annealed conditions. The hydrogen content near the fracture surface was the highest along the gauge length of the specimens.

High-temperature low-cycle fatigue behavior of a NIMONIC PE-16 superalloy—Correlation with deformation and fracture

Low-cycle fatigue (LCF) responses of NIMONIC PE-16 for various prior microstructures and strain amplitudes have been evaluated and the fatigue behavior has been explained in terms of the operative deformation mechanisms. Total strain-controlled LCF tests were performed at 923 K on samples possessing three different prior microstructures: alloy A in solution-annealed condition (free of γ′ and carbides), alloy B with double aging treatment (spherical γ′ of 18-nm diameter and M23C6), and alloy C with another double aging treatment (γ′ of size 35 nm, MC and M23C6). All three microstructures exhibited an intial cyclic hardening followed by a period of gradual softening at 923 K. Coffin-Manson plots describing the plastic strain amplitudevs number of reversals to failure showed that alloy A had maximum fatigue life while C showed the least. Alloy B exhibited a two-slope behavior in the Coffin-Manson plot over the strain amplitudes investigated. This has been ascribed to the change in the degree of homogeneity of deformation at high and low strain amplitudes. Transmission electron microscopic studies were carried out to characterize the various deformation mechanisms and precipitation reactions occurring during fatigue testign. Fresh precipitation of fine γ′ was confirmed by the development of “mottled contrast” in alloy C. Evidence for the shearing of the ordered γ′ precipitates was revealed by the presence of superdislocations in alloy C. Repeated shearing during cyclic loading led to the reduction in the size of the γ′ and consequent softening. Coarser γ′ precipitates were associated with Orowan loops. The observed fatigue behavior has been rationalized based on the micromechanisms stated above and on the degree of homogenization of slip assessed by slipband spacing measurements on tested samples.

Effect of boron on high-temperature creep behavior of austenitic stainless steel DIN 1.4970

Samples of austenitic stainless steel DIN 1.4970 containing about 40 ppm of boron and a boron-free version of this steel were creep tested at 700 °C in both the solution-annealed and aged conditions to determine their creep ductility and strength. The microstructure before creep tests was determined using transmission electron microscopy. The distribution of boron in both the steels after solution annealing (SA) and aging was mapped by means of α-autoradiography. It has been observed that in the solution-annealed condition, the creep strength of 1.4970 steel is higher than that of the boron-free version; whereas after aging, the strength of 1.4970 steel is lower than that of the boron-free version. The creep ductilities were hardly influenced by the presence of boron. The results are discussed in terms of microstructure and boron distribution in the matrix.

Tensile properties and fractography of aged hastelloy B2 (550–850°C for up to 1200 H)

The 0.2% yield strength and elongation at fracture have been measured at 25°C for Hastelloy B2 samples that were aged for up to 1200 h from 550 to 850°C. This alloy is very similar in composition to the binary Ni-20 at.% Mo alloy. At 550 and 750°C, the strength increased significantly (approximately tripling after 1200 h at 550°C). Upon aging at 750°C, the strength passed through a maximum (of about twice that of the solution-annealed condition) then decresed. In the solution-annealed condition, the ductility is approximately 60%, but after aging at 550 and 850°C for 400 h or more, the ductility was <5%. Upon aging at 750°C, it was <5% for aging 200 h or more, even though the strength decreased. In the ductile condition, the fracture surface topology was dimpled. As aging increased and ductility decreased, the fracture became increasingly intergranular and transgranular. The fracture surface topology is related to the microstructural changes that occur upon aging.