Novel hafnium containing steels for power generation

Abstract

There is clear evidence that creep damage in power plant steels is associated with grain boundary precipitates. These particles provide favourable nucleation sites for grain boundary cavities and microcracks. The formation of M₂₃C₆ carbides as grain boundary precipitates can also lead to grain boundary chromium depleted zones which are susceptible to corrosive attack. Such precipitates are the causing loss of creep life in the later stages of creep because of their very high coarsening rate. Through Monte Carlo based grain boundary precipitation kinetics models, combined with continuum creep damage modelling it is predicted that improvements in creep behaviour of power plant steels can be achieved by increasing the proportion of MX type particles. Studies of a Hf containing steel have produced improvements in both creep and corrosion properties of 9%Cr steels. Hf has been ion implanted into thin foils of a 9 wt-%Cr ferritic steel to study its effect on precipitation. Two new types of precipitates are formed, Hf carbide, (an MX type precipitate) and a Cr–V rich nitride, with the formula M₂N. The Hf carbide particles were identified using convergent beam diffraction techniques, and micro-analysis. The nanosized particles are present in much higher volume fractions when compared to VN volume fractions in conventional power plant ferritic steels. Furthermore it is confirmed that the Hf causes the removal of M₂₃C₆ grain boundary precipitates. This has led to an increased concentration of Cr within the matrix, reduced chromium depleted zones at grain boundaries, and increased resistance to intergranular corrosion cracking.     

The assessment of creep damage in certain aged alloys based on Al-Cu-Mg system

The mechanism of creep failure was investigated for three different test alloys namely Al-4.0%Cu-0.3%Mg, Al-4.0%Cu-0.3%Mg-0.4%Cd and Al-4.0%Cu-0.3%Mg-0.4%Ag at 125 and 150°C in the fully hardened condition. The room temperature tensile properties of these alloys increased in the order of ternary alloy, Cd-containing alloy and Ag-containing alloy. The creep performance of these alloys also improved in the similar order. The present studies revealed the dominance of intercrystalline creep failures in all the alloys at both the test temperatures. The grain boundary microstructures contained precipitates with narrow Precipitate Free Zones (PFZ’s) with large difference in particle spacings. Ag-containing alloy recorded minimum grain boundary particle spacing as compared to that of ternary and Cd-containing alloys. The creep damage assessment in terms of damage distribution in the gauge portion showed maximum damage in the Ag-containing alloy as compared to other two alloys. In all these alloys, failures occurred by the coalescence of several cracks and the negotiations of few boundary junctions rather than the propagation of single major crack.     

Microstructure of a Creep-Resistant 10 Pct Chromium Steel Containing 250 ppm Boron

The microstructure of a trial martensitic chromium steel containing a high content of boron (250 ppm) was characterized in detail in the as-tempered and aged conditions. This steel has a similar composition and heat treatment as the TAF steel that still is unsurpassed in creep strength among all 9 to 12 pct chromium steels. Characterization was performed by using scanning electron microscopy, energy-filtered transmission electron microscopy, secondary ion mass spectroscopy, and atom probe tomography. Focus was placed on investigating different types of precipitates that play a key role in improving the creep resistance of these steels. The low tempering temperature of 963 K (690 °C) is enough for the precipitation of the full volume fraction of both MX and M₂₃C₆. A high boron content, more than 1 at. pct, was found in M₂₃C₆ precipitates and they grow slowly during aging. The high boron level in the steel results in metal borides rather than BN with the approximate formula (Mo_0.66Cr_0.34)₂(Fe_0.75V_0.25)B₂. Two families of MX precipitates were found, one at lath boundaries about 35 nm in size and one dense inside the laths, only 5 to 15 nm in size.     

Improved Creep Behavior of a High Nitrogen Nb-Stabilized 15Cr-15Ni Austenitic Stainless Steel Strengthened by Multiple Nanoprecipitates

Austenitic stainless steels are expected to be a major material for boiler tubes and steam turbines in future ultra-supercritical (USC) fossil power plants. It is of great interest to maximize the creep strength of the materials without increasing the cost. Precipitation strengthening was found to be the best and cheapest way for increasing the creep strength of such steels. This study is concerned with improving creep properties of a high nitrogen Nb-stabilized 15Cr-15Ni austenitic alloy through introducing a high number of nanosized particles into the austenitic matrix. The addition of around 4 wt pct Mn and 0.236 wt pct N into the 15Cr-15Ni-0.46Si-0.7Nb-1.25Mo-3Cu-Al-B-C matrix in combination with a special multicycled aging-quenching heat treatment resulted in the fine dispersion of abundant quantities of thermally stable (Nb,Cr,Fe)(C,N) precipitates with sizes of 10 to 20 nm. Apart from the carbonitrides, it was found that a high number of coherent copper precipitates with size 40 to 60 nm exist in the microstructure. Results of creep tests at 973 K and 1023 K (700 °C and 750 °C) showed that the creep properties of the investigated steel are superior compared to that of the commercial NF709 alloy. The improved creep properties are attributed to the improved morphology and thermal stability of the carbonitrides as well as to the presence of the coherent copper precipitates inside the austenitic matrix.     

Microstructure and creep behaviour of Mg-12Gd-3Y-1Zn-0.4Zr alloy

Microstructure and creep behaviour of Mg-12Gd-3Y-1Zn-0.4Zr alloy prepared by squeeze casting were investigated. Transmission electron microscope (TEM) observation revealed that a kind of lamellar-shaped morphologies 14h long-period stacking order structure (LPSO) and dense β’-phase precipitates were formed by heat treatment. The alloy exhibited good creep resistance. It was shown that the creep-resistant performance kept stable because of the restriction of dense β’-phase precipitates and LPSO phases to the movement of dislocations, and the formation of β-phase plates took responsibility for the softening of material during creep. Stress and temperature dependence of the steady-state creep rate were studied over stress range of 50-100 MPa and a temperature range of 250-300 oC, and a dislocation creep mechanism was proposed for the alloy.     

Mechanical and microstructural properties of alloy 800

The evolution of microstructural and mechanical properties of alloy 800 with respect to operating conditions of the steam generator tubings of fast breeder reactors have been analyzed and presented. On the microstructural side two phenomena have important influence on the mechanical properties, namely γ′ and carbide precipitation. Gamma prime precipitation occurs in alloy compositions containing ≳0.50 pct Ti + Al, inducing mechanical property changes and, in particular, improving the long term creep resistance. Its growth rate follows the exponential law which, when extrapolated, yields an overaging time beyond 4 × 10⁴ h at ≲550°C. M₂₃C₆ carbide precipitation starts in early stages of exposure at 500 to 600°C, being of heterogeneous nature and forming mainly on the grain boundaries. The M₂₃C₆ carbides advance perpendicular to the surface of one of the austenite grains, commonly having a <110>_γ//<110>M₂₃C₈relationship, and occasionally develop into well defined cellular precipitates. On the mechanical side, a sharp creep ductility decline is observed when either, or in particular both, strong γ′ strengthening and discontinuous precipitation develop in the matrix. It is argued that this decline is principally due to the latter phenomenon and is accentuaged by matrix strengthening. Formerly Associate Professor, Metallurgical Engineering Department, Arya Mehr University of Technology, Tehran, Iran.     

Effect of alloying additions on the structure and mechanical properties of Fe<Subscript>3</Subscript>Al -1C intermetallic alloy

Effect of titanium and nickel on the structure and properties of Fe₃Al intermetallic alloy containing about 1.0wt.% C have been investigated. The composition of the alloying element was substituted for Iron. The alloys were prepared by melting commercial grade raw materials iron, aluminum, titanium or nickel in air induction furnace with flux cover (AIMFC). Further these ingots were refined by electroslag refining (ESR) process. These ingots could be successfully hot-worked using conventional hot-forging and hot-rolling techniques. The hot-worked material was sound and free from cracks. ESR hot-rolled alloys were examined using optical microscopy, X-ray diffraction (XRD), scanning electron micrograph (SEM) to understand the microstructure of these alloys. The electron probe micro analysis (EPMA) studies were carried out to determine the matrix and precipitate compositions and to identify the phases present in the alloys. The base alloy and the alloy containing Ni exhibited a two-phase microstructure of Fe₃AlC_0.5 precipitates in Fe₃Al matrix. The alloy containing Ti exhibits three-phase microstructure, the additional phase being TiC precipitate. Ti addition resulted in no improvement in strength at room temperature and at 873 K whereas Ni addition has resulted in greater improvement in strength at room temperature and at 873 K and also improved the creep life significantly from 66 hrs to 111 hrs.     

Small-angle neutron scattering investigation of creep damage in type 304 stainless steel and alloy 800

Creep damage accumulation in fully precipitated type 304 stainless steel and alloy 800 was investigated using small-angle neutron scattering. The small-angle scattering decreased initially and then slowly increased during the intermediate stages of creep. The scattering is thought to be dominated by stress assisted coarsening of carbide precipitates during the early stages of creep. This result and the direct observation of cavities by transmission electron microscopy indicate that at least two damage mechanisms, precipitate coarsening and cavitation, were occurring simultaneously. Coarsening dominates the small-angle scattering measurements during the early stages of creep, but cavitation appears to be detectable during the intermediate stages.     

Microstructure and crystallography of unidirectionally solidified Ni-W eutectic alloy

A Ni-W eutectic alloy was subjected to a process of unidirectional solidification (UDS) by the Bridgman-Stockbarger technique. Three phases were identified by transmission electron microscopy, namely: W fibers, a solid solution matrix of W in Ni, and Ni₄W precipitates of the Dla structure in the matrix. The growth axis of the W-fibers was found to be <111> and the orientation relationship between them and the Ni(N) matrix was identified as the Bain type, so that (100)_bcc ‖ (100)_fcc. The shape of the Ni₄W precipitates varies from equiaxial at high solidification rates to elongated plates at low rates. The orientation relationship between the precipitates and the matrix is the same for all solidification rates. The microstructure of specimens subjected to creep deformation was studied and the deformation modes were identified. These include dislocations and microtwins that originate mainly at the boundary between the Ni(W) matrix and the W-fibers. In some specimens the creep test was carried out after solution treatment at 1030 °C followed by quenching, which resulted in a Ni(W) matrix reinforced with W-fibers without Ni₄W precipitates. The microstructural changes during this creep process and the fracture surface were studied by SEM and TEM.     

Topologically Close-Packed μ Phase Precipitation in Creep-Exposed Inconel 617 Alloy

Two creep-exposed Inconel 617 alloy samples [923 K (650 °C) for 45,000 hours and 973 K (700 °C) for 4000 hours] have been studied using analytical electron microscopy and X-ray diffraction techniques. The thermodynamically predicted equilibrium phases in Inconel 617 alloy were compared with the phases observed which are molybdenum-enriched, topologically close-packed μ-phase, along with precipitates of gamma-prime (γ′), M₂₃C₆ and Ti(C,N). The μ-phase precipitates were in the size range 60 to 500 nm (with some larger agglomerates); they were situated both within the grains, along twin and grain boundaries, and near intra- and intergranular carbides. The stacking faults in the μ-phase were observed in high magnification electron microscopy. The precipitation of the μ-phase in these samples is significant for the potential use of this alloy in future generation steam power plants as the appearance of the μ-phase is associated with an increased tendency for cracks and voids to initiate. The μ-phase has not been previously identified in the literature relating to creep or thermal exposure of Inconel 617 alloy.     

Mechanism of cyclic softening and fracture of an Ni-Base γ′-Strengthened alloy under low-Cycle fatigue

Evolution of the microstructure and its impact on the mechanical behavior of a γ′-strengthened Ni-base alloy under low-cycle fatigue (LCF) at 760 °C, 871 °C, and 982 °C were studied. A cyclic softening was observed, the extent of which increased with temperature. This was attributed to the coherency loss of the primary γ′ particles and partial shearing and dissolution of the secondary γ′ particles. The precipitates and carbides were resistant to coarsening during thermal exposure alone or LCF at the lower two temperatures; the primary γ′ particles, however, lost coherency and spheroidized during LCF at 982 °C. Fatigue cracking initiated at the surface was mainly responsible for failure, although cavities were found in localized areas of a few test specimens. The creep damage was attributed to an accumulation of plastic strains of high amplitude at inhomogeneities, either macroscopic or microscopic.     

Effect of Solution Temperature on the Microstructure and Mechanical Properties of a Newly Developed Superalloy TMW-4M3

The influence of solution temperature on the microstructure and mechanical properties of TMW-4M3 superalloy has been investigated. Comparisons of mechanical properties have also been made between the heat-treated TMW-4M3 variants and the commercial U720Li. The key microstructural variables examined were grain size and the volume fraction and size of the strengthening γ′ precipitates that control the mechanical properties of these alloys. By increasing the solution temperature from 1373 K to 1393 K (1100 °C to 1120 °C), the volume fraction of primary gamma prime dropped from 16.9 pct to 14.5 pct, whereas the average grain size increased from 8.7 μm to 10.6 μm. Compared with an alloy with a smaller grain size, the alloy with a larger grain size exhibited superior resistances to creep and fatigue crack growth without the expense of reduced tensile strength and low-cycle fatigue resistance. This suggested that a higher solution temperature may benefit TMW-4M3 in terms of superior overall properties. The greater overall properties of TMW-4M3 variants than that of commercial U720Li were also demonstrated experimentally. The possible explanations for the improvement of mechanical properties were discussed.     

Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

An evaluation of the creep properties of two Al-Si alloys produced by rapid solidification processing

Tensile creep tests were conducted on two Al-Si alloys produced by rapid solidification: an Al-Si-Ni-Cr alloy and an Al-Si-Cu-Fe alloy, designated alloys A and B, respectively. The creep curves of these two alloys in the temperature range from 493 to 573 K were markedly different, with alloy A exhibiting a normal creep curve with a very short tertiary region and alloy B exhibiting an extended tertiary stage associated with strain localization. The minimum creep rates varied, with the applied stress raised to exponents of ∼9.0 and ∼8.5 for the two alloys, respectively. The hardness of alloy B decreased with time during the creep testing, but there was little or no change in the hardness of alloy A. These differences in the creep and hardness characteristics are attributed to the evolution of precipitates within the two alloys during creep testing. A detailed analysis shows that, over the temperature range examined experimentally, alloy A exhibits a creep strength that is superior to conventional Al-based alloys and comparable to, or even higher than, some SiC-reinforced Al composites.     

Shock Synthesis of Ternary Diamond-Like Phases in the B ― C ― N System

Nanodispersed powders of the ternary graphite-like phases BC_1.28N and BC_2.14N have been used along with mixtures of powders of turbostratic BN and C in high-temperature shock compression followed by sharp quenching. The yield of diamond-like phases has attained 50% by volume. The excess of the graphite-like phase has been removed by treatment with molten alkalis containing nitrates and then with concentrated HClO₄. The shock and chemical treatments alter the compositions of the diamond-like phases towards BCN in both of the BC_xN specimens on account of the segregation of the excess carbon followed by the dissolution of it. Precision lattice-parameter measurements and line shape analysis indicate that the diamond-like phase is a BN ― C substitutional solid solution. Treatment of a mechanical mixture of the graphite-like phases of carbon and BN under the same p and T conditions led to the formation of a mixture of cubic boron nitride and diamond.     

Creep and rupture of an ods alloy with high stress rupture ductility

The creep and stress rupture properties of an oxide (Y₂O₃) dispersion strengthened nickel-base alloy, which also is strengthened by γ′ precipitates, was studied at 760 °C and 1093 °C. At both temperatures the alloy YDNiCrAl exhibits unusually high stress rupture ductility as measured by both elongation and reduction in area. Failure was transgranular, and different modes of failure were observed including crystallographic fracture at intermediate temperatures and tearing or necking almost to a chisel point at higher temperatures. While the rupture ductility was high, the creep strength of the alloy was low relative to conventional γ′ strengthened superalloys in the intermediate temperature range and to ODS alloys in the higher temperature range. These findings are discussed with respect to the alloy composition; the strengthening oxide phases, which are inhomogeneously dispersed; the grain morphology, which is coarse and elongated and exhibits many included grains; and the second phase inclusion particles occurring at grain boundaries and in the matrix. The creep properties, in particular the high stress dependencies and high creep activation energies measured, are discussed with respect to the resisting stress model of creep in particle strengthened alloys. RICHARD M. ARONS, formerly a Graduate Student at Columbia University     

Scavenging additions of boron in low C – low Al steels

Scavenging of boron on nitrogen is almost complete on the hot strip for B/N ≥ 0.7–0.8. Nitrogen ageing does not appear and low coiling can be used (650°C). BN precipitates as polycrystalline aggregates (spherulites) and in association with other precipitates or inclusions. Solute carbon is somewhat reduced in B steels coiled at 650°C but a more important reduction is achieved when coiling at 750°C due to an Fe₂₃ (C, B)₆ precipitation in the optimum range 1.0 ≤ B/N ≤ 1.5. Yield stresses (YS) lower than 190 MPa are obtained for optimized compositions of low Al-B steels coiled at 650°C and continuously annealed (825°C – 60 s – cooling rate: 50°C/s). Further softening is achieved when coiling at 750°C (YS < 160 MPa). Lower ageing (≤ 20 MPa) of B steels coiled at 650°C is also for c.a. steels when compared to low Al grades; bake hardening remains at a high level (> 50 MPa). No ageing is observed in B steels coiled at 750°C of which the bake hardening is reduced (25 MPa). Deep drawability of B steels is improved at reduced C, Al, Mn contents and at higher coiling temperatures; a detrimental effect of a B excess is observed. Low nitrogen contents and reduced Al additions are needed to reach best B steel properties.     

Evaluation of microstructural changes in alloy 713 LC by neutron small angle scattering and analytical electron microscopy

Small angle neutron scattering (SANS) studies in combination with transmission electron microscopy (TEM) were performed on the cast nickel base superalloy Alloy 713 LC and on a Hf-modified version of the same alloy. With the aid of the TEM results the profile of the scattering curves was correlated with the M₂₃C₆ carbide and the γ′ precipitates. A coarsening of the γ′ precipitates with increasing creep deformation to a larger plate-like shape was observed. The small axis of these precipitates averaged over the grains was parallel to the stress axis. The γ′precipitates start to become anisotropic even in the primary stage of creep. In the vicinity of the fracture surface the volume fraction of the cavities and microcracks caused by creep deformation was 10⁻³ to 10⁻² The influence of the cavities was indicated by the smaller anisotropy factor measured near the fracture surface in comparison to the rest of the cylindrical part of the specimen. The variation of the anisotropy factor as determined by the SANS-method may be used to nondestructively measure the accumulated damage in the material. formerly at Oak Ridge National Laboratory, Oak Ridge, TN     

Effects of shock loading and cold rolling on the structure and high temperature creep properties of γ′ strengthened Ni-18.6 pct Cr-4.3 pct Al

The effects of shock loading and cold rolling on the structure and high temperature creep properties of precipitation strengthened Ni-18.6 pct Cr-4.3 pct Al have been investigated. The creep properties of this alloy depend on whether the testing is conducted in forming gas (90 pct N₂ + 10 pct H₂) or in vacuum. It is found that cold rolling invariably increases the creep rate and decreases the creep life when testing is conducted in forming gas. This unexpected result is believed to be caused by strain concentrations at grain boundaries which are associated with cold rolling and which contribute to creep by promoting premature intergranular fracture. In order to minimize the effects of intergranular fracture and to determine the intrinsic effects of prestraining it is necessary to conduct creep tests in vacuum. Taking this precaution, we find that the creep strength of this alloy can be enhanced only slightly by cold rolling a few percent. Cold rolling by a greater amount or shock loading to about 70 kbar (6.99 × 109 Pa) results in higher creep rates than the asaged condition. It is shown that these unexpected weakening effects can be correlated with, and are probably caused by, the presence of misshaped γ′ precipitate particles. The irregular shapes of these precipitates are believed to be caused by the line tension effects of surrounding dislocations.