As-cast microstructures and behavior at high temperature of chromium-rich cobalt-based alloys containing hafnium carbides

Hafnium is often used to improve the high temperature oxidation resistance of superalloys but not to form carbides for strengthen them against creep. In this work hafnium was added in cobalt-based alloys for verifying that HfC can be obtained in cobalt-based alloys and for characterizing their behavior at a very temperature. Three Co–25Cr–0.25 and 0.50C alloys containing 3.7 and 7.4 Hf to promote HfC carbides, and four Co–25Cr– 0 to 1C alloys for comparison (all contents in wt.%), were cast and exposed at 1200 °C for 50 h in synthetic air. The HfC carbides formed instead chromium carbides during solidification, in eutectic with matrix and as dispersed compact particles. During the stage at 1200 °C the HfC carbides did not significantly evolve, even near the oxidation front despite oxidation early become very fast and generalized. At the same time the chromium carbides present in the Co–Cr–C alloys totally disappeared in the same conditions. Such HfC-alloys potentially bring efficient and sustainable mechanical strengthening at high temperature, but their hot oxidation resistance must be significantly improved.     

Microstructure and oxidation behaviour at 1100°C of HfC containing Cr rich iron based cast alloys

Three alloys containing 0·25–0·50 wt-%C, 26–28 wt-%Cr and 4–6 wt-%Hf were elaborated by foundry. They contained a dendritic matrix and HfC carbides, which are expected to strengthen the alloy at high temperatures. They were exposed in air at 1100°C during 46 h. The aged microstructures displayed coarsened chromium carbides but no significant changes to the fraction or morphology of the HfC carbides. The surface characterisation of the oxidised samples showed that the alloys behaved well despite some localised instances of fast oxidation. The hardness was modified by the microstructure stabilisation achieved during the high temperature exposure. A preliminary test showed that the reinforcement by HfC may indeed lead to interesting creep resistance at 1100°C as had been hoped.     

Effect of selective oxidation of chromium on creep strength of Alloy 617

Abstract

In high temperature alloys, the selective oxidation of chromium to form a chromia scale leads to subsurface depletion of chromium in the alloy which in turn leads to the dissolution of chromium carbide in the depleted zone. The effect of this carbide depleted subsurface zone on the creep properties of Inconel Alloy 617 (Ni–22Cr–12Co–9Mo–1Al–0·08C; wt-%) has been determined. The specimens were subjected to heat treatments before creep testing to simulate long term service exposure of a thin walled heat exchanger tube operating at high temperatures. It was found, surprisingly, that in creep tests carried out at 900°C, specimens having extensive chromium depleted and carbide free subsurface zones exhibit higher creep strength than specimens thermally aged for the same durations, but having no chromium depleted zone. As chromium was removed from the matrix owing to selective oxidation, the carbon, released as the carbides in the chromium depleted zone dissolved, migrated to the centre of the specimen, producing enhanced carbide precipitation. This led to an increase in the creep strength of the specimen core, which offset the loss in creep strength of the subsurface zone. The expected detrimental effect of chromium depletion was therefore not observed.

MST/1487     

Dependence of oxidation behaviour on silicon content of 20%Cr austenitic steels

Abstract

In this paper, the role of silicon in the oxidation of a series of 20Cr–25Ni niobium stabilised steels containing silicon in the range 0–2·25 wt-% is explored. Oxidation tests were carried out in thermobalances at 900°C in a CO₂ based gas at 1 atm (~100 kPa) total pressure for exposure periods extending to 1·12 × 10⁶ s (310 h). The oxidation kinetics were parabolic in all cases with a rate constant that achieved a minimum value in alloys containing ~0·6 wt-%Si. By contrast, the extent of oxide spallation on cooling was least in the silicon free and dilute alloys and increased progressively with increasing silicon level. The results are discussed in relation to the morphology of the oxide layers. Comparison is also made with a previously published complementary study on steels of similar composition, but containing a dispersion of titanium nitride particles.

MST/1018     

Austenite grain coarsening in low–carbon manganese steels containing niobium and aluminium

Abstract

The effect of microadditions of niobium, aluminium, and a combination of the two on the austenite grain coarsening behaviour of 0·08C–0·25Si–1·5Mn steels whose nitrogen content was varied/from 0·002 to 0·02 wt-% has been studied. Modified methods of surface oxidation and thermal grooving were used in order to determine the precise prior austenite grain size in these steels with low interstitial elements. The optimum concentration of niobium which could be effectively used for restraining austenite grain growth when steels are austenitized at high temperatures has been determined. It is also demonstrated that steels containing. NbC or NbN particles exhibit similar austenite grain coarsening behaviour. Increasing the nitrogen content from 0·002 to 0·02 wt-% to give nitrogen–rich Nb(CN) particles does not help restrain austenite grain growth. However, when nitrogen was added to steels containing aluminium or aluminium and niobium, significant improvement in grain restraining behaviour was observed.

MST/120     

Microporosity reduction in modified RR2072 single crystal superalloys

Abstract

The effect of the addition of grain boundary strengthening elements (carbon, hafnium) on the microporosity of an experimental single crystal superalloy RR2072 has been investigated. Quantitative examination showed that the volume fraction of microporosity decreased in the alloys modified by these additions, and the reduction was associated with MC phase/carbon distribution and/or high hafnium content. Excessive carbon (i.e. >0.05 wt-%), however, did not reduce microporosity as efficiently as expected. Detailed analysis has revealed that carbon atoms, whether they stay in MC carbides or in the matrix as interstitial atoms, cause lattice expansion and thus reduce volume contraction during solidification. This mechanism is responsible for microporosity reduction in the carbon bearing alloys. Based on this mechanism of interstitial induced lattice expansion, theoretical calculation is consistent with experimental results. In the carbon and hafnium bearing material, however, high hafnium is another important reason for the reduction of microporosity.     

Influence of microstructure and chromium content on oxidation behaviour of spin cast high speed steels

Abstract

The influence of the chromium content and of the volume fraction of primary carbides on the thermal oxidation behaviour of spin cast high speed steels and semi-high speed steels used for the production of hot mill rolls was studied at 700°C. Oxidation nucleates at the carbide–matrix interface and carbides have a higher oxidation resistance than the matrix. Moreover carbides dissolve a higher amount of chromium than the matrix. As a consequence of these effects, the oxidation rate of these steels decreases by increasing the chromium content of the matrix and by decreasing the carbide volume fraction.     

Creep properties and design curves for nitrogen enhanced grade of 316LN stainless steel

Abstract

316L(N) stainless steel (SS) containing 0·02–0·03 wt-% carbon and 0·06–0·08 wt-% nitrogen is the principal material for the high temperature structural components of the prototype fast breeder reactor in India. In order to increase the economic competitiveness of sodium cooled fast reactors (SFRs), there is a strong desire to increase the design life from the current level of 40 years to at least 60 years for the future reactors. As a part of the efforts to develop materials with superior mechanical properties suitable for longer design life, the influence of nitrogen at concentrations higher than 0·07 wt-%, on the high temperature mechanical properties of type 316L(N) SS is being studied. Four heats of 316L(N) SS, containing 0·07, 0·11, 0·14 and 0·22 wt-% nitrogen have been evaluated extensively in terms of their tensile, creep, low cycle fatigue and creep fatigue interaction properties. Based on these studies, the nitrogen content has been optimised at 0·14 wt-%. This nitrogen enhanced grade of steel (NE316LN SS) was found to have significantly better tensile, creep, low cycle fatigue and creep-fatigue properties as compared to the PFBR grade of 316L(N) SS. This paper presents the influence of nitrogen on the creep deformation, damage and fracture behaviour of NE316LN SS. Design of high temperature SFR components is made on the basis of RCC-MR design code. The creep properties of NE316LN SS have been analysed in terms of the procedures for generation of the design code. Time–dependent design curves have been generated.     

Production and properties of steel–TiC composites for Wear applications

Abstract

Fe–(WTi)C composite granules containing up to 80 wt-% carbide have been produced by a selfpropagating high temperature synthesis reaction. These can be readily distributed in conventional steel melts. Additions up to 17 wt-% carbide have been made to a 0·4 wt-%C steel which was subsequently cast and hot rolled to plate. The microstructures of cast, rolled, and heat treated. samples display a homogeneous distribution of carbides which do not significantly affect the rolling performance of the steels. The carbides and grain refinement in heat treated samples result in a marked improvement in mechanical properties. The most significant improvement as a fraction of carbide additions is seen in abrasive wear performance.

MST/3196     

Low density steel 1·2C–1·5Cr–5Al designed for bearings

Abstract

A novel alloy design, designated as 1·2C–1·5Cr–5Al, has been proposed with high aluminium(～5 wt-%) and more carbon(～1·2 wt-%) addition into the classical 1C–1·5Cr bearing steel for lowering density and improving performance simultaneously, which is approximate 8 wt-% lighter than convention. In order to understand preliminarily the suitability of the novel alloy for bearing application, the martensite starting temperature and hardness, related to microstructure evolution and mechanical properties, respectively, after partial austenitisation treatment with undissolved carbides have been investigated carefully. The martensite starting temperature is comparable with conventional 1C–1·5Cr alloy. The hardness of 860±3 HV20 achieved is much higher than convention.     

Oxide adherence of Fe-20Cr-4Al alloys with small amounts of sulfur and reactive elements (Y,Hf)

Abstract

Oxide adherence of Fe–20Cr–4Al alloys with small amounts of sulfur, yttrium and hafnium was studied in air for 360 ks at 1,373, 1,473 and 1,573K by mass change measurements, X-ray diffraction, scanning electron microscopy and electron probe microanalysis. After oxidation at 1,373K, spalling of oxide scales on 7ppmS, 53ppmS and 1,300ppmS alloys was recognized. However, spalling of oxide scales on the other alloys was not observed. After oxidation at 1,573K, spalling of the oxide scales on the alloys with sulfur increased roughly with increasing contents of sulfur, and spalling of oxide scales on the alloys containing yttrium was scarcely recognized, however, oxide scales on all of the alloys containing hafnium spalled at the entire surface. Oxide adherence on the alloys may relate to morphologies of oxide scales and oxide–alloy interface, size and distribution of chromium sulfide, Y₃Al₅O₁₂ and HfO₂ particles at the oxide–alloy interface and temperature of oxidation.     

High strength and ductile low density austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides

Abstract

We introduce the alloy design concepts of high performance austenitic FeMnAlC steels, namely, Simplex and alloys strengthened by nanoscale ordered κ-carbides. Simplex steels are characterised by an outstanding strain hardening capacity at room temperature. This is attributed to the multiple stage strain hardening behaviour associated to dislocation substructure refinement and subsequent activation of deformation twinning, which leads to a steadily increase of the strain hardening. Al additions higher that 5 wt-% promote the precipitation of nanoscale L′1₂ ordered precipitates (so called κ-carbides) resulting in high strength (yield stress ～1·0 GPa) and ductile (elongation to fracture ～30%) steels. Novel insights into dislocation–particle interactions in a Fe–30·5Mn–8·0Al–1·2C (wt-%) steel strengthened by nanoscale κ-carbides are discussed.     

Insoluble phase in Al–1·52Cu–0·75Mg alloys containing silicon

Abstract

Insoluble particles present in solution treated, quenched and aged Al–1·52Cu–0·75Mg alloys containing 0·23, 0·49, 0·76, and 1·03 wt-% Si have been investigated. Back-scattered scanning electron microscope images of polished sections of the alloys showed the presence of very small, nearly spherical particles at the grain boundaries of all alloys except that containing 0·23 wt-% Si. Alloys with 0·76 and 1·03 wt-% Si also contained large particles of well defined geometrical shapes. An electrolytic technique was used to extract the insoluble particles from the matrix. The extracted particles were analysed by energy dispersive X-ray analysis in a scanning electron microscope. They contained 20·6±0·4%Cu, 32·6±0·4%Mg, 30·2±0·4%Si, and 16·6±0·1%Al (all wt-%). X-ray diffraction analysis of these particles showed that they have a hexagonal structure with lattice parameters a=1·036±0·004 nm and c=0·404±0·004 nm. The insoluble particles were identified as quarternary phase Q. Aging for extended periods at 190°C did not alter the composition or morphology of the insoluble particles.

MST/661     

Enhancement of mechanical properties of Al–Mg–Si alloys by means of manganese dispersoids

Abstract

The effects of Mn dispersoids on the enhancement of mechanical properties in Al–Mg–Si(–Mn) alloys have been studied to develop a new high Mn alloy which does not need an aging heat treatment after a shaping process (i.e. extrusion process). By adding Mn to Al–Mg–Si alloys, sphere- or rod shaped Mn dispersoids of a size ranging from 0·05 to 0·5 μm are formed by the use of proper heat treatments. The as extruded alloys containing 1·0 wt-%Mn are measured to have higher tensile properties with good ductility, as compared with those of the commercial Al alloy 6N01 (Al–0·69Mg–0·79Si–0·48Cu–0·27Zn–0·37Mn–0·3Cr– 0·11Ti, wt-%). These phenomena are obtained from the dispersion hardening effect and homogeneous deformation by Mn dispersoid particles acting as obstacles to dislocation movement. Comparing the fatigue crack growth behaviour between the high Mn alloys and the commercial 6N01 alloy in the as forged condition, high Mn alloys are shown to have higher fatigue crack growth resistance and show a more tortuous crack path. This result can be explained by the increasing energy absorption through crack deflections and tortuous crack paths by the Mn dispersoids.     

Segregation of reactive elements at oxide grain boundaries in FeCrAlRE alloys

Abstract

Extensive previous research has established that the oxidation of FeCrAl alloys at temperatures ≥1000°C results in the formation of α-Al₂O₃ oxide scales, and that minor alloy constituents (particularly Reactive Elements (RE) such as Y, Hf, Zr, etc.) can change the oxide growth mechanism. A knowledge of the segregation behaviour of these REs is thus central to our understanding of the oxidation behaviour of these, technologically important, range of alloys.

The new SuperSTEM microscope at the Daresbury Laboratory offers considerable potential for a detailed study of the segregation to oxide grain boundaries at the atomic level. The microscope has an aberration corrector fitted to the objective lens, allowing the formation of sub-Angstrom probe for simultaneous ultra-high resolution high angle annular dark field (HAADF) imaging and atomic-column electron energy loss spectroscopy (EELS). This paper reports on an initial study of oxide grain boundary segregation in commercial and model FeCrAlRE alloys containing controlled additions of the reactive elements, yttrium, zirconium and hafnium oxidised at 1250°C, in air, for 50 hours. Both yttrium and hafnium are shown to segregate to the grain boundaries while hafnium rich particules form in the outer region of the scale.     

Phase Transitions and Oxidation Behavior During Oxyacetylene Torch Testing of TaC–HfC Solid Solutions

Tantalum carbide (TaC) and hafnium carbide (HfC) have some of the highest melting temperatures among the transition metal carbides, borides, and nitrides, making them promising materials for high-speed flight and high-temperature structural applications. Solid solutions of TaC and HfC are of particular interest due to their enhanced oxidation resistance compared to pure TaC or HfC. This study looks at the effect of Hf content on the oxidation resistance of TaC–HfC sintered specimens. Five compositions are fabricated into bulk samples using spark plasma sintering (2173 K, 50 MPa, 10 min hold). Oxidation behavior of a subset of the compositions (100 vol% TaC, 80 vol% TaC + 20 vol% HfC, and 50 vol% TaC + 50 vol% HfC) is analyzed using an oxyacetylene torch for 60 s. The TaC–HfC samples exhibit a reduction in the oxide scale thickness and the mass ablation rate with increasing HfC content. The improved oxidation resistance can be attributed to the formation of a Hf₆Ta₂O₁₇ phase. This phase enhances oxidation resistance by reducing oxygen diffusion and serving as a protective layer for the unoxidized material. The superior oxidation resistance of TaC–HfC samples makes these materials strong contenders for the development of high-speed flight coatings.     

Characterisation of two Al–Fe based high temperature alloy powders

Abstract

Aluminium alloys containing additions of iron and cerium are among the alloys being developed as potential replacements for titanium based alloys for moderately high temperature applications. Development of these alloys is possible using rapid solidification technology, which results in a very fine distribution of dispersoids in the aluminium matrix. The microstructures of two rapidly solidified high temperature alloy powders of composition (wt-%) Al–6·7Fe–5·9Ce (alloy A) and Al–6·2Fe–5·9Ce–1·63Si (alloy B) have been characterised using transmission electron microscopy and the results are explained on the basis of some of the major solidification parameters, such as nucleation undercooling and recalescence. It was observed that most of the powder particles in the +10 to ?20 μm size range contained both microcellular and cellular regions, which could be explained in terms of an initial large undercooling followed by recalescence. The decomposition of the powder microstructure after exposing the powders to temperatures of 350, 420, and 500°C for 1 h was investigated using transmission electron microscopy. This work was complemented by phase identification studies using X-ray diffraction. The equilibrium precipitates Al₁₃Fe₄, Al₈Fe₂Si, and Al₃FeSi were detected in the powder microstructure of alloy B, whereas Al₁₃Fe₄ precipitates were detected in alloy A after high temperature exposure (500°C).

MST/1571     

Investigation of WC–Co alloy properties based on thermodynamic calculation and Weibull distribution

The influence of alloy composition and sintering temperature on the mechanical properties and reliability of WC–Co cemented carbides was studied theoretically and experimentally. For the first time, through a hybrid approach of thermodynamic calculations and Weibull distribution, the comprehensive performance of ultrafine WC–Co cemented carbides with different C contents and inhibitor type was investigated in detail. The carbon content of WC–10?wt-% Co–0.5?wt-% Cr cemented carbides was carefully controlled within the range of 5.38?5.52?wt-%. The contents of Cr and V are chosen to be in the range of 0–1?wt-%. It is found that WC–10?wt-% Co–0.5?wt-% Cr alloys with 5.46?wt-% C or 5.5?wt-% C show excellent mechanical properties and high reliability. WC–10?wt-% Co alloys with 0.5?wt-% Cr and 0.4?wt-% Cr–0.2?wt-% V demonstrate high mechanical property and reliability. The results of this study can be used to design process parameters during the manufacture of WC–Co cemented carbides.     

Influence of silicon and magnesium on grain refinement in aluminium alloys

Abstract

Grain refinement in Al–Si alloys with silicon contents in the range of 0·2–30 wt-% has been studied in detail with conventional as well as higher level additions of a Al–5Ti–1B master alloy. A poisoning effect was observed with Al–Si alloys containing ≥7 wt-%Si and the extent of poisoning increased with an increase in the silicon content. Silicon improves the grain refining behaviour of aluminium when added in small quantities (0·2%). Magnesium can counteract the poisoning effect of silicon. The optimum level of magnesium required to overcome the poisoning effect depends on the silicon content of the alloy. Higher level additions of a grain refiner could overcome the poisoning effect of silicon and the level required to achieve good grain refinement is a function of the silicon and magnesium contents of the alloy. The present paper also reports the influence of degasser and melt temperature on the grain refining response of Al–Si alloys.     

Influence of alloying elements on thermal conductivity and high temperature strength of copper based alloys

Abstract

The effects of alloying elements and annealing temperature on thermal conductivity and softening behaviour of Cu – 0·1Ag – xP–yMg and Cu – xSn – yTe alloys (all compositions are in wt-%) have been investigated. The Cu – 0·1Ag – xP–yMg alloys, in spite of greater amounts of P and Mg, had a higher electrical conductivity and a higher softening temperature than those of a Cu – 0·1Ag – 0·031P alloy. A Cu – 0·032Sn – 0·023Te alloy had the same levels of electrical conductivity and softening temperature as those of Cu – 0·040Sn. The conductivity and softening temperature of the Cu – 0·032Sn – 0·023Te alloy are comparable with those of the Cu – 0·1Ag – 0·013P alloy currently used for continuous casting mould materials.