High-pressure nitrogen gas alloying of Fe-Cr-Ni alloys

The melting of Fe-Cr-Ni alloys under 200 MPa nitrogen pressure has shown that nitrogen significantly improves the mechanical properties. Tensile strengths of these high-nitrogen alloys were found to be proportional to interstitial nitrogen concentration. The tensile strengths of nitrogen-alloyed steels could be significantly increased over comparable carbon-alloyed steels. The increase in tensile strength was found to be proportional to the square root of the interstitial nitrogen concentration indicating that the strengthening may be controlled by thermal effects.     

Metallurgical reaction and deposit fusion boundary microstructure under a stationary plasma arc

Alloy element loss, decarburization in melted region and microstructural change of a deposit fusion boundary under a stationary plasma arc have been investigated by a first-order kinetic and interfacial microstructure. The cylindrical specimens of iron-based alloys, Fe-C, Ni-C, and Co-C alloys were locally melted by a plasma arc using argon plasma gas and Ar+H₂ shielding gas, and the rates of alloy element loss and decarburization in the melted region were measured. Moreover, the fusion boundaries experienced when nickel, iron, Ni-Fe filler metals were deposited on iron, Fe-C, nickel and cobalt base metals, were evaluated metallographically. The initial rate of alloy element loss decreases as follows: Fe-Al>Fe-Mn>Fe-Cr>SGI-Mg>Fe-Ni. The loss reaction mechanism is metallic evaporation and the rate seems to be limited by transport in the gas boundary layer. The magnitude of decarburization is as follows: Ni-C>Fe-C> Co-C. The decarburization rate in Fe-C alloy is assumed to be governed by a process involving mass transfer in the gas phase and the molten metal. However, at low carbon concentrations, the rate appears to be limited by transfer in liquid metal. Fusion boundary deposited with nickel filler metal on iron is regular, but with carbon added to iron, an infiltration of deposit metal into adjacent base metal occurs. The fusion boundary with iron deposited on nickel is irregular where thin Ni-Fe solid solution is formed. In a deposited fusion boundary of cobalt with iron, nickel, and Ni-Fe filler metals, FeCo compound formation occurs, with cobalt dissolving into the nickel deposit metal resulting in a tongue-like structure produced by nickel penetration and a fine columnar substructure formation produced by Ni-Fe diffusion.     

Mössbauer effect study of nitrogen-implanted iron foils

⁵⁷Fe Mössbauer conversion electron scattering was used to establish that nitrides are formed when iron foils are implanted with nitrogen ions at doses above 10¹⁷ ions cm^?2. The formation of nitrides may be linked to the maximum reduction in wear rate which occurs for nitrogen-implanted steel surfaces at about this dose. The stability of the nitrides produced was studied as a function of annealing temperature.     

Development of wear resistant composite surface on mild steel by laser surface alloying with silicon and reactive melting

The present study concerns laser surface alloying with silicon of mild steel substrate using a high-power continuous wave CO₂ laser with an objective to improve wear resistance. The effect of surface remelting using nitrogen as shrouding environment (with and without graphite coating) on microhardness and wear resistance has also been evaluated. Laser surface alloying leads to formation of a defect free microstructure consisting of iron silicides in laser surface alloyed mild steel with silicon and a combination of silicides and nitrides when remelted in nitrogen. Carbon deposition prior to remelting leads to presence of a few martensite in the microstructure. A significant improvement in microhardness is achieved by laser surface alloying and remelting to a maximum of 800 VHN when silicon alloyed surface is melted using nitrogen shroud with carbon coating. A detailed wear study (against diamond) showed that a significant improvement in wear resistance is obtained with a maximum improvement when remelted in nitrogen atmosphere followed by carbon coating.     

Nitrogen addition to iron powder by mechanical alloying

Nitrogen was alloyed into iron (a) by mechanical processing in a nitrogen gas environment, and (b) by mechanically alloying with iron-nitride powders to characterize resulting nano-structure and nitrogen distribution. Although the infused nitrogen concentration was significantly greater than the thermodynamic equilibrium solubility of iron, no nitrides formed, even for nitrogen concentrations as high as 4.1 wt.% However, a bctFe phase did form. Lattice expansion calculations indicate that the sum of the interstitial bcc-Fe and bctFe nitrogen concentrations was significantly less than the total measured nitrogen concentration. A considerable portion of the mechanically infused nitrogen was determined to be associated with nanograin boundaries.     

Alloying element nitride stability in iron-based alloys; denitriding of nitrided Fe–V alloys

Recognising the various types of absorbed nitrogen in nitrided iron-based alloys and the lack of accuracy in local composition analysis of the individual nano-sized nitride precipitates (e.g. by atom probe tomography), precise determination of the nitride composition requires so-called denitriding treatments. This study proposes two routes to determine the nanonitride stoichiometry, which are demonstrated by experimental analysis of the denitriding behaviour in hydrogen gas of nitrided, model Fe–V alloys. The minimal nitrogen activity to stabilise the alloying element nitrides MeN in Fe–Me–N alloys is calculated. A strategy is offered to stabilise the alloying element nitrides and to determine the nitrogen content strongly bonded in the nitrides.     

The occurrence of austenitic layers in laser surface-melted iron alloys

In iron alloys surface melted by power laser treatments, thin, relatively soft layers mainly constituted by austenite, can form in the upper part of the solid state transformed regions. The mechanism of formation involves both carbon diffusion from the melt, in amounts sufficient to stabilize austenite, and the high thermal gradients typically produced in the alloys by high-energy sources.     

Fabrication of Lotus‐Structured Porous Iron by Unidirectional Solidification under Nitrogen Gas

A novel technique has been developed to fabricate lotus‐structured porous iron in which long cylindrical pores are aligned in one direction. The iron is melted and unidirectionally solidified in a pressurized gas mixture of nitrogen and argon. The process involves the dissolution of nitrogen in molten iron and the evolution of nitrogen pores due to the decrease in solubility of nitrogen during solidification. The porosity is controlled by adjusting the partial pressures of nitrogen and argon during melting and solidification. The nitrogen concentration in solid iron increases with increasing partial pressure of nitrogen at a given total pressure, leading to improvement of the mechanical properties of the porous iron.     

Dispersion and reaction of TiB2 in liquid iron alloys

Abstract

The degree of reaction and dispersion achieved when TiB₂ powders are melted in contact with liquid iron based alloys has been assessed via a levitation dispersion test which had been developed earlier. Both Fe₂B and TiC were observed to form as a result of dissolution and reaction of TiB₂. The formation of TiC occurs during the reaction of commercial grade TiB₂ with liquid iron alloys containing as little as 0·08 wt-%C. The reaction of high purity TiB₂ with liquid iron alloys containing 0·24 wt-%C does not however lead to TiC formation. The formation of Fe₂B was observed for all conditions tested, owing to the effectively zero solubility of boron in solid iron. The TiB₂ remaining after dissolution and reaction was found to produce relatively good dispersions in the iron matrix and therefore additions of TiB₂ to liquid iron alloys may provide a means of producing Fe–TiB₂ composite materials. However, the brittle properties of Fe₂B will mean that, whereas such materials may be very hard, they are likely to lack toughness.

MST/1477     

Studies of ceramic-liquid metal reaction interfaces

An investigation has been made of the nature and extent of chemical reactions between various liquid metals and a range of engineering-grade ceramics typically used as cutting tool inserts. Such possible reactions are relevant to chemical wear effects during metal cutting but also relate to liquid metal containment by ceramics and ceramic-metal joining. The experimental procedure has involved immersing pre-polished ceramic sections in liquid metals for controlled times with subsequent sectioning and examination of the reaction interface. The ceramics studied were two alumina-based materials and five silicon nitrides and sialons. The metals were pure iron, pure nickel and four iron-nickel alloys (a mild steel, a stainless steel and two nickel-based superalloys) and span a range of Fe-Ni compositions. The reaction rates of the alumina materials were found to be much lower than those of the silicon nitride-based materials and reflect the chemical stability of the Al-O bond array. Zirconia-toughened alumina showed little evidence of reaction with clean iron alloys but substantial attack by oxygen-containing iron-based materials was found resulting in the formation of iron-aluminium spinel reaction products. Al₂O₃-TiC/N exhibited preferential metal attack of the carbonitride phase with dissolution and/or replacement of the TiC/N dispersion. Within the silicon nitride-based group, ferrous alloys were found to be more damaging than mainly nickel alloys and silicon nitrides were more readily attacked than sialons. The difference in behaviour between the sialons and silicon nitrides is attributed to alumina additions in the former group of materials increasing resistance to attack by molten metals. A detailed mechanism of attack for these mixed-phase ceramics is proposed whereby a silicon concentration gradient is established from the crystalline ceramic phases, through the glassy binding phase, to the metal. The result is dissolution of the crystalline phase and an increase in volume fraction of the glassy binder at the metal-ceramic interface with concomitant progressive disruption of the ceramic microstructure.     

The interaction between silicon nitride and several iron,nickel and molybdenum-based alloys

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile.Trade Mark of the United Kingdom Atomic Energy Authority.Trade Mark of Henry Wiggin and Co. Ltd.Trade Mark of Union Carbide Corporation.     

Synthesis,electron transport properties of transition metal nitrides and applications

To understand the electron transport properties of transition metal nitrides (MN), electronic structure relationship between metal and corresponding nitrides is important. In binary nitrides, when nitrogen atoms occupy interstitial sites of metal lattice, volume expansion started initially without changing structure of metal lattice. Above certain concentration of nitrogen into interstitial sites of lattice, the system starts stabilizing its energy to minimum that in turn changes to another crystal structure. The chemical bonding in MN is due to the mixing of d-orbitals of M and p-orbitals of N. This is confirmed theoretically and experimentally such as X-ray photoelectron spectroscopy. The Fermi energy is generally lowered by the introduction of vacancies. However, reports on the particle size effect in the electrical resistivity of nitrides are scanty. One reason is that the role of the particle size in resistivity is difficult to determine because there is a need to understand N concentration. It poses a challenge to the synthesis of nanostructured transition metal nitrides. The transition metal binary nitrides show unusual electron transport, optical and magnetic properties as compared to their metal counterparts. Electronic properties of all transition metal nitrides known till date are discussed. Different ways of synthesis of nitrides and their applications are mentioned.     

Morphological studies of coking on heat-resistant alloys

The deposition of coke from a propylene-hydrogen mixture on to a range of austenitic Fe-Cr-Ni base alloys has been studied. Morphological investigations reveal the formation of a chromium-rich carbide layer on the surface of the alloys. This layer is initially protective, but eventually develops defects from which coke filaments grow. The formation of these filaments is catalysed by the chromium-depleted metal which becomes accessible to the gas following failure of the carbide layer. Once catalytic coke formation commences, it is maintained by the presence within the coke of small metal particles rich in iron and nickel.     

High-Pressure Processing and Characterization of Fe–High-C/N Alloys

A new process has been developed that results in (i) enhanced nitrogen addition to ferritic iron–carbon alloys and (ii) melt-casting in a single operation. This new processing technique enables Fe–C alloys to retain high nitrogen interstitial concentrations and to reduce significantly, and possibly eliminate, carbide formation. In this study two commercial-grade, steel alloys were cast under elevated nitrogen pressures, resulting in solid solution (austenite, ferrite, and martensite) high-carbon and high-nitrogen iron alloys that were, within detection limits, carbide- and nitride-free. These alloys were subsequently thermally processed to transform part of the retained austenite to martensite. The microstructure and mechanical properties of the alloys were studied as a function of carbon and nitrogen composition and as a function of thermal processing. The retain high nitrogen concentrations in these cast and processed iron–carbon alloys resulted in a substantial improvement in compression strengths.     

Nitrogen Addition to an O-1 Tool Steel

A new processing technique makes nitrogen alloying possible by adding nitrogen under elevated nitrogen pressure to prealloyed Fe-C ingots during continuous casting, producing a whole new class of precipitation-free, iron–carbon–nitrogen alloys. When both carbon and nitrogen bulk concentration levels exceeded 0.5 wt%, a duplex fcc-/(bcc-bct-) Fe microstructure resulted that is iron carbide- and nitride-free. With increasing carbon and nitrogen concentrations, there was an increase in the retained fcc-Fe phase. In cooling rate studies, increasing carbon and nitrogen concentrations shifted the knee of the fcc-Fe-to-bcc-Fe phase time–temperature–transformation (T–T–T) curve to longer times. Hardness, compression strength, and wear resistance increased with increasing carbon and nitrogen concentrations and were superior to iron–carbon alloys without the nitrogen addition.     

Surface layers produced by ion nitriding of austenitic Fe-Mn-Al alloys and the effects on hardness and corrosion resistance

Austenitic (fcc) Fe-Mn-Al alloys were nitrided in the temperature range 723 to 973 K by a glow discharge method using H₂-N₂ gas mixtures, and the microstructures and compositions of the nitrided layers were investigated in detail. A thin surface layer and a thick nitrogen-diffused layer (internal nitrided layer) were produced; the former consisted of both manganese nitrides and a ferritic (bec) phase, while the internal nitrided layer had an austenitic structure with the incorporated excess nitrogen atoms. X-ray photoelectron spectra revealed that the incorporated nitrogen atoms have a strong interaction with aluminium lattice atoms. By nitriding, hardness values of the Fe-Mn-Al alloys were increased fromH _v = 350 to ca. 1000, and the corrosion resistance to sulphuric acid solution was enhanced by a factor of 30. These improvements are associated with a uniform distribution of the incorporated nitrogen and also with the presence of nitride-like interactions between the nitrogen and aluminium atoms.     

原料配比对多孔MgO/Fe-Cr-Ni复合材料性能的影响

铁铬镍合金具有良好的高温强韧性和抗蠕变性,广泛应用于航空发动机、工业燃气轮机等设备中。利用原位合成和无压烧结工艺制备nano-MgO(34.9%)/Fe-Cr-Ni(质量分数,下同),micro-MgO(34.9%)/Fe-Cr-Ni,micro-MgO(25.7%)/Fe-Cr-Ni,micro-MgO(17.0%)/Fe-Cr-Ni 4种多孔复合材料。采用DTA,XRD和SEM等测试方法,研究不同MgO晶粒大小和含量对多孔金属基复合材料的常温和高温抗弯强度、高温抗氧化性能的影响。结果表明,在相同的烧结工艺条件下,不同原始粉末引起不同的反应热焓,导致烧结后得到的试样具有不同的气孔率和相组成。制备出的多孔复合材料主要由Cr_0.7Fe_0.36Ni_2.9,Cr-Fe固溶体的金属基体和MgO陶瓷增强相组成。当温度从室温升至1000℃左右,4种复合材料的抗弯强度值均随着温度的升高而降低,但其抗氧化性能优异。在4种多孔复合材料中,多孔nano-MgO(34.9%)/Fe-Cr-Ni复合材料的高温抗氧化能力优于其他3种多孔复合材料。     

真空退火对原位Al2O3/Fe-Cr-Ni增强复合材料性能的影响

铁铬镍合金具有良好的高温强韧性和抗蠕变性,被广泛应用于制造航空发动机、工业燃气轮机等设备。利用原位合成和热压烧结工艺制备Al2O3/Fe-Cr-Ni复合材料。为减少脆性相对复合材料性能的影响,将热压烧结试样在1000℃下真空保温2h后退火。采用XRD和SEM等测试方法,研究热处理后Al2O3/Fe-Cr-Ni复合材料的微观结构和常温力学性能。结果表明:Al2O3/Fe-Cr-Ni复合材料主要由Fe-Cr-Ni合金相、Fe-Cr相和Al2O3陶瓷增强相组成。热压烧结试样的维氏硬度、抗弯强度和断裂韧度分别为4.16GPa、298.31MPa和8.04MPa·m1/2。经1000℃高温热处理后,复合材料中Fe-Cr相发生奥氏体转变和合金基体晶粒长大,导致硬度下降至2.98GPa。Fe-Cr-Ni合金基体中韧性相含量和基体连续性增加,使该复合材料的抗弯强度和断裂韧度明显上升,其值分别为459.33MPa和12.81MPa·m1/2。     

Fabrication of Lotus‐type Porous Metals and their Physical Properties

Lotus‐type porous metals whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification in a pressurized gas atmosphere. The pores are formed as a result of precipitation of supersaturated gas when liquid metal is solidified. The lotus‐type porous metals with homogeneous size and porosity of the evolved pores produced by a mould casting technique are limited to the metals with high thermal conductivity. On the other hand, the pores with inhomogeneous pore size and porosity are evolved for metals and alloys with low thermal conductivity such as stainless steel. In order to obtain uniform pore size and porosity, a new “continuous zone melting technique” was developed to fabricate long rod‐ and plate‐shape porous metals and alloys even with low thermal conductivity. Mechanical properties of tensile and compressive strength of lotus‐type porous metals and alloys are described together with internal friction, elasticity, thermal conductivity and sound absorption characteristics. All the physical properties exhibit significant anisotropy. Lotus‐type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength.     

Corrosion resistance enhancement of marine alloys by rapid solidification

Copper-nickel alloys used in marine applications, are known for their anti-fouling properties. However, they are generally of low strength and are moderately susceptible to corrosion when used in a marine environment. Attempts at adding iron to Copper-nickel alloys by conventional ingot metallurgy, to improve their mechanical and corrosion resistant properties, have met with limited success. In this work, rapid solidification technology was employed to produce rapidly solidified (RS) Cu-10Ni and Cu-10Ni-8Fe. It was found that both the RS Cu-10Ni and Cu-10Ni-8Fe exhibited superior mechanical and corrosion resistant properties, compared with their sand-cast counterparts. Furthermore, the addition of iron to Cu-10Ni alloy, produced by RS, increased the corrosion resistance of the alloy, whereas the addition of iron to Cu-10Ni alloy produced by conventional means, had an adverse effect.