The formation of tungsten carbide in cobalt-base wear-resistant coating alloys

Tungsten carbide-cobalt alloys cannot be produced by melting because of a peritectic reaction in the W-C system; they are produced by sintering. Tungsten carbide-cobalt powders (mixed, agglomerated, sintered) can be plasma sprayed or deposited by other techniques but they cannot be fused afterwards without decomposition of the tungsten monocarbide that provides the best mechanical properties for many applications.Wear-resistant cobalt alloys were developed 60 years ago and are based on cobalt-chromium-tungsten-carbon. During studies of the CoCrWC system with various carbon concentrations and at various temperatures we identified MC, M₂C, M₇C₃, M₂₃C₆, M₆C, M₁₂C and M₂₈C carbides. The solidifying M₆C carbide is unstable over a certain concentration range of chromium and decomposes to form tungsten carbide (WC). On heat treatment the tungsten-containing M₆C forms WC in a cobalt-chromium matrix if the chromium content is less than 5 wt.%. It is therefore possible to produce a sprayed and fused or welded layer of WC-cobalt alloy. The rate of WC formation depends on temperature and time. Time-temperature-decomposition diagrams have been established for four alloys. The structures of the heat-treated alloys are similar to those of sintered tungsten carbide-cobalt alloys.     

SURFACE ALLOYING OF MILD STEEL BY LASER MELTING OF AN ELECTROLESS NICKEL DEPOSIT CONTAINING CHROMIUM CARBIDES

It is possible to realize surface alloys by laser melting an electroless nickel layer containing chromium carbide particles predeposited on a mild steel substrate. By this way the surface alloy is expected to have not only a high nickel content but also an important chromium content in order to improve the corrosion resistance. The presence of chromium in solid solution results from the dissolution or melting of the carbide particles. Typical laser solidification microstructures are obtained. Dendrites consist of an austenitic Fe-Ni-Cr solid solution and interdendritic regions are constituted by an eutectic mixture containing the same austenitic solid solution and complex Fe, Ni, Cr carbides and phosphides. In comparison with a surface alloy obtained by laser melting of an electroless nickel layer without carbide particles, the corrosion resistance was slightly improved in saline aqueous solutions. The limited effect was due to the fact that the final chromium content in the present experimental conditions was not as high as that initially expected.     

Effect of thermal aging on pitting corrosion resistance of 16Cr – 5Ni – 1Mo precipitation hardening stainless steel

Abstract

Specimens of precipitation hardening 16-5-1 stainless steel were solution treated at 1050°C for 1 h followed by aging at temperatures in the range 400 – 750°C for various holding times (1 – 16 h). After heat treatment, two types of corrosion test (accelerated and immersion testing) were conducted in 6% ferric chloride solution. The results showed that the pitting corrosion resistance was affected by austenite content, δ ferrite and precipitation of molybdenum and chromium carbides. Three critical temperature ranges were identified, which were related to the phases formed: (a) high corrosion rate at 475°C (δ ferrite and Mo₂ C); (b) low corrosion rate at 550 – 625°C (reversed austenite and Laves phase); (c) intermediate corrosion rate at 750°C (Cr₂₃ C₆ and TiC). The morphology of the pitting was dependent on the form of the δ ferrite and carbides.     

Fe-Cr-C hardfacing alloys for high-temperature applications

The microstructure and phase chemistry of a Fe-34Cr-4.5C wt% hardfacing alloy has been investigated using transmission electron microscopy and microanalytical techniques. The microstructure is found to consist of large primary M₇C₃. carbides in a eutectic mixture of austenite and more M₇C₃. The results indicate that the microstructure of the undiluted alloy becomes configurationally frozen at a temperature of about 1150° C during deposition by the manual metal arc welding technique. This allows the metastable austenite phase to contain a large chromium concentration ( 16 to 17 wt %), thus imparting good corrosion and oxidation resistance. Experimental data on the partitioning of chromium, manganese and silicon between the carbide phases are discussed in the context of the high-temperature stability of the alloy.     

High chromium cobalt-base coatings for low temperature hot corrosion

Accelerated oxidation tests have been conducted on a number of cast Co-Cr-Al alloys coated with a thin film of Na₂SO₄ and exposed in an O₂-0.15%(SO₂-SO₃) gas mixture at 750°C. These tests show that the corrosion resistance of binary Co-Cr alloys increases with the chromium content of the alloys and that even small concentrations of aluminum have detrimental effects on their corrosion resistance. Tests on Co-Cr coatings on a René 80 substrate under the same conditions indicate that there is a transition from a high to a low corrosion rate at a chromium level of about 37.5 wt.%. These results can be explained on the basis of a mechanism that we have already proposed in the literature. Based on our laboratory studies, we have developed new Co-Cr coatings that provide excellent resistance to low temperature hot corrosion and are also acceptable for conventional, high temperature, hot corrosion.     

Nickel-based wear-resistant coatings by vacuum melting

In various coating processes nickel-based hard alloy powders are applied in either an atomized or a mechanically pulverized form. The coatings show good corrosion resistance, a high abrasion resistance and a relatively low melting point. Since boron is sparingly soluble in nickel, the boride Ni₃B forms a low melting eutectic at about 1000 °C with the nickel solid solution. Boron and silicon act simultaneously as deoxidizers and improve both the properties of the coating and the bonding to the substrate. Diffusion into the substrate occurs during the coating procedures.In this paper we discuss the behaviour of heterogeneous powder compounds with nickel hard alloys. For many applications mixtures with various carbides are used. During coating, reactions take place between the Ni-Cr matrix and the added carbides. These heterogeneous or quasi-alloys, which are used because of their abrasion resistance, are metastable.The abrasion resistance depends primarily on the phases as well as on their grain size, the grain size distribution and alterations to the matrix. When carbidic quasi-alloys are exposed to abrasion and corrosion at high temperatures, reactions of the existing phases during use cannot be completely excluded. Heat treatment causes changes in the structure and abrasion resistance of carbide-containing quasi-alloys.It is difficult to follow reactions which take place during coating either in the fused mass of quasi-alloys or in heterogeneous compounds. Because of their relatively low melting points nickel-based hard alloys can be coated by furnace melting. Hence carbide compounds with Ni-Cr-B-Si powder alloys are most suitable for research also. In these mechanical alloys segregation, of relevance to practical applications, can be studied as well as the formation of various phases during the coating or heat treatment processes.The behaviour of mixtures of Ni-Cr-B-Si powder alloy with different amounts of a number of carbides is reported. The structures of the resulting phases were studied and we tried to correlate these with the results of our abrasion tests.For tungsten carbide-nickel hard alloy mixtures the formation of the ? phase is influenced by the coating parameters and the matrix as well as by the diffusion of iron from the substrate into the coating.We also investigated mixtures of an Ni-Cr-B-Si matrix with TiC (Ti, W)C and NbC. The wear resistance against steel and SiC was measured. Various wear mechanisms and the properties of the carbide-matrix interface the wear results.     

Effect of cobalt content on microstructure and property of electroplated nickel‐cobalt alloy coatings

Nickel‐cobalt alloys were electrodeposited on copper sheets in sulfate bath containing 288.5 g/l NiSO₄·6H₂O, 30 g/l CoSO₄·7H₂O, 40 g/l HBO₃, 15 g/l NaCl and 0.08 g/l lauryl sodium sulfate. The effects of cobalt content on microstructure, microhardness, and wear resistance of electroplating nickel‐cobalt alloys were studied by using SEM and XRD techniques, and microhardness tester and wear tester. The relationship between the microhardness of nickel‐cobalt alloy coatings and heat treatment procedures was also investigated. The experimental results show that cobalt content (W_t) in coating increases with Co²⁺/(Co²⁺ + Ni²⁺)% (X) in plating solution. Fitted regression equation is as following: W_t = –0.7399 + 2.2847X – 0.0133X². The increase of cobalt content leads to that the longitudinal section morphology of coating transforms from the cone into sphericity and at last into the shape of willow leaf, and its structure transforms from face centered cubic (fcc) nickel solid solution into fcc cobalt solid solution and at last into hcp cobalt solid solution. The increase of cobalt content results in the increase of microhardness of nickel‐cobalt alloy coatings, and the hardness reaches a maximum value (363 HV) when cobalt content is 54.9%. After heat treatment at 400°C and 600°C, the microhardness of coatings begins to decrease except the coating containing 79.2% Co. Moreover, the wear resistance of electroplated coatings increases with the increase of cobalt content.     

Oxidation and corrosion behaviour of mild steel laser alloyed with nickel and chromium

Surface alloys were made on mild steel, coated with nickel and chromium using laser surface alloying. Mild steel was coated with a composite layer of nickel and chromium using the plasma technique. This was followed by laser irradiation using a continuous carbon dioxide laser. Oxidation and corrosion behaviour of these alloys was then determined by carrying out oxidation in air at 800 °C and corrosion tests at room temperatures in 1 n H₂SO₄. With a 75 m layer of nickel and chromium each, it was possible to make surface alloys on mild steel, which had a chromium concentration of 6–7 wt%, but the nickel concentration varied from 10–20 wt%. Oxidation behaviour improved significantly over the as-coated specimen and aqueous corrosion improved considerably.     

Long-term ageing characteristics of some commercial nickel-chromium-molybdenum alloys

The long-term ageing characteristics of some commercial Ni-Mo-Cr alloys (the high-temperature HASTELLOY alloy S and the corrosion resistant HASTELLOY alloys C-4 and C-276) at 810 K were investigated. It was found that the three alloys undergo the following long-range ordering reaction: disordered f c c lattice ordered orthorhombic, Pt₂Mo-type superlattice. Ordering was found to cause considerable strengthening without severe loss of tensile elongation. Deformation in the ordered state occurred predominantly by twinning. The corrosion rates of alloys C-4 and C-276 in boiling sulphuric-ferric sulphate solution did not seem to be greatly affected by the long-range ordering reaction. In addition to ordering, the three alloys were also found to undergo grain boundary reactions. The resulting phase in alloys S and C-4 assumed a dispersed morphology and was identified as carbide, probably of the Type M₁₂C. In alloy C-276, however, which contains higher amounts of iron and tungsten, the boundary precipitate was in the form of a continuous layer consisting of M₁₂C and Mu-phase. This could account for the reduced tensile elongation of alloy C-276 relative to alloys S and C-4 and also to its high corrosion rate.     

Carbides in iron-rich Fe-Mn-Cr-Mo-Al-Si-C systems

Duplex microstructures of iron-base superalloys, consisting of an austenitic matrix and a M₇C₃ carbide, can be produced within the Fe-Mn-Cr-Mo-Al (Si)-C systems. The stability regions of the carbides were inspected by means of isothermal sections of alloys in the quinary combination Fe-Mn-Cr-Mo-C for 70 at% metal and 30 at % carbon. For 35 at % iron the competing carbides are found to be M₂C, M₃C and the molybdenum cementite (MoFe₂C) in the arc-melted state, with M₂3C₆ in the annealed state. In the quaternary system, Fe-Mn-Mo-C, a M₂C carbide forms, presumaby derived from a solid solution carbide, (Mn, Mo)₂C. In extracted carbides of cast alloys containing Fe-Mn-Cr-Mo-Al (Si)-C a considerable amount of the -phase carbide occurs.     

Simulation of road salt corrosion in austenitic alloys for automotive exhaust systems

Abstract

Cyclic oxidation tests in air with intermittent salt spraying have been performed to simulate the conditions of road salt (NaCl–CaCl₂) enhanced corrosion in automotive exhaust systems.

Tests were carried out at 600 and 700°C on three austenitic alloys, including two stainless steels currently employed for exhaust components (AISI 316 Ti and AISI 302B) and a higher nickel heat resisting alloy.

The presence of salt causes internal corrosion, both along a regular front beneath an outer oxide scale and down alloy grain boundaries. An increase in temperature accelerates the corrosion rate and particularly enhances intergranular penetration.

The results of micrographic and microanalytical investigations are in general agreement with an active oxidation mechanism in which Na_xCl_x vapour species, and not only chlorine, appear to play an important role. The regions directly affected are depleted in chromium and iron and enriched in nickel.

Although internal oxidation of silicon is observed, a high silicon content (2%) does not necessarily ensure effective protection against this type of attack.     

New results on the oxidation and hot corrosion of silicide overlay coatings on nickel-based alloys

Silicon-containing coatings with a matrix consisting of the saturated γ solid solution Ni₆Cr₂Si₂ and which also contain intermetallics of complex tantalum silicides as precipitates show great promise as protective layers for nickel-based alloys with improved oxidation and hot corrosion resistance. We present results of oxidation tests performed at 1000°C for up to 4000 h in air and of hot corrosion tests in burner gas with intermediate dipping in Na₂SO₄ and V₂O₅ melts. From silicon diffusion data, lifetimes greater than 10 000 h at 1000°C can be expected.     

Passivity,pitting and corrosion of anodically polarized Fe-Ni alloys in 0.5 M H2SO4 containing Cl−

Iron-nickel alloys containing 0, 20, 40, 60, 80 and 100% Ni (wt%) have been anodically polarized in 0.5 M H₂SO₄ containing Cl^?, and the conditions for passivity, pitting and corrosion with respect to alloy composition and Cl^? concentration broadly defined. Breaks occur in the values of the corrosion properties at about 30% and 70% Ni. It is considered that the corrosion properties of alloys containing up to 30% Ni are determined by the ferrite in the alloy and the low corrosion resistance of its surface film, that alloys containing 30 to 70% Ni have a corrosion resistant film probably similar to a nickel ferrite spinel, and that alloys containing over 70% Ni have properties similar to nickel and probably have a surface film based on a solid solution of iron in NiO.     

Alkaline corrosion resistance of slag-free self-shielded titanium-added metal-cored welding overlay

The corrosion resistance of slag-free self-shielded metal-cord welding overlay with different ferrotitanium additions in alkaline solution (pH=14) was investigated. The results show that the self-corrosion potential E_corr is the highest and the self-corrosion current density I_corr is the minimum when the addition of ferrotitanium is 6 wt. %. titanium carbide corrodes before other carbides in alkaline solution due to the absence of iron. A ferrotitanium addition of 6 wt. % can provide the best corrosion resistance to the fabricated welding overlays, as an increased ferrotitanium addition will result in a decrease in chromium carbide and an increase in titanium carbide, of which titanium carbide is the most prone to corrosion, followed by chromium carbide, and finally the austenitic matrix.     

Thermisches Spritzen vanadiumkarbidverstärkter Eisenbasisschichten

Thermal spraying of vanadium carbide reinforced iron based HVOF sprayed cermet coatings for wear protection like WC‐Co(Cr) and Cr₃C₂‐NiCr have found a broad range of applications in the past. By using chromium containing matrices, they exhibit good corrosion resistance along with outstanding wear resistance. In present research in the area of powder metallurgy and PTA welding iron based alloys with high content of chromium and vanadium have been developed, revealing similar properties and therefore being a cost efficient alternative to established cermet coatings. HVOF sprayed coatings of these iron based alloys are investigated regarding their economic applicability and their corrosion and wear properties.     

Oxidation of chromia forming molybdenum-tungsten based alloys

The oxidation behaviour of tungsten and molybdenum based, chromia-forming alloys prepared by powder sintering activated with group VIII metals has been investigated. The influences of the alloy composition, nature of the sintering agent and oxidation temperature have been studied. A good oxidation resistance is observed with palladium as sintering agent. This metal is rejected at the grain boundaries and allows a fast diffusion of chromium to the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample. The formation of a two-phase interface enriched in nickel leads to a non-protective oxide layer constituted with Cr₂O₃ and NiWO₄.Catastrophic oxidation is observed when the refractory metals are oxidised into volatile oxides, i.e. in the case of the alloys with a high molybdenum content. Contrary to molybdenum, a high tungsten level leads to high oxidation resistance, even at temperature as high as 1300°C. In this latter case, alloys are two-phase: this result has led to the investigation of the ternary section of the Cr-Mo-W system at 1300°C.     

Carbothermic reduction of cobalt and nickel tungstates

Data are presented on the formation sequence of metallic, carbide, and intermediate oxide phases in the carbon reduction of CoWO₄ and NiWO₄. The surface reaction between CoWO₄ particles and solid carbon yields the mixed carbide Co₆W₆C, while the reaction with the CO originating from carbon vaporization yields the intermetallic phase Co₇W₆. The initial stage of the solid-state and gas-phase reduction of NiWO₄ yields a solid solution of tungsten in nickel (～10 at % W), Ni₄W, and, presumably, the NiWO₄-WO₃ eutectic. The solid solution reacts with carbon to form Ni₂W₄C and with CO to form filamentary tungsten crystals.     

Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

Solid state phase transformations in Ti–Al–C and Ti–Al–Nb–C alloys

Abstract

Results are reported of an investigation of solid state transformations in a series of α₂ based alloys having an aluminium content of 26 at.-% with carbon up to 3 at.-%; two α₂ basedquaternary Ti–Al–Nb–C alloys with 5 and 12 at.-%Nb and 3 at.-%C were also studied. Ordering occurs in the ternary Ti–Al–C alloys and also in the 23Al–5Nb–3C alloy on quenchingfrom 1250°C. Additional carbide precipitation was not observed in the ternary Ti–Al–C alloys on reheating to 750°C. Additions of niobium resulted in the presence of the β phase at 1050°C in the 5%Nb alloy and at 1050 and 750°C in the 12%Nb alloy. In the quaternary Ti–Al–Nb–C alloys, (Ti, Nb)₃AlC was found to be the primary phase and was present in the microstructure over the temperature range studied. In the 21Al–12Nb–3C alloy, the ordered β phase transformed to α″₂ martensite on quenching from 1250;amp;#x00B0;C.

MST/1306     

The influence of vanadium on fracture toughness and abrasion resistance in high chromium white cast irons

The influence of vanadium on wear resistance under low-stress conditions and on the dynamic fracture toughness of high chromium white cast iron was examined in both the ascast condition and after heat treatment at 500 °C. A vanadium content varying from 0.12 to 4.73% was added to a basic Fe-C-Cr alloy containing 2.9 or 19% Cr. By increasing the content of vanadium in the alloy, the structure became finer, i.e. the spacing between austenite dendrite arms and the size of massive M₇C₃ carbides was reduced. The distance between carbide particles was also reduced, while the volume fraction of eutectic M₇C₃ and V₆C₅ carbides increased. The morphology of eutectic colonies also changed. In addition, the amount of very fine M₂₃C₆ carbide particles precipitated in austenite and the degree of martensitic transformation depended on the content of vanadium in the alloy. Because this strong carbide-forming element changed the microstructure characteristics of high chromium white iron, it was expected to influence wear resistance and fracture toughness. By adding 1.19% vanadium, toughness was expected to improve by approximately 20% and wear resistance by 10%. The higher fracture toughness was attributed to strain-induced strengthening during fracture, and thereby an additional increment of energy, since very fine secondary carbide particles were present in a mainly austenitic matrix. An Fe-C-Cr-V alloy containing 3.28% V showed the highest abrasion resistance, 27% higher than a basic Fe-C-Cr alloy. A higher carbide phase volume fraction, a finer and more uniform structure, a smaller distance between M₇C₃ carbide particles and a change in the morphology of eutectic colonies were primarily responsible for improving wear resistance.