Cr 3C Research Articles

High-temperature corrosion of recuperators used in steel mills

High-temperature corrosion is a serious problem on the heat exchanger pipes of recuperators used in steel mills, because they encounter severe corrosive environments at high temperatures. These pipes are found to be corroded via oxidation, sulfidation, and molten salt corrosion. Molten salt corrosion occurs by the formation of alkali-iron sulfate as reaction products of sulfur oxide in fuel gas and sodium chloride from seawater. Many integrated steel mills are located near the sea for convenience of transportation. Salt from the seawater can affect the corrosion of the heat exchanger pipes by inducing hot corrosion. Through inspection of the used pipes of recuperators, a corrosion mechanism of the heat exchanger pipes is proposed. To evaluate the corrosion resistance of possible protective coatings for recuperators, nickel- and cobalt-based self-fluxing alloys, iron-based alloys, and chromium carbide cermet coatings were tested. The coatings with high chromium content, either as a form of carbide or as an alloying element, had excellent corrosion resistance. A Cr 3C 2–NiCr coating in carbide form, and Ni–45Cr–4Ti as an alloy coating can be recommended as promising coatings for the heat exchanger pipes, even in cases where they may encounter molten salt corrosion attack. The microstructure of a coating is as important as the chemical composition of the coating material. The inter-lamella pores may act as passages for molten salts. Therefore, the coatings should be carefully sprayed so that the molten salts do not penetrate into the coating layer.

Wear rates and specific energies of some ceramic, cermet and metallic coatings determined in the Coriolis erosion tester

A series of ceramic, cermet and metallic coatings of known resistance to slurry jet erosion has been subjected to sliding bed wear using a 10 wt.% aqueous slurry of 80 grit crushed alumina particles in the Coriolis erosion tester. Wear scar cross-sections have been measured and specific energies for material removal calculated for these coatings as well as for some bulk solid materials for reference purposes. Thermal-sprayed coatings included high velocity oxygen-fuel (HVOF)-sprayed WC–12Co, Cr 3C 2/NiCr and 316L steel, plasma-sprayed (PS) Al 2O 3 and Al 2O 3/TiO 2 and arc-sprayed 440C steel. Pulsed laser clad coatings of two FeCrB based alloys were also tested. The Coriolis erosion tester consists of a 150 mm diameter rotor with a diametral passage in which a flat plate specimen is located on each side, equidistant from the rotation centre. Slurry, delivered to a central orifice, is constrained to flow outwards through the channel as the rotor is turned at speeds up to 7000 rpm, the erodent particles being pressed onto the test specimen surface by the Coriolis force. The cross-sectional area of the scar is measured as a function of distance from the rotation centre and, through a summation of an expression for frictional work done, a value for the specific energy for material removal (essentially a measure of erosion resistance) is calculated. The test method seeks to simulate the wear environment in slurry pumps and cyclones in which particles move rapidly over the test surface either singly or in a bed. The wear rate of a WC–Co cermet was found to be ∼1000 times lower than that of 316L stainless steel. The remarkable ability of the Coriolis erosion tester to discriminate in the erosion performance of different materials is discussed in terms of particle trajectories and the probable wear mechanism. Coriolis test results are compared with those for slurry jet erosion and the applicability of the two tests for erosion resistance ranking of hard coating materials is discussed.

Influence of abrasive particles on wear mechanism and wear resistance in sliding abrasion tests at elevated temperatures

The metal matrices (MMs) X2CrNiMo17-13-2 and NiCr20AlSi and the metal matrix composites (MMCs) of the latter MM with 30 v/o WC/W 2C or Cr 3C 2 as hard particles (HPs) were tested in sliding abrasion. The tests were carried out against flint, corundum and silicon carbide as abrasive particles (APs) at temperatures up to 900°C in argon. An increasing hardness ratio, H MM/ H AP, improves the wear resistance at room temperature. At elevated temperatures, the wear resistance of the MMs depends upon the formation of a self-protecting layer of embedded AP-fragments. The tests against flint and corundum revealed an increasing wear resistance up to a critical temperature T C and a sharp decrease above T C which is lower for corundum than for flint. MMs tested against SiC show a constant low wear resistance versus temperature. An important improvement in the wear resistance is achieved by the utilization of HPs in MMCs with a hardness ratio, H HP/ H AP, greater than 1.2.

Thermodynamic description of the system Ti-Cr-C

A thermodynamic description of the Ti-Cr-C ternary system is obtained based on a critical evaluation and limited experimental verification of the literature data reported on the ternary system and its binary subsystems. Using a minimum of only 3 adjustable parameters these calculations allow us to reproduce the experimentally determined solidification sequence, the invariant reaction temperatures, and the compositions of the participating liquid phase. The only exception is the reaction temperature for the ternary eutectic L ↔ TiC + Cr 7C 3 + Cr 3C 2. This discrepany seems to be connected with the difficulty to calculate the binary eutectic L ↔ Cr 7C 3 + Cr 3C 2 indicating that the simple model used for the description of the liquid phase should be replaced by a more complex model to obtain a better fit for this reaction temperature.

Sintering and microstructural development in WC/Co-based alloys made with superfine WC powder

Superfine WC powder can be sintered into WC/Co materials with an average grain size of 0.2 μm, coercivity of 600 Oe and hardness of 2200 kg/mm 2 using conventional processing methods. The characteristics of this WC powder are discussed along with the effects of milling time, cobalt particle size, grain growth inhibitor content and sintering temperature on microstructural development and mechanical properties of a WC–Co alloy. A processing package for superfine WC powder, consisting of a series of contour plots, summarize how the properties of a WC/Co material varied as a function of vanadium carbide (VC) and chromium carbide (Cr 3C 2) content and sintering temperature. The effects of admixed versus co-carburized grain growth inhibitor additions in a superfine WC/Co formulation are also investigated. Admixed grain growth inhibitors of VC and VC (>0.3 wt%)/Cr 3C 2 combinations are just as effective as their co-carburized counterparts.

High temperature sliding abrasion of a nickel-base alloy and composite

In previous work, sliding abrasion by interfacial flint particles was studied for a nickel-base MMC under argon up to 900°C. In the present investigation flint is replaced by corundum and silicon carbide. The hot hardness of the latter is above that of the NiCrAlSi-matrix and the WC/W 2C particles, which dramatically lowers the wear resistance of the MMC compared to abrasion by flint. Corundum is softer than tungsten carbide and the drop of wear resistance is less severe. In air, WC/W 2C is readily oxidized, lowering the wear resistance to matrix level. In comparison, Cr 3C 2 shows lower hot hardness but much better resistance to oxidation. A self-protecting particle layer of material debris and abrasive particles is formed under argon as well as in air, but at 800°C the oxidation of the debris in air entails a highly dense `glaze' layer.

Comparison of slurry and dry erosion behaviour of some HVOF thermal sprayed coatings

The performance of 10 high velocity oxy-fuel (HVOF) sprayed coatings has been evaluated under both dry particle and slurry erosion conditions at 90° and 20° impingement angles. The coatings were six WC cermets with Co- or Ni-based matrices, a Cr 3C 2–NiCr composite and three metallic alloys. Slurry jet erosion experiments were carried out using a 9 wt.% alumina/water slurry with 35 and 200 μm angular alumina particles, at an impingement velocity of 15 m/s and flow rate of 18 l/min in a re-circulating loop apparatus. Dry erosion was effected in a once-through mode with 50 μm diameter alumina particles at a velocity of 84 m/s and feed rate of 2 g/min. Dry erosion rates three orders of magnitude greater than slurry erosion rates were due mainly to the differences in actual particle-target impact velocities. Coating composition, microstructural integrity and hardness were the major determinants of relative erosion resistance in all tests, with matrix corrosion resistance influencing the surface damage resistance of cermets in longer duration aqueous slurry tests.

Carbothermal synthesis of vanadium and chromium carbides from solution-Derived precursors

Vanadium and chromium tartrate precursors prepared from aqueous solutions have been used as preceramic materials for carbothermal reactions with and without simultaneous nitridation. Their thermal behaviour has been investigated by TG/DTA, X-ray diffraction and measurement of surface areas. Under pyrolysis up to about 600°C, reactive composites consisting of intimately mixed carbon and amorphous M 2O 3 oxides are formed by salt decomposition. The subsequent processes of crystallization of oxides and carbothermal reduction render both the composites and the reduction products porous. The carbothermal reactions leading either to V 8C 7, a V(C,N,O) solid solution, and Cr 3C 2, respectively, proceed at moderate temperatures between 800 and 1100°C. The final products result as assemblages of fine carbide or carbonitride particles, however, with extensive heat treatment the particles grow and the surface areas diminish. Air oxidation of the final products has been studied by simultaneous TG/DTA.

Powders of Chromium and Chromium Carbides of Different Morphology and Narrow Size Distribution

Powders of Cr and Cr carbides (Cr 2C, Cr 7C 3, and Cr 23C 6) have been prepared by hydrogen reduction of CrO 1.9 and Cr 3C 2 powders of different morphology, texture and specific surface area. The Cr and Cr carbides particles have the morphology of the parent compound and a narrow size distribution. Depending on the shape, the average size is of the order of some micrometers or some tens of micrometers. Starting with Cr 3C 2 allows the preparation of Cr powders at a temperature (1000°C) markedly lower than that required when CrO 1.9 is used as the precursor (1300°C). However, a Cr 3C 2 specific surface area of the order of 10 m 2/g is necessary to achieve this within a reasonably short time (6 to 12 h). Powders of the lower carbides are prepared by using a milder hydrogen thermal treatment.

Wetting of silicon carbide by chromium containing alloys

We have studied the wetting behaviour of Cu–Cr, Cu–Ni–Cr, Cu–Si–Cr and Cu–Ni–Si–Cr alloys on sintered β-SiC. The addition of Cr to liquid Cu or Cu–Ni alloys significantly improves their wettability on SiC. The reactions between the Cu–Cr or Cu–Ni–Cr alloys and SiC lead to the formation of a duplex layer containing Cr 3C 2 and a mixture of Cu and C phases. The formation of this duplex layer can be suppressed by the addition of sufficient Si to the Cu–Cr alloys and the addition of Si does not significantly alter their behaviour. Cu–Ni–Si–Cr alloys also suppressed the formation of the duplex layer below some critical Cr concentration. A thermodynamic model developed for metal/oxide systems can be used to explain the behaviour of their alloys on SiC and also their behaviour on Si 3N 4.

Tribological properties of calcium fluoride-based solid lubricant coatings at high temperatures

Silver, calcium fluoride (CaF x with x=1.85) and chromium–carbon (Cr 3C 2) thin films have been deposited on various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF x single layers as well as Ag/CaF x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium–carbon films sputter-deposited on alacrite disk and counterfaces were found to be of interest to reduce the formation of alacrite wear debris in the wear tracks whereby reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF x /Cr 3C 2 thin bilayer structures was also investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C.

An investigation of the friction and wear behaviors of ceramic particle filled polyphenylene sulfide composites

The composites of polyphenylene sulfide (PPS) filled with 1–3 μm size ceramic particles of Al 2O 3, SiC, Si 3N 4, and Cr 3C 2 were prepared by compression molding. The hardness and the friction and wear behaviors of these composites were studied. For the latter a pin-on-disc friction and wear tester was used and sliding was performed between 5 mm × 6 mm pins and AISI 02 tool steel counterface at a speed of 1.0 m s −1 and a normal load of 19.6 N. The worn composite pin surfaces and the transfer films formed on the couterface were examined by scanning electron microscopy. An X-ray photoelectron specrometer (XPS) was used to characterize the chemical states of the elements in the transfer film for the possible tribochemical changes which occurred during sliding. It was found that SiC, Si 3N 4, and Cr 3C 2 as the fillers increased the wear resistance of PPS considerably while Al 2O 4 decreased it. The fillers promoted the decomposition of PPS and generated compounds which accounted for the changes in wear resistance of the composite. The fillers also caused abrasion and thereby disrupted the transfer film in some cases. XPS and SEM observations have been used to develop understanding for the changes in wear resistance of the composites with the use of fillers.

Hardness to toughness relationship of fine-grained WC-Co hardmetals

The hardness to toughness relationship of fine-grained WC-Co hardmetals was studied based on Palmqvist indentation toughness measurements. Sixty-five commercial and lab-sintered hardmetals of different composition, microstructure and manufacturing history were investigated to build up a representative hardness/toughness measurement band. This band is then used to discuss the influence of the various alloy- and process-related parameters on the hardness to toughness relationship of WC-Co composites. Beyond that, optimal hardness/toughness combinations can be assessed for the hardness range of 1400–2200 HV30. In general, the higher the hardness of the alloys, the longer were the indentation cracks, indicating a decrease in fracture toughness with increasing hardness. However, at a certain hardness, the toughness of individual alloys varied significantly. For example, at HV30:1670, the sum of crack lengths varied between 287 μm (high toughness) and 449 μm (low toughness), which corresponds to fracture toughness values of 11.5 and 9.2 MNm − 3 2 , respectively. Very fine-grained hardmetals (ultrafine grades) were shown to be not necessarily tougher than coarser grained alloys (submicron grades), in particular in the hardness range of 1450–2000 HV30, although they exhibit significantly more binder at a given hardness. Only in the high hardness range of > 2000 HV30 might they be of advantage. Samples, exclusively doped with Cr 3C 2 as growth inhibitor exhibit more favorable hardness/toughness combinations than comparable VC-doped alloys. However, other parameters, such as sintering temperature, sintering time, or the gross carbon content of the respective alloys must be taken into consideration for obtaining optimal hardness/toughness combinations.

Processing and properties of a 85%Cr 3C 2−10.5%Ni-4.5%Fe cermet

Sintering of the 85%Cr{sub 3}C{sub 2}-10.5%Ni-4.5%Fe cermet was carried out in both the solid-state (T 1,275 C) regimes. Samples sintered at temperatures above 1,275 C exhibit better density and hardness, owing to the liquid phase formed during sintering. The best mechanical properties and fine microstructures were obtained from the samples sintered at 1,300 C. Properties of solid-state sintered specimens are poorer, but improve with sintering time. Abnormal and elongated growths of carbide grains are favored by high sintering temperature and sintering time. The binder phase which is a solid solution of Ni-Fe-Cr{sub 3}C{sub 2} is distributed as separate islands in the skeleton of almost pure Cr{sub 3}C{sub 2} phase.

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF x with x = 1.85) and chromium-carbon (Cr 3C 2) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF x single layers as well as Ag/CaF x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF x /Cr 3C 2 thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF 2-CaF 2 eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.

Effect of milling conditions on the synthesis of chromium carbides by mechanical alloying

The synthesis of chromium carbides, Cr 7C 3 and Cr 3C 2, by mechanically alloying chromium and carbon powders has been investigated. Milling conditions were found to have a strong influence on the evolution of microstructure, with high collision energies being required to form carbide phases. Milling at intermediate energy levels resulted in the formation of an amorphous phase, and with low energy conditions only grain size refinement of Cr occurred with no evidence of any reaction between Cr and C. The amorphous phase was found to be the precursor to carbide formation.

Electrochemical corrosion behavior of SHS-synthesized d.c. magnetron composite TiC-based targets and sputtered thin films

A number of dense composite materials, based on titanium carbide, were synthesized using a coupled self propagating high temperature synthesis (SHS)-consolidation technique. These composite materials have been shown to posses both high hardness and good corrosion resistance. Thin films were produced from these dense composite materials by using them as targets in d.c. magnetron sputtering. The corrosion resistance of both the dense composite targets, and the resulting thin films in aqueous H 2SO 4 and HCl solutions was studied for the following systems: TiC 0.48; TiC 0.98: 80%TiC-20%TiB 2; 70%TiC-20%Ti-10%Mo; 60%TiC-10%TaC-20%Ti10%Mo; 60%TiC-12%Fe-8%Si-20%Mo; 72%TiC-18%Cr 3C 2-10%Ni; 60%TiC-10%Mo 2C-6%TiN-16%Ni 3Al-5%Mo-2%Cr. Increasing the TiB 2 content in either the target material or the thin film resulted in a decrease in the corrosion resistance, while the corrosion resistance of the thin films was two to three orders of magnitude higher than that of the corresponding bulk target material from which they were deposited. Thin films of 80%TiC-20%TiB 2, TiC 0.98, 60%TiC-10%TaC-20%Ti-10%Mo, and 60%TiC-12%Fe-8%Si-20%Mo were also compared under abrasive wear conditions in an aqueous medium containing SiC particles. The 80%TiC-20%TiB 2 film exhibited the highest corrosion resistance.

Propriétés tribologiques de revêtements et couches minces anti-frottement à haute température

Low friction coatings of tenth μm in thickness operating at high temperatures are composed of either metal oxides (PbO, CdO) or alkaline earth metal fluorides (CaF 2, BaF 2). These oxides and fluorides can be associated with other solid lubricant materials such as graphite or silver to extend the service temperature range from room temperature to about 800°C. Sliding contacts can also be lubricated by thin films of solid lubricant materials (silver, calcium fluoride) and Ag/CaF x ( x − 1.85) multilayer structures of 0.1 to a few μm thick produced by sputtering. The friction coefficient of these thin films was deduced from ball-on-disk tribological tests conducted at room temperature and 500°C in room air. The average friction coefficient value of silver and CaF x films in the range 0.14 to 0.42 was dependent on the film thickness and test temperature. The friction coefficient of Ag/CaF x multilayer structures was stable at about 0.35 at room temperature and 500°C. Furthermore, plan-to-plan tribological tests of thick (300 μm) plasma-sprayed coatings (PS-212 type) and CaF x thin films deposited on Cr 3C 2 underlayers were performed in room air at 500°C and 700°C. The friction properties of these thick coatings and thin films were determined and compared. The excellent performance of lubricant thin films demonstrates the interest of sputter-deposited Ag and CaF x films in lubrication of precision tolerance mechanical components operating in hostile environment.

Encapsulation of carbides of chromium, molybdenum and tungsten in carbon nanocapsules by arc discharge

Encapsulation of Cr, Mo and W in multilayered graphitic cages by arc evaporation of metal/carbon composites under different pressures of helium gas (100, 600 and 1500 Torr) has been studied. Electron microscopy and X-ray diffraction of carbonaceous products revealed that encapsulated crystallites were carbides, i.e. Cr 7C 3 and Cr 3C 2 for Cr; Mo 2C, δ-MoC 1− x (NaCl type) and γ′-MoC (AsTi type) for Mo; W 2C and β-WC 1− x (NaCl type) for W. Effect of the pressure of helium gas on the formation of filled nanocapsules was markedly found for tungsten: Encapsulated tungsten carbides were formed at the highest pressure, but not in the lower pressures.

Production capability and powder processing methods for nanostructured WC-Co powder

Industrial scale production of nanostructured WC-Co powders (NANOCARB ® Powder) with WC grains of < 50 nm uniformly dispersed in the metallic Co binder phase has been established with the spray conversion process. The patented production technology has demonstrated process capability to average Cpk levels > 1.5 for critical parameters, such as, grain size, carbon balance, impurities and composition. Conditions have been established for milling and sintering to yield flaw-free products with porosity rating of < A01 B00 C00. Use of VC and Cr 3C 2 grain growth inhibitors results in mean sintered grain sizes of < 0.25 μ and down to 0.15 μ in specialized sintering conditions. The hierarchical structure of millions of nanometer WC grains uniformly mixed with the metallic binder results in enhanced stability via reduced reactivity on exposure to air. The friable particle structure is milled to particle fragment sizes of < 1 μ for consolidation. Oxygen pick-up during milling is selectively removed via a hydrogen reduction step in order to eliminate residual porosity.