Carburization of ferrochromium metals in chromium ore fines containing coal during voluminal reduction by microwave heating

Chromium ore fines containing coal (COFCC) can be rapidly heated by microwave to conduct the voluminal reduction, which lays a foundation of getting sponge ferrochromium powders with a lower content of C. Under the conditions of COFCC with n(O):n(C) (molar ratio) as 1.00:0.84 and n(SiO₂):n(CaO) as 1.00:0.39, the samples were heated by 10 kW microwave power to reach the given temperatures and held for different times respectively. The results show that the low-C-Cr ferrochromium metal phase in the reduced materials forms before the high-C-Cr ferrochromium metal phase does. With increasing temperature the C content of ferrochromium metals is in a positive correlation with the content of Cr. The C content of ferrochromium metal in reduced materials is 0–10.07% with an average value of 4.68%. With the increase of holding time the Cr content in ferrochromium metals is in a negative correlation with the content of C, while the content of Fe changes in the contrary way. In the microwave field the kinetic conditions of carburization are closely related with the temperature of microwave heating, holding time and carbon fitting ratio. Foundation item: Project(50474083) supported by the National Natural Science Foundation of China; Project supported by the Baoshan Iron & Steel Co. Ltd. of China     

Effect of vanadium and chromium on the microstructural features of V–Cr–Mn–Ni spheroidal carbide cast irons

The objective of this investigation is to study the influence of vanadium（5.0wt%–10.0wt%） and chromium（0–9.0wt%） on the microstructure and hardness of Cr-V-Mn-Ni white cast irons with spheroidal vanadium carbides. The alloys＇ microstructural features are presented and discussed with regard to the distribution of phase elements. The structural constituents of the alloys are spheroidal VC, proeutectoid cementite, ledeburite eutectic, rosette-shaped carbide eutectic（based on M7C3）, pearlite, martensite, and austenite. Their combinations and area fraction（AF） ratios are reported to be influenced by the alloys＇ chemical composition. Spheroidized VC particles are found to be sites for the nucleation of carbide eutectics. Cr and V are shown to substitute each other in the VC and M7C3 carbides, respectively. Chromium alloying leads to the formation of a eutectic（γ-Fe ＋ М7С3）, preventing the appearance of proeutectoid cementite in the structure. Vanadium and chromium are revealed to increase the total carbide fraction and the amount of austenite in the matrix. Cr is observed to play a key role in controlling the metallic matrix microstructure.     

高铬铸铁的正火组织与性能研究

研究正火温度对新型高铬铸铁组织、硬度(HRC)及冲击韧性的影响。实验结果表明高铬铸铁在800~950℃正火时,其组织由珠光体+少量铁素体+网状共晶碳化物组成;在1 000~1 050℃正火时,碳化物溶解析出,珠光体球化,得到铁素体基体上分布的粒状碳化物+共晶碳化物+少量珠光体,硬度略有降低;在1 100~1 150℃正火时,共晶碳化物溶解得更多,冷却过程中奥氏体中析出弥散碳化物,冷却后得到马氏体(过饱和铁素体)基体上弥散碳化物+共晶碳化物,硬度明显升高,并在1 150℃时硬度达到最大值,在800~1 150℃正火加热温度范围内,冲击韧性在4.2~4.7 Jcm2之间波动,总体变化不大。     

Erosion-corrosion investigation of high chromium cast irons using newly designed jet type tester

A jet type erosion-corrosion tester was developed for the erosion-corrosion investigation of high-chromium cast irons. During tests the size and the shape of particles in the slurry can be maintained stable. The jet velocity and attack angle can be accurately controlled. The repeatability and ranking consistency of the test results are satisfactory. The test parameters can be adjusted in a wide range, so that the tester can simulate various practical working conditions. Electrochemical test data can be automatically collected and processed. Dynamic polarization curves can be obtained during erosion-corrosion test, which can be used to study the dynamic corrosion characteristics.Two high chromium cast irons were studied in hot concentrated alkaline slurry. The results show that the erosioncorrosion mass loss rate and dynamic corrosion rate of 295Cr26 iron is lower than that of 185Cr13 under the conditions similar to alumyte processing. The mechanism of erosion-corrosion of 295Cr26 and 185Cr13 was studied by using the tester. The interaction between erosion and corrosion was also quantitatively evaluated.     

热变形后低铬耐磨铸铁的组织与耐磨性的关系

研究了低铬耐磨铸铁经40％热变形及不同方式冷却后的组织与耐磨性的关系。结果表明：该铸铁经热变形后的组织与耐磨性有良好的对应关系，其耐磨性随硬度的增加而增大，同时也与组织中碳化物的形状、大小及分布有关。     

一种高铬马氏体耐热钢的显微组织和力学性能

研究了高铬马氏体耐热钢(0.09C-10.2Cr-0.52Ni-1.52Mo-0.22V-0.07Nb-3.0Co-0.01Ti-0.0129N-0.0033B)的显微组织和力学性能.结果表明该钢经1100℃×1h空冷正火处理 750℃×1h空冷回火处理后的显微组织为板条状马氏体,在原奥氏体晶界和板条间分布有M23C6碳化物,并有少量1～3μm的M3B2颗粒,马氏体板条内部有10～30 nm的MX型析出相.短期蠕变试验数据显示其在650℃的蠕变性能优于P92钢.     

Initial oxidation of Cr at 300–730 K in H2 18O/H2 16O/16O2 gas mixtures studiedin situ with18O/SIMS and XPS

The initial oxidation of pure, polycrystalline-chromium metal with gas mixtures of oxygen-labeled water, H₂ ¹⁸O, and oxygen,¹⁶O₂, at temperatures 300–730 K has been studied in situ with secondary ion mass spectrometry, SIMS. The fraction of O in secondary ion species: CrO⁺, Cr₂O⁺, CrOH⁺, Cr₂OH⁺, CrOH₂ ⁺, and Cr₂OH₂ ⁺ originating from water and oxygen was found. The reaction of water and oxygen in the oxygen/water gas mixture with Cr is then revealed. The dissociation of water in its reaction with Cr in such a gas mixture was analyzed. Oxide growth on cleaned Cr surfaces during the reaction with H₂O, O₂ and H₂O/O₂ gas was also studied in situ with X-ray photoelectron spectroscopy, XPS. A correlation between a chemical shift in XPS and the OH content in the oxide is discussed.     

高铬铸铁变温马氏体相变与摩擦诱发马氏体相变关系的研究

对三种含Mn、Si、Mo亚共晶高铬铸铁的变温马氏体相变进行了DSC分析,并对表层的摩擦诱发马氏体做了X射线衍射分析。结果表明,变温马氏体相变前期,马氏体形核、长大阻力较大。在M_s温度相近的情况下,变温马氏体相变形核、长大速率快的,其摩擦诱发马氏体量也多。     

High temperature oxidation of a (Cr2N−Mo2S3)/Cr double-layered coating deposited on steel by D.C. magnetron sputtering

A coating consisting of (Cr₂N−Mo₂S₃) overlay coating and an underlying Cr coating was deposited on a steel substrate by D.C. magnetron sputtering. The oxidation characteristics of the deposited double-layered coating were studied at temperatures ranging from 400 to 900 °C in air. The oxidation product was primarily Cr₂O₃. The unreacted coating beneath the oxide scale had some dissolved oxygen, sulfur, and iron. Oxidation of the coating occurred via complex routes such as the outward diffusion of chromium and nitrogen from Cr₂N and iron from the substrate, and the inward transport of oxygen from air, chromium from Cr₂N, and S from Mo₂S₃. This counter diffusion of various ions occurred easily via fine crystallites that constituted the coating, which had some solubility of S, O, and Fe.     

Chromium-nitride in situ composites with a compositional gradient formed by reactive DC plasma spraying

Nitrides of transition metals have good wear- and corrosion-resistant properties because of their high hardness and chemical stability. Chromium-nitride coatings can be deposited by ion plating; however, the thin thickness due to the slow deposition rate must be improved for severe wear-resistant applications. The main objective in this paper is to realize good structural control in the processing of chromiumnitride in situ composite coatings formed at a high deposition rate. They were synthesized by reactive low-pressure plasma spraying using elemental chromium powder as a spray material. The transferred arc between the gun electrode and the substrate was used to accelerate the nitriding reaction. The sprayed coatings consist of chromium, Cr₂N, and CrN, which have a composition gradient from the substrate interface to the surface. The volume fraction of Cr₂N increases with transferred arc current, and nonreacted chromium concurrently decreases, except close to the substrate. The CrN phase, however, only exists as a surface layer of 20 to 30 μm because it is decomposed to Cr₂N above 1420 K. The hardness of the composite coatings depends on the volume fraction of Cr₂N, and it increases to 1300 HV at a Cr₂N volume fraction of 0.98. The seizure stress with lubricant depends on the coating hardness. The maximum seizure stress of 24.9 MPa is obtained at a hardness of 1300 HV. The composite coatings also show a superior wear resistance. Hence, the Cr₂N in situ composite coatings synthesized by reactive plasma spraying with transferred arc are expected to be good candidates for wear-resistant applications.