A microstructural study of local melting of grey cast iron by a static plasma arc

The microstructural changes produced by plasma arc local-melting method in the fusion region or deposited metal, fusion boundary region and heat-affected zone (HAZ) of flake graphite cast iron were investigated. Cylindrical base metal specimens were locally melted at fixed time intervals under a stationary plasma torch using argon plasma gas and Ar + 10% H₂ shielding gas. The welds were produced autogenously and with filler metals. The cooling rate in the fusion region was recorded. Evaluation of the fusion boundary area included metallurgical analysis, microhardness and electron probe microanalysis. In the absence of filler metal, the structure of fusion region where completely melted was ledeburite. In the centre of the fusion region the structure appears to be that of hypoeutectic white cast iron, and in the fusion boundary of this region the structure appears to be fined ledeburite. In the fusion boundary region where supercooled phenomena can occur, fine martensite precipitates appear along the fusion line for small heat input and secondary graphite is seen for large heat input. The HAZ is composed of white martensitic, dark martensitic and martensitic-fine pearlitic layers for small heat input, and of finely laminated pearlitic and finely laminated pearlitic-ferritic layers for large heat input. Using filler metal, the structure of the deposited metal is found to be that of a nickel austenitic matrix precipitating tiny graphite nodules or slim graphite for nickel and Ni-Fe filler metals, but to be a pearlitic matrix for iron filler metal. In the fusion boundary region, nickel-martensite, eutectic or slim graphite and fine nickel-martensite are precipitated from the nickel system filler metal, and the columnar structure of ferrite and pearlite is obtained from iron filler metal. The HAZ is composed of thin ledeburitic, martensitic, martensitic-fine pearlitic and finely laminated pearlitic layers for the nickel filler metal, of ledeburitic and finely laminated pearlitic layers for the Ni-Fe filler metal, and of thick ledeburitic, eutectic graphite-crystallized and finely laminated pearlitic layers for iron filler metal. The hardness of the ledeburitic layer and the nickel-martensitic portion is very high where the liquid existed. The diffusion of nickel from the deposited metal into the HAZ can occur at least until the fused base metal of HAZ.     

On the weldability of grey cast iron using nickel based filler metal

Shielded metal arc welding process using nickel based filler metal was used to join grey cast iron. The effect of post weld heat treatment (PWHT) on the microstructure and hardness was studied. PWHT included heating up to 870 °C, holding for 1 h at 870 °C and then furnace cooling. By using nickel based filler metal, formation of hard brittle phase (e.g. carbides and martensite) in the fusion zone is prevented. Before PWHT, heat affected zone exhibited martensitic structure and partially melted zone exhibited white cast iron structure plus martensite. Applied PWHT resulted in the dissolution of martensite in heat affected zone and graphitization and in turn the reduction of partially melted zone hardness. Results showed that welding of grey cast iron with nickel based filler metal and applying PWHT can serve as a solution for cast iron welding problems.     

Formation of stainless steel layer on mild steel by welding arc cladding

The discrete microstructural characterization and the formation of stainless steel layer on mild steel where produced in cladding deposits, and fusion boundary region were investigated using tungsten inert gas (TIG) arc, high current pulsed arc and constricted plasma arc. The experimental procedure involved making bead-on-plate method for controlled travel speed, employing filler metal by using tungsten inert gas arc, pulsed current gas tungsten arc and transferred plasma arc, with subsequent sectioning and examination of the reaction interface. For TIG arc cladding, using filler metal of small diameter the deposit does not become stainless steel, but on using 3.2 mm diameter filler metal it becomes stainless steel with less than 50% dilution. For pulsed arc cladding, the complete stainless steel is not obtained on account of the existence of an incomplete mixture, particularly at the fusion boundary region. However, on using a large diameter filler metal at a pulse frequency of 500 Hz, the complete stainless steel microstructure has been accomplished. The plasma arc cladding can be achieved in such a way that the conversion into stainless steel on the mild steel surface — which is the microstructures of cellular austenite in cladding deposit and cellular dendritic austenite containing δ or σ-phase in fusion boundary region — is possible irrespective of the melt penetration and the dilution. The following conditions were found to be beneficial for the formation of stainless steel microstructure layer on the mild steel: using large diameter filler metal, below 50% dilution, and further rendering arc localized and constricted.     

The oxidation of binary alloys of chromium with metals of the first long period

Binary alloys of chromium with titanium, vanadium, manganese, iron, cobalt and nickel respectively have been oxidation-tested in air up to 1100° C. From the results the temperature corresponding to a weight change of 1 mg cm^–2 over 4 h was determined by interpolation, and this temperature has been plotted against composition.With the exception of a 50% cobalt alloy, alloys of chromium with either iron, cobalt or nickel withstand air oxidation at much higher temperatures than the constituent metals. Oxidation resistance in the other systems is generally lower.An additional note describes the determination, during oxidation, of comparative electrical resistance of scales on iron, chromium, and an iron-20% chromium alloy.     

Microstructural transformations in austenitic-ferritic transition joints

The relatively complex microstructures developed at the interface between ferritic steel and weld metal on austenitic-ferritic transition joints have been examined by metallographic observation and by hardness tests in the as-welded condition and in the as-welded-and-tempered condition. Both austenitic stainless steel and nickel-based filler metals were used in welds. On as-welded specimens a sharp change of hardness in low-alloy steel has been measured, with increasing distance from weld metal; the hardness values have been related to the observed metallographic constituents. On post-weld heat treated specimens, the behaviour is different according to the composition of filler material, either austenitic steel or nickel-based alloy. In the case of austenitic filler material, a dark-etching narrow diffusion region of carbon toward weld metal is formed, with an adjacent markedly decarburized zone, exhibiting the minimum microhardness values in a narrow band of about 60 micrometres. Since this sharp structural variation is recorded just in the zone where often failures occur, the final post-weld heat treatment appears to be proposed with due caution. In the case of nickel-based filler material, carbon diffusion is inhibited by the precipitation of alloy carbides at the weld interface. This determines a more homogeneous heat affected zone (HAZ) in the ferritic steel and a reduced decarburization near the fusion line after a post-weld heat treatment, confirming the reasons of the preference recognized to this filler material, especially when service temperature is elevated and submitted to frequent changes, or whenever a post-welded heat treatment is required.     

Microstructural evolution in AZ91D magnesium alloy during electron beam welding

Vacuum electron beam welding can have a low heat input, which means there is a minimum heat affected zone during welding of AZ91D magnesium alloy. From the observed microstructure, the weld of the AZ91D magnesium alloy can be divided into four regions, which are the weld metal zone, a partially-melted zone adjacent to the fusion boundary, a partially-melted zone adjacent to the base metal and the base metal zone. A sharp transition from the fusion zone to the non-melted zone, especially the characteristic partial melting microstructure and nature of the alloy elements, was observed. It was found that significant partial melting had taken place in the very narrow region around the weld metal of the AZ91D magnesium alloy. The Al content of eutectic β-Mg₁₇Al₁₂ in the partially-melted zone adjacent to the fusion boundary was close to the content in the continuously precipitated eutectic β particles in the fusion zone and much lower than the eutectic β in the base metal. The fully melted eutectic β-phase coexisted with the partially melted eutectic β phase in the partially-melted zone adjacent to the base metal.     

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

The concentration of nitrogen in molten Fe-Cr-Ni alloys has been substantially increased by melting under nitrogen overpressures. Total nitrogen concentrations exceeding 26 at % were obtained when melted under 200 MPa. Nitrogen is present in solidified alloys as interstitial nitrogen and as metal nitrides. The nitrogen concentration depends upon the alloy composition; nickel decreases the nitrogen concentration, whereas chromium increases the concentration. When iron and Fe-Ni alloys were melted under high nitrogen pressures they produced iron nitrides and when Fe-Cr-Ni alloys were melted they produced ON dendrites and precipitates.     

Study on filler metal (Ni-Fe-C) during GTAW of WC-30Co to 45″ carbon steel

In this study, WC-30Co cemented carbide is welded to carbon steel by the gas tungsten arc welding (GTAW) using Ni-Fe filler metal and Ni-Fe-C filler metal. The butt joints manifest more embrittling η-phase carbides with Ni-Fe filler metal, while less even no η-phase carbides with Ni-Fe-C filler metal. The η-phase carbides morphology and formative factors were further discussed using Backscattered Electron Imaging (BEI) method; Electronic probe microanalysis (EPMA) is used to determine the distribution of elements Ni, Fe, C, W and Co across the HAZ (Heat Affected Zone) near WC-30Co/welded-seam interface. The hardness profile is determined using micro-hardness measurements and bend strength value of butt joint with different filler metal is tested by four-point bend strength test. The hardness profile and bend strength value agree with the information obtained from microstructure analysis, BEI analysis and X-rays phase analysis very well. The results show: (1) butt joint of WC-30Co/carbon steel can be obtained using GTA with Ni-Fe-C filler metal; (2) the addition of carbon content to Ni-Fe filler metal leads to less even none η-phase multi-carbides strongly, and mechanical property of butt joint can be improved.     

Casting of TiC-Reinforced Steel Matrix Composite

Titanium carbide varying from 6.34 to 21.60 volume fraction was reinforced in to iron matrix by the direct reaction of pure titanium in molten Fe-C alloy. A simple casting technique has been developed successfully to improve the precipitation and distribution of titanium carbide particles in iron matrix. Lime powder is used as a protective layer on the molten metal to avoid oxidation of titanium during melting and casting. The thermodynamic and kinetic studies support the precipitation of TiC in molten Fe-C alloy at the reaction temperature, 1600°C. The size and shape of the in-situ TiC particles depend on the amount of titanium present in Fe-C molten alloy.     

Vacuum heat treatment and brazing of metallic materials

Vacuum heat treatment is applicable to most common metals. Vacuum heat treating processes show a distinct advantage over conventional processes in that vacuum heat treatment obtains a bright surface free of defects such as decarburization and intergranular oxidation. Distortion of the parts normally occuring during conventional processing is minimized in vacuum. Parts may therefore be processed in the fully machined condition. Vacuum furnace brazing renders the use of flux unnecessary; it allows the use of a wide variety of brazing filler metals; improves the physical properties of the joint by eliminating such defects as porosity and oxide inclusions; it enables the successful brazing of the nickel and cobalt base superalloys, a difficult task to achieve by conventional means. A cost comparison between vacuum and conventional heat treating processes indicates that vacuum is the most economical method of processing. The paper intends to relate the above to actual processes and to indicate where considerable savings in labour and time may be obtained by the application of vacuum processing.     

Surface wetting and interfacial behaviour in arc brazing of titanium alloy

Abstract

The spreading behaviour of Cu, CuSi, CuMnNi, and TiCuNi filler metals with Ti6Al4V titanium alloy base metal has been studied under gas tungsten arc welding heating. Test results showed that the wet angle of CuMnNi filler metal was the smallest (15 ° ) and that of Cu was the largest (30 ° ). With spreading, the liquid filler metal solidified and crystallised simultaneously, and formed a spherical crown brazing seam. The wettability and spreadability of the liquid filler metal were related to oxide scale actions with arc heating. These actions included the 'cleaning action of the cathode' and lashing activation action of electrifying ions and electrons. Furthermore, the very high temperature of the activation spot zone partly melted the base metal surface and near surface metal, and a thin liquid film was accordingly formed that created a strong heating activation. It was also discovered that there was a column solid - liquid incongruent compound η (MeTi₂) at the interface of filler metal and base metal (Me + Cu, Cu + Ni, Cu + Mn). This compound grew to form an interface with the centre of the brazing seam and was embedded in it. The particular crystallising and growing mode in the arc heating helped grow the column solid - liquid incongruent compound η (MeTi₂).     

Atomic layer deposition of transition metals

Atomic layer deposition (ALD) is a process for depositing highly uniform and conformal thin films by alternating exposures of a surface to vapours of two chemical reactants. ALD processes have been successfully demonstrated for many metal compounds, but for only very few pure metals. Here we demonstrate processes for the ALD of transition metals including copper, cobalt, iron and nickel. Homoleptic N,N'-dialkylacetamidinato metal compounds and molecular hydrogen gas were used as the reactants. Their surface reactions were found to be complementary and self-limiting, thus providing highly uniform thicknesses and conformal coating of long, narrow holes. We propose that these ALD layers grow by a hydrogenation mechanism that should also operate during the ALD of many other metals. The use of water vapour in place of hydrogen gas gives highly uniform, conformal films of metal oxides, including lanthanum oxide. These processes should permit the improved production of many devices for which the ALD process has previously not been applicable.     

The metallurgical behaviour during brazing of Ni-base alloy Inconel 600 to Si3N4 with Ag71Cu27Ti2 filler metal

Detailed observations were carried out on the metallurgical behaviour of joint-brazing of nickel based alloy Inconel 600 to Si₃N₄ with Ag71Cu27Ti2 filler metal, with emphasis on the interface between the filler metal and the Inconel 600 and the effects of nickel which was the predominant element in the base metal. Based on the experimental results, the mechanism of bonding Inconel 600 to the filler metal is attributed to the diffusion of silver and copper along the grain boundaries of the Inconel 600, which results in mechanical anchoring. The effects of nickel on the metallurgical behaviour of filler metal are summed up as enhancing the separation of silver- and copper-rich liquid phases from the molten filler metal; combining titanium and decreasing its activity in the reaction with Si₃N₄ at the interface with ceramics; promoting the diffusion of silver and copper into Inconel 600; and facilitating the flow of filler metal over the surface of Inconel 600.     

低含量钴对镍合金镀层结构和性能的影响

研究了低含量的钴对镍合金镀层结构和性能的影响,通过扫描电镜观察了镀层形貌、X射线衍射仪测量了镀层内应力,并用电化学极化曲线和腐蚀失重方法分析了镀层的耐蚀性.结果显示:镀液中硫酸钴的含量为0～4 g/L内时,随着钴盐的加入,镍合金镀层结晶细化、结构紧密、耐蚀性和硬度提高,内应力缓慢增长;硫酸钴含量超过5 g/L后,镍合金镀层结晶不再继续细化,但拉伸内应力明显变大,耐蚀性开始降低.     

Effect of nitrogen and nickel on the microstructure and mechanical properties of plasma welded UNS S32760 super-duplex stainless steels

Super-duplex stainless steels present excellent combination of mechanical and corrosion resistance, due to their strict composition control and ferrite–austenite phase balance. This balance may, however, be disturbed during welding in both the fusion and HAZ due to the rapid cooling rates and may lead to loss of the good corrosion and mechanical properties of the weldments. The present investigation is studying the effect of nitrogen addition in the plasma operation gases and of the increase of nickel in the filler metal, on the microstructure and on the mechanical properties of super-duplex stainless steels welded by the plasma transferred arc (PTA) technique. Results have shown that nitrogen addition in the plasma operation gas affects the mechanical properties of the weldments. It is shown that nitrogen addition in the plasma and protective gas and higher nickel content in the filler metal have both a positive effect on the elongation of the welded specimens and after optimization of the welding parameters very good results may be obtained in terms of tensile strength.     

Temperature field and flow field during tungsten inert gas bead welding of copper alloy onto steel

The temperature field in the base metal and bead-on-plate weld during tungsten inert gas bead welding Hs201 copper alloy onto 35CrMnSiA steel was studied and the fluid flow of the partially melted steel in the molten pool was analyzed. The results show that the thermal cycle changes remarkably and the peak temperature and the cooling speed in the fusion interface are higher than those far away from the interface. In the liquid overlay, the velocity of the fluid flow of the partially melted steel in the edge of the overlay points to the fourth quadrant, while in the puddle of the base metal, the velocity vector points to the second quadrant. Based on the calculated flow field, the distribution of the Fe-rich phase is predicted. The melted steel is driven to the boundary of the overlay. The distribution of Fe-rich phase is largely more in the outside periphery of the overlay than in the middle of the overlay.     

Work of adhesion in ZrO2-liquid metal systems

Using the sessile drop technique for the measurement of contact angles, the work of adhesion of polycrystalline cubic ZrO₂ in contact with various liquid metals was determined. Based on these experimental values, a model for the evaluation of the work of adhesion is proposed. According to this model the wetting of ZrO₂ by the metals indium, tin, bismuth and lead is established by Van der Waals dispersion binding forces. For the transition metals silver, copper, nickel, cobalt and iron a chemical equilibrium bond between the liquid metal and the oxygen ion of the surface oxide is created at the interface. In this case the value of the work of adhesion is related to the enthalpy of formation of the oxide of the metal.     

A simple route to electrophoretic deposition of transition metal-coated nickel oxide films for electrochemical capacitors

Nickel hydroxide films coated with transition metals such as nickel and cobalt were fabricated directly by a one-step electrophoretic deposition (EPD) in the presence of charging additives (transition metal salts). A nickel hydroxide particle with a weakly charged surface in an isopropanol solution was found to be detrimental to EPD and dispersion. When a small amount of charging additive was added to the suspension, the adsorption of dissolved metal ions on the nickel hydroxide resulted in a more positively charged particle surface, facilitating EPD and dispersion. When nickel hydroxide particles migrated to the negative electrode during the EPD process, the metal ions adsorbed on the particle were reduced electrochemically to form a metal layer. The as-deposited nickel hydroxide film converted to nickel oxide following heat treatment at 300 °C. Our results revealed that nickel oxide films coated with nickel and cobalt showed better capacitive behavior than the bare film. The improved capacitive behavior was attributed to the co-deposition of transition metals, which provided additional active sites on the nickel oxide surface for the electrochemical reaction to occur.     

厚板高强钢激光填丝多层焊接头特征及断裂机制研究

本文在11CrNi3MnMoV低合金高强钢激光填丝多层焊工艺优化的基础上利用电子万能试验机、HVS-5维氏硬度计、扫描电子显微镜和能谱分析仪等对焊缝接头进行了力学性能测试,并重点对断裂机制进行了分析.结果发现:激光填丝焊接头的HAZ很窄,约为1 mm.焊缝平均硬度值高出母材30%左右;顶层焊道硬度高于内层焊道;最高硬度值出现在熔合区附近;接头断裂机制为韧窝断裂;能谱分析发现韧窝中第二相粒子成分主要由母材和焊缝的合金元素决定.焊缝金属与母材界面处存在一层Fe、Mn等合金元素的氧化物,其导致侧壁未熔合,这是接头断裂的主要原因.     

WC颗粒增强铁基复合材料的性能研究

通过离心法制备了外径290mm,内径130mm,高72mm,WCp/Fe-C复合材料工作层厚度25～30mm的厚壁环形试样.通过光镜、扫描电镜和性能试验设备研究了两种不同WCp体积分数WCp/Fe-C复合材料的力学性能、耐磨损性能和抗热疲劳性能,并与硬质合金和高铬铸铁进行比较.结果表明WCp体积分数在80%和65%左右的两种WCp/Fe-C复合材料,其抗拉强度达到了320和348MPa,冲击韧性均＞4J/cm^2,硬度为HRC63.5和HRC61.5.20和40N载荷下的耐磨性分别达到208.33、90.91和127.06、57.14,抗热疲劳性能优良.与硬质合金和高铬铸铁相比,WCp/Fe-C复合材料的冲击韧性、抗热疲劳性能以及20和40N载荷下的耐磨性均有大幅度提高.