High-temperature thermal barrier coating formation by laser alloying of CP-Ti with pre-placed Ni-SiC coating

High-temperature thermal barrier coating was created on CP-Ti using a pre-placed Ni-SiC layer by laser alloying technique. The coating was developed using 80% Ni + 20% SiC, 50% Ni + 50% SiC and 60% Ni + 40% SiC, and the latter two compositions are found to be efficient in producing a uniform layer. The 100% SiC pre-placement was also used. A flaw-less coating of 0.4–0.6 mm thickness was produced at a lower power density of 1.3 to 1.9 × 10⁵ W cm^–2. Very high power density of 2.5–3.0 × 10⁵ W cm^–2 is inefficient to produce uniform coating. The laser alloyed coating consists of dendrites and intermetallic precipitates. The degree of dendrite population depends upon the coating composition and laser processing conditions. The coating hardness was 600–1200 HV, which is three to six times higher than the base titanium. Uniform hardness was obtained for the coatings produced at a laser power density of 1.3 × 10⁵ W cm^–2. The titanium silicide (TiNiSi, Ti₅Si₃, TiSi) and nickelide (NiTi₂) phases formed on the laser-alloyed coating surface was confirmed by X-ray analysis. These intermetallic phases can improve high-temperature properties of titanium and its alloys. The effect of laser power density and coating composition on the alloying depth alloying width, hardness and microstructure are discussed. The present work investigated the microstructure evolution, hardness and compound phases by means of optical and scanning electron microscopy, Vickers hardness testing, EDXRD and SIMS analysis. A 5 kW CW CO₂ laser was used for laser alloying experiments.     

不同电源等电位离子钨钼共渗的研究

利用脉冲单电源和直流单电源,分别在Q235钢表面进行等离子钨钼共渗.通过对不同电源钨钼共渗试样的渗层组织、渗层成分分布、晶体结构、渗层硬度分布的检测和比较,分析了2种电源对等离子钨钼共渗的影响.结果表明,利用单一电源均可在Q235钢表面形成明显反应扩散层;在相同工艺下,采用脉冲电源所得到的试样渗层厚度较采用直流电源的渗层厚度增加了18.1%;脉冲电源试样表面W、Mo含量(质量分数,下同)分别约为8.4%和9.8%,直流电源试样表面W、Mo含量分别为8.2%和8.9%,可见2种电源渗层表面含W、Mo量相差不大;2种电源渗层相结构均为Fe_7W_6和Fe_3Mo金属间化合物相;钨钼共渗后渗层硬度提高不明显.     

Solidification microstructures and mechanical properties of vertical centrifugal cast high speed steel

Abstract

This study was undertaken to investigate the influence of Nb and V alloying elements and manufacturing conditions on the microstructural behaviour and mechanical characteristics of HSS (high speed steel) roll manufactured by a VCC (vertical centrifugal casting) process. In the Fe - 2C - 6Cr-1.5W - 3Mo - 4V alloy, the amount of MC carbide was increased and the the amount of M₇C₃ carbide decreased with an increase in V and Nb content. In steel containing 3%Nb, primary NbC carbide was formed within the cell in the matrix. The hardness of steel containing 6.5%V but no Nb was increased a little but when 9%V was added, the hardness decreased in the specimen owing to the soft ferritic matrix. The hardness of the matrix in steel containing 1.5%Nb increased, but decreased for 3%Nb addition. In wear tests, wear loss decreased with increasing rotational wear speed.     

Laser surface alloying of case hardening steel with tungsten carbide and carbon

Abstract

The laser surface alloying process was used to introduce two different alloying materials, tungsten carbide (WC/Co) and carbon, into the molten surface of a case hardening steel (16MnCrS5), to improve its hardness and wear resistance. The chemical composition and the resulting microstructure in the alloyed layers were of particular interest in this investigation, because the strengthening mechanism was strongly dependent upon the type and amount of the alloy material. For laser alloying with carbon the increase in hardness and wear resistance was based on the martensitic transformation in the composition range concerned. For alloying with tungsten carbide it was necessary to consider two different strengthening mechanisms, namely, martensitic transformation and precipitation of carbides. In both cases the grain refinement in the laser affected zone had an additional effect. Resistance to dry abrasive sliding wear was measured using a conventional pin-on-disc wear testing machine. For both alloy materials the wear rate was substantially lower than that of a substrate that had been laser remelted without alloying additions.

MST/1556     

Formation of alloying layers in a carbon steel by compression plasma flows

The main regularities and features of steel surface alloying by means of compression plasma flows treatment of coating/substrate systems are investigated in this work. Ti, Zr, Mo and Cr were used as alloying elements. The thickness of the alloyed layer is about 8-15 μm. The mixed layer contains iron solid solutions, nitrides and possesses mechanical properties improvement. The peculiarities of alloying elements distribution are discussed.     

深冷处理对碳钢表面Mo-Cr合金层摩擦性能的影响

介绍了一种在Q235钢表面进行等离子合金化及热处理工艺,获得表面高性能强化层的技术方法.通过该技术方法的处理,使Q235钢表面含有Mo,Cr,C合金元素,成分达到或接近冶金高速钢.该工艺技术的基本原理是在真空容器中,利用辉光放电的溅射现象,首先在Q235钢表面渗入合金元素Mo,Cr,表面含量分别达到12%(质量分数,下同)和4%左右,随后进行超饱和渗碳,使表面含碳量达到2.0%以上,合金化层成分接近钼系高速钢.合金层中的碳化物细小、均匀、弥散,无粗大的共晶莱氏体组织.Q235钢表面合金化后分别采用淬火 低温回火,淬火 2h深冷处理 低温回火两种工艺.结果发现,经深冷处理的试样表面硬度达到1600HV,明显高于未经过深冷处理试样的表面硬度.摩擦磨损实验表明,经深冷处理试样的滑动摩擦系数较未经深冷处理试样的要小,经深冷处理试样的耐磨性是未经深冷处理的1.6倍.     

Surface electron beam melting and alloying of tool steels

Abstract

Surface melting and alloying of D3 steel using an electron beam has been carried out to improve its surface microstructure and properties. The solution of primary carbides, together with rapid solidification and subsequent cooling, enhance the solubility of alloying elements in the γ Fe phase and thus influence the behaviour of the steel on subsequent tempering. The surface melted zone consists of dendrites without primary carbides, which is also the case for samples alloyed with WC, SiC, or Al₂O₃. When alloyed with TiC or TiB₂, the materials contain TiC or TiB₂ primary phase respectively in addition to the iron rich dendrites. Some unmelted TiB₂ particles are also present. On tempering, both electron beam melting and alloying change the secondary hardening characteristics, increasing the peak hardness and the peak hardness temperature.

MST/1194     

Effect of vanadium on cast carbide in high speed steels

Abstract

The effect of vanadium (0–4%) on the morphology and amount of eutectic and eutectoid carbides in high speed steels has been investigated using scanning electron microscopy and image analysis. It was found that vanadium promotes the formation of MC carbide and M₂C carbide, but inhibits the formation of M₆C carbide. In the vanadium free steels, the eutectic carbide consists solely of skeletal M₆C. For each steel composition, there is a critical vanadium content at which the skeletal eutectic changes to lamellar eutectic and the critical value decreases as the molybdenum content of steel increases. The effect of vanadium on the total amount of eutectic carbide differs in tungsten alloyed and molybdenum alloyed high speed steels. The δ eutectoid has a rodlike morphology in tungsten high speed steels; δ eutectoid is not present in Mo–W or molybdenum high speed steels. Increasing the vanadium content leads to an increase in the size of eutectic and eutectoid carbides.

MST/1264     

Improving the wear resistance of AA 6061 by laser surface alloying with NiTi

To improve the wear resistance of a popular aluminum alloy AA 6061, a 1.5 mm thick hard surface layer consisting of Ni–Al and Ti–Al intermetallic compounds was synthesized on the alloy by laser surface alloying technique. NiTi powder was preplaced on the aluminum alloy substrate and irradiated with a high-power CW Nd:YAG laser in an argon atmosphere. With optimized processing parameters, a modified surface layer free of cracks and pores was formed by reaction synthesis of Al with Ni and Ti. X-ray diffractometry (XRD) confirmed the main phases in the layer to be TiAl₃ and Ni₃Al. The surface hardness increased from below 100 HV for untreated AA 6061 to more than 350 HV for the laser-treated sample. Accompanying the increase in hardness, the wear resistance of the modified layer reached about 5.5 times that of the substrate.     

20Cr_（2）Ni_（4）W钢钴粉涂层激光合金化的试验研究

本文采用２ｋＷＣＯ_２激光器对２０Ｃｒ_（２）Ｎｉ_（４）Ｗ钢进行了激光表面钴合金化的试验研究。用光学显微镜、扫描电镜、电子探针、Ｘ射线衍射、透射电镜、显微硬度计等手段分析了合金化区域的成分、组织和性能。结果表明，激光表面钴合金化可获得含钴均匀的合金化层，合金化层的高温硬度和热疲劳性能均有提高，在热作工具钢表面性能优化领域有着良好的应用前景。     

Influence of alloying elements and density on aqueous corrosion behaviour of some sintered low alloy steels

Low alloy steels produced through powder metallurgy route of sintering followed by forging are promising candidate materials for high strength small components. Porosity in such steels poses a real challenge during acid pickling treatment, which is one of the processing steps during manufacturing. The present research work attempts to investigate the mechanism underlying the acid corrosion behaviour of some sintered low alloy steels under induced acid pickling conditions. Sintered-forged low alloy steel samples containing molybdenum (Mo), copper (Cu) and titanium (Ti) were subjected to aqueous corrosion attack by immersing the samples in 18% HCl (Hydrochloric acid) solution for 25 h. Sample weight loss and Fe (Iron) loss were estimated for the corroded samples. The morphology of the corroded surfaces was studied through metallography and scanning electron microscopy. Higher porosity alloys underwent enhanced corrosion rates. Both corrosion rate and iron loss are found to decrease linearly with reduction in porosity in all cases of the alloys. The alloying elements Mo, Ti and Cu, when added in combination, have played a complementary role in the reduction of corrosion rate by almost one order of magnitude compared to unalloyed steel. Presence of carbides of the carbide forming elements Mo and Ti played a positive role on the corrosion behaviour of the low alloy steels.     

Mechanical milling of fullerene with carbide forming elements

Mechanical alloying of Ti, V, Cr, Mo and W with fullerene (C₆₀(C₇₀)) and graphite reveals that fullerene is more reactive than graphite. The formation heat of carbide is the driving force for reaction in the mechanical alloying process. Higher heat of formation results in the direct formation of carbide in Ti-C systems, and the formation of carbide in V-C systems during the subsequent heating of milled powder. In the systemsc with lower carbide heat of formation, a mixture of metal with carbon is obtained by ball milling. No carbide was obtained even after heating the milled powders up to 973 K. Small amount of fullerene remained when milled with Mo and W for 10 hours.     

X-ray investigation of solid solution partitioning in 2.25Cr-1Mo steel after extended elevated temperature service in power station

Abstract

Carbide phase transformation and alloying element partitioning in service exposed (about 45 000-160 000 h) 2.25Cr-1Mo boiler tubes from a thermal power station were studied using X-ray diffraction and X-ray fluorescence techniques on electrochemically extracted carbide precipitates. The objective of the present investigation was to develop a faster method of health assessment of this steel at elevated temperatures on a routine basis. Various carbide phases were detected with different lengths of service. The molybdenum content in extracted carbide specimens showed a systematic decrease with aging. Replenishment of molybdenum in the matrix may be a useful indicator in life assessment of this steel. When the dissolved molybdenum content in the matrix was less than ~0.8 wt-%, the hardness of service exposed specimens was found to decrease. However, carbide precipitates extracted from new specimens subjected to accelerated heat treatment at 600 and 700 ° C showed an increase in molybdenum content with increase in temperature. The X-ray data were supplemented by microscopy, EDAX and hardness measurements.     

Phase evolution in mechanical alloying and spark plasma sintering of AlxCoCrCuFeNi HEAs

ABSTRACT

Al_xCoCrCuFeNi high-entropy alloys were synthesised through mechanical alloying and spark plasma sintering. Different alloys were produced by varying the aluminium content (x?=?0.5, 1.5, 2.5 and 4). The influences of the milling duration on the evolution of microstructure, constituent phases and morphology were studied. Increasing milling time resulted in grain refinement and higher solid solution homogenisation characterised by a high internal strain. As a consequence of aluminium addition, the microstructure of materials evolved from face centered cubic (FCC) and body centered cubic (BCC) phases to FCC, BCC and ordered BCC phases. Both mechanical alloying and SPS conditions as well as aluminium content led to grain refinement and variations of mechanical properties. In particular, hardness increased with increasing aluminium content. The aluminium percentage and the evolution of consequent phases are responsible for the microstructural stability at high temperatures. In addition, with Al content increase, the further synergy of strength and ductility along with a more pronounced strain hardening was obtained.     

Boriding kinetics of H13 steel

It is known that boriding has been employed to increase the service life of parts such as orifices; ingot molds, and dies for hot forming made of AISI H13 steel. In this study, case properties and kinetics of borided AISI H13 steel have been investigated by conducting a series of experiments in Ekabor-I powders at the process temperature of 1073, 1173 and 1273 K for periods of 1-5 h. The presence of borides FeB and Fe₂B of steel substrate was confirmed by optical microscopy and scanning electron microscopy (SEM). The results of this study indicated that the morphology of the boride layer has a smooth and compact morphology, and its hardness was found to be in the range of 1650-2000 HV. Transition zone observed between the hard boride coating and the matrix was relatively softer than the substrate. The kinetics of boriding shows a parabolic relationship between layer thickness and process time, and the calculated activation energy for the process is 186.2 kJ/mol. Moreover, boriding parameter BOP, which is only a function of boride layer thickness and activation energy, has been suggested for the prediction of layer thickness in boriding of AISI H13. There is a reasonable correlation between the progress of boride layer thickness and proposed time-temperature-compensated parameter. Similar findings have been found when it is applied to another steels including tool and low alloy steels, as well as Armco iron.     

High-temperature properties and microstructural stability of hot-work tool steels

Indexable insert tools for machining operations are in service exposed to high temperatures and cyclic mechanical loads. Secondary hardening steels such as hot-work steels are commonly used for tools subjected to thermal exposure. However, these steels, highly alloyed with strong carbide forming elements as Cr, V and Mo, are generally difficult to machine and machining represents a large fraction of the production cost of a tool. Thus, the present study concerns the development of a new steel with improved machinability and meeting the requirements for high-temperature properties.Softening resistance of the THG2000 and QRO90 tool steels, commonly used in hot-work applications, and a newly developed tool steel MCG2006 with lower alloying content of carbide forming elements, was investigated by tempering and isothermal fatigue testing. Mechanisms of high-temperature softening of the tested tool steels were discussed with respect to their microstructure and high-temperature mechanical properties. Carbide morphology and precipitation as well as dislocation structure were determined using transmission electron microscopy and X-ray line broadening analysis.No difference in softening behaviour was found among the QRO90 and MCG2006 regarding hot hardness measurements. The THG2000 indicated some stabilization of the hardness between 450 and 550 °C and a considerable hardness decrease at higher temperatures.The short-time cyclic softening in isothermal fatigue was controlled by dislocation rearrangement and annihilation. The alloying composition of the steels presently tested had no influence on the dislocation density decrease.The long-time softening was affected by the material's temper resistance and strongly depended on the carbide morphology and their over-ageing resistance. The QRO90 with greater molybdenum and lower chromium contents than in the THG2000 show the best resistance to softening among the tested grades at all temperatures. The MCG2006, leaner alloyed with the carbide forming elements and alloyed with 4 wt% nickel, has better temper resistance than THG2000 at higher temperatures and longer tempering times.     

Microstructural characterization of oxide scales formed on steels P91 and P92

Abstract

In this paper, an in-depth study of steam oxidation of two 9Cr ferritic-martensitic steels (P91 and P92), for advanced power plant, has been carried out. The steels investigated were exposed to a 100% flowing steam environment at 650°C for 1,000 – 3,000 hours. Metallographic analysis showed a multilayered scale was formed on both substrates consisting of an outer Fe-rich phase and inner Fe – Cr spinel, of varying oxide stoichiometry, as well as Cr –Mn-rich bands. The results of scale thickness measurements showed that the oxide formed on the P92 steel was thicker compared to that on the P91 steel. Crystallographic analysis showed that the P91 steel exhibited a martensite to ferrite transformation adjacent to scale – substrate interface which was not observed on the P92 steel. The outer oxide layer on both substrates exhibited a region of equiaxed grains followed by a region of columnar crystal growth. However, on the P91 steel the longer exposure time (3,000 hours) resulted in the outer layer having a region with coarsened equiaxed magnetite grains. The roles of alloying elements (Cr, Mo, W and Mn) were investigated to provide a better understanding of the oxide growth behaviour.     

Laser surface alloying of medium carbon steel with SiC(P)

Laser surface alloying (LSA) is increasingly recognised as a powerful surface modification tool to enhance the wear and corrosion resistance of engineering components. The present work deals with laser alloying of medium carbon steel with silicon carbide using a high-power CO₂ laser. A processing regime, identifying an appropriate laser power-scan speed combination for achieving defect-free alloyed layers, has been established during the study. The influence of repetitive scans on the alloyed layer properties was also subsequently investigated in a comprehensive manner. Repetitive scanning was found to affect substantially the laser-alloyed zone (LAZ) and heat-affected zone dimensions, as well as the phase constitution of the laser-alloyed layers. The microstructure in the LSA layers was also observed to vary significantly with the number of scans, besides changing quite distinctly with depth in the LAZ. Tribological tests revealed considerable improvement in abrasive wear performance by laser surface alloying, and the results also emphasise the need to control the number of repetitive scans to achieve optimum performance.     

The role of alloying elements in the design of nickel-base superalloys

The constituents of nickel-base superalloys have been classified into solid solution formers, precipitate formers, carbide formers and surface stabilizers. The characteristics of solutes which would make them most suitable in each category have been specified and appropriate alloying elements have been identified. Nickel-base superalloys are hardened primarily by the precipitation of Ni₃X type compounds. The occurrence and crystallography of precipitation of various kinds of Ni₃X type precipitates have been considered. The role of substitution by alloying elements on mismatch and stability of phases has been discussed. The free electron model and the Engel-Brewer model have been applied for evaluating the stabilities of precipitates, and the role of the alloying elements in determining the stabilities of external and internal surfaces such as grain boundaries have been briefly outlined.     

Metallurgical changes and mechanical behaviour during high temperature aging of welds between Alloy 800 and 316LN austenitic stainless steel

Abstract

In the use of ferritic to austenitic stainless steel transition joints for power plant applications, the difference in coefficients of thermal expansion constitutes a serious problem. One way to mitigate this is to use a trimetallic configuration by interposing a material with a coefficient of thermal expansion intermediate between the ferritic and austenitic steels. Modified 9Cr - 1Mo steel has been joined to 316LN austenitic stainless steel using Alloy 800 as an intermediate piece. In the work herein reported, welds between Alloy 800 and 316LN have been produced using Inconel 182 filler material. These have been subjected to high temperature exposure for up to 5000 h at 625 ° C. Results have shown that up to 500 h of aging the structure and mechanical properties remain unaffected. On treatment for 2000 and 5000 h, however, there is a noticeable increase in hardness and reduction in toughness. These have been found to be caused by precipitation of Ni₃Ti and carbide phases including NbC and M₂₃C₆.