Smithells Metals Reference Book Part 15 pdf

Wrought PM materials 23-25 Superalloys owe their good strength properties to precipitation hardening by a titanium- aluminium compound, but as the temperature rises, this compound begins to go into solution and loses its effect. This puts a ceiling on the working temperture. The mechanical alloying process to produce a !he dispersion of a ceramic material-yttria is favoured-that is virtually insoluble in the metal matrix, significantly increasing the temperature at which useful strength is retained. A list of ODS superalloy compositions is given in Table 23.18, and Figure 23.11 shows the improved performance of one of these alloys compared with that of a similar composition made conventionally. 23.143 Copper One of the main users of copper is the electrical industry where conductivity is the primary consideration. Increasingly, power plant is required to operate at temperatures well above ambient where pure copper recrystallizes and mes very soft. Increasing the strength and recrystallization temperature by alloying drastically reduces the conductivity. The inclusion in pure copper of a small percentage of hely dispersed aluminium oxide provides significantly better elevated temperature strength with only a small reduction in conductivity. An atomized powder of a dilute aluminium copper alloy is internally oxidized to give a very he Al,O, particles, the powder being then processed by compaction, sintering, and working. The improved strength at room temperatures enables smaller sections to be used in, for example, miniaturized systems. Figure 23.12 gives some results for a range of alumina contents. 23.14.4 Lead This is another soft metal whose strength can be increaskd by a dispersed oxide phase. In this case lead oxide is used. The chief application is to chemical plant especially for handlingsulphuric acid. Table 23.18 NOMINAL COMPOSITIONS (W%) OF MECHANICALLY ALLOYED OXIDE DISPERSION STRENGTHENED SUPERALLOYS Ni Fe Cr AI Ti C Y,O, Mo W Ta B Zr INCOLOY. alloy MA 956 Bal 20 4.5 0.5 0.05 0.5 INCOLOY alloy MA 957 Bal 14 1.0 0.05 0.25 0.3 INCONEL. alloy MA 954 Bal 1.0 20 0.3 0.5 0.05 0.6 INCONEL alloy MA 758 Bal 1.0 30 03 0.5 0.05 0.6 INCONEL alloy MA 6ooo Bal 15 4.5 2.5 0.05 1.1 2.0 4.0 2.0 0.01 0.15 INCONEL alloy MA 760 Bal 20 6.0 0.05 0.95 2.0 3.5 0.01 0.15 * INCOLOY end INCONEL arc hademarks of the Inco family of companies. Iro=hromhtm afioys: INCOLOY alloy MA 956 sheet, plate, bar, spinnings, rings and forgings have applications in the hot sections of gas turbines and dim1 engines where the resistance of the alloy to creep, oxidation and sulphidation allow higher metal temperatures and longer component life. The alloy is being used to replace molybdenum in high-temperature vacuum furnaces for fixtures and heat-treatment trays. INCOLOY alloy MA 957 is intended for nuclear power applications, especially fuel cladding in liquid metal cooled reactors. Compared with 316 type stainless steel it has higher strength at 700°C and considerable resistance to irradiation damage. Nickel-chromium alloys: INCONEL alloy MA 754 is used for brazed nozzle guide vane and band assemblies in advanced military aero endnes. The principal advantages of the alloy for these applications are thermal fatigue resistance, long term creep strength and a high melting point. INCONEL alloy MA 758 is highly resistant to attack by molten glass and is used in spinnercttes for the production of fibre glass. The parts are formed by hot spinning plate. Gm p'me ODs alloys: The immediate applications for INCONEL alloy MA 6ooo are for first- and second-stage turbine vanes and blades machined from solid bar. Forced airfoil components have also been developed. The characteristics of INCONEL alloy MA6OOO allow blade cooling to be reduced or eliminated as the metal temperature CUI be increased by 100 K or more in engines where the stresses are medium or low. INCONEL alloy MA 760 is an industrial gas turbine derivative of INCONEL alloy MA 6M)o having greater resistance to corrosion and oxidation. Initial applications are for machined vanes and blades but forged components are under development. 23-26 Sintered materials 200 100- w- E 80- z - 60- E ,,l - In 40 20 - - - r 1093 C Rupture strength Mechanically alloyed ODS (InconelMA 754) I I I1 I I 1 II (Inco Alloys International) Figure 23.11 Properties of mechanically alloyed Income (Inconel b a trakmark of Inco Alloys Intemarwnal) compared with a conventional super alIoy Composition of Inconel alloy MAIM: Fe Al Cr C Y,Q, Ti Ni 1 .o 0.3 20 0.05 0.6 0.5 bal. 23.145 Aluminium Section 23.3.1 referred to the mechanical alloying of AI with graphite to produce powder containing a dispersion of aluminium carbide. This powder containing a dispersion of aluminium carbide. This powder compacted into billets and extruded gives a product with much improved strength at elevated temperatures (see Figure 23.13). This may have applications in aircraft where weight saving is of significant value. 23.15 Spray Forming This process is not powder metallurgy in the strict sense of the term in so far as the metal is at no stage in the form of powder. However, for reasons that will be apparent the PM world has adopted it. The process involves gas atomization of a liquid metal, but instead of allowing the droplets to solify as powder, the spray is caused to impinge on a solid surface where the droplets are collected as a semi-solid layer which solidifies as a layer of dense metal. This layer may be built up to any desired thickness, and by suitable choice of design of the original target, the angle of the spray, and other parameters, near-net shapes can be produced. For example if the target is a cylinder rotating horizontally and capable of being moved in a controlled manner in the axial direction, a tube of dense deposited metal is formed. The deposit has aII the advantages of dense metal produced from powder, is. complete absence of macro-segregation and pipe-related defects. A further merit of the process is that by injecting fine refractory powder particles, i.e. by entraining them in the atomizing gas stream, ODS material m be deposited. Injection moulding 23-27 Aluminum oxide content, YO)% 0.45 0.93 1.35 1.80 225 2.70 3.15 m C I vi 0 C P X 59 0 0.10 0.20 0.39 0.40 0.59 0.60 0.70 Aluminum content, WO Figure 23.12 Properties of clirprswn-strengthened copper (CI5715 and C15760 ore compositions made by SCM Metd Products) 23.16 Injection Moulding Commonly referred to as MIM (metal injection moulding), this proprietary process consists of mixing fine G20pm metal powder with a thermosetting organic material to form a plastic mass that can be injected under pressure into a mould to form the equivalent of a compact. The part is then carefully treated by solvents and/or heat to remove the binder and then at a higher temperature eventually to sinter the metal. Shrinkage of the order of 10% linear occurs but this can be predicted accurately and parts with very close dimensional tolerances and of quite complex shape can be made. It is an expensive process but the advantages in eliminating expensive machining operations make it viable for a number of applications. 30 25 s 20 2 0 W 0 I m 15 iii 10 %v, U s .o 0- 0 ZOO LOO 600 TemDerature loci Figme 2313 Strengthllempemhae retatiomhip of ahanin- iwn with a dispersion of AI,C, produced by mechicd alloyhg compared with mgot-based high strength alloys. Dkpd b a tra&tnnrk of Smtermetdlwerk Krebscge 23.17 Hardmetals and related Hard Metals This family of PM materials consists of fine, hard, and usually brittle particles bonded with a relatively soft and tough binder phase which is normally metallic. The hard particles are generally between 1 and 5pm, but even finer grades with particles below 1 jm are now being made. The original hard phase was tungsten monocarbide (WC) and the preferred binder phase was cobalt. The name cemented carbide was, and to a large extent still is, used to describe these materials, but the official name is now hurdmetuL Carbides other than that of tungsten were later added, e.g. those of tantalum, niobium, and titanium, and binder metals other than cobalt have been used, but the WC/Co-based compositions still have the lion’s share of the market. More recently, products have been developed in which the hard phase is not carbide, but nitride, boride, carbo-nitride, oxide, or combinations of these. Such materials may not strictly be called hardmetals since hardmetal has been defined in IS0 standard 3252 as ‘sintered material characterized by high strength and wear-resistance comprising carbides of refractory metals as the main component together with a metallic binder phase’. Uses Hardmetals were originally developed as a substitute for diamond as wire drawing dies for tungsten, and they are stdl used for that purpose, but the largest single use today is as cutting tools. It is for this application that the noncarbide alloys have been developed, but no cost effective substitute has been found for the WC/Co hardmetal where straightforward wear resistance is the primary requirement, and this includes cuffing tools for non-ferrous metals and non-metallic materials. Other such cases are wire drawing dies, dies for the compaction of metal powders for the manufacture of PM parts, rolls for metal rolling miIls, and other large abrasion-resistant parts. Munufacture The process for the production of hardmetal is a classic example of liquid phase sintering: a mixture of WC and cobalt powders is pressed and sintered at a temperature above the melting point of cobalt. However, to get good results special procedures in the preparation of the powder mix are necessary. The indents are wet milled together in order to coat each carbide particle with cobalt, and to facilitate this, the cobalt powder must be extremely fine. It is well known that very fine powders do not flow readily, if at all, and to overcome this problem the WC/Co mixture is ‘granulated‘, by which is meant the production of agglomerates. A favoured method of granulation Hardmetals and related hard metals 23-29 is the spray drying of a slurry of the powder with a liquid containing also a pressing lubricant. During sintering, the compact shrinks by as much as 50 vol% to become nearly 100% dense. The sintering temperature used in practice varies with the composition, being lowest (1400°C) when the cobalt content is high, and rising to 1600°C) or higher with compositions having high proportions of the carbides of Ti, Ta, and/or Nb and low cobalt contents. In cases where it is not posible to get close to the required shape by direct pressing and sintering, the compact may be presintered at a lower temperature so as to remove the lubricant and provide sufficient strength for handling. In this state, the object can be machined using tools of bonded diamond or other hard material. Although it is usual to refer to the binder phase as being cobalt there is some mutual solubility between it and the carbide, and great care is needed to ensure that the carbon balance is maintained such that neither the brittle W/Co (eta) phase nor free graphite is formed. The toughness is affected also bq the size of the carbide particles; the her they are the harder but less shock-resistant is the final product, However, grades with carbide particle size well below 1 pn are reported to combine high hardness with toughness. -Hot Iso-static Pressing Although the porosity of conventionally produced hardmetal is normally low, porosity can be completely eliminated by hot iso-static pressing (HIPping). Toughness is, thereby considerably increased and the possibility of the rejection of large and expensive components at a late stage of grinding or polishing no longer presents a problem. HIPping is now routinely applied to a large number of hardmetal parts including indexable cutting tool tips that are a major product. Recently it has been found possible to combine HIPping with the sintering stage. The parts are sintered in vacuum to a density such that the porosity is sealed, and then high pressure gas, usually argon, is introduced into the furnace. The pressure required for full densihtion is lower than that needed for the HIPping of already sintered parts, and the new process, referred to as Sinter-HIP or Pressure Assisted Sintering is rapidly replacing the original two-stage process of vacuum sintering followed by HIPping in a separate furnace, at least for cutting tool tips. Compositions Straightforward WC/Co hardmetal appears to be the most cost-effective material for many applications where wear resistance is the primary requirement, including the machining of non-ferrous metals. Additions of e.g. TaC improve the already good wear resistance, perhaps by acting as grain growth inhibitors, and are especially valuable in applications involving high temperatures. When the use is the machining of ferrous materials at high speeds, the situation is different. In addition to abrasive wear, reaction between the carbide particles and the steel results in what is known as crater wear. The substitution of more stable carbides such as those of Ta, Nb, Ti, and Hffor some or all of the WC considerably improves the cratering resistance. Wear resistance is, as would be expected, markedly iniluend by the amount of binder phase as shown in Figure 23.14, but reduction in the cobalt content also makes the alloy more brittle, so a compromise between toughness and wear resistance is necessau. Table 23.19 gives compositions and properties of one manufacturer’s range. Table 23.20 lists the IS1 classification of carbides according to use. Alternative Binders Because of the high price of cobalt and the perceived instability of the countries that produce the bulk of it, continuing efforts have been made to find alternatives. Ni, Fe, and Ni/Mo have been used successfully in certain applications, and a recent entry into the field is Ni/Cr which now appears in some commercial grades. One grade is based on Tic with MoC and a Ni/Mo binder. Good results have been reported also with a superalloy binder which combines conspicuous high-temperature strength with toughness. Useful resuls have been reported also with a nickel binder containing ruthenium, but this member of the platinum group is, of course, relatively very costly. Coatings For tools for the machining of steels the most dramatic improvement has been the development of surface coatings. An ideal tool material fr.om the cutting point of view would be a pure, very 23-30 Sintered materials Table 23.19 COMPOSITION AND PROPERTIES OF TIZIT GRADES OF HARD METAL Chemical composlrlon-wdghf % Average Fmmerse Size strength Densify Ta grain ruphae IS0 codes TIZT (Nb) grades WC Tic C Co NqCr pm MPa plcm' m5 PI0 m25 P%PM P3&P40 P40 MI0 MICLM20 K10-K20 M15 M2O KOl-KO5 KO5 K10 KID-KZO K20 K2D-K30 K30 K40 KM KO5-K.20 POSP25 KMK20 K05-K20 P15-P30 KIC-K20 P15-PU) K10-K20 P25-P40 P25-P40 POSPZS P35-P45 m5-~20 W5T SlOT s2n. S26T S36T S40T UlM U16T u17T UZOT H03T HOST HlOT H16T H20T H25T H30T H40T HMT H60T H70T H80T BlOT B30T B4OT B5@T TCRlO TCR30 Sr16* Sr17* Gm IS** Gm 25** Gm26*=* om 35" 60.5 69 71.5 69.5 16.5 17 83.5 84 a6 77 96.7 81.5 94.2 93.7 932 902 90.2 81.2 842 80 76 72 94 91 88 85 94 91 86 86 83.5 71 17 76.5 GmM*** 76.5 Gm176*** 86 Gm306*** 91 22 16 9 6.5 4 4 5 7.5 2.5 4 1.5 - - - 0.2 0.2 0.2 0.2 - - - - - - - - - - 2.5 2.5 5 4 4 4 4 2.5 - 11 8 11 14 7.5 8 5.5 1 5.5 10 0.3 6 0.3 0.3 0.6 0.3 0.6. 0.6 0.6 - - - - - - - - - 5.5 5.5 5.5 IO 10 7.5 7.5 5.5 - 6.5 7 8.5 10 12 11 6 7.5 6 9 3 5 5.5 6 6 9.5 9 12 15 20 24 28 6 9 12 15 - - 6 6 6 9 9 12 12 6 9 2-3 2-3 2-3 2-3 2-3 3 1-2 1 3 3 1 1 1 1-2 2 1 2 2 2 3 3 3 2-3 3-4 3-4 34 1-2 2 3 3 1-2 3 3 2-3 2-3 3 3-4 1400 1650 1800 1900 2200 2400 1800 1800 1950 2050 1600 1800 1900 1950 z000 2100 2200 2550 2800 3000 2900 2800 20.50 2500 2550 2600 2000 2300 - - - - - - - - - 10.0 11.3 12.4 12.7 13.1 13.3 13.5 12.9 14.0 13.3 15.3 14.5 14.8 14.9 14.1 14.7 14.6 14.3 14.0 13.5 133 12.8 14.9 14.6 14.2 14.0 14.8 14.4 - - - - - - - - - * Coating/thickness 8 /rm. **Coatinglthickness liC, Ti(C, N), Ti llL12p. ***Coatinplctbickness Tic, Ti(C, N), TiN 5p. Hardmetals and refated hard metals 23-31 Co@cient Young's Thermal ofthermal Compressive modulus Coerciw Electrical conductivity expansion Hardness strength of elasticity Poisson's force restiuity (20°C) (20-400"C) HV30 GPa GPa ratio wm pcm w/mK 10"-6/K 1705 1675 1575 1555 1390 1420 1685 1730 1575 1460 1850 1830 1750 1675 1615 1545 1470 1340 1225 1030 920 855 1500 1215 1115 990 1500 1400 Hardness of coating >2mv 5.3 5.3 5.2 5.1 5 .O 4.9 5.7 5.7 5.3 5.1 6.1 6.1 6.0 5.8 5.7 5.5 5.4 5.0 4.5 3.8 3.5 3.2 5.5 4.2 4.0 3.8 5.5 5.0 - - - - - - - - - 490 530 540 550 560 540 620 600 590 550 660 650 650 640 640 630 620 600 570 520 500 480 650 630 580 560 630 600 - - - - - - - - - 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.22 0.23 0.24 0.24 0.24 0.22 0.22 0.22 0.23 0.19 0.21 - - - - - - - - - 11.94 11.94 11.94 13.13 11.14 11.94 14.92 19.89 12.73 10.5 24 19.5 18.70 17.11 15.12 14.72 12.33 10.74 9.55 7.00 5.81 5.25 11.95 7.3 6.5 6.0 - - - - - - - - - - - 91 58 37 30 25 24 24 26 25 28 20 23 20 20 18 18 17 17 17 16 16 19 19 19 17 m - - - - - - - - - - - 25 30 40 50 60 60 60 50 60 60 100 100 100 100 100 90 90 90 80 70 60 60 100 90 80 80 70 61 - - - - - - - - - 7 6 6 6 5.5 6 5.5 5.5 5.5 5.5 5 5 5 5 5 5 5 5.5 6 6 6.5 6.5 5 5 5.5 6 5 4.8 - - - - - - - - - Figure 23.14 Relationship between cobait content, hardness and wear of WC/Co hard metal Table WO IS0 CLASSIFICATION OF CARBIDES ACCORDING TO USE Steel, steel casting8 Steel, steel castings Steel, steel castings Malleable cast iron with long chips Steel, steel castings Malleable cast iron with long chip Main groups of chip remoual categories of materio1 Finish turning and boring, high cutting speeds. small chip sec tion, accuracy of dimensions and he finish, vibration-free operation Turning, copying, threading and milling, high cutting speeds, small or medium chip sections Turning, copying, milling, medium cutting speeds and chi, sections, plaoing with small chip sections Turning, milling, planing, medium or low cutting speeds, me- dium or large chip sections, and machining in unfavourable conditions* Groups of application 1 Distin- Steel castings with sand inclusion and. I tions with the wssibilitv of lam cuttina and- for machin- Muterial to be machined Steel Steel castings of medium or low tensile strength, with sand inclusion and cavities I use ami working comiitiom For operations demanding very tough carbides: turning, plan. ning slotting, low cutting speeds, large chip sections, with the possibility. of large cutting angles for machining in unfavour- able conditions. and work on automatic machina - cavities ing &I unfavourible con&tions*-and woyk on automatic whines I Direction of lncrease in characteristic o/ f cut carbide M - K Steel, steel castings, mangaoese stecl Grey cast iron, alloy cast iron Steel, steel casting;, austenitic or manganese steel, grey cast iron Steel, steel castingi, austenitic steel, grey cast iron, high temperature resistant alloys Mild free cutting steel, low tensile steel Non-ferrous metals and light alloys lo M 20 M 30 40 Ferrous metals with long or short chip and non-ferrous metals Turning, medium or high cutting specds. Small or medium chip t sections Turning, milling. Medium cutting speeds and chip sections Turning, milliog, planing. Medium cutting speeds, medium or Turning, parting off, particularly on automatic machines 7 I I t large chip sections Ferrous metals with short non-ferrous metals and non-metallic materials Chips Very hard grey cast iron, chilled castings of over 85 Shore, high silicon aluminium alloys, hardened steel, highly abrasive plastics, hard cardboard, K 01 1 It I II finish brin& ,,,ang, scraping ceramics Grey cast iron over 220 Brinell, malleable cast iron with short chips, hardened steez silicon ahm- iuium alloys. copper alloys, plastics, glass, hard rubber, hard cardboard, porcelain, stone. rey cast iron up to rine , non-ferrous cast iron, low tensile steel, 2o :et& copper, b-i%iniul T,,,&,& &jlin& drilling, bring, broachin& Turning, milling, planing, boring, broaching, demanding very tough carbide 51, fondit!ns* id with k bosPifbility :.irgecut& urning, m ing, paning, s ttmg, or mac inlog in u avour- P a 3 7 7 I I angles Turning, milling, planing, slotting, for machining io unfavour- able conditions* and with the possibility of large cutting angles I I I Rcprodnd from IS0 recmcndstion 513 by permidon of the British Standards institution, 2 Park Street, London, WIA 2BS [...]... (0-34.5 M Pa) 260 10 300 1 5 151 0 1610 1 700 017 2 0.127 016 2 125 - 5 44 37 28 3 1340 - 35 none 300 Polydimethyl (1oooCS 17 6 14 3 12 2 1 870 1910 2 loo 010 5 018 4 016 4 - 277 5 18 1 18 1 19 4 155 0 - 0.162 019 5 015 5 177 316 30 Medium phenyl Chlorophenyl 10 7 0170 0.060 0.0225 15 7 -50 20 19 19 - 016 4 145 60 10 1 0.046 0.027 0. 0156 197 < -73 20 20 1 490 155 0 1 670 0 .150 015 4 0.140 =-273 - 70... Austenitic stahkss steel Carbon-graphite Copper Iron Nickel Silicon nitride T o steel (15Mo15Co) ol 15 2 02 6 - - - Zinc 04 3 03 9 0.52 77K 295K - - 14 5 03 5 07 8 08 4 11 0 07 0 07 5 11 2 650°C 980°C - - 0.99 08 1 09 7 10 6 315 C 14 9 - - 0.28 03 0 - 04 8 026 - The friction behaviour of polymers differs from that of metals in three respects First, the coefficient of friction tends to decrease with increasing... Alcohol Benzene Glycerine 100°C 0.16 0.125 0 .15 0.195 0.18 0.09-0.1 0.13 0.08 0.33 0.43 0.48 0.2 0.19 0 .15 0.2 0.205 0.22 0.09-0.1 0 .15 0.08 - - 0.25 LUBRICATION OF METALS ON STEEL STATIC FRICTION Rape oil Castor oil Bearing surface Ir, H Long chain Mineral oil fatty acids H R Hard steel (axle steel) Cast iron Gun metal Bronze Pure lead Lead-base white metal (Sb 15, Cu 0.5, Sn 6, Pb 78.5) Pure tin Tin-base... PTFE (low speeds) FTFE (high speeds) Filled PTFE (15% glass fibre) Filled PTFE (15% graphite) Filled PTFE (W/, bronze) Rubber (polyurethane) Rubber (isoprene) Rubber (isoprene) Dry Wet Dry Dry Dry Dry or wet Dry or wet Dry or wet Natural Natural Dry or wet Dry or wet Dry Dry B Y Dry Dry W t (water-alcohol e solution) 0.4 0 .15 0.5 0.5 0.5 0.4 0.1 0 .15 0.25 0.1 0.06 0.3 0.12 0.09 0.09 1.6 3-10 2-4 Table... hardness (kg mm-') Metal Metal ~~ Silver Tn i Aluminium zinc 20 26 5 15 35 cpe opr 40 Gold Iron Chromium plate 120 800 Oxide 1650 1800 200 130 150 - Load (g) at which appreciable merallic contact occurs 0 003 0 0.02 02 0.5 1 10 Never The static coefficients of friction of a number of metals and alloys on steel are shown in Table 25.3 Of particular note are the values for indium and lead,which are the same... 0.00287 11 4 15 4 < -65 -60 14 0 12 8 10 5 Mixed C3-G C-l 4CO pentaerythritol ester dipentaerythritol ester Triaryl phosphate Fluorocarbon ester Polyglycol at 25°C) 1 0 0 0.032 0.012 0.005 144 -60 10 2 007 8 0.025 0.083 132 11 3 19 5 007 8 0.280 0 0 85 3 0.0195 0.0053 0.0103 0 -21 - 18 -15 10 2 0.220 0.070 0.022 5 164 -25 09 7 0.140 003 8 0.045 200 -55 -50 - 33 13 8 11 8 20 1 2 - - 1960 2 100 0 .154 019... Table 25.12 STATIC FRICTION OF VARJOUS METALS (SPECTROSCOPICALLYPURE) LUBRICATED WITH 1% SOLUTION OF LAURIC ACID (M.P UT) IN PARAFFIN OIL AT ROOM TEMPERATURE Co@cient offriction p, Metal Unlubricated Lubricated Aluminium Cadmium Chromium Copper Iron Magnesium Nickel Platinum Silver 1.3 0.5 0.4 1.4 1.0 0.5 0.7 1.3 1.4 0.3 0.05 0.34 0.10 0 .15 0.10 0.3 0.25 0.55 Reference: 4 Table 2 3 5 1 LUBRICATION... cost of the tool insert is a small part only of the total machining cost, and if the cutting rate can be doubled and the tool life halved in consequence, the overall efficiency may well be much greater than that of prolonging the life of the tool Acknowledgements Associazione Industriali Metallurgici Meccanici m i (AMMA) American Society for Metals (ASM) British Powder Metals Federation (BPMF) and Powder... performance of a number of coated and uncoated metal pairs 25.2.3 Erosive wear Erosive wear due to the impact of a stream of solid particles is dependent on the size, hardness, f velocity and angle of impact o the particles Wear rate generally increases rapidly with increasing particle size and hardness and impact velocity For strong and tough materials the maximum wear rate occurs at an impact angle of... viscous effects occur together, the conditions are said to be those of ‘mixed‘ lubrication W e sliding speeds are very low indeed a ‘stick-slip’ or jerky motion arises due in part to the hn elastic response of the drive and in part to the coefficient of static friction exceeding the coefficient hs of dynamic friction T i undesirable effect can be suppressed by the use of special lubricants containilrg . 10"-6/K 1705 1675 157 5 155 5 1390 1420 1685 1730 157 5 1460 1850 1830 1750 1675 1 615 154 5 1470 1340 1225 1030 920 855 150 0 1 215 1 115 990 150 0 1400 Hardness of. speeds, medium or Turning, parting off, particularly on automatic machines 7 I I t large chip sections Ferrous metals with short non-ferrous metals and non-metallic materials. 1.76 1.43 1.22 1 870 1910 2 loo 0 .150 0.148 0.146 - 277 5 0.97 0.140 0.083 0.045 200 -55 1.81 1.81 1.94 - 155 0 - 0.162 0 .159 0 .155 177 316 30. * AveraEe value over