Volume 01 - Properties and Selection Irons, Steels, and High-Performance Alloys Part 13 pps

Light 208 30.2 45.1 6.54 0.4 606 87.9 2.7 2.0 . . . . . . 112 Heavy 205 29.8 44.9 6.51 0.7 738 107 2.7 2.0 . . . . . . 110 Standard 35 None 345 50 . . . . . . <1.0 620 90 2.7 2.0 275 40 61 None 461 66.8 47.2 6.85 1.7 952 138 8.8 6.5 . . . . . . 95 Light 405 58.8 48.5 7.03 1.2 924 134 5.8 4.3 . . . . . . 111 F-208 Heavy 383 55.5 47.6 6.9 0.3 1040 151 5.7 4.2 . . . . . . 108 Standard 35 None 434 63 . . . . . . <1.0 903 131 7.5 5.5 360 52 75 Source: Ref 9 (a) None, as sintered; Light blackening, 2 h exposure in 538 °C (1000 °F) steam; heavy blackening, 4 h exposure in 538 °C (1000 °F) steam. (b) Unnotched Charpy test at room temperature. References cited in this section 8. H. Ferguson, Heat Treatment of P/M Parts, Met. Prog., Vol 107 (No. 6), June 1975, p 81- 83; Vol 108 (No. 2), July 1975, p 66-69 9. L.F. Pease III, J.P. Collette, and D.A. Pease, Mechanical Properties of Steam Blackened P/M Materials, in Modern Developments in Powder Metallurgy, Vol 18-21, Metal Powder Industries Federation, 1988 Ferrous Powder Metallurgy Materials Revised by Leander F. Pease III, Powder-Tech Associates, Inc. Re-pressing As a secondary mechanical forming operation performed at room temperature, repressing is done primarily to increase density, which increases mechanical and physical properties and hardness. Improvements in part dimensions can also be achieved by re-pressing. The amount of material deformation achieved with repressing is greater than in sizing because the forces used are greater than the sizing forces. The reduction in height of a ferrous part generally ranges from 3 to 5%. As with sizing, part tolerance after re-pressing depends on material type and part size. Re-pressing generally refers to the application of high pressures on a sintered part at room temperature, while powder forging refers to processes in which a P/M preformed part is kept at an elevated temperature during the application of high pressure (see the section "Powder Forging" ). At room temperature and at pressures as high as or higher than the compacting pressure, re-pressing increases the strength of a sintered P/M part by decreasing its porosity and by cold working the metal. The part is considerably strengthened, but at the expense of ductility. Resintering after re-pressing increases the ductility and toughness of the part without diminishing its strength. Those materials that are difficult to re- press after sintering usually can be re-pressed if the sintering is done at a low temperature at which alloying cannot take place; this low-temperature sintering is called presintering. For iron alloys, presintering is done at 845 °C (1550 °F). The effect of re-pressing on the density of ferrous P/M materials is shown in Fig. 14. The density that is achieved by re- pressing depends on the density of the sintered or presintered compact, the re-pressing pressure and lubricant, and whether the powder used was prealloyed or mixed from elemental powders. Fig. 14 Effect of re- pressing on density of powder metallurgy compacts. Alloy steel powders (4640 composition) were compacted at various pressures, then sintered, re- pressed, and resintered. For each specimen, the final density is indicated by the intersection between the curve that indicates the re- pressing pressure and the grid line that indicates the green compacting pressure. (a) Prealloyed powder. (b ) Diffusional alloy made from elemental powders Ferrous Powder Metallurgy Materials Revised by Leander F. Pease III, Powder-Tech Associates, Inc. Powder Forging * Powder forging is a process in which unsintered, presintered, or sintered powder metal preforms are hot formed in confined dies. The process is sometimes called P/M forging or P/M hot forming, or is simply referred to by the acronym P/F. Powder forging is a natural extension of the conventional press and sinter (P/M) process, which has long been recognized as an effective technology for producing a great variety of parts to net or near-net shape. Figure 15 shows the powder forging process. In essence, a porous preform is densified by hot forging with a single blow. Forging is carried out in heated, totally enclosed dies, and virtually no flash is generated. Fig. 15 The powder forging process The shape, quantity, and distribution of porosity in P/M and P/F parts strongly influence their mechanical performance. Powder forging is a deformation processing technology aimed at increasing the density of P/M parts and thus their performance characteristics. There are two basic forms of powder forging: • Hot upsetting, in which the perform experiences a significant amount of lateral material flow • Hot re- pressing, in which material flow during densification is mainly in the direction of pressing. The form of densification is sometimes referred to as hot restriking, or hot coining While P/F parts are primarily used in automotive applications where they compete with cast and wrought products, parts have also been developed for military and off-road equipment. The economics of powder forging have been reviewed by a number of authors (Ref 10, 11, 12, 13, 14, 15). Some of the case histories included in the section "Applications of Powder Forged Parts" in this article compare the cost of powder forging with that of alternative forming technologies. Material Considerations The initial production steps of powder forging (performing and sintering) are identical to those of the conventional press and sinter P/M process. Certain defined physical characteristics and properties are required in the powders used in these processes. In P/M parts, surface finish is related to the particle size distribution of the powder. In powder forging, however, the surface finish is directly related to the finish of the forging tools. Typical pressing grades are -80 mesh with a median particle size of about 75 μm. The apparent density and flow are important for maintaining fast and accurate die filling. The chemistry affects the final alloy produced, as well as the compressibility. Green strength and compressibility are more critical in P/M than they are in P/F applications. Although there is a need to maintain edge integrity in P/F preforms, there are rarely thin, delicate sections that require high green strength. Because P/F preforms do not require high densities (typically 6.2 to 6.8 g/cm 3 ), the compressibility obtainable with prealloyed powders is sufficient. However, carbon is not prealloyed because it has an extremely detrimental effect on compressibility. The two principal requirements for P/F materials are a capability to develop an appropriate hardenability that will guarantee strength and to control fatigue performance by microstructural features such as inclusions. Hardenability. Nickel and molybdenum have the advantage that their oxides are reduced at conventional sintering temperatures. Alloy design is therefore a compromise, and the majority of atomized prealloyed powders in commercial use are nickel/molybdenum based, with manganese present in limited quantities. The compositions of three commercial P/M steels are: Composition, wt% (a) Alloy Mn Ni Mo P/F-4600 0.10-0.25 1.75-1.90 0.50-0.60 P/F-2000 0.25-0.35 0.40-0.50 0.55-0.65 P/F-1000 0.10-0.25 . . . . . . (a) All compositions contain balance of iron. The higher cost of nickel and molybdenum, along with the higher cost of powder, compared with conventional wrought materials, is often offset by the higher material utilization inherent in the P/F process. More recently, P/F parts have been produced from iron powders (0.10 to 0.25% Mn) with copper and/or graphite additions for parts that do not require the heat-treating response or high-strength properties achieved through the use of the low-alloy steels. Inclusion Assessment. Because the properties of material powder forged to near-full density are strongly influenced by the composition, size distribution, and location of nonmetallic inclusions (Ref 16, 17, 18), a method has been developed for assessing the inclusion content of powders intended for P/F applications (Ref 19 , 20, 21 , 22). Samples of powders intended for forging applications are re-press powder forged under closely controlled laboratory conditions. The resulting compacts are sectioned and prepared for metallographic examination. The inclusion assessment technique involves the use of automatic image analysis equipment. The compact used for inclusion assessment may also be used to measure the amount of unalloyed iron powder particles present. Process Considerations The development of a viable powder forging system requires the consideration of many process parameters. The mechanical, metallurgical, and economic outcomes depend to a large extent on operating conditions, such as temperature, pressure, flow/feed rates, atmospheres, and lubrication systems. Equally important consideration must be given to the types of processing equipment, such as presses, furnaces, dies, and robotics, and to secondary operations, in order to obtain the process conditions that are most efficient. This efficiency is maintained by optimizing the process line layout. Examples of effective equipment layouts for performing, sintering, reheating, forging, and controlled cooling have been reviewed in the literature (Ref 10). Figure 16 shows a few of the many possible operational layouts. Each of these process stages is reviewed in the following sections. Fig. 16 A powder forging process line. Source: Ref 23 Preforming. Preforms are manufactured from admixtures of metal powders, lubricants, and graphite. Compaction is predominantly accomplished in conventional P/M presses that use closed dies. The control of weight distribution within preforms is essential to produce full density and thus maximize performance in the critical regions of the forged component. Excessive weight in any region of the preform may cause overload stresses that could lead to tool breakage at forging. Successful preform designs have been developed by an iterative trial-and-error procedure, using prior experience to determine the initial shape. More recently, computer-aided design (CAD) has been used for preform design. Preform design is intimately related to the design and dimensions of the forging tooling, the type of forging press, and the forging process parameters. Among the variables to be considered for the preforming tools are: • Temperature, that is, preform temperature, die temperature, and, when applicable, core rod temperature • Ejection temperature of the forged part • Lubrication conditions, that is, influence on compaction/ejection forces and tooling temperatures • Transfer time and handling of the preform from the preheat furnace to the forging die cavity Correct preform design not only entails having the right amount of material in the various regions of the preform, but also is concerned with material flow between the regions and prevention of potential fractures and defects (Fig. 17). Fig. 17 Configuration for the ring preform (a) for forging the part shown in (b). (b) Cross section of the part under consideration for powder forging Sintering and Reheating. Preforms may be forged directly from the sintering furnace; sintered, reheated, and forged; or sintered after the forging process. The basic requirements for sintering in a ferrous P/F system are: • Lubricant removal • Oxide reduction • Carbon diffusion • Development of particle contacts • Heat for hot densification Oxide reduction and carbon diffusion are the most important aspects of the sintering operations. For most ferrous powder forging alloys, sintering takes place at about 1120 °C (2050 °F) in a protective reducing atmosphere with a carbon potential to prevent decarburization. Typical P/M sintering has been performed at 1120 °C (2050 °F) for 20 to 30 min. Increases in temperature will reduce the time required for sintering by improving oxide reduction and increasing carbon diffusion. Chromium-manganese steels have been limited in their use because of the higher temperatures required to reduce their oxides and the greater care needed to prevent reoxidation. Any of the furnaces used for sintering P/M parts, such as vacuum, pusher, belt, rotary hearth, walking beam, roller hearth, and batch/box, may be used for sintering or reheating P/F preforms. The sintered preforms may be forged directly from the sintering furnace; stabilized at lower temperatures and forged; or cooled to room temperature, reheated, and forged. All cooling, temperature stabilization, and reheating must occur under protective atmosphere to prevent oxidation. Induction furnaces are often used to reheat axisymmetric preforms to the forging temperature because of the short time required to heat the material. Difficulties may be encountered in obtaining uniform heating throughout asymmetric shapes because of the variation in section thickness. Powder forging involves removing heated preforms from a furnace, usually by robotic manipulators, and locating them in the die cavity for forging at high pressures (690 to 965 MPa, or 100 to 140 ksi). Preforms may be graphite coated to prevent oxidation during reheating and transfer to the forging die. Lubrication of the die and punches is usually accomplished by spraying a water-graphite suspension into the cavity. The forging presses commonly used in conventional forging, including hammers, high energy rate forming machines, mechanical presses, hydraulic presses, and screw presses, have been evaluated for use in powder forging. The essential characteristics that differentiate presses are contact time, stroke velocity, available energy and load, stiffness, and guide accuracy. Metal Flow in Powder Forging. Draft angles, which facilitate forging and ejection in conventional forging, are eliminated in P/F parts. This means that greater ejection forces on the order of 15 to 20% of press capacity as a minimum are required for the powder forging of simple shapes. However, the elimination of draft angles permits P/F parts to be forged closer to net shape. Tool Design. In order to produce sound forged components, the forging tooling must be designed to take into account: • Preform temperature • Die temperature • Forging pressure • The elastic strain of the die • The elastic/plastic strain of the forging • The temperature of the part upon ejection • The elastic strain of the forging upon ejection • The contraction of the forging during cooling • Tool wear Secondary Operations. In general, the secondary operations applied to conventional components, such as plating and peening, may be applied to P/F components. The most commonly used secondary operations involve deburring, heat treating, and machining. The heat treatment of P/M products is the same as that required for conventionally processed materials of similar composition. The most common heat-treating practices involve treatments such as carburizing, quench and temper cycles, and continuous-cooling transformation. The amount of machining required for P/F components is less than the amount required for conventional forgings because of the improved dimensional tolerances. Standard machining operations may be used to achieve final dimensions and surface finish. One of the main economic benefits of powder forging is the reduced amount of machining required. Improved machinability can be accomplished by the addition of solid lubricants such as manganese sulfide. Mechanical Properties Wrought steel bar stock undergoes extensive deformation during cogging and rolling of the original ingot. This creates inclusion stringers and leads to planes of weakness, which affect the ductile failure of the material. The mechanical properties of wrought steels vary considerably with the direction test pieces are cut from the wrought billet. Powder forged materials, on the other hand, undergo relatively little material deformation, and their mechanical properties have been shown to be relatively isotropic. The mechanical properties of P/F materials are usually intermediate to the transverse and longitudinal properties of wrought steels. The rotating-bending fatigue properties of P/F material have also been shown to fall between the longitudinal and transverse properties of wrought steel of the same tensile strength. While the performance of machined laboratory test pieces follows the intermediate trend described above, in the case of actual components, P/F parts have been shown to have superior fatigue resistance. This has generally been attributed not only to the relative mechanical property isotropy of powder forgings, but also to their better surface finish and finer grain size. This section reviews the mechanical properties of P/F materials. The data presented represent results obtained on machined standard laboratory test pieces. Data are reported for four primary materials. The first two material systems are based on prealloyed powders (P/F-4600 and P/F-4200; see the section "Hardenability" in this article). The third material, based on an iron-copper-carbon alloy, was used by Toyota in 1981 to make P/F connecting rods; Ford Motor Company introduced powder forged rods with a similar chemistry in 1986. Mechanical property data are therefore presented for copper and graphite powders mixed with an iron powder base to produce materials that generally contain 2% Cu. Some powder forged components are made from plain carbon steel. This is the fourth and final material for which mechanical property data are presented. Forging Mode. It is well known that the forging mode has a major effect on the mechanical properties of components. With this in mind, the mechanical property data reported in this section were obtained on specimens that were either hot upset or hot re-press forged. Heat Treatments. There were three heat treatments used in developing the properties of the prealloyed powder forged materials: case carburizing, blank carburizing, and through-hardening (quenching and tempering). Hardenability. Jominy hardenability curves are presented in Fig. 18 for the P/F-4600, P/F-4200, and iron-copper- carbon alloys. Testing was carried out according to ASTM A 255. Specimens were machined from upset forged billets that had been sintered at 1120 °C (2050 °F) in associated ammonia. Fig. 18 Jominy hardenability curves for (a) P/F-4600, (b) P/F-4200, and (c) iron-copper- carbon materials at various forged-carbon levels. Vickers hardness was determined at a 30 kgf load. Tensile, Impact, and Fatigue Properties. Tensile properties were determined on test pieces with a gage length of 25 mm (1 in.) and a gage diameter of 6.35 mm (0.25 in.). Testing was carried out according to ASTM E 8 using a crosshead speed of 0.5 mm/min (0.02 in./min). Room-temperature impact testing was carried out on standard Charpy V- notch specimens according to ASTM E 23. Rotating-bending fatigue (RBF) testing was performed using single-load, cantilever, rotating fatigue testers. The tensile, impact, and fatigue data for the various materials are summarized in Tables 9, 10, and 11. Table 9 Mechanical property and fatigue data for P/F-4600 materials Sintered at 1120 °C (2050 °F) in dissociated ammonia unless otherwise noted Ultimate tensile strength 0.2% offset yield strength Room-temperature Charpy V-notch impact energy Fatigue endurance limit Forging mode Carbon, % Oxygen, ppm MPa ksi MPa ksi Elongation in 25 mm (1 in.), % Reduction of area, % J ft · lbf Core hardness, HV30 MPa ksi Ratio of fatigue endurance to tensile strength Blank carburized Upset 0.24 230 1565 227 1425 207 13.6 42.3 16.3 12.0 487 565 82 0.36 Re-press 0.24 210 1495 217 1325 192 11.0 34.3 12.9 9.5 479 550 80 0.37 Upset (a) 0.22 90 1455 211 1275 185 14.8 46.4 22.2 16.4 473 550 80 0.38 Re-press (a) 0.25 100 1455 211 1280 186 12.5 42.3 16.8 12.4 468 510 74 0.36 Upset (b) 0.28 600 1585 230 1380 200 7.8 23.9 10.8 8.0 513 590 86 0.37 Re-press (b) 0.24 620 1580 229 1305 189 6.8 16.9 6.8 5.0 464 455 66 0.29 Quenched and stress relieved Upset 0.38 270 1985 288 1505 218 11.5 33.5 11.5 8.5 554 . . . . . . . . . Re-press 0.39 335 1960 284 1480 215 8.5 21.0 8.7 6.4 . . . . . . . . . . . . Upset 0.57 275 2275 330 . . . . . . 3.3 5.8 7.5 5.5 655 . . . . . . . . . Re-press 0.55 305 1945 282 . . . . . . 0.9 2.9 8.1 6.0 . . . . . . . . . . . . Upset 0.79 290 940 136 . . . . . . . . . . . . 1.4 1.0 712 . . . . . . . . . Re-press 0.74 280 1055 153 . . . . . . . . . . . . 2.4 1.8 . . . . . . . . . . . . Upset 1.01 330 800 116 . . . . . . . . . . . . 1.3 1.0 672 . . . . . . . . . Re-press 0.96 375 760 110 . . . . . . . . . . . . 1.6 1.2 . . . . . . . . . . . . Quenched and tempered Upset (c) 0.38 230 1490 216 1340 194 10.0 40.0 28.4 21.0 473 . . . . . . . . . Re-press (c) . . . . . . 1525 221 1340 194 8.5 32.3 . . . . . . . . . . . . . . . . . . Upset (d) 0.60 220 1455 211 1170 170 9.5 32.0 13.6 10.0 472 . . . . . . . . . Re-press (d) . . . . . . 1550 225 1365 198 7.0 23.0 . . . . . . . . . . . . . . . . . . Upset (e) 0.82 235 1545 224 1380 200 8.0 16.0 8.8 6.5 496 . . . . . . . . . Re-press (e) . . . . . . 1560 226 1340 194 6.0 12.0 . . . . . . . . . . . . . . . . . . Upset (f) 1.04 315 1560 226 1280 186 6.0 11.8 9.8 7.2 476 . . . . . . . . . Re-press (f) . . . . . . 1480 215 1225 178 6.0 11.8 . . . . . . . . . . . . . . . . . . [...]... Standard composition ranges for austenitic manganese steel castings ASTM A 128 grade Composition, % C A Mn Cr Mo Ni Si (max) P (max) 1.0 5-1 .35 11.0 min 1.00 0.07 B-1 0. 9-1 .05 11. 5-1 4.0 1.00 0.07 B-2 1.0 5-1 .2 11. 5-1 4.0 1.00 0.07 B-3 1.1 2-1 .28 11. 5-1 4.0 1.00 0.07 B-4 1. 2-1 .35 11. 5-1 4.0 1.00 0.07 C 1.0 5-1 .35 11. 5-1 4.0 1. 5-2 .5 1.00 0.07 D 0. 7-1 .3 11. 5-1 4.0 3. 0-4 .0 1.00 0.07 E-1... 0.07 E-1 0. 7-1 .3 11. 5-1 4.0 0. 9-1 .2 1.00 0.07 E-2 1.0 5-1 .45 11. 5-1 4.0 1. 8-2 .1 1.00 0.07 F 1.0 5-1 .35 6. 0-8 .0 0. 9-1 .2 1.00 0.07 The available assortment of wrought grades is smaller and usually approximates ASTM composition B-3 Some wrought grades contain about 0.8% C and either 3% Ni or 1% Mo Large heat orders are usually required for the production of wrought grades, while cast grades and their... oxide imparts extra strength for handling Sintering takes place in a closed reaction vessel in a controlled atmosphere of argon and hydrogen The 60% dense as-molded parts shrink 14 to 20% and achieve near-full density (>95% dense) The parts have very fine, noninterconnected porosity and much better elongation, toughness, and dynamic properties than conventionally pressed and sintered materials Such parts... tolerances and quality control are very significant Re-pressed or coined P/M parts are made to a tolerance of ±0 . 013 mm (±0.0005 in.) on a 25 mm (1 in.) diam part As-sintered tolerances on well-behaved alloys are ±0. 001 mm/mm (±0. 001 in./in.) Most MIM producers are achieving ±0.003 mm/mm (±0.003 in./in.) with some companies offering 0. 001 mm/mm (0. 001 in./in.) on selected dimensions The industry as a... binders and lubricants from MIM parts has been developed It is an outgrowth of vacuum furnaces, which have long been used to dewax and liquid phase sinter carbide parts The MIM parts are then heated to sintering temperatures of 1100 to 131 5 °C (200 to 2400 °F) Because the parts are not oxidized, the process works well with sensitive chromium- and manganese-containing materials, as well as low-alloy... pressure manifolds and the fabrication of gun sight parts with a special nongalling cam locking mechanism that could not be machined on the parts The first two MIM production parts in automobiles were a part for an ignition lock (Fig 24) and a single -part replacement (Fig 25a) for a two -part turn signal lever assembly (Fig 25b) Both have been in service since July 1988 Fig 24 MIM part (upper left)... 1025 149 625 91 10 19.2 2.7 2.0 337 Upset(b) 0.85 280 1130 164 625 91 10 16.6 4.1 3.0 343 525 76 0.46 Re-press(a) 0.81 200 1040 151 640 93 10 16.2 2.7 2.0 335 Re-press(b) 0.82 (a) Still-air cooled (b) Forced-air cooled 220 1170 170 745 108 10 12.8 2.7 2.0 368 475 69 0.41 The iron-copper-carbon alloys were either still-air cooled or forced-air cooled from the austenitizing temperature of 845 °C... single MIM part The lower portion of Fig 25(a) shows the first version of the MIM part, and the upper view shows the final 19.0 g (0.670 oz) MIM part that replaced the assembly The MIM material is iron with 2% Ni, sintered and then case hardened It replaced AISI 4037 and SAE 1018 case hardened The MIM part succeeded because of its superior strength compared to the two-piece assembly The core properties. .. dredging, lumbering, and in the manufacture of cement and clay products Austenitic manganese steel is used in equipment for handling and processing earthen materials (such as rock crushers, grinding mills, dredge buckets, power shovel buckets and teeth, and pumps for handling gravel and rocks) Other applications include fragmentizer hammers and grates for automobile recycling and military applications... 455 °C (850 °F) 135 5 196 0.77 410 Tempered at 695 °C (1280 °F) 700 101 Effect of Porosity on Mechanical Properties The mechanical property data summarized in the previous sections are related to either hot re-press or hot upset forged pore-free material The general effect of density on mechanical properties is presented in Table 14 Table 14 Tensile and impact properties of P/F-4600 hot re-pressed at two . Composition, wt% (a) Alloy Mn Ni Mo P/F-4600 0.1 0-0 .25 1.7 5-1 .90 0.5 0-0 .60 P/F-2000 0.2 5-0 .35 0.4 0-0 .50 0.5 5-0 .65 P/F-1000 0.1 0-0 .25 . . . . . . (a) All compositions. for (a) P/F-4600, (b) P/F-4200, and (c) iron-copper- carbon materials at various forged-carbon levels. Vickers hardness was determined at a 30 kgf load. Tensile, Impact, and Fatigue Properties. . mechanical properties is presented in Table 14. Table 14 Tensile and impact properties of P/F-4600 hot re-pressed at two temperatures Re-pressing temperature Re- pressing stress Re-pressed