Fatigue

Fatigue Technical Publications

193. Chromium Steels for High Performance PM Applications Chromium steels have long dominated the landscape of the wrought industry because of their high performance capabilities at modest cost levels. Historically, chromium steels have had difficulty penetrating the powder metallurgy market because of oxidation issues. Recent powder manufacturing advances, however, have resulted in low-oxygen chromium steels such as Ancorsteel 4300, which enables the production of high performance parts with conventional processing techniques. The current work reviews the capabilities of this Cr-Si-Ni-Mo alloy and its derivatives. Static properties, dynamic properties, and dimensional stability data are reviewed with an emphasis on a sintering temperature of 1120 ºC (2050 ºF). Comparisons are made to traditional powder metallurgy materials in both the as-sintered and heat-treated conditions as well as to heat-treated wrought alloys.

192. Stainless Steel AISA Grades for PM Applications Applications requiring stainless steels are growing at a rate of about 5% annually. Opportunities for using powder metallurgy (PM) exist, but additional grades not covered by MPIF Standard 35 are required. The American Iron and Steel Institute (AISI) has standards for a broad range of stainless steels that can be used in many applications, but the compositions of these grades must be modified to be conducive to manufacture by conventional PM techniques. Several of these grades have been produced as standard press and sinter powders. The physical properties, mechanical properties and microstructures of these various grades are reviewed to serve as a guideline for PM parts manufacturers and potential applications of these grades are addressed.

189. Capabilities of Two Chromium Powder Metallurgy Steel for High Performance Applications at Conventional Sintering Temperatures Ancorsteel 4300, a high performance Cr-Si-Ni-Mo steel, was unveiled two years ago as the first in a series of powder metallurgy alloys that will simulate wrought steel compositions. Advantages of this alloy include good compressibility, high hardenability, and excellent dimensional stability. More important, however, is that this alloy has the ability to be effectively sintered at 1120 °C and maintain oxygen contents below 500 ppm. This unique blend of performance and processing capabilities provides static and dynamic properties that exceed those of conventional powder metallurgy alloys and approach wrought gearing materials. A second Cr-Si-Ni-Mo alloy has now been developed that offers complimentary performance levels at a lower Mo content. This manuscript reviews properties of the two chromium steels with comparisons to traditional sinter-hardened and heat-treated powder metallurgy alloys.

185. Precipitation Hardening PM Stainless Steels Applications requiring high strength stainless steels are growing rapidally. Precipitation-hardening stainless steels have seen limited use in powder metallurgy despite their high strength. Strengthening of these alloys is achieved by adding elements such as copper and niobium, which form intermetallic precipitates during aging. The precipitation-hardening grades exhibit corrosion resistance levels comparable with those of the chromium-nickel (300 series) grades. The physical properties and microstructures of two precipitation hardening PM stainless powders are presented: 17-4 PH, a high-chromium, martensitic precipitation hardening stainless steel, has been optimized for use in PM applications; and a new low chromium (12 w/o) alloy that utilizes copper in the precipitation reaction. This alloy (410LCu), is considered to be a cost-effective alternative in applications that require high strength and moderate corrosion resistance.

183. Performance Capabilities of High Strength Powder Metallurgy Chromium Steels with Two Different Molybdenum Contents The desire for advanced ferrous powder metallurgy materials has led to the development of alloys that simulate wrought steel compositions. A recently commercialized Cr-Si-Ni-Mo steel can be effectively sintered at conventional temperatures and provides good compressibility, high hardenability, and excellent dimensional stability while maintaining oxygen contents below 500 ppm. This alloy has demonstrated static and dynamic properties that exceed those of conventional powder metallurgy alloys and approach wrought gearing materials. A second Cr-Si-Ni-Mo alloy has now been developed to offer complimentary performance levels at a lower Mo content, while still providing excellent sinter-hardenability. The current work reviews static and dynamic properties of the two Cr steels with an emphasis on accelerated cooling rates after sintering at 1120 ºC (2050 ºF). Comparisons are made to traditional powder metallurgy materials processed in both the sinter-hardened and heat-treated conditions as well as to heat-treated wrought alloys.

182. Lower Molybdenum Steels for High Performance Powder Metallurgy Applications Molybdenum has long been known to be a useful alloying element in powder metallurgy steels because of its enhancement of hardenability at relatively modest alloying levels. Since their introduction to the industry two decades ago, hybrid alloys based on prealloyed Mo steels such as FLN2-4405 have often been used in the heat-treated state to provide high performance properties. However, as raw material prices have become increasingly unstable in the last few years, the economic benefit to such alloys has reduced, forcing parts manufacturers to seek alternative materials systems. The current work provides a comparative analysis of a recently developed 0.3 wt.% Mo steel with more conventional 0.85 wt.% Mo and Mo-free steels. Property data and metallography are presented for crankshaft sprockets processed in the as-sintered, oil quenched & tempered, and induction hardened conditions.

178. Effects of Residual Surface Stress and Tempering on the Fatigue Behavior of Ancorsteel 4300 In this study, the microstructure, residual stress and fatigue behavior of Hoeganaes’ Ancorsteel 4300 sintered steel were characterized. Samples were tempered at either 205°C or 315°C and then machined by low-stress grinding. The effects of residual surface stress (measured by XRD) on the fatigue behavior were studied by either application of a stress-relieving heat-treatment after machining or by polishing away the machined surface. Based on this study, samples tempered at 315°C had slightly higher fatigue strengths than samples tempered at 205°C. Although significant compressive stresses were induced by machining on the surface of samples, these residual stresses did not affect the fatigue behavior. Rather, the fatigue behavior was controlled by porosity inherent to these sintered steels.

174. Fatigue Crack Growth of Prealloy Fe-0.85Mo-2Ni-0.6C Steels with a Homogeneous Microstructure The fatigue crack growth behavior of powder metallurgy steels (P/M steels) is strongly affected by the nature of porosity and microstructure of steel matrix. Our previous work has focused on a premix P/M steel prepared from Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6% graphite. In this study, we have studied the fatigue crack growth behavior of a prealloy steel of similar composition. Use of the prealloy powder resulted in more homogenous microstructure than the premix steel. The alloys were tested at three different densities: 7.0 g/cm³, 7.3 g/cm³, and 7.5 g/cm³. Microstructure characterization was conducted by optical and scanning electron microscopy (SEM). Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆K_th. The degree of crack closure was measured for both premix and prealloy steels at different R-ratios, and is discussed.

171. Development of a High-Strength-Dual-Phase P/M Stainless Steel Applications requiring high strength stainless steels are growing at a fast pace. Typical alloys used for these applications are either highly alloyed materials such as 17-4PH or materials that require a secondary heat treatment such as SS-410HT. A new dual-phase stainless steel has been developed as a lower cost option. The microstructure of the dual-phase stainless steel consists of a mixture of ferrite and martensite, the proportions of which are dictated by the chemical composition of the alloy. This unique microstructure results in high strength and hardness, while maintaining ductility. The mechanical properties of this new alloy are compared with those of competing materials such as 17-4PH, SS-409LE and SS-410HT. Potential applications for this new material are reviewed.

166. A New CR-Bearing Alloy for High Performance Applications Ancorsteel 4300 was recently introduced to the marketplace and is the first in a series that will simulate wrought steel compositions. This new alloy represents a technological breakthrough with low sintered oxygen contents in a system that employs both chromium and silicon. Its main advantages include the ability to be effectively sintered at 1120 °C (2050 °F), good compressibility, high hardenability, and exceptional dimensional stability across a variety of processing conditions. These characteristics make this material an attractive cost-effective alternative to alloys that require secondary quench hardening treatments and enable the penetration of P/M into higher performance applications. The current work reviews the effects of density, cooling rate, and carbon content on the static and dynamic properties of this new product, along with comparisons to Q&T properties for wrought AISI grades 4340 and 8620.

163. Effect of Heat Treatment and Case Carburizing High-Density P/M Steels Recent advancements in high-density lubricants enable P/M steels to be processed to densities approaching 7.40 g/cm³in a single compaction step using heated compaction tools via the AncorMax D® process. This broadens the number of suitable applications for P/M steels, including high performance gears. However, in addition to high-density, the microstructural and mechanical property requirements vary depending on the application. This objective of this paper is to quantify and understand the mechanical property differences obtained by subjecting high-density P/M steels to various heat-treatments. Mechanical properties, fatigue properties, and microstructural analysis will be presented in the as sintered, quenched and tempered, and carburized states.

162. Effect of Case Carburizing on Mechanical Properties And Fatigue Endurance Limits of P/M Steel Case carburizing has long been a basic technique for the improvement of the wear and fatigue resistance and of PM steel components. The key to the successful improvement in carburizing, however, is understanding, and interpreting the microstructure of the carburized case. The main area for growth in the PM industry is in the high performance gearing applications. The success of penetrating this area depends upon the ability to understand the key components that effect the fatigue endurance limits of PM materials. This paper will examine and illustrate how tensile properties and the fatigue endurance limits of a P/M hybrid alloy are affected by alloying additions and carburizing.

153. Fatigue Crack Growth of Sintered Steels with A Heterogeneous Microstructure Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm^3,7.36 g/cm³, and 7.53 g/cm³. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆K_th. In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, contributed to toughening and crack deflection. These findings are supported by quantitative measurements of crack growth rate, through the various microconstituents.

142. Chromium Containing Materials for High Performance Components Recently developed silicon-bearing alloys were engineered to replace malleable and ductile cast irons, and have shown excellent property combinations at high sintering temperatures. A modification to these alloys merges the power of silicon and chromium in one system, and allows for extraordinary performance. The presence of chromium improves both static and dynamic properties with the added benefit of being close to die-size after sintering. The current work details extensive laboratory data that show the effects of compaction pressure on this modified alloy processed at high sintering temperatures. Also presented is a field experience on a heat-treated production component that combined the high performance alloy system with warm compaction technology. Static and dynamic properties are presented for samples sintered in both laboratory and production scale furnaces.

139. Effect of Microstructure on the Rotating Bend Fatigue Response of a Prealloyed and Two Hybrid P/M Steels The effect of microstructural inhomogeneities on the rotating bend fatigue response of a prealloyed (FL-4405) and two hybrid (FLC2-4405 and FLN2-4405) steels was evaluated. Different microstructures at a nominal density of 7.4 g/cm³were developed by conventional sintering, high temperature sintering, quenching and tempering and sinter hardening followed by tempering. In previous studies on these steels, tensile and impact properties, hardenability, fatigue crack growth rates, pore characteristics and residual stress distributions were quantified. For each steel, the highest fatigue limit but the lowest fatigue ratio is obtained in the quenched + tempered condition. Sinter hardening of the steels containing copper and nickel increases the fatigue limit relative to the as-sintered condition. High temperature sintering reduces the fatigue limit relative to conventional sintering. The fatigue ratio is a function of microstructure and is lowest in the three steels in the quenched + tempered condition. The inferior fatigue behavior of the copper-containing steel is attributed to the large pores resulting from the coarse copper powder.

138. Fatigue Crack Growth of Fe-0.85Mo-2Ni-0.6C Steels with a Heterogenous Microstructure (Experiments and Microstructure-Based Simulations of the Axial Fatigue Behavior of P/M Alloys) Powder metallurgy processing of steel alloys typically results in a material with heterogeneous microstructure and residual porosity. The fatigue crack growth behavior of these materials is strongly affected by the nature of porosity and heterogeneous microstructure. Notched fatigue specimens were prepared from a Fe-0.85Mo prealloy mixed and binder-treated with 2%Ni and 0.6%C. The alloys were tested at three different densities: 6.98 g/cm³, 7.36 g/cm³, and 7.53 g/cm³. The microstructure at each density was characterized to determine the porosity, microconstituents, and phase fractions. Fatigue testing was performed at various R-ratios, ranging from –2 to 0.8. Increasing porosity and increasing R-ratio resulted in a decrease in ∆Kth. In situ observation of crack growth showed that the cracks propagated through Ni-rich regions. It appears that pearlite regions, and, to some extent bainite regions, however, contributed to toughening and crack deflection.

137. Rollong Contact Fatigue of Surface Densified Material: Microstructural Aspects (Surface Densification Approach to High Density Gears) Automotive gearing applications have material requirements combining static strength, bending fatigue, and rolling contact fatigue durability. Advances in P/M alloys and processing can produce as-sintered densities greater than 7.4 g/cm³ in complex gearing geometries. This high sintered density results in high static and fatigue resistance. However, at less than full density rolling contact fatigue performance is compromised. For high duty cycle gearing, pore free density is needed in the tooth contact region and in the area where the tooth flank intersects with the gear tooth root radius. This paper will investigate the effects of part processing and surface densification on the rolling contact fatigue properties of a high density FLN2-4405 material. Variables studied include: depth of densification, sintering conditions, surface microstructure, and post densification heat treatment practices. The results will demonstrate effects of residual porosity, case microstructure, and soft-nickel rich regions on rolling contact fatigue. Metallographic analysis will illustrated the cause of the failures associated with these variables.

132. High Density Processing of Ancorloy MDC Materials Previous experimental work has shown that silicon containing steels exhibit high tensile properties and impact strength at relatively low densities ranging from 7.0 to 7.1 g/cm3. Higher densities via AncorMax D^® processing has shown that sintered densities in excess of 7.3 g/cm3 are possible at compaction pressures ranging from 550 to 760 MPa. (40 to 55 tsi) This paper will examine the metallurgical and mechanical enhancements achieved through the AncorMax D process and high temperature sintering of the Ancorloy^® MDC and Ancorloy^® MDCL materials at densities ranging from 7.0 to in excess of 7.3 g/cm³.

131. Methods to Improve the Fatigue Life of Sinter-Hardened Components Previous experimental work showed that fatigue performance is affected by the alloy system, heat treatment method, and microstructural features of test specimens. The present study will present information concerning the effects of varying the sinter-hardening cooling rate (and subsequent microstructure features) on the mechanical properties sinter-harden steels and the Ancorloy MDCL™ material system. Emphasis will be given to the rotating bending fatigue performance of these systems and how this experimental data correlates with the fatigue performance of the actual component in accelerated life testing.

128. Chromium Additions to the Ancorloy MD Series Ancorsteel 41AB, introduced several years back, demonstrated the benefits of chromium and manganese additions to molybdenum steels. The more recently developed Ancorloy MD series provides enhanced ductility and strengths in P/M steels. This paper examines the mechanical properties achieved through the combination of high performance materials with chromium additions and high temperature sintering. Two chromium-modified materials were developed by replacing a portion of admixed nickel with a high carbon ferroalloy to improve dimensional properties and hardenability. Reviews of properties such as tensile, impact, transverse rupture, rotating bending fatigue, hardenability, and compressibility are presented.

127. Powder Metallurgy of High Density Helical Gears Powder Metallurgy is a proven technology to produce high strength gears for the automotive industry. Advances in powder production, compaction, and sintering combined with double pressing have enabled overall part densities up to 7.5 g/cm³ in spur gears. However, helical gears are more difficult to produce to these same densities because the geometry does not lend itself to the DP/DS process. Described in this paper is a P/M parts making technology capable of producing single pressed and sintered helical gears with core densities approaching 7.4 g/cm³. Description of a prototype run will be presented with the resulting sintered part densities and part-to-part variability. To further enhance the performance and geometry of these helical gears, they were subsequently surface densified via rolling. Improvements in the surface density and gear quality will be described.

125. Effect of Density on the Microstructure and Mechanical Behavior of Powder Metallurgy FE-MO-NI Steels The microstructure and mechanical properties of Fe-0.85Mo-Ni powder metallurgy (P/M) steels were investigated as a function of sintered density. A quantitative analysis of microstructure was correlated with tensile and fatigue behavior to understand the influence of pore size, shape, and distribution on mechanical behavior. Tensile strength, Young’s modulus, strain-to-failure, and fatigue strength all increased with a decrease in porosity. The decrease in Young’s modulus with increasing porosity was predicted by analytical modeling. Two-dimensional microstructure based finite element modeling showed that the enhanced tensile and fatigue behavior of the denser steels could be attributed to smaller, more homogeneous, and more spherical porosity which resulted in more homogeneous deformation and decreased strain localization in the material. The implications of pore size, morphology, and distribution on the mechanical behavior and fracture of P/M steels is discussed.

117. Properties of High Density Sinter Hardening P/M Steels Processed Using an Advanced Binder System Sinter-hardening P/M alloys offer an excellent opportunity for a part manufacturer to produce hardened components in an economical fashion by eliminating secondary heat-treatments. Unfortunately, sinter-hardening P/M base iron grades are prealloyed with substantial levels of Ni, Mn, and Mo which increase hardenability but reduce compressibility. Furthermore, Cu and graphite are added to further increase strength and hardness. These alloying additions all reduce compressibility limiting the maximum attainable green and sintered densities. This paper explores how processing sinter-hardening alloys with a new proprietary binder system can improve compressibility and lead to higher densities and mechanical properties. The data show green density increases of 0.05-0.15 g/cm3 and be achieved and can result in tensile strength and hardness improvements.

116. Advanced Sinter-Hardening Materials and Practices Sinter hardening is a well-established production technique utilized in the manufacture of P/M components with hardness and tensile strengths that approach the values of quench and tempered materials. The potential drawback of the sinter hardening process is the uniform carbon content of the case and core. This uniformity of carbon content does not promote a desirable compressive stress condition on the surface of the component leading to less than optimum fatigue strength. Experimental work was performed in which several sinter-hardening materials were produced with lower core carbons and subsequently carburized after the sintering process to produce a carburized case. Mechanical properties including tensile and fatigue of the non-carburized and carburized material will be presented plus the effect of the carburizing cycle on the carbon gradient of the new sinter hardening materials.

112. Processing of Hybrid Alloys to High Densities Premixes containing prealloy molybdenum, such as Ancorsteel 85HP, nickel and graphite have exceptional mechanical properties. This presentation will highlight the properties of these materials processed to densities of 7.25 to 7.45 g/cm³ by single press, single sinter techniques. The exceptional green strength of these materials in combination with density provides a unique opportunity to convert more automotive components to P/M.

108. Effect of Microstructural Inhomogeneties on the Fatigue Properties of a Prealloyed & Two Hybrid P/M Steels In the first phase of this study, the effect of microstructural inhomogeneities on the tensile and impact response of a prealloyed (FL-4405) and two hybrid (FLC2-4405 and FLN2-4405) P/M steels based on prealloyed Ancorsteel 85 HP was evaluated. In phase two we assess crack propagation response. The base powder and additions were mixed with 0.75 w/o Lonza Acrawax in 227 kg (500 lb) batches. A density of 7.4 g/cm³ was obtained by double pressing (550/550 MPa). Sintering temperatures of 2050 °F(1120 °C) and 2300 °F (1260 °C ) were utilized. A group of sintered compacts of each alloy was heat treated by quenching from 1650 °F (900 °C) into warm oil at 160 °F (70 °C) followed by tempering at 375 °F (190 °C) for 1 h. Two groups of sintered compacts of the FLC2-4405 and FLN2-4405 alloys were sinter hardened and tempered at 375 °F (190 °C) for 1 h. Experimental data showed that the P/M steels exhibit comparable fatigue crack growth rates (1.1207E-4 to 3.0185E-4 mm/cycle) at a stress intensity range of 1000 MPa (mm)^1/2. Quenched and tempered microstructures resulted in the highest fatigue crack growth rate. Sinter hardening of FLC2-4405 and FLN2-4405 lowered the fatigue crack growth rate. High temperature sintering reduced the fatigue crack growth rate in FL-4405 but increased it in FLC2-4405 and FLN2-4405.

107. Effect of Copper and Nickel Alloying Additions in the Tensile and Fatigue Behavior of Sintered Steels (Axial Fatigue Behavior of Sintered Ferrous P/M Alloys: Experiments and Modeling) The influence of Cu and Ni on tensile and fatigue behavior of Fe-0.85Mo prealloy with 0.6% graphite addition was investigated. The transient liquid phase formed by the presence of Cu resulted in more rounded pores in the Fe-Ni-Cu alloy compared to Fe-Ni. While the total porosity was similar in both alloys, a larger fraction of secondary pores was present in the Fe-Ni-Cu. The addition of Cu resulted in an increase in proportional limit stress, ultimate tensile strength, and fatigue strength over the Cu-free alloy. The higher fatigue resistance was attributed to the enhanced solid solution strengthening provided by Cu in Fe, and the higher proportional limit in the Fe-Ni-Cu alloy, caused by the more rounded nature of the pores, and stronger Fe-Ni-Cu matrix surrounding the pores. The proportional limit stress appears to be a good indicator of fatigue strength, since it quantifies the onset of localized plasticity in these materials.

103. The Development of Engineered Binder-Treated Alternatives to Duffusion-Alloyed Powders Engineered binder-treated premixes have been developed as alternatives to diffusion alloyed powders. The binder-treated materials meet the chemical composition limits for the diffusion alloyed materials listed in MPIF Standard 35, Materials Standard for P/M Structural Parts.

At an equivalent combined carbon content the binder-treated materials exhibit higher strength than the diffusion alloyed materials. When the combined carbon content of the binder-treated materials is reduced, to provide an equivalent strength level, the binder-treated materials match the tensile ductility and impact energy of the diffusion alloyed products.

The as-sintered and the quench-hardened and tempered performance of the new materials is reviewed and compared with diffusion alloyed materials of similar chemistry. These recently developed materials represent the first in a new family of high performance ferrous P/M materials.

101. Fatigue Crack Initiation and Propagation in Ferrous Powder Metallurgy Alloys Many of the targeted applications for powder metallurgy materials, particularly in the automotive industry, undergo cyclic loading. It is, therefore, essential to examine the fatigue mechanisms in these materials. The mechanisms of fatigue crack initiation and propagation in ferrous powder metallurgy components have been investigated. The fatigue mechanisms are controlled primarily by the inherent porosity present in these materials. Since most, if not all, fatigue cracks initiate and propagate at the specimen surface, surface replication was used to determine the role of surface porosity in relation to fatigue behavior. Surface replication provides detailed information on both initiation sites, and propagation path of fatigue cracks. The effect of microstructural features such as pore size, mean pore spacing, as well as the heterogeneous microstructure on crack deflection was examined and is discussed. Fracture surfaces were examined to elucidate a mechanistic understanding of fatigue processes in these materials.

69. The Effect of Microstructure and Pore Morephology on Mechanical and Dynamic Properties of Ferrous P/M Materials The objective of this study was to quantify and understand the combined role of microstructure and pore characteristics on the transverse rupture strength (TRS), tensile properties and rotating bend fatigue response of conventional and ANCORDENSE processed FLN2-4405 premixes. To this end, the premixes were made with samples using fine (4µm) and coarse (50µm) nickel powder to promote differences in pore size and diffusion characteristics. Compacts were sintered in synthetic DA (75 v/o H2 / 25 v/o N2) at 2050°F (1120°C) or 2300°F (1260°C) to densities in the range of 6.8 g/cm³ - 7.2 g/cm³. Pore size and spacing, cumulative pore size and number of pores per unit area were determined by stereological analysis, and the crack path was monitored by means of optical microscopy. The static and dynamic properties of the materials made from the two premixes are interpreted in terms of the attendant microstructures and pore characteristics, as dictated by the premix type, sintering temperature and sintered density.

59. The Effect of Microstructure and Pore Morphology on Mechanical and Dynamic Properties of Ferrous P/M Materials Fatigue testing was performed on FN-0205 premixes in order to evaluate the effect of pore structure and processing method on the fatigue properties. The premixes were made with two nickel sources:

· mean particle size of 4 mm

· mean particle size of 50 mm

Metallographic analysis was performed to quantify the pore structure. The following parameters were examined: pore size, pore shape, mean pore spacing and average pore size. Previous work, which examined a variety of materials, indicated that predicting the fatigue strength of a material is a complex relationship between the type and strength of the microstructural constituents, as well as stereological parameters such as mean pore spacing and pore size. This paper attempts to determine to what extent each of the above parameters influences the fatigue strength of P/M materials.

52. The Effect of Microstructure on Fatigue Properties of Ferrous P/M Materials Fatigue testing (rotating bending fatigue) was performed on several materials in order to evaluate the effect of several microstructural elements. Metallographic analysis was performed to characterize the microstructures of the materials and attempt to identify failure mechanisms.

Previous work indicated that predicting the fatigue strength of P/M materials is a complex relationship between the grain size of the material, the type and strength of the microstructural constituents present and, primarily, the mean pore spacing. [1,2] This study examines these relationships in more depth.

45. The Development of High Performance P/M Steels Ferrous powder metallurgy has continued to displace competing cast or wrought technologies in automotive applications. This required the development of materials systems with higher, more consistent performance than those available previously. However, competing technologies are not static. The paper examines the materials development and microstructural control required to meet the challenges and opportunities offered by the development of new P/M parts.

42. The Effect of Microstructure on Fatigue Properties of High Density Ferrous Materials Fatigue testing (rotating bending fatigue) has been performed on several high performance ferrous P/M material systems. Detailed metallographic analysis was performed to determine differences in the failure mechanisms for various material and process combinations. A variety of material compositions were single compacted to high density via the ANCORDENSE™ compaction system. This was followed by conventional and high temperature sintering and testing in the as-sintered and heat treated conditions. The analysis provides information as to the relationships between density, structure and composition with fatigue life.

38. Powder Metallurgy Gears - Expanding Opportunities Powder metallurgy (P/M) is a precision metal forming technology for the manufacturing of parts to net, or near net shape. The powder metallurgy process is illustrated schematically in Figure 11. There are three basic steps to producing parts; mixing, compacting, and sintering. Variations to these basic steps such as infiltration, double pressing/double sintering, and powder forging may be used to achieve higher density parts. A sizing operation may be used to qualify critical part dimensions. Alternatively, a machining step may be added for the same purpose or to achieve a geometric feature not possible during rigid die compaction. P/M parts may be through hardened or surface hardened as required by the intended application.

11. Tensile, Impact and Fatigue Performance of New Water Atomized Low-Alloy Powder - Ancorsteel 85 HP A new water atomized, prealloyed powder has been developed containing 0.85% molybdenum as the alloying addition. The as-sintered and heat treated tensile, impact and fatigue performance have been determined for a range of graphite additions using both single and double pressing techniques. Results indicate that the new powder, Ancorsteel 85 HP, has a unique ability to be compacted and repressed to densities not attainable with existing prealloyed nickel-molybdenum powders. The higher densities achieved produce performance equivalent to or better than Ancorstee12000 or Ancorstee14600V using conventional single compaction techniques. However, the additional density increment achieved during repressing results in mechanical properties in excess of what is possible with the existing low-alloy steels. It is expected that the new prealloyed powder will be used in high-density applications requiring good hardenability. It will also provide a base for our high performance ferrous material development program.

7. High Performance Ferrous P/M Materials: The Effect of Alloying Method on Dynamic Properties A comparison has been made between fully prealloyed, partially prealloyed, and elementally admixed alloys in the "as sintered" condition in order to assess the influence of microstructural and chemical homogeneity on the tensile, impact, and fatigue properties. Elementally admixed and completely prealloyed powders were prepared with chemistry similar to that of the diffusion bonded Distaloy 4600A (nominally 1.8 wt. % Ni, 1.6 wt. % Cu, and 0.55 wt. % Ho). An addition of 0.6% graphite was made to each of these materials. In one series of experiments, test pieces were prepared from each of the materi`1als pressed to a green density of 6.9 g/cm3. Sintering was carried out at 2050°F for 30 minutes at temperature in a dissociated ammonia atmosphere. A second series of experiments was carried out in which a fixed compaction pressure of 45 tsi was applied to each of the materials. Sintering was carried out in a similar manner to the first series. An additional prealloyed material, Sumiron 4100S, was included in this second series of experiments. However, this material was sintered at 2300°F to reduce the tendency for oxidation of this chromium and manganese steel alloy. For the samples pressed to a fixed green density of 6.9 g/cm3, the tensile strength of the partially prealloyed material was significantly higher than the other materials. The impact energy of the partially prealloyed material was also higher. A similar trend was found for the samples pressed at a fixed compaction pressure of 45 tsi. The partially prealloyed product, sintered at 2050OF, was even superior to the Sumiron 4100S; sintered at 2300°F. The differences in the tensile and impact properties were significant at the one percent level. The rotating bending fatigue performance of the partially prealloyed material was superior to that of the Sumiron 4100S.

4. Ferrous Powders - How Alloying Method Influences Sintering The mechanical properties of P/M materials are directly related to their microstructure and the size, distribution, and morphology of the porosity they contain. Alloying additions are made to develop specific material performance characteristics. However, the manner in which the alloys are constituted has a significant effect on the porosity and microstructure of the final sintered product (1,2).

3. Fatigue Properties of P/M Materials The tensile properties and fatigue endurance limits of several widely used P/M steels have been tested. Statistical estimates of the 99.9% survival stress have shown that fatigue endurance ratios can vary from 0.16 to 0.47. Thus the use of 0.38 as a rule of thumb for estimating the fatigue endurance limit from static tensile property data can result in large errors. The single most effective method of improving fatigue properties is to increase the part density. Fractographic observations were made on some of the fatigue failures, including stable and unstable crack growth.

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