Sinter-Hardening

Sinter-Hardening Technical Publications

200. The Effect of Sintering Conditions and Composition on the Mechanical Property Response of Cr Containing PM Steels The desire for advanced ferrous PM materials has led to the development of alloys that more closely simulate wrought steel compositions. Traditional wrought steel alloying elements, such as Cr, are used to improve mechanical properties, hardenability and are cost effective. Chromium has historically been avoided in PM due to oxygen related issues, but newer PM alloy systems now contain this element. While providing good mechanical properties and overall product cost, proper sintering of these alloys is the critical challenge. This paper presents the role of sintering and composition on the mechanical properties and microstructure of a chromium containing PM steel.

199. Alloy Development of Sinter-Hardenable Compositions Market forces in the PM industry are challenging the traditional compositions typically used in PM alloys. Mo, Ni and Cu are the predominant alloying elements used in ferrous PM due to their low affinity for oxygen. In addition, Mo has little effect on compressibility and copper rapidly alloys by way of a liquid phase at sintering temperatures. All three elements also increase the hardenability of steels, allowing for sinterhardening of parts produced with a combination of the elements. Price pressures are causing a reevaluation of powder chemistries utilizing these elements, and the challenge of alloy development is to advance alloy systems that optimize the balance between mechanical properties and overall production cost. Sinter-hardenable compositions play a key role in this regard. This paper will evaluate and discuss alternative alloys to those primarily used in the European market.

196. Sintering of Powder Premixes - A Brief Overview Advances in the understanding of the sintering of powder premixes have contributed significantly to the growth of the ferrous powder metallurgy industry. This includes sintering both in the solid state and in the presence of a liquid phase. In this article, the sintering of iron powder premixes containing: 1) graphite; 2) nickel and graphite; 3) copper and graphite; 4) Phosphorus as ferrophosphorus; and, 5) boron as ferroboron are discussed. The evolution of microstructure and mechanical properties are discussed as well.

194. Effect of Post Sintering Thermal Treatments on Dimensional Precision and Mechanical Properties in Sinter-Hardening PM Steels Dimensional precision is a critical parameter in net shape processing of ferrous PM components. PM parts producers continue to pursue larger parts, but absolute tolerances dictated by the end user generally do not scale with part size. Therefore, in larger parts, the variation in percentage change in size, or dimensional change, must be reduced. Beyond the dimensional changes associated with pressing and sintering of typical low alloy PM steels, sinter-hardenable alloys present some unique challenges and opportunities for PM part manufacturing. The ability to harden a part in the sintering furnace eliminates the need for a secondary quenching operation. The resulting microstructure of untempered martensite is, however, not ideal for dimensional stability and mechanical properties. Tempering hardened steels results in improved mechanical properties and dimensional shrinkage, as the martensite converts to a more stable ferrite and carbide microstructure of higher density. In addition, many sinter-hardening grades contain high Cu and C contents that result in relatively high amounts of retained austenite. Retained austenite can improve impact and ductility properties, but contributes to dimensional instability as it can transform to lower density bainite and/or martensite with thermal fluctuations. Proper thermal treatments of sinter-hardened steels are necessary to obtain the best combination of mechanical properties and dimensional control. This paper reviews the effects of different post-sintering thermal treatments on the dimensional, microstructural and mechanical property changes of sinter-hardened PM steels.

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.

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.

187. Enhanced Machinability of Sinter-Hardenable PM Steels Machining of sinter-hardened PM steels provides a challenge for part makers. The machinability of PM steel already differs from that of wrought steel due to the presence of porosity and the often heterogeneous microstructure. In addition, hardened wrought steels are generally machined prior to hardening, whereas in sinter-hardened PM steels, the only options are green or pre-sintered machining and machining in the hardened condition. To facilitate machining of sinter-hardened materials, a new additive (MA) has been developed to increase tool life during the machining process. Hard turning tests were performed to evaluate the effect of this new additive. Sintered compacts with the MA additive were compared to compacts without a machining aid and to compacts that contained the MnS additive. This paper discusses the improvement in machinability with this new additive in sinter-hardenable PM steels.

186. Dimensional Precision in Sinter-Hardening PM Steels Dimensional precision is a critical parameter in net shape processing of ferrous PM components. Beyond the dimensional changes associated with pressing and sintering of typical low alloy PM steels, sinter-hardening alloys undergo a transformation from austenite to martensite. The formation of martensite results in a large expansion during cooling, as martensite is the lowest density phase in steels. Tempering hardened steels results in shrinkage, as the martensite converts to a ferrite and carbide microstructure of higher density. Both of these transformations have a large impact on the dimensional change. In addition, martensitic regions with high Cu and C contents may contain large amounts of retained austenite. As austenite is the highest density phase, retained austenite results in less growth of the compact. The presence of martensite and retained austenite, in addition to the tempering step, all play a role in the final dimensions of a component. This paper reviews two sinter-hardening grades and investigates the dimensional and microstructural changes of those grades through different post-sintering thermal treatments.

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.

179. The Effect of Post Sintering Cooling Rate on Microstructure and Machinability of a PM Sinter Hardened Steel The effect of post sintering cooling rate on the microstructure of a PM sintered steel was investigated by assigning three different cooling conditions to the same alloy. The various microstructures produced by these different cooling conditions were evaluated and quantitatively analyzed for volume fraction of bainite, martensite, and retained austenite. Machinability tests were also performed on samples from all three cooling conditions to determine how these microstructures affected tool life. This was done using a turning operation and the tool life machinability criteria. It was found that the volume fraction of martensite was the most important factor in determining machinability. Lower martensite contents produced much higher machinability and vice versa. Lower martensite contents also resulted in decreased hardness. Bainite was found to be a much more favorable phase from a machinability standpoint. It was also observed that the trajectory of the chips during the turning operation was an indicator of the condition of the tool edge.

177. High Density Processing of Cr-Si-Ni-Mo Containing Steel Ancorsteel 4300, an iron alloy containing Cr-Si-Ni-Mo, was recently introduced and is capable of achieving high mechanical strength with exceptional dimensional stability. With the ability to be sintered at conventional temperatures, this alloy offers a unique blend of performance capabilities that can provide an economic advantage over alloy systems requiring high temperature sintering or secondary quench hardening. The current work discusses the performance of the new chromium steel at densities above 7.2 g/cm³at various cooling rates using an advanced lubricant/binder system. Comparisons to a hybrid Ni-Mo steel and a diffusion alloyed Ni-Cu-Mo steel are presented.

176. Sinter Hardening Response of a Cr-Si-Ni-Mo Containing Steel A number of sinter-hardenable materials have been developed over the past several years. Notable among these are Ancorsteel^®737 SH (prealloyed Mo, Mn, Ni steel), Ancorsteel 4600V (prealloyed Mo, Ni Steel) and hybrid alloys containing prealloyed molybdenum with copper and admixed nickel. All these materials use copper to enhance hardenability. Ancorsteel 4300 (Cr-Si-Ni-Mo containing steel) exhibits excellent hardenability without the addition of copper, with as-sintered hardnesses greater than 20 HRC utilizing cooling rates typically found in production sintering furnaces. A quantitative study to assess the hardenability of this alloy system has been undertaken and a comparison will be made with more traditional Mo-Ni-Cu alloys. Continuous sinter cooling transformation curves will be presented along with apparent hardnesses and metallographic analysis of various phase fractions.

168. Effect of Composition and Cooling Rate on Physical Properties and Microstructure of Prealloyed P/M Steel The composition of P/M steels plays an important role on the microstructure and physical properties of sintered parts. The different levels of alloying elements prealloyed into base powders change the hardenability of the material. In addition, copper and graphite additives also play an important role. Further, different cooling rates will have a profound effect on the microstructure. Three base alloys with different levels of copper and graphite were sintered at 1120°C and cooled at either conventional rates or under sinter-hardening conditions. The effect of these variables on microstructure and mechanical properties will be discussed, with an emphasis on dimensional change.

167. High Performance Applications of Chromium Steels Sintered at Conventional Temperatures To meet the rigorous demands of automotive gearing, sprocket and other power transmission applications, double-press / double-sinter (DP/DS) techniques are often used to achieve the desired level of static and fatigue performance. Ancorsteel 4300, a new Cr-bearing material, has shown improved strength levels compared to traditional P/M steels. By employing such an alloy, core property requirements can be met at densities around 7.0 g/cm³. Replacing the pre-sinter and secondary press operations with selective densification, which will ensure sufficient contact fatigue resistance, can provide an economic benefit. The current work demonstrates the viability of processing a sprocket for a power transmission application with this high performance alloy system sintered at 1120 °C (2050 °F) in a production furnace. Subsequent selective densification and surface carburization provides a wear resistant and durable case layer. The results are compared with those achieved using FLN2-4405 processed through the traditional route of DP/DS, heat-treat.

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.

165. High Density Processing of a New CR-Bearing Steel The application of P/M steels in highly stressed applications requires both high density processing and high performance alloys. Ancorsteel 4300, a new high performance alloy that contains Cr, Si, Mo, and Ni, utilizes the hardenability and mechanical property enhancement of chromium while maintaining low sintered oxygen contents. The elevated mechanical properties compliment the high compressibility of the powder allowing for use in high density applications. When combined with high density processing techniques such as the advanced lubricant/binder system AncorMax D, high densities and excellent properties can be achieved. This paper presents the effects of processing on density and mechanical properties with this new material, with a comparison to FLN2-4405 and FD-0405.

154. A Review of Current Sinter-Hardening Technology Sinter-hardening has developed into a highly cost effective production method for the production of through hardened P/M parts without the need for additional heat-treatments. Over the last several years advances have been made in sinter-hardening material systems and furnace technology. This paper reviews these advances as well as some key processing parameters required to produce high quality sinter-hardened components. Specific topics included are proper alloy selection, mechanical and fatigue properties, microstructural development, optimization of furnace cooling rates, and proper tempering practices.

149. An Investigation Into the Effect of Copper and Graphite Additions to Sinter Hardening Steels Sinter-hardenable powders have been used as replacements for traditional quench-hardened and tempered materials due to their ability to transform to a martensitic microstructure upon cooling during a typical sintering operation. In the manufacture of the base sinter-hardenable powders, alloying additions are made to the melt (prealloyed), with graphite added to the base powder as the carbon source. However, additions of copper to further improve the hardenability of the mix are commonplace. The combined admixed additions of graphite and copper to the prealloyed powder can conceivably lead to increased retained austenite contents. In the present study, metallographic techniques have been developed to resolve retained austenite in a predominately martensitic material. Etching and staining techniques, automated image analysis, and scanning electron microscopy were the metallographic tools used in this study.

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.

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.

129. Effect of Cooling Rates During Sinter-Hardening Sinter-hardening is becoming a more widely used process for the production of high strength P/M parts. The ability to produce martensite directly from the sintering furnace enables the process to produce parts, with properties close to those of quenched and tempered steels, more efficiently by omitting a separate heat treatment operation. The success of sinter-hardening depends upon the ability to produce microstructures of high martensite content consistently during accelerated cooling after sintering. This paper will examine and illustrate the effects of changes in cooling rate from sintering temperature upon the microstructure, hardness and properties of a hybrid P/M steel. It will show how comparison of cooling curves of instrumented Jominy and sintering furnace can be used to improve the sinter-hardening process.

124. Properties and Applications of High Density Sinter-Hardening Materials Sinter-hardening materials are characterized by their high hardenability which enables the formation of >80% martensite during accelerated cooling. However, these moderately alloyed materials often exhibit lower compressibility and the resulting lower density limits their use in potential high strength applications.

What is needed is a method to improve the green and sintered density of current sinter-hardening materials that will enable these materials to be utilized in new high strength applications. This paper describes how the green and sintered density of standard sinter-hardening alloys can be improved using new alloy systems coupled with advanced binder technology. The resulting improvements in mechanical properties will be presented as well as the potential use of high density

120. Statistical Approach to a Leaner Sinter Hardening Alloy A popular sinter-hardening alloy is based on pre-alloyed Fe-Ni-Mo-Mn powder to which 2% copper and 0.9% graphite is added. Data in the literature suggests that reduced copper and graphite contents may provide equal hardenability and higher tensile properties. Experimental results demonstrate that a fully martensitic microstructure and higher tensile properties may be obtained with leaner alloy chemistry. Reduced additive (copper and graphite) content improves pore free density. Response Surface Methodology (RSM) is used to illustrate the results of the statistically based experimental design.

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.

110. Enhanced Processing of Silicon-Containing High Performance Materials In 2001, an extensive program was initiated to evaluate new silicon-containing materials designed to compete with various grades of ductile and malleable cast irons. These bindertreated, press-ready premixes were compared to a standard FLN4-4405 in a production environment on a complicated, high volume application. This year’s work investigates both double pressed / double sintered and heat-treated performance of the new silicon-containing materials. Mechanical properties and dimensional stability information are presented and compared to several standard material candidates containing no silicon.

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.

105. Advances in P/M Gear Materials Powder Metallurgy is an efficient manufacturing process for the production of gearing and similar net shape components. Because of limitations arising from the inherent porosity and limited alloy systems available, the traditional uses for P/M gearing was in relatively low stress applications. The recent introduction of new compaction techniques and new alloy materials has produced P/M components with significantly higher yield and tensile strengths approaching the strength levels of wrought gearing materials. This paper will review the new P/M processes and materials and their suitability for gear type applications. Mechanical property comparisons will be made to the common automotive gearing materials including ductile and malleable cast irons and wrought low alloy steels.

104. Application of Sinter-Hardenable Materials for Advanced Automotive Applications Such as Gears, Cams, and Sprockets Recent demands within the automotive industry have been for applications requiring high apparent hardness, high hardenability, and increased mechanical performance. These often-conflicting requirements necessitated the development of new materials that offer high as-sintered apparent hardness and good static/dynamic mechanical properties without the added expense of a secondary heat treatment. Traditionally, sinter-hardening materials have offered acceptable apparent hardness but at the expense of mechanical properties and sintered density. This paper will document the mechanical properties of a series of sinter hardening materials that offer good compressibility, high apparent hardness and enhanced mechanical properties. The discussion will focus on utilization of these materials in automotive applications (within both the engine and transmission) such as gears, cams and sprockets that are currently produced by either the press, sinter, and heat treat process or by conventional machining of a casting or wrought material. Enhanced processing through high temperature sintering also will be discussed.

90. Advanced Processing of Sinter-Hardening Materials The sinter-hardening process has been shown to provide excellent mechanical properties and part-to-part size control. Previous work has indicated that exceptional mechanical properties and high apparent hardness values can be achieved by sinter hardening in a high temperature furnace with standard cooling. This work focuses on combinations of advanced techniques intended to optimize mechanical properties for stringent applications. While warm compaction is utilized to increase density, various material alloy combinations are blended together in an effort to study green density variables. Where applicable, specific market opportunities are identified.

80. Application of Sinter-Hardenable Materials for Advanced Automotive Applications such as Gears, Cams, and Sprockets Recent demands within the automotive industry have been for applications requiring high hardness, high hardenability, and increased mechanical performance. These often conflicting requirements necessitated the development of new materials that offer high as-sintered hardness and good static/dynamic mechanical properties without the added expense of a secondary heat treatment. Traditionally, sinter-hardening materials have offered acceptable hardness but at the expense of mechanical properties and sintered density. This paper will document a series of sinter hardening materials that offer good compressibility, high hardness and enhanced mechanical properties. The discussion will focus on utilization of these materials in automotive applications (within both the engine and transmission) such as gears, cams and sprockets that are currently produced by either the press, sinter, and heat treat process or by conventional machining of a casting or wrought material.

77. Field Experience on a New Sinter-Hardening Material Traditionally, the processing of sinter-hardening materials has been limited to conventional sintering temperatures. Hence, very little sinter-hardening research has been conducted at higher sintering temperatures. However, the superior hardenability of Ancorsteel^® 737 SH allows for sinter-hardening at temperatures in excess of 1180 ºC (2150 ºF) without the need for accelerated cooling.

This paper will both present field experience on conventional sinter-hardening processes and investigate the effect of copper, nickel, and graphite additions on the properties of Ancorsteel 737 SH sintered at 1260 ºC (2300 ºF) utilizing conventional cooling. Particular attention will be paid to dimensional change characteristics, mechanical properties, apparent hardness values, and martensite content in sintered parts.

74. Improved Efficiency by Use of Sinter-Hardened P/M Automotive Components Sinter-hardening, accelerated cooling, of P/M components directly from the sintering furnace is an increasingly popular production process. Sinterhardened P/M steels possess similar macrohardness and strength to heat treated P/M steels processed by quenching and tempering. Where design permits, sinter-hardening enables P/M fabricators to improve process efficiencies by omitting a separate heat treatment operation. This paper examines the interaction of material selection and process conditions required to develop a sinterhardened P/M component for an automotive application.

66. A Superior Sinter-Hardenable Material Sinter-hardening technology has been assisting the P/M parts fabricator by improving processing efficiencies and reducing costs. Furthermore, the barriers to attaining good sinterhardenability and part performance have been reduced through improvements in materials and equipment developments. Recent material advances have focused on new alloys with increased hardenability and compressibility.

A new sinter-hardenable alloy has been introduced which provides improvements in hardenability and compressibility over the well-established FLC-4608 composition. These improvements will allow fabricators to reach higher densities and mechanical performance under typical compaction and sintering conditions. Mechanical performance and material capabilities are investigated as a function of density and admixed composition. Additional processing to achieve higher green densities and mechanical performance will also be reviewed.

62. What is SInter-Hardening ? The mechanical properties of ferrous powder metallurgy (P/M) materials are directly related to their density and microstructure. Many PIM parts are heat treated, in a secondary operation, to develop a tempered martensitic microstructure either in a surface layer, or throughout the part. The need for a secondary quenching operation may be avoided by "sinter-hardening" the parts.

Ferrous P/M materials with sufficient hardenability will develop microstructures containing significant percentages of martensite in the as-sintered condition. Accelerated cooling · techniques for sintering furnaces have been developed which permit larger parts to be sinter-hardened, or materials with lower hardenability to be used to produce sinter-hardened parts with smaller cross-sections.

The difference between hardness and hardenability will be explained and a review presented of how the alloying method selected for ferrous P/M materials influences hardenability. Examples of sinter-hardenable materials will be provided and the benefits and disadvantages of the sinter-hardening process will be discussed:

60. Application of High Performance Materials and Processes - Alloy Systems Significant advances have been made in the past several years in developing Low alloy materials for highly stressed applications. A review of these material and processing developments will be made. Recent material developments focus on developing high apparent hardness and tensile strength in P/M parts without the need for a secondary quench-hardening operation. The effect of alloy type, alloy content, and cooling rate on hardness and other properties will be discussed.

54. Sinter-Hardening P/M Steels The use of P/M structural parts is growing in part due to the use of the sinter-hardening process which utilizes high performance materials in combination with an accelerated post sintering cooling rate. The sinter-hardening process offers improved mechanical properties over conventional sintering without a separate heat treatment operation. Thus, where the part design permits, sinter-hardening offers considerable economic benefits to the part producer.

Sinter-hardening typically requires that the P/M steel substantially transform to martensite during cooling. A variety of microstructures and properties can be obtained by varying the post sintering cooling rate. By controlling this rate, the microstructure can be manipulated to produce the required amount of martensite to obtain the desired mechanical properties.

Alloying elements such as molybdenum, nickel, and copper promote hardenability in P/M parts. By increasing the hardenability of the material, the parts can be cooled at slower rates and still produce large amounts of martensite. The ability to increase the amount of martensite, leading to increased strength and hardness, through the use of proper alloy selection and accelerated cooling rate will be discussed.

50. Effect of Process Conditions Upon Sinter-Hardening Response of FLC-4608 Materials Sinter-hardening provides for improved efficiency and competitiveness by eliminating a separate, secondary heat treatment operation. Materials options, process flexibility, and application requirements demand a better understanding of process, microstructure, and mechanical property relationships in order to fully capitalize on the opportunity afforded by sinter-hardening.

This study investigates the effect of cooling rate on the material properties of an FLC-4608 material processed under production conditions. Variables used to control accelerated cooling are related to componet microstructure and mechanical properties. Production cooling rates and microstructural comparisons are made with a laboratory developed Continuous Cooling Curve (CCT) for the FLC-4608 alloy.

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.

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.

27. Recent Developments in Ferrous Powder Metallurgy Alloys A systems approach to engineered ferrous powder metallurgy (P/M) materials is described. The approach encompasses the use of high compressible, high performance powders in premixes produced using proprietary mixing technology that employs patented binders. To ensure that an appropriate microstructure is achieved to suit the functional requirements of a particular application, alloys are formulated based on knowledge of the compaction and sintering cycle that will be used to make the P/M parts. These premixes have improved flow and die filling characteristics that result in greater consistency throughout the entire P/M part manufacturing process. In addition, the use of binder treated premixes leads to reduced dusting and segregation of alloy additions. Binder treated premixes produced using high compressible, prealloyed molybdenum steel powders are shown to be particularly well suited for quench-hardening, sinter-hardening, and high temperature sintering. They also form the basis for a series of chromium, manganese, and chrome-manganese P/M 'Steels. The systems approach will be augmented during 1994 by the introduction of new material and process technology that enables part densities of 7.3 to 7.5 g/cm3 to be achieved through single compaction processing.

17. Sinter-Hardening Low-Alloy Steels The availability of prealloyed steel powders employing molybdenum as the major alloying element offers new levels of compressibility and mechanical properties. When the prealloyed powders are combined with conventional P/M additives such as copper, nickel and graphite, it is possible to develop high strength martensitic microstructures directly from the sintering cycle. The impact and tensile properties of copper, nickel, graphite premixes based upon the prealloyed molybdenum steels are compared under controlled cooled conditions. The ability to balance tensile strength, toughness and hardness by control of alloy chemistry is illustrated.

13. Performance Characteristics of a New Sinter-Hardening Low-Alloy Steel A martensitic microstructure can be developed in some powder metallurgy materials without the need for a secondary heat treatment operation provided the material is cooled sufficiently rapidly from the sintering temperature. These P/M materials are termed "sinter-hardening" steels. The partially alloyed powder, Distaloy 4800A, and nickel-molybdenum prealloyed steels such as Ancorsteel ® 4600V with copper additions are capable of being sinter-hardened. Ancorsteel 85 HP, a new highly compressible low-alloy powder employing molybdenum as the primary alloying element, is also capable of being sintered-hardened when copper and graphite additions are made to it. Ancorsteel 85 HP has a higher compressibility than nickel-molybdenum prealloyed powders. The effect of cooling rate has been studied on the microstructure and mechanical properties of Ancorsteel 85 HP + 2% copper + 0.9% graphite. Tensile and impact properties have been evaluated for a range of material densities and compared with those obtained with samples based on Ancorsteel 4600V.

12. Improved Dimensional Control and Elimination of Heat Treatment for Automotive Parts The automotive industry has expressed concern about the general quality of heat treatment (austenization and quenching) and the desire to reduce or eliminate dependence upon this process whenever possible. Therefore, in a continuing effort for improvement during the past year, a process has been developed that eliminates the conventional heat treating operation for some applications. Some of these finished parts require both a high impact strength and a hardened wear resistant surface. The Charpy impact, tensile and TRS properties of a binder treated premix based on a partially alloyed powder have been evaluated utilizing a variety of processing conditions. These include various carbon contents, sintering temperatures and sintering times. Quantitative metallography was used to evaluate the pore size, pore shape and microstructural constituents present as a result of the various materials and processes. These factors were then correlated with the measured properties.

10. Steering Column Tilit Lever - P/M Material Development Automotive steering columns use a variety of levers to lock the flit mechanism in position. A new P/M material has been developed to withstand the impact and hardness performance requirements of this application. The new material is currently subjected to a brief surface carburizing and tempering treatment to impart wear resistance. The P/M part only requires honing of the pivot hole to meet the specified tolerance.

Long term plans are to achieve the desired performance requirements using a modified version of the new P/M material, with a higher graphite addition, which can be used after tempering the" as-sintered" product. The Charpy impact properties of three P/M materials, each based on a partially alloyed powder (Distaloy 4800A) but with different percentages of added graphite, have been tested for a variety of processing conditions. Neutral hardening, carburizing, and sinter-hardening treatments have been compared. The influence of tempering temperature and the incorporation of a cryogenic treatment in the process cycle have been reviewed. Quantitative metallography has been used to compare the pore size, pore shape, and percentage of microstructural constituents present in the different P/M materials. The measured impact properties are discussed in relation to these factors.

1001 Taylors Lane • Cinnaminson, NJ 08077-2017 • USA • 856-829-2220