Boron Steels

Boron Steels Technical Publications

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.

172. Pre-Alloyed Boron in Powdered Metal (P/M) Stainless Steels The demand for high-density stainless parts continues to grow as P/M continues to battle conventional stainless. The compressibility and hence the final sintered density of P/M stainless steels are limited due to their high alloy content. The addition of boron allows P/M to produce nearly full density parts that achieve mechanical properties similar to wrought materials. By adding boron to the melt prior to atomizing, the segregation of boron is minimized leading to a more uniform microstructure and properties. The present work will examine the role of pre-alloyed boron in both austenitic and ferritic stainless steels. Experimental work will involve the measurement of green and sintered properties. Using the laboratory experiments as a guide, specimens will be sintered in production furnaces to determine the performance of various grades of boron containing stainless steels. The specimens will be evaluated for density distribution, dimensional stability and mechanical properties.

152. Production of Stainless Steel Powders by Advanced Steelmaking Technology Advanced melting technology is now being employed in the manufacture of stainless steel powders. The new process currently includes electric arc furnace (EAF) technology in concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA) and hydrogen annealing. The new high performance processing route has allowed Hoeganaes Corporation to provide not only a more consistent product, but has allowed enhanced properties, such as improved green strength and green density. This paper will review the potential to use this processing route to provide products with improved properties and performance.

145. Development of Stainless Steel and High Alloy Powders (Improved Stainless Steel Processing Routes) Advanced melting technology is now being employed in the manufacture of stainless steel powders. The new process currently includes electric arc furnace (EAF) technology in concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA) and hydrogen annealing. The new high performance processing route has allowed Hoeganaes Corporation to provide not only a more consistent product, but has allowed enhanced properties, such as improved green strength and green density. This paper will review the potential to use this processing route to provide products with improved properties and performance.

135. Improved Stainless Steel Processing Route Advanced melting technology is now being employed in the manufacture of stainless steel powders. The new process currently includes electric arc furnace (EAF) technology in concert with Argon Oxygen Decarburization (AOD), High Performance Atomizing (HPA) and hydrogen annealing. The new high performance processing route has allowed Hoeganaes Corporation to provide not only a more consistent product, but has allowed enhanced properties, such as improved green strength and green density. This paper will review these processing changes along with the potential new products that can be made with this technology.

113. Effect of Small Additions of Boron on Mechanical Properties & Hardenability of Sintered P/M Steels Low levels of boron (0.01-0.15w/o) may induce sufficient hardenability and strength in powder metallurgy steels to permit a decrease in the level of the alloying elements, increase powder compressibility and reduce the as-sintered hardness. These lean alloys may be sufficiently ductile to coin and be hardened by subsequent heat treatment. The goal of this study was to identify the boron level in FLN2-4400 (Fe + 0.85w/oMo, 2.0w/oNi, 0.3w/oC) which yields the optimal combination of strength, ductility, and hardenability. Tensile, transverse rupture, hardness, and Jominy end quench tests were performed on this alloy with six different levels of boron added Sintered strength and ductility increase up to 0.05w/oB, but decrease beyond this level, even though sintered density increases significantly. Jominy hardness traces show that the hardenability is not increased substantially until the concentration of boron reaches 0.05w/o. The microstructures of the Jominy bars show that with an increase in boron level, the depth to which martensite is retained increases, but that grain boundary segregation occurs. A level of boron ~ 0.05w/o gives the optimum combination of strength, ductility, and hardenability in FLN2-4400.

73. Hardenability of Sintered Fe-B-C Alloys The objective of this study was to evaluate and interpret the effect of small additions of boron (<0.09 w/o) on the hardenability of sintered Fe-B-0.3 w/o C alloys. For comparison, in terms of hardness response, binary Fe-B alloys were also evaluated. The alloys were prepared by mixing gas atomized Fe-12 w/o B with Ancorsteel 85 HP powder; carbon was added in the form of graphite. Jominy bars were fabricated from the alloy powder by cold isostatic pressing (414 MPa) in a polyurethane bag and sintering at 1120 °C or 1230 °C (30 min) in dry hydrogen (dewpoint – 40 °C). Hardenability was quantified by means of the standard Jominy end quench test. Small additions of boron enhance both the hardness and hardenability of the base Fe-C alloy, particularly at the higher sintering temperature. This effect is attributed to inhibition of the nucleation of ferrite by the boron which retards the formation of pearlite.

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.

51. Processing Experience of Green Strength Enhanced Material Systems Green strength enhanced material systems have been developed for iron and Low alloy as well as stainless powder metallurgy applications. Relative to normal processing, the increase in green strength is 50-100%. The nature of green strength with respect to both materials and processing conditions is reviewed. The processing variations designed to meet target properties such as apparent density, flow and compressibility are compared with conventional material systems. Manufacturing experience with a mechanical press is presented.

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.

43. Single Compaction to Achieve High Density in Ferrous P/M Materials in Automotive Applications The continued growth of ferrous powder metallurgy in automotive applications is dependent on the development of higher density and improved dynamic properties. New powder metallurgy applications also must be cost effective through the continued use of the process's, net shape forming capabilities and a reduced number of manufacturing steps. The processes utilized to manufacture some of these new parts also must provide the ability to produce thin walled parts with complex geometries.

The use of the warm compaction process (ANCORDENSE™) will be shown to develop high density levels with a single compaction process. The process also provides increased green strength and reduced ejection forces. The dependence of mechanical properties on density will be demonstrated.

An example of a potential application of the warm compaction technology is an output shaft. The capability of manufacturing this part with the warm compaction process is outlined and compared with the same part made by the double press/double sinter (DPDS) process. Part density and performance from both processes are compared.

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.

31. Advanced Properties of High Density Ferrous Powder Metallurgy Materials The introduction of the ANCORDENSE™ system has provided significantly higher density levels than previously possible in a single press/single sinter operation. This paper will explore the role that higher density has on mechanical properties. Various properties will be evaluated, including transverse rupture strength, tensile strength, and impact. Additionally, the effect of other processes, such as high temperature sintering and heat treatment, will be addressed.

9. Properties of Parts Made from ANCORBOND Processed Carbon-Nickel-Steel Powder Mix(FN-0208) Studies were conducted to determine the effects on property variability of parts made from a bonded Ancorsteel 1000 mix containing 0.95% graphite, 2.0% nickel, 0.6% Acrawax and 0.3% zinc stearate. The part geometry studied was that of a cylindrical bushing. The treatment effects on powder properties and on several parts properties were determined. The powder properties included the traditional green and sintered properties and the graphite and nickel dusting resistance’s. The parts properties surveyed included both green and sintered properties and sintered carbon and nickel contents. Similar studies of a companion regular mix of nominally the same composition were conducted for purposes of comparison.

Compared with the regular mix, the bonded mix exhibited marked improvements in graphite and nickel dusting resistance and in powder flow properties. In the parts-making study, the bonded mix showed significant differences in mean dimensional change characteristics but was otherwise reasonably similar to the regular mix in terms of mean property values. In the case of variability, the bonded mix was statistically equivalent in green and sintered dimensional change characteristics and sintered nickel content but otherwise significantly improved relative to the regular mix in all of the other properties of interest. The latter included green weight and density and sintered density, hardness, crush strength and carbon content.

The variability improvements in the parts from the bonded mix were attributed both to the effects of the binder treatment in improving powder flow properties and reducing carbon segregation. The difference in mean dimensional change characteristics between the parts from the bonded mix and those from the regular mix were also attributed to the effect of the binder treatment in improving alloy admix uniformity.

The fact that the parts variability improvements of the bonded mix did not extend to the dimensional change characteristics was suspected to be due largely to a purely statistical effect arising from the dissimilarity in the mean dimensional change values relative to those of the regular mix. A limited laboratory-scale study was subsequently conducted to examine this possibility. In general, the findings supported the idea. In addition, they also presented a strong indication that it may be possible by virtue of the ANCORBOND process to effect significant reductions in the nickel contents of graphite-nickel mixes without sacrificing either mechanical 2 properties or dimensional change characteristics.

5. Statistical Process Control in Iron Powder Production and New Product Development SPC is discussed with a view to indicating its implications not only to manufacturing and quality but to research and product development as well. In the manufacturing/quality area, the efforts and methods attending full-scale implementation of SPC are briefly reviewed with special reference to the differences inherent in powder making versus manufacturing of parts. SPC charting techniques suitable for powder making are described and discussed. In the product development area, it is shown how SPC influenced a major research program. The objective of the program was to improve premixed products with a view to reducing variability in parts manufacturing. ANOVA studies of production mixes generally showed that the main sources of premix variability were mix to mix differences and within mix differences arising from demixing subsequent to premix manufacture. It was recognized that SPC is especially applicable to dealing with the first of these and subsequent efforts to implement it to premixes are described. In the case of demixing, the developments of a new premixing process is reported in which the alloy admix ingredients are bonded to the iron.

The general findings of extensive studies, which show the benefits of the new process in terms of reduced variability in parts manufacturing, are reviewed and the results of a study of a FC-0208 premix are presented as an example. SPC theory and concepts are used to indicate the significance of the results and their potential applicability to the production of parts.

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.

1. Impact and Fatigue Characterization of Selected Ferrous P/M Materials Dynamic property data on pressed and sintered ferrous powder metallurgy materials have come under increasing demand as the P/M industry has grown into areas of application involving more highly stressed components. Data collected from relatively simple dynamic property tests will provide new avenues for P/M alloy development. Un-notched Gharpy impact energy and rotating bending fatigue tests have been used to characterize commonly used P/M steels. The endurance ratios of porous steels have been found to be relatively insensitive to processing, with higher strength materials giving proportionally higher endurance limits. Since impact energy was not strongly affected by varying the carbon content up to the eutectoid composition, increasing the carbon content of low alloy steels is a viable way of increasing endurance limit. Impact energy transition temperature has been found to be a factor in carbon-free phosphorus steels, but not in conventional low alloy steels. As has been indicated in the literature, sintered density is crucial to both impact energy and fatigue endurance limit. Metallographic examination of the fatigue cracks has provided some insight into the nature of the R.B.F. test.

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