Hardenability

Hardenability Technical Publications

141. Effects of Cooling Rate on the Hardenability of Chromium Containing P/M Steels Two chromium containing grades, Cr-modified Ancorloy^®MDC and a developmental CrMnSiMo steel, have been investigated to develop an understanding of the phase transformation behavior of these steels during cooling. The cooling rates studied are in the range typically found in sintering furnaces. A quantitative study to assess the hardenability of these materials has been undertaken and the results of dilatometric, metallographic and hardness evaluations to determine the effects of cooling rate on the various Cr containing steels are presented.

134. Ancorloy Hardenability The Continuous Sinter Cooling Transformation diagrams of five proprietary admix compositions including Ancorloy^® 2 and Ancorloy^® 4 at sintered carbon contents of 0.50 w/o and Ancorloy^®MDB, Ancorloy^® MDC and Ancorloy^® MDCL at sintered carbon contents of 0.60 w/o are presented for sintered densities in the neighborhood of 7.10 g/cm³.

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.

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.

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

123. Effect of Porosity on the Thermal Response, Hardness, Hardenability and Microstructure of P/M Steels The pores present in sintered steels affect their thermal characteristics. In the present study the role of porosity was examined in two hybrid P/M steels in relation to cooling rate, hardness, hardenability, and microstructure utilizing an instrumented Jominy test. To establish a base line for thermal response in the absence of pores, Jominy bars were hot isostaticlly pressed after sintering. Pores enhance slightly the measured cooling rate near the water quenched end of the Jominy bar. This behavior is predicted on the basis of a recent simple heat transfer model in which a decrease in thermal diffusivity results in faster cooling near the water quenched surface. Alternatively, simulation of penetration of water into the pores by capillary action predicts that this is a viable mechanism to enhance cooling rate.

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.

98. Effect of Porosity on The Hardenability of P/M Steel Pores in sintered P/M steels influence their thermal response and thus hardenability. Porosity decreases thermal conductivity and attendant cooling rates, and it reduces the mass of the steel from which heat is removed during quenching. The latter effect is quantified by a factor (1-), where e is the fraction of porosity; in contrast, the influence of e on thermal conductivity is more complex. In the present study, the hardenability of three sintered steels (Fl-4405, FLC2-4405 and FLN2-4405) with levels of porosity in the range 7v/o-16v/o has been determined experimentally using an instrumented Jominy test in which thermocouples gave direct readings of cooling rate as a function of distance from the water-quenched end of the bar. The cooling of the Jominy bars was also simulated by means of a three – dimensional model using the finite difference method. Cooling curves are given for the three steels as a function of the level of porosity at distances in the range 5 mm to 65 mm from the water-quenched end of the Jominy bars; the corresponding hardness traces define the 50% martensite distance. The model predicts a decrease in cooling rate with an increase in porosity, hence hardenability should decrease whereas the experimental data show clearly that the P/M steels with a level of porosity > 12v/o cool faster than a baseline pore-free wrought steel. This is attributed to penetration of the water via the interconnected pores in the sintered steels.

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.

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.

22. High Density Processing of CR-Mn P/M Steels The use of chromium and manganese as alloying elements in P/M steels offers several potential advantages over copper and nickel that are used in conventional P/M alloy steels. The paper will illustrate how the principles used to improve the hardenability and performance of wrought steels can be applied to P/M chromium and manganese alloy steels using a systems approach. The use of chromium-manganese for P/M applications was made possible by binder-treated premix technology of a highly compressible prealloyed low alloy base material. The flexibility of alloy design will be illustrated by examples of through hardening, high strength low alloy steels.

15. Surface-Hardenable Heat Treated P/M Steels The addition of fine particles (< 20µm) of high-carbon ferroalloys to the high compressible prealloy powders, Ancorsteel® 85 HP and Ancorsteel 150 HP, has been shown to be a practical way of producing ferrous low-alloy steels containing chromium and manganese. Increased sintering temperatures improved the mechanical properties of the materials and the effect was particularly noticeable at 2350°F.

The ferroalloy additions significantly enhanced the hardenability of the base low-alloys. Materials based on the low-alloy powder containing 1.5% molybdenum were more hardenable than those based on the 0.85% molybdenum alloy. These materials are well suited for plasma nitriding and should find use in gears and cams that require a hard wear-resistant surface coupled with a strong, tough core.

8. Powder Forging is a process in which unsintered, presintered, or sintered powder metal preforms are hot formed in confined dies. The process is sometimes called P/M (powder metallurgy) forging, P/M hot forming, or is simply referred to by the acronym P/F. When the preform has been sintered, the process is often referred to as "sinter forging."

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