Machinability Technical Publications
195. The use of Machining Additives in Ferrous PM Parts to Improve Machinability: Although powder metallurgy (PM) alloys are often more difficult to machine than their wrought counterparts, PM has the advantage of easily admixed machinability additives. The most successful and widely used additive, manganese sulfide (MnS), can greatly improve tool life in many applications. A new machining additive (MA) has been introduced to compliment MnS in powder metallurgy PM steels. This new additive is chemically inert under production processing conditions and does not produce discoloration and rusting that has been occasionally found with the MnS additive. The MA additive also reduces tool wear compared with MnS containing mixes in sinter-hardening applications. In this paper, an industrial scale trial was conducted to determine the preferred additive in an induction hardened crankshaft sprocket. Tool wear measurements were made throughout the machining process, surface corrosion was evaluated and the physical properties were tested for each machining additive condition.
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.
181. Machinability Additives for Improved Hard Turning of PM Steel Alloys: The machining of ferrous PM alloys differs considerably from wrought materials. The role of porosity and heterogeneous microstructures complicates the machining process, often making it more challenging. In addition, the presence of martensite in the microstructure of more highly alloyed and/or sinter-hardened PM components increases tool wear. One advantage of PM is that machinability additives can be easily admixed into the powder and therefore into the final part. Manganese sulfide is a well known additive for improving machinability. A new machining additive, designated MA, has been developed to compliment MnS in PM steels. Hard turning tests were performed to evaluate the effect of both additives on tool wear in different material systems. The MA additive was found to improve machinability beyond that of MnS in sintered compacts containing martensite. It additionally reduced rusting on the part surface. This paper discusses the improvement in machinability with these additives, with an emphasis on sinter-hardenable PM steels.
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.
121. Machinability: A Material Property or Process Response?: Although PM is considered a net, or near-net shape process, many pressed and sintered ferrous parts are machined prior to final assembly. During part conversions, the potential differences in machining response between PM and conventional, particularly cast iron parts may cause concern. These concerns are frequently supported by personal experience when prototype PM parts are machined using the tools and conditions used for the conventional part. Alternately tables of relative machinability for cast irons and steels are used to justify concern over the machinability of PM steels. This approach to comparative machinability ratings is questioned in Machining Data Handbook, " There appears to be an endless desire to provide machinability ratings for materials" (Ref. 1). A more radical view of machinability was expressed by Trent who suggests," There is no clear cut unambiguous meaning to this term" (Ref. 2). However, changes in tool life under defined conditions clearly change manufacturing costs and lead to the classification of materials as having good or poor machinability. An alternative viewpoint is that the microstructures of PM steels differ considerably from those of wrought steels or cast irons of equivalent strength, so that we should anticipate a difference in machinability and allow for such differences at the design stage. This paper will try to illustrate that the "machinability" of PM steels is a response to the machining process, not an inherent material property, and that by changing the cutting process we can reduce cutting forces, reduce tool wear and increase tool life.
114. Role of Additives in PM Machining: Many PM parts are machined before final assembly. The increasing use of dedicated lines, or cells, in which many sequential operations are conducted on an assembly means that the time required to machine a PM component will need to be reduced if it is the bottleneck in the assembly process. This paper examines the effects of three freemachining additives: boron nitride, manganese sulfide and pre-alloyed, or resulfurized, sulfur, upon the machining response of an FC-0205 PM steel in a turning operation. The effects of the additives on tool wear, cutting forces, chip form and surface finish are compared.
94. Silicon Containing High Performance Alloys - Machinability and Mechanical Properties: With the introduction of several silicon-containing materials, the potential for replacing ductile and malleable cast irons with PM alloys has never been greater. These materials exhibit extremely competitive property combinations when conventionally compacted and sintered at 1260 °C (2300 °F). However, additional opportunities exist if advanced densification processes such as double press / double sinter or warm compaction are employed. This paper investigates several important manufacturing processes including advanced densification techniques, vacuum sintering, and machinability.
63. Productivity Enhancement through Improved Machinability Materials: Machinability of powder metal components (PM) requires a better understanding of material composition, microstructure and machining process conditions. By employing the appropriate machinability additives and base powder design, machining productivity can be optimized. Through proper application planning, the material supplier, PM fabricator, and end user can design the P/M system to balance part performance and machinability requirements.
35. Machinability of PM Steels: This paper will examine the potential to improve the machinability of sintered PM steels by the addition of freemachining agents. Testing will examine the effects of freemachining agents upon the sintered properties and machinability in drilling of commercial PM steels, including FC-0208 or FN-0205.