锻造技术英文文献和中文翻译(9)

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VOL. 2. NO. 2. JANUARY 1982 9 I

and galling, when a better surface n»ish is required to be generated, or when increased capability is sought in terms of speed or pressure.

Successful application was cited in the case of indexable inserts, form tools for screw machine dovetails (fourfold increase in tool life), class C gear hobs (class A hobs are not feasible because the 1900 °F deposition temperature distorts the tool beyond its strict tolerances), parting tools for turn- ing (tenfold increase in life), piercing punches (permitting six resharpenings as opposed to only one in the case of chrome-coated punches), trim dies for the fastener industry (which run longer at higher speeds creating much downtime in the event of wear, thereby justifying coating), and form- ing tools such as draw dies for transmission parts. The average gain in tool life was reported to be three- to four- fold. The mechanism of adherence of the coating to the substrate, according to Mr. Vagle, is one whereby the coat- ing adheres preferentially to the carbides in the substrate. Chromium carbides were noted to be especially good in this regard. H-series die steels and D2 tool steel were mentioned as good substrate materials. Scientific Coatings’ procedure is to obtain premachined tools from their customers, coat the tools, and then final-heat-treat them. The cost of coating was quoted to be approximately $5 to 10 per pound, de- pending on the shape and size of the tool to be coated.

LOW-COST ALTERNATIVES

Of the talks falling under this category, the first was by

R. Cellitti of International Harvester Company on the sub- ject of controlled cooling of steel parts from the forging temperature. Mr. Cellitti described an International Har- vester project aimed at demonstrating that control-cooled forged gear blanks are a viable alternative to gear blanks made by normalizing and tempering after forging. In order to realize benefits such as reduced energy, reduced labor cost, and reduced thermal process overhead costs, a primary requirement of the new method was that the resulting microstructures should be uniform and reproduc ible, and accompanied by a hardness of 170-207 BHN for optimum machinability. Mr. Cellitti’s talk covered hard- ness distributions in control-cooled planetary gear blanks, microstructures, and machinability data, as well as design considerations for a controlled cooling system, and the energy and cost benefits of such a system. The conventional process for planetary gear manufacture involves induction heating the billets, forging them, trimming the forgings, room-air cooling, normalizing, tempering, shot peening, and, finally, gear cutting. The new process eliminates room-air cooling, normalizing, and tempering from the above sequence and replaces them with a single step: con- trolled cooling. In both processes, gear cutting is followed by case carburizing which is unaffected by the type of initial heat treatment.

92 VOL. 2, HO. 2, JANUARY 1982

The effect of process parameters on the end product was examined by control-cooling planetary gear blanks from various forging temperatures and at various cooling rates. The optimum cooling cycle was found to require blowdown, from the trim press, to 1500 °F, at which point the blanks are muffle-cooled in a furnace at a cooling rate of 35 °F per minute. The selection of 1500 °F as the temperature to begin slow-cooling was to allow adequate latitude above the 1200 ° to 1250 °F transformation temperature so that vari- ations in table speed or air flow rate did not result in blow- down to below the transformation temperature. Controlled cooling resulted in fairly predictable hardness in the range of 187 BHN (microstructure of pearlite and blocky ferrite) to 207 BHN (onset of Widmanstatten structure). Normalized and tempered blanks, for reference purposes, averaged 163 BHN, typically. Machinability evaluation was done by mea- suring the wear on the tip of a gear hob after a specified duration of cutting time. Mr. Cellitti reported that the control-cooled parts were at least as machinable as their normalized and tempered couterparts. He added that a con- trolled cooling unit is being readied for production.