锻造技术英文文献和中文翻译(8)

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PRECISION RING ROLLING

In his talk on the precision ring rolling process, W. Han- sen of the General Electric Company noted that approxi- mately 15 pct of the cost of today’s gas turbine engines is associated with rings rolled from various high-temperature alloys, including nickel-base superalloys such as Inconel 7 18 and Rene 41, several cobalt-base and iron-base super- alloys, and stainless steels. Precision (cold) ring rolling was developed in 1976 to overcome the very poor utilization of these materials (10 : 1 typical buy : fly ratio) in the traditional practice of machining rings made by hot forging or by flash welding followed by hot rolling. The new process starts with a controlled weight of bar stock (maintained within 0. 2 pct) which is equivalent to the desired finished weight of the part. Each inpidual length of the bar is hooped and flash- welded into a ring preform, which is then incrementally cold-rolled using successive sets of roll dies. The upper roll die is hydraulically down-fed to reduce the cross section of the preform. The reduction in section thickness is accom- panied by a diametral growth and a lateral spread in the

J. APPLIED METALWORKING

preform. Intermediate annealing and pickling are frequently required between stages.

The ring-rolled parts are precise in cross section (TO.003 inch), but their as-rolled diameters may be off the mark by as much as half an inch. To overcome this, the rings are precision-expanded, enabling GE to maintain a diametral tolerance of U0.010 inch for large rings and TO.003 inch for smaller rings. The remarkable feature of their process, according to Mr. Hansen, is the flatness of precision—rolled rings — a 0.005 inch shim will not enter beneath a 50 inch diameter ring when the latter is placed on a surface plate. GE reportedly uses a $3OO,0O0 Grotnes machine capable of developing 60 tons of closing force on the roll dies. The dies, which are fabricated from D2 grade of tool steel and hardened to HRC 58, were said to cost approximately

$5,000 per set. Regarding cycle time, Mr. Hansen said that GE requires two shifts to turn around a lot size of 60 rings, of which a half—shift goes into roll die changeover and other set—up tasks. They do not, as yet, use CAD/CAM techniques in their operation, beyond the use of empirical formulae to predict metal movement assuming a certain ratio of di- ametral growth to lateral spread. In Mr. Hansen’s words, the precision ring rolling process is still an art and not a science. GE has negotiated license agreements with several compa— nies, including Amweld and Kelsey-Hayes in the United States and Rolls Royce in the United Kingdom, for commer— cial use of the process.

COATINGS

In his talk on the use of hard coatings to extend tool life,

R. Vagle of Scientific Coatings, Inc. , described the chemi- cal vapor deposition (CVD) process for depositing the vari- ous coatings, its application to the coating of steel tools, types of coatings, and performance results. The CVD pro- cess utilizes a high temperature ( 1900 °F) in a chamber consisting of either a vacuum or a controlled atmosphere, with chemical action bj/ a combination of gases, in conjunc- tion with a catalyst, giving a molecular bond with the sub- strate (tool) material. Available coatings include tungsten carbide, titanium carbide, titanium nitride (which is sol’ter but more lubricious than TiC), aluminum oxide (whose in- eriness makes it ideal for high-speed turning applications), hafnium nitride, silicon carbide, and combinations of the above. The typical coating thickness is 0.0003 inch or 8 pm, with thinner coatings preferred for sharp edges and somewhat heavier deposits desirable on large radii. Accord- ing to Mr. Vagle, CVD-coating of tools has been around since the 1950’s, but its application to the coating of steel components is very recent. Steel tools coated with TiN and TiC were the subject of attention as recently as the 198 l Hannover machine tool show. Their use appears to be best justified in applications seeing excessive wear, cratering,