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,