Steel Tube Manufacture

This article first appeared in Practical Engineering 1940 Vol1 No24. The information contained within the article is accurate as of 1940. This article describes new methods in Engineering at the time.

Background

In modern manufacture the tendency is to concentrate all operations from raw material to finished product on one-unit works. Some time ago, at a well-known British steelworks, steel tube manufacture was organised in this way on the largest scale. We find the spectacle of blast furnaces, coke ovens, Bessemer steel plant and rolling mills organised to the conversion of iron ore and coal into millions of feet of small steel tube. At one point on these works there are two welding machines which turn steel strip, moving through at nearly ten miles an hour, into steel tube.

Through these two bottlenecks flows half the total steel production of the works. It is a triumph of modern organisation that so much material can move smoothly through such a bottleneck, hour after hour, 24 hours at a stretch without one single hitch.

To understand steel tube production, one should go right to the heart of the process, that is to the tube-making benches. At these works steel tube is made by two methods. Some tube is welded up automatically from strip steel, but larger tubes are made by pushing out steel ingots on push benches.

Fretz-Moon Welded Tubes

The principle of the Fretz-Moon welding method is that coiled steel strip comes from the rolling mills and is unrolled and straightened through rolls before going to the heating furnaces. Coil is fastened to the end of coil as they unreel by a flash-welding machine. As this process takes a minute or more to carry through, an accumulating device has to be incorporated in the flow schedule. The strip therefore doubles back on itself in a big loop, and while the strips are being joined, the loop becomes smaller. It again extends and becomes bigger as the new coil unreels. Although it is a simple device, it enables the mill to run smoothly with perfect continuity.

The Loop Pit

From the loop pit the steel strip is drawn through a long gas-heated furnace. Temperature and time of travel in the furnace are exactly controlled so that the strip leaves the furnace at the correct temperature for welding. It immediately enters the welding rollers. One pair of rollers is curved to bend the tube round into a circular cross-section, the next pair of rolls forces the edges of the strip together, and a third pair gives the final seal to the weld. In one foot of travel, at the rate of ten miles an hour, strip has become tube.

The continuous worm of steel is then operated on by a flying saw which cuts it off into lengths as it continues to travel on its way. As soon as one cut is completed, the saw lifts, flies back, drops again and makes another cut. It is a very unusual kind of mechanical synchronisation. The cut lengths are cooled, scaled, brazed, then threaded at the ends on a multi-threading machine, the sockets at the end are spun on and the tubes are all ready to be bundled into batches for despatch.

Wellman Push-Bench

The Fretz-Moon process is used for tubes up to 2in. gauge. Tubes larger than this, up to 6in., are  made by drawing   steel ingots over mandrels. It starts with steel billets: that is, steel blocks cut off to exact size and weight from bar. Each billet is heated under pyrometer control in a rotary hot furnace, and the billet is put in one side. By the time the furnace has rotated to bring that particular billet to working heat, it is alongside the push bench. That billet is seized by tongs and put in a press arm pierced with a blind hole. Then the mandrel bar is pushed into the hole, bar and billet are slung on to the push bench positioned in front of a hydraulic ram. In front is a run of rings of gradually-diminishing diameter. Power is put on the ram, and at a pressure of thousands of pounds the red-hot billet is shot down through the rings with a clattering rush. As in all pressing operations, mechanical force raises the temperature of the steel and the metal flows smoothly round the mandrel bar in a tube of even diameter and thickness.

Smoothing and Polishing

The newly-formed tube, still on the mandrel, shoots out from the last ring on a set of spinning rollers which smooth and polish it. It cools here and shrinks on the mandrel. To release it, a quick reheat is applied, which expands the, tube from the mandrel so that the latter can be slid out free of the tube. The mandrel travels back to the head of the pushing bench along an automatic conveyor. The tube itself goes off to the finishing benches.

Steel Production

Behind all these operations are the steel mills, the Bessemer steel converters, the blast furnaces and coke oven batteries. These take raw coal and carbonise it to coke in great vertical gas-heated ovens built of fire-resistant brick.

But coke is only one of the products. There are the by-products of tar, ammonia and benzol, and most important of all, coal gas which is sufficient in quantity to supply all the gas needed for the many soaking pits and reheating furnaces of the whole series of processes. Coke from the ovens goes to the blast furnaces, where it meets iron ore from the iron fields and lime from lime-burning kilns. When pig-iron is tapped from these great furnaces, it is taken red hot in ladle cars to the steel plant. The blast furnaces also have a by-product, blast-furnace gas, which is used for heating the coke ovens and for steam raising in the boilers which power the whole works.

The pig-iron is turned into steel by blasting it with air in Bessemer-converters. The process is essentially the burning-out of the impurities in the iron, such as carbon and phosphorus, until these reach their low concentrations of fractional percentages which are allowable in steel. The Bessemer plant described is of great interest as it represents a revival of this process in England.

When steel is poured from the converters it is ready for casting into ingots, and these ingots are then ready for rolling down to bar or strip to feed the tube mills. Thus, in, full circle we are back to the making of the tubes themselves.