Abstract

Cold-rocking is a production method that is frequently used by the manufacturers of seamless tubes in Europe. High reductions (∼ 60%) can be achieved by cold-rocking even in high strength steels and in this way very high strength levels are reached.

SKF Steel has for many years cold-rocked mechanical tubing of C-Mn steel and of microalloyed HSLA steel. In this way minimum yield strengths of 107 000 psi (740 MPa) and 123 000 psi (850 MPa) were exceeded which is equal to that of quenched and tempered steels Elongation AS is 10%. Annealing improves elongation to 15% and reduces yield strength with about 7 000 psi.

The Charpy-U impact strength is minimum 22 ft.-lb. so that the material is quite tough. Due to aluminum addition all nitrogen is bound as aluminum nitrides and therefore ageing never appears in these steels, not even after one year of storing. This is shown both by hardness curves and by impact tests. The steels have good weldability because of low contents of carbon and alloying elements.

A decrease in hardness sometimes appears in the heat affected zone. The micro-structure is ideal for machining and the steels can be turned 50% faster than SAE 4140 of the same strength.

Compared to AISI the 00- and wall tolerances on these cold-rocked tubes are better than cold-drawn and much better than hot rolled tubes. Application examples are given.

Cold-pilger mills

Cold-rocking or cold-rolling of tubes is a production method that is frequently used by the manufacturers of seamless tubes in Europe. In a cold-rocking mill or cold-pilger mill the deformation is made by a pair of roll halves with a profile that rocks along the tube (Fig 1). The roll halves and the whole stand have a weight of about 6 tons and go back and forward about 120 strokes per minute (Fig 2). Between every stroke the tube s fed one inch and twisted 60 °. The outer surface of the tubes is formed by the roll halves and the inner surface is formed by a mandrel inside.

By cold-rocking high reductions, 60% and more can be achieved even in high strength steels. This leads to considerable deformation hardening and very high strength levels are reached.

Mechanical properties in cold-rocked material

SKF Steel has for many years cold-rocked mechanical tubing of C-Mn and of vanadium microalloyed HSLA steels. In this way minimum yield strengths of 107 000 psi and 123 000psi are exceeded. These figures are results from tensile tests. In comparison, however, the yield point is reduced about 17 000 psi due to the Bauschinger effect. The Bauschinger effect is eliminated by stress relieve annealing at 400–500 °C and we get the same yield strength both in tension and compression, minimum 100 000 psi and 115 000 psi respectively. Thus in the annealed condition coldrocked tubes have their highest resistance against bending.

All mechanical properties are given in table I.

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