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Seamless tubes are manufactured by various processes, such as the piercing mill, the Diescher mill, the Pilger mill, the Conti mill, the Assel mill, and the push bench mill. All processes are based on a common technological concept:
The piercing of a heated billet to a hollow bloom, the stretching of the hollow bloom to a shell (thick walled tube) and finally the rolling of the reheated shell into a tube. The rolls and tooling required for each of these process stages can be manufactured from cast materials and tungsten carbide as well as tool steels.
The sizing rolls in push bench mills require a material with a high hot strength, good toughness and a high wear resistance; the hot working tool steel Märker® WM 28 has proved very successful for these rolls. The tensile strength required for these rolls is in the range of 1400 - 1700 N/mm², depending on the working conditions.
In detaching plants the tube is released from the mandrel bar by light expansion and the mandrel is drawn out of the tube in the mandrel extraction unit. Depending on working conditions and plant design, detaching rolls are made from either cast materials or forged tool steels. For cast rolls we recommend centrifugally produced bi-metal castings with a hard wearing alloyed outer shell and a grey iron soft core. It is recommended that forged rolls are made from either the hot working tool steel Märker® WM28 or in the special heat treated tool steel Märker® ZESEK with high wear resistance.
PUSH BENCH ROLLS
Made of centrifugally cast bi-metals
B&J supplies rolls made from centrifugally cast bi-metal consisting of an alloyed outer shell and a grey iron soft core. These materials have been developed to meet specific requirements and offer advantages over the lower grade chilled cast irons. With these simpler chilled cast irons the chill depth comprises 8-10% of the barrel diameter and even though the chilled shell is hard it drops significantly in the transition area.
For rotary piercing a number of roll materials are used. Depending on plant design, operating conditions and mechanical load, low alloyed to medium alloyed tool steels are used, offering good grip and if necessary, a good pressure resistance. The special tool steel Märker® S455 has been specially developed for this application and offers a good compromise of grip, pressure resistance and wear resistance. For the piercing rolls in Diescher mills the tool steel Märker SRSE, with a higher tensile strength, has proved successful.
Stretch reducing mills enable elongation; change in the diameter and, depending on the design, reduction of the tube wall thickness without the use of internal tooling.
STRETCH REDUCING AND SIZING ROLLS
Advantages of the centrifugally cast bi-metal rolls made by B&J are:
Rolls made of forged tool steel
In addition to rolls made from cast materials, forged rolls in the tool steel Märker® ZESEK, offering high wear resistance and improved toughness, have proved successful, particularly for thick walled tubes. These rolls are specially heat treated to meet the specific requirements of this hot rolling process.
ROLLS FOR SEAMLESS TUBE PRODUCTION
Piercing plugs produce a hollow bloom, from a heated billet, that is rolled over a mandrel bar. Depending on the process, either tool steels, with a good high temperature strength, or special steels, with a high compressive strength, are used which additionally generate an oxide scale improving wear protection.
High integrity – no porosity
Choice of composition and depth of shell
Hardness consistent throughout depth of shell
High resistance to crack propagation
High thermal conductivity
Good fire cracking resistance
Improved toughness due to grey iron core
OTHER ROLLS FOR SEAMLESS TUBES
Bowers & Jones manufacture rolls for all types of hot formed and seamless tube production. Examples of which are shown below.