Roll Management

As roll is the most costly material in rolling, the efficient management system helps in reduction in roll inventory and efficient campaign planning of section to be rolled. In addition it also helps in regular monitoring and to take better purchase decision. It includes:

  • • Estimate the Requirement of rolls,
  • • Procurement of rolls,
  • • Receipt of Rolls,
  • • Stacking of Rolls-Mill wise,
  • • Turning of Rolls,
  • • Supply Rolls to Mills,
  • • Redressing of Rolls and its resupply,
  • • Claim Settlement,
  • • Reclamation/Salvaging of Rolls,
  • • Scrapping/ Recycling the Rolls for sale.

Objective of Roll Management

  • • To supply suitable roll to rolling mill to produce good quality rolled product,
  • • To guarantee supply of safe roll to the mill avoiding cracks, spalling,
  • • To minimize consumption of roll through minimization of dressings,
  • • To choose most suitable quality of roll for different application,
  • • To minimize storing of rolls in roll shop,
  • • To make required modification to the roll drawing in order to minimize production cost,
  • • To achieve reduction of roll cost target.

Roll Inventory

The minimum number of three sets is required for normal rolling. One in rolling, one hot spare and one in redressing. Since they are critical items, judicious planning and procurement method is to be employed for continuous rolling. For this future-rolling plan is a must so that rolls are available on time.

Proper Layout of the Shop

Objective of Roll Shop is as follows:

  • • To supply suitable roll to rolling mill to produce good quality rolled product as concerning profile and surface aspect.
  • • to guarantee supply of safe roll to the mill avoiding cracks, spalling and so on.
  • • to minimize consumption of roll thr ough minimization of dressing.
  • • to choose most suitable quality of roll for different application.
  • • to minimize storing of rolls in roll shop.
  • • to make required modification to the roll drawing in order to minimize production cost.
  • • to achieve reduction of roll cost target.

To achieve above mentioned objectives, the ideal layout should be such that rolls can be supplied and retrieved fr om mill at a short notice. A typical layout is shown below:

Layout of a Roll Turning Shop

Fig. 3.12 Layout of a Roll Turning Shop.

Investigation of Roll Failures

Pictorial view of Roll Failure

Fig. 3.13 Pictorial view of Roll Failure.

Following steps are to be followed up during the investigation

1. Examination of fracture surface

Roll Failure-Brittle Fracture

Fig. 3.14 Roll Failure-Brittle Fracture.

Roll Failure-Fatigue Fracture

Fig. 3.15 Roll Failure-Fatigue Fracture.

Roll Failure-Torsional Stress

Fig. 3.16. Roll Failure-Torsional Stress.

Roll Failure-Crystalline features

Fig. 3.17 Roll Failure-Crystalline features.

Roll Failure-fatigue failure having ratchet marks and brittle fracture

Fig. 3.18 Roll Failure-fatigue failure having ratchet marks and brittle fracture.

Investigation of roll failure should be conducted at the site only. It should be done just after the roll breakage. Delay in the examination of fracture surface may lead to the loss of useful information. Ann of investigation is-

  • (a) Whether the roll failed prematurely due to fatigue cracks with ductile fracture or failed in a brittle maimer.
  • (b) Whether fracture is coarse or fine.
  • (c) Whether fire cracks are present over the surface and if present, then what is the depth of it.
  • (d) The point at which fatigue cracks get originated.
  • 2. Metallurgical Investigations

Once the fracture surface has been examined, a metallurgical investigation of a roll failure is to be conducted to collect data of fractures such as:

  • (a) Histoiy of rolls.
  • (b) What types of heat treatment given to roll before use.
  • (c) Nos. of redressing of roll and its diameter.
  • (d) Amount of draught given, rolling temperature at that tune of failure, composition of roll and its hardness.

It is also to know chill depth and uniformity of chill, if it is made of chilled cast iron.

  • 3. Roll investigator has to compare this collected information with the available past data.
  • 4. Photogr aph of fracture surface should also be taken for investigation report.
  • 5. Sample of fracture portion of rolls has to be collected to assess the microstructure investigation and the hardness of rolls.
  • 6. The establishment of the cause of failure is utmost important, after going through all above mentioned procedure for the investigation of roll failure.
  • 7. Finally to take collective actions for future rolling and from manufacturer.

Reclamation of Rolls

Automatic Submerged Arc Welding

Submerged Arc Welding. Automatic sub-merged arc welding is used for following

Fig. 3.19 Submerged Arc Welding. Automatic sub-merged arc welding is used for following: (a) to build up of worn out portion of part

Welding of Simple Shape

Fig. 3.20 Welding of Simple Shape.

Welding of Complex Shapes

Fig. 3.21 Welding of Complex Shapes.

(b) Reclamation of old and manufacture of new part with improved surface properties such as corrosion resistance, acid resistance, thermal resistance etc. by deposition of metal

Welding to improve surface properties at different parts

Fig. 3.22 Welding to improve surface properties at different parts.

(c) Rational selection of the blank required to make a part with a view to economize the material and machine hours.

Submerged arc welding is essentially an arc welding process. The working area is shielded by a steam of granular flux. The electrode may be bare or Cu coated and in the form of coils. It is fed into the blanket of flux; the rate of feed is controlled automatically to give correct arc length. DC current supplied by a welding generator to produce the hidden or submerged arc between the electrode and article. Heat generates melts the electrode and base metal. The flux adjacent to the arc gets melted and will float on the surface of the molten metal. This thin film of liquid flux protects the molten metal from the contact of ah and also prevents the splashing. Steel rolls of less than 0.8% C content are usually considered for the building up. It is not advisable to build up the full roll to the original size, since thick layer of weld will induce severe internal stresses leading to the formation of cracks. The passes which are built up for the first time, should be first turned in a roll turning lathe and in such a way that all wear out marks and fire cracks get removed from the roll. The thickness of deposited metal must be of a min. thickness of 5 mm at any portion of the working surface of roll after the finish of the turning. For reconditioning, deposit should be made sufficient to restore the dimension of pass. Generally, working passes are built up repeatedly upto 10 times. Further building up of metal is restricted due to the fatigue of the parent metal of the roll. If there are also a deep circular cracks on the surface of the roll or on the roll neck, then welding is not advisable.

The weld building up technology for eveiy work roll should be selected in such a way to ensure the perfect formation of eveiy deposited weld.

 
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