Reason for use:
This problem occurs mainly where a large bite angle occurs in the roll pass design, that is where the size of the billet entering the stand is relatively large compared to the size of the roll pass profile of that stand. This occurs mainly in the roughing stands at both the rod and bar mill. It was found out that certain metal sprayed coatings applied to the rolls can alleviate this problem by providing the necessary surface roughness on the rolls.
Properties of such a coating would have to fulfil the following criteria:
Metallisation Mark 61 / Mark 66E Flamespray System
Metallisation T405/1 material exotherms during spraying, which produces very high bond
The time saving is based on stoppages booked at a bar and rod mill against the biting problem. These times were averaged over two months, prior to any coatings being applied.
Material savings are based on the assumption that on average half a billet gets scrapped on start up in order to overcome this problem.
Rolls should be checked for previous deposits and any other major faults.
Degrease using any approved industrial solvent may be used to completely remove grease or oil from the surface.
Where grit blasting facilities are available, it is recommended that they be used. The standard of surface cleanliness required is as Swedish Standard SA3. Bearing surfaces not being treated should be masked before blasting.
Where grit blasting facilities are not available, pre-machining by rough threading or grinding should be carried out before spraying.
Application of spray coating
Spraying should begin as soon as possible after preparation and before any visible sign of deterioration occurs. The Roll is mounted in between centres. The surface speed should not be less than 60 feet per minute. (18 metres/minute).
(A) The Flamespray Equipment should be set up in accordance with the Metallisation Manual for the spraying of T405/1 Flamespray Material.
(B) The Area to be sprayed should be cleaned with a vacuum cleaner or clean air blast to remove any loose particles of grit.
(C) The Flamespray Pistol should be set so that the spray stream is at 90° to the surface being coated and traversed at an even speed giving a uniform coating.
(D) Apply T405/1 Coating to the required thickness.
(E) Spraying Parameters
(i) Range 100mm (4”)
(ii) Nozzle Air Pressure 3.37 Bar (55 psi)
(iii) Oxygen Pressure 2.0 Bar (30 psi)
(iv) Acetylene Pressure 1.02 Bar (15 psi)
Gas 5.5 Oxygen 2.2
There should be the minimum of interruption from commencement of preparation to completion of spraying. At all times, the prepared surface should be protected from dust, dirt, moisture etc.
No finishing is required as the coating is used as sprayed.
REFERENCE TECHNICAL BULLETIN N°S :- 2.3.17 Metallisation Bonding Material T405/1
1.1.8 Metallisation Type IV Flowmeters
HE-FC-002 Arcspraying of Mill Rolls