Engineered Surfaces for Exceptional Performance
Engineered Surfaces for Exceptional Performance

Surface Enhancement

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Page 12 of 14

Flame Spraying of Mill Rolls

Various components in rolling operations are exposed to rigorous wear and corrosion that can be protected by thermal spray coatings to maintain their service life.

The production of steel subjects the mill components to high load stresses and vigorous corrosion by heat and corrosive chemicals that condense and attack the heat transfer surfaces. This has the potential to cause many problems throughout the steel processing lines.

Thermal sprayed components are designed to allow the steel to be processed more rapidly and efficiently thus creating the required resistance to wear and negate corrosion at high temperatures.

With over 30 variations of roller types - such as Bridle Sections, Dancer Rolls, Steering Rolls, Catenary Rolls, High Temperature Seal Rolls, Water and Non-Cooled Rolls keeping the quality, maintenance costs and production requirements are paramount to the continued production of steel. Thermal spray has a multitude of coatings that suit each specification application such as Plasma applied Ceramic thermal barrier coatings or Arc Sprayed MSSA 75 Arc Bond with its Exotherm process during spraying which produces very high bond strength coatings. Resistant to oxidation, high temperature and thermal cycling.

Reason for use: Grip Coating

In the past, tremendous problems have been associated with billet ‘biting’ on newly machined mill rolls.
The rolls have a very smooth finish after being machined and therefore offer less surface friction on the billet to pull it through the mill stand.

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:

  1. Provide the necessary surface roughness.
  2. Provide the necessary hardness to withstand the rolling forces involved.
  3. Provide sufficiently high bond strength to the parent metal in order to prevent any failure in shear of the coating.
  4. The coating must also offer enough life until the necessary roughness is worn into the parent metal of the roll.
  5. The coating should eliminate all biting problems, in order to eliminate plant stoppages and scrap tonnages due to bite problems.

Equipment: Metallisation Flame Spray System

Materials: Metallisation T405/1 material exotherms during spraying, which produces a very high bond.

Economic Evaluation

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.

Method

Preliminary Inspection

Rolls should be checked for previous deposits and any other major faults.

Cleaning

Degrease using any approved industrial solvent may be used to completely remove grease or oil from the surface.

Blasting

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.

Pre-Machining

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).

  1. The Flame spray Equipment should be set up in accordance with the Metallisation Manual for the spraying of T405/1 Flame spray Material.
  2. The Area to be sprayed should be cleaned with a vacuum cleaner or clean air blast to remove any loose particles of grit.
  3. The Flame spray 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.
  4. Apply T405/1 Coating to the required thickness.
  5. Spraying Parameters
    1. Range: 100mm (4”)
    2. Nozzle Air Pressure: 3.37 Bar (55 psi)
    3. Oxygen Pressure: 2.0 Bar (30 psi)
    4. Acetylene Pressure: 1.02 Bar (15 psi)

Flowmeter Settings: Gas 5.5, Oxygen 2.2

General

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.

Finish Grind

No finishing is required as the coating is used as sprayed.