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Preparation of ceramic-metal coatings by plasma spraying to improve the performance of roller mill and forging tools
D. JUDAS
CRITT « Céramiques Fines »
Centre Régional d’Innovation et de Transfert Technologique, Maubeuge (F)

Of the many methods of fabricating weldless tubes, the process including the based on back extrusion is one which allows tubes to be produced in a most efficient manner.

In the initial step of the process, commonly known as Mannesmann piercing, a sample tube is produced from a billet or circular steel piece. The types of steel used by the Vallourec company are low carbon steels to high alloy steels.

The fabrication of weldless tubes in high alloy steels and more particularly in the Z20C13 composition make high demands on the hot forming tools such as the drill heads.

These forging and rolling tools are subjected to thermomechanical stresses of compression, friction, shear and thermal shock, as well as to oxidising and hot corrosive atmospheres. Tools are generally exposed to a heat treatment in an oxidising atmosphere which results in the formation of a film on the surface of the tool head which ensures adequate lubrication at elevated temperatures during the piercing process.

In order to extend the lifetime of the drill heads, CRITT Ceramiques Fines (Maubeuge, France) have developed a number of protective coatings deposited by plasma spraying. A number of tests have been used to assess the quality and performance of the coatings before subjecting the coatings to industrial trials. These tests were 3-point flexure, thermal shock, assessment of degree of carburisation and hot shear.

The tests showed that the most encouraging technical solutions were among the ceramic-metal composite coatings with incorporation of a lubricant phase.

The mixed ternary coating, yttria doped zirconia, NiCrAlY and nickel graphite, on a drill head of 30NC11 proved to be the most efficient for piercing high alloy steels of the type Z20C13. It appears that the abradable nickel-graphite particles released during firing and during wear of the coating ensured adequate lubrication.

The performance of the NiCr – chrome carbide coating on a V20NCDW10 head was non-conclusive as the head became jammed in the former during initial extrusion.

The test demonstrated that the ceramic-metal composite coatings possess good thermomechanical properties and also ensured good lubrication of the head during the extrusion operation.

However, as the products should not become carburised by the lubricant, nickel-graphite in this case, other lubricant coatings need to be assessed in order to solve this carburisation problem.


Ceramic composite coatings on forging tools
by plasma spraying (CRITT Maubeuge)
          
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