TPT November 2018

T E CHNOLOG Y

Considerations for pipe quenching QUENCHING a pipe consists of fixing the martensite on at least 95 per cent of the pipe section at room temperature, according to the experts at Barni SpA in Italy. To obtain the perfect result two different technologies are available depending on the pipe thickness that is to be quenched. For pipe thickness equal or less than approximately 16- 18mm it is possible to fix the percentage of martensite required using spray quenching. For a higher thickness it is necessary to use a quenching tank. continuous steam source (calefaction). The steam, when it continuously covers the surface, prevents the cooling fluid from being fully effective. It is therefore necessary to eliminate this insulating film. The solution consists of cooling the pipe surface with tangent jets so that the fluid dynamic pressure removes steam from the surface.

technology that is eventually to be used. The water flow is essential in order to allow the dynamic pressure to remove the scale from the pipe surface. In order to obtain proper quenching it is vital that the pipe temperature never decreases below a certain value. In spray quenching, due to the close distance to the heating oven, the pipe temperature decrease is insignificant. In addition, a considerable quantity of water from the descaler will not have had enough time to cool down the external surface before the intervention of the spray nozzles of the quenching machine. In conclusion, during the tank quenching process it is important to lower the water flow for cm 2 of external surface and increase the dynamic pressure to remove the scale. For spray quenching the process is slightly less complicated.

The surface that is to be cooled must be almost completely free from scale, which, if present, acts as insulation and interferes with the controlled cooling process. For this reason, before both the quenching head and the quenching tank processes begin, it is important to remove the secondary slag (descaling) that has formed in the quenching oven during the pipe heating phase. This next phase is delicate and the parameters that result in the descaling (water flow for cm 2 of surface and water dynamic pressure on the external surface) must be different and carefully measured according to the quenching

It is necessary to have the ‘Bain diagram’ of the steel that is to be quenched, in order to correctly identify the gradient of the cooling speed and the necessary water flow. It is important to make sure it does not intersect with the curve identifying the phases of transition during the cooling process. Since the pipe temperature that is to be quenched is about ten times the boiling point of the quenchant during the cooling process, it generates a

Barni SpA – Italy Email: web@barnispa.com Website: www.barnispa.com

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