TPT September 2013

Technology News

Tube producers benefit from quenching and tempering

as well as in the process control from heating and quenching or cooling. After many heat treatment facilities for steel bars and tubes have been installed with induction technology, particularly in the last few years when the owners were able to realise the production cost savings described, they are now looking for possibilities of transferring the advantages of induction to even more complex components. Currently, Elotherm has developed an innovative induction solution allowing seamless tubes to be heated with upset ends so homogeneously that the desired material properties are achieved. Since 2012, this process has been in use at a world market leader for OCTG products in its new plant in Saudi Arabia for the production of tubes with upset ends and strongly stressed jacket tubes for oil exploration. The exploration of some oil and gas deposits requires jacket tubes for lining the actual borehole, which have to be highly stress-resistant. For example, particularly deep boreholes, low ambient temperatures or saltwater require the use of jacket tubes with especially high demands on strength or leakage protection. Such jacket tubes typically have outer diameters from 5 to 10¾" and are upset at the end to diameters up to 16". A special challenge for the induction process is a significant difference in weight between the straight tube body and the upset ends which may be up to 65 per cent according to the relevant API specification 5CT. To reach a uniform austenitising temperature and homogeneous hardness and microstructure conditions after tempering, additional heating of the ends is required. For this reason, inductive preheat coils are provided upstream of the actual austenitising system. The position of the upset ends is precisely sensed via an integrated position control system. This ensures that in fact only the upset, ie thicker tube areas, are heated in the preheat units and the remaining tube body almost stays at ambient temperature.

the so-called “tempering brittleness”, reducing the notch impact toughness to a large extent. This effect results from the precipitation and recrystallisation of carbides on the grain boundaries during transformation of the microstructure, in particular from nickel-chromium steels susceptible to it. In contrast to this is the inductive quench and temper process where the critical temperature range between 400 and 600°C is passed so quickly that the tempering brittleness described above cannot even arise. A precipitation of carbides does not take place and the material retains its good strength properties and notch impact toughness without becoming brittle. Long dwell times at high temperatures, above all in the austenite area of the material to be treated, normally cause an undesired decarburisation of the surface layer. This negative effect weakens the work piece within the subsurface resulting in an inhomogeneous hardness over the entire material cross-section. For inductive quenching and tempering such decarburisation does not take place since the material is retaining a high temperature only for a very short period. A result of the properly synchronised inductive Q&T process is long products whose straightness is better than the standard requirement according to API guidelines. The reasons for that are as diverse as complex and can be found above all in the work piece transport

INDUCTIVE quenching and tempering of steel bars has gradually become established in the industry and SMS Elotherm has further developed this process for tube products such as seamless tubes due to the booming market in the OCTG product area for oil and gas exploration. The present requests for a reduction of stock and a large variety of materials result in a flexible manufacturing strategy in the Q&T shops with small batch sizes. In many cases, this means that batches of different material dimensions and qualities have to be heat-treated at different quenching and tempering temperatures. For a batch change the entire induction plant can be emptied within a few minutes and newly set up for the next batch with a minimum of effort. An inductively heat-treated work piece is characterised by a highly homogeneous microstructure and hardness. The evaluation of the Rockwell hardness over the entire cross-section and along the entire long product normally reveals a variance of less than 1 HRC. In conventionally fired plants the tempering process takes place over a long period in a temperature range between 400 and 600°C to make sure that a reduction of hardness or tensile strength via microstructure transformation after soaking is reached. An undesired side effect is, however,

Quenching and tempering tube products

SMS Elothern GmbH – Germany Website: www.sms-elotherm.de

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S eptember 2013