All about DICREST Ex Stock – Part 1

Known as the steel for sour-gas-resistant pressure vessels, DICREST has been specially developed for fabrication of pressure vessels for handling of sour gases. It is resistant to HIC (hydrogen-induced cracking), an advantage achieved by a very special production route. During the steelmaking process, particular attention is paid to ensure exceptional cleanness and the generation of a homogenous structure. A specially-controlled rolling process, and subsequent normalizing and stress-relieving annealing after fabrication work, guarantees HIC-resistance throughout the plate.

Find out some main aspects of this special steel in part 1 of our “All About DICREST” series.

1) What was the reason behind the development of DICREST ex stock?

dicrestThe production of HIC resistant steel has been sharply increased due to the use of greater quantities of H2S containing crudes. Many customers have been requesting small quantities with very short delivery times, quite often for the purpose of repairs to existing equipment. Meeting both of these demands is difficult to achieve in large-scale steel production as it disrupts the regular production that serves as the company’s economic basis. But these clients need to have their urgent needs fulfilled.

So the idea was born to create and stock a specially designed steel in order to be able to supply steel plates that meet the highest standards of HIC resistance while providing the very short delivery times that can only be achieved by a stock holding company. The steel was designed to simultaneously fulfill as many of the numerous HIC specifications demanded by the market as possible at the same time. Obviously, some compromises were required, as the specifications vary widely. But the final result is a product that meets the highest standard of excellence, based on the experience of renowned manufacturer Dillinger Hütte – supplier of more than a quarter of a million tons of HIC-resistant pressure vessel and line pipe steel plates per year.

2) How many DICREST stock yards are there?

At the moment, there are two DICREST stock yards: AWS, based in the Netherlands and DIME, based in Dubai (Jebel Ali Free Zone, UAE).

3) What grades of HIC resistant steels do AWS and DIME hold in stock?

The DICREST stockyards hold SA516-60, SA516-65 and SA516-70, which is based on long term demand.

4) Is it possible to buy HIC resistant steels that meet the harmonised European standards ex stock?

Unfortunately, this is not possible ex stock. That is one of the sacrifices that had to be made when designing this steel. Due to the long term demand, it was decided not to offer material in accordance to European standard specification as it is seldom asked for. For larger tonnages, however, specially designed steel plates that meet the European standards can be supplied, but such orders are subject to longer delivery times. For small quantities and short delivery times, this is not possible. You may wish to consider an individual evaluation that may allow you to use the ASME grades in conformance with the European regulations upon special survey report.
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5) Is there a standard heat treatment for DICREST ex stock?

Yes. First of all, it is important to know that it is generally necessary to apply a threshold value of stress relieving or PWHT to DICREST material to activate the HIC resistance. This is also necessary for thicknesses of less than 38 mm for which the ASME VIII codes don’t formally require a PWHT treatment. For this reason, all mechanical values in the AWS DICREST test certificates are measured after simulated heat treatment. The heat treatment is carried out at 600 °C -7/+10 °C in accordance to the requirements of the ASME VIII codes. In practice, this entails a holding time of 2.36 minutes per millimeter of thickness up to 50.8 mm (2 inches) plus 15 minutes of holding time per additional inch of thickness. For example, a thickness of 70 mm requires 135 minutes of holding time and 95 mm requires 150 minutes.

6) Can standard holding times be exceeded or can higher temperatures be applied?

Unfortunately, in most cases, the answer to both questions is no, because both the tensile and toughness properties will be affected as a result. Toughness requirements at fairly high temperatures and the use of grade 60 or 65 may make it possible to meet these wishes.

7) Are very low carbon equivalents possible?

In general, the carbon/carbon equivalent might be slightly higher than for specially designed material. On one hand, this is the result of the compromises necessary to fulfill different specifications at the same time; while on the other hand, it has to do with the statistical variation of the chemistry in production. However, it is usually possible to achieve a Ceq of max. 0.39 for thicknesses of 10 to 25 mm and a Ceq of max. 0.42 for thicknesses of 25 to 60 mm.

8) Does this product meet the thickness reduction ratio requirements of SA 20?

Yes. SA 20 prescribes a minimum thickness reduction ratio of 3.0 to 1 for boiler and pressure vessel grades to ensure good product quality. All DICREST plates of this type produced by Dillinger Hütte will meet this requirement.

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