HIC Resistant Steel Plate is used by boiler makers and fabricators when Hydrogen Induced Cracking is a big risk.
What is HIC Resistant Steel Plate?
The most common grades of HIC resistant steel plate is A516 – to ASME/ASTM standards. It will generally be certified to grades 60, 65 and 70 so one plate can meet the requirements for any of the three grades A516 60/65/70 + HIC.
Once the A516 plate is produced a sample is then tested to see if it meets the required of TM0284, with is the NACE standard for the Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking, using one of two test solutions. In simplistic terms these are acidic solutions that mimic the corrosive effects of the conditions that induce HIC Cracking. The steel stays in the solution for a few days and then it is examined under a microscope and the cracks that have formed are recorded.
it is possible to get HIC plates to other standards – however about 90% of all HIC plates manufactured by Dillinger to client specifications are to A516 so the market for P355 plates with HIC resistance is small. As a consequence all of our lates for sour service are to the ASTM standard.
HIC Plate Performance
When assessing test results the key characteristic is CLR ratio – the crack to length ratio. This is specified in EN10028 – Annex A. Basically the smaller this is the more resistant the plates are to HIC cracking and thus the more suitable they are for use in sour service. The actual level of HIC resistance required really does depend on the equipment and process and the severity of the H2S. In many cases a CLR of 15% can be adequate – though this will obviously limit options if a feedstock with greater sulphur content is used in the future.
The best HIC plate has a CLR of less than 5%. HIC resistant steel can have a CLR of up to 15%. In some cases our plates will have a CLR of 0%. This is in part due to luck during the sample process – if there are no cracks in the test sample you get a good CLR 🙂 However a lot is also down the way the steel is made and a huge percentage of our plates – as rolled – meet the highest acceptance classes.
How is HIC Resistant Steel Plate Made?
There are two approaches to making HIC resistant steel plate.
The first is to design the steel manufacturing process around the requirements – and produce a steel that is of very high quality and very resistant to corrosion. Dillinger Hutte is a mill which takes this approach when they produce Dicrest HIC resistant steel plate. This means that HIC resistant steel plate is produced consistently and reliably and can be expected to pass any retests
The second is to produce normal boiler plate and then to test the best heats to see if they meet the requirements. This obviously allows HIC resistant steel plate to be produced much more cheaply due to the lower capital investment required – but because testing is done on a sampling basis the risk of the plates failing tests during production or commission is higher and the consequences serious.
When you deliberately aim to make HIC Resistant Steel Plate there are several key factors that help to make the best possible steel. First control of the steel’s metallurgy. This is best done by undertaking a primary metallurgy, measuring the ladle composition and then redoing the steel making process to fine tune it.
Secondly taking a very conservative test strategy when it comes to testing the slabs produced. You can often get better values by testing towards the middle of the slab. So by testing towards the end of the slab you can get more statistical confidence in all the HIC resistant steel plates produced from the heat.
Finally there are the mechanics of the rolling, stress relieving and annealing which all come together to produced steels with exceptional HIC resistance – even at great thicknesses
We’ve covered the manufacturing process of HIC material in more detail
How do you Evaluate Different HIC Steels?
The usual approach is to compare the Mill Test Certificates (MTCs) and see which meet the design and engineering technical requirements. Depending on the application this can often provide a clear technical leader.
The next question is usually one of availability – how much HIC steel is immediately available from stock, or when can it be produced from new rolling. Mills than specifically manufacture HIC plates are often more reliable in this respect as they can generally be more confident of guaranteeing lead times as they have to produce less steel due to less testing risk.
Finally there is the question of technical support. Samples of plates are tested not the whole plates – and so there is the possibility of issues with the plate in service. The tests give a high level of confidence that there will be none, but…. Many fabricators chose the mill that can offer the best technical support – and that is usually the ones who have designed their processes around making HIC resistant steel plate, and not just testing it.