There are two routes to making HIC material for sour service. You can either make HIC material in a dedicated production or you can take standard A516 grade 70 boiler plate and test it to TM0284-2003.
Steelmaking of HIC Material
HIC resistant material for pressure vessels and linepipe is produced in a special process that ensue good properties for sour service operation. This production route delivers a homogenous resistance level throughout the entire plate. This is delivered through improved production methodologies and improved quality assurance measures.
In the steel making and casting the pig iron is first desulphurised before it enters the caster to be converted to steel. It is then decarburised, denitrogenised and dephosphorised . These all make the steel very clean and removes many of the non-metallic inclusions that make steel susceptible to Hydrogen induced cracking. The metallurgy is then checked and adjusted to optimise the chemical composition of the HIC material.
There is then a process that finely tunes the chemical composition. This is done under vacuum conditions and nitrogen, sulphur and hydrogen are evaporated from the melt. This is hastened by stirring the molten steel with a lance of the inert gas, argon. The steel is then deep desulphurised and this reduces the sulphur content to approximately 10 ppm. The steel is then calcium treated form inclusion forming of the remaining impurities and for Dillinger the Ca/S ratio, after all these treatments is normally between 1.5 and 5.
Casting of HIC Material
When the steel moves onto casting great care is taken to avoid the steel from taking on oxygen and sulphur from the atmosphere. This would remove much of the good work already undertaken. It is normally done by adding a casting powder to the surface of the steel. This melts and forms an impermeable layer between the atmosphere and the steel.
During the casting process there are two main goals when producing HIC material. First is getting the geometry of the slab casting right for the plate rolling to come. Secondly is the need to avoid segregations in the steel. Dillinger’s research shows that using vertical continuous casters is the best way to achieve a homogenous low level of non-metallic inclusions and segregations. This is because as slab is cast the inclusions tend to rise. In a vertical cast the inclusions will tend to cluster around the centre line whereas in the curved caster they will rise away from the centre line. And this leads to far greater heterogeneity in the HIC material.
Rolling of HIC Material
Once the slab has cooled it is assessed for suitability and then those portions that are suitable (for example a casting incident may damage the homogeneity of part of the slab) are then taking to the rolling mill. The slabs get preheated to between 1100 and 1250 °C before rolling. The next step is crucial. To maximise the homogeneity of the final plate ‘high shape factor rolling’ is applied. This means that instead of rolling the plate little by little, a large deformation of the plate is applied on each pass through the rolling mill. This is only possible with a very powerful rolling mill (the stand at Dillingen is one of the largest in the world) AND control of that power. The effective of reducing the number of passes and increasing the rolling force is increasing the deformation in the core of the plate.
Once this is done the plates are warmed to allow for final hydrogen effusion from the steel; they are then normalised. The final stage is during production at the fabricating facility when the plates are stress relieved to achieve the full strength of the HIC material.
Comparison of HIC Material with Standard A516 70 plates tested to TM0284
There are two key issues here. First, making HIC material with a specially designed processed focuses as much on the homogeneity of the steel as the actual steel making process. This is important because testing of HIC material is another form of statistical process control. The plates and slabs are sampled and the HIC quality is dependent on that quality. For a plate that is non-homogenous it is certainly possible for a set of samples to pass the HIC test at the mill and then fail during production or commissioning because the additional samples were taken from a different location.
The second point is that we can quantify that issue by comparing the CLR values of plates made by different routes. For HIC material made with a special process we can expect that 88% of samples will have a CLR of less than 2%. For plates made by a normal method and then ‘upgraded’ to HIC you only 37% of plates achieve a CLR of 2%. So if your requirement if for an HIC plate of 5% CLR then retesting a specially produced HIC material plate will given you a 90%+ confidence of passing the retest. For the pseudo HIC resistant steel plates that is significantly lower – approximately 50%. That’s a big risk and it’s why we sell Dicrest HIC plate from Dillinger
HIC Material available EX-Stock
We have one of the largest stocks of HIC material in the world in our Netherlands warehouse with a plates of up to 100 mm thick available with a CLR of 5% or less. These plates meet typical requirements for HIC service and more specialised requirements are available from new rolling. Plates are all from Dillinger Hutte. Contact us to tell us your requirements.