HOW OUTOKUMPU’S LDX 2101 DUPLEX GRADE DOES MORE WITH LESS
When Outokumpu created LDX 2101, they were aiming, as with all duplex grades, to find the best balance between austenitic and ferritic characteristics. What they achieved, however, is a resilient, versatile stainless steel that, over the course of its life cycle, actually costs less than many competing materials.
LDX 2101 is a patented grade of duplex stainless steel with a distinct high-performance profile. It contains enough ferritic material to provide outstanding engineering properties – it’s easy to machine and with planning, simple to weld.
At the same time, its austenitic content ensures that it’s particularly corrosion resistant and the phase balance between these two characteristics gives it an unyielding strength – almost double that of equivalent austenitic or ferritic steels.
RESISTANCE TO PUNISHMENT
LDX 2101 is often called a “general purpose” stainless steel, but the label does it a disservice, because while it’s deployed in a wide variety of applications, the way it performs under harsh conditions is remarkable. For example, LDX 2101 sees extensive application in hydrometallurgy, where it is seen as a compelling alternative to austenitic grades. Stainless steel cathode plates, used in copper refinement, are commonly made from LDX 2101, precisely because the copper stripping process involves a fair amount of battering.
In fact, this ability to withstand punishment is gaining Outokumpu increased attention in the mining industry – just last year they serviced an order of 4 000 tons of duplex for the Husab Uranium mine in Namibia.
BIG PICTURE THINKING
To understand why LDX 2101 offers lower costs, when compared to other steels, requires taking a long view. Globally, budgets are tight, but this makes it all the more important to select resources that will deliver the most value.
A stainless steel – or even a carbon steel, for that matter – cannot be evaluated purely on what its raw cost will be, because so many factors have an impact on the total cost of a given project. Choosing the right material for a given application will often yield benefits that reduce costs significantly in other areas of the project – some obvious, some less so – but all adding up to less money spent over the total life cycle, see figure 1.
Priced per metric ton, LDX 2101 is more expensive than either ferritic or austenitic steels – and significantly more than carbon steels. But it is also much more robust than any of these materials and many of its cost benefits are products of this hardiness. The most obvious advantage is that the high mechanical strength of LDX 2101 enables “downgauging” of requirements. Nothing has to be as thick as it would be if built of carbon steel, or even austenitic stainless steel. This means that less material is required to achieve equivalent (if not better) quality and durability.
As a side benefit, using less material also means lower transport costs; an indirect factor that becomes increasingly relevant when working with higher volumes.
For the most part, stainless steel is a good forming material, but it’s fair to say that duplex grades tend to require higher forming forces than austenitic grades do, especially when cold forming. Some might say that this increases cost, since forming equipment will have to be calibrated to work harder, with greater risk of springback.
However, this assertion only carries weight when directly comparing identical gauges across steel grades and it would almost never make sense to do so. The force required to work such strong steel is mitigated by the thinner gauges you would typically be working with, so costs associated with forming are roughly equivalent.
Due to their increased hardness, duplex generally provides more machining challenges than austenitic grades. But in this regard, LDX 2101 is unique – unlike most duplex grades, it readily accepts machining and can be worked with even greater ease than 316L.
Machining costs, then, could be expected to be roughly equivalent compared with regular austenitics, but again, the advantages of down gauging actually imply a cost benefit.
At first glance, it would appear that welding LDX 2101 is more expensive than carbon steels or austenitics – and it is. However, variations in method make the comparison tricky.
Difficulty is not a factor – LDX 2101 requires the same flux-cored arc welding (FCAW) technique as similar steels, but there are differences in how joints need to be configured and in the type and quantity of fillers required.
On paper, LDX 2101’s welding costs per kg seem higher than other steels, which is to be expected given that less material is used overall. However, the process of welding LDX 2101 is also significantly faster. In fact, when compared to A516-60 (a carbon steel commonly chosen for fabricating large tanks), LDX 2101 is more than twice as fast. This also means total assembly time is faster, which yields labour savings.
Maintenance is a significant factor in assessing the total life cycle cost of a project – some upkeep expenses are easy to quantify, others less so, but they are all significant. Consider, for example, the common case of fabricating a large tank, intended for storing or processing something with mild to severe corrosive properties, possibly under pressure.
The traditional approach is to build the tank out of carbon steel, primarily because it offers comparatively low costs per metric ton. The steel is painted on the outside to protect it from weather and coated internally with fiberglass or epoxy to defend against corrosion.
But no coated carbon steel will survive in the longterm without periodic work. At minimum, there will need to be a full maintenance cycle every 10-15 years, during which the tank will have be emptied, the old coating removed, any damage repaired and a new coat applied.
THE TRUE COST OF MAINTENANCE
The upkeep costs on a carbon steel tank are dependent on factors like the price of labour, the coating material used and how long the coating will take to cure in the local climate. They are variable, but predictable.
What is harder to estimate and potentially far more expensive, is what it will cost to shut down a production line while the work is done. Every day the unit is offline translates to lost production and wasted labour, the effects of which are not trivial. The damage could be mitigated using a spare tank – but that requires doubling the initial material outlay, since you’d need to build 2 to begin with. It is in the area of maintenance costs that LDX 2101 comes into its own.
Provided the correct post-fabrication surface treatment is applied, a tank made from LDX 2101 will not require planned maintenance at all. Its duplex formulation contains high quantities of chromium and nitrogen, giving it strong resistance to both uniform and intergranular corrosion. Where it truly shines, however, is when it’s placed under pressure. With extremely high resistance to stress corrosion cracking, it is a great choice for environments where it will get punished. This is why it is increasingly being used for storage tanks, pressure vessels, industrial process equipment and chemical tankers.“The mechanical properties alone make it worthwhile,” says Johan Karlsson, General Manager of Outokumpu Sub-Saharan Africa. “For example, in transportation, if you have a lighter truck, you can move more materials on it. And compared with galvanised or painted steel, you don’t need maintenance. You’re not having to sand the truck down and repaint or regalvanise it on a yearly basis. That’s why we see it as delivering a low life cycle cost.”
LOW LIFE CYCLE COST
While it can be argued that projects built from LDX 2101 have higher initial outlays, basing a decision on this aspect alone is short sighted. Any saving made at the fabrication stage is rendered insignificant when compared with what the total cost will be over the course of a project’s life cycle.
Given that an LDX 2101 project won’t incur costs beyond the initial build and won’t cause production shutdowns, it offers the best value over the long term