3v Modified – The Lost Crucible Steel

I enjoy reading patents from steel companies, it reveals information about new products and research often not available otherwise. One thing I’m surprised there isn’t more talk about is a steel Crucible patented but never sold – an improved 3V: https://www.google.com/patents/US7615123

How was it improved? The primary change was that they reduced vanadium and replaced it with niobium:

Niobium carbides are also hard, MC carbides, reported to be slightly harder, in fact. So when getting the best “bang for your buck” for carbide volume, niobium is just as good if not better. That way, the steel can have a low volume of carbides for high toughness, but good wear resistance due to those carbides being extremely hard.

Why replace vanadium with niobium? In the patent they state that, “it was discovered that adding niobium to a cold-work tool steel composition results in a larger driving force for precipitation of MC type-Nb-rich primary carbides, which in turn leads to a finer distribution of carbides.” The driving force they are talking about is how “strong” of a carbide former niobium is. If you add carbon to steel you get iron carbides. If you add enough chromium you get chromium carbides instead. If you add vanadium you get vanadium carbides instead. It all depends on the “driving force” for carbide formation for the given elements. Niobium has an extremely high driving force for carbide formation and even tiny amounts of niobium leads to the formation of niobium carbides. The surprise here is that they are saying that this increased driving force led to smaller carbides. Typically, the stronger the driving force the higher the temperature the carbides form, even in the liquid steel during casting. When they form at very high temperature they tend to coarsen more, because diffusion is faster. So it is somewhat of a surprise that a higher driving force for carbide formation led to a reduction in carbide size. Niobium alloying is somewhat limited in conventional steels for exactly this reason – the carbides become too large and toughness is reduced. Therefore, apparently something different is occurring with powder metallurgy which led to a reduction in carbide size instead.

Crucible researchers created melts of the steel in two stages, a 50 lb heat which they created with the Laboratory Gas Atomizer (LGA) and a 650 lb heat which they melted with the Pilot Gas Atomizer (PGA). The carbide size was similarly small when compared with alloy “A” which is the original 3V:

This refined carbide size led to an improvement in toughness, particularly in the transverse direction (note it is mislabeled as bend fracture strength):

The original 3V transverse toughness is only 25% of longitudinal, a surprisingly poor value. The new steel, however, has near parity between transverse and longitudinal toughness, likely due to the improved carbide size and distribution. Wear resistance, however, was similar or slightly improved.

So if this steel was so much better why did it never materialize? That is hard to say since I never heard anything about it at the time. Perhaps they had issues when making full size heats, or that the properties weren’t as good in full production. Maybe they determined the improvement wasn’t enough to sell more steel. Or they were worried that customers would lose confidence in their current steel if such large improvements are possible. One clue is the date at which the patent was published: November 10, 2009, which is several months after Crucible filed for bankruptcy. Crucible sold off their research facilities and lost all of their research engineers in September 2009. So if any work was remaining to scale up the steel that likely ended abruptly. It’s too bad because it looks like a nice steel.