Materials > Steel

Steel yourself for some amazing journal articles here!

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Steel is basically a mix mostly iron (Fe) and other elements, with the most common carbon (C).  When at room temperature Fe atoms arrange themselves in a BCC crystal structure called α phase or ferrite.  When we heat it up, at a certain point the atoms rearrange themselves into the FCC arrangement called γ phase but usually called austenite.  By cooling or quenching this hot steel, we can lock in this crystal structure.  Sometimes when we heat and cool it, and cool it at certain rates, we can end up with a body centered tetragonal (BCT) structure.  This structure is like an elongated FCC and because it is elongated it has lots of strain which makes it very hard.  When we add more carbon, the temperatures at which these structures occur changes.  Traditional steel is Fe and C, and adding Cr makes stainless steel. We also age steel, and by ‘age’ what we mean is heat it up and cool it down so to get the exact amount of phases we want.

In aerospace, the steel we focus on is mainly used in the shafts of the engine.  Steel is used because a shaft rotates at high speeds in warm temperatures, thus needs to be strong and tough.  As the shaft is a critical part of the engine and therefore should never break!

 

AerMet 100

Fe, 0.23% C, 11.1% Ni, 13.4% Co, 3% Cr, 1.2% Mo

Looking at the composition of AerMet 100, it looks a lot like some of the superalloys with Ni, Co, and Mo.  Starting with a very low carbon steel of Fe and C, we add Cr.  Cr makes it stainless steel and gives it resistance to corrosion.  Adding Ni to the steel increases its strength and toughness.  The problem with having lots of Ni is that it stops the steel changing into martensite or BCT.  By adding Cr it also makes this change in structure easier to achieve.  Now when we look at the precipitates that form, they are (Mo,Cr)₂C carbides.  This steel has a strength of around 1800MPa, about double the strength of Ti-64, but operates at lower temperatures.

 

Super CMV

Fe, 0.23% C, 11.1% Ni, 13.4% Co, 3% Cr, 1.2% Mo, 0-1.3% Ti, 0-0.8% Al

Super Chrome-Molybdenum-Vanadium steel or Super CMV is a steel being used for shafts in Rolls Royce engines.  This steel gets its strength from precipitation.  If precipitates form with Ni and Ti then they will form a HCP or hexagonal close packed structure or η phase.  If it precipitates with the Al then it will become β phase.  Compared to AerMet 100, Super CMV has similar properties with the advantage of better corrosion resistance.  Currently the shafts use both AerMet 100 and Super CMV with Super CMV at the rear as it can withstand the higher temperatures.

 

F1E

Fe with almost no C.  Some Ni and about the same of Co and Cr.  With just a pinch of Mo, W, and Al

The ISM has been involved in the development of F1E.  This material is a new maraging steel that looks like becoming part of future Rolls Royce engines.   ‘Maraging steels’ are martensitic and aged (MAR from martensite + AGING = MARAGING).    These steels have very little carbon and so get their strength from growing precipitates just like superalloys.  Some might even call them iron based superalloys.  Using precipitation strengthening this material out perform its current competitors AerMet 100 and Super CMV.  The precipitates it forms are a hexagonal laves on the grain boundaries, and in the grain.  It also makes a β phase from the Ni and Al.