Do "non-iron steels" exist?

Question

There are numerous steel alloys, containing mostly iron, carbon, and some other metals. Generally speaking, we can think of them as if they were some type of steel.

My question is: do "non-ferrous steels" exist? I am thinking of pure, non-iron metals, containing a little carbon, just as is added to iron to turn it into steel. Or asked another way, are there other metals besides iron that are doped with carbon in order to form an alloy like steel?

In general, how does the addition of carbon affect the properties of these metals?

Answer 1

Steel is defined as an alloy of iron and carbon; there is no such thing as a non-ferrous steel. If you alloy some other metal with carbon, it becomes something other than steel. Looking for a steel without iron in it would be like looking for brass or bronze without copper. You can alloy things other than copper with zinc, tin, or aluminum, but those would not be kinds of brass or bronze.

As far as other alloys that contain carbon, this Wikipedia article has a good list of various kinds of alloys (as you can see, there are a lot of them), and searching through it, you'll see that there aren't a lot of other things that are alloyed with carbon besides iron. As for why this is, I don't have a good answer.

Answer 2

Iron and carbon have an interaction which make them different from most engineering alloys. This is to do with both the relative size of C and Fe atoms and their chemistry.

Carbon atoms are just the right size to insert themselves into the crystal lattice of iron, this strains the lattice enough that it is somewhat harder and stronger than pure iron. However the really important part is that the presence of carbon allows steel to be heat treated. Here it is heated above a critical temperature at which the crystal structure changes and if it is cooled rapidly the carbon content prevents it from returning to it's 'normal' structure at room temperature and instead forms a multi-phase structure which is highly stressed but chemically stable and as such is very hard with a high tensile strength. This can be further modified by controlled reheating to partially reverse this transformation and produce a material with controllable strength hardness and toughness.

Note that the above is a quick overview and there are whole books on the detailed behaviour of steels as the iron-carbon system can exist is several different states with different crystal structures and various micro-structural combinations of them.

This type of heat treatment is pretty much unique to steel and certainly very different from the way that most alloys behave and is a result of the specific interaction between iron and carbon and depends on the fact that iron can exist as both body centred and face centred cubic crystals.

It is also achieved by very low concentrations of carbon, usually less than 1.2% or so. In fact only about 0.7% carbon by mass is soluble in iron and any surplus will tend to form carbides or precipitate out as graphite (as in cast iron).

There are various metal carbides in use (such as tungsten carbide) but these are really ceramics rather than solid solution alloys.

There is also at least one type of stainless steel (H1) which is precipitation hardened and contains nitrogen instead of carbon. This is a different hardening mechanism to that of carbon steel. The purpose of eliminating carbon is to improve corrosion resistance, especially in salt water. I've only ever encountered this is a blade steel in knives. There are also low carbon stainless steels but these are not hardenable by heat treatment and are designed for improved weldability.

Answer 3

Summary: The Fe-C system, and thus steel, is unique due to a eutectoid transformation from a high-solubility phase to a low solubility phase that allows for a wide variety of microstructures and properties which are highly and relatively easily tunable. Other first-row transition metals have different, and less exploitable, behavior when alloyed with carbon.

Fe-C is the only first-row transition metal-carbon system that has a eutectoid transformation in its phase diagram.The eutectoid transformation changes austenite to ferrite and cementite on cooling. Austenite has high carbon solubility, and ferrite has low carbon solubility. I am picking on first-row transition metals as they tend to have chemical behavior "close" to that of steel, with similar cost, density, and other "obvious" properties (with the exception of scandium, which is extremely rare and expensive), and examining all 70+ metals is a fair amount of work for this answer.

The nature of the eutectoid transformation allows for many microstructures and thus a high degree of tunable properties. Consider a eutectoid steel austenitized and cooled at varying rates:

• If cooled slowly, a moderately ductile, moderately strong pearlite microstructure forms. Pearlite results from a cooperative nulceation and growth process as carbon leaves austenite during its transformation to ferrite, forming alternating lamellae of ferrite and cementite.
• If cooled moderately rapidly and then held isothermally for a period of time, a much harder bainite microstructure forms. The kinetics of bainite formation are not well understood, but the microstructure is a less-organized arrangement of cementite and ferrite, again resulting from carbon coming out of solution as austenite transforms into ferrite.
• If cooled extremely rapidly, an extremely strong and hard martensite microstructure forms. Martensite formation is a diffusionless process in which carbon is trapped in austenite while it transforms to a BCC structure, distorting the lattice into a strained BCT structure which is difficult to strain further, hence its high strength. By altering the quantity of carbon and being creative with heat treatment schedules, a wide array of microstructural combinations are available.

With appropriate alloying and heat treatment, it is possible to have a steel with retained austenite, ferrite, pearlite, bainite and martensite all in the same material. Such complex microstructures are impossible in other first-row transition metal-carbon systems.

All of the wide heat-treatability and wide array of microstructures and properties are entirely due to the presence of a eutectoid transformation which takes a high-solubility phase to a low-solubility phase. The eutectoid transformation itself is due to a phase change from austenite (FCC) to ferrite (BCC) and the resulting significant loss of carbon solubility. The answer to your question is effectively no, there are no other alloys (of which I am aware) that behave like steel during processing. The answer to your alternate question is that carbon has less useful and less exploitable effects on other first-row transition metals.

Below are the Fe-C, Ni-C and Mn-C phase diagrams for comparison. Note that the Fe-C phase diagram stops at 0.2 a/a C while the others go to 1.0 a/a C. Ni-C has no eutectoid, only a eutectic transformation, and thus can only be precipitation hardened. Any other microstructure formation would have to occur during solidification. Mn-C phase diagram has a eutectoid, but it goes from a high-solubility phase to another high-solubility phase, which means that extremely large amounts of carbon would be present in the lower temperature phase (nearly 10% a/a C compared with less than 1% a/a C in steel), which would result in extreme brittleness.

Answer 4

See comments. Based on start point of:

Super 13cr is defined as a low-carbon stainless steel. The chemical composition specified from suppliers such as Sumitomo specifies Fe min 0%- Max 0%, C is to be below 0,03.
    Commonly used in oil and gass applications to resist sour environments and some H2S. But it's expensive as... 4 chickens, in solid gold.

http://www.howcogroup.com/materials/mechanical-tubing-octg/grade-super-13-cr-13-5-2-tube.html