Terms in Metallurgy

[geofthesmith]

hello all i read more than i post here so my name doesnt pop up much, but ive got a speech im gonna be giving about the basics of metallurgy specifically dealing with bladesmithing. and its been a wile since ive forgoten all the technical lingo i learned and im having a hard time understanding some of the stuff Ive googled...im rambling long story short id like to know the laymans definitions (or closest thing to em) for bainite ferrite and perlite. anything would help please and thanks

geof

[Steve Sells]

What is wrong with using the real terms, they are simple, and accurate. anything else may only confuse issues.

[geofthesmith]

i suppose i worded this poorly, my apologies. all i needed to know was the definition or the properties of the steel in those states so i can better explain what im talking about.

[Graham Fredeen]

Hey Geoff,

How have things been lately?

Here's some of the basics for you:

"Pearlite" is the softened structure of steel that you attain through annealing steel. It is composed alternating bands of ferrite (iron) and cementite (iron carbide). When you heat steel above its critical temperature, austenite forms (simply put, the crystaline structure of the ferrite expands, allowing the iron and carbon go into solution, forming a FCC crystaline structure, a cubic crystal with the iron on the corners and the carbon centered on the face). If you cool the steel slowly the carbon will diffuse and will form pearlite again. The slower you cool, the greater amounts of carbon will be able to diffuse, the more pearlite will form and the softer the steel will become. If you cool austenite very quickly (through quenching), the carbon will not diffuse sufficiently enough to form pearlite structure and the result will be martensite (or bainite, depending on the cooling rate, but martensite is the most "common" structure refered to). Martensite being the hardened structure of steel, formed when the carbon is "trapped" within the FCC strucure of austenite when it suddenly contracts from the rapid cooling, and in its contraction distorts and forms a body centered tetragonal structure. This is a much more ridigid structure, therefore it allows for much less deformation, which is "hardness". The quicker you cool steel the less carbon will diffuse out of solution, the more martensite will form and the harder it will become. However, with higher carbon steels, you can cool them too quickly, forming too much martensite. The crystaline structure of martensite actually takes up more room than that of pearlite, resulting in expansion, (thats why differentially hardening a katana causes it to curve, since the edge is being hardened (forming martensite resulting in expansion) and the spine is not being hardened (pearlitic strucure)). This expansion of the martensite structure, paired with the rigidity of the structure will not always allow for the deformation and stress caused through this expansion, and the structure fractures (this is why quenching oil-hardening steels in water causes them to crack). In cases where there is more carbon present, more carbon will go into solution, meaning that it will take slower rates of cooling to diffuse this additional carbon. This extra time required for diffusion is why higher carbon steels will become "harder" (since within the time it takes to cool, more carbon remains in solution, so more martensite will form) than those with less carbon, but it is also why they must be cooled slightly slower durring hardening (oil and air quenching, instead of water or brine, depending on the steel). There are also alloying elements added to some steels to effect the rate of diffusion and other characteristics.

I actually came down to the shop this weekend, going to pick up a 100 lb propane tank, so if you are not busy with anything else and need to know more, you are welcome to come down.

[geofthesmith]

thanks a bundle graham this will make things a lot simpler. i regret to say that i read this on Sunday witch usually seems to be the case, otherwise i probably would have come down

[Steve Sells]

many terms are listed in BP0078

[Ben Hartley]

forming a FCC crystaline structure, a cubic crystal with the iron on the corners and the carbon centered on the face).

I know this thread is pretty old, but I just wanted to clear up the description of FCC iron.

The FCC and BCC diagrams show only atoms of one type, the atoms on the faces and the atom in the middle are still iron atoms.



The FCC structure has more room between iron atoms, allowing the carbon to diffuse into the spaces between iron atoms.

John D. Verhoeven's "Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel" explains it very well with nice diagrams, it may even be available online somewhere for free.