What Changes Occur in Steel Structure During Knife Forging?

[flava]

Hello,

I just registered to these forums but already visited this place for a while. I have a question that fits this particular subforum (I hope). It is related to another hobby of mine, which is knife collecting/making. Let me give you first some background info on the subject, in order to better explain the question.
A knife blade is usually made of steel, and brought to it's particular shape either by "stock removal" (grinder, mill, files ...) or forging. The knifemakers that forge to shape are saying this method has several advantages, like bringing the raw steel to a more homogenous state, "packing" the crystals closer and getting a crystalin structure that shows smaller "grain".
My question is: by taking a big bar of steel (like roller bearing steel 52100 or spring steel 5160) that has a diameter of let's say 20cm, getting it hot enough and forging it down to a blade size (using power hammers, repeated heat cycles), what changes in the steel structure might one expect? In particular, is there ANY change in density possible?
Thank you in advance for any clue or answer.

Flava

 

[omagdon7]

Well heat treatment can cause the formation of more grain boundaries which would lead to increased hardness. Density really isn't going to change much I don't believe depending on what state it is heated to. From what I remember at I think 900K (or celsius not exactly sure) the strucure of steel changes from BCC to FCC which causes an increase in density if I am not mistaken. But I believe when it cools it begins to return to a BCC structure. Honestly, for most civil applications of knives I don't really think it matters.

 

[flava]

Thank you, omagdon7
In both cases, the blade will be heat treated (normalized, quenched and tempered) after it has reached the required shape. A forged blade is hammered to shape, which requires multiple heat cycles. A "stock removal" blade is abraded to shape.

 

[PerennialII]

Yeah, would say the density change itself is meaningless (unless viewed really locally microstructure wise but don't think that is the case here). Due to forging you can have a highly oriented microstructure, which has both pros and cons. It can be for example a great in arresting and preventing initiation of cracks in region of ductile tearing, but due to the orientation possesses in some loading orientations poor toughness and mechanical properties, high propensity to initiate cleavage cracks etc. Naturally quantitatively as with all treatments can make a better steel for an application by forging and subsequent heat treatments or ruin the steel. Get some lose some, great if you lose something you don't really need.