Just wanting to learn more about the different phases steel goes through during the heating / cooling / tempering process.
In the as-bought state the steel is ferrite + cementite + alloy carbide. This stage is used because it is very easy to grind and machine. Ferrite is the main body of the steel, cementite is a particular type of iron carbide which forms in steel because carbon does not dissolve very well in ferrite.
When the steel is heated it will transform into austenite. The critical aspect of this phase is that it will dissolve massive amounts of the alloying elements. In fact if you soak steel hot enough the austenite will dissolve all of the alloy, but in general you don't want that. Some of the alloy will be left undissolved as you want some of the carbides to stay for wear resistance and to prevent the growth of the austenite which forms in pieces (grains).
If the steel is cooled rapidly it will not form ferrite again but instead martensite. The critical part here is that the martensite can hold very high amounts of carbon in it and the more carbon in it the harder and stronger it gets. This is the critical property that is achieved by the entire process. The maximum martensite hardness comes at about 0.6% carbon, above that is mainly for carbide formation.
The reason that cold is used is because it is necessary to ensure all of the austenite changes to martensite. Austenite is very soft and weak so you generally don't want it left in the knife, especially in the edge. If you soak for a very long time (a day) then a particular type of carbide (eta) will form which can increase wear resistance.
When the knife is tempered it will form tempered martensite, and a number of things happen to the steel. The alloy starts to come out of the martensite, the martensite starts to soften, the carbides starts to coarsen and this is why the steel gets softer and weaker.
I am simplifying it quite a lot and generalizing, but that is the basic process. There are lots of interesting things which can happen if you do things wrong which is very common as often people have no idea what they are doing which is why you can see crazy things like 5160 being more brittle than D2.
Have you ever had a knife edge which is both soft and weak but also brittle and wondered how such a thing can happen at the same time?
This is often a sign of the steel being over soaked (too high a temperature) which means the austenite grain grew very large, too much alloy dissolved and too much austenite remained and was not turned to martensite (the more alloy dissolves the harder it is to change to martensite).
-with too much alloy dissolved the wear resistance suffers and the edge wears down quickly
-with too much austenite left the edge will be weak and soft in random places
-what is worse the austenite will change to martensite over time and is now untempered and very brittle, again in random spots
-the very large aus-grain also makes the steel very weak and brittle at the same time
If you use a knife like this you find :
-edge is hard to sharpen, very gummy
-rolls and dents easy
-chips easy at the same time
-wears down and grinds too easy
and you wonder why anyone would like HYPE-X as is doesn't seem overly impressive. This is one of the things you need to be careful of when trying to figure out steels from knives because you are not just seeing the steel but also how it was processed.
SolidState wrote:Do you have any suggestions?
You only need 1000X to get the carbide fraction, in fact beyond that and your field of vision would be too low to estimate the volumes (from the areas). The critical thing to realize of course is that the carbide fractions/volumes are dependent on the austenization temperatures/times. Sandvik is fairly nice in this regard as they show the as-quenched micro-graphs in various states and show what the steels look like both under and over soaked.