martensitic steel

[Steve Hayden]

I see quite a few trheads on blade forums about cryo/cold treating 420 and other low carbon steels like AUS 6A (japanese version of 440A). These types of steel contain ~0.3% Carbon. By definition, martensite steel has at least 0.8% Carbon. Presence of alloying elements can lower the Carbon % somewhat, but not below 0.6%.
Steel with less than 0.6% Carbon will form some acicular (needle like) Martensite crystals but are not martensite steel.
They cannot be treated made into something they are not, no matter how much you freeze them. To quote an old saying: "You can't make a silk purse from a sow's ear"
The Martensite transition is a temperature dependent one. When the steel equilibrates with the ambient temperature, the transformation at that temperature is complete. No amount of added time will affect the Martensite transition further. Alloying with other elements may add some diffusion related deactions that require time at temperature and may require immediate quenching. This may be evident in the need cited by many makers to quench CPM SS immediately.

A google search on this topic is a good place to start. I can also cite some books if anyone is interested.

 

[Manowar]

To shgeo.

The steel that Buck uses in their knives are 420 HC(HC stands for high carbon). I have a Buck special 119 and a Night hawk that is made of the 420HC at Rockwell 58. By comparing them with Cold Steels 420"subzero quenched" steel,the Buck knives far out perform them.

Both are called 420,and they are related,but what makes the Bucks 420HC better(because it certanly is)than the 420 that Cold Steel uses? Is it the larger amount of carbon that Buck has? I am not an metalurgist,so some further info would be interested to get.

Manowar

 

[Elwin]

PLease consult a time, temperature, transformation curve for your favorite steel to see if it will completely harden, and how fast it has to be cooled. Some require a very fast cooling rate, while others can be cooled in a mild breeze. Also, when the temperature stops changing, martensite stops forming, like shgeo said.

 

[raker]

A time/temperature chart (for a particular steel) will show a lot more than if it will get hard. It will show the time necessary for cooling, the temperature at which the martinsite will start to form, the temperature at which it stops, and the length of time that it could form. 4140 is considered a heat treatable steel and has .4% carbon. This is as low a percent of carbon as I have heard of that will harden. 1050 has .5% and 5160 has .55% to .65% carbon and they form martinsite.
Most knife makers are only interested in the cooling time required to miss the pearlite nose of the T/T chart. I use the length of the martinsite transformation zone to make sure that the steel stays in it as long as I consider it practical ( 1 hour at 300 deg. F. for 52100). Then a slow cool to 175 deg. and that is the temperature at which martinsite will stop forming. Most people that use knives and not nesessarily make them, are interested in what will perform for their needs. That is why they have so many different steels and different heat treats on them. This is just .02 and it really depends on the heat treating procedure as to whether a steel will get hard.

 

[etp777]

As other posts on this have alluded to, steel alloy is NOTHING without the right heat treatment. I'd guess that the Buck and CS alloys are close if not the same(within QC variations), but Buck has Paul Bos heat treating their blades.

Buck knives may not be fancy, but I've never had trouble with one yet. They do the job, and keep doing it. And the HT is a very large part of that.

 

[Keith Montgomery]

I have seen steels with less than .15% carbon called martensitic steels. Steels such as AISI 405 which has a very low carbon content of .08%. These steels can be hardened, just not very much. They are useless for anything other than dive knives.

Here is an interesting article on cryogenic metallurgy.

http://lennon.pub.csufresno.edu/~rlk16/cryo.html

 

[Steve Hayden]

Any amount of Carbon in steel can form some martensite crystals. If there is less than the eutectic percentage, ~0.8%, only acicular (needle like) crystals will form that are dispersed through the ferrite or austenite matrix. At and above 0.8% martensite starts forming as lathes and plates that form interlocking masses. above 1.0% there are mostly plates that form.
Steel with less than 0.6% carbon may preferentially form bainite rather than martensite, depending on quench rates and alloy composition.
Both Bohler/Uddeholm Corp and Crucible Steel Corp recommend cold treatments at -112 F to deal with retained austenite. The diagram in the link that Kieth gave above is really interesting. I have been looking for that kind of plot. I believe it is a loglog plot of wt percent in order to get the linear representation.
I am sure that the true Cryo temps are preferable, but the extra benefit is non-linear down to absolute zero, that is the gain diminishes in proportion as the temperature gets lower. Liquid Helium would be the best of the standard cryo gasses, but is so expensive that it would be unrealistic. Dry ice achieves the recommended temperatures cheaply and is much easier to handle than LN2.