The question gets asked all the time,
"What's the best steel to make knives out of?".
Well, the answer depends on what you want the knife to do....
Yeah, yeah, I know, you want the knife to cut
stuff, but the key is to think about how the knife
will be used, and what kind of wear and tear that use will put
on the blade. For example, a general utility camp knife may have to slice
up summer sausage for lunch one minute, and then chop away
branches the next so that the tents can be erected; while a
survival knife may be called upon for self-defense, and be
subjected to violent stabbing and/or slashing maneuvers, and
perhaps even be asked to block another blade. A filet knife needs to combine razor sharpness with
flexibility in order to filet that tasty salmon you just caught,
while a hunting/skinning knife needs to be able to take a fine
edge and hold it throughout the entire gutting, skinning,
packing and butchering chores, demanding more endurance than
other applications. Blade
shape is widely recognized as being very important to a knife's
function, but it is also important to recognize that alloy and
heat treatment are critical to the knife's function as well.
Making a survival/combat knife out of a very hard,
brittle alloy makes no sense as such blades can shatter readily
when struck or stressed. Likewise, it makes little sense to make a hunting/skinning
knife out of a softer, tougher steel that would require the
successful hunter to stop and re-sharpen his knife every few
minutes while field-dressing a game animal.
How do the different alloys stack up for knife
applications, and to what hardness are they typically heat
treated? Let's
review what the different components contribute to the final
alloy, and then see which alloys are best suited for which jobs.
The most important constituent in steel is
obviously iron (chemical symbol Fe).
The bulk of all steel alloys is iron and it provides the
matrix for all the other components to work, and it also
provides the iron for the iron carbide phases that differentiate
steel from iron. Next
to iron, the most important constituent in steel is carbon
(chemical symbol C). When
iron ore is processed, it ends up with a significant amount of
carbon (about 4%) in it as a result of the processing used to
remove some of the other components of the raw ore.
This 4% C alloy is used in making cast iron products.
There are several iron carbide phases that are made when
iron and carbon are mixed.
The key phases responsible for the properties of steel
are only accessible when the carbon content is below about 2%,
so this cast iron alloy cannot be used directly to make steel,
it is necessary to get rid of some of the carbon first.
Typical steel alloys have somewhere between about 0.1%
carbon to as much as 2% carbon.
Alloys that have less than 0.5% carbon are called
"low carbon steels", and alloys that have more than
0.5% carbon are called "high carbon steels".
The amount of carbon in a steel alloy is important
because it is the carbon that allows the alloy to be hardened
through heat treatment, and more carbon allows for more hardness
(up to a point). However,
for simple iron/carbon alloys, increasing hardness also means
increasing brittleness, and alloys above about 1.2% carbon can
be unacceptably brittle for knife applications (depending on the
heat treatment protocol used).
Toughness is the ability of the blade to withstand stress
without failure (brittle fracture, chipping, etc.), and
generally speaking a softer alloys will provide better toughness
than harder alloys. Overall,
as a general statement, knife alloys will have 0.5% to 1.2%
carbon in them (there are a few notable exceptions to this).
The
next most important element in steel alloys would be chromium
(chemical symbol Cr). The
main thing that Cr contributes to steel alloys is corrosion
resistance. Moderate
Cr content of a few percent increases the corrosion resistance
of the alloy, but does not make it a true stainless steel (these
alloys are called "stain resistant").
For an alloy to be truly stainless it needs to have a Cr
content of 13% or more, and the higher the Cr content the
greater the corrosion resistance.
It is important to recognize that stainless steels can
still rust (especially if left in prolonged contact with blood),
this corrosion resistance just means that they are less prone to
rusting than are carbon tool steels.
Cr also contributes to the "hardenability" of
the alloy (by forming carbides), as well as its edge durability,
but the main reason its added to the mix is to limit rusting.
Nickel (chemical symbol Ni) also contributes to
the corrosion resistance of steel alloys, and it's not uncommon
to see small amounts of Ni added to a stainless alloy for this
reason.
Other metals from the middle of the periodic
table of elements, like molybdenum (Mo), vanadium (V), manganese
(Mn), and tungsten (W) are also commonly encountered in the
steel alloys used to make knives.
Like iron, these metals form carbides when mixed with
carbon, and like the iron carbides, these metal carbides make
the alloy harder. Each
behaves a little bit differently and each contributes in it's
own unique way, but the bottom line is that all contribute to
the overall hardness of the alloy, as well as enhancing wear
resistance. It is
important to note that these components can increase the
hardness of the alloy without necessarily increasing the
brittleness of the alloy (particularly Mo).
Mo and V are particularly valuable since they also
contribute to the edge-holding durability of the knife, as well
as the overall toughness of the alloy.
Silicon improves the strength and wear
resistance of an alloy, but it is also added because it improves
the properties of the steel during the manufacturing process.
So, in summary, carbon is what makes steel steel,
and somewhere between about 0.5% and 1.5% carbon makes for good
blades (depending on the alloy and the application).
13% (or more) Cr makes a steel stainless, and helps to
minimize corrosion issues, as well as helping hardenability. Ni also aids in corrosion resistance. Molybdenum helps to
prevent brittleness, increases hardness, toughness and edge
retention. Vanadium
forms fine-grained carbides and enhances wear resistance,
toughness and hardenability.
Manganese contributes to hardenability and wear
resistance, but also improves the properties of the steel during
the manufacturing process (hot rolling etc.). Silicon improves
strength and wear resistance, but it's like Mn in that it
improves the properties of the steel during the manufacturing
process. So the
bottom line is somewhere around 1% C is good for a knife steel,
13% (or more) Cr makes it stainless, Mo and V aid edge
retention, and small amounts of other elements like Mn and Si
can also be beneficial.
Heat treating a steel alloy allows for the
conversion of the various iron carbides formed in these alloys
to one specific phase (martensite) that is harder and more
desirable than the others.
How this heat treatment is carried out depends on which
alloy one is working with, and what final hardness one is aiming
for. In a nutshell,
the steel is heated to a specific temperature (commonly
somewhere around 2000 degrees Fahrenheit for the stainless
steels, lower for the carbon tool steels), held at that
temperature for a certain period of time, and then taken through
a very specific cooling process.
For knife applications, a final hardness of somewhere in
the range of 50-60 on the Rockwell C (Rc) scale is usually
targeted. Blades
that are hardened to over 60Rc are difficult to sharpen and tend
to chip easily. Softer
blades tend to be tougher and withstand stress and abuse better,
but will generally have poorer edge retention.
As with everything in life, compromises must be made, and
the deciding factor is what kind of use the knife will be put
to.
Tool Steels
Historically,
one of the traditional favorites for making knives is 1095, a
simple tools steel that has 0.95% carbon in it, and 0.4% Mn. Knives made of 1095 take an edge very well, but they rust
easily and are commonly coated with some sort of rust resistant
coating (e.g. phosphate). It
is reasonably tough (depending on what hardness it has been
tempered to), but for better toughness lower carbon tools steels
(e.g. 1060) are generally used.
For those who have made knives from files, file steel is
commonly 1095. 1095
can be heat treated up to about Rc 66, and then drawn down to
the desired hardness. Ka-bar
has been using 1095, heat treated to 56-58 Rc, for over half a
century.
Simple water-quenching steels like W-1 and W-2
have 0.7-1.5% C, and less than 0.5% each, of Cr, Mn, Mo, Ni, Si
tungsten and V, and are very similar to 1095 in terms of
hardness, edge-holding and ease of rusting.
Likewise, the simple oil quenching alloys like O-1 and
O-2 (both of which contain 0.9% C, and 1.2% Mn; O-1 also has
0.5% Cr, 0.5% tungsten, 0.2% V) can also be heat treated up to
about Rc 65, and then tempered down to the desired hardness.
All of these steels rust easily.
A number of tool steels that were originally
developed for more specialized applications have also been used
in the knife-makers art. For
example, L-6 is a band saw steel alloy that is composed of 0.7%
C, 1.25-2.0% Ni, 0.6-1.2% Cr, 0.5% Mo, 0.25-0.8% Mn, 0.5% Si,
0.2-0.3% V. It has
a reputation for having a good balance between workability, cost
and good working toughness.
L-6 can be heat treated to Rc 60, it is very tough and
holds an edge very well, but the low Cr/Ni content leaves this
alloy with poor corrosion resistance and it rusts easily.
5160H is a similar alloy (0.6% C, 0.6-1.0% Cr, 0.65-1.1%
Mn, 0.15-0.35% Si) which can be hardened up to the mid-50s Rc
and is respected for its toughness.
Bob Loveless reported that he made some
very nice blades out of S5 (composed of 0.60% C,
0.85% Mn, 0.25% Cr, 0.30% Mo, 0.20% V, and 1.90% Si),
that had very good toughness and edge holding properties, but
that it was an unpredictable alloy that he never figured out --
about 1/3 of the blades would just shatter.
Eventually he just walked away from S5; there was just
too much frustration.
S7 is a somewhat similar alloy to S5, but with
significantly more Cr and Mo, and less Si (S7 has 0.55% C, 0.70%
Mn, 3.25% Cr, 1.40% Mo, 0.25% V, and 0.35% Si).
S7 is becoming a popular steel for survival and combat
knives as a result of its excellent toughness.
While the Cr content makes this alloy somewhat rust
resistant, S7 will definitely rust, and so these knives are
commonly coated with some sort of rust resistant coating.
52100 is a somewhat harder, but nonetheless
similar alloy, used to make ball bearings.
52100 is composed of 1.0% C, 1.5% Cr, 0.35% Mn and 0.35%
Si. It requires
some specialized (and involved) heat treating/quenching
procedures, but can be heat treated up to a hardness of 67 Rc,
and then drawn down to Rc 56-61, depending on the tempering
temperature used. 52100
is well-liked among knife-makers for its toughness.
This alloy is popular with certain makers for survival
knives as a result of this trait (Swamp Rat Knife Works uses a
variation of 52100 they call SR-101).
These knives are also commonly coated with some sort of
coating to prevent rusting.
Die Steels
A-2 is a die steel, composed of 1.0% C, 5.25%
Cr, 1.1% Mo, 0.6% Mn and 0.25 V, and makes a very good knife
steel, with very good edge durability, and excellent
toughness. It
has some corrosion resistance but is definitely NOT stainless,
and will rust. Note
the higher carbon content along with more Mo (relative to the
tools steels above), this is where A-2's highly regarded edge
durability and toughness come from. A-2 can be heat treated up to Rc 64, and then is readily
tempered down to a usable hardness of 60 Rc.
Several knife makers (like Bark River) use A-2.
D-2 is another die steel, highly regarded for
its excellent wear resistance.
D-2 is almost (but not quite) a stainless steel, composed
of 1.55% carbon, 11.5% Cr, 0.9% V, 0.8% Mo, 0.45 Si, and 0.35%
Mn. Because of the
high Cr content D-2 has good corrosion resistance, but it's not
quite rust-proof. D-2
can be heat treated up to a hardness of 64 Rc, and then drawn
down to 55 to 61 Rc, depending on the tempering temperature used
(brittleness can be a problem if the alloy is left at 62-64 Rc).
Bob Dozier is one of the notable knife makers who
uses D-2 extensively in his hunting knives and Dozier's knives
have an excellent reputation for taking and holding a fine edge
(he heat treats them 60-61 Rc).
My friend Jim Taylor has one of Dozier's knives
and tells me that it will keep a fine edge through 3 or 4 deer
without any need for re-sharpening.
Stainless steels
Given the things that a sportsman's knife gets
exposed to during routine usage (rain, sweat, blood, etc.) it's
no surprise that stainless steels have gotten popular in order
to limit corrosion (stainless steels will still rust, just not
as easily as carbon steels). In fact, most knives made today are made with some sort of
stainless steel. In
this discussion, we'll start with the lower carbon classes of
stainless and work our way up.
One of the more popular alloys in recent years
for mass-produced knives is 420 High Carbon (HC), which is
composed of about 0.5% C, 13.5% Cr, and 0.35-0.90% Mn.
420HC is used by a number of commercial knife
manufacturers (e.g. Gerber, Buck, Kershaw, etc.), and is
typically heat treated up to a hardness in the low 50s Rc.
Knife manufacturers like this alloy because it's cheap,
has excellent corrosion resistance properties, and is easy to
grind (i.e. it isn't hard on tooling like some other alloys can
be, this is a significant issue when mass producing thousands of
blades and trying to keep costs affordable).
It's easy to sharpen, and takes a decent edge, but edge
retention isn't all that it could be and these blades typically
need to be touched up regularly.
420HC makes a decent general purpose utility blade as
it's easy to re-sharpen.
A similar alloy that one also finds in
mass-produced knives is called 425 Modified, which is composed
of about 0.5% C, 13.5%
Cr, 1.0% Mo, 0.35% Mn, and
0.35% Si. As you can see from the C and Cr content this alloy is rather
like 420HC, with the exception of the added Mo, which should
give it somewhat better edge retention.
Swedish steel has been respected knife steel for
many years. Sandvik
makes a slightly harder, but similar (to 420HC) stainless alloy,
called 12C27, that has been widely used in a number Scandinavian
knives. 12C27
contains 0.65% C, 13.5% Cr, and 0.35% Mn.
Alloys such as this are commonly heat treated to the mid
to upper 50s Rc and have excellent toughness.
12C27 has a reputation for being a very "clean"
alloy (i.e. consistent composition, with few impurities).
Perhaps
the most commonly encountered stainless steel in commercial
knife making today is the 440 class of alloys.
This class is composed of three different alloys, 440A,
440B and 440C, all of which have very high Cr content (and hence
corrosion resistance). 440A
is the softest of this series, and is composed of
about 0.55% C, 17% Cr, 1.0% Mn, 0.75% Mo, and 1.0% Si.
It is used by several knife manufacturers and is commonly
heat treated to the mid to upper 50s Rc.
440B is used by Randall to make their stainless
knives, and it is composed of about 0.85% C, 17% Cr, 1.00% Mn,
0.75% Mo, 0.75%, and 1.0% Si.
The higher carbon content of 440B allows it to be heat
treated up to Rc 59-60, if so desired.
Of the three 440 alloys, 440C is arguably the best knife
steel, both in terms of edge-holding ability and in terms of
corrosion resistance. 440C
contains about 1.1% C, 18% Cr, 1.0% Mn, and 1.0% Mo.
The higher carbon content allows this alloy to be heat
treated as high as 61 Rc (typically only taken up to about
58-60), and the very high Cr content gives 440C the best
corrosion resistance of any of the typical knife steels.
440C is a very popular knife steel because it is widely
available, has excellent corrosion resistance, is fairly easy to
grind and takes a good edge.
Edge retention is pretty good (although some have
criticized 440C for not holding up under hard use).
A
similar group of alloys has come over in recent years from
Japan. AUS6, AUS8 and AUS10 have been compared (approximately) to
440A, 440B and 440C. AUS6
is the softer of these alloys and contains about 0.6% C, 14%Cr,
1.0% Mn, 1.0%
Si, 0.49% Ni, and .25% V (there is also a variation on this
alloy called AUS 6A that has 1.2% Mo in place of the V).
AUS6 is typically heat treated to the mid 50s Rc, and it
should have good toughness.
AUS8 has gotten to be very popular lately and is being
used by a number of knife manufacturers (Cold Steel, Spyderco,
Kershaw, SOG and many others).
AUS8 contains 0.75% C, 14% Cr, 1.0% Mn, 1.0% Si, 0.49%
Ni, and 0.25%V (there is a variation on this alloy too, called
AUS8A that contains 0.95% C, 14% Cr, 1.0% Mn, 1.0% Si, 0.50% Ni,
0.20% Mo, 0.15% V, 0.40% W, which should be harder and have
better edge retention). AUS8
is generally heat treated up to 58-59 Rc and will take an
excellent edge. Edge
retention is good, but not as good as some of the higher grades
of stainless (see below). AUS10 is a harder alloy that has been compared to 440C.
AUS10 contains 1.10% C, 14% Cr, 0.50% Mn, 0.30% Mo, 0.49%
Ni, 0.27% V. AUS10
doesn't seem to be very widely used at the moment (perhaps
because it's competing head-to-head with 440C?), but it looks to
be an excellent knife steel nonetheless.
In 1972, a new crucible steel was brought to
market that was called 154CM.
Bob Loveless was one of the first to use it to make
knives, and he liked what this steel gave him and he gave it
high marks. 154 CM contains 1.0% C, 14% Cr, 4.0% Mo, 0.6% Mn, and 0.25%
Si (note the unusually high Mo content of 154CM; Mo is a key
component in alloys used for high speed tooling and contributes
wear resistance and durability to the alloy).
Loveless made knives out of this alloy for many years,
and this alloy helped establish Loveless' reputation as a knife
maker. 154CM is now
widely used by a number of custom knife makers, in top-quality
hunting knives. It
is generally heat treated up to about Rc 59-60, and will take an
outstanding edge. Edge durability is reported to be excellent.
Japan
responded to 154CM with an alloy that they called ATS34,
intended to capture the same strengths as 154CM.
ATS34 contains 1.04% C, 13.9% Cr, 3.55% Mo, 0.4% Mn,
0.28% Si, and is reputed to be a more uniform and
"cleaner" alloy than 154CM (once again, note the high
Mo content). Many
knife makers (both commercial manufacturers and custom knife
makers, Bob Loveless included) went over to using ATS34 as their
preferred blade steel after it was introduced to the market.
Like 154CM, ATS34 can be heat treated to 59-60 Rc, it
takes an excellent edge and has excellent edge durability.
Top-quality knives (and knife blanks) made from ATS34 are
available from many sources.
Recently, a new alloy has come on the scene, one
whose properties are sufficiently appealing that it has lured a
few of these knife makers (including Loveless) away from ATS34.
This new alloy is called BG-42, which is composed of
1.12% C, 14.5% Cr, 4.0% Mo, 1.2% V, 0.5% Mn, 0.3% Si.
Note the BG-42 basically has more of everything than does
ATS34 -- most notably more carbon, more chromium, more
molybdenum and more vanadium (the fine-grained vanadium carbides
add another element of durability to the alloy).
Given this composition, one might expect it heat treat
easily up to Rc 60, to take an excellent edge, and to hold it
for a long time, and indeed it does.
There aren't that many people using BG-42 at the moment,
but I suspect that that will change in the coming years.
It is an excellent knife steel.
A similar high carbon stainless that has gotten
to be rather popular recently is CPM S30V.
This alloy is composed of 1.45% C, 14% Cr, 2.0% Mo, 4.0% V, 0.40% Mn, and 0.40% Si (note the high carbon
content, and while the Mo is lower than ATS 34, it is still
pretty high, and is made up for by the unusually high vanadium
content). S30V is designed to offer an optimum combination of
toughness, wear resistance and corrosion resistance.
This alloy is processed differently than typical steel
alloys, resulting in a finer grain size and a more uniform
distribution of the alloying elements, resulting in the greater
toughness and wear resistance.
S30V was specially formulated to promote the formation of
vanadium carbides, which are harder and more effective than
chromium carbides in providing wear resistance. S30V is usually heat treated to 58-61 Rc, and is reputed to
have better toughness than highly regarded steels like 440C and
D2, and is said to have better edge-holding capability than
440C. S30V is a hard, wear-resistant steel used by knife
makers like Spyderco, Benchmade, and others.
Conclusions
So which alloy makes the best knives?
Well, that depends on what you want the knife to do.
It also depends on what your taste in steel is like.
The following discussion is one interpretation of how
these alloys are suited for the different tasks that a knife is
asked to do. Others might see things a little differently, but this can
serve as a useful starting point.
The survival/combat knife needs toughness, and
cannot tolerate brittleness, and so those knives tend to be made
from alloys with intermediate carbon content (generally 0.6 to
0.8%), and heat treated to more moderate hardness (mid 50s Rc),
as this is most conducive to blade toughness.
Corrosion resistance is important since one never knows
what kind of weather one might be up against (or for how long)
in a survival situation, so some sort of corrosion resistant
coating (and there are a number of very good ones out there) or
stainless alloy are popular options for these knives.
Since a corrosion resistant coating can also dull the
glare of a polished blade, these are particularly popular in
survival/combat knives. Alloys
that fit well in this niche include S7, 52100, and 5160.
A-2 and 1095 are also highly regarded in this
application, in spite of their higher carbon content (tempered
down to 56 or so). Toughness
is a key concept for knives in this category.
Filet knives have similar needs, but for very
different reasons. A
filet knife needs to be flexible and does not tolerate
brittleness very well, so once again toughness is a key
attribute. Carbon
content once again tends to be in the intermediate range (0.6%
to 0.8%), since more carbon might lead to unacceptable
brittleness. Filet
knives tend to be heat treated tends in the mid 50s Rc to
provide a good compromise between toughness and the ability to
take a fine edge (the narrow blade profile on most filet knives
makes them easy to re-sharpen quickly, so edge retention isn't
generally a concern). Filet
knives will get wet repeatedly (as well as bloody) in the course
of their duties, so stainless alloys are a real plus here.
Sandvik's 12C27 is an excellent fit for this category, as
are 440A and AUS6.
The general utility camp knife needs to be sort
of "jack of all trades".
The blade needs to be moderately hard to be able to take
a good edge, but it must also be tough enough to withstand light
chopping. This set of demands suggests an alloy with moderately high
carbon content (around 0.8 to 1.1%), heat treated to Rc 56-58.
A traditional favorite for this application is good ol'
1095 (just make sure to maintain it and don't let it rust), and
many folks would tell you that there's no better camp knife than
a Kabar. A-2
is also highly regarded in this role.
These days, one tends to see more stainless alloys in
this application, and 440C makes a very useful camp knife, as
does AUS8. S30V
would also make a dandy camp knife.
A hunting/skinning knife needs to take a fine
edge and hold it throughout the gutting/skinning/butchering
chores. Blade
hardness and edge durability are the key parameters here, so
high carbon content (preferably over 1%) is called for, and high
molybdenum content (2% or more) helps to hold the edge (and
added vanadium is also a plus).
Blood is three times as salty as seawater and can corrode
knife steel very rapidly, so corrosion resistance is
particularly valuable for a hunting/skinning knife, making high
chromium stainless alloys are very useful here (and added Ni
doesn't hurt any). These
knives are generally heat treated up to about 60 Rc.
Alloys that are particularly well suited to this
application include S30V, 154CM, ATS34, and BG-42.
So
which steel alloy is best for making knives?
You tell me -- what do you want the knife to do?
Glen
E. Fryxell
Meet
Glen on the "About Us" page!
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