1. Gun Steel: What is it? What are the different kinds +/-? Old days vs current times.
First, let us discuss what STEEL is, exactly.
If you look at the periodic table of the elements:
You don't see anything called STEEL. That is because STEEL is not an elemental metal - it is an ALLOY of IRON (symbol Fe, atomic number 26) and CARBON (symbol C, atomic number 6). Sometimes other elements are added as well - they can include:
COLUMBIUM (AKA NIOBIUM)
But what is an ALLOY? An ALLOY is a solid solution of two or more elements where at least one is a metal (in the case of STEEL that element is IRON). A solid solution is much like a liquid solution such as salt water except that the matrix is a solid, not a liquid. Just as salt dissolves in water you can dissolve certain elements into iron. Kind of cool, huh?
Certain alloying elements are very important. CARBON is the most critical alloying element in STEEL. Generally speaking, more carbon content equals harder steel. Thus, all things being equal a STEEL with 0.50% C will be harder than a STEEL with 0.40% C. CHROMIUM is what makes a stainless steel "stainless", provided that there is at least 12% of it in the STEEL. SULPHUR and LEAD are nice for machinists because they make cutting chips break off easier and thus extend tool life. The detrimental things they do to STEEL on a microstructural level, though, are horrendous.
There is a whole convention for naming/numbering STEEL. It is known as the AISI (American Institute of Steel & Iron) system and it consists of a 4-digit designator, sometimes with some letters tossed in. The first two numbers determine which alloy family the STEEL belongs to:
10xx Plain carbon (Mn 1.00 max)
12xx Resulfurized and rephosphorized
15xx Plain carbon (max Mn range: 1.00-1.65)
13xx Mn 1.75
23xx Ni 3.50
25xx Ni 5.00
31xx Ni 1.25; Cr 0.65 and 0.80
32xx Ni 1.75; Cr 1.07
33xx Ni 3.50; Cr 1.50 and 1.57
34xx Ni 3.00; Cr 0.77
40xx Mo 0.20 and 0.25
44xx Mo 0.40 and 0.52
41xx Cr 0.50, 0.80, and 0.95; Mo 0.12, 0.20, 0.25, and 0.30
43xx Ni 1.82; Cr 0.50 and 0.80; Mo 0.25
43BVxx Ni 1.82; Cr 0.50; Mo 0.12 and 0.25; V 0.03 min
47xx Ni 1.05; Cr 0.45; Mo 0.20 and 0.35
81xx Ni 0.30; Cr 0.40; Mo 0.12
86xx Ni 0.55; Cr 0.50; Mo 0.20
87xx Ni 0.55; Cr 0.50; Mo 0.25
88xx Ni 0.55; Cr 0.50; Mo 0.35
93xx Ni 3.25; Cr 1.20; Mo 0.12
94xx Ni 0.45; Cr 0.40; Mo 0.12
97xx Ni 0.55; Cr 0.20; Mo 0.20
98xx Ni 1.00; Cr 0.80; Mo 0.25
46xx Ni 0.85 and 1.82; Mo 0.20 and 0.25
48xx Ni 3.50; Mo 0.25
50xx Cr 0.27, 0.40, 0.50, and 0.65
51xx Cr 0.80, 0.87, 0.92, 0.95, 1.00, and 1.05
50xxx Cr 0.50; C 1.00 min
51xxx Cr 1.02; C 1.00 min
52xxx Cr 1.45; C 1.00 min
61xx Cr 0.60, 0.80, and 0.95; V 0.10 and 0.15 min
72xx W 1.75; Cr 0.75
92xx Si 1.40 and 2.00; Mn 0.65, 0.82, and 0.85; Cr 0 and 0.65
xxBxx B denotes boron steel
xxLxx L denotes leaded steel
xxVxx V denotes vanadium steel
The last two digits indicate the decimal percent carbon content. Thus, a 1040, 4140 and an 8740 all have 0.40% carbon content.
In STAINLESS STEELS you have the following families:
300-series are called AUSTENITIC STAINLESS STEELS and have the best overall corrosion resistance.
400-series are called MARTENSITIC STAINLESS STEELS and can have both good hardness and some corrosion resistance.
PH-series are called PRECIPITATION HARDENING STEELS and can be "aged" to a moderate hardness with good corrosion resistance.
Of all of these STEELS, relatively few are used in firearms. 4140, 41V45 and 4150 are the well-known ordnance grades specified by the US Military for things like barrels. Some companies, like Olympic Arms, have been known to use 416 STAINLESS STEEL for barrels. Still others use 17-4 PH STAINLESS STEEL for barrels.
But just having STEEL isn't enough. There is a reason that STEEL is so useful - that is its ability to be either soft or hard depending upon its HEAT TREATMENT. What governs this ability is the capacity of STEEL to change its crystalline structure (yes, metals are crystalline). There are three crystal structures to know with regards to STEEL:
Body-Centered Cubic (BCC) or Ferrite
Face-Centered Cubic (FCC) or Austenite
Body-Centered Tetragonal (BCT) or Martensite
Generally speaking, BCC is soft and BCT is hard. FCC is a kind of transition structure during heat treatment (when the steel is orange-red hot around 1350-1900 F). Basically, you start with a BCC structure and then when you heat it up to orange-red hot it changes to FCC. Then, when you quench it (usually into oil) the FCC changes to BCT. The reaons why it changes structure are kind of complicated but if you really wanna know I can explain that, too.
After you quench the steel and make it BCT you need to TEMPER it. This is a moderate heat operation (anywhere from 350-1100 F) that reduces the strength of the BCT structure but allows it to regain ductility and toughness (as-quenched BCT is almost as brittle as glass). The higher the tempering temperature the softer it gets. You usually pick a tempering temperature in order to hit a certain hardness range (such as HRC 33-36 for a barrel). And before you go saying that you want it to be as hard as possible you should remember two things:
1 - The machinist who is going to cut the rifling in the barrel will use the whole book of swear words on you and then write a new chapter.
2 - While a low tempering temperature might get you really high hardness that won't last after your first 30-round mag-dump. When you exceed the tempering temperature in service the STEEL will soften, possibly causing distortion as well. The last thing you want when holding off the zombie horde is a wet-noodle for a barrel.
WAY BACK IN THE BAD OLD DAYS STEEL was much harder to come by. It wasn't until the mid 19th century that blast furnaces made steel making an industrial production. Before that it was the province of skilled artisans who closely guarded their secrets. Regardless, you needed three things to make STEEL:
Combining these things with a LOT of heat (be it in a crucible or forge) resulted in a simple and dirty kind of plain carbon STEEL. The STEEL would then be forged into whatever forms were needed (hoes, ploughs, swords, whatever). Damascus STEEL was a kind of pattern-welded (welded on the forge) STEEL that allowed for some early control over impurity and carbon levels. For making gun barrels the steel was drawn into wires or cords and then twisted around a mandrel and forge-welded in that twisted pattern. Because the forge-welding technique depends greatly upon the skill of the blacksmith you should NEVER consider a damascus STEEL gun barrel to be safe to use.
In modern times, we use blast furnaces (though the 3 basic ingredients remain) to make our basic (or ladel) STEEL. We then use something called a Basic Oxygen Furnace (BOF) to "burn-down" the carbon content until it reaches the desired range. With this process we can make steel with very well-controlled carbon levels, down to a hundredth of a percent accuracy (0.01%).
STEEL is sometimes re-melted in a controlled fashion to eliminate some impurities and refine the crystal grain structure (fine grain is desirable for lots of reasons). Some of these methods include Electro-Slag Remelting (ESR), Vacuum Induction Melting (VIM) and Vacuum Arc Melting (VAR). While this remelting adds cost it is desirable for safety critical applications.
I'll get to the other proposed topics as time permits, but feel free to post any questions for more detail about the chemistry of steel or its heat treatment in this thread.