SOME BASIC FIREARMS METALLURGY

BASIC STEEL PROPERTIES & APPLICATIONS
In modern firearms, the choice of steel, and the degree to which it is hardened, is crucial to safety, dependability, and rust resistance. Matching the properties of steel to a given application is a complicated science known as metallurgy.  No knowledge of firearms or the history of ballistics can be called complete without a basic knowledge of steel and heat treating.

Early gun makers used a fairly unrefined form of iron in their guns.  There was also no efficient method for boring barrels. Instead, they used a long round iron bar called a mandrel.  Metal strips, heated in a forge, were wrapped around the mandrel.  The strips were then pounded while hot with a hammer.  These blows not only created the proper shape and appearance; it also bonded the  adjacent strips together.  This method is called hammer welding and is still used today in rare instances. This method of making rifle barrels was still in use as late as the 1850's, and practitioners included US government arsenals such as Springfield Armory.  By that time, steam had replaced muscle as the hammers' motive force.  This method was used to make Damascus barrels were made into the early part of the twentieth century of either twisted steel or steel and iron twisted together in combination.  These barrels were used mostly on shotguns and the exquisite patterns left from forming them were also etched onto solid steel barrels.  As a note, Damascus barrels are unsafe for modern cartridges.  To determined whether a barrel is a true Damascus barrel or a solid steel copy, locate a part of the barrel that is  normally hidden from view, such as under the forearm. Place a drop of  hydrochloric acid on the steel and observe the reaction. A solid steel barrel,  even with a Damascus pattern, will be turned a uniform gray color while the pattern will still be visible on a genuine Damascus barrel. When the outcome is clear, clean away the acid, then wash, dry, and oil the entire barrel.  Damascus barreled firearms were all made for black powder, with chamber pressures less than 25,000 psi and unjacketed bullets.

Around the turn of the century, along with smokeless powder and jacketed bullets, came better steels. One form was nickel steel in which nickel was added for strength. A more important advance was realized with ordnance steel.  Ordnance steel contained .45 to .55% carbon, 1.0 to 1.3% manganese, .05% phosphorous (maximum), .05% sulfur (maximum) and .25% silicon.

MODERN STEEL CLASSIFICATIONS
A brief bit of background is in order to give some context to a discussion of modern steels.  By agreement between the Society of Automotive Engineers (SAE) and the American National Standards Institute (ANSI), a numerical system is used to classify both tool and standard steels. Tool steel is used to make the dies, jigs, fixtures, and cutting tools used in the manufacture of guns and gun parts and accessories. Heat treating readily hardens it. Standard steel is not as easily hardened, except on the outer surface, and is used for many other things. It has a classification system based on a 4 or 5 digit number that works as follows:

The first figure indicates the class of steel, such as nickel or carbon.  The second figure indicates whether an alloy is present and the approximate percentage of the predominant alloying element.  The last two figures indicate the average carbon content in hundredths of percent.  Any steel with the last two figures below 30 is considered low carbon. The higher carbon grades are less ductile and harder to form and weld than the lower  carbon  steels, but they also have improved strength.

Occasionally steel numbers that are prefixed with WD are seen. This stands for War Department and was used by the military to ensure tighter control over the specifications of steel used in military weapons.  Alloy steels contain added elements for the purposes of modifying their behavior during heat treatment to result in improved mechanical and physical properties.   For example, the addition of nickel increases hardness and tensile strength.  It is used in railroad rails, as well as armor plate and ammunition.  Vanadium adds the ability to resist repeated stress through an increased elastic limit.  Tungsten adds air hardening qualities to steel and is regularly used in conjunction with nickel or chromium or both.  Manganese adds toughness in proper quantity; in small amounts (1.5 to 5.5%), the steel is very brittle and can be broken with a hammer. Additions up to twelve per cent make the steel ductile and hard. Molybdenum is commonly used in conjunction with other alloys to improve high temperature service, wear resistance, and to improve hardening qualities.  Chromium improves toughness, stiffness, and hardness.  The percentage utilized varies over a wide range from low to high chromium steel. Chromium also helps in resistance to acids, heat, and nitriding. (Iron nitride.) The addition of sulfur, and/or selenium improves the ability to machine or cut the steel in the manufacturing process.  These additions that help the machining characteristics also weaken the grain structure.  Some of the advantage gained  in one area is lost in another.

The most common steel used in modern firearms is called chrome­moly. Properly designated as 4140 by SAE grade, it is considered ultra high strength steel that, if properly oil quenched and tempered, will have a tensile strength of up to 180,000 psi.  Indeed, just cold drawn and annealed, its tensile strength is 98,000 psi. The last two figures of the SAE number (40) indicate a carbon content between .38 and .43%.  It also contains chromium  (.80-1.10%), manganese (.75-1.00%), molybdenum (.15-.25%), phosphorus (.040%), sulfur (.04%), silicon (.20-.035%) and no nickel.   4140 steel is used by some manufacturers for barrels, actions, revolver and pistol frames, etc. It is excellent steel, tough, strong, and easy to machine and heater.

HEAT TREATMENT

HARDENING
Hardening by heat treatment is an old skill that has become highly refined.  There are two basic steps to the process.  The first involves heating the steel at least 100° F. above the metal's transformation temperature (the preferred term, but sometimes called critical point). If the steel is not heated to this temperature so that austenite forms, no hardening action can take place.  The transition temperature is the point at which two solid phases exist in equilibrium. This temperature must be reached to the center of the metal piece without the outer surface being overheated. This takes care and time. The austenite is a solid solution of one or more elements and unless otherwise designated, the solute is generally assumed to be carbon. (Another  would be nickel austenite.)  The second step involves rapid cooling (quenching) of the steel at a rate faster than the critical rate to produce a martensitic (very hard and brittle) structure. The hardness will depend heavily on the carbon content.  The heating can be as simple as a torch or forge or as sophisticated as a temperature and atmosphere controlling furnace.  The quenching is determined by the type of steel and can be in oil, water, air or brine.  The heating temperature will vary by carbon content from 1,310° F. to 1,550° F, and in salt baths up to 2,400° F.

TEMPERING
Next, the steel is tempered.  Tempering (drawing) which is a process of reheating quench-hardened or normalized steel to a specific temperature that is lower than for hardening, holding it for a "soak", and then cooling it at a controlled rate. This also can involve oil and salt baths.  The purpose of tempering is to reduce brittleness and remove internal stress.

STRESS RELIEVING
Stress relieving is similar to tempering and the two are frequently done in one operation.  It is actually a separate process requiring different temperatures and conditions.  Sometimes it is required when hardening and cold-work stresses are induced from welding or from machine work.  After the First World War, a technique called autofrenage has been used occasionally for prestressing barrels. In this process, a high hydraulic pressure is used to permanently enlarge the bore about 6% and the outside about 1%. The elastic strength can be almost doubled by the joint effect of the residual compressive stress near the bore and the higher elastic limit.

CASE HARDENING
Case hardening is where just the outer surface of low carbon (.30 per cent or less) steel is hardened. The surface is impregnated with carbon through cyaniding or nitriding or another of the seven common methods of case hardening. This involves high temperature followed by quenching and controlled cooling.

SOFTENING
A heating and cooling process called annealing can soften hardened steel.

NORMALIZING
Forged pieces have a coarse grain structure and normalizing refines the grain. It is done at a temperature usually about 100° F. higher than the regular hardening process. Cooling is done in still air at room temperature.  Hardness is not desired because it is done before machining.

HEAT TREATING IN GENERAL
It is not our intent to imply that heat treatment is a simple process.  On the contrary, the heat treatment of metals is an exacting process and nobody should attempt it from this brief familiarization.  Indeed, any heat treated parts should be tested for hardness by a reputable company using either the Brinnell or Rockwell test (There are 10 other methods of testing hardness, but those two are the most common).

Whichever method is used, it will have a number that is based on resistance to indentation under the conditions imposed by the particular test. This number is meaningless unless it can be given significance by comparison to the hardness of familiar objects.  Cutting tools, such as taps, require a hardness of about 63 on the Rockwell C scale. Below 61 and they tend to be too soft and wear excessively and over 65 they will be hard and chip.  Brass can be from 40 to 95 Rockwell, but it is measured on a different scale, the Rockwell B scale.

Hardness affects tensile strength. If the 4140 steel that was discussed earlier is 20 on the Rockwell C scale, it will have a tensile strength of 110,000 psi. At 38 Rockwell C its tensile strength will be 180,00 psi.  Controlling factors are the temperature of the draw and quench. 4140 is considered to be medium in hardening ability and is frequently used "as is" at about Rockwell C-10.  Modern barrels are made of steels other than 4140. 4135 steel heat treated to about 275 Brinnell hardness (Bhn) has been used with success.  For a slight cost savings, cold drawn stress-relieved 1137 steel, with a minimum yield of 90,000 psi. can be substituted.  1137 has less distortion than 4140 in hardening, but falls short in other areas.

The correct hardness for a barrel made with one steel grade may be too hard or soft for another grade. The early Springfield .30-'06 rifles with low serial numbers (below 1,257,762 or by Rock Island Arsenal below 285,507) generally had barrels that were too hard and brittle. They were usually above Rockwell C-50 at C-54 or C-55. They used low-grade steel that would be safer at C-30 to C-40. The barrels at C-40 had a tensile strength of 145,000 psi. and were safe at 100,000 psi. Above C-50 they were too brittle. They were perhaps excellent wear for resistance but had a danger of blowup. Nickel steel Springfield barrels in the later models are safe and correct at C -45 to C-55.

A hardness test on a firearm is normally useless. It is a waste of time and money. As noted earlier, hardness is an issue of both comparison and application. A hardness number is worthless without knowing the exact steel used. Is it 4140 or 3140? Is it 1020 or 2330?  For the hardness number to have meaning as far as firearm use and safety, the part would have to be destroyed by metallurgical testing to determine tensile strength, alloy, purity, etc. A Rockwell C reading of 45 can be good or bad depending on the steel. It does not tell strength, only hardness. The cheapest scrap steel can be heat­treated or case hardened to give a Rockwell C reading anywhere on the scale between 20 and 80. It only has value when the steel designation is known.  Proof testing is the only way to know if an action is safe.

The hardness of a barrel's steel is a factor and like the composition, it is a trade-off. The harder steel has better heat resisting properties and more strength. On the other hand, it is more difficult and expensive to machine and as such, costs extra. As expected, a balance in the middle is obtained in the better guns while the cheap guns ordinarily use the softer steel with fewer alloys added.  A high percentage of gunsmiths do their own heat treatment, and some are very good at it. Nevertheless, it cannot be recommended. Improper heat treatment can result in quench cracking, uneven hardness, brittleness, distortion, and other serious problems.

Silver solder is a common method of adding sights, scope rings, etc. Modern silver solder will fuse at about 450° F. and some older silver solder requires around 650° F. Silver soldering on a firearm requires both knowledge and patience to avoid damage to the heat treatment.

BREAKS AND CRYSTALLIZATION

All metal in a solid state is crystalline by nature.  Breaks occur at flaws in the metal.  The flaw may be microscopic and unseen without instruments, but there will always be some type of weakness.  Cracks occur at weak intergranular faces and grain boundaries and can be from various sources, such as creep, intergranular corrosion, severe shock, changes in temperature, and stress fatigue. Of course, we are not speaking here of breaks from subjecting an action to three times the pressure it was designed for, but in that case, it will still rupture at the weakest point.

CHROMIUM PLATING

The hard chromium used in bores is slightly different in its application than decorative chromium as applied to the outside for appearance and rust resistance. Decorative chrome is usually applied over a thinly added layer of copper or nickel.  A chrome-plated bore can have good wear and corrosion resistance if done properly, but most have a thickness of only about .0005" to .00075." That is five to seven and a half ten thousands of an inch.  It is sometimes as thin as .0002". Military plating is usually better at .001" to .002 in thickness.  Even though it is extremely hard, it frequently does not hold up well.   Chrome plated barrels are not usually made to as close a tolerance  as a standard barrel. Plating requires the use of an electrochemical bath, which removes some of the steel. Sometimes called electro -polishing, it is needed for proper adhesion of the chromium plating (instead of the copper or nickel), then the thin chrome is added. Even under the best of conditions, extremely close tolerances cannot be held.  Chromium plated bores are excellent for hunting, military and police use; anyplace where rust can be a problem.  Chromium plating is also very good at preventing metal fouling in barrels used for rapid-fire. For high accuracy in target work, it is not usually recommended.  Also, chrome plating will not increase velocity, as is sometimes believed.  It is certainly no "slicker" than a good quality lapped barrel. Another misconception is that plating a bore will fill in and smooth out rough places. The plating follows the existing finish and all marks are still there only they are then in the plating. That is why the outside or visible parts of anything plated has to be polished so perfectly before hand. The plating wears away first near the chamber and then progressively on down the bore with the original thickness controlling how long it lasts.   Frequently, fouling does not stick as well to chrome as steel and it helps to control rust. The American Rifleman, a publication of the National Rifle Association, conducted tests with a .357 S&W Magnum. The barrel leaded about the same after chrome was added as before, although, it was reported to be a little easier to remove the fouling from the chrome.  It can be considered very good for rust prevention but only fair for other objectives.

STAINLESS STEEL

Stainless steel is very popular for firearms despite the extra cost. Stainless steel was originally a trade name used for cutlery steel and patented in 1916. It was not legally maintained as a trademark and now the term is generally used for all steels that resist rust, corrosion, acids, and high temperature scaling. There are about fifty different compositions of stainless steel, each with its own unique properties of machineability, hardening, heat resistance, strength, stress resistance, etc. Some can be hardened and others cannot As with many other things, the choice is always a compromise. For example, the best corrosion resistant steels have less strength and toughness. Generally, to get the tensile strength required in firearms, the steel used will, under some conditions, rust and corrode. Stainless is also excellent at reducing barrel wear and fouling. The grade of steel used in manufacture and how it is machined and processed is the deciding factor.  The tensile strength of stainless steel is lower than properly processed 4140. For example, stainless  type 410 is 65,000 psi.

These steels always have chromium added.  The amount can vary from as little as 4% (AlSI 501) or as high as 27% (AlSI. 446). Chromium percents below 10 are not technically considered stainless steels, but by common usage are called that. The common stainless steels contain from 11.5% to 27.0%. The iron content will be at least 50%. Alloying additions may include nickel, molybdenum, columbium, titanium, manganese, sulfur, and selenium.

STELLITE

Stellite is used as a liner in barrels that are expected to receive extremely hard use.  The lining is usually only at the throat or chamber shoulder and extended for two to eight inches.  A close visual inspection will show a gap or a faint ring. The most common use of stellite in firearms is with machine gun barrels to reduce heat and wear resistance. The material is so hard that it is used to make cutting tools for machining metal and for valves in aircraft and high RPM automobile engines. Stellite has high impact and cantilever strength and heating and cooling can be repeated indefinitely without any loss in hardness. It is resistant to corrosion and oxidation and has good tensile strength. There are several grades with each composed of varying proportions of cobalt, chromium, and tungsten.  As one would expect, it is expensive to purchase and use.

TITANIUM

Titanium parts are costly to make, but they can hold tremendous  pressure at half the weight of steel. With excellent corrosion resistance (almost 100%) and other long-term advantages, titanium will probably be used more in the future; probably as parts or components rather than as a complete weapon or as an alloy such as titanium/graphite. Pure titanium has a melting point of 3,097° F. and a modulus of elasticity of 16,500,000 psi. (No, that's not a mistake. It is in millions.) Even as an alloy, titanium has excellent qualities.  The modulus of elasticity is defined as the ratio of increase of unit deformation to the increase of unit stress within the elastic limit and is given in psi. It can be measured in tension, compression and shear.  Don't let the technical terms throw you. Modulus means a positive number expressing the measure of the function, which is elasticity (flexibility, resilience) in this case. The rest is the ability of the material to
return to its original dimensions after the removal of stress.

PLASTIC

Plastics, composites and polymers are already on the scene in a big way; not for entire guns, but for component parts. They can be engineered and molded to be strong but lightweight and corrosion proof.  They are cheap to manufacture and have but one major flaw. They cannot handle high temperature, at least not at the time this is written. No doubt, the future will change the situation. The Glock was the first gun with a polymer frame to be widely accepted. Other manufacturers, including, CZ, Smith&Wesson, Walther, and Beretta are now following Glock's lead.

EROSION & CORROSION

This may be a good place to mention that many people use the terms erosion and corrosion interchangeably. In most cases, the choice of words would make no difference, but they do mean completely different things, each affecting gun parts and bores in different ways.  Corrosion damages by chemical reaction. Moisture and chemicals produced by primers, powders, and saline agents are the main culprits. Rust is the most common result. Modern ammunition reduces the problem, compared to earlier powders and primers, which caused corrosion because of their chemical composition.

Erosion is a gradual wearing caused by the friction of the bullet.  Also, the heavy wear just forward of the chamber caused by the cutting action of the high pressure and high temperature gases.  Both damage the gun, but in different ways.
 
 
 

Note: Data for this month's trivia page was gathered from:

Rinker, Robert A., Understanding Firearm Ballistics, (Mulberry House Publishing, Apache Junction, Arizona: 2000)

Understanding Firearm Ballistics is available from Amazon.com.  Click on the image to order.

 
 
 

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