I've included this so you can get an idea of the common testing methods used on metals and what they mean. These will only be for mechanical properties which includes tensile strength, yield strength and hardness. Some people call them physical properties. This is wrong! Physical properties include: electrical conductivity, thermal conductivity, thermal expansion and vibration dampening capacity. Mechanical properties can be tested at any temperature. I routinely test artillery shells for fracture toughness at minus 65º F. I test oil-well tool joints for impact toughness at minus 40º F. I also test some stainless steels for creep strength at over 800º F. Generally, the colder the temperature, the more brittle a metal is and the higher the temperature, the softer it is. There are some exceptions to this rule like a phenomenon called hot-short. It's when some high sulfur steels become brittle over 2050º F. Sorry... moving on.
The first test I will discuss is the spark test. This is a test that anyone can perform at home. The idea is simple: the spark stream given off during a grinding operation can be used to approximate the grade or alloy of a steel. The equipment used should be a grinder with a no-load speed of 9000 rpm and a wheel size of around 2.5 inches. A semi-darkened location is necessary.
The easiest way to learn the test is to observe the spark streams from various known grades and compare them with this text. As you grind, you will see lines called carrier lines. At the termination of the carrier lines, you will see small bursts called sprigs. Low carbon (1008) is a very simple stream with few bright sprigs. The higher the carbon content, the more numerous the carrier lines and sprigs.
Some alloying elements change the appearance of the test. Sulfur imparts a flame shaped, orange colored swelling on each carrier line. The higher the sulfur, the more numerous the swellings. A spear-point shape that is detached from the end of the carrier line identifies phosphorus. The higher the phosphorous content the more numerous the spear points. Nickel appears as a white rectangular-shaped block of light throughout the spark stream. Chromium appears as tint stars throughout the carrier lines, having a flowering or jacketing effect to the carbon burst. The presents of silicon and aluminum have a tendency to depress the carbon bursts. All said, the best thing to do is make a set of standards to use as a comparison.
The next test is the hardness test. I'm going to repeat the definition of hardness for those of you who think it means more than it does. "It's the ability to resist plastic deformation." Nothing more.
When we push a dent into a material, the material plastically deforms. (See definition) What happens is the crystals of metal move out of the way of the indenter. There are several types of tests but they all do the exact same thing. They push an indenter into the metal with a known load or force. It's simple really. If you push an "X" size indenter into the material to an "X" distance, using a load of "X", for "X" time, than the material must be "X" hard. The softer the material, the further the indenter will penetrate. Harder materials need higher loads than softer materials. There are basically five types of tests. Each has several "scales". The scales are just various sizes and shapes of indenters, with various loads. The five basic tests are Rockwell, Brinell, Shore Sceler, and microhardness methods called Knoop and Vickers.
When reporting a hardness value, you absolutely must report the method. This is a pet peeve of mine and it annoys the hell out of me. If someone says that the hardness of an object is 85 Rockwell or 400 Brinell, I say they don't have a clue to what they are talking about.
For Rockwell, you must report the scale. "85 Hardness Rockwell C, or "85 HRC". Rockwell has several scales. The most common are: A, B, C, D, 15N, 30N, 15T and 30T. Each has a specific indenter and load. For Brinell, you must report the load, indenter diameter and time of loading. A report of 400 HB 3000/10/15 means 3000 Kg with a 10mm ball for 15 seconds.
The Brinell test is done by pressing a tungsten carbide ball into the material, then you measure the impression with a little microscope with a built in scale. With the Rockwell test, the machine does all of the work, and you just report the hardness and method.
The microhardness methods are performed exactly the same way as the Brinell test except that that they are done under a microscope. The "dent" made in the material is measured using a measuring device built into the microscope. Knoop micro-hardness is reported the same way. 300 HV 500. The "500" is the load in grams. Several loads to chose from here too.
The Shore scelerscope method is completely different than the others. What's cool about it is that it's portable. You can fit it in your pocket. The bad thing is that it's not as accurate. What it does is drop a tungsten carbide ball down a cylinder. When the ball bounces back up the cylinder you measure the distance it bounced back. The higher the bounce, the harder the material.
The next test, or tool, is the material's strength. There are several strength tests. Each one could have a chapter for itself. I will discuss tensile strength and yield strength. To perform these tests, a force is applied by pulling on a test specimen called a tensile bar. The bar is loaded into a machine and a load is applied at one end. The bar is pulled at a given rate until it breaks. The machine records the load as it is applied and plots it against the amount the bar stretches. The "stretch" that a bar undergoes is called strain.
As the plot is drawn, you will see the force and the strain go up at the same rate. Suddenly, a point will occur where the amount of "stretch" suddenly increases very rapidly but the force hardly increases at all. This is the yield point. The force continues to rise as the bar stretches until the bar breaks. This is the tensile strength. Tensile strength is the most load that can be applied to a material before it breaks and can be considerably higher than the yield point. .
If you release the force on the bar before the yield point, the bar will return to its original dimension. If you exceed the yield point, the bar will remain permanently deformed. Tensile strength is computed by the force, in lbs, applied divided by the cross-sectional area, in square inches, of the bar. (F/A) Brittle material will have the yield point and the tensile strength near the same point. A ductile material will have the two points much further apart.
Another bit of data you can discover using a tensile test is the Young's Modulus of Elasticity (E). This is the stress, in inches, divided into the strain, in psi. All steel has a modulus of around 28 million psi. Other data points are elongation (El) and reduction of area (RA). Elongation is the measure of stretch in percent of a gauge length. Reduction of area is the amount that the test bar thins or "necks" as the bar is pulled.
The formula for RA is:
Af - Ao
------- X 100
Ao
Where:
Af is final cross-sectional area
Ao is the original
cross-sectional area
The formula for El is:
Lf - Lo
------- X 100
Lo
Where:
Lf is the final gauge length
Lo is the original gauge length
Other tests include fracture toughness, fatigue crack growth, impact toughness, and creep-strength. These are a bit much for this FAQ and are only used by Strength of Materials Engineers. Wear tests, and even a simple bend test are common but... some other time.
4.0 Basic Metallurgy
4.1 Metallurgy of Iron Alloys (Steels)
Iron alloys are the most common ferrous alloy. Steel is a solid solution of iron and carbon. It's called a solution because the carbon is dissolved in the iron. Iron is the solvent and the carbon is the solute.
Steel, like water, can go through phase changes. With water, the phases are solid, liquid and gas. With low carbon steel, the phases are liquid, austenite, and ferrite. If we add salt to water, the temperature of all the phase changes are altered. Salt will lower the transition temperature of the liquid to gas phase change and lowers the temperature of liquid to solid as well. When we add carbon to iron, the temperatures are altered in the same way. The more carbon we add (to a point), the lower the temperature of the phase change will occur. Carbon also creates new phases that don't exist in iron by itself. Pearlite is a mixture of cementite (Fe3C) plus ferrite. The most carbon that can be dissolved in austenite is 0.80%. This is called "eutectic". Other alloys can be described as being eutectic alloys. These alloys have the maximum amount of the alloying element that can be dissolved into the parent material.
The more carbon you add to steel (above 0.20%), the more pearlite you get, up to the 0.80%. Above 0.80%, you get carbides. So if a steel has less than 0.20% carbon, all you can get is ferrite. If a steel has 0.40% carbon, you get pearlite and ferrite. If a steel has 0.90% carbon, you get pearlite and carbides.
To know the chemistry of a steel by knowing its grade remember the following rules: Plain carbon steels are 10xx grades. 10 is plain carbon and the next two numbers are the carbon content. E.g., 1045 has 0.45% carbon. All 10 grades also have Mn, P, and Si. The last two numbers of ALL grades designate the carbon content. E.g., 8620 has 0.20% carbon. The other grades can be found in the table above. Some times you will see a grade like 12L14 or 10B21. The L means it has lead for macheneability and the B means it has Boron for increased hardenability. Stainless steels and other alloys have their own grade alloy numbering system. I will cover that later. The key here is that if you know the chemistry of the alloy, you will know is hardness, strengths and if a thermal treatment will work at all.
4.2 Metallurgy of Tool Steels
Tool steels are highly alloyed steels, each having a special property.
These properties include: wear resistance, hot hardness, and toughness. All of
them are heat-treatable. See the section: Four Digit Alloy Numbering
System, for there general hardenability. Hint, an air hardenable steel is
much more hardenable than an oil hardenable one. They have hardness ranges of
40 to 65 HRC. They generally have at least one alloy, other than carbon, to
give it a special property. An example would be D2. It has 1.50% carbon,
12.00% Chromium and 1.00% Molybdenum. It is air hardenable, has excellent wear
resistance, but has low toughness. In contrast, S5 having 0.55% Carbon, 0.80%
Manganese, 2.00% Silicon and 0.40% Molybdenum, has excellent toughness, but
has only fair wear resistance.
5.0 Heat Treatment
Heat treating can be defined as the heating and cooling of metals or metal alloys in some manner that will alter their metallurgical structure and change their mechanical properties.
Hardening is usually thought of when we say heat treat. But any form of thermal process is a form of heat-treating. The goal of all thermal treating is to induce a phase change, complete a phase change or reduce stresses caused by a phase change or cold working.
Cryogenic quenching or treatments are done to steels to complete the austenite to martensite phase change, but any material that undergoes a phase change upon cooling can benefit from cryogenic treatments.
Tempering is a method to reduce the stresses induced by the austenite to martensite phase transformation, and stress relieving is usually performed after cold working.
Annealing has several categories, and includes spheroidizing, and normalizing. Each of these thermal treatments will be discussed in the next chapters. The rule of thumb is to heat the part and hold or "soak" it for one hour per inch of thickness.
I would like to mention here that any heating of a steel over 1200º F. will cause it to decarburize in an uncontrolled atmosphere. That is to say the exposed surface will lose all or part of its carbon. What happens at heat-treat temperatures is this: Carbon doesn't really like to be in steel. It would rather be with oxygen. If an oxygen atom hits the steel it will form CO. If CO2 hits the steel it will form CO + CO. If water vapor hits it you will get H2 + CO. I have seen decarb as deep as 0.020 inches deep not having ANY carbon and another 0.050 inches deep of partially decarburized steel. FYI, this is written as 0.020FF - 0.25TAD. FF means Free Ferrite and TAD means Total Affected Depth.
Decarburized steel is not good for obvious reasons. It's recommended to heat treat a part before its final dimension. That is to say, allow some tolerance to machine off or remove in some way, the decarburized material.
5.1 Hardening
The steps to hardening steel alloys are to austenitize, quench and temper. Other alloys like copper, aluminum and stainless steel, require different methods.
The only way to harden steel is to undergo phase changes. The first thing we need to do is form austenite. You cannot form any other phase without cooling from austenite. If you quench hot pearlite, you will end up with cold pearlite. If you properly quench austenite, you will end up with martensite. Austenite has the ability to dissolve up to 0.8% carbon. This is because the atoms of an austenite crystal are arranged so that they are much further apart than a crystal of ferrite.
If we have as much as 0.8% carbon dissolved in austenite, and we slowly cool it to room temperature, the dissolved carbon will precipitate out of solution, and form carbides, (Fe3C), in little striped plates alternating with the ferrite called pearlite. If we cool or quench the austenite very rapidly, we will trap the carbon atoms in the austenite crystals when they try to change to ferrite. This phase of carbon trapped in the iron crystal is called martensite.
The rate of cooling needed to form martensite varies greatly, depending on the chemistry of the steel. If you quench austenite slower than what is needed to form martensite, you will form bainite. Bainite is a microstructure between pearlite and martensite. For some applications this is desirable due to its toughness.
The rate of cooling needed for each grade of steel can be found on a Continuous Cooling Transformation (CCT) diagram or Isothermal Transformation (I.T.) diagram. Too many to list here. Bainite is a kind of phase that's in between martensite and pearlite. A steel with less than 0.20% carbon cannot form martensite at any rate of cooling. Should I repeat that? No, SAE1018 cannot be hardened.
Steels with carbon contents in the 0.20% to 0.40% range need water or brine (salt water) as a quenchant. The "speed" of a quench media is determined by the rate of heat transfer from the part to the media and is given in degrees per second. The fact that brine was a "faster" quenchant than water was discovered centuries ago when a blacksmith quenched a part in a bucket of urine. (Goat urine is best!) Higher carbon steels can be quenched in oil. There are several "speeds" of oil, but all of them are much slower than water. Some tool steels are so hardenable that the only quenchant needed is air. There are polymer quenchants on the market today but are not readily available. All quenchants work better if they are agitated vigorously.
A fully hardened steel contains martensite. Martensite is very hard. It can be as hard as 65 HRC. This means its also very brittle. The formation of martensite is so violent and brittle that many times the part cracks. This is called a quench crack. To reduce the stresses caused by the formation of martensite, we must temper it. Tempering is done by raising the temperature of the steel to a point LESS THAN the critical temperature, or austenite formation temperature. Tempering temperatures range from 800 to 1200º F. The higher the temperature, the softer the metal.
5.2 Hardening (Flame & Induction)
All of the same hardening processes happen with this method as with a normal hardening process. But it can only be done in a localized area. You heat the area above the austenitizing temperature and it's quenched. The difference is that the part "self quenches". Heat is applied to a _local_ area using a flame or an induction coil. When the heat is removed, the steel in the area of the heat conducts the heat away from the austenite fast enough that you form martensite. The part must have two things for this method to work. The first thing is that it must be hardenable. The second is that it must have a large mass in the area that can draw the heat away. As for any time you form martensite, it should be tempered. The same method used to heat the localized area can be used to temper it.
5.3 Stress Relieving
This is usually done to a material after it has been cold worked (see below). Cold working imparts a great deal of stress into the metal. This process relieves these stresses. The reasons that cold working imparts stress are way beyond the scope of this FAQ, but sufficeth to know its on the atomic level. This stress increases the hardness and brittleness, as well as the tensile and yield strength. If the stresses are uneven, and they always are, the part will distort as the stresses are relieved. A typical process is to heat the part to 600 - 800º F, soak, and cool at any rate.
5.4 Normalizing
This is a very common form of annealing. The method is to austenitize, than air cool to room temperature. You get...? Yes, pearlite. The biggest advantage is to get a uniform microstructure and to soften up the metal for subsequent operations like machining. Its done after cold-working to re-crystallize the microstructure. The stresses imparted to the metal crystals cause them to "break up" and re-form when austenitized.
5.5 Spheroidize Annealing
This is the softest state that a steel can get. There are several ways to spheroidize. The most common way is to heat the metal to a point less than the austenitizing temperature, and leave it there for up to 24 hours. Then you furnace cool it at a rate no faster than 15º F. per hour. A very expensive process. What happens is that all of the carbon above 0.20%, (remember that up to 0.20% remains dissolved in ferrite), precipitates out of solution into little spheres of iron carbide. Can you Spheroidize 1018? NO!!! (Think about it.) The cementite in the pearlite also goes from plates to spheres. This does two things. With the carbon out of solution and into spheres, the only thing resisting movement in the material is ferrite, and ferrite is soft. The other thing making it soft is that the spheres kind of act like little bearings for the ferrite to move around.
5.6 Cryogenic Treating
Cryogenic treating is usually performed by quenching a part from room
temperature, into a bath of liquid nitrogen. In steels, the temperature where
the completion of martensite formation from austenite can be substantially
low. If this temperature is never reached, the microstructure will contain
retained austenite. After tempering, the microstructure will contain tempered
martensite and retained austenite. A cryogenic quench will finish the
transformation. At this point, the microstructure will contain tempered
martensite and un-tempered martensite. You must re-temper the part after this
operation to relieve the stresses.
6.0 Forging and Forming
6.1 Cold Forming
Cold forming consists of drawing, extruding or otherwise shaping of metal at or just above room temperatures. Just plain beating the hell out of metal with a hand sledge is a form of cold forming. This process greatly increases the hardness, tensile strength and yield strength. It also increases the brittleness of the part. This is true for almost any metal alloy.
6.2 Hot Forming
Hot forming usually refers to the forging process. "Working" the metal while its hot is much easier than cold working but does not do as much work hardening to the piece.
Hot working can be defined as: plastically deforming a metal above the re-crystallization temperature. For steel, this temperature is the austenite formation temperature. To hot work a metal, you first heat the metal well above the austenite temperature. 300 to 400º F over is common. This allows plenty of time for you to form the piece while its austenite. Here is where we can do some "blacksmithing" at home. The process is basically simple. Heat the part above the austenite transformation temperature then beat the hell out of it. You will be able to "feel" if its getting to cold. As the part cools back through the austenite temperature and starts to form pearlite it will become stiffer. Just stick it back in the fire and heat it back up.
Another way to tell that it's too cold is by its Curie point. The Curie point is the temperature where a magnetic alloy becomes non-magnetic or visa-versa. For low-alloy steel, the curie point is 1414º F. A quick touch with a magnet will tell you if you are over the curie point. Typically, you should stay over 1500º F. Of course, there is the color vs temperature method to determine its temperature. Once the part is the shape you want, you can let it cool on its own to form a normalized structure or you can quench it to harden it by forming martensite. Don't forget to temper it.
When you are done, you will have a decarburized layer. The best method for removing this layer is with a file. You will know when you have removed it when the file doesn't remove as much as easily. The decarburized layer is all ferrite. Ferrite is much softer than tempered martensite. I have done this at home with small parts using a torch. If the grade of steel is not very harden able, you will need to quench it in brine (10 percent salt water). If its very hardenable, brine will crack it. Use an oil for these steels. As with any quenching operation, agitation accelerates the quenching operation. I have never done large parts at home. You would need a large hot fire. I would suggest reading a blacksmithing FAQ. However all of the metallurgy remains the same.
7.0 Safety
Although everything should be a "no-brainer" I want to add this just to cover all the bases. A lot of the FAQ discusses heat treatment. We are working with temperatures as high as 2000º degrees Fahrenheit. A heat treated part should not be assumed to be cool enough to touch. A large part can retain heat for several hours. If you think it my be too hot to touch, splash some water on it and see if the water boils. Quenching can splash hot media. Wear protective clothing for all heat treat operations. The minimum recommended is safety glasses, welding gloves and non flammable clothing.
Using oil as a quenchant is dangerous. Oil can catch fire. An all purpose fire extinguisher is absolutely necessary to have close by. If quenching in oil, the part must be fully submerged to at least several inches below the surface. If the part is not fully submerged, it WILL start a fire. Oil fires can be hard to extinguish. When heating or grinding a metal containing alloying elements like nickel or other hazardous metals, a respirator is recommended. Grinding metal can create hazardous dust and the sparks can start fires. If you attempt to cryogenically treat a part, the same protective equipment is recommended. (save the fire extinguisher.)
Suggested Reading
Avner, Sidney H.: Introduction to Physical Metallurgy, McGraw-Hill,
inc.
ASM, Metals Handbook 20 Volumes
Afterword
Questions, comments, and insults regarding this FAQ may be addressed to
misc.survivalism or (I will probably regret this) to monel@sprynet.com I also want to thank
Alan T. Hagan for all his help in releasing and hosting this FAQ.
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Metallurgy FAQ v 1.0 Copyright 1999 Drake H. Damerau, All rights reserved
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