This is the first of a planned series Ive been working on together with Terry (38Chevy454), the official HAMB metallurgist, on the fundamentals of metallurgy as it relates to our hobby. Terry and I thought that we would try to make a series of articles and discussion topics on some of the basic metallurgy that hot rodders are dealing with fabricating, purchasing and using metal. We are not going to be able to make you a metallurgist, but we hope to add to your knowledge so you can make better decisions. There are also many old time myths that we hope to point out and tell the truth to prevent these old wives tales from continuing. This first post will focus on the basics of what metal is. Well get into more detailed topics later. Im going to focus primarily on steels, as that is what 90% or more of what we deal with on hot rods is made from, but the ideas apply to all metals. So grab your favorite beverage and lets see what we can learn. As liquid metal cools, many small crystals begin to form. As the metal cools further, the crystals grow, until it eventually forms a solid composed entirely of many small, tightly packed crystals (also called grains). The solid has less volume than the liquid, so you get some shrinkage during solidification. This is opposite of water where solid water (ice) has larger volume than liquid. In metals the shrinkage from liquid to solid contributes to distortion in welding and also to porosity in castings. Back to our discussion on the packed crystals. Within each crystal or grain, the atoms are arranged in a very specific pattern, called the crystal structure. You have all heard the expression It crystallized and broke, well that is total BS. Metals are always crystal structure when solid. The crystal structure configuration may change, and that has significant effect on the properties, but it always has a crystal structure. To picture the arrangement of atoms in a crystal, imagine a stack of cannonballs, where each cannonball represents one atom of a material. There are different patterns that the cannonballs (atoms) can be arranged in. For example, when starting a new row in the stack, you can place the cannonballs in the divots between each ball in the row below, or you can stack them directly on top of the balls in the row below. In a metal, the specific pattern that the atoms arrange into depends on the type of atoms (for example: iron, aluminum, or titanium), and the arrangement of the atoms determines many of the properties of the metal. When a metal is flexed or stressed enough to cause deformation, some of the atoms inside each grain slip past each other. In some crystal structures this slip can occur more easily, which results in a less strong and more ductile metal, and in other crystal structures the slip is more difficult, resulting in a higher strength material. The boundaries between each grain help resist the deformation, thus smaller sized grains will result in higher strength. Normally the grains are very small; a typical grain size for steels would be approximately 0.002 across. Grains are not shaped as exact cubes or round balls, they are irregular shape. The crystal structure alignment within a grain is all the same orientation, but neighboring grains will have different orientations, which also contributes to making the metal stronger. A good visual example to imagine the grains and the crystal structure is rock candy, which has the various grains and the different crystal orientations easy to see. (see attached microstructure image). When you heat or cool a metal, you can cause the atoms to rearrange. This is called a phase change, and dramatically changes the properties of the metal. Most steel that we deal with is not heat treated and is in a phase called ferrite. In this phase, the arrangement of the atoms is Body Centered Cubic (BCC), which is a structure in which it is relatively difficult for the atoms to slip past each other, so the metal is fairly strong. If you heat steel to high enough of a temperature (approx 1400F), you begin to form a different phase called austenite. The arrangement of atoms in austenite allows them to slip much easier, resulting in a much softer and more ductile material. The specific arrangement of atoms in austenite, called the Face Centered Cubic (FCC) structure, is the same as that in other soft metals like aluminum, copper, gold and lead. The arrangement of the atoms in BCC and FCC structures are shown schematically in the attached images. As a point of information, FCC structure in steel is non-magnetic, so red hot steel is not magnetic like it is at room temp when it is BCC structure. Quenching steels from high temperatures can result in other phase changes, which will be discussed in a later topic on heat treating. Steel used for automobile components is a mixture of iron plus up to about 1% carbon, plus small amounts of other alloying elements. Carbon is by far the most important alloying element in steel. Carbon is very efficient at increasing the strength of iron. The carbon atoms, which are smaller than iron atoms, fit into the spaces between iron atoms, helping to lock them in place and giving a higher strength material. Carbon can also combine with other elements such as chromium or vanadium to form extremely hard carbide (ceramic) particles within the steel, which increases hardness and wear resistance. Tooling is a common application for carbide steels. Too much carbon will create problems with welding. Pre-heat and post-heat requirements as well as controlled cooling rates can be required. Fortunately, most all of the pieces that we use for fabricating brackets and frames are low carbon and can be welded without any troubles (at least due to chemistry!) Other alloying elements can be added to control specific properties of the metal such as strength, toughness, and corrosion resistance. Some elements are also added during the steel making process to control tramp elements (such as sulphur, phosphorus, and oxygen) that tend to reduce the toughness of the steel. Alloying elements can also affect the way in which steels respond to heat treatment, which will be discussed later. Some of the other most important alloying elements for steel are listed below, together with their effects: Manganese: Helps to control sulfur, which causes brittleness in steels Chromium: increases strength, forms hard carbides, increases corrosion resistance Nickel: Increases toughness (resistance to fracture) Molybdenum: Increases strength, helps maintain strength at higher temperatures Silicon: helps deoxidize the steel, oxides can cause lower strength and toughness Vanadium: helps control grain size, helps maintain strength at higher temperatures Titanium & aluminum: both help to deoxidize, and also help control grain size A list of topics that are planned for future articles include: heat treating stainless steels cast iron corrosion protection: plating, coatings, hydrogen embrittlement the metallurgical properties of different product forms, forged vs cast vs rolled cold rolled vs hot rolled welding machining nondestructive testing: magnetic particle, x-ray, ultrasonic, penetrant failure analysis, why did it break? Feel free to offer any other topics that you are interested in. Terry and I will try our best to explain the metallurgy behind it and any concerns you should be aware of. We will try to keep the technical content down low enough so you can follow, but have enough detail to still be able to understand why and how the metal behaves or reacts. Have fun reading and understanding this first introduction, feel free to ask any questions. Look for the other topics to follow at approx one per every week or two.
You can bend A36 to an inside radius of 1.5 times the thickness. I stamped some ribs in a work part and hardness testing (related to strength) showed the tensile strength increased from 66,000 PSI to 100,000 PSI. As it cold works, the material becomes less ductile, i.e. the spread between the yield and tensile decreases. Calculations put the yield at 92,000. I was happy.
What exactly is drop forging? I see this on tools usually. What does it do? Also, I have been curious about when you should quench and when not. When to quench with water versus oil. Does the weight of oil have a different effect?
Check this out: http://science.howstuffworks.com/question376.htm Check out:http://www.metalsmith.org/pub/mtlsmith/V21.3/TTTCURVE.htm Due to the grain structure created in drop forging, crankshafts that are drop forged are stronger than crankshafts that are cast. The quenching of steel is a science all to itself. The more you read about it the more you will want to know. It's like a metallic miracle.
This is outstanding. Somebody was paying attention in their Material Science and Machine Design courses.
The ideal environment for heating and cooling steel is a large room with a high ceiling, with all of the doors and windows closed so the air is relatively still. The high ceiling allows the heat to rise instead of baking you. Hot steel should always be kept out of moving air. Since the grain structure of steel is no longer stable when it has been heated to yellow or red, the still air in the room minimizes any unwanted changes in the grain structure of the work. For the same reason, keep a box of sand nearby to put the hot parts in after the work is finished so they can cool down gradually. When you move the parts to the sand, move them through the air slowly. Dave http://www.roadsters.com/
Thanks to all of you for the ideas, see some info below: Phil, your bending and increasing hardness is an example of work hardening. We will try to cover some of that under the heat treating topic, which will also include methods to make metals stronger or harder. Frank, wait for the tech on forgings and it will be covered. Real quick, drop forging is just the technique to shape the metal, but it all falls under the generic "forging" process. Quenching will be covered under the heat treating topic. Sliderule, el gringo and myself are both metallurgical engineers, so we will try to help spread the knowledge. Without causing you to overload on real specific details, to understand the basics. Hopefully you will learn enough to make good decisions on your own, or know when to ask one of us for expert help. Terry
that was great, thank you both for putting this together please let the whole series go in the tech archives thanks again
El Gringo, thanks for spending all this time on metallurgy. I have a question on the tensil and shear strengths of stainless steel vs the chassis bolts like grade 8 and such. Reason being, I want to use stainless steel bolts and nuts where ever possible on my A. I live on the coast and needless to say, corrosion and rust is a major problem. You can't really put chrome parts on your engine and car because in a year they'll be rusty and you just can't keep up with it. So I go the ss route on everything I build, however, now I'm worried about the t and s strengths from what I've been reading around the HAMB. As always, thank you for your opinion and help. It may mean the difference between a pleasant cruser and a roadside POS. Adios ya'll!
Why has single-order heat treating disappeared? I have a steel crank I want heat treated, but cannot find a service. Everyone tells me nobody does heat treating in the US anymore. Sorry if I'm jumping ahead.
Thanks for the feedback everyone. As Terry mentioned, we are already planning to address these questions at a later time. I know this post was a bit lacking in practical information, but the ideas here are important for when we get to the more detailed topics like heat treating. This wasn't originally intended to be a Tech Week post, its just that Tech Week finally got me motivated enough to get it done. Aman, while its true that most stainless bolts will be inferior to Grade 8's, there are options out there that will meet or exceed the properties of Grade 8. However, these options will be significantly more expensive. For an example, take a look at the material property charts on the ARP website: http://www.arp-bolts.com/Tech/TechMetals.html Bolts made from Custom 450 have higher strength than Grade 8, and I expect would also have much greater toughness and ductility. However, the Custom 450 steel alone will cost 10x or more than the steel used to make grade 8, and the processing is also much more expensive. If you want to spend the money you can get something that will work, but the stainless bolts you buy at the hardware store will likely be inferior to Grade 8. I can give you some assistance in evaluation the options if you want. Repoman, heat treatment is alive and well in the US, however finding a shop that will handle one piece probably is fairly difficult. I work with a shop here in Chicagoland that does cranks and camshafts for NASCAR, but you would be looking at easily $1500 to do one piece, and the total price would be the same if you did 50 pieces. You might take a look at a company called Bodycote, they've been buying up most of the independent heat treaters in the U.S. One of their locations may have the capability and desire to do your crank, or they might be able to point you to someone that will. Keep the questions coming.
Thanks guys. I had to go into the archives to find my old book from University so I can follow along. Terry, It looks like your Alliance membership trumped your official metalurgist title.
That's OK, I think most on here, at least that have been around for a while, know me as the HAMB metallurgist. Hopefully with the series El Gringo (Ryan) and I are doing it will help to consolidate this info in one place, rather than the misc posts where I have added metallurgy to the answers. BTW, congratulate El Gringo as it was his idea to make this series, and his input has as much time and value as mine.
Wow. I'm really looking forward to the rest of this series. I'm sure that this will be covered, but I'd really be curious about what makes MIG welds so much harder than TIG or oxy-gas welds. For now, I just accept it as part of life, but the curiosity in me wants to know WHY.
Uhhh, would you believe just busy on other things? Thanks for the reminder, I will try to get a new "lesson" started.
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