Well it has been a while since the first "Metallurgy for Hot Rodders" lesson 1: Introduction to Metallurgy. As a help to our fellow hot rodders, two HAMB metallurgists, El Gringo and myself have tried to provide a basic understanding of some metallurgy principles. In order to help you understand enough to recognize potential areas for concern. This lesson is on heat treating and so sit back, read it up, ask questions and hopefully we can all learn a bit. Heat Treating for Hot Rodders What is heat treating? Heat treating is a basic term that means using heat to alter the metals properties. Heat treating can be used to make the metal stronger and harder, to lower the strength and make the metal more ductile, to increase corrosion resistance, or other reasons. Heat treating of hot rod parts is typically done for higher strength and higher surface hardness. For iron-based metals (steel for example) there is a direct relationship between hardness and strength, as hardness goes up, so does strength. Not all metals have this direct relationship; aluminum is an example where strength does not follow directly with hardness. In order to understand heat treating, you must first go into a little technical science here in this lesson. Carbon is the primary element that adds strength to steel when it is heat treated. All steels have some carbon, and the more carbon, the more potential strength you can get. Chemistry tells us that there are several forms that carbon can take in the iron. What we need to know to properly heat treat steel is at what temperatures do these different forms exist for a certain carbon content. The iron-carbon phase diagram gives us this information, and serves as the basis for all heat treating of steels. The point of the iron carbon phase diagram is that you have a solubility of carbon in the iron. In other words how much carbon can the iron hold at a given temperature. Without getting too involved, notice the line going across at 1333 F (723 C), labeled A1. That line is the austenitic transformation line. Now notice the line going up to left at approx 45 degree angle, labeled A3? All heat treating for strength requires the steel to be heated above this A3 line, so that complete transformation to austenite is made. Above this line all of the carbon in the steel is dissolved into the iron, and available to provide strength when the steel is quenched to room temperature. See the vertical line at 0.83% carbon? That is the eutectoid line, do not worry about the technicality of it, it is the carbon limit for most steels you will run across. You really only need concern with the portion from 0.83% and to the left. Quenching from this high temperature above A3 forms martensite, which is the hard strong structure you want for high strength or hardness. In as-quenched martensite all of the carbon that you were able to dissolve into the austenite is still trapped in between the atoms, basically pre-stressing the steels crystal structure at a microscopic level. Because of this pre-stressing, as-quenched martensite is very strong, but also very brittle. Therefore after quenching martensite must be tempered (heated up to some point below the transformation temp and held for 2-4 hours typically) to bring down the hardness and bring up the ductility by drawing some of this carbon out to form small carbides. A file is very hard and strong, but brittle. Not a good choice for a spring where the part has to have some ductility. Nearly all heat treated steel parts are tempered martensite structure. If you heat a tempered martensite part up beyond the tempering temperature, it will lose strength as higher temperatures cause more carbon to be removed from the marteniste. Why heat treat? Heat treating is done most often to make the metal stronger and harder. Wear surfaces such as gears need high surface hardness. Bearings are also examples of hard surfaces. Springs are typically heat treated, whether leaf or coil. Any bolt of grade 8 or higher will be heat treated. Grade 5 is typically not heat treated , but stronger than a grade 2 due to the cold working process of forming the bolt head and roll formed threads. Heat treating provides the means to alter the material properties so that the part can serve the function for the life of the assembly or vehicle. However, heat treating can also be performed to make a metal more ductile so it can be formed easier. You all have heard of heating up steering arms to drop them? This is an example of heat treating in the simplest sense. A part that undergoes multiple forming operations may require an in-process anneal, to remove the work hardening and make the part more ductile again. Annealing is a heat treat operation that is used to make the metal soft. Annealing can also be used to remove residual stresses that have developed in the metal, such as those from welding. This is (obviously) referred to as a stress relief heat treatment, and is typically done at lower temperatures than a full anneal, such that the actual structure of the metal is unchanged. How to heat treat? There are many ways to heat treat. The most common commercial practice is a furnace with some type of gas environment. The type of gas environment can affect the surface condition of the metal. Pure air will actually pull carbon out of the steel surface (decarburize), making the surface less hard, which in most cases is not desirable. An atmosphere with excess carbon can diffuse carbon into the steel, making a harder surface, which is common for gears or shafts that have wear surfaces. Putting carbon into the surface is called carburizing. For most alloy steel parts the atmosphere is controlled to match the carbon in the steel, so that no real change in surface carbon occurs. Heat treating can also be done in a high temperature liquid, typically a molten salt. A vacuum can also be used as the atmosphere (although technically vacuum means no atmosphere) with special sealed furnaces, resulting in a very nice surface finish on the heat treated parts. Furnaces can be batch type, processing a load at a time. Think of your oven in your house, this is a batch type process. Furnaces can also be continuous, with a long belt that parts get loaded on one end and then through the high temp section, quench and then out the other end. This is typical for high volume production. Lets talk about quenching. Once the steel part has transformed to austenite we quench it to form martensite. Quenching is typically done in water, oil, or glycol. Water is fastest, glycol next and oil is slowest. Depending on the carbon content, too fast of quench can cause cracking. If you do not quench fast enough, you will not form martensite and the part will be low strength and too soft. The alloying content in the steel will strongly effect this range of proper cooling rates for heat treatment. Some highly alloyed steels will actually quench fast enough in air to form martensite, such as air-hardening tool steels for example. Most steels require a faster rate that is provided by a liquid to form martensite. You need to know the metal you have to determine the proper temperatures and quenching procedures to get the desired end result. Tempering is almost always done in an air atmosphere, as the temperatures are typically not high enough to cause any decarburization problems. And air is free, any other atmosphere cost money. A typical tempering time is around four hours, but can vary widely from this time depending on the alloy and the desired result. Who heat treats? Commercial heat treaters exist in any industrial area. Many companies have in-house heat treating capability. Heat treating to form martensite on a part you fabricated is not something that can be typically done at home. You can anneal at home on small pieces, especially the stress relief heat treatment. How do I use this knowledge? Since heat treating is not something you can do at home, and us hot rodders like doing stuff ourselves, we need to understand what heat treating does for us and how to use it to our benefit. This means that if a part is heat treated we need to be careful to not mess up the heat treatment. This means heating a spring up to lower your car is a bad thing to do. By heating red hot, you destroy the tempering and cause the martensite to be transformed to regular old non-heat treated ferrite and pearlite structure, thesame form of steel as most of the steel the rest of your car is made from. Which means it is low strength and will not work as designed. It also means that of you heat up your steering arms (not heat treated form factory) to drop them, do not quench them and potentially make them heat treated. You want steering arms to be ductile and bend, not break. Slow cooling is best, still air and no liquid. Another example would be to alter radius arms, such as later Ford with spring in front when you split the radius arms. Another thing to avoid is welding on any part that has been heat treated for high strength. The heat from welding will mess up the tempered martensite structure and you have the same problem as the heated spring.
I'm gonna have to read this a few times over! Thanks for all this info! I've been looking forward to Part 2 ever since part 1 was posted!
Here is Lesson 1 for those that may have missed it or want a refresher. http://www.jalopyjournal.com/forum/showthread.php?t=124334
I for one want to thank you guys for doing this. This is very valuable information. There have been may heated debates in the fabrication world about heating steel to form it and what it does to the strength. Ron Covell (for one) shows the heat and form method of bending steel tubing in his videos. What are your thoughts on this? It has always been my understanding (and experience) that if you are looking for a soft (ductile) weld on steel, Oxy/Acy would be your first choice closely followed by TIG. MIG welds cool too fast and therfore cause the welds to become hard. But what about strength? What is the heat from Oxy/Acy and TIG welding doing to the surrounding metal in the HAZ? Is a MIG weld strong and hard or hard and brittle due to it's rapid cooling? For all of the previous questions, I am solely refering to mild steel. Along these same lines, and I know that this has come up before but I feel that it should be included here, why do some say that it is OK to weld 4130 without post heat treating it and some say you do not have to post heat treat? I would also like to hear your thoughts on Oxy/Acy welding of stainless steels in terms of what it does to the structure of the steel in the HAZ. I know that these questions are not exactly about heat treating, but they are about the effects that heat has on metal so I hope they are fair to ask.
Nicely written. It's been a few years since my undergrad classes in Material Science and Strength of Materials - a good refresher. While on the subject of transforming the grain structures via heating...do you have any experience with the deep-freezing process used to strengthen the material? Quite a few drag-car runners seem to be using it but I've yet to really read anything substancial from anyone with a true engineering background. -Chad
The deep freezing, usually using liquid notrogen at -320 F, is just a method to ensure that you have complete transformation from austenite to martensite. Some steels are more susceptible to having a small amount (approx 1-2% or less) of austenite, even at room temp after quenching. So the deep freeze causes the untempered martensite to become marteniste. What can happen is the retained austenite can transform to untempered martensite due to stress or even from the heat energy of tempering. What you do not want is to have untempered martensite in a part, recall that is brittle. There is also a slight size differential between austenite and martensite, so there is a problem for real close tolerance parts as well.
Thanks for the answers. I think that maybe I have worded some of my questions incorrectly. Let me try again. Let's lump these two into the same question since they are really the same question asked twice. and What I meant was, is the metal in the HAZ , whether through proper welding or simply heating with a torch, changed to the point of altering it's strength enough to matter in the real world? I do realize that super heating or boiling the metal will have adverse effects but I'm refering to proper welding or heating for the purpose of bending. What I actually meant was, is a MIG weld strong and hard OR hard and brittle. In a previous answer, you said: I think that was the answer I was looking for with this question. So 4130 can or cannot be safely welded via TIG or Oxy/Acy without a post heat treatment? What I'm getting at is say I Oxy/Acy welded a piece of stainless. What are the properties of the metal in the HAZ after the weld is complete? I know that this is a lot of questions but I REALLY appreciate you taking the time to answer them for all of us. It really helps to have someone like you guys around to help us neanderthals.
I'll see if I can shed any more light on some of these questions. Can the HAZ be altered enough to matter - absolutely. Think of it this way: accross the HAZ the metal has been exposed to a variety of temperatures. Right at the fusion line of the weld the temperature is at the liquid point of steel. Moving further out, the maximum temperature decreases. You end up with a band of material around the weld that has been heat treated to very high temperatures, then fairly rapidly cooled. In higher alloyed steels, such as 4130 this gives a region that is as quenched martensite, which is extremely brittle. Hence the recomendation for a post weld anneal on that material. How important that anneal is depends on a alot of factors, including the application, section thickness, weld practice, etc. We'll cover the metallurgy of welding in more detail in a future lesson. When heating for bending, it depends on the material and how hot you get it. Without knowing the starting material, the starting heat treat condition, I can't really predict how much it will change the properties. Without getting into too much detail (since it will be covered later), when welding stainless you can essentially over temper some of the base material in the HAZ. The carbon combines with the chromium, reducing the amount of chromium available to provide the "stainless" properties. This is the sensitization Terry mentioned.
So what about dropped axles? Will they be fine if you cool them slowly in sand? Oh, and thanks guys for putting forth the time and effort to do this!!!!
All hail, the Gods of fire, molten iron and sparks have spoken. Take this knowledge, go forth and make hotrods... Does my heart good to see a phase diagram again. Great job guys.
its possible to heat treat at home(to an extent), all you need is your O/A setup, get it going like your going to cut something., dont use the full flow oxy lever, . then, you want to get the part cherry red then keep going till the metal has sparks coming off of it, keep it up to temp for a 30 sec to a 1 min but dont melt the part. after the time is up, go ahead and proceed to quench with you method of choice. ( Torch Hardening ) is best for smaller pieces being the fact that you can hold the temp more even over the smaller area
Great Metallurgy lesson. Easy to understand too. When you get a chance, please explain why a cast iron dropped axle may be (or may not be) as good as a forged dropped axle. For instance Magnum makes a cast iron axle which is supposed to be a high quality product with enough ductility and other physical properties to be safe and not break.
Probably the biggest benefit to the cast iron part is the price. Strengthwise, it may be adequate, but that depends on the type of iron. Good 'ol grey iron tensiles between 20-40,000 psi but Ductile Iron on the other hand can be as strong as 100,000 psi ultimate strength. The forged steel could be in various strengths too, depending on the type of steel. The question becomes what type of iron and forged steel are being compared.
I have a question. I want to make hub cap clips. I could buy them for about $50 for all 4 wheels, or make them myself if I knew what to do to make them springy. So, if I bent some let's say 1/8" x 3/4" mild steel (or?) strap into hub cap clips, how could I treat them to make them springy? Or, could I buy spring steel and bend it? Thanks, Kurt
The Jalopy Journal
