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Valve Material Heat Treatments
By Ted Tunnecliffe
Glad to see you came back after that heavy-duty stuff I threw at you last time. This time it won’t be any better I’m afraid. We’re going to talk about heat treating of valve alloys.
Let’s start with the terms that are used. Like many other things, there is sometimes more than one name for the same thing. We should begin with the terms used for the heat treating of hardenable steels. This includes hardening, quenching, tempering, drawing, stress relieving, annealing, process annealing, strain hardening and cold working. What I have just given you are, in some cases, both the technical term and the "shop term" for several heat treats or conditions.
For example, hardening may also be called quenching, but they are the same. In order to harden a steel you must first heat it up to a pretty high level and then cool it quickly – that’s called quenching.
There are different types of quenches which simply dictate how fast a part is cooled. You can quench in water, which is pretty fast, or you can quench in oil which is much slower. How you do it depends on a number of things and we’ll get into some of those. Hardening of valve steels is typically done at temperatures of 1,500° F or above, but it depends a lot on the type of steel you are dealing with.
What about tempering? The so-called "shop term" for this is drawing but it’s the same animal. In either case, you are reheating a hardened part for the purpose of softening it and relieving internal stresses. We’ll get into more detail on these after we’ve covered some of the basics.
Tempering is normally done at temperatures of 400° F or above depending on how soft you want the product to end up. The higher the temperature the softer the parts will be.
Now let’s talk about stress relieving. This is a variation of tempering. When you stress relieve all you are doing is a low temperature, short time temper. The main difference is that the hardness normally isn’t reduced measurably but the internal stresses in the part have been reduced; this means it is less likely to crack.
Annealing. I’ve noticed that this term gets misused a lot. In some instances it is used to describe a hardening process which it definitely is not. It is just the opposite – it is a softening heat treatment. So what’s the difference between it and tempering? It is essentially a matter of degree. Tempering can be done over a range of temperatures to produce various hardnesses while annealing is typically done slowly at a very high temperature, usually in combination with a slow cool.
This produces a significant reduction in hardness. In fact, full annealing reduces the hardness the maximum amount. With annealing you would typically begin at just below the hardening temperature level, hold the part there for a short time then slow cool it to get a very soft part.
Process annealing. In this case we are tempering or softening a part to accomplish a certain objective. For example, let’s say a steel part has both hard and soft spots in it from some previous operation. This can make it difficult to machine because you keep hitting those hard spots. If we process anneal it we reduce the hardness of those hard spots so they are closer to the rest of the part making the machining easier.
Strain hardening. This is also called cold working. It is called that because it happens at room temperatures rather than at elevated levels as the heat treatments we have been talking about. The technical term is strain hardening. Actually this is not a heat treatment so maybe we shouldn’t even be talking about it, but it can have a major effect on hardness which is why we plugged it in here. Strain hardening is a surface hardening that results from some deformation of that surface by rolling, hammering or any such localized mechanical loading.
Although we are talking about steels here, relating it to soft copper tubing is a good way to understand it. You know that when you bend soft copper tubing the first time it bends easily. But now try to straighten it out. Sure you can do it but it is much more difficult now. The reason is that you have work hardened or strain hardened the material during that first bending. The same thing happens to steel, but it is not quite so obvious. Different kinds of steel have different degrees of susceptibility, but we’ll get into that later.
Okay – back to the beginning. Let’s talk about just exactly what happens during quench hardening. First we’ve got to have steel. Steel is an alloy of iron and carbon, but not too much carbon or it will be cast iron. Steel can dissolve just so much carbon and, after that it won’t dissolve so it will be present in the metal usually as graphite. Graphite, by the way, is great for machinability, but lousy for strength.
We talked about "dissolving" carbon so let’s explain that. When steel is heated to harden it, and remember we said that had to be to about 1500° F or higher, it goes through what is called a "transformation." This means that the molecules in it change their form.
The transformation takes place during heating and is a change in the structure of the molecules that make up the steel. It starts out as what we call a "body centered cubic" (BCC) molecule – an atom at each corner and one in the center — and transforms into a "face centered cubic" (FCC) molecule. Figure 1 is a sketch of the BCC structure.
If you took physics in high school remember how they talked about atoms and molecules and all that jazz? And remember that the electrons that help make up the atoms have orbits? A lot of little electrons are zipping around their nuclei. The sketch in Figure 1 just shows the atoms, but doesn’t show the electron orbits so you have to imagine that more is needed for each atom than is shown.
Visualize the fact that those atoms have taken up all the space between them so that the molecule is completely full. Got it? So now you say "So what?" Well in order to understand what is really going on, we have to know all that stuff. Now let’s talk about that "transformation". As we said, a change is going to take place when we start heat treating to harden the steel and that change will have several effects. As we heat up the steel it will get to a point where some weird things happen; we call that the "austenitizing" temperature. It was named after some guy named Austen who discovered it.
Anyway, at that "magic" temperature the steel changes and it loses its magnetic properties. Don’t ask me why, all I know is that it happens. In addition, the molecules change from BCC to face centered cubic (FCC). Now the molecule has an atom at each corner and one on each face of the cube. This means that it has 14 atoms rather than nine as in the BCC shape. It‘s larger because of this and bigger, too, because we heated it up causing it to expand thermally. In addition, it is now hollow — there is no atom in the center. So where is all this going? Hang in there it will all make sense soon. I hope it will anyway. Figure 2 shows what the molecule looks like now.
Okay so now we’ve got a big, hot molecule with a hollow center. Well, because of these things, that molecule will now "dissolve" the carbon that is in the steel, but outside of the molecules. We call that type of dissolution an "interstitial" solid solution. You really needed to know that didn’t you?
It might help to remember some of the terms but that one you can forget because you will probably never use it again. All it really means is that the carbon is inside the molecule. But one thing you should remember — this is taking place in the solid state — not as a liquid even though we talk about dissolving and solutions. We are talking about a solid solution. Okay — so now we’re getting someplace.
So what happens next? Glad you asked. Now that we have the carbon dissolved, we want to keep it there. If we slow cool the metal the carbon will just sneak back out of the molecule and we’ll be right back where we started. So we need to trap it in there and we can do that by quenching or rapidly cooling it so it doesn’t have time to get back out. Neat, huh?
So what happens when we quench it? Well we now have the carbon trapped inside the molecule. Upon cooling, it is going to transform back into the BCC shape. The only trouble is – there is no room for the carbon in that smaller molecule.
As we said earlier, the atoms of steel (or iron actually) take up all the space available. Since the carbon is trapped in there it is going to stay, but because there is no room for it, the molecule will become distorted. So we now have a tetragonal shaped molecule. That is, it is deformed BCC shape. Figure 3 shows what I mean.
So now we have hardened steel. That’s what happens when the molecule gets distorted like that. This distortion causes the molecule to be under a lot of stress because it wants to be back to its original form, but it can’t due to the carbon being in the way. Those atoms are all trying like crazy to get back to that BCC form so they are exerting a lot of pressure. Now if you check the hardness it will be very hard. After all what is hardness? It is a tiny point pushing a dent into the metal right? That means moving those highly stressed molecules and they ain’t about to move easily.
Heat treat cracking
So why do steels sometimes crack when they are hardened? Good question. When we quench the steel we usually put it into a liquid which cools it rapidly. As the part is going into the quench, one side hits the quench first — right? That side will begin to transform back to the BCC structure while the other parts of the piece are still in the FCC state. Since the BCC is physically smaller, it can be so highly stressed that — bing — you’ve got a crack. There are other reasons, too, of course, but that’s a biggie.
So how do we avoid cracking? Well first of all, use the right quench. And as I’m sure you know, there are many different quenches which simply amount to different rates of cooling. This therefore means keeping more or less carbon trapped in solution depending on how fast the quench rate is.
Hope you’re still with me. I know this stuff can get a bit hairy, but I think it helps in the understanding of heat treating if you know the fundamentals of what is happening inside the steel. And, believe it or not, we have just scratched the surface.
This is going to be a lot easier. In tempering we are reducing hardness and stress. Remember we said that tempering was accomplished by heating the part back up? It is, but it is after hardening. If you do it before hardening then it would have a different name like stress relieving or annealing. So what is going on? When the hardened part is heated up it expands a little. Because of that expansion the molecules are a bit bigger and that lets the dissolved carbon sneak out of the solution.
Also, as we said before, the higher the heat – or longer the time – the more carbon will sneak out and the softer the part will be. But we don’t want to go above the transformation temperature (the austenitizing temperature) or we will just reharden the piece. Now that wasn’t so complicated was it?
Now that you have the fundamentals of hardening, the rest are easy. In this heat treatment we start with an alloy that is austenitic (and therefore non-magnetic) or FCC already. This state is present at room temperature and doesn’t change as the temperature goes up.
Because of this FCC condition the steel can dissolve things in addition to carbon that may be outside the molecules such as carbides and carbonitrides. These are just compounds made up of what are called "carbide formers". Such compounds are usually metals that have an affinity for carbon and they include iron, aluminum, titanium, niobium, molybdenum and even nitrogen, as well as a number of others.
The compounds are formed, perhaps in the scrap from which the steel was made or in the molten steel as it is being made, and they are soluble in the FCC molecule. It is important to realize that, especially in the U.S., the so-called "scrap circuit" where we reuse metal by remelting it to make new alloys, can contain little bits of many different metals. We truly reuse our scrap in a very efficient manner — much better than in many countries. Anyway, because of this we have a lot of those carbide forming elements in our steels.
So, getting back to the subject, in order to solution treat an iron based alloy, you must heat it up to a very high temperature and hold it there for a short period while those compounds dissolve. Since the alloy is already FCC it will not transform so you don’t have to worry about cracking at quench, even in water, as you would with many steel alloys. And when it cools quickly like that we trap those compounds in solution by thermally shrinking the molecules. The temperatures we use are usually over 2,000° F in order to expand the molecules so they can dissolve the carbides and carbonitrides. And the quench is usually water because it cools the parts very quickly. By the way — most exhaust valve alloys are austenitic and I’ll tell you why in a minute.
So why do we go through all this rigmarole? Well what we are trying to do is to strengthen the alloy. As you know there are many different kinds of strength like hardness, tensile and fatigue to name a few. The type of strength that we are primarily interested in with exhaust valve alloys is high temperature fatigue strength. Because of that concern, austenitic valve alloys are typically solution treated before being put into service.
But one of the peculiarities of this heat treat method is that, while increasing fatigue strength tremendously, it actually softens the metals because we have dissolved those hard carbides. But we don’t normally want to put a part in service in this condition because it wouldn’t wear worth a damn. So now what? Of course — we’re going to give it another heat treatment.
Another term for this is "aging" and it is done for a couple of purposes. One is to get some of the hardness back as we said earlier. And we can do that without significantly reducing the hot strength. Although it does reduce it a little, it is so little that it is difficult to measure. But the hardness will go up from perhaps 20 Rc as solution treated to about the middle 30s Rc after aging. Again as we said before, the increased hardness gives the part much better wear resistance and resistance to deformation. The other benefit is stabilization.
Any time things go on inside of a material, there is usually some effect on the outside. This aging treatment is no exception. It not only does the things we have already mentioned but, in addition, it actually shrinks the size of the part. For example, on a valve, the overall length would be perhaps 0.003" to 0.004" shorter than before the valve was aged. So, if you didn’t do this heat treatment before you put the part in service, you might wish you had. We just picked out one characteristic to illustrate our point, but the whole part is going to be dimensionally affected. But that is what we mean by stabilization.
I think we have given you enough to chew on for this time so we’ll say "so long" and be back at you next time with an article on valve designs.
Ted Tunnecliffe is the president and owner of Met Tech, Inc., Engine and Metallurgical Engineering Consultants. Previously, Tunnecliffe was a chief engineer for the aftermarket at Eaton Engine Components. In this capacity, he worked on valve engineering projects for most major North American OEMs, as well as a variety of international clients. Tunnecliffe brings more than 45 years of experience in the areas of valve materials, designs and processes to the readers of Automotive Rebuilder magazine.