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Valve Alloys-What Makes Them So Special?
By Ted Tunnecliffe
Those of us who have been in the valve business all of our career tend to forget that everybody else hasn’t. So in this article we’re going to try to cover some of the basics as to just what types of alloys are used for valves and why. In future articles we’ll get into various other valve associated topics.
In the first place, most valve alloys are not stainless steel. In fact, in the U.S., there is only one that we know of that is truly a stainless steel used by original equipment manufacturers (OEMs). That was not true a number of years ago when several stainless steels were used.
Sure, if your valves come from overseas, and if the supplier hasn’t caught up with the latest materials, they could very well be stainless steel products. But as time went on during the early development years, it was found that we could modify a stainless by changing its chemistry somewhat, to come up with an alloy that was much better than the original stainless.
When we see someone claiming to use stainless steel valves, either they are behind the times, or they are not really using a true stainless - with that one exception of course. That alloy incidentally is 422 stainless which Eaton Corp. was instrumental in developing. I’ll tell you that story in a future article.
Valve alloy families
There are mainly two types of valve steels used these days. One is called martensitic steel by metallurgists, and the other is called austenitic steel. Don’t be afraid of the fancy metallurgy terms - if I can learn them, anybody can. Superalloys are also used, but we’ll talk about them later; technically speaking, they’re not steels anyway.
A martensitic is simply one that can be hardened by heating and quenching. An austenitic is one that cannot be hardened in this manner. In addition, the martensitic steel is magnetic and the austenitic is non-magnetic, so it’s easy to tell the families apart.
So what’s the big difference?
I hope you don’t think that I am taking the long way around, but if you understand some of the background, you’ll see how we’ve gotten to where we’re at and why we got there.
One of the big differences in valve alloys is caused by the typically different operating characteristics of intake and exhaust valves. Remember that intake valves operate at about 1000° F maximum; exhaust valves typically run about 1450° F maximum. Valve alloys are selected for their properties at these conditions.
If you hear of someone using exhaust valve materials in intake holes, this isn’t necessarily good. In fact, it usually isn’t good at all. Because of the lower operating temperature, intake alloys are actually much stronger than exhaust materials at intake temperature levels. Now let’s take a look at the relative strength of intake and exhaust alloys.
One of the best and easiest ways of measuring strength is by hardness. Generally, the harder a material is, the stronger it is. Also, remember that room temperature hardness is only one condition and isn’t where the valve normally operates so we should also know what its hardness is at elevated temperatures.
There are a number of other ways to check alloy strength, but let’s just look at hardness for now. We’ll get to the others at another time.
The curves in Chart 1 show the relative hot hardness of intake and exhaust valve alloys. As you will note, we are checking the hardness at a temperature which starts at room temperature and goes upward to the levels normal for intake valves and exhausts. Remember, hardness is a definite measure of strength as well as wear resistance.
The alloy classes shown in Chart 1 include the typically martensitic intakes, the usual austenitic exhausts, the superalloys and the hard facings. The superalloys are only used on exhaust valves if nothing else will work. The hard facings are welded onto valve seat faces to improve wear and corrosion resistance. Because the classes of valve alloys that are used for different purposes also have different characteristics, these curves show four families of materials and the range of variation within each.
For example, you’ll notice that the hard facing alloys are at the top of all the groups and pretty much stay that way at almost all temperature levels. So why not use them for everything? Good question. The answer: they are too darned expensive. Typically, they’re either nickel or cobalt-based and these metals are not cheap. Compared to the iron-based alloys, they can easily be several times as costly.
Martensitic alloy compositions
Now let’s look at the iron-based martensitics, or usual intake valve alloys. The typical light duty alloy is SAE 1547. So what do all the numbers mean? It’s not as complicated as it sounds. The "15" is for the type of material (which is 1.5% Mn, but does not relate to that) and the "47" is for the points of carbon. This simply means the percentage. In other words, the 47 in 1547 is for 0.47% carbon. Chart 2 shows some of the others:
The 1547 is the most commonly used passenger car intake valve alloy in the U.S. The heavy-duty engines use another alloy that we will talk about shortly. The alloying contents in any of these valves amounts to only a total of about two percent or less as you can see, and it is there mostly for processing reasons rather than any effect it has on the performance or durability of the valve. It really just makes them easier to heat treat.
Martensitic steel hot hardness
So what’s the real difference between the hot hardness of the martensitics? Actually not very much. As you can see by the curves in Chart 1, they all start out quite high in hardness at room temperature. That 35-45 Rc level, by the way, is used because that is where it will be as the valves are put into service, not at their maximum heat-treated hardness as the other groups are. By the time the temperature gets to 1100°-1200° F they are all really soft and have lost most of their strength.
Austenitic steel hot hardness
Now take a look at the austenitics. As we said, they are typically used as exhaust valve materials. The main reason being that they tend to retain their strength or hardness at elevated temperatures much better than the martensitics do. At the lower temperature levels, however, you can see that the martensitics are definitely harder and therefore better.
That’s why it usually doesn’t make a whole lot of sense to use exhaust valve alloys in intake holes. Of course, there is a lot more to it with regard to other properties, but this hot hardness data is very important.
Superalloy hot hardness
These materials are not steels, and like the austenitics, they are also non-magnetic. They are nickel-based alloys of several metals. They usually contain a lot of chromium, roughly 15-20%, plus other metals that are added to improve their high temperature strength. Such elements as titanium, aluminum, tantalum and niobium are added in very small amounts. But these little amounts have a big effect in the improvement of hot strength as you can see from Chart 1.
The chromium in superalloys is mainly there for one purpose - corrosion resistance. That is the same reason it’s in the martensitics and austenitics as well. It does add hardness when it combines with carbon, but that is not its major use in valve steels. Its main purpose is corrosion resistance.
Martensitic stainless steels
Maybe it would help at this point to back up a little and take a look at our comment earlier about the fact that stainless steels don’t usually make good valve materials. They don’t because of one unfortunate fact: carbon and chromium like each other too much. This affinity will cause the chromium to be combined with the carbon in the steel so it can’t form enough chromium oxide to protect the surface. That is what "stainless steel" is all about.
Generally, magnetic stainlesses have low carbon, except for valve steels. Another big exception are the "cutlery" grades, i.e., the stuff they make stainless steel knives out of. It has a lot of carbon as well as chromium in it to get the hardness as well as the corrosion protection necessary just as we do with valve alloys.
So martensitic stainless steels are alloys of iron, carbon and chromium and it’s the chromium that gives the corrosion or oxidation resistance. It takes about 12% of chromium or more to prevent rusting. The carbon, if it is there in the right amount, will also allow it to be hardened, and now we will have a martensitic valve steel that is similar to that cutlery grade we spoke of earlier.
The only actual stainless steel valve alloy that is used in the U.S. by an OEM engine builder is 422 stainless. Chart 3 shows its nominal composition in percentages of the significant elements:
The strongest intake valve alloy available at 1000° F is 422 SS. Its strength may not always be necessary, however. For most heavy-duty intake applications an alloy called Silcrome 1, or just Sil 1, is typically specified. It holds its strength better at elevated temperatures than the other intake alloys we showed you earlier. Its composition is a lot like 1547, except that it has 3.25% Si and 8.5% Cr, so technically it’s not a stainless.
What about exhausts?
So now we understand a little about why we use intake alloys in intake holes but what about exhausts? On that side of the engine, as we said earlier, the operating temperature is considerably hotter. Since most of the intake alloys fall on their faces above about 1000° F, and we saw how the exhaust alloys stay harder there, why is that? The main reason is the composition of those materials. Let’s take a look at a couple of common ones.
Exhaust valve alloys
The most popular exhaust valve alloy used in the U.S. by far is an alloy called 21-2N. With certain exceptions, such as high performance or heavy-duty engines, 21-2N is it. This is an austenitic alloy (see you’re getting used to the terms already!) made up of 21% chromium, two percent nickel and two percent nitrogen. You say nitrogen is a gas? What’s it doing in a valve steel? We’ll get to that.
21-2N was derived from an alloy called 21-4N. The two are pretty much the same except that 21-4N had four percent nickel rather than only two percent. Why change? Because nickel is a rather expensive alloying element and if two percent is almost as good as four percent why not cut down the cost, especially if the engine can’t tell the difference? This is essentially the case.
So how did 21-4N come about? A number of years ago, when I was just a kid in this business, we used leaded gasoline in this country almost exclusively. Leaded gasoline is made by adding tetraethyl lead to gasoline to up its octane. When the engine ignites that gasoline, the tetraethyl lead also burns producing lead oxide and leaves it behind as a deposit on any parts exposed to it. That obviously includes the exhaust valves.
The deposit has a tan or yellowish color so you can recognize it if you see it. Figure 1 shows a combustion face of an exhaust valve corroded by lead oxide. The center area is black because the lead oxide burned away leaving a corroded black spot. In Figure 2 the throat or underhead area of that same valve is shown. This valve appears to have stretched, but it did not. The reduction in diameter is due strictly to corrosion.
As we all know, almost any reaction tends to be accelerated if heat is added and exhaust valves are certainly hot. Lead oxide is very corrosive and corrodes the valves, especially in their hottest areas, which are the center of the head, and in the underhead area.
The exhaust valve alloys that were available at the time were stainless steels highly susceptible to corrosion. They would actually "rot" away with exposure which shortened their life. That is until one smart cookie at a steel company came up with a new alloy that was more resistant to corrosion. He found that if he reduced the amount of silicon in an alloy he could improve its resistance to lead oxide corrosion.
Silicon is often intentionally added because it helps oxidation resistance. But that is resistance to corrosion from oxygen, not from lead oxide. In fact there still is a whole family of "Silchrome" alloys based on that principle.
Silicone may be added to certain things to expand them but valve alloys aren’t one of them. So, anyway, the steel company patented 21-4N and it quickly became the most commonly used exhaust valve alloy until 21-2N was developed as a less expensive version.
Exhaust valve compositions
There are a lot of exhaust valve alloys used around the world, but let’s just concentrate on the principal ones in the U.S. This means six alloys - two for light- and medium-duty and four for heavy-duty.
One thing you have to be aware of, however, is that nearly all of the heavy-duty jobs are of bi-metallic construction. That is they have an austenitic or superalloy head material and a hardenable or martensitic steel stem.
Most of the light-duty jobs are single-piece valves. Chart 4 shows the nominal chemical compositions of those six alloys. The first two are light-duty and the last three are heavy-duty. 23-8N is an "in betweener."
We’ve already talked about why we use a number of the different elements in valve alloys, but we haven’t covered everything. For example, we mentioned earlier that nitrogen was used in a number of valve alloys which seems rather peculiar given that it is normally a gas. Here is the explanation.
Usually retained austenite is a bad situation, i.e., in most steels we really don’t want it even in very small amounts. But in exhaust valve steels we want it all to be austenite to take advantage of its superior hot strength.
There are certain elements that tend to retain austenite in a steel if they are present. There are actually four: carbon; nitrogen; nickel; and manganese. They all have differing strengths with regard to that tendency.
We know that carbon must be present to give us strength, so that’s a given. The others may or may not be added. But if we want to keep the alloy as austenite to take advantage of that hot strength tendency, we need the austenite retainers.
In addition, certain ones like nitrogen tend to combine with carbon to form what are called "carbonitrides" and they add strength and hardness to the alloy as well as to retain austenite.
But there is a limit to just how much can be dissolved, so we add some amounts of other austenite retainers like nickel and manganese. They also have less tendency to retain the austenite, so we have to add more of them.
For instance, manganese has only about half the strength of nickel so we must add double the amount to do the same job. And it costs less than nickel. Take another look at Chart 4 and you’ll see what I mean.
Hard facing alloys
The hard facing alloys, as we said before, can be welded onto valve seat faces, usually exhausts, to give them wear and corrosion resistance. Historically, there have been only two families of hard facing alloys, cobalt-based Stellites and nickel- based Eatonites.
Recently during the cobalt crisis in the late ’70s, the price of cobalt went from about $6 a pound to about $60 a pound in a very few months. The reason was that most of the world’s cobalt comes from Zaire and Zambia in central Africa.
A civil war, as I remember it, resulted in a work stoppage and so, with no more of this material coming into the pipeline, the price went sky high. I was still at Eaton at the time, and having most of the heavy-duty valve business, we stockpiled weld rod in our vault. If I remember right, we had about $4 million worth at one time. That situation stimulated a very active program in the development of nickel-based alloys to replace the cobalt types.
Hard facing alloy compositions
As we’ve said, historically, there have been two basic groups - cobalt and nickel types. Chart 5 shows the nominal compositions of the most popular of those valve facings.
You’ll notice that Eatonite 6 is not cobalt or nickel-based, but rather iron-based. Let me expand on that story of the cobalt crisis. When the program was undertaken to develop new nickel-based hard facings, we also thought about intake valve hard facings since there were a few such applications.
Again, because of the operating temperature difference, typically lower alloy properties were believed to be tolerable. So we looked at the possibility of an iron-based alloy for such applications and developed Eatonite 6 for that purpose.
One of the interesting things about this business is that just when we think we know it all, something jumps up and bites us. I don’t even remember why we tried it, but we put some Eatonite 6 faced valves into exhaust holes and they worked like a charm.
So, all of a sudden, we had an alloy that is considerably less costly, but works about as well as the more expensive materials.
There are just two types of valve stem coatings used in the U. S. One, and by far the most popular, is hard chromium plating. The other is nitriding, and only one engine builder that I know of uses it.
Chromium is popular for several reasons. It’s hard, it has a low coefficient of friction, it’s a low temperature process and it contains surface cracks which are at a controlled frequency which aid in retention of lubricants. Figure 3 shows the chrome plated surface at 400x magnification. It can be applied in several thicknesses depending on the durability required.
Nitriding is a high temperature process (about 1100° F) and the coating is very thin, hard and wear resistant. It is a cyanide producing process so it must be very carefully controlled. It is also alloy limited in that it will not take on some valve materials.
That about wraps it for this time. In our next article we plan to address some of the details of heat treating of valve alloys. Again, we’ll probably start with the basics of hardening, tempering, annealing, etc., and then move into some of the more complex methods.
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.