Click on a thumbnail to see the full-size image
How to Make a Valve
By Ted Tunnecliffe
The steps involved with manufacturing heavy duty and passenger car valves
During the 2000 calendar year, you read a considerable amount about the design, testing and operational considerations of engine valves. To complete the educational process, we will discuss a very important step in the lifecycle of a valve – its manufacture.
When developing the general processing steps needed to make a valve, the very first steps are to decide what alloy should be used and how to heat treat it. That’s not as easy as you might think. We need to know the operating conditions that the valve will be exposed to before we can make such a judgment.
Such things as temperature, stresses and corrosive environment are very important if we are to provide a part that will do what it’s expected to do for the length of time it’s supposed to. If we don’t have that information we tend to overdesign just to be safe and end up with a valve that is in great shape when the engine is scrapped because all the other parts in it are shot.
We also need to know the nature of the various valve alloys as well, which means there is a lot of homework to do before we even begin to consider just how we will actually make it. If you’ve been reading these articles in Automotive Rebuilder (See June, September and December 2000 issues) you know that we have dealt with many of those issues already.
If we take a look at the testing and manufacturing steps needed, this is what we get:
Material – What alloy or alloys will best suit our needs? What about coatings – stem, head, seat face?
Heat treat – How shall we heat treat the valve and where on the valve should we do it?
Forming – Forged or cast? If cast: sand, hot wax, permanent mold, etc.? If forged: cold headed, gathered and upset, or extruded and upset?
Construction – One-piece, mid-stem welded, tip welded, internally cooled (and closure method), seat welded?
In-process handling – Automated, semi-automated or batch processing?
Inspection details – In-process and final inspection? Frequency and standards?
Let’s start by looking at each of these to see what we will do and why. To do this we are going to have to make some assumptions or we’ll be here all day.
We’ll look at a typical passenger car exhaust and also at a heavy-duty exhaust valve for comparison. Exhaust alloys are the toughest to work with which is why we chose them. Also, that will give us a broader look at valve manufacturing in general.
The passenger car exhaust is relatively simple and the heavy-duty job is among the most involved from the viewpoint of manufacturing and from the application and durability standpoint as well. But there will be a lot of commonality between the two types.
See the Assumptions chart for those things we "know" about our valves. That’s a lot of assuming, but you have to start someplace.
So now we’ll get down to the nitty gritty. And, I’m going to throw out some possible alternatives in many of the steps and let you decide what you would do in each case.
We’re being a little facetious about this because you don’t have nearly enough information to make decisions, but just play along. Also, the steps below don’t necessarily line up with the basic steps referred to earlier.
After we’ve decided on the alloy and bought the material, it will come to us as bar stock, probably in about a one inch round size. It will have a "cert" with it (certificate of analysis) that tells us, in addition to the exact chemical composition of every significant element, a number of the physical and mechanical properties of this actual "heat." A heat is the batch of material that was poured and rolled at the mill to our particular requirements. That heat will probably be somewhere between 40,000 and 80,000 lbs. The cert may also tell us the tensile strength, hardness potential, grain size or any number of characteristics of this exact batch of material. We will keep the cert as a part of our permanent records for possible future reference.
What about the surface condition? Should it be hot rolled, which means there will be some scale on it, or should it be centerless ground? A hot rolled surface can tend to hide seams and such so if our customer is at all fussy about the parts he gets, we had better use ground stock since there is less of a chance of surface imperfections getting by us. Hot rolled stock may be quite acceptable in many cases and is certainly less costly than ground material. What do you think?
Since we already know how we’re going to forge it (by extrusion and upset method – see assumptions), we now need to know how long the slug should be that we’re going to forge it from. That means calculating the volume of the finished valve and adding material to allow for forging and machining losses. If we don’t add enough, we can end up scrapping a lot of parts simply because they didn’t fill. If we add too much we’re not only wasting material but also metal removal operations will take longer.
Then we must decide if it will be better to shear the material or to cut it off with an abrasive wheel. That decision will relate to what the final finish condition of the valve will be. If it is to have a non-machined head face, the original "slug pattern" (or diameter of the forging slug) may be visible and sometimes customers don’t like to see that.
In addition, if there is a burr on the slug going into forging, it may produce a lap of material on the combustion face and that can be death to the engine because it can cause preignition. On the other hand, shearing is much faster than abrasive cutting and therefore a lot less costly. Either cut-off method will use fully automated handling. So what did you decide to do?
Forming the valve
Let’s take a closer look at the forming options and why we decided to forge by extrusion.
Casting is a truly inexpensive way of forming parts because you can do a lot of them at a time. One pour and you’ve got a whole bunch of blanks for machining. So why isn’t it used more? For a number of reasons – and I’m not going to give you a choice as to casting or forging. The valve is going to be forged.
Probably the biggest reason that the casting process is not used is its lack of inherent quality. Remember that a forged part whether it is a valve, a piston, a con rod or whatever, is generally much stronger and for good reason. When the alloy is rolled at the mill from that huge, white hot ingot that starts out maybe three feet in diameter, it goes through many passes to get it down to the size we need. Those passes are essentially squeezing the metal together so that if there are any internal openings, blowholes, porosity, and so forth, they don’t stay there very long. Not so with a casting. There are more likely to be such openings in a casting and they can be real trouble.
In this method the valve is forged to shape without even heating the material up. It will be hotter after forging, but that is only from the energy of metal movement. Such forming can be done on relatively small valves like single cylinder lawn mower sized engines, but it is not usually practical for larger valves. The alloys used on larger valves simply do not lend themselves to this forming method so that’s out.
Gather and upset forging
OK, so we’re going to forge it. By what method? If we gather and upset it, we start out with stock that is a little larger in diameter than the finished stem. We heat one end to the forging temperature and "gather" it into a knob.
The character of that knob is a very important item. If it is smooth all over it should make into a good valve. But if it is a little wrinkly on the surface, when it is upset (forming the head shape) it can produce those wrinkles at the interior of the head.
We call those internal indications of metal movement "flow lines" and they tell us just how the metal moved as it was being formed into the part. If the flow lines are wrinkled it will usually take place at about the mid-radius of the head – right where the valve is under the combination of high stress and temperature when running in the engine. And that can be deadly since those wrinkled lines represent a weakened zone.
That gathered red hot knob is then put into a closed die shaped like the final valve head and it is smacked with a ram that forces it into the final valve head shape. It may have to be reheated before this forging blow or it may not. If it is still in the forging temperature range of the alloy, which can be pretty narrow with some materials, you may get away with just one heating.
Two blow impact extrusion forging
This is the other method by which valves are made. In it, we start out with stock that is perhaps an inch or so in diameter, heat it to the forging temperature and drop the slug into a closed die which has a small hole in the bottom. The hammer comes down whacking the slug and forcing part of it through that bottom hole. That portion will eventually become the valve stem.
It’s sort of like squeezing toothpaste out of a tube. You start out with a large diameter and squeeze it to a smaller one. But don’t try this in your bathroom.
Now we have a still red-hot forging that has a small diameter at one end and a larger knob at the other end. In fact, at this point, it looks much like the "as-gathered" part does before it is upset. The forging is now moved to a second die and smacked again "upsetting" it. This second blow is done in much the same way as it was in the gather and upset method but there is usually only one heat required.
The advantage to this method of forging is that the flow lines are always well shaped. You don’t have to depend on an inspection step to assure a good forging. The disadvantage is that extrusion requires rather expensive dies that may not last too long. So extrusion is generally more costly than is the upset method, but it produces product of a more consistent quality.
In both of these forming methods the handling is usually automated or semi-automated. Inspection checks both the dimensions, done on the floor, and the internal characteristics done in the met lab.
Heat treatment, straightening, car tip welding
For the passenger car types of valves, we will typically solution-treat and age after forging, then straighten. Straightening of the head and the stem is required because the valves will distort from the forging operation, but it may be done in a different sequence.
For example, you can straighten after forging or after doing the solution treat but before the aging, or you can straighten after the full heat treat. The valve can also distort from the heat treating so it may be a tough call. However you decide to do it, the valve is now ready to begin the tip welding and machining operations.
After welding the tip disk on, usually by resistance welding, the tip must be hardened. It will usually be fully hardened to give it the needed wear resistance the austenitic or non-quench hardening head alloy does not have. You may or may not stress relieve it after hardening – it’s up to you.
The heavy-duty valve is where we separate the men from the boys. Since we have assumed a mid-stem welded construction, we have only an extruded head to deal with at this point. But it must be cut to the proper length so that the weld ends up where we want it.
Shear or abrasive wheel cut? Another decision to be made. The head will be treated much like the passenger car part – solution treated and aged. No straightening will be necessary at this time, but will usually come later after the stem has been welded on. But on this heavy-duty job we now have to heat-treat the stem portion so we will harden it – remember it is a quench hardening steel as opposed to the age hardening (or non-quench hardening) head material. And we may temper it to the specified hardness (usually 30-40Rc) at this point, or temper it after the welding operation. Which do you want to do?
Back to passenger car valves for the moment. We now have a valve that is forged, heat treated, straightened, tip welded and tip hardened. So, essentially, only the machining operations remain.
We’re only going to hit the high points here and not all the subtleties that are involved. Some of this machining will involve turning and some will be grinding operations. Generally the more precise metal removal operations will be grinding.
The seat face will always be a ground surface. You may have a rough cut made by turning just to remove excess stock, but the precision required at this sealing surface, at least in my experience, is always done by grinding. We’ll usually take at least two operations to grind this surface, sometimes three. Watch out for out-of-roundness as well as runout on the seat face.
We could probably do an entire article on dimensional characteristics and the techniques used in determining them, but we don’t have the space here. So let’s just say that the dimensions, microfinish and roundness are very important, especially on this and the stem surfaces. The same is true of the cylinder head seat but, because these are valve articles, that’s somebody else’s problem.
The combustion face may or may not be machined, and it can be either turned or ground. If it is not machined, the valve will be less costly but more tolerance from the seat face to that surface will be required. In all diesels that I know of, for example, the combustion face is machined due to the close proximity of the piston because of the high compression ratios normal to such engines. I know of one in which, if you stack up the tolerances, the piston to valve clearance can be as close as 0.014˝.
The head O.D. is always machined, but it can be turned or ground. The important point is that a radius or chamfer should always be present at the outer corner of the combustion face to avoid any possibility of inducing pre-ignition.
The underhead area may or may not be machined. And, once again, it can be turned or ground, but turning is more common. That is particularly true with the advent of computerized lathes that vary their turning rates with the different diameters so that the tool is always moving against the stock at an idealized cutting speed and therefore giving excellent surface finishes.
The stem, which requires a very precise clearance with the guide, will always be ground, at least in my experience. It will probably be done in several passes and end up with a diametrical tolerance of a thousandth or less. Roundness is also essential here. Remember that roundness and runout are two entirely different things, but each is very important. A flash of chromium plating will be added to the stem at a point near the final manufacturing operation. Flash chromium is only about 0.000005˝ thick but it helps resist stem scuffing and galling, especially on these sticky austenitic exhaust valve steels.
The keeper groove is another important area. On valves made in the U.S., this surface is normally ground, but I have seen imported parts that are turned and that can be trouble if they’re not really smooth. Remember that this area, because it is the smallest diameter on the valve, is highly stressed. Turning marks are stress raisers that can lead to fractures, so be wary of turned keeper grooves.
The valve tip surface is usually very highly stressed unless the valve gear is a direct acting Type I system. All the other types have a radius or line contact with the rocker pad or other opening gear. True, the tip surface is normally under compressive rather than tensile stress, but it must be very smooth and flat — flat relative to the rocker pad or other opening mechanism – in order to avoid premature wear. And also remember that wherever there is compressive stress there must also be a related tensile stress. We’ll be talking more about that later in another series of articles on valve failure analysis.
Heavy-duty valve stem welding
When we left this subject earlier, we had gotten to the point of having forged and heat-treated the head and stem. Now what do we do? Well, since we don’t have the two pieces together yet, we’d better see to that.
The most common methods of stem welding these days are friction or inertia welding, and they are both very similar and sophisticated systems.
They are essentially the same, except that in friction welding the two pieces being joined are continually pushed together as only one of them is being rotated. The pressure causes the friction, which raises the temperature to the melting point, so that welding can take place. When the weld is complete, the rotation stops and the valve is removed. Timing on this operation is critical.
With inertia welding, on the other hand, a large flywheel is brought up to speed and then released from the driving mechanism. The inertia in that wheel produces continued rotation that makes the weld as the two pieces are pushed together.
With either system, there is no external heat added. The friction of contact at the point where the pieces touch produces the heat for the weld. Pretty clever, huh? I seem to remember that the Russians were the first to develop this system back in the ’60s. You have to give credit where credit is due.
Prior to these methods, resistance welding was the technique used. In a resistance welder, the two pieces were brought together touching each other and the juice was passed through them. Sparks fly all over the place as the joint is heated and the welding takes place as the pieces are pushed together. This method still has some limited use especially on very large valves.
Heavy-duty seat face welding
In our assumptions we said that a Stellite hard facing would be applied. Now is the time to do that.
There are essentially two methods of applying a seat weldment that are in production use today. One, and by far the most common, is the plasma transferred arc (PTA) system, and the other is the oxy-acetylene method. One common thread between the two is the fact that a recess is cut into the seat area to receive the weldment in either case.
In the PTA method a powdered metal is used and the "plasma" melts it. Plasma is a superheated gas formed by an internal arc ionizing it and producing that "fourth state of matter." The melted powder flows onto the valve through an arc struck between the welding head and the seat face. A cover gas protects the work surface from oxidation. It is an automated and very rapid welding system with the operator simply loading, unloading and visually inspecting parts to confirm their acceptability.
Oxy-acetylene welding is a hand operation done in a method that requires several preheat and post heating stations. An operator uses a solid weld rod that he melts in the torch flame and drops onto the rotating work piece thereby applying the weldment itself. In this method the operator is much more in control and is therefore much more of a factor.
Heavy-duty valve machining
Even though there are a lot of similarities in machining operations with either kind of valve, there are certain dissimilarities and we’ll touch on those. A major factor is the different head and stem materials that are necessary to deal with in our heavy-duty valve example.
The first metal removal operation that needs to be done is to remove the weld flash in the middle of the stem. It can be sheared (friction weld only), turned or ground off, but it has to go.
If the valve was friction welded the flash will look like a couple of rings, a smaller one of head alloy and a larger one of the stem material, near the center of the stem. If it was resistance welded the flash will not be nearly as neat in its appearance. It will be rough and have a spattered appearance. Either method can give good welds, but there are some advantages to the friction method and one is that you don’t have to provide that external heat.
After removing the weld flash we can now proceed with the remaining machining operations. Those different head and stem alloys cause some headaches for the manufacturing guys because the two machine so differently. The head material is comparatively gummy, tending to load up grinding wheels or stick to machining tools while the stem alloy machines much easier. When the stem is being machined, one side of the weld may be one diameter and the other side a different diameter.
Another machining problem can be the seat facing. Such alloys are not easy to machine because they are very hard, which is of course one of the very reasons we put them there. They are also very corrosion resistant but that has nothing to do with their machinability.
Inspections - routine and special
The rest of the processing is pretty much the same for both types of valves, and that includes inspection.
All manufacturing operations are normally under statistical process control (SPC) and all parts get a 100% visual at final inspection. SPC requires that sampling be done at each step of processing to confirm that the particular operation is running normally and not producing defective or questionable parts. In many cases this inspection is fully automated.
There are a number of inspection techniques used for stem and seat weld examination such as ultrasonic for internal integrity and fluorescent penetrant for possible surface imperfections, but outside of those the visual and dimensional stuff is pretty routine. Just what is done, and its frequency and acceptance levels, are established earlier in agreement with the customer.
Today’s ultrasonic units are much more sensitive than was the old x-ray used years ago. At the time the simple x-ray was the only technique available so that’s what was used. Today x-ray still has limited usage for examination of internally cooled valve cavity characteristics and coolant content, but not much more.
In-process and/or final inspection checks are made for evidence of residual magnetism in the parts, microfinish at various surfaces, eddy current of tip hardness, and seat face sealability – and those are only some of the non-destructive tests.
Any questions about alloy make up or dissolution at weld points, heat affected zones, residual stress from seat welding or tip hardening may be addressed by emission dispersive x-ray (EDX) analysis – an attachment to the scanning electron microscope (SEM) or other special tests. In addition, the lab may do microstructural checks for grain size, forging flow lines, weld penetrations, dissolution of carbides and various other internal characteristics of the valve.
That’s it for this time folks. We hope that you understand valve manufacturing and material content from these articles and have a better understanding of the valves that should be used in your specific engine rebuilding applications.
You may e-mail Ted at email@example.com.
Passenger Car Exhaust Valve
Material - Solid (not internally cooled) 21-2N with a tip of SAE 8645.
Heat treatment - Head/stem alloy will be solution heat treated and precipitation hardened (aged). Tip alloy will be fully hardened.
Forming - Forged by the extrusion and upset method.
Construction - Single-piece head and stem but containing a tip welded on between the keeper groove and the end of the value. Stem will be flash chromium plated.
Handling - Fully automated processing.
Inspection - Statistical process control applied throughout, plus 100% visual inspection at final.
Heavy-Duty Exhaust Valve
Material - Solid 23-8N head with SAE 8645 stem, Stellite seat facing and heavy chromium plated stem
Heat treatment - Solution treated and aged, plus hardened stem and rehardened tip.
Forming - Forged by extrusion and upset process.
Construction - Two-piece mid-stem welded with seat facing weldment.
Handling - Non-automated processing.
Inspection - Statistical process control applied throughout plus 100% visual at final.