Stronger than ductile iron, capable of withstanding high speed pounding and loads that would destroy piston rings made of ordinary grey cast iron, and able to be formed into extremely narrow bands for bridging gaps between pistons and cylinder walls. Yes, we are talking rings of steel. Automotive top compression rings that is.
Steel compression piston rings have been used for 30 years or more in many heavy-duty truck applications because steel is the best material for withstanding the extreme loads experienced inside high compression turbocharged and supercharged diesel engines. But steel rings did not make the transition to automotive applications until the auto makers started downsizing ring dimensions to reduce friction and weight. The Japanese were the first, switching to steel top compression rings about ten years ago. Ford and General Motors followed suit, using steel in a couple of applications (the Ford 1.9L and Buick 3800 V6, then more and more applications as engines have been downsized and made more efficient.
To understand why the change to steel is taking place, we need to take a closer look at the basic materials from which pistons rings are made.
Grey cast iron is a perfectly adequate ring material for most passenger car applications as long as the rings are of sufficient size to handle the loads. But the change to thinner low tension rings combined with efforts to squeeze more power out of smaller displacement engines has increased the operating loads on the rings, especially the top compression ring that receives the brunt of the punishment. Uncoated gray cast iron is compatible with cast iron cylinder walls and will not gall or scuff, but it is also brittle. Bend a ring that is made out of gray cast iron too far and it will snap. The material has little "give" because of its microstructure. When you examine the grain structure of gray cast iron under a microscope, it has sharp rectangular grains that easily fracture if the metal is shock loaded or bent too far (a good reason for always using a ring expander when installing rings on a piston).
With narrow low tension rings (1.5 mm or 5/64 inch) gray cast iron rings can break if the engine is subjected to heavy or continuous detonation. The hammer-like blows produced by the colliding flame fronts shock loads the rings and can break them resulting in a loss of compression, cylinder damage and oil consumption problems. Though this danger can be minimized to a large extent in engines with computerized engine controls by using a knock sensor to retard spark timing, there is no guarantee it can protect the rings under all circumstances.
Various alloys of gray cast iron are available, including "intermediate" alloys that are somewhat harder (28 to 38 HRC) and stronger. Rings made out of these alloys are used uncoated for the second compression ring in many engine applications as well as the top ring in two cycle engines. Chrome or moly coated intermediate gray cast iron rings are also used for top compression rings.
That brings us to "ductile" iron rings. Ductile iron has been used for years for heavy-duty truck gas and diesel rings because ductile iron is roughly twice as strong as gray cast iron. Ductile iron is also called "nodular" iron because its microstructure contains rounded or nodular shaped grains. These increase strength and allow the metal to bend without breaking. Consequently, ductile iron compression rings can take a lot more pounding than gray cast iron rings without breaking. In fact, you can bend a ductile iron ring like a pretzel and it will not snap. That is why the domestic vehicle manufacturers have been using ductile iron compression rings in many turbocharged and high output engine applications in recent years. Ductile iron has also been a popular choice for racers because of its ability to hold up in a high rpm, high stress racing environment.
But ductile iron is more expensive than gray cast iron. Ductile iron is also not as compatible with cast iron cylinder walls as gray cast iron. It tends to scuff and gall unless it is faced with chrome or moly.
Although ductile iron rings are definitely superior to gray cast iron, they are not always necessary. Ductile iron rings are used in all turbocharged engines and most of the engines developed since the early 1980s with 1.5 mm top compression rings to reduce the danger of ring breakage. But the higher cost of ductile iron does not always required in such applications. There is absolutely no advantage to using high strength ductile iron in an engine that does not require it. It is like putting 100 octane fuel into a car with a 7.5 to 1 compression ratio.
The one advantage ductile iron does offer is that it has a lot of bending strength and is very resistant to breaking. It is also has greater hardness, but this does not necessarily mean it is more wear resistant. Most of the abrasives that cause premature ring wear will wear a ductile iron ring just as fast as an ordinary gray cast iron ring. It is the moly or chrome coating on the ductile ring that helps retard the wear rate.
Most aftermarket piston ring suppliers use ductile iron top compression rings in their replacement ring sets for engines that were originally equipped with the same type of rings. Others offer less expensive standard gray cast iron rings.
How do you tell a ductile iron ring from one that is made of gray cast iron? You cannot tell by appearance alone because both materials look the same. In addition, both kinds of rings are factory coated with a protective black phosphate coating. So the only way to tell one from the other is to either bend a ring to see if it breaks (in which case it would be gray cast iron), or to see if it "rings." A gray cast iron ring will makes a dull thud if tapped or dropped on the floor. Ductile iron (as well as steel), however, rings like a bell.
The next step up from ductile iron is steel. Ductile iron is roughly twice as strong as grey cast iron, but steel is roughly twice as strong as ductile iron. So steel rings can really take a pounding without failing.
Here is how the three alloys compare:
Material Hardness Tensile Strength Fatigue strength
Grey cast iron 22-23 HRC 45,000 psi 30,500 psi
Ductile iron 38-40 HRC 180,000 psi 87,300 psi
Steel (SAE9254) 44-53 HRC 240,000 psi 138,600 psi
As you can see, steel is harder, has a higher tensile strength and higher fatigue strength that either ductile or grey cast iron. How this actually translates into ring strength and wear resistance depends on the size and shape of the rings themselves. But generally speaking, steel rings provide:
Steel piston rings can solve a lot of problems in highly stressed engines. They are stronger, harder, seal better and resist breakage and wear under load. They are ideal for any application that involves higher combustion temperatures, higher compression loads and tougher emission standards. "The SAE 9254 high alloy steel that some ring suppliers use in their rings also lowers engine oil consumption because a lighter ring provides a more effective seal against the bottom of the ring groove. The smaller cross section, permitted by the greater strength, also improves the ability of the ring to conform to less-than-perfect cylinder bores. And compared to ductile or cast iron, the inherent strength of steel creates less chance of ring breakage. Steel also provides longer service life and a reduction in ring side wear and ring groove pound out.
Steel has become the ring material of choice among many racers, including most NASCAR engine builders.
Like ductile iron, steel is not compatible with cast iron cylinder walls, so it must be coated with either chrome or moly, or gas nitrided. The rings are made from preformed steel wire, much in the same fashion as the steel rails for oil rings. The wire comes from the steel supplier coiled like a Slinky, which is then cut to form the rings. The rings are slightly distorted (like a lock washer), however, from being coiled, so after they are heat treated and shaped the sides must be ground flat. The steel ring is then chrome plated or face coated with plasma moly that is inserted into a recess in the face of the ring.
Most of the steel rings currently in production have a width of 1.2 mm (0.047 in.). Some are as small as 1.0 mm. Such rings are found in many late model Asian, European and domestic engines . The 1.2 mm rings are about as thick as two oil ring rails stacked together, so there is not a lot of space to machine a groove for a moly facing. That is why the rings are usually chrome plated or gas nitrided.
The amount of machining that is required to finish a steel ring is far less than that which is required to finish gray cast iron or ductile iron rings, so steel rings are actually less expensive to manufacture, at least in large batches. In smaller batches, however, the special wire that is required can be costly.
Some piston ring suppliers say there is no reason to use steel in a full size ring. Steel lends itself best to the narrow low tension ring applications because it is too stiff for the wider rings.
Steel rings are usually barrel faced, having contoured outside diameters which gives the ring a center contact with the cylinder wall. The extremely narrow 1.0 mm rings usually have a tapered face.
Most of the ring manufacturers we interviewed say steel piston rings are being used and more and more late model engines, and is the ring material of choice for demanding applications such as turbocharged engines, performance engines and diesels, especially for the top compression ring. Like ductile iron, it is very resistant to breakage.
According to most ring manufacturers, steel and ductile iron rings can be considered virtually interchangeable as far as rebuilding most passenger car gasoline engines is concerned. So if steel replacement rings are not available for a stock engine you are rebuilding, you can usually substitute ductile iron piston rings. But performance engines are another matter. In these kind of applications, you should always replace same with same (steel with steel).
You should never substitute ordinary gray cast iron rings for ductile iron or steel top compression rings in a late model engine because the cheaper rings will not hold up. Many of these newer engines are designed around the ductile or steel rings they use. If you do not use the right type of ring, you may have ring problems 20,000 miles or so down the road.
Along with the change to steel rings for high output engine applications, many Japanese and European engines use rings that have been treated with gas nitriding. Gas nitriding (which should not be confused with the black phosphate coating that is currently used on most rings to prevent rust during shipping and storage) is a heat treatment process that impregnates the surface of the metal with nitrogen to case harden the metal. When used on piston rings, it case hardens the entire surface of the ring to a depth of about .001 inches which greatly improves its resistance to side wear as well as face wear. Gas nitrided rings have a hardness of about 1100 on the Vickers scale which translates into about 68 HRC which is almost 50% more than steel rings and four times that of gray cast iron rings! The rings are so hard that ring wear is virtually nonexistent. In fact, the cylinders will wear out long before the rings will.