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 have followed suit, using steel in a couple of applications (the Ford 1.9L and Buick 3800 V6).
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 has two drawbacks. One is that it is more expensive than gray cast iron. The basic material costs more and it is more expensive to machine. The other is that ductile iron is 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.
Gene Hailey of Enginetech says that 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 translate to better. 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."
Hailey says Enginetech uses ductile iron top compression rings in its premium ring sets for every engine application that uses ductile iron as original equipment.
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.
RINGS OF STEEL
The next step up from ductile iron is steel. As we said earlier, ductile iron is roughly twice as strong as grey cast iron, and 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:
Jesse Jones, marketing specialist with Perfect Circle/Dana Corp., says steel rings can solve a lot of problems. They are stronger, harder, seal better and resist breakage and wear under load. He says 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 we use in our rings also lower engine oil consumption. The 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."
Jones said steel has become the ring material of choice among many racers today. "Of the last 29 NASCAR races of the 1992 season, 16 winners were running our Speed Pro steel top compression rings."
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 which we will describe later. 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 Japanese engines (see accompanying application list). 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, getting steel supplier to provide the special wire that is required can get costly, which is why the domestic ring manufacturers are taking a "wait-and-see" attitude towards expanding their coverage of steel rings for applications that do not use steel as original equipment.
Roger Borer of Muskegon Piston Rings said there is really no advantage of going to 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 said steel is unquestionably the ring material of the future, especially for the top compression ring. Like ductile iron, it is very resistant to breakage. But it is also less expensive to manufacture.
Most of the new engines that are being introduced are going to steel. The Ford 4.6L modular V8, which was introduced several years ago and initially used ductile iron top compression rings have been refitted with steel rings for 1993.
Though steel rings are starting to be used in newer domestic engines, most of the domestic ring manufacturers are currently using ductile iron for applications that require a premium ring material.
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 a steel replacement ring is not available for a certain application that uses steel as original equipment, you can substitute ductile iron.
Mike Lynch of Sealed Power says, "We feel it is very important to replace these vital parts with the same high quality parts. Sealed Power has chrome plated steel replacement rings for the engine applications that require them."
Lynch also said engine rebuilders should never substitute ordinary gray cast iron rings for ductile iron or steel top compression rings 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, your customer is going to have ring problems 20,000 miles or so down the road."
Along with the change to steel rings for high output engine applications may come another new technology: 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.
To date, the Japanese and some of the Europeans are the only ones using gas nitriding to coat steel rings. There are no domestic production engines that yet use gas nitrided rings.
As mentioned earlier, steel rings have to be coated so they will not scuff, so gas nitriding may someday replace chrome plating as the coating of choice for steel rings.
Roger Borer of Muskegon said domestic manufacturers are looking at the gas nitriding technology but are currently invested in chrome plating. "The future of gas nitriding rings depends on what kind of environmental restrictions the EPA puts on chrome plating. I would guess that within the next 5 to 10 years, gas nitriding will replace chrome plating. The question then becomes is it more economical to do the gas nitriding in house (as we do chrome plating now) or do we farm it out to an outside vendor.
Ring manufacturers have also been tinkering with various combinations of plasma moly and ceramics (such as chromium carbide). Ceramics are extremely hard and wear resistant, but do not conduct heat well. So the amount of ceramic in the mix has to be limited to match the application. Ceramic faced rings have been developed for drag racing applications, and Volvo currently is using a "Moly Cermet" (80% moly/20% chromium carbide ceramic) faced ring in a turbocharged heavy-duty truck engine. But for everyday passenger car applications, gas nitriding appears to have the best chance of being universally accepted.
JAPANESE STEEL RING APPLICATIONS (older engines)
Accura 1.6L 1986-89 Integra
Honda ** 2.1L 1990-91 Prelude
Hyundai 3.0L 1990-91 Sonota
Isuzu 2.0L 1988-89 Impulse
2.3L 1988-90 Trooper
Mitsubishi 2.4L 1990-91 Colt
3.0L 1991-92 GT3000/Dodge Stealth
Madza 1.8L 1990-92 323, Protege
2.2L 1988-92 MX6, 626, Ford Probe
2.6L 1987-89 B2600
3.0L 1988-92 MPV, 929
Nissan 1.6L 1989-91 Pulsar, Sentra
1.8L 1988-91 Pulsar
2.0L 1991-92 Sentra
2.4L 1989-91 Stanza, 240SX
3.0L 1990-91 300ZX
Toyota 1.6L 1988-89 Corolla, MR2
1.6L 1989-90 Geo Prism
1.6L 1988-91 Geo Storm
2.0L 1988-91 Celica
2.2L 1990-91 Celica, MR2
2.4L 1990-92 Previa
4.0L 1988-90 Land Cruiser
** Honda includes others not listed