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Ask The Experts: What’s the difference between ductile iron, cast iron, and stainless steel?

Ductile iron, cast iron, and stainless steel have different costs but are used to make the same parts. Why does it matter which one I use? Is spending the extra money on ductile iron or stainless steel going to make a difference?

What is cast iron?

This is a cast iron gate valve, available through Tork Systems, Inc.

In the past, U.S. Coast guard and Navy regulations have changed to substitute these metals for one another when dealing with valves. Gray iron is upgraded to ductile iron, and ductile iron is upgraded to stainless steel. We want to help you understand the distinction so that you don’t replace a cast iron valve with another cast iron valve when you should be using stainless steel. As always, make sure to reference your CRF and regulation manuals before replacing parts to ensure you’re using the right material.

Cast iron, commonly referred to as gray iron, is an alloy made with iron and 2-4% carbon, and may contain trace amounts of other impurities like sulfur or phosphorus. It’s not new by any means. Cast iron has been around for centuries, and it’s known as a very durable material. It has excellent damping qualities, meaning that when intense vibrations disturb the metal, it is unlikely to suffer any damage. However, this alloy has low ductility. Any bending of the material will cause it to crack.

Ductile iron is a newer alloy than gray iron. While cast iron has been around as far back as 5th century B.C., ductile iron was first created in 1943. Ductile iron is more elastic than gray iron. It has twice the tensile strength of cast iron and a yield strength of 40k while cast iron has no yield strength at all. This means that when ductile iron is bent, it can conform a little, even when gray iron simply cracks. Ductile iron is also significantly less likely to corrode than cast iron.

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This is a stainless steel trim, 150 LB globe valve

Stainless steel is a shiny, silver-colored metal that is minimally porous and very durable. It has a thin, transparent layer that protects and even heals the metal on it’s outer sides when scratched or damaged. It is highly resistant to corrosion, and is very hard. Some schedules of stainless steel may be ferrous, which means that it has iron mixed into it. These ferrous stainless steels alloys may have magnetic properties and be less resistant to corrosion and lose ductile strength. A good way to test your steel to find out if it is ferrous or not is to put a magnet on it! Grades of stainless steel in the 300 series, such as stainless steel 316 pipe, are non-ferrous. Grades of stainless steel that are 400+ series are likely to be ferrous.

What are they used for?

Cast iron is normally used for automotive and structural purposes such as mounting heavy machinery. It isn’t likely to be damaged from the high vibrations of a big truck or a large machine shaking around. It is normally impractical for marine uses because pipes and valves tend to bend slightly as a ship moves on water, and this can cause the iron to crack.

Ductile iron is better for marine uses than cast iron in most circumstances. It has a higher dutile rating (thus, the name) and will conform more to the swaying of a ship. While ductile iron has good ratings for dampening, it is not as highly recommended as cast iron for use on loud or heavy machinery because it doesn’t soundproof as well and is not as durable when vibrated.

Stainless steel doesn’t have the same dampening qualities as ductile or gray iron, but it is very durable and doesn’t require protective treatment before use in marine applications. It is naturally resistant to corrosion, bends easily with proper treatment, and is very unlikely to crack. It can handle high pressure and extreme temperature. Stainless steel is often used in marine, aircraft, and space applications because it is sturdy, flexible, heat and corrosion resistant, and is easy to maintain.

At Tork Systems, Inc, we typically see the marine industry use stainless steel in the 300 series (304 & 316 are the most common). If you are working on a ship, we recommend sticking to the 300 series. Parts in the 400+ series may crack during ship movement or corrode faster than steel in the 300 series.

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This is a wafered ductile iron butterfly valve

Coloring and Mil-Spec:

At Tork Systems, Inc. we have different colors painted on our valves to help you tell the difference between materials. Cast iron is painted black,
ductile iron is painted green or blue, and stainless steel stays that nice, shiny, silver color. Most cast and ductile iron valves have bronze trim, which means that the inside of the valve is bronze. These colors are fairly typical in the industry, but other manufacturers may mark their valves with a different color, so make sure and check all markings before assuming the material type.

Some variations that we have seen include iron that has been painted silver and blue valves that are ductile iron with varying trim styles. Always double check the numbers on the valve itself. The colors are not always the same!

Tork Systems, Inc. carries mil-spec valves, pipe, and fittings, even the tough-to-find ones! So, when you’re looking for something that meets your standards make sure to connect with an expert here!

Jeremy Bice
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Jeremy Bice

Customer Success at Tork Systems, Inc.
Jeremy is a former sergeant in the United States Marine Corps. His specialty is engaging customers in creative ways and building relationship with key players in marine PVF industry.
Jeremy Bice
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