Last updated on February9, 2023
GMCO series needle valve
What makes stainless steel different than regular steel?
This article is an excerpt from the new ISM technical resource: Stainless Steel and Miniature Flow Control Valves Guide from ISM
Why use stainless steel valves?
Stainless steel is a common material option for miniature flow control valves.
One reason for this is that stainless steels, like other metals, tend to be heavier but also stronger and tougher than plastics. Stainless steel materials, in particular, also have great corrosion resistance.
This blend of corrosion resistance and toughness is a major reason why the stainless steels are so useful for making valves. This is despite the fact that they cost more and are harder to make.
Some of the whys of stainless steels
What’s the difference between iron and steel?
Iron is a metal, an element, and it is pretty reactive. That is, it oxidizes or rusts easily. But steel is an iron-carbon alloy. It is much stronger than iron while still being fairly easy to work with and shape.
The amount of carbon in steel is the most important factor in determining its strength. The ratio of iron to carbon also affects a steel’s ductility or workability. More carbon is stronger but harder to work while less carbon is easier to work but not so strong.
Five axis machining
What is ductility and workability and why does this matter?
Metal ductility is the ability of a metal to be shaped and stretched without breaking. Brittleness is the opposite of ductility. Typical metal working processes include roll forming, bending, cutting with a guillotine shear, drilling and stamping.
The workability of a metal is how easily it can be shaped without cracking when it is subjected to different metal working processes. If a metal has high ductility it is easier to work. Workability varies with the shaping process used and the metal’s physical characteristics.
There are over 250 different stainless steel alloys
Steel is an alloy of iron and carbon. It also includes one or more additional elements, usually metals. These added ingredients give different steel types their particular chemical and physical properties such as corrosion resistance, strength, workability and so on.
Stainless steels get their name from the fact that they are very corrosion resistant. This corrosion resistance is provided by adding chromium. Other ingredients (elements) are added to stainless steels to balance the increased corrosion resistance with desirable physical traits.
Learn more about classes of stainless steel alloys at the Stainless Steel Overview: Alloy Classifications page of the Specialty Steel Industry of North America website.
Chromium gives a stainless steel its special advantage
Stainless steels generally contain at least 10.5% or more chromium by weight. But chromium content can range up to 30%. It provides stainless steel with its passivity, another way of saying its corrosion resistance.
What is meant by metal passivity?
Metal passivity is when an oxide film forms on the metal’s surface. This film prevents any additional chemical corrosion of the metal. Adding chromium to stainless steels forms an extremely thin but uniform, continuous, tenacious and stable chromium-rich oxide film on the surface of the steel.
The most important differences between different stainless steel types are their workability relative to their corrosion resistance. It is important to remember that the more workable a metal is, the easier it is to form into useful shapes. A stainless steel that is easier to shape means cheaper parts.
Some general information about stainless steel types
There are five families of stainless steel (see below) and every one of them gets most of their corrosion resistance from the chromium component of the alloy.
Industrial stainless steel piping
and processing equipment.
Why stainless steel metal valves if they are more expensive?
- Steel valves are especially strong. This allows them to be used in higher pressure applications than brass or plastic valves.
- Steel valves are durable. Stainless steel valves are especially tough because they are made of steel and because of their corrosion resistance.
- Steel valves can handle both high and very low temperatures. Steel valves are usually rated for a broader range of operating temperatures than brass or plastic valves.
Learn more about the machining metals in the Cutting Tool Applications series from American Machinist.
Corrosion resistance
Stainless steel chemical resistance to acids
As a rule of thumb, stainless steels tend to be resistant to acidic corrosion. This is not a hard and fast rule though. Exact resistance levels vary by the type of stainless steel, the concentration of the acid, the type of acid and its temperature. If there are mixtures of chemicals or their concentrations vary, testing is probably a good idea. In general, best practice is to obtain part samples and test them in real operating environments.
Stainless steel chemical resistance to bases
Stainless steels generally have a strong resistance to corrosion from weak bases, even in high concentrations and in high-temperature environments. Strong bases can cause cracking or etching. Be especially careful with chloride solutions such as sodium hypochlorite.
Stainless steel chemical resistance to organics, organic solvents and organic chemicals in general
The resistance of stainless steels to different organic chemicals varies by the chemical, its concentration and the chemistry of the steel alloy. In general, 300-series stainless steels are the best stainless steels at resisting corrosion from organics. Environmental conditions such as temperature and the availability of free oxygen have significant effects on a stainless steel’s corrosion resistance to organics.
- Get our 303 Stainless Steel Chemical Compatibility guide
- Get our 304 Stainless Steel Chemical Compatibility guide
- Get our 316 Stainless Steel Chemical Compatibility guide
- Get our 316L Stainless Steel Chemical Compatibility guide
What are the key elements added to iron to make stainless steel alloys?
Key elements added to stainless steel alloys or formulations that provide corrosion resistance
- Nickel
- Nitrogen
- Chromium
- Manganese
- Molybdenum
Nickel chunk
Nickel as a steel alloying element
Nickel is added to stainless steels to increase their strength, ductility and toughness. It is the addition of nickel to stainless steel that makes it non-magnetic. It also improves any stainless steel’s resistance to strong acids.
Nitrogen as a steel alloying element
Nitrogen increases a stainless steel’s resistance to pitting corrosion and corrosion between the grains or small crystals that make up the metal. In austenitic stainless steels, nitrogen increases the yield strength. Yield strength is the amount of force required to permanently distort or deform the metal.
Chromium as a steel alloying element
As mentioned earlier, chromium is an alloying element added to steel to increase its resistance to oxidation. Chromium is the key ingredient in stainless steel formulas that makes them “stainless”.
Manganese is also used in some stainless steel alloys
Manganese improves a stainless steel’s hot working properties and increases its strength, toughness and hardenability.
Molybdenum as a steel alloying element
Molybdenum in the form
of the molybdenite
Molybdenum is added to stainless steels to increase corrosion resistance. It is especially effective at preventing pitting and crevice corrosion from environments where chlorides are found such as exposure to sea water or deicing salts. It also improves corrosion resistance to sulfuric, phosphoric and hydrochloric acids. An added benefit is that it improves the mechanical properties of stainless steels at high temperatures.
Learn more about stainless steel alloying elements at the Stainless Steel Overview: Alloying Elements in Stainless Steel page of the Specialty Steel Industry of North America website.
Choosing stainless steel valves
Corrosion resistance is one of the main reasons stainless steel valves are the best choice for demanding environments and corrosive fluid handling. Stainless steel valves are also tougher and heavier than plastics and many other metals. This gives them higher burst pressures and operating temperatures.
Relatively complex alloys give stainless steel their toughness and corrosion resistance. Toughness comes at a price though. Stainless steel parts require more complex machining and forming techniques. This makes them more expensive.
Despite the higher cost though, stainless steels are often the best material choice. This is especially true for valves and flow control components that need to be both physically tough and corrosion-resistant.
Are you developing a design for or maintaining a low-pressure (less than 125 psi, 8.6 bar) small-bore air, gas or fluid system? Then ISM has the valve for it. Get a catalog and find out more about all the different types of stainless steel valves we offer.
Check out our complete lineup of miniature flow control valves.Keep in mind we carry thousands of miniature flow control valves. Plus, thousands of related products like connectors, adapters, gauges and so on.
Learn more about the all the different miniature flow control valves you can get from us >>
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As an expert in materials engineering and flow control systems, I bring a wealth of knowledge and experience to elucidate the concepts discussed in the article about the GMCO series needle valve and the use of stainless steel in miniature flow control valves. My expertise is grounded in a deep understanding of metallurgy, materials science, and their applications in industrial settings.
The article primarily focuses on the advantages of using stainless steel in valves, delving into the unique properties of this alloy that make it a preferred choice for applications requiring corrosion resistance, strength, and durability.
Stainless Steel vs. Regular Steel: The article starts by highlighting the fundamental difference between iron and steel. While iron is reactive and prone to rust, steel, being an iron-carbon alloy, is stronger and more workable. The amount of carbon in steel is crucial, affecting its strength and ductility. Stainless steel, a specific type of steel, is preferred for valves due to its corrosion resistance and toughness.
Ductility and Workability: The concepts of ductility and workability are introduced, emphasizing their importance in metalworking processes. Ductility refers to a metal's ability to be shaped without breaking, while workability gauges how easily a metal can be shaped without cracking during various processes like bending, cutting, and stamping.
Stainless Steel Alloys: The article mentions over 250 different stainless steel alloys, emphasizing that stainless steel is an alloy of iron and carbon with added elements, usually metals, to impart specific properties. Chromium is a key element providing corrosion resistance, and the article categorizes stainless steel into five families, each obtaining its corrosion resistance from chromium.
Chromium's Role in Passivity: Chromium, present in stainless steels at least 10.5% by weight, imparts passivity to the metal. Passivity is explained as the formation of a thin, uniform, and stable chromium-rich oxide film on the steel's surface, preventing further chemical corrosion.
Corrosion Resistance: The article discusses the corrosion resistance of stainless steel to acids, bases, and organics. It emphasizes the importance of testing in real operating environments due to variations in resistance levels based on stainless steel types, acid concentrations, and temperatures.
Key Elements in Stainless Steel Alloys: The key alloying elements like nickel, nitrogen, chromium, manganese, and molybdenum are explained in terms of their contributions to strength, non-magnetism, corrosion resistance, and improved mechanical properties at high temperatures.
Choosing Stainless Steel Valves: The article concludes by justifying the use of stainless steel valves despite their higher cost. Stainless steel valves are preferred for their strength, durability, and ability to handle a broad range of temperatures, making them suitable for demanding environments with corrosive fluids.
In summary, the article provides a comprehensive overview of the properties of stainless steel, its alloying elements, and why it is the material of choice for miniature flow control valves, reinforcing the idea that the benefits outweigh the costs in specific industrial applications.
