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What makes stainless steels 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.

Want to learn more about miniature valve stainless steels? The new ISM technical resource, Stainless Steel and Miniature Flow Control Valves Guide, provides insights into

     • Stainless steel advantages

     • What makes it corrosion resistant

     • Why it’s a more expensive material

     • Which types are used to make miniature valves

Read more about it

 

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.
 

A five-axis machining tool by Heller Maschinenfabrik.
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.

Chromium crystals and a cubic centimeter cube of refined chromium.
© Heinrich Pniok under GDFL


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.

Typical industrial stainless steel piping and processing equipment.
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.

 

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
     
A chunk of refined nickel.
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.

Electrolytically refined manganese chips and a cubic centimeter cube of refined manganese.
© Heinrich Pniok under GDFL


Molybdenum as a steel alloying element

Molybdenum in the form of the mineral molybdenite.











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 steels 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.
 

Steven C. Williams headshot March, 2018.About the author
Steven C. Williams is the technical writer and an inbound marketing specialist at Industrial Specialties Manufacturing (ISM), an ISO 9001-2015 supplier of miniature pneumatic, vacuum and fluid circuitry components to OEM's and distributors all over the world. He writes on technical topics related to miniature pneumatic and fluidic components as well as topics of general interest at ISM.       
Steven C. Williams my LinkedIn profile link button.

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.

Front cover of the ISM Flow Control Valves catalog.


The ISM Miniature Flow Control Valves catalog includes

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  • Links to product pages in our e-commerce catalog

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