What was I made for?
What was I made for?

("What Was I Made For?", Billie and Phinneas Eilish O'Connell, 2023)

 

THE BENEFITS OF ALLOYED METALS QUICKIE REVIEW:

PTOA Readers and Students learned in PTOA Segment #246 that a base metal like Iron can be alloyed with one or more metals ... or a non-metal like Carbon ... to change the characteristics of the metal.

For example, the alloy incorporated into the base metal can make the final metal ...

• easier to form into a desired shape.
• stronger and less susceptible to embrittlement.
• less prone to rust.
• less prone to sulfur corrosion.
• able to withstand a different temperature range.

 

 

As the nearby graphic illustrates, the alloyed metal may be incorporated into the base metal by substituting some atoms of the base metal with atoms of alloyed metal or by placing alloy atoms within the interstitial spaces between base metal atoms.

A base metal can be alloyed with more than one type of alloy using both alloy methods, substitution and interstitial placement.

This PTOA Segment explains the differences between Carbon Steel Alloys and the specialty non-ferrous Nickel alloys.

Carbon Steel Alloys and non-ferrous metal alloys are made for specific process service applications. The long-term operating safety and mechanical integrity of the plant depends upon accurately matching the process stream service to the metallurgy of all components in the plant's Piping Network.

 

CARBON STEEL AND CARBON STEEL ALLOYS

Low Carbon Steel 

Low Carbon Steel is the most popular metal pipe because it is relatively inexpensive and will work well in most processing services which have manageable PV Temperatures and PV Pressures.

The definition of Low Carbon Steel is a Steel which is Alloyed with less than 0.3% Carbon.

Integrating the Carbon atoms between Iron atoms promotes ductility and "weldability."

A Low Carbon Steel may also contain small amounts of Silicon or Aluminum. Manganese may be incorporated to mitigate the corrosive effects of Sulfur and Phosphorus.

Low Carbon Steel is not compatible with process services which experience:

• processing Temperatures that exceed 650°F or
• a hydrogen gas atmosphere processed at high PV Temperatures or
• a PV Temperatures lower than -20°F.

 

Furthermore ...

At 950 °F, the strength of Carbon Steel decreases to one third of its manufactured strength. As the PV Temperature increases just 50°F more to 1000°F, the strength of Carbon Steel decreases to 1/6th of its original strength!

 

Low Alloy Carbon Steel

The limitations of Low Carbon Steel that are listed above can be mitigated by increasing the amount and type of alloy integrated into the base Iron metal.

The typical Low Alloy Carbon Steel has less than 10% total alloy content. However, the foundry specifies the metal content in the Low Alloy Carbon Steels produced.

Low Alloy Carbon Steels contain less than 0.5% Molybdenum (Mo) which fortifies the metal's strength above 900°F. Chromium (Cr) is also added to resist sulfur corrosion.

For some industrial process services, the Molybdenum and Chromium content is increased.

The Low Alloy Carbon Steel that is destined for the fabrication of Vessels, Furnace Tubes, Exchangers, and their associated piping will have up to 9% Chromium alloy due to their prolonged exposure to high PV Pressures and PV Temperatures.

Some industrial facilities use natural gas as a feedstock to produce solid fertilizers and hydrogen gas (PTOA Segments #44 and #45 featured steam reforming of natural gas to produce Syn Gas and Hydrogen gas).

The high PV Temperatures and PV Pressures required for the industrial production of hydrogen gas requires a higher Molybdenum alloy content in the Low Alloy Carbon Steel recipe for the purpose of mitigating embrittlement. The embrittlement of metal was featured in PTOA Segment #246.The mechanism of "metal embrittlement" is illustrated in the nearby graphic.

 

High Alloy Steels

High Alloy Steels contain significantly more total alloy content than Low Alloy Carbon Steel does. The specified limit in alloy content between "Low Alloy Content Steel" and "High Alloy Content Steel" will be set by the foundry.

The nearby specification sheet from a foundry lists the Alloying Content ranges for the steels produced at the foundry.

At this foundry:

• Low Carbon Steel has less than 0.2% Carbon.
• Medium Carbon Steel has greater than 2% Carbon but less than 0.5% Carbon.
• High Carbon Steel has greater than 0.5% Carbon.

 

Furthermore, at this foundry, the Low Alloy Steel product will have less than 8% Total Alloying content which is a more conservative upper limit than the 10% total alloy limit for Low Alloy Carbon Steel stated in the previous section.

The High Alloy Steel manufactured at this foundry will have greater than 8% total alloy content.  Note that the upper limit to the High Alloy Steel product is not listed. The upper limit of High Alloy Steel is expensive and made-to-order for specific process services.

 

Chromium Steels

Chromium puts the "stainless" in "Stainless Steel."

Seventeen percent (17%) Chromium Steels are used to manufacture pump internals and compressor parts so that they can resist sulfur corrosion when the process PV Temperature ranges from 550°F to 1000°F.

Twelve percent (12%) Chromium Steel is used in protective linings found in steel equipment, thermowells and valve trims that are likewise exposed to sulfur-induced corrosion.

 

Austenitic Stainless Steel

The behavior of Carbon Steel changes abruptly once Nickel and Chromium are added to Steel in amounts that total over 20%.

The addition of the Nickel changes the structure of steel to form austenite, which has significantly different properties than the steels previously mentioned in this PTOA Segment.

As is indicated in the nearby chart, Type 304 Stainless Steel has 18%-20% Chromium and 8%-10.5% Nickel.

Type 316 Stainless Steel has 2% less Chromium yet 2% more Nickel, then 2-3% Molybdenum alloy is incorporated and voila! ... the 304 Stainless Steel is now 316 Stainless Steel.

One big difference in behavior of Austenitic Stainless Steels is that they are not magnetic, even though they contain Iron.

A ferrous metal that is not magnetic is a huge change in behavior!

If a Process Operator suspects a contractor is substituting a cheaper grade of steel pipe for Type 316 Stainless Steel, a magnet can be used to verify the presence or lack of magnetism. A slightly more involved test requires a bit of nitric acid which the processing plant's laboratory may have in stock.

To recap ...

The reasons to invest in Austenitic Stainless Steel are resistance to rusting and chemical-induced corrosion.

For example, Austenitic Stainless Steel will be used to manufacture the tube support hardware found in Fired Furnaces. Why? This type of metal resists oxidation at very high PV Temperatures while maintaining strength.

On the other end of the PV Temperature spectrum, these metals can retain their strength and ductility at very low PV Temperatures.

THE NICKEL ALLOYS
(NON-FERROUS, VERY EXPENSIVE METALS) 

The Carbon Steel family of metals cannot handle process fluids which are thermally hot and acidic (like hydrochloric acid, HCl) or strongly corrosive (like aqueous NAOH, ... aka saltwater). This type of process service will be compatible with the Nickel Alloys.

Nickel Alloys are just like what they sound like: The base metal, Nickel, is alloyed with lesser amounts of Iron, Copper, Aluminum, Chromium, Cobalt, and Molybdenum.

Monel (For Corrosion Resistance and Heat Transfer at Elevated Temperatures)

Monel is a Nickel-Copper alloy manufactured to resist corrosion at high PV Temperatures while maintaining excellent heat transfer and electrical conductivity characteristics.

For one example, Monel will be used to fabricate the internals of the radial reactors that are used to turn hydrotreated naphtha into gasoline feedstocks. Naphtha reforming was featured in PTOA Segment #37.

Monel is needed in this process service because hydrochloric acid is injected into the heated feed stream (hydrotreated naphtha and hydrogen gas) to improve the performance of the catalyst contained in the reactor.

Furthermore, Monel's optimal heat transfer property is balanced between the endothermic and exothermic reactions of the process which results in the efficient use of the fuel gas utility.

Inconel (For Sustained Performance at Extremely High PV Temperatures)

Inconel is a Nickel-Chromium alloy manufactured for even greater corrosion resistance than Monel at Temperatures up to 2200°F (1093°C). The strength and durability of Inconel is sustained at these high Temperatures, plus Inconel is easy to form and has a good "weldability factor. "

Inconel will be used to manufacture Gas Turbine internals and the tube supports in a Hydrogen Plant's Reaction Furnace.

Hastelloy (For Sustained Highly Chemically Corrosive Environments)

Hastelloy is a Nickel-Molybdenum-Chromium Alloy which has superior corrosion resistance performance in sustained corrosive environments such as seawater or hydrochloric acid mixtures. Because its strength is sustained at high PV Temperatures, Hastelloy is used in power plants and chemical processing where continuous high PV Temperature performance is a priority.

The next PTOA Segment alerts PTOA Readers and Students and future Process Operators about metallurgy phenomena that can impact their safety and well-being.

 

TAKE HOME MESSAGES: The long-term operating safety and integrity of a chemical processing plant depends upon accurately designing the metallurgy of the Piping Network components to be compatible with the flowing process stream.

The family of Carbon Steel Alloys includes:

• Low Carbon Steel - less than 0.3% Carbon content, the most popular Piping Network fabrication metal. Other included alloys are Silicon, Aluminum, and Manganese.
• Low Alloy Carbon Steel - less than 10% Total Alloy Content. Additional incorporated alloys include Chromium, Molybdenum, and Nickel.
• High Alloy Carbon Steel - has over 10% Total Alloy Content. For example:
  • "Stainless Steel" has over 11% Chromium alloy.  12% Chromium alloy Stainless Steel is used for protective linings, thermowells, and valve trims when sulfur-induced corrosion is a concern. Some internal pump hardware can have 17% Chromium alloy.
  • Austenitic Stainless Steel has over 16%-20% Chromium alloy and 8%-14% Nickel alloy, depending on the Type of SS manufactured.  Austenitic Stainless Steel is used when there is a need to resist chemical-induced corrosion, rusting, and oxidation at high PV Temperatures. Austenitic Stainless Steel is also used where sustained strength and ductility is needed at very low PV Temperatures. The addition of Chromium and Nickel in amounts that total 20% and above greatly changes the behavior of Austenitic Stainless Steel because, although still ferrous (containing Iron), these types of steel are non-magnetic.

 

Expensive Nickel Alloys are non-ferrous and are used when the process fluid includes thermally warm acids or highly corrosive fluids like saltwater.

• Monel is a Nickel-Copper Alloy used when resistance to corrosion, efficient thermal heat transfer, and good electrical conductivity are required for the process service.
• Inconel is a Nickel-Chromium alloy used when PV Temperatures up to 2200°F are sustained.
• Hastelloy is a Nickel-Molybdenum-Chromium alloy with superior corrosion resistance and strength even when compared to Monel.

 

©2023 PTOA Segment 0247

PTOA PV FLOWRATE FOCUS STUDY AREA
PIPING NETWORK HARDWARE

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