Differences Between the 300 Series Stainless Steels
The 300 series austenitic stainless steels are a set of iron-based chromium-nickel alloys designed to resist corrosion. This in combination with excellent formability, resistance to wear, and strength at temperature make them common materials of construction within piping systems.
Differences between the alloys are slight but deliberate. While they can be used interchangeably in many applications, sometimes there is an ideal solution. Substitutions in such situations could mean compromised service life.
As corrosion resistance is one of the primary reasons end users opt for metal hose, application media typically guides alloy selection. 304 is often used as it is the most cost-effective option, though 321, and 316 especially, offer better corrosion resistance. For this reason, most Penflex hoses are made using 321 or 316L.
Braid is usually 304L as it will not be in contact with flow media, though 316L is an option if the application is in a corrosive environment—like in, on or near the ocean—or if the outside of the hose will be subject to corrosive media via drips, spray, run-off, etc.
For especially corrosive applications, superior corrosion resistant properties can be found among higher-nickel alloys like Monel® 400 and Hastelloy® C276.
300 Series Stainless Steels: Chemical Composition
The chart below shows the chemical composition of the most common 300 series stainless steels used in the metal hose industry. Single figures signify the maximum percentage allowable under ASTM 240 requirements.
|Chromium||18% – 20%||18% – 20%||16% – 18%||16% – 18%||17% – 19%|
|Nickel||8% – 10.5%||8% – 12%||10% – 14%||10% – 14%||9% – 12%|
|Molybdenum||2% – 3%||2% – 3%|
|Titanium||5 x (C + N) min – .70%|
304 is considered the baseline when it comes to corrosion resistance. Various alloying components have been added to the 321 and 316 grades to increase corrosion resistance.
In the case of 304L and 316L, carbon has been taken out. The “L” stands for “low carbon.” Lower carbon alloys are less susceptible to carbide precipitation in the Heat Affected Zone (HAZ) than their standard type counterparts.
Chromium and carbon can mix under the heat of welding to create chromium carbides at the grain boundaries. This reaction depletes the chromium layer that gives stainless steel its corrosion resistant properties, ultimately making the HAZ a target for chemical attack. One way to combat carbide precipitation is to reduce the amount of carbon in the parent material.
Another more effective way is to add titanium to the metal, as is the case with 321. With Type 321, rather than being attracted to the chromium, carbon is attracted to the titanium. This helps ensure the passive chromium layer remains intact.
With both 316L and 321, post-weld cleaning processes can eliminate corrosion due to residual carbide precipitation.
Resistance to Pitting Corrosion
Molybdenum is added to the 316 grades to increase resistance to pitting corrosion, especially in the presence of chlorides. To help in the selection of an appropriate alloy, an equation based on chemical composition was developed. PREN, or the pitting resistance equivalent number, is a theoretical way of comparing pitting corrosion resistance among various alloys.
|304, 304L, 309, 310, 321||18.0 – 20.0|
|316, 316L||22.6 – 27.9|
|317, 317L||27.9 – 33.2|
|AL-6XN||39.8 – 45.1|
|Inconel® 625||46.4 – 56.0|
|Hastelloy® C-276||64.0 – 73.8|
Taking precautions to ensure the HAZ more closely resembles parent materials in terms of corrosion resistance and planning for pitting corrosion is important if corrosion resistance is a priority. In applications where corrosion is not an issue, any of the 300 series alloys will likely deliver similar results.
Rates of Corrosion Among 300 Series Stainless Steels
Another way to demonstrate differing levels of corrosion resistance among these alloys is to consider expected rates of corrosion. Rates vary from chemical to chemical and are illustrated in Penflex’s corrosion resistance chart. In thinking about how much metal will be worn away each year, the difference between corrosion resistance capabilities can be seen more easily.
And when it comes to corrosion resistance, it’s not just the alloy that must be considered, but the wall thickness of the alloy as well. We’ve pulled together another bulletin to specifically address this topic.
Derating Factors at Elevated Temperatures
No other materials can maintain their properties through such a wide temperature differential as metal. Anything below 0°F will likely require metal so cryogenic applications are a common use case for metal hose. Some of the austenitic stainless steel mechanical properties actually increase at low temperatures! Anything above about 400°F will also require metal so applications with super saturated steam or those within steel mills or furnaces are also likely scenarios for metal hose.
It’s important to remember that with increased temperatures comes a reduction in pressure ratings, and there are some differences in those factors among the common 300 series stainless steels. Derating factors are based on braid alloy.
The temperature reduction factors for 321 and 304 are higher than 316 until about 700°F and then the reverse is true with 316 having the higher reduction factors than 321 and 304. The higher the derating factor, the higher the pressure ratings will remain.
For example, to calculate the maximum working pressure for a P4 Series ¾” 321 stainless steel corrugated hose with one layer of 304L braid that will be operating at 800°F, multiply working pressure (940 PSIG) by appropriate derating factor (.73).
The working pressure for the hose at 800°F is 686 PSIG.
Penflex developed its derating factors after gathering raw data on tensile strength at elevated temperatures from major material suppliers and taking the lowest values in each category for the various alloys. For this reason, they may be more conservative than derating factors published by NAHAD or ISO 10380.
It’s important to remember the maximum working temperature of the end fittings and their method of attachment also needs to be considered when working with increased operating temperatures.
For application temperatures above 1000°F, we often suggest Inconel® 625.
Considering the Entire Application
As mentioned above, in many applications substitutions in hose alloy will have little impact on hose performance. However, when temperatures rise, pressures increase, or hoses cycle frequently, we must pay closer attention.
The impacts of temperature, pressure and movement can be compounded leading to corrosion sooner than anticipated had application media been the only factor in our corrosion calculations. While the differences between the 300 series stainless steels may seem small, we can begin to see how quickly they could become magnified.
Please contact us with any questions.
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