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 * Who We Are
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20
Oct


STAINLESS STEEL FOR HEAT RESISTANCE

October 20, 2023
Casting Design, Continuous Improvement, Steel Foundry Industry

By: Nick Knotts, Industrial Engineer, The Lawton Standard Company




WHY STAINLESS STEEL?

When it comes to castings for heat resistant applications, stainless steel is
one of the most commonly selected materials. Heat resistant steels are made up
of a variety of different microstructures, ranging from ferritic to austenitic
and everything in between. Heat resistant steels also display a wide variety of
mechanical properties, ranging from an Ultimate Tensile Strength (UTS) as high
as 107ksi down to 65ksi at room temperature and a Yield Strength (YS) of 81ksi
to 36ksi at room temperature.


HOW TO MEASURE PERFORMANCE

However, the key component of a heat resistant steel is its durability at high
temperatures, which can be measured in a variety of different ways. One way to
measure the performance of a steel at high temperatures is to measure the UTS
and the YS at elevated temperatures, typically more than 1200F. Many heat
resistant steels can hold a UTS of 30-50ksi at 1400F and a YS of up to 30ksi.
Typically, alloys that have a sufficiently high chromium and nickel content
perform the best in this category of elevated temperature tensile and yield
strength, including HL, HP, HU, and HK. Alloys in this category typically have a
fully austenitic structure. Due to the higher presence of alloying elements,
these alloys also tend to be more expensive.

Another way that performance of heat resistant steel is measured is in terms of
its creep and stress rupture strength. Creep is extremely common in heat
resistant steel castings. For those who are unfamiliar, creep is the stress that
occurs to castings that are under strain at high temperatures. While entirely
preventing creep is not possible, most heat resistant steel alloys are designed
to minimize the effect of creep to some degree, which in turn, prolongs the
service life of the casting. Where creep becomes the most problematic is in
select cases where it leads to casting deformation and can even lead to
fractures due to the strength of the casting being compromised such that it
fractures below the properties defined in an elevated temperature tensile test.


ALLOY SELECTION

Creep can be accounted for in casting design and in alloy selection, an engineer
can select a casting design that will allow the casting to continue to perform
for an extended period in the event of creep and may also select an alloy that
is more resistant to creep. In terms of alloy selection, an engineer should
select an alloy that slows the process of plastic deformation and has a high
rupture stress, prioritizing one or the other based upon the application. In
terms of deformation control, the best bet is typically to go with an alloy that
contains at least 30% nickel and 15% chrome to obtain a fully austenitic
structure, HT, HU, and HP are great examples. Some iron-chromium-nickel alloys
such as HK also perform well in this arena.

When it comes to the rupture stress, controlling carbon content to be in the
0.3-0.7% range will be the most important variable. In the 0.3-0.7% carbon
range, the metal will be much more resistant to rupture stresses than those that
are 0.2% and below. Other alloying elements are also key, particularly enough
nickel to form an austenitic structure (At least 18%, preferably 22%+) and a
chromium content more than 15% are key, HK, HN, and HP are quality examples.
Some of the most rupture resistant alloys will contain some content of
specialized alloying elements such as tungsten or niobium, though carbon content
remains the most impactful variable to control.


AVOIDING OXIDATION

Another key in stainless steel is resistance to oxidation at high temperatures.
For this reason, a heat resistant stainless steel must contain a minimum of 12%
chromium to resist iron oxide formation at high temperatures. Further oxidation
resistance can be obtained through a higher chromium and nickel content.


THERMAL FATIGUE

If a casting is subjected to thermal cycling or shock, that must also be taken
into account when it comes to alloy selection for a heat resistant steel. There
is not a great way to measure thermal fatigue in a casting, there are thermal
fatigue tests, but they do not greatly carry over to reality.


HOW TO RESIST CARBURIZATION

Carburization resistance is another consideration to be made, especially for
castings that will be involved in an application like commercial heat treatment.
Higher nickel and chromium contents largely increase the resistance of the metal
to carbon penetration into the surface of the casting. Silicon plays a vital
role in carburization resistance as well, small increases in silicon can make a
drastic difference on the ability of the alloy to resist carbon penetration,
typically around 2% silicon is used in castings that are meant to resist
carburization. Other alloying elements have been added to stainless steels to
resist carburization though are not widely used and their effectiveness remains
debatable.


OTHER CONSIDERATIONS

In rare cases, considerations need to be made for a high sulfur environment that
will cause oxidation in the steel castings. High nickel content heat resistant
alloys are very prone to corrosion in a high sulfur environment due to their
fully austenitic structure, so alloys that are entirely ferritic are typically a
better selection.


CONTACT US!

Have any questions about stainless steel? Need heat resistant castings? Reach
out to a foundry expert today!


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 * Home
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RECENT POSTS

 * Man of Steel: Basic Carbon and Low Alloy Steel
 * Man of Steel: Corrosion Resistant Stainless Steel
 * Customized solutions for any industry
 * Man of Steel: Austenitic Manganese Steel
 * New research on penetration vs. thickness in washes




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