This document discusses metals and alloys for high temperature applications. It describes various steels and their suitable temperature ranges, such as plain carbon steels being suitable up to 400-500°C, low alloy steels up to 500-600°C, and chromium steels up to 800°C. Austenitic stainless steels can be used up to 450°C. Nickel and nickel chromium alloys dominate the 600-1100°C range. Refractory metals like tungsten, molybdenum, and tantalum are available up to 1200°C but are very costly. The choice of metal depends on factors like temperature, oxidation resistance, strength, and cost.

1. Metals and Alloys for High
Temperature Applications
Prepared By Mohanad Hameed
Master of electrical engineering
Unversity of technologe

2. 8.7 STEELS AT HIGH
TEMPERATURE

3. Plain carbon steels containing 0.2–0.4% carbon and small
amounts of silicon and manganese are ferritic or pearlitic.
They are:
● Cheap.
● Weldable.
● And available in all forms.
They oxidize above 400–500°C and are suitable for low or
medium (elevated) temperature parts such as structural
frames, supports, panels, etc. Painting improves corrosion
resistance.

4. Low alloy steels containing 0.15% carbon and 1.2%
molybdenum or chromium show slightly better properties
and can be used for parts requiring intermittent service at
temperatures up to 500–600°C.
Chromium steels containing up to 20–24% Cr have good
oxidation resistance but will show spalling under thermal
cycling. They can be used up to 800°C.

5. Austenitic stainless steels are resistant to corrosion by
oils, fumes, moisture, and many gases up to 450°C.
They retain their surface shine and cleanliness up to
450°C after which the surface may tarnish or blacken,
but corrosion Resis-amounts of chromium and nickel
along with small amounts of Mo, Cb, Ti, Al, etc. They
can be used up to 1150°C.

6. Table 8.5
Shows the properties
of selected heat
resistant steels.
Reference should be
made to handbooks
as the number of
commercially
available heat
resistant steel is very
large.

7. 8.8 SELECTION OF METALS
FOR HIGH TEMPERATURE
APPLICATION

8. We have seen in previous sections that at high temperatures,
metals will fail by:
● Excessive stress.
● Or by reaction with their Environment.
Hence, these two cases become the deciding factors in the
choice of metals.
The following general remarks will help to make a
preliminary
choice. Note that there can be exceptions due to some
combinations of temperature, stress, and environmental
attack.

10. 1. Aluminum is the cheapest choice if the temperature
does not exceed 300°C. It is quite resistant to oxidation
and attack by CO2 and moisture, and finds application
in ovens, dryers etc.
1. Ordinary (plain carbon) steels have adequate strength
up to 400–500°C, but do not possess a high oxidation
resistance. They are cheap and easy to fabricate in a
variety of forms such as rods, strips, and wires, and
structurals.

11. 3. Low alloy steels containing 1–3% chromium and
small amounts of molybdenum and vanadium can be
used up to 600–700°C.
● They can be easily fabricated and have better
oxidation resistance than plain carbon steels or cast
irons.
● They should be used in normalized or annealed
conditions.

12. 4. Low corrosion resistance of steel at high temperature can
be improved by plating or impregnation. Chromium plating is
frequently used. It helps to reduce oxidation and improves
reflectivity. Impregnation of the surface with an oxide layer
improves resistance to oxidation and attack by molten salts
and carburizing chemicals.
5. In some applications, steel parts can be cooled by water
or air circulation. This keeps their temperature at an
acceptable level.

13. 6. Metallurgically, the metal to be used should have a single
phase (e.g., ferrite and austenite) and a large grain size.
Generally, alloys hardened by heat treatment are not suitable
because a prolonged high temperature breaks the hardening
phase such as with martensite.
The exception is dispersion-strengthened alloys.

14. 7. For design purposes, short time tensile test data
(e.g.Figure 8.3) and combined creep-rupture data
(e.g.,Figure 8.2(C)) are useful.
Such data are available in handbooks or manufacturers’
publications. Before making use of such data it is
necessary to make an estimate of the desired ‘‘life’’ of the
part and expected stresses.

15. 8. In design and fabrication keep in mind the linear expansion.
Some useful hints are:
● Avoid sharp corners or edges.
● There should be no welds at corners or edges.
● Corrugated sheets are better than flat ones.
● Use the same section throughout.
● Allow free expansion at the ends to prevent bending, buckling,
distortion, or sagging.
● Baskets, fixtures, trays, etc. used in heat treatment undergo
repeated heating and cooling under a stressed condition. The
effects of expansion and thermal fatigue are more severe in
such cases.

16. 9. Next in cost to plain and low alloy steels are the various
stainless steels (S.S.). There are two main types of S.S. that
are suitable for our purpose.
● Ferritic S.S :These contain 8–25% chromium and about
0.15% carbon. The chromium forms an oxide layer and
protects from oxidation and furnace gases. Ferritic steels
are used up to 600°C. The chromium oxide layer breaks off
after repeated heating and cooling.

17. ● Austenitic S.S.—These contain 18–25% chromium and 8–
20% nickel with about 0.08–0.1% carbon. These steels can
be used up to 700°C without appreciable oxidation.
However, they are not suitable for sulfur bearing gases.
Austenitic stainless steels have excellent resistance
to oxidation up to about 400°C and are not affected by gases
and substances associated with food processing. Hence, they
find major applications in ovens for baking, etc.

18. 10. Nickel and nickel chromium alloys dominate the tempe-
rature range from 600 to 1100°C and are the costliest.
● They offer several advantages if properly fabricated and
used and thereby prove economical.
● There are many alloys based on Ni and Cr.
● They contain small amounts of aluminum, titanium,
molybdenum, and balance iron.
The properties of some nickel alloys frequently used in high
temperature furnaces are given in Table 8.6.

20. 11. Beyond about 1200°C, the only metals available are refractory
metals—tungsten, molybdenum, and tantalum.
● They are very costly compared to nickel alloys.
● Their availability is limited to simple shapes such as sheets,
rods, and wires.
● They oxidize easily beyond about 600°C and have to be
protected by special gases, or inert gases, or vacuum.

21. ● At temperatures beyond 1200°C they have low strength. The
maximum useful temperature when properly protected are
tungsten–2560°C, molybdenum–1900°C, and tantalum–
2400°C.
● Their main use is in radiation shields, resistors, furnace tubes,
boats, crucibles, etc.
● For processing high purity metals as in semiconductor
manufacture, platinum or platinum alloys are used up to
1600–1700°C.

22. 12. Metals have high specific heat and conductivity. Heat
absorbed or conducted away represents thermal loss.
Hence, the amount of metal fittings and furniture in the
heating zone should be the minimum required.

23. Summing Up
The choice of metals and alloys should be made by considering the
temperature, type of service (continuous or intermittent), expected life,
oxidation, and possible corrosion, the most important criteria being the
cost. In many cases the choice is a compromise.

24. References
Yeshvant V. Deshmukh. ( 2005). Industrial Heating Principles, Techniques,
Materials, Applications, and Design. Published by CRC Press.

25. THANK YOU
For listening