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Pipes made of austenitic, heat-resistant steels
Heat-resistant steels were specially developed for use at high temperatures.
In the form of pipes, they are used in the construction of heat exchangers, for example.
Characteristics of heat-resistant steels
Heat-resistant steels are steels possessing good mechanical properties for short and long-term
loading due to their higher alloy content of chromium, nickel, silicon, and aluminium and with special
resistance to the effects of hot gases and combustion products as well as molten salt and metal at
temperatures above approximately 550°C. The level of their resistance depends enormously on the
reaction conditions and cannot be determined using any test method.
Scaling Resistance in the Air
|Type of Steel||Material||Temperature*|
|X12 CrNiTi18 9||1.4878||850°C|
|X15 CrNiSi 20 12||1.4828||1000°C|
|X 12 CrNi 25 21||1.4845||1050°C|
|X 15 CrNiSi 25 20||1.4841||1150°C|
|X 10 NiCrAlTi 32 20||1.4876||1100°C|
|Material||C %||Si %||Mn max.||P max||S max||Al %||Cr %||Ni %|
The scaling resistance the high-alloyed chromium-nickel steels is achieved using a
protective top layer consisting primarily of chromium oxide.
Additional additives, especially of aluminum and silicon, provide additional protection.
Oxidation, sulfurization, carburization, nitrogenization, and reactions with ashes and
other solid or molten deposits are particularly important for the scaling resistance from
a technical standpoint. The reactions can occur individually or simultaneously depending
on the prevailing conditions and may have correspondingly different reaction rates.
The scaling limit temperatures specified in Table 1 apply to air and are an approximation for
sulfur-free combustion gases. For high water vapor contents, the actual scaling limit may be lower.
For completely combusted, sulfur-free gases, a reduction of the scaling resistance by 100 to 200°C
must be taken into account depending on the composition of the gas.
In combustion gases containing sulfur, there is no significant impact on the scaling resistance
when a surplus of air is available.
In complete combusted, sulfurous gases, though, the scaling limit is significantly reduced
due to the formation of sulfide. Alloys with high nickel contents can exhibit strong scaling
above the nickel-nickel sulfide eutectic point, which is approx. 640°C.
When exposed to incompletely combusted gases, carburization of the heat-resistant
steels can occur. In this case, bonding with chromium can result in the depletion of this
element as a mixed crystal, which is indicated by a reduced scaling resistance.
The austenitic chromium-nickel steels, especially those with a high nickel content, are less
sensitive than the corresponding ferritic chromium steels.
For reductive combustion gases containing nitrogen, the behavior of the steel is similar
to that during carburization.
For deposits from the combustion gases, low-melting eutectics can form on the steel
due to reaction with the scale layer, which quickly leads to the destruction of
the material. The permissible temperature limits depend greatly in this case on the
composition of the deposits and are generally very low, for example like when
alkaline sulfates, phosphates, metals and/or heavy metal oxides are present.
Sulfidation is increased the most by hydrogen sulfide. Aluminum and silicon
improve resistance against sulfidation.
Nickel and silicon Improve the carburization resistance.
When starting up and shutting down systems and during downtimes, combustion products may
condense. If this condensate contains sulfurous acid or sulfuric acid, then you must
expect a stronger reaction.
Heat-resistant steels are generally used at temperatures at which the material creeps when
stressed. When calculating for systems, you must use the creep strength and elongation time
values provided in Table 4.
Comparison of Standards
|1.4878||321||Z 6 CNT 18-10||X 6 CrNiTi1811||12 Ch 48 N 10 T||A700|
|1.4828||309||Z15 CNS 20-12||-||20 Ch 20 N 14 S 2||H550|
|1.4845||310S||Z12 CN 25-20||X 22 CrNi 25 20||-||H522|
|1.4841||314||Z 12 CNS 25-20||X 16 CrNiSi 25 20||20 Ch 25 N 20 S 2||H525|
|1.4876||-||Z 8 NC 32-21||-||ChN 32 T||H500|
(*)=Manufacturer's Code Schöller-Bleckmann Böhler
When using heat-resistant steels, you must expect changes in the material in certain temperature
ranges that, after cooling down to room temperature, can lead to a reduction of the ductility.
The behavior of the material at the operating temperature is generally not affected by this.
|Type of Steel||Hardness||Elastic Limit*||Tensile Strength||Fracture Elongation**|
|1.4878||130-190||min. 210||500-750||min. 40%|
|1.4828||150-210||min. 230||500-750||min. 30%|
|1.4845||130-190||min. 210||500-750||min. 35%|
|1.4841||150-210||min. 230||550-800||min. 30%|
|1.4876||139-190||min. 210||500-750||min. 30%|
The values apply to cold formed pipes with wall thicknesses of 0.5 to 5 mm
(*)=0.2% elastic limit
(**)=The values apply to sample thicknesses ≥ 3 mm.
In austenitic steels with higher Cr content, the Ω phase can form the temperature range from 550
to 900°C. The Ω phase is a brittle, intermetallic compound between iron and chromium and other
transition metals that do not exhibit any non-permissible changes to the ductility at operating
temperatures, but that can cause the material to become brittle after cooling down to room
temperature. Si and Cr promote these precipitation processes, while Ni and Al hinder them.
The Ω phase is only relevant in actual practice for 1.4821 and 1.4841.
The Ω phase can be dissolved again by annealing at temperatures > 900°C.
Characteristic values of the long-term behavior at high temperatures
1% Elastic Limit*
|Material||Temperature||for 1,000h||for 10,000h|
|1.4878||600 °C||110 N/mm²||85 N/mm²|
|900 °C||13 N/mm²||5 N/mm²|
(*)=The stress, based on the initial diameter, that leads to a permanent elongation of 1%
after 1,000 or 10,000 h
|Material||Temperature||for 1,000h||for 10,000h||for 100,000h|
|1.4878||600 °C||185 N/mm²||115 N/mm²||65 N/mm²|
|900 °C||20 N/mm²||11 N/mm²||4 N/mm²|
(*)=The stress, based on the initial diameter, that leads to breakage after 1,000, 10,000 or 100,000 h.
Average linear coefficient of expansion between 20°C and ...
(10⁻⁶ mm) : (m x °C)
(W) : (cm x °C)
Other Characteristic Values
(**)=J : (g x °C)
(***)=Specific electrical resistance for (O x mm²) : m
Heat-resistant austenitic CrNi steels are characterized by a high temperature strength in addition
to their good scaling resistance. For this reason, they can generally be used for purposes in which
a high mechanical strength is required in addition to scaling resistance.
The high temperature strength of the material 1.4876 is improved through the addition of titanium
and aluminum so that the long-term values for this material at temperatures over 600°C are
Due to the NI content, these steels are more sensitive to sulfurous gases, especially in
non-oxidizing atmospheres. On the other hand, they have better resistance to carburization
and nitrogenization in comparison to ferritic steels.
The material 1.4841 should not be used in continuous operation at temperatures below 900°C
due to its tendency to become brittle in the Ω phase.
It should only be necessary in a few cases for the user to hot-form the heat-resistant
The hot forming temperature is 1150 - 800°C.
Due to their low yield strength and high elasticity, austenitic steels have good
cold forming properties. After very strong deformation, the resulting cold hardening
effects can be undone through subsequent heat treatment with fast quenching.
Annealing the austenitic steels at 900°C air temperature offers advantages
In terms of cutting operations over the quenched state.
In solution annealing, the steel is cooled in water or air, and for thinner walls,
in air or inert gas.
When machining austenitic steels, adequate cooling must be ensured due to their low thermal
conductivity. Its strong cold hardening behavior, which can make the use of dull tools or machining
at the cutting depth more difficult, requires the use of sharper tools and the correct specification of
the cutting depth and cutting speed.
The heat-resistant austenitic steels are, assuming the corresponding qualifications are available,
suitable for welding using all of the known methods. However, arc welding should be preferred
over gas fusion welding.
Welding slag must be removed. Its presence will lead to high removal rates, especially for
Sulfurous oven gases, due to the formation of low-melting corrosion products.
Preheating and heat treatment after welding is generally unnecessary.
|Base Metal||Electrode or Welding Rod|
We supply seamless hot-rolled and cold-processed pipes made of heat-resistant steels as well as
welded pipes with dimensions and tolerances based on DIN 2462 and DIN 2463.
An acceptance test certificate according to DIN 50049/3.1 can be made available for the
heat-resistant pipes. Acceptance is performed according to Steel-Iron Material Data Sheet 470.