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Kanthal APMT (Construction materials)

Datasheet updated 2014-06-11 16:08:31 (supersedes all previous editions)

Kanthal APMT is an advanced powder metallurgical, dispersion strengthened, ferritic iron-chromium-aluminium alloy (FeCrAlMo alloy) recommended for continuous use up to 1250°C (2280°F) in oxidizing and reducing environments.

Kanthal APMT has high creep strength and excellent form stability up to 1300°C. Kanthal APMT forms a protective and non-scaling Al2O3 surface oxide when exposed to high temperature, which gives good protection in most furnace and combustion environments, i.e. oxidizing, sulphidizing and carburizing, as well as against attacks from deposits of coke, ash, etc. The combination of excellent oxidation properties and form stability makes the alloy unique.
The superior resistance of Kanthal APMT to oxidation and carburization makes it very suitable for high temperature construction applications in various atmospheres.

Applications

  • Radiation tubes for electrical and gas heated furnaces
  • Retorts and muffles for heat treatment and sintering of powder metallurgical components
  • Shielding tubes
  • Burner components, nozzles and flame detectors
  • Load carriers, furnace furniture and load carrying trays
  • Radiant cracker tubes in ethylene furnaces
  • Uncooled components in coal, gas, and biofuel fired power plants
  • Thermocouple protection tubes in power plants and high temperature petrochemical processes

Values and diagrams are representative for all product forms in delivery condition unless otherwise stated. Values presented in imperial units are interpolated from tests made in SI-units.

Forms of supply

Form Dimensions (mm) Dimensions (inch)
Plate width ≤ 1200 ≤ 47,24
thickness 3 - 20 0.12 - 0.79
length ≤3000 ≤ 118,44
Extruded tubes Outer dia. 26 - 260 1.05 - 10.24
Wall thick. 2.87 - 11.0 0.11 - 0.43
length** 3000 - 13000 118.11 - 511.81
Cold Rolled Strip* width ≤ 205 ≤ 8.07
thickness 0.2 - 3 0.01 - 0.12
Wire Ø 0.2 - 9.5 0.01 - 0.37
Rod Ø 5.5 - 12 0.22 - 0.47
Round bar Ø ≤ 100 ≤ 3.94
Length ≤ 4500 ≤ 177.17
Forging blanks width ≤ 500 ≤ 19.69
thickness 35 - 170 1.38 - 6.69
length** ≤ 3000 ≤ 118.11
Square bar ф ≤ 150 ≤ 5.91
length ≤ 4500 ≤ 177.17

Other sizes and forms can be discussed on request

*) Cold rolled strip can be delivered as cut to length products

**) Length depending on cross section

Chemical composition

C % Si % Mn % Mo % Cr % Al % Fe %
Nominal composition 3.0 21.0 5.0 Balance
Min - - - 20.5 -
Max 0.08 0.7 0.4 23.5 -

Microstructure

Structure is ferritic with typical average grain size 30-50mm in delivery state. Grains are typically elongated in the long direction in wire and bar and are generally extended in the plane of flat products. Some product forms are subject to a secondary recrystallization after exposure to temperature exceeding 1000°C that typically leads to long and flat grains with length or width up to a few hundred µm.

Microstructure APMT Microstructure APMT
Polished and etched micrograph, from 8 mm hot rolled plate, delivery state TEM section showing grain boundaries and particle dispersion

Physical properties

Density g/cm3 7.25
Electrical resistivity at 20°C Ω mm2/m 1.40
Poisson's ratio 0.30

Temperature factor of resistivity, Ct

Temperature
[°C]
100 200 300 400 500 600 700 800 900 1000 1100 1200 1300
1.00 1.00 1.01 1.01 1.01 1.02 1.02 1.02 1.03 1.03 1.03 1.03 1.04

Young's modulus

Temperature [°C]
[GPa]
20 100 200 400 600 800 1000
220 210 205 190 170 150 130

Thermal conductivity

Temperature [°C]

Thermal Conductivity
[W m-1 °C-1]

Temperature [°F]

Thermal Conductivity
[Btu ft-1h-1°F-1]

50

600

800

1000

1200

11

21

23

27

29

1200

1400

1600

1800

2000

2200

12.5

13

14

15.5

16

17

Coefficient of thermal expansion

Temperature [°C]

Thermal Expansion (x106) [°C-1]

Temperature [°F]

Thermal Expansion (x106) [°F-1]

20 - 250

20 - 500

20 - 750

20 - 1000

20 - 1250

12.4

13.1

13.6

14.7

15.4

68-400

68-600

68-800

68-1000

68-1200

68-1400

68-1600

68-1800

68-2000

68-2200

6.8

7.0

7.2

7.3

7.4

7.6

7.9

8.1

8.4

8.6

Specific heat capacity

Temperature [°C]

Specific Heat Capacity
[J kg-1 °C-1]

Temperature [°F]

Specific Heat Capacity
[Btu lb-1 °F-1]

20

200

400

600

800

1000

1200

480

560

640

710

670

690

700

68

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.16

0.16

0.16

0.17

0.17

Melting point 1500°C (2732°F)
Magnetic properties Ferromagnetic, Curie point approximately 600°C (1112°F)
Emissivity - fully oxidized material Ɛ 0.70

Note: All values shown in Imperial Units are extrapolated

Mechanical properties

Tensile properties at room temperature 20°C (68°F)

Proof strength
Rp0.2
MPa (ksi)
Tensile strength
Rm
MPa (ksi)
Elongation
A
%
Hardness
Hv
510-600 (74-87) 725-780 (105-113) 10-25 250

Note: Material in heavy sections generally has higher tensile strength and lower elongation values

Mechanical properties at elevated temperature

All values are representative average values in delivery condition. The samples are taken in the longitudinal direction from tube and in length and cross directions on hot rolled plate.

Hot tensile test (deformation rate 10-3 s-1)

Temperature °C (°F)

Tensile strength
Rm
MPa (ksi)
600 (1112) 420 (61)
 800 (1472) 120 (17)
 1000 (1832) 42 (6)
 1200 (2192) 16 (2.3)

Hot tensile test - Gleeble test (deformation rate ~1 s-1)

Creep strength - 1% elongation
Time Temperature/ Stress (MPa)
h  700°C 800°C
 900°C 1000°C
1100°C
1200°C
 1300°C
 100  39,9  26,2  19,7  12,7  7,0  3,4  2,1
 1000  36,8  23,4  16,2  9,9  5,0  2,3  1,5
 10000  34,0  21,0  13,2  7,8  3,6  1,6  1,2
 100000  31,4  18,8  10,8  6,1  2,6  1,1  0,9

1% elongation data are calculated from minimum strain rate data. In general, there is an initial amount of primary creep in the order of 0,3 - 1% depending on product form, temperature and stress level. Total elongation to rupture depends on temperature and stress but is typically in the order of 3 to 12% where the lower range is representative for low stress levels.

Creep strength - rupture
Time Temperature/ Stress (MPa)
h  700°C 800°C
 900°C 1000°C
1100°C
1200°C
 1300°C
 100 45,0 29,2 21,6 14,4 8,7 4,6 2,7
1000 39,7 24,8 17,0 10,8 5,5 2,5 1,5
10000 35,0 21,1 13,4 8,1 3,5 1,4 0,9
100000 30,8 18,0 10,6 6,1 2,3 0,8 0,5

Creep rupture data are representative average values for tube, bar and hot rolled plate based on creep tests performed within the time and temperature range indicated by the length of the solid lines in the diagrams (test times less than 50 000 hours). It should be pointed out, that component lifetime on thinner sections may be limited by oxidation/corrosion rather than creep rupture at low stress levels and long exposure times as indicated by the shaded area in the tables. 10 000 hour rupture compared to some other alloys are given in diagram.

Creep rupture stress

Larsson-Miller diagram

Secondary creep rate

Corrosion resistance

High Temperature Oxidation and Corrosion Properties

Kanthal APMT exhibits excellent high temperature corrosion properties due to the spontaneous formation of a thin layer of aluminium oxide (Al2O3) that protects the base material from corrosion attack. The most important properties of the scale are summarized below:

  • Thermodynamically stable – forms also in protective atmospheres at very low dew points
  • Inert – once formed, it is very stable with respect to chemical reactions
  • Dense – forms a very effective barrier against carbon diffusion and penetration from contaminants
  • Thin – very small amounts of aluminium is consumed to form and maintain the scale which results in very long oxidation life time
  • Adherent – resistant to spallation during thermal cycling

Oxidation Properties

The oxidation resistance of Kanthal APMT is superior to that of Ni-base and high alloyed austenitic chromia forming alloys due to its alumina protection. It gives less scale spallation, and alumina is, in contrast to chromia, not sensitive to emit volatiles in humid atmospheres.

Kanthal APMT is recommended for service in air and in most oxidizing and reducing gases up to 1250°C (2280°F) which is approximately 100°C (180°F)higher than that of the best performing chromia forming alloys. For shorter periods, temperature up to 1300°C (2372°F) is acceptable without substantial detrimental effects.

Corrosion resistance in dry N2 or H2/N2 is very good when DP is higher than -25°C (-13°F). Below this level of water content, the alloy might be susceptible to nitriding in certain situations.

The figure below shows mass change during intermittent oxidation in air at 1100°C. After an initial period, cycle times were approximately 100 hours. Kanthal APMT shows a sub-parabolic weight gain, while the oxide scale on the Fe25Cr35Ni alloy starts to flake after less than 24 hours.

Oxidation limited lifetime

Oxidation limited lifetime is determined by the gradual consumption of Al within the alloy. The protective alumina scale breaks down after extended time at high temperature when the level of Al has reached beween 1 and 3wt% depending on temperature and thermal cycling. Comparative oxidation lifetime may be estimated according to ASTM B78-81. The test is performed on Ø 0,7 mm wire and results are shown in the diagram for APMT and some other high temperature alloys.

Carburization Properties

The formation of a protective alumina scale gives Kanthal APMT superior resistance to carburization compared to chromia forming alloys.

In the figure below the average depth of carburization was calculated based on total loss of material during the test.

Heat Treatment

Preoxidation

Pre-oxidation treatment results in a protective ~1 µm thick alumina scale that is ideal for further exposure in corrosive environments. Recommended pre-oxidation parameters are 8 hours at 1050°C. Cooling rate: 50°C/h down to 500°C followed by air cooling.

Stress relieve

Stress relieve can be made after further forming processing or welding. Recommended parameters are 30 minutes at 875° in air. Cooling rate: 100°C/h down to 500°C followed by air cooling.

Fabrication

Kanthal APMT is ductile at room temperature with elongation to rupture between 10 and 25% depending on product form. Since room temperature impact strength is comparatively low, we recommend nevertheless that plastic deformation is performed using a preheating to T≥250°C (480°F) when possible.

Bending over edge with radius give less localised stress compared to V-bending and is preferred when possible. Using V-bending, preheating is generally needed.

For plates and strip the edge bending radius Rmin > t is generally possible.

For tubes, inductive bending with heating to 875°C is recommended. Normally no stress relieving heat treatment is necessary after the bending process. Minimum radius of bending, with acceptable change of wall thickness ± 10 %, can be calculated from

Rmin = 3 O.D.

Where O.D. is the outer diameter of tube and radius of bending is defined from the centreline of the tube.

Machining

To be added

Joining

Recommendations are for guidance only, and the suitability of a material for a specific application can be confirmed only when the actual service conditions are known. Continuous development may necessitate changes in welding technical data without notice.

The most commonly used method for joining APMT is conventional TIG/GTAW-welding. Laser welding is also used successfully and gives excellent weld joints and small HAZ. Alternative joining methods like brazing and mechanical joining like riveting and threading has been tested and may be useful for certain applications and has the advantage that the strengthening particle dispersion remains intact.

Welding consumables

For TIG/GTAW welding, wire Ø1.6 or 2.4mm may be used. Ø1.6 mm is recommended for the root pass and for subsequent passes 1.6 or 2.4 mm can be used.

Weld preparation

The recommended joint preparation for welding TIG/GTAW with material thickness between 5 and 20 mm can be seen in the figure.

a) Joint preparation with dimensions. Reccomended welding settings. b) Welding can best be done without root gap.
Current(A) Voltage(V) Current type(polarity) Shielding gas Gas flow(l/min) Root gas Root gasFlow (l/min)
80-150 11-14 DCEN Ar 8-10 Ar 10-20

Preheating

Preheat to 250 ± 50 °C (480 ±90°F) is recommended. If possible, preheating with open flame/torch should be avoided, or if the only option, be performed by personnel experienced in welding APMT due to the difficulties of attaining an even temperature distribution on the weld area.

If necessary, reheating of the weld area should be performed in order to maintain the temperature of the weld at minimum 200°C (390°F) during the welding procedure.

Post weld heat treatment - PWHT

A post weld stress relieve has to be carried out directly after welding. The weld is not allowed to cool below 200°C (390°F) before the post weld heat treatment is performed. For parameters, see heat treatment. We recommend a combined stress relieve and pre-oxidization of the weld area in conjunction with the post weld heat treatment in case of APMT to APMT welding.

Initial cleaning and post weld cleaning

Degreasing of the joint faces should be done prior to welding. Post weld cleaning can

be done mechanically by stainless steel brush.

Dissimiliar welding

When welding APMT to other materials, APMT is usually the more sensitive of the two materials and the welding can be carried out as when welding APMT to APMT.

Recommended welding consumables for APMT towards some common alloys

Weld towards Filler
Kanthal APMT Kanthal APMT
Ni-base alloys, 600, 601 Sandvik 25.20L
High alloy autenitic stainless steel, 310, 353MA, 253MA, 800, HK40, HP Sandvik 25.20L
Austenitic stainless steel, 304, 316, 347 Sandvik 308L
Ferritic stainless steels, 409, 430, 446 Kanthal APMT
Carbon and low alloy steel Kanthal APMT

Retained strength in the weld

Kanthal APMT is an advanced powder metallurgical dispersion strengthened alloy and welding will have a negative impact on the mechanical and high temperature creep properties of the material. The reason is the disruption of the grain structure and the distribution of the dispersion. Representative data from creep strength of TIG welds made with Kanthal APMT filler can be seen in the diagram. For example, at 1000°C (1830°F), the weld rupture strength can be compared to unaffected material at 1100°C (2010°F).

Additional information

These guidelines/recommendations take oxidation and corrosion properties during prolonged exposure to high temperature into consideration. Therefore our recommendations may differ from traditional welding recommendations used in construction welding. For further advice, contact your local Sandvik Heating Technology sales representative.

Disclaimer: Recommendations are for guidance only, and the suitability of a material for a specific application can be confirmed only when we know the actual service conditions. Continuous development may necessitate changes in technical data without notice. This datasheet is only valid for Kanthal materials.

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