Therma 4724
EN 1.4724

General characteristics

Low-alloyed ferritic heat resisting grade with improved scaling resistance. Uncritical regarding embrittlement at service temperatures. Applied for e.g. furnace equipment, thermal boiler components, grids, burner nozzles, conveying parts, thermowells.

The high temperature (HT) ferritic stainless steels complement Outokumpu austenitic heat and creep portfolio. The main alloying element in the ferritic grades is chromium. Its positive effect on the scaling resistance is enhanced by silicon and aluminium. The characteristic properties are excellent resistance to oxidising and reducing sulphur containing atmospheres, good resistance to oxidation and high thermal conductivity with low thermal expansion.

4713 does not form brittle phases but should only be exposed to moderately corrosive atmospheres owing to its low chromium content.

4724 is a truly stainless high temperature grade with 13% chromium. It is not critical in terms of embrittlement. 4742 shows better scaling resistance than 4724 and can be subjected to reducing sulphur environments without risk. It is subject to 475°C embrittlement and grain coarsening at temperatures above 950°C. σ-phase may form after long time exposures to temperatures around 650°C.

4762 - with the highest chromium content- is the most resistant to reducing sulphurous gases. It is susceptible to the same embrittlement phenomena as 4742 whilst σ-phase forms during long exposures in the range 600°C to 800°C.

Characteristic temperatures

The characteristic temperatures for the different grades are shown in the table below.

Steel grade Maximum service temperature in dry air,ºC  Hot forming1,ºC  Annealing2,ºC
4713 800 1100-750 750-800
4724 850 1100-750 800-850
4742 1000 1100-750 800-850
4762 1150 1100-750 800-850
       

1 Cooling still air        2 Cooling forced air or water

Creep strength

Creep properties of 47XX, Rp0,1 N/mm2 (mean values) are shown in the table below.

Time,h Temperature
  500 600 700 800 900
1 000 80 27.5 8.5 3.7 1.8
10 000 50 17.5 4.7 2.1 1.0

Typical applications

Outokumpu Stainless ferritic high temperature steels are mostly used in high temperature applications with sulphurous atmospheres and/or low tensile loads such as for installations within:

  • Furnace equipment
  • Thermal boiler components
  • Grids
  • Burner nozzles
  • Conveying parts
  • Thermowells
  • Chemical industry (drums)
  • Power industry (coal burners)
  • Metalworking industry (heat treatment boxes)

 

Product forms, available sizes and finishes

Flat

Product typeFinishesThicknessWidth
Black hot band1U3,65-6,001000-1300
Cold rolled coil2B, 2BB, 2C, 2D, 2E, 2G, 2J, 2R0,05-4,003-1500
Cold rolled sheet2B, 2BB, 2C, 2D, 2E, 2G, 2J0,50-4,00350-1500
Hot rolled coil, pickled1C, 1D, 1U3,50-6,0050-1530
Precision strip2R0,05-1,503-649
Chemical composition

The chemical composition is given in the table below.

The chemical composition is given as % by weight.

CMnCrNiMoNOther
Typical 0.0712.5Si:1.0 Al:0.9
EN 10095 ≤0.12≤1.0012.00-14.00Si:0.70-1.40
Mechanical properties

Mechanical properties at room temperature are shown in the table below.

 

StandardRp0.2Rp1.0RmElongationImpact strengthRockwellHBHV
 MPaMPaMPa%J
Product type: Cold rolled coil and sheet
Typical (thickness 1 mm)480510600
Product type: Hot rolled coil and sheet
Typical (thickness 4 mm)
Product type: Hot rolled quarto plate
Typical (thickness 15 mm)34037051530
EN 10095 ≥ 250450-650

1)Elongation according to EN standard:
A80 for thickness below 3 mm.
A for thickness = 3 mm.
Elongation according to ASTM standard A2” or A50.

Corrosion resistance

Oxidation
In oxidising environments, a protective oxide layer is likely to be formed on the metallic surface. If the layer is tight and adherent, it can prevent other aggressive elements in the environment from attacking and reacting with the steel. However, the layer can grow in thickness due to constant oxidation. The resulting porous layer will allow the gas to penetrate through to the base material through pores or cracks. Silicon and aluminum are both beneficial for oxidation resistance. Low thermal expansion and high thermal conductivity of the ferritic base material reduce changes in volume and thus spalling of the protective layer.
Sulphur attacks
As a rule, ferrites perform better than austenites in oxidising and reducing sulphurous atmospheres. SO2 or H2S are possible compounds in sulphur containing process gases or fuels. In terms of resistance to carburisation, austenitic grades show more favorable results than ferritic ones due to their high Ni-content. Formation of chromium carbides or chromium nitrides, respectively, embrittles the material. Additionally, the surrounding matrix becomes chromium depleted and thus less able to form an oxide layer, which consequently reduces the scaling resistance of the material. Silicon has a beneficial effect on both carbon and nitrogen pick-up. Aluminum is only favorable in terms of carburisation. The high nitrogen affinity of aluminum results in aluminum nitrides retarding formation of a protective alumina and leading to premature failure of the material.
Molten metals
In molten metals, Nickel is the most susceptible element to dissolution. Austenitic material is bound to fail when e.g. molten copper penetrates the grain boundaries. HT ferrites - on the other hand - are expected to show good compatibility with molten copper. Final resistance will, of course, depend on the composition of the molten metal.
For more information, see Outokumpu Corrosion Handbook.

Pitting corrosion resistanceCrevice corrosion resistance
PRECPTCCT
13

PRE Pitting Resistant Equivalent calculated using the formula: PRE = %Cr + 3.3 x %Mo + 16 x %N
CPT Corrosion Pitting Temperature as measured in the Avesta Cell (ASTM G 150), in a 1M NaCl solution (35,000 ppm or mg/l chloride ions).
CCT Critical Crevice Corrosion Temperature is the critical crevice corrosion temperature which is obtained by laboratory tests according to ASTM G 48 Method F

 

 

Physical properties

Data according to EN 10095.

 

DensityModulus of elasticityThermal exp. at 100 °CThermal conductivityThermal capacityElectrical resistanceMagnetizable
kg/dm3GPa10-6/°CW/m°CJ/kg°CµΩm
7,710,5215000,75Yes
Fabrication

Hot forming
Hot working should be carried out within the temperature ranges given under the headline Characteristic temperatures earlier in this datasheet.
Formability/Machining
Generally, ferrites are difficult to form in the cold condition. They are formable at room temperature when sheets are no thicker than 3 mm; 4713 even 6 mm. Thicker 4713 and 4724 plates must be preheated and formed within the temperature range 250 - 300°C. 4742 and 4762 should even be heated up to 800 - 900°C to avoid formation of any brittle phases. Generally, the minimum radius for bending deformation can be taken as “double thickness”. Machining is considered to be less problematic due to their low strain hardening rates.
Welding
The same precautions as for carbon steels are normally required. Preheating of the joints to 200-300°C is necessary for plates thicker than 3 mm. Due to grain growth in the heat affected zone, the heat input should be minimised. Gas shielded welding methods such as GTA (TIG), plasma arc and GMA (MIG) are preferred. Pure argon should be used as the shielding gas. Matching filler material has detrimental effect on the ductility why austenitic welding consumables, e.g. 307, 309 or 310 are recommended. If the weld will be exposed to a sulphurous environment, overlay welding with the matching ferritic filler will be necessary.



 

Standards & approvals

The most commonly used international product standards are given in the table below.

 

StandardDesignation
EN 100951.4724