Therma 4713

General characteristics

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:
• Chemical industry (drums)
• Power industry (coal burners)
• Metalworking industry (heat treatment boxes)
• Furnace technology (walls, doors)

 

Chemical composition

The chemical composition is given in the table below.

 

StandardGradeCMnCrNiMoNOther
Typical47130.066.5Si:0.8 Al:0.8
Mechanical properties

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

 

StandardGradeRp0.2Rp1.0RmElongationImpact strengthRockwellHBHV
  MPaMPaMPa%J
Product type: Cold rolled coil and sheet
Typical (thickness 1 mm)
Product type: Hot rolled coil and sheet
Typical (thickness 4 mm)3754005002679
Product type: Hot rolled quarto plate
Typical (thickness 15 mm)32035047530

 

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
6

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
7,711,5234500,70Yes

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