Ferrite number control in Austenitic Stainless Steel Welding

Ferrite is defined as the solid solution of carbon in iron. What we perceive from ferrite number in welding? Ferrite number is the amount of retained delta ferrite into the weldment of austenitic stainless steels, which may support mechanical and corrosion properties (if present within the acceptable range). Ferrite number approximates 5-20. Lower limit is required for protection from aggressive corrosion while the higher limit is required to assure the minimum solidification cracking in weld deposits.

        1.    Stainless steel (austenitic)
        2.    Solidification modes
        3.    Problems
        4.    Factors affecting the ferrite content
        5.    Measurement of ferrite number
        6.    Ferrite control
        7.    Heat Treatment
        8.    Conclusion


Stainless Steel Austenitic
Stainless steel family includes three types; austenitic, martensitic and ferritic. Austenitic stainless steel is known for best welding properties and excellent corrosion resistance. It has face centered cubic structure with the alloying elements of chromium, nickel, manganese, carbon, nitrogen, chromium, molybdenum. Their strength can be increased when applied for cold working. Unlike other carbon steels they don’t undergo martensitic formation on quenching and similarly not subjected to cold cracking. However they can be subjected to hot cracking which is the concerning problem. On the basis of alloying elements it is classified as full austenitic or austenitic ferritic. Elements nickel, carbon, manganese and nitrogen constitutes austenite while the chromium, silicon, niobium, molybdenum form ferrite.


Solidification modes
The solidification modes are classified on the basis of austenite and ferrite phases. The modes are named as A (austenitic), AF (austenitic + ferrite), FA (ferrite + austenite) and F (ferrite). These modes depends on Crreq / Nireq ratio. Lower ratio will favor the formation of A mode while the higher ratio will favor F mode. In AF mode austenite is primary phase while in FA mode ferrite is the primary phase.


Problems
Weld deposit with less ferrite number will lead to hot cracking while on the other hand more ferrite content can also lead to other mechanical problems. Lower amount is required to protect the weldment from corrosive media.


Factors affecting the ferrite content
Cooling rate of weld metal, is the major one that controls the ferrite number in austenitic stainless steel weld deposits. So the selection of electrode diameters, arc length, amperages (current) and heat treatments are the controlling parameters that influence the ferrite number. As we know the common practice is to select the current according to the electrode diameter. Right selection of all above mentioned parameters assists in achieving controlled cooling rate which is required to obtain desired ferrite content.


Measurement of ferrite number
Following are the common methods which are being utilized for the evaluation of ferrite number into the weldment. 


Ferrite Control
Special electrodes are being used called controlled ferrite consumables in order to achieve required amount of delta ferrite into the weld metal. As it is stated above cooling rate has an amazing effect on ferrite number. As the cooling rate decreases, ferrite content tends to reduce while on the other hand cooling rate increases, the ferrite content tends to increase and (appear as fine particles into the microstructure). We should know that an acceptable range of ferrite is required for particular service conditions when 

  • High toughness is required at cryogenic temperature
  • Low magnetic permeability 
  • Protection from corrosive media.

However the factors like multipass welding, heat input, penetration and cooling rate are the keys for controlling ferrite number in austenite welds.


Heat Treatment
Some post weld heat treatments may embrittle the ferrite in microstructure . Delta ferrite has body centered cubic structure. Heat treatment within 1040-1100 C and holding for 10 minutes can dissolve the delta ferrite. Hence reducing the ferrite content up to 7-8%.
Heat treatment of austenite welds by annealing has a damaging effect by increasing delta ferrite content that may lead to the brittleness in weld. However the decomposition – transformation depends on the annealing temperature and time.


Conclusions

     1)    Stainless steels (austenitic) have worldwide applications in chemical plants, brewing, food industries, nuclear plants and power plants. They has excellent welding mechanical properties.

     2)    Control of ferrite number is the ultimate requirement in austenite welds. An acceptable range of ferrite number is required in austenite welds that provides resistance to hot cracking and corrosion.

     3)    WRC 1992 provided some measurement techniques and methods for the evaluation of weld ferrite content. Ferrite number can be predicted by magnetic permeability, Severn gage and ferrite meter.

     4)   Austenite composition in filler materials are responsible for the control of ferrite in weld deposits.

    5)    Ferrite number can be controlled by cooling rate, heat input and multipass tempering. Moreover Heat treatment can also be used to maximize and reduce the ferrite content.

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