Reheat cracking in low Alloy and Stainless Steel

By: Publisher Team | Oct 06, 2020 00:53 AM
Reheat cracking in low Alloy and Stainless Steel

Definition

Reheat cracking is also called stress relief cracking. From the definition itself it is clear that reheat cracking is related with PWHT after welding, where PWHT is compulsory to soften the martenstic micro structure or to reduce the residual stress. So that PWHT cannot avoided for reheat cracking prevention.This type of cracking observed in austenitic stainless steel and Cr-Mo-V steels. 

Reheat cracking Mechanism

Mostly reheat cracking found in low alloy steels containing Cr,Mo,V (secondary carbide formers).Austenitic stainless steel having Nb,Ti with high carbon content also susceptible to reheat cracking in elevated temperature. Materials having strong precipitation reaction during PWHT also susceptible for this type of cracking, but this reheat  cracking not yet reported in aluminum or titanium alloys.

Reheat cracking Mechanism in Low Alloy steels

Reheat cracking found in austenitic grain boundaries of the coarse grained region of the HAZ. In case low alloy steel five conditions to be present in order to occur reheat cracking.

1)  Elevated temperature thermal gradient

This means the HAZ heated up to a temperature where austenitic phase forms and to a temperature where grain growth occurs. This temperature particularly in the range of 350-550⁰C

2)  Carbide dissolution

It is found that very low carbide dissolution occur at above mentioned temperature range, so that there is high precipitation reaction occurs.

3)  Residual Stress

During the cooling there exist some residual stress in the structure.

4)   Reheating into critical temperature range

During cooling period the microstructure again experience a temperature (350-550⁰C) where again carbide precipitation reaction occurs.

5)   Creep or stress relaxation during reheating

Additional stress gained during reheating will promote this cracking.

During weld thermal cycle HAZ get heated to a temperature where grain growth occurs, that is austenitic phase, at this time carbide dissolution is high. During the cooling time austenitic phase get converted to martensitic and carbide solubility decrease. This two condition will result stress and precipitation hardening, this leads to reheat cracking along the grain boundaries. This cracking normally found in HAZ than in weld metal.

Reheat cracking Mechanism in Stainless steels

Reheat cracking well observed in austenitic stainless steel especially in Type 347 stainless steel and it is found in HAZ and fusion zone of thick members.

As we know that Type 347 and 321 are stabilized stainless steel due to the presence of Nb and Ti to reduce the inter granular corrosion. Reheat cracking also observed in type 308 filler welded by FCAW process.This crack formation is due to the presence of bismuth in the slag.

During the heating cycle niobium carbide will get precipitated and at the same temperature stress also developed at the grain boundaries which finally ends in cracking. One interesting thing about this cracking is that it will found in weld metal and HAZ, but in low alloy steel it found only in HAZ.

Thick section of weld formed by using type 308 filler in FCAW process are also susceptible to reheat cracking. This is because flux cored electrode contains Bi, this reduces the ductility of weld metal at a temperature above 700⁰C and reheat cracking develops at austenite-delta ferrite interface.

Under clad cracking

This is a type of reheat cracking found during cladding of low alloy steel. This is because of reheating of coarse grained micro structure produced during cladding.

Relaxation cracking

It is the another form of reheat cracking when austenitic stainless steel subjected to elevated temperature during service. Its mechanism is similar as reheat cracking but it will occur after a long time.

Prevention of Reheat cracking

1)   Composition control

We know that reheat cracking is due to the presence of secondary carbide formers. So cracking can be eliminated or reduced by selecting steels with low susceptibility to reheat cracking

2)   Effect of welding condition

Grain growth is the one cause for reheat cracking to occur, as the grain growth increases the susceptibility to reheat cracking increases. In order to reduce the grain growth we need some control over weld heat input to reduce the grain growth.

3)   Control of residual stress

We need to control the stress, that is formed during welding of thick sections. Preventive steps include preheating, control of bead size and welding sequence.

4)   Control of stress relaxation

It is possible to avoid the formation of crack by managing the stress relaxation time even though if there any residual stress present. This is not easy for low alloy steel but it can be applied to austenitic stainless steel because the carbide precipitation is at higher temperature in stainless steel. So it is possible the weld to hold below this temperature for some time to reduce the stress.

5)  Effect of stress concentration

Reduce the stress concentrated area in the weldment like removing sharp edges, blending toe and so on.

6)   Buttering

Apply a resistant weld metal to the susceptible base metal where the composition of buttered layer is resistant to reheat cracking.

Source:https://www.theweldingdiary.co.in/2020/10/reheat-cracking-in-low-alloy-and.html


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