WELDABILITY OF STEEL IN WELDING

WELDABILITY OF STEEL

HYDROGEN CRACKING:

During fabrication by welding, cracks can occur in some type of steel, due to the presence of HYDROGEN. The technical name for this type of cracking is “HYDROGEN INDUCED COLD CRACKING”.

Hydrogen cracking is also called as:

HAZ Cracking: Cracks tends to occur mainly in HAZ.

Cold Cracking: Cracking occurs when weld metal cools down.

Delayed Cracking: Cracks occur sometimes after welding is done.( after 48 hours)

Under Bead Cracking: Cracks occur in the HAZ beneath weld bead.

HYDROGEN CRACKING MECHANISM:  

             Hydrogen enters the molten metal when hydrogen containing molecules are trapped & when hydrogen containing molecules break down into hydrogen atoms. During welding they diffuse into solid steel and while the weld metal & HAZ is hot they tend to diffuse into atmosphere. But at low temperatures hydrogen cannot escape into atmosphere and get entrapped into the HAZ.

            If the HAZ has susceptible micro-structure indicated by being relatively hard & brittle, there are also relatively high tensile stresses in the weldment & tends to hydrogen cracking.

FACTORS INFLUENCING SUSCEPTIBLLITY TO HYDROGEN CRACKING:

            Hydrogen cracking occur in HAZ when 4 conditions exists at a same time

  • Hydrogen is present above a certain level (i.e. 15ml/100gm of weld metal).
  • A tensile stress is acting on the weld (more than half of yield strength)
  • A hard & brittle HAZ microstructure is present (martensite).
  • The weld ment is at low temperature (i.e. below 300 degree centigrade).

MEASURES TO AVOID HYDROGEN CRACKING:

            Hydrogen cracking can be avoided by ensuring that at least one of the “4” factor is not active during welding.  

HYDROGEN:

  • Ensure that coated electrodes, flux cored wire & SAW fluxes are low in hydrogen content when welding.
  • By using low hydrogen welding process like TIG, MIG etc.
  • Low hydrogen electrodes must be stored & backed or supplied in vaccum sealed packages to protect them from atmosphere.
  • Ensure that weld zone is dry & free from oil, grease, rust & scales as they may contain hydrogen.
  • Check the amount of moisture present in shielding gas or use dry shielding gas.
  • Use bakes electrode.

TENSILE STRESS:

  • Avoiding stress concentration due to poor fit-up.
  • Avoid poor weld profile & sharp weld toes.
  • Apply a stress relief after welding.
  • Keep weld volume as low as possible.

HARD & BRITTLE HAZ MICROSTRUCTURE:

  • Processing the steel with a low C.E.V that is at low end.
  • Using moderate welding heat input so that weld does not cool quickly.
  • Apply pre-heat so that HAZ cools more slowly.
  • Maintain inter-pass temperature for multi-run welds.

WELD MENT AT LOW TEMPERATURE:

  • Apply a suitable pre-heat temperature.
  • Maintain the pre-heat temperature to 250 degree centigrade when welding is finished & hold it at same temperature at least for 2-hours to facilitate escape of hydrogen.

 SOLIDIFICATION CRACKING:

            The technical name for cracks that forms during weld metal solidification is called “SOLIDIFICATION CRACKING”.

Solidification Cracking is also called as:-

Hot Cracking: They occur at elevated temperature.

Centerline Cracking: Cracks may appear at the center line of the weld bead.

Crater Cracking: Small cracks in weld crater are solidification cracks.

SOLIDIFICATION CRACKING MECHANISM:

            Weld metal solidifies over a temperature range and since solidification starts from the fusion boundary towards the center line of the weld pool, during the last stages of the solidification there may be enough liquid film present to form a weak zone in the center of the bead. This liquid film is the result of the low melting point constituent being pushed ahead of the solidification front.

            During solidification, tensile stresses starts to build-up due to contraction of the solid parts of the weld bead and it is these stresses that causes the weld bead to rupture. These circumstances result in a weld bead showing a centerline crack that is present as soon as the bead has been deposited.

FACTORS INFLUENCING SUSCEPTIBILITY TO SOLIDIFICATION CRACKING:

            Solidification cracking occurs if 3 conditions occur a same time namely:

  • The weld metal has susceptible chemical composition.
  • The welding condition used gives an   un-favorable bead shape.
  • High level of restraints or tensile stresses present in the weld.

MEASURES TO AVOID SOLIDIFICATION CRACKING:

      Solidification cracking can be avoided by ensuring that at least one of the “3” factor is not active during welding.

WELD METAL COMPOSITION:

  • Ensure weld joints are cleaned thoroughly before welding.
  • Ensure the sulphur contaminations like grease or oil are not present on the parent material and weld zone.
  • Ensure copper contaminations are not there in the weld zone as it has low solubility in steel & can form film that can be a source of cracking.

UN-FAVOURABLE WELDING CONDITION:

  • Reducing the welding current to give shallower bead.
  • Reducing welding speed to give a wider weld bead.
  • For TIG welding use a slope-out device to make a favorable weld bead (width to depth ratio >>2).

RESTRAINTS OR TENSILE STRESSES:

            Cannot be completely removed but can be reduced.

  • Apply Pre-heat which may be helpful in some cases
  • Ensure balanced welding to avoid distortion
  • Consider welding sequence to avoid or take account of the build up of high restraints.

LAMELLAR TEARING:

            Lamellar tearing is a type of cracking that occur only in steel plates or other rolled products underneath a weld.

Characteristics of Lamellar Tearing:

  • Occur only in rolled plates.
  • Cracks usually form close to, but just outside the HAZ.
  • Cracks tend to lie parallel to the plates having a stepped aspect.

LAMELLAR TEARING MECHANISM:

            High stresses in the through thickness direction, that are present as welding residual stresses causes the inclusion stringers to Open-Up and the thin ligament between individual De-Cohesed inclusion then tear & produce Lamellar Tearing.  

FACTORS INFLUENCING SUSCEPTIBILITY TO LAMELLAR TEARING:

            Lamellar tearing occurs when two factors are active at same time.

  • A susceptible rolled plate is used to make a weld.
  • High stresses act on the through thickness direction of the susceptible plate.

MEASURE TO AVOID LAMELLAR TEARING:

Susceptible Rolled Plates:-

  • Ensure that the material used have a good through thickness ductility ( 20% elongation & above ).
  • By using clean steel with low sulphur content ( less than 0.015% ) & with few inclusions.
  • By using the plate which has been subjected to Through-Thickness Tensile Testing to ensure good Through-Thickness Ductility.

Through-Thickness Stresses:-

  • Reducing the size of the weld by using partial penetration joint instead of full penetration joints.
  • Using fillet weld joint instead of the full penetration or Partial penetration weld joint.
  • By applying a buttering layers
  • By changing joint design.

MCQ IMPORTANT POINTS

    1. Hydrogen cracking is caused if C.E.V Value is high (0.48%).
    2. Low alloy steel is very much prone to hydrogen cracking
    3. High strength low alloy (HSLA) is very much prone to hydrogen cracking.
    4. Carbon percentage is related to hardness
    5. High hardness will cause hydrogen cracking
    6. Pre-heat is applied to slow down cooling rate.
    7. Pre-heat & Post-heat is applied to avoid hydrogen cracking.
    8. Material with high sulphur & copper content are prone to Solidification cracking.
    9. Solidification cracking is caused if W: D ratio is less that 2:1
    10. SAW Welding process is prone to Solidification cracking.
    11. Lamellar tearing is caused just outside the HAZ in parent metal.
    12. Buttering / cladding layer is applied to avoid lamellar tearing.
    13. Short Transverse Reduction Area ( STRA) test is used to measure through thickness ductility in Z-direction.

 

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