Lamellar Tearing in Welding & carbon- Low alloy steels

What is Lamellar Tearing?

Lamellar tearing or Lamellar Cracking  is a cracking phenomenon that occurs in thick Al-treated plate that is subjected to high transverse (through-thickness) welding strains. The problem is related to the presence of flattened (rolled) MnS inclusions in Al-treated parent steel, lying parallel to the plate surface and giving low ductility in the through-thickness (short transverse) direction. The elongated inclusions on different planes at the edge of the visible HAZ crack or de-cohere, joining to form stepped cracks, which progress approximately parallel to the plate surface as lamellar tears.

Lamellar Tearing

Stages of Lamellar Tearing

A lamellar tear takes place in three stages as:

  1. Start of separation and void formation at inclusions in the base metal.
  2. Formation of crack “terraces” by the linking together of individual separations.
  3. Joining of terraces through shear failure. The tear itself exhibits a characteristic terraced profile, and it is this step-like terrace look that distinguishes a lamellar tear from an ordinary discontinuity.

How Lamellar Tearing affect welding Quality?

Lamellar tearing is mainly a problem when depositing T-butt or heavy fillet welds, as opposed to straight butt welding because the orientation of the weld means that the shrinkage strains act though-thickness. Where good ductility is needed in the through-thickness direction, this may be specified when ordering steel, as indicated in codes, eg BS EN 10164:2004. Short transverse reduction in area (STRA)   tests are required to give a 20-  30%  minimum. Often such steels are called   Z-grade steels   (having guaranteed properties in the Z or through-thickness direction). This improvement can be produced by a sufficient reduction in sulphur content and by inclusion shape control,  achieved by adding  Ca or rare earth metals to the molten steel.

Lamellar Tearing

Causes or reason of lamellar tearing

Lamellar Tearing occurs when below three conditions exist such as:

  1. Use of susceptible rolled plate having inclusions for making weld joints and,
  2. High transverse stress acting along the through thickness or Z (short) direction of the plate.
  3. Weld orientation.

Susceptible rolled plate refers to base material as a plate having very low ductility in the short transverse (Z)  direction, which causes tearing of material even in normal welding stress as the area in the inclusion lacks the strength to withstand stresses.  This low ductility is due to the presence of non-metallic inclusion (MnS) which will get flattened and elongated during hot rolling of material.

Mechanism of lamellar tearing/cracking

Below mentioned factors contribute lamellar cracking or tearing.

1) Low through thickness ductility of parent material also called short transverse ductility.

2) Thickness of material.

3) High quantity of stringers or inclusion along the rolling direction.

4) Weld joint geometry.

Lamellar tearing occurs in the HAZ region of C-Mn or low alloy steel, just outside the transformed region between ferritic-pearlitic structures. Impurities such as sulfur will get converted to manganese Sulphide during the solidification,  deformation process.  It is oriented along the rolling direction.  if the sulfur content is relatively high that is greater than 0.05% wt, then the stringer of MnS will be continuous along the rolling direction. If the sulfur content is less than 0.05% then MnS becomes discontinuous. 

Lamellar Tearing

Quantifying Lamellar cracking/tearing (STRA Test)

Most of the tests used to quantify Lamellar tearing in the material are based on self-restraint type, where welds produce high thickness stress.

Another type of quantifying lamellar tearing is to perform an STRA test (Short transverse reduction area). This is a simple test that gives some indication of lamellar cracking susceptibility. In this,  the test specimen is subjected to a tensile test with their axis perpendicular to the plate surface. It is measured as a percentage in reduction area, the higher the value of %R, The greater resistance to lamellar tearing or cracking. t measures through-thickness ductility of a steel plate (the Z direction). Steels with low  STRA are commonly associated with a high level of rolled sulfide or oxide inclusions and there is no grade of steel prone to lamellar tearing like steels with low STRA.

The general rule states that steels with STRA over 20% are resistant to tearing while steels with less than 10 to 15% STRA should be used only in lightly restrained joints. Steel suppliers can provide steel that has been through-thickness test with an STRA value of over 20%. 

Susceptible & improved Joint designs for Lamellar tearing

Various measures can be taken to avoid lamellar tearing, the main aspects is the improved weld design as shown in below figure.

Lamellar Tearing design

Prevention of Lamellar Tearing

  1. Use low-hydrogen electrodes when welding large T- and corner joints. Absorbed hydrogen is not deemed to be a principal cause for lamellar tearing initiation, but the use of low-hydrogen electrodes on large joints (longitudinal, transverse, or through-thickness) to minimize the tendency for hydrogen-induced cold cracking is good practice in any case. The use of non-low-hydrogen electrodes may invite trouble.
  2. In large joints, sequence weld passes in a manner that builds out the surface of base metal stressed in the longitudinal direction prior to depositing weld beads against the face of the base metal stressed in the through-thickness direction. This procedure allows a significant part of the weld shrinkage to take place in the absence of restraint.
  3. On corner joints, where feasible, the beveled joint preparation should be on the base metal stressed in the through-thickness direction so that the weld metal fuses to the base metal on a plane into the thickness of the base metal to the maximum degree practical.
  4. Double-V and double-bevel joints require deposition of much less weld metal than single-V and single bevel joints, and therefore, reduce the amount of weld shrinkage to be accommodated by approximately one-half. Where practical, use of such joints may be helpful.
  5. In weldments involving several joints of different thickness base metal, the larger joints should be welded first so that the weld deposits which may involve the greatest amount of weld shrinkage may be completed under conditions of lowest restraint possible. The smaller joints, although welded under conditions of higher restraint, will involve a smaller amount of weld shrinkage to be accommodated.
  6. The area of members to which large welds will transfer stresses in the through-thickness direction should be inspected during layout to assure that joint weld shrinkage does not apply through-thickness strains on base metal with preexisting laminations or large inclusions (see ASTM A578).
  7. Properly executed peening of intermediate weld passes has been demonstrated to reduce the potential for lamellar tearing. Root passes should not be peened in order to avoid the possibility of introducing cracks in the initial thin weld passes which may go undetected and subsequently propagate through the joint. Intermediate passes should be peened with a round-nosed tool with sufficient vigor to plastically deform the surface of the pass and change the tensile residuals to compressive residual stresses, but not so vigorously to cause a chopped-up surface or overlaps. Finish passes should not be peened.
  8. Avoid the use of over-strength filler metal.
  9. When practical, use base metal with low ( < 0.006%) sulfur or base metal with improved through-thickness properties.
  10. Critical joints should be examined by RT or UT after the joint has cooled to ambient temperature.
  11. If minor discontinuities are detected the Engineer should carefully evaluate whether the discontinuities can be left unrepaired without jeopardizing the suitability for service or structural integrity. Gouging and repair welding will add additional cycles of heating and cooling and weld contraction under restraint conditions that are likely to be more severe than the conditions under which the joint was initially welded. Repair operations may cause a more detrimental condition.
  12. When lamellar tears are identified and repair is deemed advisable, the work should not be undertaken without first reviewing the WPS and an effort made to identify the cause of the unsatisfactory result. A special WPS or a change in the joint detail may be required.

Acceptance standards for Lamellar Tearing/ Cracking

As lamellar tears are linear imperfections which have sharp edges, they are not permitted for welds meeting the quality levels B, C and D in accordance with the requirements of BS EN ISO 5817:2017.

Design calculation for Material Selection

DIN EN 1993-1-10 (Eurocode 3) Tab.3 specifies criteria for the calculation and selection of the required material.
Starting with a sheet metal thickness > 30 mm, the required ZEd must be calculated, using Table 3.2. 

The risk of lamellar tearing can be neglected if the following conditions are met:
ZEd ≤ ZRd

This follows that
ZEd = the required Z-value, which results from the magnitude of the strain of the base material due to obstructed weld shrinkage.
ZRd = the available Z value of the material pursuant to EN 10164, that is to say Z15, Z25 or Z35.

The required Z value ZEd can be determined using the following formula and Table 3.2 from DIN EN 1993-1-10:
ZEd = Za +Zb +Zc + Zd + Ze

Refer below Table for Criteria affecting the target value of zEd.

Lamellar Tearing design

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