Wednesday 28 December 2011

WELDING DEFECTS : CRACKS

Cracks:

WELDING DEFECTS : CRACKS

Process Cracks

  •  Hydrogen induced cold cracking (HICC)
  •  Solidification cracking (Hot Tearing)
  •  Lamellar tearing
  •  Re heat cracking

When considering any type of crack mechanism, three elements must be present for it’s occurrence:
  • Stress: stress is always present in weldments,through local expansion and   contraction.
  • Restraint: may be a local restriction, or through the plates being welded.
  • Susceptible: microstructure: the structure is often made susceptible to cracking through welding, e.g high hardness

Hydrogen Cracking:

Hydrogen causes general embrittlment and in welds may lead directly to cracking.

The four essential factors for cracking to occur

  • Susceptible grain structure
  • Hydrogen >15ml
  • Temperature less than 200°C
  • Stress

Remedies for Hydrogen Cracking:

Precautions for controlling hydrogen cracking:

  1. Pre heat, removes moisture from the joint preparations, and slows down the cooling rate
  2. Ensure joint preparations are clean and free from contamination
  3. The use of a low hydrogen welding process and correct arc length
  4. Ensure all welding is carried out is carried out under controlled environmental conditions
  5. Ensure good fit-up as to reduced stress
  6. The use of a PWHT or Post Weld Heat Treatment

Solidification Cracks:

Essential factors for solidification cracking:

  • This type of cracking is referred to as Hot Cracking
  • Susceptible microstructure: Columnar grain growth
  • Impurities, sulphur, phosphorous and carbon
  • The amount of stress/restraint
  • Most commonly occurs in sub-arc welded joints
  • Joint design depth to width ratios,
  • Combinations of both stress, deep narrow welds and sulphur

Precautions for controlling solidification cracking:

  • Low dilution welding process
  • The use of high manganese and low carbon content fillers
  • Maintain a low carbon content
  • Minimise the amount of stress / restraint acting on the joint during welding
  • The use of high quality parent materials, low levelsof impurities
  • Remove laminations
  • Clean joint preparations, free from oil, paints and any other sulphur containing product.
  • Joint design selection depth to width ratios  


Solidification cracking in Austenitic Stainless Steel:

  • Austenitic stainless steel is particularly prone to solidification cracking
  • This is due to the large grain size, which gives rise to a reduction in grain boundary area
  • High coefficient of thermal expansion, with high resultant stress
  • A structure that is very intolerant to contaminations, sulphur, phosphorous and other impurities.
  • The precautions against cracking are the same as for plain carbon steels with extra emphasis on thorough cleaning and high dilution controls.


Lamellar Tearing:

  • Lamellar tearing has a step like appearance due to the solid inclusions such as sulphides and silicates linking up under the influences of welding stresses
  • It forms when the welding stresses act in the short transverse direction of the material (through thickness direction)
  • Low ductile materials in the short transverse direction containing high levels of impurities are very susceptible
  • The short tensile test or through thickness test is a test to determine a materials susceptibility to lamellar tearing.


Factors for lamellar tearing to occur:

  • Low quality parent materials, high levels of impurities
  • Joint design, direction of stress
  • The amount of stress acting across the joint during welding
  • Hydrogen levels in the parent material

**Note: very susceptible joints may form lamellar tearing under very low levels of stress.





Precautions for controlling lamellar tearing:

  • The use of high quality parent materials, low levels of impurities
  • The use of buttering runs
  • A gap can be left between the horizontal and vertical members enabling the contractional movement to take place
  • Joint design selection
  • Minimise the amount of stress / restraint acting on the joint during welding
  • Hydrogen precautions


In-Service Cracks:

  • Fatigue cracks
  • Weld decay in austenitic stainless steels
  • Creep failure
  • Stress corrosion cracking

Fatigue Cracks:

  • Fatigue cracks occur under cyclic stress conditions
  • Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area
  • All materials are susceptible to fatigue cracking
  • Fatigue cracking starts at a specific point referred to as a initiation point
  • The fracture surface is smooth in appearance sometimes displaying beach markings
  • The final mode of failure may be brittle or ductile or a combination of both

Precautions against Fatigue Cracks:

  • Toe grinding, profile grinding.
  • The elimination of poor profiles
  • The elimination of partial penetration welds and weld defects
  • Operating conditions under the materials endurance limits
  • The elimination of notch effects e.g. mechanical damage cap/root undercut
  • The selection of the correct material for the service conditions of the component


Weld Decay:

  • Weld decay may occurs in austenitic stainless steels
  • Also know as knife line attack
  • Chromium carbide precipitation takes place at the critical range of 600-850oC
  • At this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content
  • Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur


Precautions for Weld Decay:

  • The use of a low carbon grade stainless steel e.g. 304L, 316, 316L
  • The use of a stabilized grade stainless steel e.g. 321, 347, 348 recommended for severe corrosive conditions and high temperature operating conditions
  • Standard grades may require PWHT, this involves heating the material to a temperature over 1100oC and quench the material, this restores the chromium content at the grain boundary, a major disadvantage of this heat treatment is the high amount of distortion


2 comments:

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Unknown said...

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