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