Defects of Steel
Defects of Steel
Several types of defects can be found in steel, including:
- Inclusions: These are non-metallic particles that are present in the steel, such as slag, oxides, and other impurities. These particles can cause weakness in the steel and can also affect its surface finish.
- Porosity: This phenomenon is the presence of small holes or voids within the steel. Porosity can occur due to improper casting techniques, and it can also cause weakness in the steel.
- Cracks: Cracks can occur in steel due to various factors, including improper heat treatment, welding, and mechanical stress. They can weaken the steel and make it more susceptible to failure.
- Laminations: These are layers of steel that have separated from one another, creating a layered structure. Laminations can occur due to improper rolling or forging techniques, and they can also cause weakness in the steel.
- Seam: These are surface defects that occur due to improper welding or casting. They appear as lines on the surface of the steel and can weaken it.
- Scabs: These are raised or depressed areas on the surface of the steel that is caused by improper rolling or forging. They can weaken the steel and affect its surface finish.
- Laps: A lap is a type of surface defect that occurs when two layers of steel overlap one another, causing a raised area on the surface. Laps can weaken the steel and affect its surface finish.
- Cold Shut: A cold shut is a type of defect that occurs when two streams of molten steel do not fuse together properly during casting, resulting in a partial fusion. They can weaken the steel and affect its surface finish.
Steel can exhibit a variety of defects. It is important to detect and remove these defects during the manufacturing process to ensure the integrity and quality of the steel.
Further readings.
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Some lesser known Defects of Steel
1. Lamellar Tearing
Lamellar tearing is a rare but critical defect that occurs in welded steel plates, especially those with high sulfur content.
Cause:
- Inclusion of non-metallic materials (like manganese sulfide) aligned parallel to the rolling plane.
- Poor through-thickness ductility.
Impact:
- Creates subsurface cracks during welding.
- Reduces load-bearing capacity, especially in thick plates.
Prevention:
- Use steel with low sulphur content.
- Apply welding techniques that minimise through-thickness stress.
- Consider Z-quality steel plates.
2. Segregation Bands
Segregation bands are uneven distributions of alloying elements visible in rolled steel sections.
Cause:
- Uneven cooling during solidification leads to concentration differences in carbon, manganese, and phosphorus.
Impact:
- Causes inconsistent mechanical properties across a steel section.
- Weak points may form under cyclic loads.
Prevention:
- Improve ingot casting and cooling control.
- Use secondary refining techniques.
3. Hydrogen Flaking (Flakes)
Also known as fish-eye defects, hydrogen flakes are tiny internal cracks caused by trapped hydrogen.
Cause:
- High hydrogen levels during steelmaking.
- Rapid cooling after forging.
Impact:
- Reduced toughness.
- Risk of sudden brittle failure.
Prevention:
- Vacuum degassing to remove hydrogen.
- Controlled cooling rates.
4. Banded Structure
This occurs when steel grains align into bands due to rolling in one direction.
Cause:
- Segregation of pearlite and ferrite during hot rolling.
- Incomplete homogenization.
Impact:
- Unequal tensile properties along different directions (anisotropy).
- Reduced fatigue life.
Prevention:
- Controlled rolling and heat treatment.
- Avoid excessive cold work.
5. Pipe Defect
A pipe defect is a central cavity in ingots, often unnoticed until machining.
Cause:
- Solidification shrinkage during ingot casting.
Impact:
- Weakens structural integrity after machining.
- Increases scrap rates.
Prevention:
- Controlled feeding during casting.
- Use of hot tops and risers.
