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21 Cards in this Set
- Front
- Back
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Definition of Welding |
Joining of two parts using strong, primary, metallic bonds |
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Advantages of welding |
- Uniform Join - Less Materials |
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Is fusion welding solid or liquid state? |
Liquid |
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What is the heat source in Fusion welding? |
Electrode |
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What’s the purpose of the shielding gas in fusion welding? |
To prevent oxidation of the metal surface |
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Define the Heat Affected Zone (HAZ) |
Area either side of the fusion zone which heats up, but doesn’t melt, undergoing changes in microstructure (different to parent metal and weld metal) |
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What is the weakest region of a weld? |
HAZ |
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parameters which increase the size of the HAZ |
- increasing weld/ heat energy - decrease the speed of the weld (slow heating) - increase the initial weld temperature - lower thermal conductivity of base metal - thinner base material |
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Why is the HAZ the weakest region? |
- Ductile to brittle transition temperature is increased - notch impact energy decreases |
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What defects are found in the HAZ? |
- Phase transformations - recrystallisation - grain growth - precipitation/coarsening - embrittlement - cracking |
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What is the Fusion Zone? |
Zone directly either side of the weld metal which has similar properties to a cast metal |
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What defects do you get in the Fusion Zone? |
Casting Defects: - Porosity - Segregation - shrinkage - cracks - orientation variation |
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What factors contribute to casting defects? |
- impurities - base metal dilution of filler - turbulence and mixing - large temperature gradients - cast and mould interactions - dynamic moving process |
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4 types of grain structure variation |
- coarse grains - fine grains - equiaxed grains - dendritic grains |
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What’s the ideal grain structure in a fusion weld? |
Fine, equiaxed grains |
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Factors which influence grain structure |
- cooling rate - weld/parent metals - shape of workpieces |
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Define multi pass weld |
Multiple heat treatment cycles with melting and remelting of weld metal |
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What is hydrogen cracking? |
Decrease in ductility causing embrittlement and cracking in martensitic steels |
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Methods to reduce distortion |
- reduce weld heat input - faster welding - few weld passes as poss - restrain components to plastically deform weld - allow base material freedom to move - adjust initial position to allow for distortion - stagger welds - shot peening - stress relief heat treatment |
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Effect of post weld heat treatment |
Tempers martensite to reduce residual stress |
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Effect of preheating weld |
Reduces cooling rate, distortion, shrinkage stress and cracking Allows hydrogen to escape |
