Welding Flashcards
What is welding?
A manufacturing process where 2 or more metal parts are joined together. The parts are heated to a temperature high enough to cause melting or softening.
Welding can be done with or without pressure and with or without the use of a filler material.
State the two types of joints and give some examples of each.
- Butt joints - square and closed square.
2. Groove joints - single bevel, single-V, Tee, Flange, Double-J.
What are the three weld zones?
- Fusion zone (FZ)
- Heat affected zone (HAZ)
- Parent material (PM)
What five factors do the microstructures present in a weld depend on?
- Parent material composition
- Filler rod composition
- Temperature to which the material is heated
- Time at this temperature
- Cooling rate
What is hydrogen cracking? How does it occur and how can it be prevented?
Hydrogen cracking (aka cold cracking) requires all of these three factors; hydrogen, stress and susceptible microstructure (high hardness).
It occurs below 300C and can be prevented by preheating and conducting a low hydrogen welding process.
What are the 5 main factors which influence the risk of cracking?
- Weld metal hydrogen
- Parent material composition
- Parent material thickness
- Stresses acting on the weld during welding or imposed soon after welding
- Heat input
Which formula measures the ability to form martensite?
The carbon equivalent (CE) measures the ability to form martensite, which is necessary for hydrogen cracking.
What are the ranges of CE which dictate the heat treatment required?
Below 0.35% - no preheat or post weld heat treatment required
Above 0.55% - both preheat and post-weld treatments required
Between 0.35% and 0.55% - only preheating is required
Why would you preheat the materials to be welded?
Preheating reduces the temp difference between the weld region and the base metal.
The effects of this are:
- Reduces cooling rate = reduces chance of forming martensite
- Reduces distortion and shrinkage stress
- Reduces the danger of weld cracking
- Allows hydrogen to escape
Why would you do a post-weld heat treatment?
A post-weld heat treatment allows martensite to be tempered. This reduces hardness and strength but increases ductility and toughness.
Residual stresses are also restrained by the post-weld heat treatment.
What will be influenced by the thickness of the parent material?
- The cooling rate and therefore the hardness level
- The microstructure evolved in the HAZ
- The level of hydrogen retained in the weld
The combined thickness of the joint with dtermine (along with joint geometry) the cooling rate of the HAZ and its hardness.
Why would you do multi-pass welding?
The heat from subsequent passes affects the structure and properties of previous passes.
- Tempering
- Reheating to form austenite
- Transformation from austenite upon cooling
