Lesson 5 - Welding Of Steels Flashcards
What is welding?
A fabrication process that joins materials by causing coalescence.
What is coalescence of materials?
A localised fusion of metals or non-metals produced either by heating the materials to the welding temperature, with or without the application of pressure, or by the application of pressure alone.
What is solvent welding?
The process used in plastic fabrication to join pieces of plastic together:
- The solvent breaks apart the molecules of two plastic surfaces
- New bonding at molecular level is formed after solv ent evaporates
- The solvent-welded joint has the same strength of the parent material
What are the three most common types of fusion welding (welding of plain carbon steels)?
- Electric arc welding (more versatile; various materials)
- Gas welding (welding/cutting of ferrous/nonferrous metals; no electricity)
- Resistance spot welding (thin plates; quick)
Describe the process of arc and gas welding.
- After welding has been done, the weld puddle solidifies;
- During the solidification of the weld pool, heat is transferred along the plates by thermal conduction, and the welded parts cool down
- There will be a temperature profile induced, with the highest temperatures in the vicinity of the welded joint;
- Within the HAZ, the temp. of the work pieces will be very close to the melting point of the steel in question
What is the HAZ?
The zone in which the austenitic transformation takes place as a result of heat transfer from the weld pool.
What is the material in the joint called, and what should its composition be?
- Material in the joint is provided by means of a filler rod
- Should have a very similar composition to the plates being joined together.
What happens to critical cooling rate as carbon content increases?
It decreases
What happens if the cooling rate exceeds the critical rate? And why is it an issue?
- The martensitic transformation will take place in the HAZ
- Issue as embrittlement and craving may occur which are both very damaging to the mechanical properties of the weld and HAZ
What are the factors affecting the fatigue/crack behaviour in welded metals?
- Notch shape
- Metallurgical and property change in HAZ
- Residual stress due to the local temperature change
- Welding defects
- Environmental effects
Explain the Brittle-Ductile transition
- B-D transition depends on the competition between the cleavage separation (brittle failure) and the dislocation and void growth (plastic deformation/damage)
- Such competition depends in turn on material properties, environments and loading conditions
- Transition may occur when the above conditions are changed, and thus, the definitions of ‘ductile’ material and ‘brittle’ material are related to the environment and loading conditions.
What is hydrogen embrittlement?
- There is a small amount of moisture in the coating on the welding electrodes. This breaks down into hydrogen and oxygen during welding.
- A number of metals including steel become brittle as a result of exposure to hydrogen
- Arc welding is particularly high risk when it comes to hydrogen embrittlement.
How does hydrogen enter into metal?
At elevated temperatures, hydrogen molecules split into t separate atoms, which diffuse into metal. When the temperature drops they reform into hydrogen molecules.
How does hydrogen cause the cracking?
- Formation of hydrogen gas bubblkes crtreates voids and pressure points within the metal, which may lead to a crack.
- The cracking process is driven by the application of external tensile stresses
What steels are more vulnerable than others to hydrogen embrittlement?
Low alloy and high strength steels are much more vulnerable than plain carbon steels
Is hydrogen embrittlement easy to detect?
No, very difficult. The first indication is usually when the failure takes place. The formation of gas bubbles within the steel is not an instant process, it can take up to 24 hours before cracking begins. If there is a risk of hydrogen embrittlement then a baking treatment can be used to drive off hydrogen before cracking starts.
