Structural Steels Flashcards
What are some properties of structural steels?
Used in equipment for higher strength
Can be obtained in plates or solid bars
Fe-C-Mn system
Can increase strength through grain refining
It’s ferrite and pearlite microstructure means can weld without compromising integrity
Name the pros of structural steels
Cheap raw materials and to form
Stronger and tougher than strip steels
Name the cons of structural steels
No corrosion resistance
Can’t be cold formed
Name three applications of structural steels
Cranes
Submarines
Oil rigs
Bridges
What is the typical composition of structural steels?
1.5wt% C with Mn
<0.2 of Nb, V, Ti when undergone precipitation strengthening
Si, P, N present when undergone solid solution strengthening
Ferrite and pearlite microstructure
It’s microstructure is visibly similar to LCSS
Name 5 methods of strengthening structural steels
Grain refining
Solid solution strengthening
Precipitation strengthening
Work hardening
Phase balance
What is the role of Mn in structural steels?
Mn is an austenite stabiliser
It keeps austenite at lower temperatures
The equilibrium transformation temperature between austenite and ferrite lowers
Which limits austenite recrystallisation
A lower temperature means there is less energy for diffusion
Therefore decreases the growth of ferrite grains
Describe how grain refining increases strength
To increase strength, you want small grain sizes
This is achieved by creating more nucleation sites
Nucleation sites are places where crystal can form from a solution, bubble of vapour from boiling liquid, drop of liquid condense from a vapour
They assist in the physical separation of solid liquid and gas
This usually happens in the early stage of phase changing due to boiling, freezing, evaporation or condensation
You can increase nucleation sites by reducing the size of austenite phase
Or introduce alloying to lower the austenite to ferrite temperature through alloying
Or cooling faster after hot rolling
Definition of grain refining: reduces grain size during the solidification phase
Describe how solid solution strengthening increases strength in ferrite
Alloys promote SSS
The process involves dissolving one metal into another
Can still be classed as low alloy steels but contain more alloys than in strip steels
Why does increasing alloy content not always lead to increasing strength?
Increasing alloys can lead to a higher chance of forming martensite upon cooling
The cooling rate after welding is relatively quick, which means heat dissipates quicker
Weld ends up harder and brittle and not the soft ferrite and pearlite structure
Which leads to cracking at the interface (weld cracking)
