Stainless Steel Flashcards
What are the constituents of SS and their percentage ?
Iron 72%
Carbon 0.3%
Chromium 18%
Nickel 8%
Titanium 1.7%
What is the two functions of chromium in SS ?
Forms passive oxide layer on surface - increases corrosion resistance.
Lowers austenite to martensite temperature and conversion rate.
What is the two functions of nickel in SS ?
Improves strength.
Provides corrosion resistance.
What is the function of titanium in SS ?
Prevents precipitation of chromium carbides at grain boundaries during heating - i.e. prevents weld decay.
What ratio of austenitic SS wire is used for orthodontic wire ?
18:8 (chromium : nickel).
What are the four failures of SS wire ?
Overworking.
Mechanical abrasion.
Fatigue.
Weld decay.
Explain weld decay.
Intergranular corrosion created by overheating (500-900 degrees) = chromium carbides precipitates at grain boundaries allowing oral fluids to penetrate and weaken structure.
What are two ways to overcome weld decay ?
Addition of Ti into steel = stabilised steel.
Low carbon steel.
What is the function of stress relieving annealing ?
Allows material to settle to equilibrium.
What temperature does stress relieving annealing occur ?
450 degrees for 2 mins.
What are the advantages of SS ?
Thin and light material.
Fracture resistant.
Corrosion resistant.
Good heat conduction.
Abrasion resistant.
Good strength.
Spring back ability.
Stiffness.
Ductility.
Ability to be welded/soldered.
What are the disadvantages of SS ?
Inaccuracy.
Damage due to high pressure during swaging (leads to wrinkling).
Difficult to gain uniform thickness for denture base.
How is springback ability measured ?
EL / YM.
Define sprinback ability.
Allows material to be subjected to large deflections without permanent deformation.
Define stiffness.
Appropriate rigidity to maintain required force.
