Stainless steel

Question 1

is soft or hard wire used for orthodontic appliances

Answer 1

hard

Question 2

how is hard wire made

Answer 2

drawing the metal in a cold state through a series of dies of successively smaller diameter. This process also causes work hardening which gives the wire its spring properties

Question 3

how can we increase the spring properties of the wire further

Answer 3

by further work hardening during the bending process of constructing component parts for ortho appliances

Question 4

what is the bauschinger effect

Answer 4

when a coil is bent in a wire, it is differentially stretched so that the outer surface becomes more work hardened and thus better spring properties than the inner surface.

If the coil is activated in the same direction as the previous bending, its elastic recovery is greater than if it is deflected in the opposite direction

Question 5

when do fractures occur in stainless steel wire in orthodontics

Answer 5

1. overworked
2. mechanical abrasion crushed or marked
3. fatigue
4. weld decay

Question 6

what type of stainless steel is most commonally used in orthodontics

Answer 6

18/8 austenitic stainless-steel alloy

(austenitic is the crystalline space lattice structure of the alloy. The alloy can’t be heat hardened and remains in a soft condition unless it has been cold worked)

Question 7

what makes up 18/8 austenitic stainless steel alloy

Answer 7

• 72% iron
• 18% chromium
• 8% nickel
• 1.7% titanium
• 0.3% carbon

Question 8

what is the function of iron

Answer 8

main constituent and when combined with carbon forms steel

Question 9

what is the function of chromium

Answer 9

• lowers the content of the carbon present
• lowers the temperature at which martensite forms
• forms a passive oxide film over the surface of the metal that enables the stainless steel to be corrosion resistant

Question 10

how is martensite formed

Answer 10

the principle component of hard steel, formed by quenching from high temperatures. It consists of intergrown plate-like crystals with a distorted cubic structure arising from the presence of carbon atoms in the iron structure.

Question 11

what is the function of nickel

Answer 11

• alongside chromium assist in achieving the austenitic structure at room temperature by lowering the critical temperature at which the austenitic structure breaks down on cooling
• improves corrosion resistance of the alloy and increasing its strength

Question 12

what is the function of titanium

Answer 12

• prevents precipitation of chromium carbides at the grain boundaries when the alloy is heated during welding or soldering
• carbon combines with titanium in preference to chromium

Question 13

What is the difference between austenite and martensite

Answer 13

Austenite

• cube

Martensite

• cube distorted by interstitial carbon atoms that don’t have time to diffuse out during displacive transformation (quenching)