Wrought Metals And Alloys
Question 1. Write a short note on 18 – 8 stainless steel.
Or
Write a short note on 18 – 8 steel.
Or
Write in brief on 18 – 8 stainless steel.
Answer:
18-8 stainless steel was introduced by Dr. W.H. Hatfield in 1924.
At temperatures between 912°C and 1394°C the stable form of iron is a face-centered cubic (FCC) structure called austenite. The austenitic stainless steel alloys are the most corrosion-resistant of the stainless steels. This alloy is also called 18-8 stainless steel based on a percentage of nickel and chromium in its composition.
Read And Learn More: Dental Materials Question And Answers
Composition Of Stainless Steel:
Uses: These are used most commonly in orthodontics and pedodontics in the form of bands and wires.
Alloy Constituents of 18-8 Stainless Steel and their Function:
Advantages Of Stainless Steel:
- It withstands the occlusal forces because of its high stiffness.
- It has high ductility and malleability, so it is easily drawn into wires or hammered into stainless steel sheets
- Cold working can be done easily with 18-8 stainless steel without the risk of fracture.
- This increases the strength of the metal.
- The formability of 18-8 stainless steel is good.
- Due to this many small stainless steel wires are twisted or braided together to form multistranded arch wires.
- Due to corrosion resistance it is stable in an oral environment.
- It resists distortion so tipping and torquing can easily be done by brackets.
- Due to the presence of a smooth surface, stainless steel brackets lead to less friction which makes orthodontic treatment faster and more comfortable.
- 18-8 stainless steel is biocompatible
- 18-8 stainless steel is economical.
Disadvantages Of Stainless Steel:
- It has a low spring back as compared to nickel–titanium alloys which makes it a poor initial alignment archwire.
- Due to its high stiffness more frequent activations are required.
- It can lead to discomfort to the patient as it delivers more force compared to the wires made of other alloys of the same diameter.
- In this soldering is needed, if the temperature becomes more than 500°C the corrosion resistance of the alloy is decreased.
Properties Of Stainless Steel:
- Sensitization: 18-8 stainless steel may lose its resistance to corrosion if it gets heated between 400 and 900°C.
- Stabilization: It is the method to minimize sensitization. In this, some element is introduced which precipitates as carbide in preference to chromium. For this titanium is used commonly.
- Annealed and partially annealed wires: Both these wires can be used as orthodontic wires. When these stainless steel wires are fully annealed they get soft and highly formable. When stainless steel wire is partially annealed its yield strength is increased and formability is decreased.
- Mechanical properties: In cases of orthodontic wires, strength, and hardness can decrease with a decrease in diameter.
- Tensile strength is 2100 MPa
- Yield strength is 1400 MPa
- Hardness is 600 KHN.
Question 2. Write in brief on stainless steel.
Answer:
Steel is an iron-based alloy that consists of less than 1.2% carbon. When 12 to 25% of chromium is added to steel the alloy is known as stainless steel.
Types of Stainless Steel:
Based on the different crystalloid configurations of structure, stainless steel is of three types, i.e.
- Ferritic
- Martensitic
- Austenitic
- Ferritic of Stainless Steel:
- Pure iron at room temperature has a body-centered cubic (BCC) structure and is referred to as ferrite which is stable up to 912°C.
- Ferric alloys have good corrosion resistance, but less strength and hardness. So they fid little application in dentistry.
- Martensitic Stainless Steel
- When austenite (face-centered cubic structure) is cooled very rapidly (quenched), it will undergo a spontaneous, diffusionless transformation to a body—centered tetragonal (BCT) structure called martensite.
- This is a highly distorted and strained lattice that results in a very hard and strong but brittle alloy.
- The corrosion resistance of the martensitic stainless steel is less than that of the other types.
- Because of their high strength and hardness, martensitic stainless steels are used for surgical and cutting instruments. Bur shanks are also made from this steel.
- Advantages of Stainless Steel:
- Its heat treatment can be done.
- Due to its high strength and hardness, it is used for making surgical as well as cutting instruments, bur shanks, and orthodontic pliers.
- Disadvantages of Stainless Steel:
- Bitterness is very high owing to distorted and strained lattice structure.
- It has less corrosion resistance as compared to other types.
- Austenite Stainless Steel:
- 18-8 stainless steel was introduced by Dr. WH Hatfield in 1924. At temperatures between 912°C and 1394°C the stable form of iron is a FCC structure called austenite.
- Austenitic stainless steel alloys are the most corrosion-resistant of stainless steel.
- This alloy is also called 18-8 stainless steel based on the percentage of nickel and chromium in its composition.
- Uses: These are used most commonly in orthodontic and pedodontics in the form of bands and wires.
Composition of Austenite Stainless Steel:
Advantages of Austenite Stainless Steel:
- It withstands the occlusal forces because of its high stiffness.
- It has high ductility and malleability, so it is easily drawn into wires or hammered into stainless steel sheets.
- Cold working can be done easily with 18-8 stainless steel without the risk of fracture. This increases the strength of the metal.
- The formability of 18-8 stainless steel is good. Due to this many small stainless steel wires are twisted or braided together to form muti stranded arch wires.
- Due to corrosion resistant it is stable in an oral environment.
- It resists distortion so tipping and torquing can easily be done by brackets.
- The presence of smooth surface stainless steel brackets leads to less friction which makes orthodontic treatment faster and more comfortable.
- 18-8 stainless steel is biocompatible
- 18-8 stainless steel is economical.
Disadvantages of Austenite Stainless Steel:
- It has a low spring back as compared to nickel–titanium alloys which make it a poor initial alignment archwire.
- Due to its high stiffness more frequent activations are required.
- It can lead to discomfort to the patient as it delivers more force compared to the wires made of other alloys of the same diameter.
- In this soldering is needed, if the temperature becomes more than 500°C the corrosion resistance of the alloy is decreased.
Uses of Austenite Stainless steel:
Various stainless steel alloys are used for manufacturing:
- Bracket and buccal tubes
- Archwires
- Bands
- Ligature wires
- Attchments
- Springs
- Microimplants
- Wires for removable appliances
- For manufacturing orthodontic instruments and pliers.
Question 3. Write a short note on orthodontic wires.
Answer:
Various types of orthodontic wires are used in both fixed and removable prosthodontics for tooth movement and stabilization.
Classification Of orthodontic wires:
- On Basis of Elastic Behavior
- Type1: Wires displaying linear elastic behavior during unloading at temperatures till 50°C
- Type 2: Wires displaying nonlinear elastic behavior during unloading at temperatures till 50°C.
- On Basis of Material Used
- Stainless steel
- Co– Cr– Ni alloy
- β – titanium alloys
- Ni – Ti alloys
- Gold alloys.
- Based on Cross Section
- Round
- Square
- Rectangular
- Multistranded.
- Based on Gauge Size
- 0.018”
- 0.016”
- 0.07”.
Properties of Orthodontic Wires:
- Force generated:
- Force generated by the wire on the tooth depends on its composition and design.
- For a given design, force generated is proportional to stiffess of the wire.
- Elastic deflection and working range: Large elastic deflection produces low constant forces which are less damaging. This also leads to greater working range.
- Springiness: Springiness is the measure of how far a wire is deflected without leading to permanent deformation.
- Stiffess: It is the amount of force required to produce specific deformation. It is inversely proportional to springiness.
- Resilience: This is the energy storage capacity of wire which is the combination of strength and springiness.
- Formability: It represents the amount of permanent bending the wire tolerates before it breaks.
- Ductility of wire.
- Ease of joining: Most of the wires are soldered or welded together
- Corrosion resistance and stability: It is important for the durability of the appliance and biocompatibility.
- Biocompatibility: Most of orthodontic wires satisfy this property. People who are allergic to nickel may get allergic reactions from nickel-containing orthodontic wires.
- Cost factor: Titanium alloy wires are expensive as compared to stainless steel or cobalt-chromium nickel wires.
Manufacturing of an Orthodontic Wire:
Following are the steps in the manufacturing of an orthodontic wire:
- Melting and Casting: Raw materials from the ore are melted by an electric furnace for several hours.
- Metal is poured into dies from which an ingot is manufactured.
- Drawing: Set metal ingot on the machine which will pull the ingot via a die with a hole in it. Here the shape of the hole is determined by the cross-section of wire. Ingot should be pulled via various dies with holes of progressively decreasing size till the desired gauge is reached.
- Annealing: In this, heating and cooling under controlled conditions should be done. This leads to the removal of stress in the wire by recrystallizing metal. This softens the metal and produces back its original flexibility
- Finishing: Now wire is finished by fier abrasives and buffing. This provides a mirror-like luster to the wire.
Question 4. Write briefly on Beta-Titanium.
Answer:
A titanium–molybdenum alloy known as beta-titanium is used as a wrought orthodontic wire. Beta-titanium was introduced in orthodontics by Burstone and Goldberg in 1979.
At a temperature of more than 885°C pure titanium rearranges into a body-centered cubic lattice referred to as the beta phase. By the addition of elements such as molybdenum, columbium, vanadium, niobium, cobalt, copper, and silicone, a titanium-based alloy can maintain its beta structure when cooled to room temperature such alloys are called as beta stabilized titanium.
Composition Of Beta-Titanium:
Advantages Of Beta-Titanium:
- Beta-titanium alloys are much better as compared to the other alloys, they have the balance of properties, a clinician needs, i.e.
- They have high elasticity, low stiffess, formability, and ease of joining.
- Delivery of force is 0.4 times the force of stainless steel and still, they are deflected twice without any deformation.
- Beta-titanium wires are the only major orthodontic wires which possess true weldability and clinically satisfactory joints which are made by electrical resistance welding.
- Formability is excellent due to the body-centered-cubic structure which allows many of the slip systems which are available for the dislocation movement.
- Beta-titanium wires deliver low biomechanical forces when the comparison is done between stainless steel and cobalt-chromium-alloys.
- As these wires have the passivating effect of titanium dioxide they have excellent tarnish and corrosion resistance.
- The potential for allergy and hypersensitivity is low with beta-titanium alloys.
- In an oral environment, beta-titanium alloys are biocompatible and stable.
Disadvantages Of Beta-Titanium:
- Beta-titanium alloys have high surface roughness which leads to high frictional forces.
- Their cost is high.
Clinical Applications Of Beta-Titanium:
- These wires are used for the majority of procedures at the time of orthodontic treatment.
- Beta-titanium wires are used for orthodontic procedures such as bite opening, space closure, retraction, and correction of various minor.
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