#288 Name the three types of stainless steel

Name the three types of stainless steel - Mechanical Engineering

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Free Chegg Question

(A) Name the three types of stainless steel categories and their respective crystalline structure at room temperature. Described the general properties of each category.

(B) Describe the process of cold working and its effect on grain structure. Illustrate the expected effect of % cold working on the ductility and yield strength of a metallic material.

(C) Sketch a typical Stress-Strain-Temperature diagram for a Nitinol material, identifying the key features of the diagram and indicating the phase of the material at different points on the diagram. On the diagram, indicate the region that corresponds to shape memory (thermal recovery).

(D) Sketch two stress-strain curves for a superelastic Nitinol material: one for the case of the material being loaded to failure, and one for a sample being completely unloaded after reaching approximately 8% strain. Describe how the characteristic unloading behavour of superelastic Nitinol is particularly beneficial for orthodontic archwire applications

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Free Chegg Answer

(A)
The three types of stainless steels are:
1. Austenitic
2. Ferritic
3. Martensitic

Their respective crystal structures are:

1. Austenitic - Face - centered cubic
2. Ferritic Body centers cubic
3. Martensitic - Orthorhombic

Austenitic steel consists of 18% Cr + 8% Ni structured around the iron. They are not hardened by the conventional heat-treatment process. They are non-magnetic in nature.

Ferritic steel has ferrite as its main phase. Ferritic steel is relatively less ductile than austenitic steel. It is cannot be hardened by heat treatment.

The martensitic structure is a microstructure with a needle-like pattern. It is a supersaturated solution of carbon in iron. In provides hardness and brittleness to the steel but reduces its toughness.

(B)
Cold Working
The metalworking process in which metal is heated below recrystallization temperature is known as cold working squeezing, drawing, bending and shearing are the main categories of cold working.

The advantage of cold working over hot working are:

No requirement of heat
Better surface finish
Dimensional accuracy
Improved strength
Directional properties are minimized

The disadvantages of cold working over hot working are:

The requirement of heavier equipment
Higher forces required and hence, power consumption is more
Less ductility
Undesirable residual stresses produced
Strain Hardening

Effect on Grain Structure

The increase in the amount of cold working increases the hardness before annealing can take place. This decreases the temperature required for recrystallization and increases the grain size.

Ductility and Yield Strength
During cold working, the dislocation density increases the hardness before annealing can take place.

(B)
Cold Working
The metalworking process in which metal is heated below recrystallization temperature is known as cold working squeezing, drawing, bending and shearing are the main categories of cold working.

The advantage of cold working over hot working are:

No requirement of heat
Better surface finish
Dimensional accuracy
Improved strength
Directional properties are minimized

The disadvantages of cold working over hot working are:

The requirement of heavier equipment
Higher forces required and hence, power consumption is more
Less ductility
Undesirable residual stresses produced
Strain Hardening

Effect on Grain Structure

The increase in the amount of cold working increases the hardness before annealing can take place. This decreases the temperature required for recrystallization and increases the grain size.

Ductility and Yield Strength
During cold working the dislocation density increases. This interferes with the general dislocation mobility. The reduction in dislocation movement limits the shear strain which can be produced during loading. Reduced shear strains mean the ductility will be less.

The dislocations also provide resistance to deformation. This means an increased strength of the material.

Hence, the ductility reduces, and yield strength increases with increases in % cold working.

 

(C)
The stress stain temperature diagram of Nitinol is shown in the figure. 1 The various phases are labeled and the thermal shape memory is shown.

Thermal shape memory means that the deformation can be recovered thermally.

(D)
The required loading and unloading curves are shown in figure 2.

Nitinol wires can change back to their former shape upon heating (thermal shape recovery) to use nitinol as an archwire, the orthodontist mols the wire to the current shape of the patient's teeth After placing the wire, it will be heated up and try to reform into a straighter shape.

Hence, nitinol can be used for orthodontists are wire.