Q1: Required properties

Strength

This is important as the gears will experience quite high loads and must not yield or deform. The overall strength of the gear will be controlled by composition and processing but the more critical property of surface hardness will be controlled by additional surface treatments. Try another property.

Toughness

Toughness is important as when the gears engage together there may be a little mismatch which could result in a degree of impact loading and it is important that the gear does not fracture. However it is not one of the top three properties, try another property.

Corrosion Resistance

Corrosion resistance is important in that any corrosive product will degrade the operation of the gears and increase the wear rates. However, the gears operate in a protected environment (for example within the gear box) thus minimising the opportunity for aggressive corrosive species to come in contact with the gears. Also lubrication (grease etc.) is used which acts as protection against corrosion. These measures are generally sufficient for us not to worry greatly about added corrosion resistance measures. Try another property.

Magnetic properties

Not really important for this application. Try another property.

Aesthetics

The appearance of the gear is less important than many other properties as gears are not on view. Try another property for one of the three most important.

Cost

Correct, cost is always an issue with design and materials selection, especially of mass produced items. In this case cost will be minimised because the steels are used in large quantities and, as low strength levels are all that are necessary, the alloying costs will be low.

Steel cleanliness

This is important as customers want increased consistency in their products and consistency is at least partially controlled by cleanness, for example fatigue life is influenced by inclusion presence. However modern steel making procedures gives relatively clean steels and this is an ongoing concern rather than an over riding one for this application. Try another property.

Fatigue resistance

Correct, this is important as the gears are constantly experiencing varying loading cycles. The fatigue resistance can also be related to the surface hardness of the gear.

Formability

This is important as the gear has to be fabricated. Gears can be fabricated by a number of methods, the most common is forging followed by machining and heat treatment. There are requirements for a reasonable level of formability so that the gears can be forged however this is less critical than other properties. Try another property.

Machinability

This is quite an important property but not one of the top three. For example when gears are hot forged to final shape some finish machining is required. In some cases gears are produced by net shape cold forging and in this case machinability is less important but still a requirement. Try another property.
COST - Cost is always an issue with design and materials selection. In this case cost will need to be considered but is not one of the top three choices as there are other properties that are more critical. Try another property.

Weldability

Weldability is not an issue here as the gears are usually fabricated as a single component that do not require welding. Try another property.

Electrical properties

Not really important for this application. Try another property.

Wear resistance

Correct, wear resistance is extremely important in determining the overall life cycle of a gear and its performance. Wear resistance can be related to the surface hardness of the material hence surface hardness is another critical property.

Creep

Not really important for this application as creep in steels is only an issue at elevated temperatures and long exposure times, for example power plants where components may experience temperatures of 400�C+ for 20+ years. Try another property.

Recyclability

This is less important for these applications as they are designed to have a long life time, however steel is an easily recycled material in all its forms. Try another property.

Surface hardness

Correct, for a gear the surface hardness helps control the wear resistance of the material and hence its overall performance and lifetime.

When you have correctly identified all three, they will be displayed in a 'Notepad', from where you can link to pages giving more detail.