Steel or Semi-Steel Wheels
Algood steel wheels are made from pig iron which is a semi-finished metal produced from iron ore in a blast furnace and containing high amounts of carbon. Pig iron is then further refined in a furnace for conversion into steel. The wheels are formed through a casting process in which metal is heated until liquid and then poured into a mould.

The capacity of Steel wheels  is 2,500 lbs. They are porous and prone to chipping and cracking but are the most economical choice in steel wheels.

Ductile Iron Wheels
Also known as ductile cast iron, is a type of graphite-rich cast iron. It has much more impact and fatigue resistance because of the graphite and in particular because the of nodular shape of the graphite that is used. Ductile iron wheels are also formed using a casting process. 
 
The capacity of Ductile Iron wheels is 15,000 lbs. They are less porous than steel wheels but are still susceptible to cracking and chipping. They are more expensive than steel wheels.

Forged Steel Wheels
Forged Steel is an alloy of carbon and iron that is compressed under extreme pressure to make a very hard and strong substance. In the forging process heat or force is applied to steel billets or ingots, causing the material to change shape while in a solid state. The resulting highly compressed material is exceptionally strong.
 
The capacity of Forged Steel wheels is, at minimum, a whopping 20,000 lbs. They are not porous and are virtually failure-free. Not surprisingly these wheels are the most expensive steel wheel option.

V-Groove Wheels
V-Groove wheels can be made from either cast iron steel or forged steel. They are manufactured with a groove that guides the wheel along a track and are often used to support entrance gates or vault doors.

304 Stainless Steel Wheels
Stainless Steel is a mixture of steel and a minimum of 10.5% chromium. The chromium keeps the steel from oxidizing, rusting, and corroding. Stainless steel wheels are made from round bars of stainless steel material that are lathed to form wheels and are then machined for tread and diameter. 

Stainless wheels are moisture resistant and, unlike all other steel wheels, they will not rust. That makes them ideal for applications that are subject to the elements such as agriculture as well as food, medical and pharmaceutical environments. They will wear better than ductile iron but not as well as forged steel and are in the mid to high price range.