This is a huge and complex subject and there is the internet out there with all its’ endless in depth information (and misinformation), so I will not attempt to get technical.
In the context of our sort of work mild steel is the most readily available and economical of the alternatives. Its advantages are that it is relatively cheap, easily available, can be worked readily and without detriment in a number of ways, both hot and cold, and is generally of fairly consistent quality/composition.
Its disadvantages come to the fore with its employment in restoration situations where it has to replace or live alongside old wrought iron pieces. In such circumstances its’ ‘granular’ composition (as opposed to the almost wood like and fibrous composition of proper wrought iron) tends to limit the subtlety of the forged work. A close look at some of the wonderful examples of old wrought iron artefacts that are still in existence around the country will reveal a fineness of line that is hard to mimic in mild steel.
The fact that some of these old works are still standing is a testament to the staying power of the material from which they are made; Mild steel, particularly when formed into intricate shapes, must be very carefully finished if it is to stand a chance of surviving as well, for its’ ability to withstand oxidisation is very much less than that of wrought iron.
Stainless steel, which comes in different grades, has the advantage suggested by its’ name that it is rust resistant to a high degree. Its’ cost, however is high, – although it can sometimes be offset in the budget of a project by the fact that there is the option of less cost in the finishing stages. It can be worked hot and cold to much the same degree as mild steel (though is intolerant of overheating), but is very susceptible to contamination from ferrous tools. This last can be quite an issue and has to be thought about carefully before commencing.
Some of the bronzes are forgeable, but need a lot of care when hot working as the temperature range in which they can be worked is usually narrow. Aluminium bronze and Silicon bronze are two that I have come across a certain amount and both of which I have come to feel fairly comfortable with. But these are expensive materials to make mistakes with and they would normally be more at home in either engineering or founding environments
Aluminium is also a readily available material; again there are many different grades and compositions; some of these can be hot worked (forged) but it is more frequently found in engineering, fabrication and founding situations. It is also dearer than mild steel but lighter and does not oxidise to the same degree.
There are also many alloys, both ferrous and non ferrous, with widely varying specific properties which can be used as required. The simpler carbon steels we use a certain amount for tooling, tongs etc. and the press requires some fairly specific alloys for its’ tools.