Cold Steel Forging

[Daryl]

Without getting too technical:
Forging high carbon or specialty-steels / iron at too low a temperature will create internal fractures that will not be seen unless x-rayed or section-cut after cooling.
This is caused from a large section thickness having the outside portion brought up to a forging temperature too rapidly, while the internal portion is still at a temperature not yet hot enough to become sufficiently plastique for movement. What happens is the material that has reached the upper transformation temperature now moves (relatively) more freely than that which has not - this creates small internal fractures which can not be removed save having the entire piece of material being re-puddled.
The solution is to ensure that the material is thoroughly 'soaked' up to forging temperature. A method of checking is to pull the iron from the fire when at a medium yellow and observe how quickly the temperature changes to red - if it begins to change rather quickly (assuming you are not dealing with thin stock), then it is not completely heated, or heated correctly. The same if the iron changes after only a few blows of the hammer.
An analogy would be pulling a frozen loaf of bread from the freezer, throwing it in the microwave at high for a minute and the trying to squeeze the entire loaf through an opening smaller than itself - do this and look at the results and you have a very rough idea as to what I am talking about.