Cold Forming

I'm certainly no expert in metallurgy but cold forging causes work hardening of the metal. This could be desirable in some instances and undesirable in others. You can do your own demonstration of this by bending a piece of metal back and forth until it breaks. The work hardening makes it brittle until instead of bending it fractures.

I agree with rcapper here. Grain size, distribution and orientation are strongly affected by forge temperature. Tool forces and wear are also an issue. There's a very nice comparison of these two methods at http://www.qcforge.info/cold_warm_hot.html.

Cold forging will produce a harder, stronger metal, only it may be more brittle. When you cold forge a metal, it introduces dislocations. Normally the atoms in a metal are in a crystal, where the atoms are repeated in a pattern. An easy way to explain dislocations is that attoms get squeezed into positions that are not normal, and stress builds up. this internal stress makes the steel harder and stronger, since it takes even more force to move the atoms (for example if you bend a piece of metal, then try to bend it back, that part that was originally bent is too hard, so it will bend beside it instead). Heat relaxes the atoms and allows them to go back into their normal positions, so there is less internal stress. (if you heat the metal before trying to bend it back, there is a better chance of it doing what you want). So hot forging will not be as strong, but it will be less brittle. -Frank ----------- http://snowboard.skule.ca

Dear Frank,

Thats really a good analysis and example about the stress build up and the reason behind more strength in cold forged parts, i would like to share one of the practical difficulty which we are facing now a days in our cold forging section that " there is a under filling defect which use to come at the start of the machine on spline part of the two wheeler kick spindle shaft which we are producing on sakamura BPF560 SSL part former machine. What is happening that, in the starting this under filling defect use to come on spline portion on which kick leaver use to get mounted and after some time say after forging of 500 components that under filling goes automatically (even the volume of the billet has not changed) , what we are assuming that after the dies gets heated up, this problem automatically goes off , but i am just not convinced with the assumption, so can u please provide some relevant authentic answer / study material/ or any web site regarding this.

Regards

Priya Ranjan