Magnetic Core Charateristics

In general there is no change for steel cores unless you do something to physically damage them. I have disassembled inductors more than 50 years in use and found them just as made. Well designed (and used) ferrite cores also show no change. What will change over the years are the coils on them. You can see huge changes in distributed capacitance.

Thanks for your reply. can u please tell me some literature or any mathematical formulation if possible to support the above said answer. your answer seemed quite interesting to me but at the same time I will like to put one point which I was thinking in morning was, sice the temprature of core rises so there must be some cubical expansion factor attached to it thus but as that cools down may be (I don't know) it does regain its orignal size and thus a slight change in dimension may happen or not? and if happen then whether core charateristic will change.

you should ask for emperical data?

Let me see if I have this right. You want a formula for something that doesn't happen?

TVP45

actually I asked same answers from professors of some very reputed college in world and I will post here what his reply was: "Gee. I don't know how steel characteristics change with time in a transformer. The hysteresis loop is affected by stress, hardening and annealing. So it is reasonable to think that the steel in a transformer might change a bit over a long time. It does not, however, appear to be a big effect. I suspect the people who make steel sheets might know something about this." "I agree that the magnetic characteristic of the core material must be getting affected in long run by the factors you have mentioned. However, the experience shows that in a good material the changes must be within the permissible limits even after decades of service. Transformers are giving satisfactory service even after half a century of installation. Good alloy (antimony) steel for core were developed long long ago in Sweeden." so any if can help then please help,