Delayed Stress Releiving / Normalising

[mohanmohit]

Identically Deep drawn and welded cups, made from hot rolled steel sheets of same cast are divided into two equal lots. One lot is stress relieved or normalised immediately and the other say after a year. Would there be a difference in the micro-grain structure of the parent metal of the two lots? If yes, the reasons for the same may please be intimated. Further is there any defined relationship between the difference and such time delay.

 

[mcguire]

Response to stress relief may minutely differ because of the minor amount of room temperature creep that could occur over a year, but normalizing is really a history-erasing process in which new austenite grains are formed from which new ferrite grains are precipitated. There is nothing in a time delay to alter the driving forces which determine grain size of the final material in the two lots.
That said, being from the same cast does not make two lots identical; their chemistries can differ, but more importantly their processing paths could have differed greatly. For example, hot reduction, hot rolling temperature, cold reduction and annealing temperatures; all these can influence the as-drawn part you have some issue with.
There is a lot of variability in steel that does not derive from chemistry. That is the most controllable variable.
Tell us the issue and let us deal with specifics.