Stress Relieving to Achieve Desired Geometry 2

[Philbur]

My question is idealized as follows:

I take a thin section rod of cast iron say 1 mm square and clamp both ends in a manner that causes the rod to bend to a particular predefined curvature that has uniform distributed stress along the section. Then I heat it to a stress relieving temperature of say 600 deg. C. in order to relieve the induced stresses to “freeze” in the predefined curvature.

1) Will the induced stresses be relieved at the point they where induced, freezing in the desired curvature or will most of it be relieved at some arbitrary point according the metallurgy, uneven heating or whatever. My intuition says that stress relieving will not commence at all places at the same time and that stress relieved at the "first" point will change the stress values and therefore the curvature at all other points. Or do I misunderstand the metallurgy involve.

2) Do I go for rapid uniform heating or slow.

3) Would heating to an annealing temperature of say 800 deg. C be better or worse in the context of my objective?

To put my question into perspective the described (in use) process relates to the manufacture of small none commercial IC engine piston rings.

Any help would be most appreciated
Phil Burman
Stavanger, Norway

 

[metman]

To: Philbur-- 1) The areas of higher stress are going to be at a higher potential energy or maybe it is better to say these areas are more unstable in mechanical metallurgy terms hence yes these areas will be affected first and most by the added thermal activity yet at the temperature suggested in your question 3) I suspect that the lower stressed areas will also be largely affected so that the overall effect will be a homogenoously stress relieved article even if all the stresses are not relieved in which case you will have some springback or elastic recovery but I am guessing residual stress will be nil at that temperature.
2) Go for uniform slow otherwise you could induce thermal stresses which could tend to negate your purpose.
3) Don't know about optimum temperature. I am way rusty on Cast Iron metallurgy. You will definitely change the microstructure at that temperture since it is above the lower critical.

Leonard Jesus is the WAY

 

[mcguire]

Stress relieving is really nothing but allowing creep to occur. Generally it relieves residual rather than applied stresses, but the casting doesn't know the difference.
Use 675F for one hour should get you 95% stress relief. You'll have to judge if that gets you the desired geometry.