Mild steel micro structure changes at around 200¦C 2

[TitanSteel]

Dear Experts,

I have noticed some kind of heat treatment taking place on a mild steel part where I work and I would like to know what is actually happening.

The part is a very large retaining ring, about 25" in diameter with a relatively small cross section. Before the part is painted it has very little spring and fails our in-house "spring test", but after the painting process it passes. The painting process is basically a electrophoretic process which is cured in an oven at around 200°C for 1hour and left to air cool.

To confirm it was the heat that was causing this to happen not the paint, I placed one of the unpainted retaining rings straight onto the oven track missing out the EP paint track and the same pass result occurred when tested after it had cooled down.

The spec. for the steel is as follows:

It's a hot rolled section, Carbon 0.16 Max, Manganese 1.5 Max, Phosphorus 0.025 Max, Silicon 0.45 Max, Sulphur 0.02 Max, Aluminium 0.02/0.05, Nitrogen 0.009 Max, Copper 0.2 Max , Chromium 0.10 Max, Niobium 0.09 Max, Titanium 0.22 Max

Thanks for any light you can shed on this matter.

 

[blacksmith37]

If it is cold rolled, you may be getting some strain aging, especially if it contains Nb and/or Ti. Although 1 hour is short time. You could check the actual chemistry and do a series of aging time and temperatures. You may want to age your stock to enhance properties.

 

[TitanSteel]

Thank you very much blacksmith37 for your reply.

After looking into your reply it seems to makes great sense. Just to double check what you are saying; through cold rolling the amount of dislocations in the part is increased. Then when the part is put in the oven and kept at that 200°C the interstitial carbon and nitrogen atoms migrate to the dislocations and pin them, thus increasing strength and reducing ductility.

I hope I have understood you correctly.

Thanks again for your advice and knowledge.

 

[TVP]

In the automotive industry this is referred to as Bake Hardening. Here is an excerpt from the Automotive Steel Design Manual (AISI, 2002):

http://www.a-sp.org/database/pdf/CarsAsdm/Chapter2/section2-02.pdf

Here are some other links with more technical descriptions:

http://www.jfe-21st-cf.or.jp/chapter_1/1b_2.html

http://www.thyssenkrupp-steel-europ...2/00/00/2789094/Bake_hardening_steels_eng.pdf

http://www.mat.ensmp.fr/Pages/sf/publi/ballarin09I.pdf