Measuring Upper Yield Point in Bake Hard Steel

[metallurgist1]

I freuently tensile test bake hardening steel, which should exhibit a distinct "upper and lower yield point" after standard baking treatment. On certain occasions the tensile test does not record a distinct upper yield point but simply plateaus to the lower yield point. Based on what I have read, this problem can occur due to test alignment, poor fixturing, or slow test speed. I have tried to optimize these but still frequently have problems registering the upper yield point. If anyone has any suggestions please let me know,...

- thanks

 

[kenvlach]

The yield point behavior depends upon many metallurgical variables: the alloy composition (overall as well as local segregation), impurities, prior cold working, time at austenizing temperature, etc. The initial elastic elongation of a steel (after casting or high T heat treatment) generates dislocations, and after a critical number are reached, plastic yielding begins, dropping the stress necessary for elongation. The transition is somewhat analogous to the transition from static friction to sliding friction. Of course, steel is more complex in that Luder's bands can form, work hardening occurs, etc.

The yield point gap is usually most evident for low carbon steels. One factor long known to reduce the difference between upper and lower yield point is coarsening of the grain size, e.g., H. Conrad, Acta Metall., vol. 11, pp. 75-77 (1963). So either excessive time at temperature (coarsens grain size) or too little time at too low a temperature (fails to remove all cold working), can minimize the yield point drop.

I didn’t find any ‘great’ discussions on-line, but these lecture notes are good:

http://www.imperial.ac.uk/mechanicalengineering/study/Notes/2MSM/lecture3.pdf

You seem pretty conscientious with regard to testing procedures. If you are confident that the ASTM E8, A370 and test equipment manufacturer’s procedures are being follows, it is likely that you are observing an actual material characteristic rather than a testing artifice.

You probably have all necessary procedures, but here are some
ASTM E8-03 Standard Test Methods for Tension Testing of Metallic Materials

http://www.astm.org/cgi-bin/SoftCar...NE_PAGES/E8.htm?L+mystore+swhh8376+1074899568

ASTM A370-03a Standard Test Methods and Definitions for Mechanical Testing of Steel Products

http://www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/REDLINE_PAGES/A370.htm?L+mystore+swhh8376

A brief article on possible testing problems: “Watch Out for Tensile-Testing Pitfalls”

http://www.industrialheating.com/CD...ures/BNP__Features__Item/0,2832,75348,00.html

A test equipment manufacturer’s site, with the various yield points given on a schematic curve. They also calibrate equipment, if that is a concern.

http://www.tiniusolsen.com/results/results-metals-tens.html

Two sites for those w/o yield testing backgrounds:
A glossary and schematics re yield:

http://nsm.eng.ohio-state.edu/Stamping_Glossary/html/y.html#Y1

“TENSION OF DUCTILE METALS” [brief test procedure using pre-machined samples].

http://www.engr.csufresno.edu/~eest/tension.htm

For any future metallurgy questions, I suggest posting at
Metal and Metallurgy engineering Forum
forum330

Hope this helps.
Ken

 

[TVP]

kenvlach provided excellent information. I agree with him that this sounds like a potential material/hardening problem. Compare the chemical compositions (including Al & N) of the suspect tests to those that tested ok. Grain size will definitely have an effect, so if you have any data on this, compare with the chemical composition, and you may see that the suspect tests were not really "bake hardening".