Aging of Test Specimens? 4

[salmon2]

Per ASTM A370 8.4, it says "Unless otherwise specified, it
shall be permissible to age tension test specimens". What is the purpose of this specimen aging possibly?

 

[swall]

Just a guess--low carbon deep drawing steels are subject to strain ageing, which can result in fluting and luders lines. This is caused by an abrupt yield point and subsequent uneven yielding during drawing. To eliminate this condition, the steel is normally given a temper rolling pass. However, the strain ageing phenomenon would make it easier to pick out a yield point on a tensile test. Consequently, if one were tensile testing sheet steel that had been temper rolled, ageing at 212F would cause the sharp yield point to return, which would be easier to spot on the tensile test.

 

[metengr]

In addition to the above, you have variability associated with machining methods that are not defined in A 370. From the literature I have seen ASTM conducted an evaluation into machining methods and variability associated with tensile testing.

http://books.google.com/books?id=Rd...9K1OMid8oDONP1Ptg9E&hl=en#v=onepage&q&f=false

 

[blacksmith37]

Likely gives the mills a little "wiggle" room (ASTM has a strong mfgr component)to meet spec. Control Rolled steels (also "micro-alloyed", strain aged, and other names) can have significant property changes with aging. Look for small amounts of Nb, Ti, V and maybe others in a low carbon flat rolled product. This is the basis of high strength ERW line pipe.

 

[salmon2]

Thanks a lot everyone.

 

[salmon2]

For information only, the paragraph about this in a370 is:

8.4 Aging of Test Specimens—Unless otherwise specified, it shall be permissible to age tension test specimens. The timetemperature
cycle employed must be such that the effects of previous processing will not be materially changed. It may be accomplished by aging at room temperature 24 to 48 h, or in shorter time at moderately elevated temperatures by boiling in water, heating in oil or in an oven.

 

[bhollands]

For cast materials the aging is to remove hydrogen. The hydrogen will naturally leave the steel over time. The thicker the section the longer it takes for the hydrogen to be removed. In quench and tempered materials above about 240HBN an un-aged bar will show lower % reduction of area than an aged bar from the same heat and heat treatment batch. You can also allow the machined test bar to sit at room temperature for a couple of days and achieve the same results.

Bob

 

[salmon2]

Thank you, Bob, that makes sense.