Yieild strength of cast iron

[sansvk]

Hi,
I would to know the yield strenght for various grades of gray CI.
I know people use 0.2%y , can someone indicate the values or indicate any website. This for an analysis project, using a software which requires yeid strength.

thanking you
san

 

[rnd2]

A pretty good start would be zero

 

[NickE]

rnd2 is right. GCI is generally regarded as having no measurable yield strength. In the tensile test grey iron fails catastrophically by brittle fracture. The may be microscopic yielding behavior but there are few devices with the resolution necessary to show the plastic deformation.

nick

 

[EdStainless]

If your deveice really will be loading the casting in tension you need to look at using DI in place of gray.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[SMF1964]

I would avoid using "0" or zero as a yield strength, since you can get division by zero errors. In truth, the yield strength of cast iron would not be zero but would be at or very close to the tensile strength of the alloy, since the material behaves elasticly right up until failure. In practice, I agree with EdStainless that your really want to use a ductile iron or malleable iron instead of a gray iron because it wall have some ability to withstand plastic deformation without failure.

The "0.2%y" is a reflection of the practice of using a yield point as defined by the applied stress at which the steel has plastically deformed 0.2%. Graphically, your run a tensile test (which starts at an initial stress/strain of 0 [stress units]/0%). Then you draw a line starting at an initial stress/strain of 0 [stress units]/0.2% that runs parallel to the stress strain curve's elastic line. Where the two lines intersect, you get your 0.2% off-set yield strength.

As far as links or tabulated data, I would recommend ASM International's metals handbooks in your local university library or a 'google' search of 'gray cat iron' will turn up something.

 

[metengr]

sansvk;
I checked several sources and could not find any reportable yield strength for gray cast iron automotive classes SAE J431 or ASTM standard gray iron Class 20, 40, etc or ASTM A159.

I would suggest for your software to use either .80 * UTS as a value for the fictitious yield strength, if you are also required to input the gray iron UTS as separate but distinct values. Or assign YS=UTS for data inputs. Just curious, what is this software analysis for?

 

[rnd2]

Yield is supposed to be a definitive,logical, measurable point where plasticity commences as a direct result of an external applied stress.
It makes no sense to assign a value to a material that doesn't exhibit plastic deformation for any practical purpose.

 

[SMF1964]

I agree with rnd2 that gray iron won't show any practical purpose, but the yield is not necessarily a measurable point, which is why the 0.2% offset method is used to estimate the yield strength.

If the material does not exhibit any plastic deformation in a practical sense, then the yield strength is essentially equal to the tensile strength. That is, the material elastically deforms until it breaks, with no plastic deformation. Setting yield strength to zero would indicate that the material starts plastically deforming immediately upon loading.

 

[rnd2]

SMF1964
My argument is this: What is the point of assigning a value for something that for practical purpose does not exist?
Gray cast iron is a brittle material. It has zero yield strength in the same sense that there is no yield strength because it does not plastically deform.
There is elastic strain only, no plastic strain. There is only UTS to measure.

 

[SMF1964]

From the ASM Metals Handbook Volume 1, 10th edition, p18-20: "Gray iron does not obey Hooke's law, and the modulus [of elasticity] is usually determined arbitrarily as the slope of the line connecting the origin of the stress-strain curve with the point corresponding to one-fourth the tensile strength (secant modulus). Some engineers use the slope of the stress-strain curve near the origin (tangent modulus)."

At the same time, "Elongation of gray iron is very small, on the order of 0.6%".

I hadn't realized that the material was non-linear elastic, which makes yield strength meaningless, since it cannot be accurately measured (without risking damage to your extensometer if the material fails before you reach 0.2% elongation), as rnd2 indicates.