Grey Cast Iron And Permanent Deformation 1

[sdra2]

Hello.

I have seen a number of stress-strain curves for grey cast iron indicating permanent deformation after application and removal of a load. In all of the instances I have seen, however, the first load application gives rise to a stress that is some significant proportion, say 40% or more, of the UTS of the material.

What I should like to know is whether there are lower stress regions where all of the strain is recoverable for grey iron.

Thanks.

 

[metengr]

You might want to start with the web site below from the Casting Institute. There is a section on mechanical properties with particular reference to stress strain curves;

"Steel shows a rather minor influence of tensile strength and hardness on the modulus of elasticity, since it is mostly in the range of 29,000,000 to 30,000,000 psi. For gray iron, the modulus of elasticity not only varies with tensile strength but also with the stress level. As a result of these factors, the modulus of elasticity will vary from around 12,000,000 psi for a very soft iron to over 20,000,000 psi for a high strength iron. The stress-strain curve for gray iron in tension is almost a curved line from the origin. This has been reported by many investigators, and Morrogh[14], in reporting some work by Gilbert, suggests that the curve is a result of some volume changes in the spaces occupied by the graphite. They have also shown that some microcracking takes place between flakes. Some investigators have used resonant frequency measurements and also sound velocity measurements which are dependent on modulus of elasticity to predict tensile strength."

http://www.ironcasting.org/Gray Iron - A Unique Material.htm

 

[metengr]

Here is some additional information from ASM Handbook, Volume 1 for gray iron;

Modulus of Elasticity. Typical stress-strain curves for gray iron are shown in Fig. 10. Gray iron does not obey Hooke's law, and the modulus in tension 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). The secant modulus is a conservative value suitable for most engineering work; design loads are seldom as high as one-fourth the tensile strength, and the deviation of the stress-strain curve from linearity is usually less than 0.01% at these loads. However, in the design of certain types of machinery, such as precision equipment, where design stresses are very low, the use of the tangent modulus may represent the actual situation more accurately.

 

[sdra2]

Thanks metengr for all of the information above.

I think the "...secant modulus is a conservative value suitable for most engineering work; design loads are seldom as high as one-fourth the tensile strength, and the deviation of the stress-strain curve from linearity is usually less than 0.01% at these loads..." quote has largely allayed the practical concerns that I had.

The ASM handbook to which you refer may well be the source of information that I am after. I have looked at the ASM website, and I see that this volume covers steels and irons. In the past I have found that such publications tend to be weighted 95:5 in favour of steels information, which may prevent me from asking my boss to purchase it! Assuming you are familiar with it, could you please give an indication as to how much information there is directly related to irons rather than steels. Thanks.

 

[TVP]

ASM HANDBOOK Volume 1 has 104 pages on irons, 926 pages on steel, etc. Looks to be about 90:10 in favor of steels. It is an excellent reference on both subjects though, and I certainly recommend it if you want authoritative information on irons & steels.

 

[unclesyd]

Here is a used booklet, some what dated, that has very good data on Grey Iron. The booklet is The Properties of Grey Iron from the Grey and Ductile Iron Founders Society edited by C.F. Walton.

http://www.alibris.com/booksearch.detail?S=R&bid=8715977054&cm_mmc=shopcompare-_-base-_-nonisbn-_-na