Flexibility of steel alloys

[jgannon]

I'm trying to make a purchasing decision on steel rods from McMaster, and I don't have nearly enough background in materials science to make the best choice. We already bought a piece of 1006, and found it to be too flexible. The other options in the size we need are 1018, A2, D2, M2/M7, O1, S7, and W1. McMaster lists hardness and yield strength for these materials, but my understanding is that these are not directly related to the property I'm interested in. Is there a chart or something somewhere that will tell me which alloy will flex the least under a fairly constant load? My gut says that the one labeled "very hard" (the M2/M7) is what I'm looking for, but I'd like numbers or an expert opinion to back it up.

[SNelson]

The properties that defines a materials stiffness are the modulus elasticity or the modulus of rigidity. Which property (mod. of elasticity or mod. of rigidity) matters depends on your particular loading scheme.

Also, a material's hardness generally refers to a material's ability to resist indentations, scratching, etc.

A good resource for material properties is matweb.com. However, most steels have the same modulus of elasticity (~200 GPa). You may consider using a different material, possibly a carbon composite.

If you give me a few more details about how the steel part is being loaded, I might be able to make suggest an alternative material.

[Kevin Sevcik]

Another good resource is efunda.com. Tooooons of materials and other information. But a limited number of page views per day unless you pay membership.

Agreed with SNelson, though. Alloying materials primarily affect hardness, yield strength and tensile strength. To increase stiffness, you'll need to move to a different material, or change the cross section of your part. Tubes with larger diameters are much stiffer per pound than rods. So if you can get away with a larger diameter tubular shaft/strut/whatever, it will be much stiffer. Though at that point, it's prone to buckling. So you have your choice of lighter, larger, stiffer, etc.

[Tristan Lall]

Kevin's suggested website is good...and this probably makes it even more useful.

[Dick Linn]

The modulus of elasticity for steel varies only slightly between various alloys. Stainless is typically on the low side (28,000 ksi) and something like M4 on the high side (31,000 ksi). There is still not a lot of difference.

[Rick TYler]

To expand slightly on what Kevin wrote, stiffness varies as the fourth power of cross-section. Making it bigger will pay off faster than making it from different steel alloys.

Now, if you replaced that steel bar with a carbon fiber rod, it would be much stiffer. Not as resistant to side loads, and likely to fail in much more spectacular ways when its load limit is exceeded, but it would be stiffer.