I need a 1/4" x 1 1/2" x 80" steel flat bar. I will omit the details of what I need it for because this question is basically asking which composition yields a stronger steel.
www.discountsteel.com has a wide variety of steel bars, but I am not sure how to read the ratings regarding tensil strength and hardness. Here are all the products:
Stainless Steel
Cold Rolled Steel
Hot Rolled Steel
If you click the ASTM material Specifications tab at the bottom of the pages and scroll to the bottom, you will see mechanical data for which I have the following questions:
First off, what is "minimum tensil strength"? The stainless steel 304 has a minimum of 75, but the hot rolled and cold rolled appear to have ranges of 58-80 and 55-70 respectively. Why does stainless have a single number and the others have ranges? Why does it says minimum? Does a higher number mean stronger steel?
What is minimum yield strength?
Second is the hardness scale which uses the Rockwell scale which I've looked into a bit. The stainless rating for 304 is 88, but the rating for hot-rolled is B76. For cold-rolled, it seems to be broken into two: Hot rolled is B67-B80 and cold drawn is B80-B90. This confuses me even more because this looks like the steel is cold rolled hot rolled? Why is the stainless rating just 88 while the others seem to be a range and use the B scale? Does stainless just default to a scale since it's just represented by a raw number?
Best Answer
OK, a few definitions:
Yield strength is the amount of force required to cause the steel to yield, which means permanently deform (i.e. permanently stretch).
Tensile strength (a.k.a. "ultimate strength") is the amount of force required to cause the steel to actually break. This will be equal to or greater than the yield strength.
Minimum just means that the steel will be at least that strong.
Hardness is a measure of how resistant the steel is to scratching and denting. For structural usage it's probably not important, but would be important if you were looking for a durable finish, e.g. a workbench top or a tool bearing point.
Stiffness (you didn't ask about this, but it's another way of looking at the strength of a material) is a measure of how much something deflects when you put a force on it. Steel alloys tend to be pretty similar in this regard.
As you can see, "strongest" doesn't really have a specific definition, it depends on what you're looking for.
Here's an analogy for the difference between yield and tensile strength: Imagine you have a spring. You pull on it a little, and when you let go it returns to its original shape. This is "elastic deformation", and no damage has been done. Now you pull hard on the spring and it doesn't return to it's original shape anymore. The material has yielded and you have "plastic deformation". This may or may not be considered "failure", depending on the application. Now pull really hard and the spring breaks. That's the ultimate strength. Clearly the spring has failed now.
As for the ranges: "steel" is a non-specific name for several alloys and it can be made in several grades, hence the ranges you've found. The material is usually designated with an alloy number. "Cold rolled" and "hot rolled" are methods for shaping the steel, and don't really tell you anything about the strength.
I should also point out that all of these properties that I've mentioned are for the steel material itself. If you want to know the behavior of an actual piece of steel, you need to know both its material and it's shape.