Side Project Questions: What Steel?

[Dick Linn]

Stainless? Brittle heat-treated steel? Take it from someone who knows that he doesn't know, it's not as simple as all that.

[DonRotolo]

OK, let's dispel a myth here: Heat-treated steel is NOT STRONGER, it is HARDER. What you want is a very "Tough" steel (that is a real term used to describe steels) not a very "hard" steel. Strong (Tensile strength) isn't the ultimate goal, you want a high yield strength with a relatively low elongation (like less than 50% or so).

While a 1045 or 4340 alloy will be way strong, even annealed, these alloys may be hard to find or expensive. 1040 is nearly as good and a bit easier to work/machine. Make it thick enough and even 1010 won't break.

Case-hardening can give you high surface hardness while maintaining the tough interior body, but it still fractures more easily.

Ultimately you want a steel that is tough without heat treatment, other than annealing or perhaps normalizing (depending on the steel), both of which can be done at home easily enough (heat it up hot, and let it cool. Done). If rust is a problem, Rustoleum paint will fix that easily enough.

[Brandon Holley]

Originally Posted by Dick Linn

Stainless? Brittle heat-treated steel? Take it from someone who knows that he doesn't know, it's not as simple as all that.

I wasnt getting in to specifics. There are a lot of variables to consider for Craigs application. If this is going to be a shaft used on a road surface that may be moist, damp, etc...stainless would be something I would consider...as I stated above in my original post I am not going to recommend a specific grade. I was simply recommending he look into stainless grades.

And heat treating makes steel harder...thats a fact, I didn't say it wouldnt work for his application, I was just throwing out ideas for him to consider. If you read the point that I stated in parentheses, I was simply stating "hardened" steel does NOT make it the "better" steel.

Take it from someone who knows what he's talking about...I was simply giving him things to consider.

[Kevin Sevcik]

I haven't run any numbers on this yet, but I think it's entirely possible you'll want a harder steel, rather than a tougher steel. Here's the difference between the two:

Tough = more energy required to ultimately fail the material. For identical tensile specimens, a tougher steel will require more total energy to ultimately break the specimen. Typically, this is because it stretches and deforms a LOT, so you exert a reasonable force over a long distance. Tough steel is good for things like car crumple zones, buildings, and other things where you'd rather the member yield and deform while absorbing energy than suddenly break.

Hard = more force required to ultimately fail the material. This typically implies less toughness, because while the material requires more force to break it, it doesn't stretch near as much. Think of the difference between breaking off a pice of cold, hard taffy versus warm stretchy taffy. High hardness is good in situations where any yielding/bending will be just as catastrophic as a clean fracture. Typically, this is in shafts and suchlike. Additional benefits to high hardness are a higher fatigue limit. This means that the steel will better resist repeated low level stresses on it. Such as from bumpy pavement at 60mph, or a slightly misaligned rotating shaft.

So.... Long story short, I think you need to decide if you'd rather have an axle bend somewhat while you're going downhill at 60mph, or break off completely. Also, if you have or can find pictures of the remains of a board that HAS failed, it can be remarkably useful in determining what it was that caused the axles to fail.

[Brandon Holley]

Originally Posted by Kevin Sevcik

So.... Long story short, I think you need to decide if you'd rather have an axle bend somewhat while you're going downhill at 60mph, or break off completely. Also, if you have or can find pictures of the remains of a board that HAS failed, it can be remarkably useful in determining what it was that caused the axles to fail.

My hesitation for a hard steel is the fact you just mentioned. If i were moving 60 miles an hour, i'd prefer for the axle to bend rather than just snap off...but again, this is at the discretion of Craig

[Dick Linn]

A properly heat-treated steel can be stronger in both tensile and yield strength. The hardness and "brittleness" are factors to control, and different alloys respond quite differently.

For example, I ran across this a while back during a discussion of heat treat requirements for 4140 steel over at Practical Machinist. I found the information on embrittlement effects interesting, especially the fact that the notch toughness decreased in the midpoint tempering ranges (figure 2).

http://www.nhml.com/resources/2007/4...ment-of-steels

Some of you may find this discussion on bolt strength of value:
http://www.rockcrawler.com/techrepor...ners/index.asp

As far as sourcing metal, you can get 1144 Stressproof, 4130, 4140, or 4340 fairly inexpensively at places like www.onlinemetals.com. I've seen some Grade 8 bolts specified with AISI 4037 or 4042.