How would there be a yield Temp if there is no thermal stress induced?

The same way a cold piece of steel has a yield strength with out any loads applied to it. It's just a characteristic of the steel.

So, how does the Yield Strength change as Temperature changes?

may be the temp. change the material property by add heat the energy add to the particle so the impact between particle increse the deformation happen before when the material under low temp.

Let us start from "basics":

Let:

E= modulus of elasticity; ε= Strain; σ= Stress; α= coefficient of thermal expansion; L=Length; ΔL=change in length; ΔT=change in temperature; σ_y=yield stress

ΔL=α*ΔT*L ; ε= ΔL/L = α*ΔT ; σ =E*ε = E*(α*ΔT) = (E*α)*ΔT

When the material yields we have: σ = σ_y = (E*α)*ΔT

From the above ΔT = σ_y / (E*α)

In other words, the above equation calculates the change in temperature (ΔT) that you will need to let the material yield.

This is good stuff, but isn't it describing the case of a sample rigidly constrained and subjected to a change in temperature?

I got the impression that the original post was asking about the relationship between proof stress and temperature. Here is a random extract from one of my data books giving the proof stress v's temp for 316 stainless steel:

400 deg F - 29000 psi

800 deg F - 25000 psi

1000 deg F - 23000 psi

1100 deg F - 22000 psi

1200 deg F - 21000 psi

1300 deg F - 20500 psi

1400 deg F - 19500 psi

1500 deg F - 18000 psi

I will leave you to work out the law/relationship here, but do you need to?

Unless you are perhaps trying to extrapolate results in uncharted territory, there are plenty of tables around to use.

Hope this helps.

Drew

I agree that's what the post was asking about. If it's any help, pressure vessel design standards give maximum design stress (as opposed to yield stress) over a range of temperature. The one I'm looking at is an old version of BS5500, but I'm sure ASME XIII has something similar.

I understanded that you'd like to know the effect of the temperature on the physical propertie of the steel, right?

As you may know, the temperature is actually an atomic particles cinetic measurement. As the molecules increase their movement, the more the temperature.

You may also know that the yeld stress is a measurement of the amount of energy a material can hold, that will cause the activation of its deffect sliding systems in atomic crystaline structure scale (at least that's how they call it in Brazil, in direct translation. Unfortunately, I dont know the exact technical terminology in english). When this amount of energy is reached, the material starts to slide its atomic arrange planes along each other until the material discontinuities block it (let's say, grain boundaries and alloy elements, among others). When a limit is reached, the tension increases until the material breakes.

With this figure in your mind, it's easy to evaluate that, if you have less particles energy in the material, you have less sliding systems activated (or, in other words, is harder to move particles along). YOu will observe less material deformation, and more fragility.

If I took the correct approach to your doubt, let me know and I can give you some more references.

This makes the most sense to me...

an increase in thermal energy, will decrease the mechanical energy required to yield the steel.

so with a bit of math/algebra/calculus, knowing the yield strength of a particular steel should be enough to develop a theoretical curve for the relationship?.. what else would i need to know? I'm not sure of the terminology, but the energy storage capacity of the material???

There are dedicated publications that gives you the theoretical development for the material characteristics from the atomic structure and cristaline arrangement. Look for books on materials science and mechanical construction. This is a kind of advanced matter, normally covered in master achievement level, so another suggestion is to look for monographies that will also tell you the best sources to look for. Of course you can find this relationship empirically, but there are some developments.

PS: The capacity of teh material to store energy before breaking is called tenacity. In a tensionXdeformation diagram, it can be understanded as the area below the curve. Thus, you can evaluate how many energy a material supports by this area. For example, high strengh steel may have a high yeld strengh, but will absorb less energy before break than a mild steel, that will deform a lot.

At what temperature does steel start to stress while under thermal load. I understood that once a steel shell reaches 630oF that the change starts. Can anybody elaborate and/or give me some references.

thanks

you should ask new questions in new threads...

Refer to the ASM Handbook, Vol. 4, Chapter 1, Stress Relief. It gives examples of the relation between the yield strength ant the temperature for several steels.

Has anyone published numbers for the change in yeild strength due to change in temperature of AISI 1020?

looking the same for A2017

Keep in mind that the reduction to yield strength at a given temperature also may be affected by total amount of time it has been at that temperature.

I've read the topic, but there is no answer for the simplest question: where can we find the relationship between strength properties and temperature?

I think that something "free" on the net must exist, 'cause in hot steel forming industry these formulae are essential.

"essential"? no not really... I work for a major Integrated Steel Mill and from what i've been able to find we only know this information for our own products, based on real world testing. It would seem that there should be some sort of fomula though...

In the past I worked for a steel hot forging company, but the needed force of deformation was always empirical - I agree, there is a kind of relation: but, why no one can tell us what is this formula?!

IMO, there must be some literature on this!