Steel Exposed to High Temperature for Short Term

[jimmytwotimes]

I am reviewing a 1/2" diameter steel flatbar hook. It is used to drop another material into an oven, and thus is exposed to 1500 degrees fahrenheit for a quick second. My question is if anyone can point me in the direction of some resources which will give me an indication of how this heat exposure will effect the steel over time, what kind of stress the steel will face from this very short exposure, etc..

Thanks

 

[rb1957]

you're obviously (?) using annealed, possibly normalised, steel ... ie not heat treated for higher strength.

i'd wonder how hot the hook actually gets, exposed for such a short time.

there's data in Mil Hdbk 5 (or MMPDS-01), both available online, but for exposure to 1/2hr.

if anything, i'd suspect that might just be a thermal fatigue thing happening, but i don't think so ...

Quando Omni Flunkus Moritati

 

[amendale]

To calculate the steel temperatures reached by a member you can use the step-by-step method or best fit method for steel temperature change (research this - it depends on several exposure factors). A change in temperature will change your steel yield strength and elastic modulus. You can find graphs of this online that plot this against temperature. If the ends of the member are restricted you will develop stresses due to expansion (strain = temperature * coefficient of thermal expansion), but doesnt sound like you need to consider this for your case.

If you want to be safe use the full temperature of 1500 farenheit which would correspond to about 80% reduction in your yield strength.

 

[dhengr]

JB:
What is a “1/2" diameter steel flatbar hook”? And, are you loading a heat treating oven, or some such, where the hook isn’t exposed to the 1500̊F temp. for an extended period of time? What is its temp. after many cycles? You can usually take steel up to 600 - 700̊F without appreciably changing its Fy or Fult.; its modulus of elasticity will be about 90 - 85% at those same temps. As long as you can keep its core temp. in the 600̊ range, or less, you should be o.k. The biggest thing I would expect in working in that hot environment would be accelerated scaling and loss of net section over time. People who work their equipment at these temps. use much higher factors of safety to account for these things, and then inspect it regularly.

 

[jimmytwotimes]

What I mean by 1/2" dia. steel flatbar hook is basically a fishing hook, that is 1/2" in diameter all over. Its not exposed to 1500 degrees for very long, only say < 5-10 seconds (conservatively)

I'm just looking for direction to a resource which can give an indication of the effect (on strength mostly) of very short term exposure to such temperatures, over time (many cycles). The hook is also carrying ~600 lbs (dropping that load into the 1500 degree oven).

For a very short exposure to that heat, an 80% reduction in yield strength seems pretty severe..

Thanks for the responses

 

[SteelPE]

When I was at graduate school I took a class called "Structural Design for Fire Conditions". Basically we did the step analysis that amendale suggested. You would have to figure the temperature exposure of the rod as the temperature around the rod increases from room temperature to 1500 degrees and back down again. The class we took addressed fires, so we were talking temperatures that went from room temp to 1200-1400 degrees over 5-10 minutes and coming back to room temperature in about 6-8 hours or so not a few seconds. Your exposure is going to be less. However, how many times/hour do you plan on subjecting this rod to 1500 degree temps? 1, 30, 60, 100? Are you dipping the rod into a fluid that is 1500 degrees?

 

[rb1957]

the key is to figure out the temperature the hook gets to.

you can do a bunch of thermodynamic modelling to figure it out, or you can paste a bunch of thermocouples (or heat sensitive paint) on the hook. or maybe there's some sort of viewing aparatus that detects surface temperature ? i'd've thought that the hottest location would be the ingot (?) contact area.

are you trying to figure out if the hook'll break ?

Quando Omni Flunkus Moritati

 

[dik]

Do you have access to thermal imaging or a laser thermometer, and take a quick reading after to see what the temperature is?

Dik

 

[rb1957]

guess that'd be my "viewing aparatus that detects surface temperature" !?

but i've got to ask "what's the value of the hook ?" it's a simple 1/2" barstock. if there's a question about how much load can it pull, try it out ... you might want to have a larger hook standing by if you have to pull a load from the oven (you wouldn't want to spoil a load 'cause of the hook breaking).

maybe it's a question from OH&S ? so now the value of the work isn't the value of the hook, and you Have to get a number (guessing is not an option !?)

Quando Omni Flunkus Moritati

 

[BAretired]

Over-sizing the hook seems like a pretty simple and inexpensive precaution to take. Or you could have an ice bucket the hook must be dropped in before and after each lift.

BA