Steel and Earthquake

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Do you want low carbon steel to withstand earthquakes? Low carbon means more ductile, right? Am I right?
 

Astronuc

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Generally, low carbon in steels implies greater ductility.

You might try - American Institute of Steel Construction and enter "seismic" in the search engine. Quite a few documents (pdf) are available including some supplements.

Mild steels and high strength steels are used, but I'll have to get back with regard to specific grades in seismic areas.

There are also design issues depending on the type of structure, e.g. steel column/beam vs reinforced concrete (i.e. rebar).
 
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Can you explain to me how come as you lower the temperature on a isothermal transformation diagram for a eutectoid plain-carbon steel, you get finer and finer microstructures? Like you get as you lower it from coarse pearlite to fine pearlite to the finest bainite? Also, I don't understand what diffusion of concentration gradient is and what is the difference between toughness and strength. Is toughness also the hardness(stiffness)?
 
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Gokul43201

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This is off the top of my head so it may not be the best explanation. Essentially, the grain size is determined by the extents of nucleation and diffusion. Clearly, greater nucleation will cause smaller (finer) grains, while more diffusion results in bigger (coarser) grains. Also, we know that the diffusion rate increases with temperature (exp[-E/RT]), so at lower temperatures, diffusion is inhibited, making nucleation dominant. Also, I believe - meaning that it seems logical to me - that during non-equilibrium cooling, the lower you go below the equilibrium nucleation temperature (the liquidus, in the case of a melting transition, for instance) the greater will be the nucleation rate, once begun. Some of this is speculation on my part (I don't have any Phys Metallurgy books nearby, and it's been several years since I dabbled in it), so I would wait for a better authority like Astro or Perennial to come along.


Toughness is not hardness or strength. It is in fact the area under the stress-strain curve (and usually under impact loading), or the energy absorbed by the material until fracture. Think automobile design. If you slam you car into a wall, the kinetic energy you had goes into the impact, and if this energy is not absorbed by the material of the car, you might end up with more than a few broken bones.
 

PerennialII

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physicsss said:
Also, I don't understand what diffusion of concentration gradient is and what is the difference between toughness and strength. Is toughness also the hardness(stiffness)?
There was a thread while ago where these differences in toughness/ductility and strength were addressed in some detail:

https://www.physicsforums.com/showthread.php?t=59152

#7 over there has how I perceive this, as Gokul stated toughness and strength are very different things (although some very workable relations and correlations can be identified). Overall one could say that toughness is an energy related parameter, while strength is coupled with deformation.

Actually the issues are addressed from a similar angle also here :

https://www.physicsforums.com/showthread.php?t=54157

Hardness is essentially a measure how a material is able to resist plastic deformation (think about typical hardness tests where a small sharp indenter is pushed into a material) and as such correlates to some extent both with strength and toughness, best however to tensile strength (since it is the only similar typical material characteristic available).

Stiffness then again is typically reserved to contain both structural and material "stiffness" elements, i.e. the Young's modulus of a material (if we limit ourselves to linear-elastic behavior) and structural stiffness (think for example different beam cross-section under bending) ... which when put together form what we typically understand as (structural) stiffness.

People have typically very differing views about what they perceive as toughness, ductility, strength, hardness etc., so in general quite a bit of care is required in order not to mix anything (and somewhat case dependently different definitions may be correct, or better applicable).

Diffusion of concentration gradient .... well a concentration gradient does change via diffusion, and the presence of a concentration gradient is overall important in diffusion processes ... are you looking for definitions related to diffusion or something else?

physicsss said:
Can you explain to me how come as you lower the temperature on a isothermal transformation diagram for a eutectoid plain-carbon steel, you get finer and finer microstructures? Like you get as you lower it from coarse pearlite to fine pearlite to the finest bainite?
I'd go with Gokul's explanation on this one ... at high temperatures (= near the austenite transformation temperature) the nucleation rate is low, whilst the grain growth rate is high and vise versa at low temperatures. The resulting structures will then have characteristics which have resulted either from nucleation or growth dominated transformation processes (thinking about the extremes of ferrite - pearlite structures for example). So it is to great extent a matter of temperature dependency of diffusion and the greater driving force for nucleation the lower the temperature (the higher the undercooling).

There was some structured some here :

http://mse-pma1.eng.ohio-state.edu/~peterand/mse205/chapter10/chp10.pdf [Broken]

Overall I prefer continuous cooling transformation diagrams over isothermal ones ... more practical, don't have to think about the constant temperature thing.
 
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Astronuc

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Grain size and to some extent microchemistry are determined by quenching temperature and quench rate. Faster quenches and lower quenching temperatures produces finer grains and more homogeneous microchemistry precisely for the reason that Gokul stated. As a liquid metal solidifies, elements/compounds with the highest melting points will precipitate and the grains will nucleate around them. Also lighter elements, like carbon, diffuse at faster rates than heavier elements, so it is desirable to quench quickly so the chemical gradients are flat, and so that mechanical properties, which are dependent on alloy chemistry are as uniform as possible.

Toughness (the ability to plastically deform without failing) is somewhat related to yield and ultimate tensile strengths. One can see some general trends that show the stronger the material, the lesser the toughness.

Example of http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTA-0204-1125/$FILE/MMTA-0204-1125F.pdf?OpenElement (one page of an article)

Also under certain conditions, http://doc.tms.org/ezMerchant/prodtms.nsf/ProductLookupItemID/MMTA-9803-781/$FILE/MMTA-9803-781F.pdf?OpenElement (one page of an article)

An example of widely different strength/toughness relationships.

Another good source of information is http://www.key-to-steel.com/Articles.htm

Fracture Toughness of High-Strength Steels at Low Temperatures
 
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Astronuc

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Structural, Carbon and HSLA Steel Plate
http://www.chapelsteel.com/A709-grade50-grade-345.html

A709 is the specification used for steel plate used in the manufacture of bridges. Chapel Steel stocks A709 Grade 36, A709 Grade 50 and A709 Grade 50W. These grades are often specified with charpy tests referencing the specific zone (or climatic region) in which the bridge will be in use. The charpys are specified as non-fracture critical which is a per heat test (frequency “H”), or fracture critical which is a per plate test (Frequency “P”). Bridge steels can also be ordered to the ASSHTO M270 Spec referenced as AASHTO M270 Grade 36, AASHTO M270 Grade 50, AASHTO M270 Grade 50W.
and read the article

http://www2.arch.uiuc.edu/aamin/451/download/New_Steel.pdf [Broken]

and

ASM International - High-Strength Low-Alloy Steels Compositions of various high strength, low alloy steels.


The geometric (dimensional) design is the other aspect besides choice of material. Basically a structural engineer designs to code, which limits loads/forces in the structure. The materials are pretty much dictated. Compositions can be found in the last link. If one is outside US, one will have to find equivalent specifications to one's country or geographic area, as in EU.
 
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