|
|
|
Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
|
Strength, Hardness and Brittleness are all different things.
From my limited knowledge of metallurgy hardness and brittleness are mutually exclusive.. The harder something gets, the more brittle it becomes.. Grade 8 Bolts have a higher yield strength than grade 5, but you shouldnt use them in place where shock loads are likely to be experienced or breakage will result in catastrophic failure.
Often a slightly stretched bolt is preferable to a snapped one.. I know that in building ultra-light aircraft, Grade 5 bolts are usually specified, since snapped bolts are not a good thing. I saw a Gyrocopter once that had been tipped over on landing with the Rotors spinning, and the main rotor head spindle bolt had stretched so much that its diameter was down to half the original at the thinnest part, but it hadnt broken !
Someone like the Kero's or MytQik might be able to speak with more authority on this subject, but in the meantime, heres a web link that looks useful.
http://aluminium.matter.org.uk/aluselect/tempers.htm
_________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
|
Wed Dec 01, 2004 10:10 pm |
|
|
|
|
kkeerroo
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane
|
Ok, this is the second time I've started writing this as I was trying to make it quick and easy. Now I'll try for some thing you can understand.
Every metal is made up of crystals. When I talk about crystals I don't mean the shiney see through ones they charge a fortune for to silly women. What I mean is the way the atoms in the metal are stacked on top of each other. The atoms might be in a cube shape, or a pyrimid, or hexagon shape or what ever, and the pattern repeats it self through the metal. Now when liquid metal is cooling into a solid the atoms start to stack them selves into crystals. But because it doesn't happen straight away there will be lots of starting points all though the metal where this happens. Now none of these crystals will be facing the same direction so when they grow and bump into each other they don't join properly, so what we end up with is lots of lumps of crystals all joined together. If you've ever looked at the surface of zinc coated steel (galvabond) you'll see lots of different colors, those are the differently orriantated crystals.
Now the differnt crystals facing differnt directions are called Grains, like in wood. Where the grains (blobs of crystals) touch is called the Grain Bounderies. Now the bonds of the atoms between each other at the grain bounderies are weaker then the bonds in the crystals, so when a crack passes though the metal it'll travel along the grain bounderies. If you had a bit of metal with lots of little grains and lots of grain bounderies it would be easy for a crack to find its way though the metal, but if you big grains with not many bounderies its harder for a crack to find its way though the metal. Also crystals are able to stretch easily, but not if there are other grains getting in the way. So metal with big grains can stretch more then metal with lots of little grains. This is why soft metals are ductile and hard metals are brittle.
To get confussing we all know about phase changes in materials, eg. Solid to Liquid to Gas. Well some metals under go a Solid to Solid phase change when heated up to a certain temperture. For example plain carbon steel changes from a Body Centered Cubic crystal structure to a Face centered Cubic structure when heated up to 750 degrees.
Some
aliminium alloys are the same. When they change to this hotter crystal structure I beleive they become one big grain. When it cools down different points cool quicker, forming grains first, causing all those little grains to form again. Depending on how fast you cool you end up with different size grains.
Basically tempering is a process to control how big the grains in the material are. Big grains = Soft (bad) but Ductile (good). Small grains = Hard (good) but Brittle (bad).
Its lucky you didn't ask about the heat trement of steel as the post would be 4 times this length. But steel is more fun. _________________ Get Some!!!
Secretary of the Queensland Robotics Sports Club inc.
|
Thu Dec 02, 2004 8:32 am |
|
|
Nick
Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
|
I found some useful sites for further reading:
http://www.key-to-metals.com/Articles.htm
general stuff for tech heads.
http://www.principalmetals.com/properties/step2.asp?Family=Aluminum
very specific infor on all alloys including heat treatment. recommended!
Back to my suggestion about just using a more suitable alloy: I started out by trying to get 6061 like all the US guys recommend. That didn't go so well as its expensive and rare in Australia, so I looked up alternatives. As I've said before, 5083 alloy (which is used mostly for boat building) is as good as, or better choice for combat robots, particularly if the frame is welded. To quote:
quote:
When non-heat-treatable alloys are welded, microstructural damage is incurred in the HAZ (heat affect zone). Unlike the case of heat-treatable alloys (ie 6061), whose strengthening precipitates may dissolve or coarsen, the HAZ damage in non-heat-treatable alloys is limited to recovery, recrystallization and grain growth. Thus, loss in strength in the HAZ is not nearly as severe as that experienced in heat-treatable alloys.
... so 5083 has slightly higher tensile strength and makes a better welded frame than 6061 unless you spend $$$ on heat treatment. My other fave quote about 5083:
quote:
An important application for alloy 5083 is the construction of tactical military vehicles. The hulls and turrets of vehicles such as the M113 armored personnel carrier, the M2/M3 infantry and cavalry fighting vehicles, the M109 self-propelled howitzer, and AAV7A amphibians all consists of welded 5083 aluminum structures.
2024 allow would be even better for internal parts like motor and bearing mounts, just be aware that it is not weldable and usually comes in rods or bars and not in extruded sections or thin sheet. Some suppliers and fabricators like O'briens Aluminium have offcuts of 2024 that are useful sizes. O'briens will not sell cut to size 2024 or 6061, but will make waterjet parts in those alloys. _________________ Australian 2015 Featherweight champion
UK 2016 Gladiator champion
|
Thu Dec 02, 2004 11:47 am |
|
|
|
|
|
Spockie-Tech
Site Admin
Joined: 31 May 2004
Posts: 3160
Location: Melbourne, Australia
|
Off-topic Hardox posts moved to another Thread..
My fault, I should have known that people who like it get all defensive about it whenever its criticised, and I bought it up.. sorry.. we now return to you the on-topic subject of aluminium hardening...
To steer things back on track, when you buy your heat treated aluminium, it usually comes with a "T" number after the alloy indicating its "temper". Eg 6061-T6, 2024-T3..
As Far as I know, "Temper" is different to "hardness" although both are created by a heating/cooling process.. My understanding of the "temper" of a metal is the amount it can be deformed or bent and still return/spring back to its original form. If you exceed its temper rating, then the bent/deformation becomes permanent. The higher the T number, the greater the temper..
Question is, is this necessarily related to hardness ? does a higher temper automatically mean a higher hardness or are they different properties ? _________________ Great minds discuss ideas. Average minds discuss events. Small minds discuss people
|
Fri Dec 03, 2004 10:27 am |
|
|
Nick
Experienced Roboteer
Joined: 16 Jun 2004
Posts: 11802
Location: Sydney, NSW
|
"Temper" is both a noun and a verb, so I take it to be a process and not a rating or measurement like hardness is. You temper metal to alter it's characteristics, so a high temper is a description of more than just hardness.
I think you are confusing temper with "modulus of elasticity"
quote:
Definition
: When a material is subjected to an external load it becomes distorted or strained. With metals, provided the loading is not too great, they return to their original dimensions when the load is removed, i.e. they are elastic. Within the limits of elasticity, the ratio of the linear stress to the linear strain is termed the modulus of elasticity or more commonly known as Young's Modulus.
This rating is what makes titanium such great armour; it bends, absorbing and distributing the energy, then springs back. Steel can do that too, but not as much and it weighs 1.7 times more.
I liked the Fedur composite steel more than Hardox as you get a really hard surface with a tough backing. Given the excellent performance of the tooth on Plan B's weapon, which has hard facing steel welded on the front edge, Fedur would go well as armour or weapon parts. _________________ Australian 2015 Featherweight champion
UK 2016 Gladiator champion
|
Fri Dec 03, 2004 12:03 pm |
|
|
kkeerroo
Experienced Roboteer
Joined: 17 Jun 2004
Posts: 1459
Location: Brisbane
|
quote:
Originally posted by Spockie-Tech:
As Far as I know, "Temper" is different to "hardness" although both are created by a heating/cooling process.. My understanding of the "temper" of a metal is the amount it can be deformed or bent and still return/spring back to its original form. If you exceed its temper rating, then the bent/deformation becomes permanent. The higher the T number, the greater the temper..
First I've heard of this term.
Sounds more like you refering to the eleastic limit of a material. Springs are the best way to describe it as they amplify whats going on in the material.
If you aply a small load to a spring it will stretch a certain distance. Apply a bigger load and it'll stretch further. Take away the load and it spring back to its origonal length. The same thing happens in everything including rope, steel bars, glass, ect... Now the load we put on the spring is called Stress and is calculated by:
Stress = Force being applied / Cross section area it being applied on
So for some thing hanging by a rope the force is the weight of the object and the area is the cross sectional area of the rope. Pretty simple. Stress is measured in Pascals.
Back to the spring. How much the spring stretches is called Strain. Strain is actually the percentage the sample has stretched. For steel it is less then 0.01%, but for a rubber band it be over 500% .Strain has no units by the way.
If you were to put a load on a spring and measure the distance, then try a different load and keep doing that till you are able to make a pretty graph you'd be a boring person, but you'd also notice the the graph is linear (in a straight line), untill you reach a certain point. At that point the spring wont return to it origional shape. If you are like me you would have pulled a spring out of an out ball point pen once and stretched it till it was almost straight. Well the point where the spring starts to want to be straight is the point of the graph where it stops being straigh. This point is called the Maximum Yeild Strength. Before this point the material was eleastic (went back to it origional shape) and after this it was Plastic (it didn't).
Even though the material can take more load after this point Engineers consider a part to a failed if it hit this point. Ductile items tend to keep stretching after this point, but britle items break.
Sorry, this had nothing to do with Aliminium or Andrew question which I forgot about. _________________ Get Some!!!
Secretary of the Queensland Robotics Sports Club inc.
|
Fri Dec 03, 2004 1:09 pm |
|
|
|