case hardened or weakend steel?

[capybara]

if i have a bracket that is always bending due to load,
what would happen if:
i heat it read hot with a blow torch, then throw it in a bucket of cold water?
would it be stronger (less likely to bend)
or weaker (more likely to crack)?
what is the proper way to harden metal?

 

[Fandu]

Depends what the bracket is made of. Cold-Rolled Steel (CRS) will become slightly more brittle and is likely to warp when thrown in cold water. Any types of cast metal will most likely crack if cooled that fast, usually a cooling period of hours is required for cast steel.

The strength of a metal is largly based on the elemental makeup of the metal. Hardening through heating and cooling can be effective if done correctly, but it takes many repetitions and long cooling periods usually.

 

[Snooper]

It doesn't take that long. Usually, steel is tempered in one pass, not multiple passes. The trick is heating it up to the right temperature and holding it there for the right amount of time to get the molecular changes you need. Then you have to cool it off at the right rate to get the rest of the hardening effect you are looking for. When I am tempering allen wrenches, I usually heat them up to a bright red glow on the last 1/4" of the tip and then let them cool in the air until they drop to a very dark red. Then, they get dumped into a pail of water to quench them. It works very well. If I quench them too soon, they will be very hard, but also very brittle. I have never heard of metal cracking from quenching them unless it is very large peices of metal. I have heard of it cracking from work hardening though...

 

[rgwalt]

I did some forging and heat treating in high school metal shop. We had some chisles and screw drivers that cracked upon cooling in water. The pieces were just dropped into water instead of being slowly lowered in, and the rapid cooling caused cracks to form in the metal.

As was mentioned, the heat treating process depends on the type of metal you are working with. If the bracket can easily be remade or bought, try taking your current piece, heating until it is a bright orange, and keep it there for 10 or 20 minutes. Slowly lower it into water or motor oil (for slower cooling). It will be less likely to bend, but more brittle. So, if this bracket is holding something that could cause lots of damage if the bracket failed, I would seriously look at the benefits of heat treating before making any changes.

Ryan

 

[mAdD INDIAN]

Something interesting to add in this topic that I learnt in my material's science course (so its finally useful now):

When you hit a metal (like bang your hand on the table), you are actually strenghting it. Its called "cold working" the material. What happens is you cause dislocations in the atomic structure. An increase in dislocations increases the repulsive force between them (something to that effect), which prevents the material from further deformation. The more you hit it, the stronger it gets (until a certain point ofcourse).

This post also acts as a review for me since I have my final exam for this course in two days.

 

[CTho9305]

It gets stronger, but more brittle. Right now, the bracket bends. If you harden it and the load is too great, it will fracture suddenly.

 

[Wintermute76]

I assume that your bracket is plain mild steel. Heat treating won't help you much as it probably has a low carbon content like 1018 CRS or something. Your best bet if you want to make a new bracket would be something along the line of annealed 4140, a good all around chrome-moly steel with good toughness. Heat treating will make it hard, but more brittle like a couple of the other guys said.

the simplest thing would be to increase the cross section or put in some type of reinforcement.

 

[drag]

hehe, if you want to strengthen it and don't want to go thru to much trouble just do this:

sand it down, you want to just mark up the metal and get rid if the paint (if any) use a rough grit of paper like 80 grit.

now find a small peice of wood, something flat or will follow the contours of it, I don't know how big, but you won't need anything realy thick or anything. I bet a pop sickle stick would work great for anything you'd find in a computer case.

Now take some epoxy from a epoxy tube you can get from any hardware store, you know the 2ton or the 5 minute stuff that comes in a big syring that is divided into 2 parts. Mix up a little bit really really well! If you don't mix it up well it won't dry evenly. And be conservitive, you can always add more glue, but too much will make a mess. I like to use a old magazine with shiny pages and a plastic spoon. That works the best IMO.

Now just glue the stick to the peice of metal. You can mask off what you don't want glue on with some scotch tape, or clear packing tape if it is a large area.

The more you prep, the better it will look. Remember to make sure that you know exactly what you are going to do before you do it. If have a small wood clamp (you know the spring types for small stuff) or a pair of vise grips, use them to pin the bit's together. Dried epoxy by itself is brittle, but as long as you have a nice tooth etched into the metal and it's clean and there is no exess gap it is actually very very strong. Hell, stratch and goudge the metal to get something for it to grip into if you want.

Now if you want it to look nice, just sand everything down, get rid of any sharp edges or anything that looks ugly. Use some fine grit paper, moving from rough to fine, eventually to like 150 to 180 grit. Now go and get some primer and spray paint, I suggest rustoleum. They have nice color and is strong. Plus it has a nice "hammered" line of paints that leaves a metal-like texture... Primer it, give it a quick sand, then primer it again, then use the paint.

If it has 90 degree bend or something that needs to be reinforced, just take a square peice of wood and put it into the corner, or maybe take a bit of sheet metal and bend it around the corner and glue the bits to the back. If you do it nice you can probably make it look like one peice, if it works...

Or if you can do that just bend a thicker bit of metal to match the curve or bend and glue it to the back...
If you prep it correctly epoxy is some s**t, some day after engineers figure out the long term effects they will be gluing houses together instead of nailing them, it's quicker and much stronger. That's how they put together F1 race cars! It's a carbon fiber mesh with it all glued together in forms, with the epoxy being squished imbetween all the fibers. Composite materials you know


If it is to big for gluing:


But then again you can probably make your own bracket with a Big Freaky Hammer, pair of large vise grips, and table vise, a drill and a thicker bit of metal. Maybe some files to form it and get rid of burs, enlarging holes etc etc. I've helped out making brackets for car alternators and such and as long as you line everything up well (usually with washers for shims) it'll last for a long long time.

Why dosing may or may not work, I wouldn't try it on anything that can't be replaced.

If its just steel, heating it up and dosing it will help de-crystilizing the metal. If it cools slowly large grains (metal crystals- sort off) will form, making it more prone to bending, but more resistant to catastrophic failure (cracking). Dosing it rapidly will scramble the molecular structure, making it stiffer, but more brittle. Cooling it too rapidly can cause it to cool unevenly in a violent fasion. It would have to be fairly thick, but anything thin like sheet metal won't do that, it would have to be pretty thick or heated up very very much...


Another possible fix:

If it is something seriously large or hefty, I would try to reinforce it with bits of metal brazened or soldered on. Brazing is a proccess were you melt brass rods into a joint between to bits of metal. You need to have a special flux (acid that cleans and etches the metal rapidly when heated). You can do it with a propane torch as long as the metal bits ain't to big. You won't need to heat the metal up much hotter then a very dull glow. It is actually very very strong, only physically welding will be stronger and only if the welds are very high quality. You see the brass melts into the surface of the steel and forms a alloy (just a fancy term for 2 or more types of metal mixed together) Brass has a higher tensile strength then steel so a failure would happen in the surrounding metal instead of the joint itself.

Just my 2 cents. Nothing I like better then amature engineering using materials and junk I have at hand

 

[Wintermute76]

you can't just grab a piece of steel and expect to heat treat it. even then, if you heat it and cool it, you still need to anneal or temper it if you expect to do anything with it.

generally steels with higher carbon content get harder when heat treated, but also more brittle.
low carbon steels, won't heat treat, but can be case hardened, tending to be tougher, but not as hard.

as a rule, more toughness(flexibilty, resistance to cracking) more hardness
<-------------------------------------------------------------------------------------------------------->
less hard more brittle


When tool steels are heat treated they're bought up to their heat treat temp 1500-1900 F depending on the alloy, held at that temp for a certain time period per inch of thickness then quenched (air, oil or water) then brought up to anealing temperature to temper the steel and relieve stress. Full hard steel has a needle-like structure called martensite, which is rather brittle (I once saw a small cavity block shatter on the floor when it came back without annealing), tempering converts some of the martensite to austenite a more stable crystal structure. Also some steels need to be hardened in a controlled atmosphere to control carbon loss and surface quality.



heating red hot won't be enough, you need to get it to a good orange/yellow before any structure change.


about any bracket that I can think of should just be annealed steel (unhardened) nad if it's bending, weld on an angle or something.

 

[PrinceXizor]

1. The appropriate "color" of a material is dependent upon the molecular make-up of the steel. Personally when we did some lab work on heat treating, the steel samples we used were bright red, not orange, yellow. But, it all is relative to the temperature needed to change the crystalline structure to what you want. The basic idea behind heat treating is that you raise the temperature of the material past a certain point to where the molecular (crystalline) structure of the material is in the more "desirable" form. Then you want to cool it quickly so that it remains in this form instead of returning back to its original crystalline structure at room temperature.

2. Any steel, from light carbon steels like 1018 to more robust steels like 1042, can be heat treated. The reason lighter steels are often not is the cost/benefit/result tradeoff to heat treating as opposed to other processes such as case hardening. Lighter steels are not that strong, and if you needed such strength you would have specified a stronger steel. So heat treating to get more strength is kind of pointless. Case hardening allows a greater impact and wear resistance for this lighter steel while still keeping its good ductility and machinability (which is usually why it is chosen). That being said, lighter steels can be heat treated, but the increase in strength is usually counterbalanced by a steep decline in ductility.

3. For your application (strengthening a bracket), you might want to consider oil quenching as opposed to water quenching. Oil dissipates heat slower than water and thus there is less risk of cracking. By oil, we're talking about motor oil types, not cooking oil. You also could consider just letting it air cool. This process also strengthens the steel (although not as much).

Hope this helps.

P-X

Edit One: As was mentioned though, unless this is just a semi-experiment, your safest bet is to reinforce the bracket in some manner. Heat treating is done in a controlled envronment and using Eutectic charts to determine the various crystalline structures and the temperatures needed to achieve those. If this is a truly important bracket, I would reccommend against any type of heat treating since you are not certain of the type or quality of the material used.

 

[Mingon]

I would suggest oil quenching and then tempering to purple - this should allow it to retain some degree of flexibility without the risk of cracking - been a while since I made scribers and centre punches.