Intro to heat treatment of steel (hardening and tempering)----make money online

hey everyone I wanted to talk about heat

treating steel a little bit in this

video so if you search for this topic on

the internet you'll find there's a lot

of theoretical and background

information and then also a lot of

practical information without much

explanation of why the things are done

so I'm hoping to bridge the gap a little

bit in this video when we talk about

heat treating steel we're mostly

interested in increasing the materials

strength by exposing it to different

temperatures in sort of a prescribed way

but first a note about what strength

actually is there's a few different

quantities to keep in mind here the main

ones being stiffness and strength so

here's a graph here and it says strain

on the bottom and stress on the vertical

axis and what we're showing here is how

hard we're pulling on the material here

the stress is the force divided by the

area and the strain is how far the

material is moved so this is the the

Delta how much the material has actually

elongated or contracted divided by its

total length so this graph takes into

account whatever size material you might

have so for steels and most ductile

materials when we start yanking on the

material that will deform a little bit

and if we don't yank too hard we can let

go and the material wills elastically

returned back to its original shape so

if we take this steel bar here and I

flex it a little bit it snaps back to

its straight shape with no problem and

so what's happening here on the graph as

I'm taking it up to about this point and

letting it back down instead if I take

this coat hanger and stretch it a little

bit now when I let go the curve actually

stays in the coat hanger so on the graph

what's happened is I've stressed it so

the stress has gone up and eventually

we've gone so far that it's gone past

its yield point and now we're in a

region called the plastic region so we

put a plastic deformation in this coat

hanger by bending it like this what's

important to note is that we can have a

very stiff material that's not very

strong or we could have a very

material that's not very stiff the two

quantities are actually not dependent on

each other so let's pretend this line

represented steel aluminum on this graph

might look something more like this and

there's a lot to talk about with this

graph but I'm just going to do sort of a

general kind of overview but

hypothetically let's say we had an

aluminum alloy that looked like this its

strength could actually be equal to the

steel now it's not going to be as stiff

because the stiffness is actually a

material property and we can't easily

change the stiffness through heat

treatment we would probably have to use

a different material if we needed a

different stiffness but if we had this

this great aluminum alloy we could say

well it's just as strong as steel

because the stress-strain point where

the material gives out and doesn't

spring back anymore is actually at the

same stress however the material has

moved more in that in that loading

because it's not as stiff the x over

here indicates the point of breakage and

so if we keep pulling this material

farther and farther eventually we're

into this plastic region where we can

change the shape of the material without

putting in all that much additional

stress and then eventually the material

gives out and it breaks so when we heat

treat steel we're actually staying on

this same slope here because we can't

change the stiffness of it but what we

can do is move the yield point up

through heat treatment so let's just say

we had a material that looked like this

now this material is just as stiff as

the original one it's still steel but it

has a much higher yield point so we say

that it's stronger one of the downsides

though is that this line doesn't go over

into the plastic region as far so what

happens with this material is we load it

and it's still elastic it's still

elastic and then there's a tiny amount

of plastic deformation but it breaks

right away this is characteristic of

extremely hard steels and it's also

characteristic of

brutal materials like glass so if we

took a piece of glass just like a window

and flexed it we could let go and it

would snap back to its you know flat

shape but eventually if we flexed it too

far it would just break all of a sudden

without really much warning and we can't

take a piece of glass and bend it and

then let go and expect it to retain that

shape so glass is a brittle material

because it doesn't have this plastic

region you'll also note that it says

maximum tensile strength here instead of

here so what's the deal with that like

why why do we actually count this as the

material strength the reason is that

let's say we were building like an

airplane part or something that you

wanted to support a load in an important

way if the material is in this plastic

region the part has deformed enough

where it might be causing problems so

let's say this or an airplane landing

gear if you're up in this region over

here the landing gear is not going to be

the same shape anymore so it's true that

you might get a little bit of additional

strength out of the material but you

really can't count on that part being

sound anymore so for engineering this is

the point where we say the material has

failed it's yielded so the weight a

hardened steel is to heat it up until

it's glowing red and then very quickly

reduce the temperature by plunging it in

water or oil typically and what happens

this is the crystalline structure inside

the steel changes so it's very different

from letting the steel cool down slowly

and that rapid cooling is actually what

causes us to make the graph that looks

like this instead of like this but we

have a problem

I just said that this is behavior is

like glass where you load the material

and then suddenly it breaks really

without much warning and we don't really

like that behavior in very many

materials and another problem is that

the really freshly hardened steel like

if you heat the steel up dump it in

water take it out it's so incredibly

hard and brittle that you can break it

very easily even with your hands at the

steel if the piece is small enough so

typically all hardening operations are

followed by a tempering operation and

the tempering operation actually

lowers the strength of the material but

it increases the toughness so there's a

very distinct trade-off there and the

tempering process can be tailored to

give us any sort of a a strength versus

toughness trade-off so for example let's

say we tempered it so that we had a

material that looked like this instead

of going all the way to full hardness we

could temper the material and maybe we'd

end up with something like that so now

we've got all this extra room here in

the plastic region and it's not quite as

strong as the as the full hard but the

tempered steel is much much more easy to

use in an engineering application

because it's not like glass it's more

like a normal metal so to test this out

I bought some w1 steel this is an eighth

of an inch in diameter and w1 means

water hardening so this steel is meant

to be heated up and then tossed in water

to quench it to cool it down and harden

it and then you can temper it to give

you any sort of a curve a desired

toughness and strength and to test it I

came up with this little test jig here

so that I could load the samples in

bending and carefully apply more load by

hanging a bucket from it and I filled

the bucket up with sand and bits of

metal to see how much load I could hold

with each with each piece of steel with

each sample and what I did as I started

off these samples are untreated so this

is probably not fully annealed when I

talked about cooling the steel down and

you have a couple options you could heat

it up to red-hot and then cool it down

really really slowly by like putting it

in an oven or in an insulator and that

will give you full anneal that's the

softest you can get if you heat the

steel up and just let it cool down in

air that's called normalized and so even

that will give you some amount of

hardening over the full and yield state

and I don't know how this is sold for

mcmaster this is probably normalized so

they heated this up and then let it cool

down at ambient temperature I'm guessing

but it's it's relatively soft and so I

was able to bend it by

this just by applying 16 kilograms so

note that we actually didn't get to

breakage on this piece what would happen

to us since we went up the graph and

then stopped somewhere around here so it

was plastic and eventually just slipped

out at the fixture if we kept bending it

eventually we get to fracture and it

would break so next I tested one of

these full hard pieces and this one I

heated up to you know cherry red and

then dropped it in water and took it out

and put it in the loading jig and this

one only held six kilograms and also as

you can see there's no bending at the

fracture so that we have this sort of a

situation where it elastically deformed

you can see it bending a little bit when

we load it and then suddenly it

fractures and snaps back there's very

little if any plastic deformation at the

breakage point now you might be saying

well this only held six kilograms and

the soft one held you know sixteen point

two kilograms you know what's the deal

with that I thought we were supposed to

be getting a lot more out of this and

the answer is that point loading is a

very complex thing and so if we have a

bar like this with a steel cable loading

it like this right at the point where

the steel cable is touching it there

could be an additional stress caused by

this loading scheme this is also the

reason that glass is not considered a

structural material because you can't

really clamp a piece of glass without

introducing a lot of local stresses that

would break it so you can really think

of super hard steel like this as a piece

of glass where it's very um it's very

touchy and so small small amounts of of

local stress will cause it to fat to

fracture which is why it's basically

never used so now we've covered the

extreme ends of this spectrum we've gone

from normalized or very soft to full

hard which is almost unusable because

it's just so brittle so to temper the

steel what we do is we heat it up a

little bit and then let it cool down

slowly and what happens here is we give

up some of this hardness because we're

letting

that crystalline structure changed by

heating it up a little bit and if we

heat it up to a very specific

temperature we can control how much

strength were actually trading for

toughness very conveniently steel will

change color in air based on how how

high we heat it up and the color change

comes from an oxide layer that's forming

on the steel and it's interfering with

light and we can see what color or what

temperature the steel is based on what

color we see off that because the oxide

layer is forming an optical interference

pattern there so as we heat it up we'll

see a straw yellow color and then kind

of an orange color and then brown and

purple and then blue and then light blue

and the hotter we heated up the more

strength we give up in return for

getting more toughness and so there's

quite a bit of research and fine-tuning

to be done here but for home shop

hardening and tempering it's actually

quite ineffective and decent means of

setting up tooling of course if you have

access to a kiln it also makes a lot

more sense to just set the temperature

that you want to temper your steel to

and put it in the kiln and leave it for

the prescribed time which is actually

like an hour - usually and then take it

out of the kiln and let it cool down so

interestingly enough I started with the

the 300 degree Celsius piece that I kiln

tempered and this piece held about 55

kilograms in fact my bucket became

overloaded I put all of the sand in

there and that it was holding fine and

then I put all kinds of random scrap

bits of metal in there and it was still

holding and I had to push down on it

with my arms so I I completely didn't

expect how strong I could actually make

this steel compared to the full hard and

the normalized state the results for the

other tempered pieces were pretty

similar except for this one this one i

tempered only two straw yellow which is

less tempering which means more brittle

and stronger so I stopped recording how

much weight these things held because my

system was woefully in

but what was interesting is that this

one broke in a brittle sort of a

fracture whereas these other tempered

pieces that were tempered to higher

temperatures did not break like that

these yielded another really handy trick

is to use the file to determine how hard

the material is that we're working with

so these normalized pieces if you just

lightly run a file along and I'm hardly

pressing down on the file I'm just

pushing it along very gently you can see

that it sort of grabs and after you do

this a few times so we get a very good

feel for what different steals at behave

like but this is very grabby and if we

take one of the full hard pieces the the

file just absolutely glides along like

it's on glass it's not even biting into

the material at all and that's because

this is actually harder than the file so

when we drag the file teeth across there

and the teeth don't dig into the metal

at all whereas with a softer one the

file teeth actually bite in and that's

what's causing the drag also I should

point out that hardness is related to

strength so when we say a material is

really hard what we mean is it's

actually very strong and files are quite

hard it's actually one of the hardest

tools that you'll find in a common

machine shop and the fact that we can

run it across this and this is actually

even harder than the file seemed would

indicate that this is something that

this is a hardness that you generally

not encounter here's a graph that shows

what's actually happening when we cool

down a piece of steel so this is the

first part of the process the hardening

part of the process and we've got

temperature on the y-axis and time and

the x-axis and we're starting off at

about 800 degrees C which is the cherry

red color and what we want to do is get

down into this phase down here we want

to get below this line without going

through this part of the graph so this

whole deal with cooling it down quickly

is because we need to get down to this

part of the graph without interfering

with this area this graph is called the

time temperature transformation graph

and we talked about going past the

those of the TTT graph like this and so

there's this critical cooling rate where

we have to get down into here around so

we don't get this hardening effect so if

we take too long if we if we spend ten

seconds cooling down from 800 we're

gonna end up in this region and that

means that we'll get some hardness so

there'll be some hardening effect but it

won't be anywhere near getting down to

here and if you're curious the M is

martensite which is the crystalline

structure that gives us that really high

hardness in steel when we temper the

steel we're actually starting out down

here and we take it up into this region

so we're basically giving up some of

this really hard crystalline structure

and gaining some of this less hard but

tougher structure and there's a lot of

terminology involved here that probably

won't help you understand it but if you

go searching for this stuff you'll find

quite a depth of information so you

might be wondering what can I do this

trick with a coat hanger if I heat it up

and then cool it down and do this sort

of transformation no the answer is nope

you need to have steel that is hardened

abaut and not all steels are hardened

Abul and the thing that determines

whether they're hardened Abul or not is

the carbon content and to a lesser

extent the other alloying ingredients so

this w 1 water hardening steel that I've

been using today has a carbon content

fairly close to 1 1 % so this graph

shows us temperature on the y axis and

carbon content as a percent on the x

axis and most tool steels are pretty

close to about 1% and the reason for

that is that it makes this crystalline

structure that's very beneficial for

having a very hard structure if we have

tons and tons of carbon what we actually

have is cast iron and if we have very

little carbon we have cheap steel

basically ok well I hope that was

helpful

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