- Joined
- Sep 9, 2003
- Messages
- 2,361
A bit of confusion in other threads indicates that we have some fresh faces eager for knowledge and I may have shortchanged some folks in my lecture at the Ashokan seminar. The same tired old TTT curve gets drawn at every heat treat lecture so I decided to dispense with it in mine, but there is a reason it is so often used, it is a very valuable visual aid in understanding some of the concepts and transformation products. So I decided to make up for it now and try to help clear things up with a little discussion about some charts (those with more experience, please cut some slack here as my intentions are to keep it basic for any folks who may have not discussed this stuff before, I would rather get the basic concepts across than debate the technical details) :
This is an I-T diagram for 1095. I-T stands for isothermal transformation, but I prefer to call it by its other name, a TTT curve. TTT stands for time-temperature-transformation and that is why I like the name because it describes it all more simply and completely.
Even though TTT curves are what many use to determine heat treatments for their steel, it is worth mentioning that it is based upon holding temperatures, and a more accurate resource for quenching would Continuous Cooling Diagrams but C-T diagrams seem to be a real @$$#% to get your hands on, so we use what we got. Each steel has its own distinct curve and will look different than this one for 1095 but I like this one because it so clear to read and explain.
An excellent resource for this information is an Atlas of Isothermal Transformation Diagrams do a book search for one and get it, you wont be sorry.
These diagrams consist of a column of numbers up the left side and a row of numbers along the bottom. The left column refers to the temperature of the steel and the bottom row indicates the time in seconds at that temperature. The first thing you should take note of is that in order to fit the chart on a page the time numbers are logarithmic and pile up very fast.
In the middle of the chart is this series of curved lines that start at the right hand side, come all the way over to the left hand side and then recurve around and down to the bottom. In the area above and to the left of this, which I have taken the liberty of shading pink is austenite. You heat steel to the critical temperature and you have transformed things on the inside to austenite.
Now if you chart a line the diagram that corresponds with your rate of cooling, i.e. how many seconds the steel is at a given temperature you can see what changes will happen to your austenite. The different lines in the main curve indicate the amount of ferrite (iron) and cementite (carbon) separate out of your austenite solution. To the right where the field is open is complete transformation of the austenite by diffusion into other products.
The most prominent feature of the curve is the tip that comes closest to the left side of the graph at around 1000F. this is what is often referred to as the pearlite nose. As you cool austenite in simple iron carbon alloys like 1095, it becomes very unstable around 1100-1000F. and wants to revert back to a BCC (body centered cubic) atomic stacking, this promotes diffusion of pearlite which is made up of separated bands of iron and carbon. The closer you are to 1000F, thus the quicker you cool it, the less the separation and the finer will be the pearlite. Slow cooling will take the austenite into the top of the curve ands allow wider separation for coarse pearlite, as you would get in a traditional anneal.
This graph is off from most since it shows more than ¾ of a second to beat the nose and 1095 is quicker that that. But that narrow stretch of austenite to the left at around 1000F is the reason we sorry about cooling 10XX series and W2 so fast; any pearlite made is martensite lost and overall hardness lost. But if you manage to miss it you will notice that the field then opens up again and you will still have austenite all the way down the line around 420F that says Ms. This is when martensite (indicated by green) will begin to form and the steel will actually harden. You will also see a line that says M50, this indicates 50% martensite formed. Since martensite doe not rely on diffusion it does not care about time. If you stop cooling at M50 no more martensite will form until cooling resumes regardless of how long you wait, but you could give the austenite time to stabilize that resist transformation so we want to keep going to a line that is missing on this graph, but is very important, called Mf or the martensite finish line.
This is an I-T diagram for 1095. I-T stands for isothermal transformation, but I prefer to call it by its other name, a TTT curve. TTT stands for time-temperature-transformation and that is why I like the name because it describes it all more simply and completely.
Even though TTT curves are what many use to determine heat treatments for their steel, it is worth mentioning that it is based upon holding temperatures, and a more accurate resource for quenching would Continuous Cooling Diagrams but C-T diagrams seem to be a real @$$#% to get your hands on, so we use what we got. Each steel has its own distinct curve and will look different than this one for 1095 but I like this one because it so clear to read and explain.
An excellent resource for this information is an Atlas of Isothermal Transformation Diagrams do a book search for one and get it, you wont be sorry.
These diagrams consist of a column of numbers up the left side and a row of numbers along the bottom. The left column refers to the temperature of the steel and the bottom row indicates the time in seconds at that temperature. The first thing you should take note of is that in order to fit the chart on a page the time numbers are logarithmic and pile up very fast.
In the middle of the chart is this series of curved lines that start at the right hand side, come all the way over to the left hand side and then recurve around and down to the bottom. In the area above and to the left of this, which I have taken the liberty of shading pink is austenite. You heat steel to the critical temperature and you have transformed things on the inside to austenite.
Now if you chart a line the diagram that corresponds with your rate of cooling, i.e. how many seconds the steel is at a given temperature you can see what changes will happen to your austenite. The different lines in the main curve indicate the amount of ferrite (iron) and cementite (carbon) separate out of your austenite solution. To the right where the field is open is complete transformation of the austenite by diffusion into other products.
The most prominent feature of the curve is the tip that comes closest to the left side of the graph at around 1000F. this is what is often referred to as the pearlite nose. As you cool austenite in simple iron carbon alloys like 1095, it becomes very unstable around 1100-1000F. and wants to revert back to a BCC (body centered cubic) atomic stacking, this promotes diffusion of pearlite which is made up of separated bands of iron and carbon. The closer you are to 1000F, thus the quicker you cool it, the less the separation and the finer will be the pearlite. Slow cooling will take the austenite into the top of the curve ands allow wider separation for coarse pearlite, as you would get in a traditional anneal.
This graph is off from most since it shows more than ¾ of a second to beat the nose and 1095 is quicker that that. But that narrow stretch of austenite to the left at around 1000F is the reason we sorry about cooling 10XX series and W2 so fast; any pearlite made is martensite lost and overall hardness lost. But if you manage to miss it you will notice that the field then opens up again and you will still have austenite all the way down the line around 420F that says Ms. This is when martensite (indicated by green) will begin to form and the steel will actually harden. You will also see a line that says M50, this indicates 50% martensite formed. Since martensite doe not rely on diffusion it does not care about time. If you stop cooling at M50 no more martensite will form until cooling resumes regardless of how long you wait, but you could give the austenite time to stabilize that resist transformation so we want to keep going to a line that is missing on this graph, but is very important, called Mf or the martensite finish line.
.
. In this situation you get at blade that hardens and forms martensite but lacks the full strength of a blade that didn't have that fine pearlite contaminating those beautiful fields of martensite.
, if folks would like to talk about where bainite rests on these charts I would be more than happy to cover that, I just want to make it perfectly clear that there are no magic bullets and if you can't first figure out the basics of a simple quench bainite is not going to save your blades, or even equal them- heck fine pearlite and bainite would REALLY suck!
