tratamente termice diagrame

7/27/2019 tratamente termice diagrame

http://slidepdf.com/reader/full/tratamente-termice-diagrame 1/8

Carbon steel and the hardening thereof, weren't sufficiently understood until well

into the last century. Before that time, and especially centuries before, the whole

process was wrapped in secrecy, myth, ritual and magic. All knowledge was empirical

and it was difficult, if not impossible, to know which parts of a forging ritual were

vital and which weren't. Did moon phase have a bearing? Were swords forged in thewinter necessarily better than those in the summer? Did the wood source for charcoal

really make much difference? Does speaking the correct words at the right moment

matter? What could you do if your anvil got hexed?

Chemical composition was uncertain, if not unknown. So the best smiths could do

was follow a forging ritual which had an acceptable rate of success. That parts of

forging rituals were silly, irrelevant and possibly counter-productive shouldn't

preclude the process of having a ritual in the first place. What I'm presenting in this

tutorial is a ritual of sorts. It's a way to understand, think about, organize information

and activity toward a forging outcome. I suppose what I'm saying is that knowledge is just about updating the ritual not replacing it.

Steel is a crystalline substance and as it's temperature changes, so too does it's

crystalline structure. That's what heat treatment does--it changes the crystalline

structure by way of time and temperature.

There are a lot of different structures in steel but only three need your attention:Austeni te, Ferr ite and Martensite. Austenite exists at high temperature and to the left

in the graphic below. Ferrite is to the right and Martensite at the bottom. We will use

the term, Ferrite, to include Pearlite and Cementite as Pearlite is a form of Ferrite.

Mostly, we' re using Ferr ite to mean any non-Austeni te/Marti nsite structur e. So you

only need to learn three steel words.

http://www.navaching.com/forge/heat.html






We're now ready to grapple with the central mystery of carbon steel--the Cooling

Path. Above are five Cooling Paths--A through D. Each line signifies a different rate

of cooling and thus different resultant steel structures. Steels with less than 0.3 %

carbon cannot be hardened effectively, while the maximum effect is obtained at about

0.76 % due to an increased tendency to retain Austenite in high carbon steels.

But first, let's back up and first look at some other diagrams. Following is a phasediagram of carbon and iron. Indicated is our area of interest--1/2 to 1 percent of

carbon and a temperature range of maybe 1800F down to room temperature.



The Iron Age is based on this diagram--it's also color coded for temperature.

Cast iron and steel exist because of the strange and wonderful chemical interactions

between carbon and iron.



Our area of interest, the blue line is 0.76% carbon.The thing to take away from this graphic is that as steel cools it experiences changes

in crystalline structure.



Once heated, how steel cools (the path it takes) will determine it's properties. Cooling

Paths are a graphic representation of the function between time and temperature. For

each steel there is a graph called a TTT (time, temperature, transformation) graph or

sometimes it's called a Isothermal Transformation Diagram. Let's call it TTT. Above

is a portion of a generalized TTT graph. The red line starting at the upper left denotes

a partial cooling path.

Two things to notice here: First, the time axis at the bottom is a log (as in logarithmic)

scale. TTT graphs always use a log time scale and it may take some getting used to.

Notice the time scale at the top of the graph, it's the same, time-wise as the scale at the

bottom. Now does a log scale make more sense? And finally, second, the cooling path

is demonstrating a Austenite to Ferrite transformation--which, during final heat

treating is the transformation we'll try and avoid.



OK, we've got a Cooling Path for knife hardening, the blue line. This is a fairly typical

TTT graph for simple carbon steel. As previously mentioned, every steel has it's own

graph and by reading the graph you can come to conclusions about how to best heat-treat the steel.

Here's the deal. When steel is heated above Critical the crystalline structure changes to

Austenite. Once steel is withdrawn from heat and cools to about 1350F, the graph

above comes into play. The goal is to get the temperature to fall fast enough to miss

the Nose and get into the Martensite range before the Time Limit. In this case, that's

less than a second (green dotted line). If you can get from Austenite to Martensite

without crossing the Nose you'll have a hard, strong steel. The structure in the graph

depicted by red lines is usually called an S Curve. We want to avoid it except at the

bottom--the Martinsite range. That's it! That's the whole thing--creating a cooling path, with quenchants, which avoids the S Curve and doesn't cause your blade to

strain and crack. You have less than a second--one Mississippi--to get the blade's

temperature under a thousand degrees. And to do it in a way that doesn't wreck the

steel through heat shock. Do that and you've won the game! That's what the Cooling

Path is, that's what it's about.







In the graphic above, there's a line called the Eutectoid Temperature, it's also called

the Lower Transformation Line. It's the same for all iron and steel and this

temperature, 1333F, is the temperature at which the clock starts in TTT graphs. When

the steel temperature falls below 1333F the Time axis becomes active.

Returning to our original graphic, there are two other cooling paths of interest:

Annealing and Normalization (A & B). Both of these cooling paths purposely cross the

S curve. Annealing is cooling very slowly, over a period of a day or so. Normalizing

is cooling a little faster, maybe 2 or 3 hours. At the bottom of the cooling paths is a

description of the resultant steel structure. It becomes apparent that the cooling path

chosen determines the structure and properties of the steel.



Line E is the Critical Cooling Path, the one that just touches the Nose. Anything to the

left of Line E will produce proper hardening. Line D is a generalized path for water

quenching, somewhere between Lines E and D would be the path for oil quenching.

Notice that every cooling path is just a graph of Temperature versus Time and that

Time is represented in log units. Getting these two things set in your mind will clear up most of the heat-treating mysteries.

Documents

tratamente termice diagrame