13: Isothermal Transformation Diagrams

What is martempering?

A cooling process designed to relieve stress and reduce the chance of cracking and distortion in martensite steel. The steel is rapidly quenched like normal to achieve martensite steel except its temperature is then held a few seconds longer just above the martensite temperature, being careful not to allow enough time to reach another isothermal region, and then rapidly quenched again. The few extra seconds of soaking just above the martensite temperature reduces a little more stress and distortion.

What is an isothermal transformation diagram?

A diagram that identifies the different steel structures that are transformed from austenite over a period of cooling time. This diagram is a very useful tool.

Bainite is named after who?

E.C. Baine, a metallurgist who conducted research on the transformation of austenite.

Why is the cooling time on an I-T chart graduated with logarithmic values?

because in quenching, most of the transformation from austenite takes place in the first 10 seconds. Therefore, the amount of time represented by the first two intervals on the diagram is much smaller than the incremental times farther to the right on the x-axis. After the first few seconds of quenching, less transformation will take place and at lower rates of time.

Bainite presents an advantage over other transformation structures because why?

because it combines fairly high strength with fairly good ductility.

In the iso-thermal diagram, in order for the steel to transform completely to austenite, it must be heated above what?

both the lower and upper transformation transfer lines.

I-T diagrams are used to determine how the steel structure is effected by what?

different cooling procedures.

If a certain type of steel is heated above its upper transfer temperature until it becomes austenite and is then cooled slowly, it becomes _____________, ____________, or _____________.

ferrite/peralite/cementite

With industrial I-T diagrams, an exact ____________ value cannot be determined, but a hardness ____________ can be established by identifying the minimum and maximum hardness values.

hardness/range

What are the limitations of the I-T diagram?

it does not plot the percentage of carbon present in the steel. This also makes it necessary for a new I-T diagram to be used every time the percentage of carbon changes. In fact, a different type of I-T diagram must be plotted for every type of steel.

If a steel is heated above its upper transfer temperature and is quenched rapidly, it becomes ________________.

martensite

Bainite is superior to ______________ in ductility and toughness, but it is not as hard or strong. It has less ductility than _____________, but is stronger and harder.

martensite/fine pearlite

What is isothermal conditions?

meaning equal or constant temperature.

As the temperature-time line passes through the isothermal regions, the structure will not ___________ any percentage of structure it attains even as the temp-time line changes direction to go back the opposite way it came, but will add what to its composition?

reverse/it will add more of a different type of structure if the temp-time line passes through another isothermal region.

What is one important thing missing from an iron-phase diagram? What can't we predict well as a result?

the amount of time involved in the cooling process. The diagram does not differentiate between slow and rapid cooling rates in terms of time, so there is no way to know or predict the cooling process or the type of steel structure that is produced.

The minimum possible hardness value of a structure corresponds to what? What does the maximum hardness value of a structure correspond to?

the point where the temp-time line first crosses the left C curve on the diagram/the point where the temp-time line crosses the right C curve.

I-T diagrams are also called what?

time-temperature-transformation (T-T-T) diagrams or C curves; C-curves because of the shapes the curve often make.

Describe the iso-thermal diagram layout

1. Temperature is plotted against time in Fahrenheit and Celsius; temperature on the left with time along the bottom. 2. There are two horizontal lines near the top of the diagram. The top line represents the upper transformation temperature and the lower line represents the lower transformation temperature. 3. The cooling time along the bottom x-axes is measured in seconds of logarithmic values meaning that time is measured in increasingly larger (or exponential) intervals along the x-axis. 4. There is another time-line that follows the path of the temperature of the steel after quenching begins and is called the temperature-time line. The line shows the progress of each quenching stage.

Describe the parts of a plotted I-T diagram

1. There are two c-curves; one on the left and one on the right. When the temperature-time line is to the left of both C curves, the steel is 100% austenite and has not begun its transformation. This region is called the austenitic region. 2. The region in between the left C curve and the right C curve is called the region of transformation and represents the structural changes that take place and where austenite is converted to some other structure, such as pearlite or martensite. 3. The dotted line between the C curves is the 50% transformation line. When a temperature-time line reaches this point, 50% of the austenite has been transformed to another structure and so only 50 % of austenite remains. 4. The isothermal regions: The temperature-time line crosses one or more regions at some point called isothermal regions. There are four different iso-thermal regions and is where the steels structure is held at a constant. a)Coarse pearlite (large grain) region: used to describe not just coarse pearlite, but also coarse ferrite, coarse cementite, or any combination of these structures. Again, coarse pearlite does not literally refer to 100% pearlite. It can be used to describe any combos of the other structures. Coarse pearlite is produced when steel is cooled extremely slowly. b)Fine pearlite (fine grain) region: represents steel that is transformed into fine pearlite, fine ferrite, fine cementite, or any combination of these types of steel and fine pearlite. c)Martensitic region: the lowest region of transformation on the diagram. If the temp-time line crosses into the martensitic region, martensite will be formed. This type of steel is formed when the material is quenched very rapidly. The full transformation to martensite does not occur until the fast-cooled steel reaches approximately 300 F. (between 200 F and 500 F depending on the type of alloy). d)Bainite region: between the martensitic region and the fine pearlite region.

Bainite typically reaches hardnesses of ________ to ________ Rc.

50/55

When 1060 steel is plotted on an I-T diagram, what structure cannot be obtained and why?

Martensite/because no space exists between the nose of the curve and the y-axis of the diagram. Thus, no matter how fast the quenching speed is, 100 % martensite cannot be obtained with 1060 steel.

How are I-T diagrams made?

Metallurgical and steel corporations that produce I-T diagrams obtain their data by conducting many series of quenching tests on steel. In order to plot an I-T diagram, many small, identical samples (perhaps 100 or more) are heated and quenched. A typical sample is about 1" in diameter and 1/16" thick. Each sample is heated above the upper transformation temperature and quenched using a specific program. The cooling methods differ slightly for each sample. Isothermal conditions are established by holding temperatures during quenching. The samples may be plunged into hot molten lead that is maintained at a constant temperature, such as 500 F, 800 F, or 1000 F. One by one, the samples are heated and removed from the furnace, quenched, and then tested for hardness. The data these samples generate is used to plot the actual shapes of the C curves. This data is also used to determine the hardness values for the hardness scale on the diagram.

What are industrial I-T diagrams?

Similar to the basic type of I-T diagram except a scale along the right side of the diagram is added to show hardness values in Rockwell.