TTT Diagram : Introduction

TTT diagram 

TTT stands for Time-Temperature-Transformation . In modern engineering and technology everywhere metals are used mainly steel , but direct use of steel were in past years but now it's exchanged from steel to alloy steel. But now a days scientist think about lowering the product weight and increasing the strength by mixing alloys in steel.

        As metallic products and parts pass from different mechanical process like extrusion,pressing,forging,rolling, cutting, straightening forming etc, they gets stressed and their microstructure changes ,to regain it's microstructure,change and increase strength as well as lowering the weight parts or products are heated and process is known as Heat treatment process. 

आसान भाषा में कहें तो लोगों को  हर सामान चाहे वो छोटा हो या बड़ा उसमें reliability, आसानी, मजबूती और हल्की से हल्की चाहिए। इसके लिए वैज्ञानिकों ने एक तरीका खोजा कि प्योर लोहे,स्टील में कुछ अलग धातु या केमिकल डाल दिया जाए जिससे वह सामान हल्का, टिकाऊ व मजबूत बन जाय । वह तरीका था ,Heat-treatment जिसे सही ढंग से करने के लिए यह TTT डायग्राम बनाया गया।

                                 To do the Heat treatment process scientist made a procedure and demonstrated the TTT diagram . TTT diagram is also known as Isothermal transformation diagram . It is used to form/design a cooling process to get a desired microstructure. It is a plot of temperature-time for a steel alloy of definite composition. 

 Eutectoid steel have  0.76% carbon.

In Heat treatment process metal is heat-treated to austenitic region, because it is the region where we can diffuse high carbon percentage (upto 2%)  into metal 

TTT diagram is also known as S-curve, C-curve isothermal diagram and Bain's curve. Diagram shows different structures and different hardness on the basis of time and temperature. The diagram is useful in understanding the transformation of an alloy steel that is cooled isothermally.

Here in this diagram green lines showing start of transformation and red line end of transformation . Dotted lines are showing the interval between these two curves indicating partial decomposition of austenite to ferrite and cementite .

Diagram shows if we heat any metal piece to 750℃ or above then it goes to stable austenitic region ( 727℃ is eutectoid temperature and stable austenitic region starts from here ) then on cooling , between 540℃ to 727℃ and right of C-curve it is pearlite region with pearlitic structure. Between 215℃ to 540℃ at the right of C-curve it is bainite region with bainitic structure. Left of C-curve and from 215℃ - 727℃ it is unstable austenite region . From 215℃ to above transformation starts but lower the 215℃ martensitic structure formation starts and complete martensite structure forms at 100℃. At temperature below eutectoid temperature , austenite decompose into prakriti structure, at 600℃ Sorbite is formed and at 500℃-550℃ Troosite is formed. At lowering the temperature from 500℃ to 220℃ acicular troosite or bainite is formed. 

        

           Pearlitic region I divided into 3 parts i.e coarse pearlite , fine pearlite and very fine pearlite. like this Bainitic region also divided into 2 parts i.e Upper or Feathery Bainitic region and Lower or Acicular Bainitic region.

● As metal structure gets denser it becomes harder and brittle like from diagram at 690℃ coarse pearlite have 15 hrc hardness on Rockwell C scale.

➡ Coming to down fine pearlite               have  denser structure than                 coarse pearlite with hardness of         30hrc . 

➡ Very fine pearlite have 40hrc.

➡ Upper or feathery bainite have             40hrc.

➡ Lower or Acicular bainite have 60       hrc.

➡ Likely on going down, martensitic       structure have higher hardness of       64hrc.

Factors which affect TTT diagram :

1. Composition of steel ( carbon            %age and alloying element %age)

2. Grain size of austenite .

3. Alloying elements like Cr,Ni,Mo            and W can shift the nose of                  Austenite to pearlite                              transformation , and  also                    formation of a separate Bainite          nose.

Some important points : 

● As the transformation                           temperature    decreases,                      Hardness    of structure increases.

● Hardness is inversely proportional      to Temperature.

● Austenite have FCC structure             while     Martensite have BCC.

● If we take a block of eutectoid              steel and heat to 750℃ (austenite      stable region) & suddenly cooled        in water or quenched within                  seconds,  it becomes martensite        which called Displasive Phase            Transformation .

           In this process structure                changed quickly from FCC to BCC      called Rapid Phase                                Transformation.

● Phase transformation (FCC to              BCC) occurs only in Austenite, i.e        only Austenite changes to                    martensite not Bainite and                    Pearlite.

● After 100% Pearlite / Bainite, on          increasing time at that                          temperature, it remains                        Pearlite/Bainite.

● To change Austenite into Pearlite        or Bainite , it includes Diffusion            process. Yo change Austenite into      Martensite, it includes                          Displacement process, it is                  diffusionless .