Using phase diagram and the following TTT diagram for a 0.5 wt% C steel alloy, d

Question

Using   phase   diagram and   the   following   TTT   diagram for   a   0.5  
wt%   C   steel   alloy,  

determine   the   final   microstructure   (in   percentage)   of   a   small  
specimen   that   has   been   subjected   to   the heat-treatment   processes   shown   in   the   graph.  
assume   that   the   specimen   begins   at   950°C   ,   and   that   it   has   been   held   at  
this   temperature   long   enough   to   have   achieved   a   complete   and   homogeneous  
austenitic   structure.  

Composition (at% C) 10 0 5 15 20 25 1600 1538°C 1493°C 1400 2500 1394°C 1200 1 147 2.14 4.30 y, Austenite 2000 1000 912°C + Fe3C 800 1500 727°C 0.76 0.022 Ferrite 600 +Fe 3 ementite (Fe3C) 1000 400 2 3 4 6.70 (Fe) Composition (wt% C)

Explanation / Answer

At .5 wt % C and 950°C, phase is present. And it has 100 wt% C, homogeneous austenite is formed.

For case 1, after cooling at 600°C  approximately 50% of the specimen has transformed to coarse pearlite. Upon cooling to 250°C and holding for 102 s 50% (out of which 50%) transforms to martensite.Hence the final microstructure consists of about 50 coarse pearlite and 25% martensite

For case 2, after cooling and holding at 640°C for 105s, approximately 50% of the specimen has transformed to coarse pearlite. Upon cooling to room temperature, the remaining 50% ( in which 90%) transforms to martensite. Hence, the final microstructure consists of about 50% coarse pearlite and 45% martensite.

For Case 3 , after cooling and holding at below 600°C for less than 104 s, approximately 50% of the specimen has transformed to coarse pearlite. Upon cooling to 100°C, the remaining 50% transforms to martensite. Hence, the final microstructure consists of about 50% coarse pearlite and 50% martensite.

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