Question

Problem 4 The TTT (isothermal transformation) diagram for an iron-carbon alloy with eutectoid 0.78 wt. % C composition is shown on the next page. Describe the microstructure present in a thin strip of 1080 steel (0.80 wt. % C, approximately eutectoid composition) after each step of the following treatments. Assume that the steel is initially 100 % in the austenite phase due to prior homoge- nization by holding at 871 °C for 1 h. (a) Quench in lead at 704 °C; hold 20 minutes Then, quench in lead at 482 °C; hold 1 second Then water quench (b) Quench in lead at 704 °C; hold 6 hours Then water quench (c) Quench in molten salt at 260 °C; hold 1 minute Then air cool (d) Quench in molten salt at 315 °C; hold 1 hour Then air cool (e) Quench in molten salt at 427 °C; hold 1/2 hour Heat rapidly to 593 °C; hold 1 hour Water quench (f) Quench in ice water Then anneal at 500 °C for 5 hours

Answer 1

871 © Fig 1 , see curve @ we can see that when quench to 104°C and hold for 20 min, no phase transformation occur. Agaun quenching at 482°c and holding for I see, no phase transformation occur. 7040 Microstructure - 100% Martensite Acid 315ch 266 © foom fig 2, curve , we can see that when quench to 704°c and hold for 6 hrs, nearly 40% of auestenite convert to fine peciolite and on water quench, semaining auestenite Convert to martensite. 60 120 36003 Microstructure : 40% fine pear lite slich Fige 1 + 704 ct 60% Martensite ② from figure I, curse ©, we can see that on quenching at 260°c for 60 see, no phase transformation occur. - Microstructure : 100-2 Martensite @ From fig 1, curve O, on quenching at 315c for I hour, nearly 20% auestenite convert to beanite & ou further cooling to Martensite Microstructure! 40% Boinite + 21600's Fig 2 60% Martensite