Before we examine further using heat treatments to harden and temper steel let s look at some numbers Yield Stress values in MPa for di erent treatments 1020 Steel Fe with 0 20 wt C 4340 Steel Fe with 0 40 wt C 1 8 wt Ni 0 80 wt Cr 0 25 wt Mo 0 70 wt Mn 1 A CCT diagram showing the transiOon from pearlite to martensite with increasing cooling rate 2 A large piece of material will experience di erent cooling rates and therefore have di erent microstructures from the center of the piece to the surface 3 Microstructure variaOon in di erent sized samples We know that the cooling rate has a tremendous in uence on the resulOng microstructure and mechanical properOes 4 Alloy composiOon is also very important This shows how addiOons of alloying elements like Cr and Mo can suppress pearlite formaOon allowing slower cooling to form microstructures like bainite and martensite 1080 Steel 4340 Steel 5 The Hardenability of an alloy refers to the ability of an alloy to form harder structures like martensite in preference to pearlite This is quanO ed using the Jominy end quench test 6 Typical Jominy Test Result in red As hardenable as it gets Reasonable Hardenability Low hardenability 7 8 Same carbon content di erent addiOves 4340 Ni Cr Mo 4140 Cr MO 8640 Ni Cr Mo 5140 Cr Same base alloy with varying amounts of carbon 9 10 11
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