Part 4 Steel 1 What is martensite in steel how does it form

Part 4. Steel 1. What is martensite in steel, how does it form, what is the impact on ductility and tensile strength, how is the material processed to improve properties with martensitic phases? 2. Problem 13-18. 3, using figure 12-14, draw the microstructure for 3 wt% C at 1250 C, 1000C and 700C. Label phases and provide the compositions and volume fractions Extra Credit: (pick one) 1. What did Dante write? 2. How has Led Zeppelin influenced music? 3. What novel did Emily Bronte write? 4. What is a slushi? 5. Draw a small picture of a german short haired pointer

Solution

1. Martensite is a very hard form of steel that is formed due to diffusionless transformation. For the formation to occur, the cooling rate of austenite needs to be so fast that the carbon atoms do not have time to diffuse out of the crytal structure in large enough quantities to form cementite.

The strength of the Martensite is mainly determined by its carbon content. With the increase in martensite volume fraction, the carbon content of the martensite phase decreases and the tensile strength increases. Thus, the tensile strength is directly proportional to the volume fraction in martensite.

Carbon content of the martensite significantly affects the ductility of martensite phase. Increasing martensite volume fraction decreases carbon content of the martensite which in turn leads to increasing ductility of martensite phase. Increasing ductility of the martensite constituent increases overall ductility of the steel.

Dual-phase steels are characterized by a microstructure consisting of martensite in a polygonal ferrite matrix. The hard martensite particles provide substantial strengthening. This microstucture acts as a particle-reinforced composite. The properties of the martensite can be adjusted by controlling the steel chemistry and intercritical annealing temperature in the ferrite plus austenite region just prior to rapid quenching. The ductility, fracture toughness and other properties can satisfactorily be optimized by developing microstructures with fine distribution of ferrite and martensite in DP steel either by introducing new alloy design or adopting an appropriate heat treatment procedure.