Iron and steel making Flashcards
What does cementite stabilise to?
Alpha and graphite
What crystal structure is austenite?
Face centre cubic
What crystal structure is martensite?
Body centre tetragonal
How is martensite formed?
Quenching prevents nucleation/ growth by diffusional transformations; produces phase in which atomic moements < interatomic spacing; forms super-saturated form of C in alpha ferrite; martensite grows in laths/ plates
What are the mechanical properties of martensite?
High TS but brittle; tempering to ~600 Celsius allows some martensite to be relieved, and carbides are precipitated
What are isothermal transformation diagrams?
Show to what extent transformations occur if a steel is held at one temperature; shows what happens with unlimited time, when all transformations are allowed to process to completion
What is a TTT diagram?
Time, temperature, transformation
What does the nose show on an isothermal transformation diagram?
Shows period of time and temperature of pearlite forming
How is a TTT diagram determined?
By study of quenched-in microstructures, which reveal extent of the transformation
What is CCT?
Continuous cooling transformation
How is a CCT diagram determined?
From study of quenched-in microstructures
What is annealing?
Leaving object in furnace after heating is finished and furnace is switched off; very slow cooling
What is normalising?
Cooled in air after removing from furnace; cooled by convection
Why is oil quenching slower than water quenching?
Oil is very viscous, so not very effective at convection
What is air quenching?
Forced air blast
What does the critical cooling rate do?
Produces 100% martensite
What happens at the martensite start?
Any pearlite remaining is transformed to martensite
Is tit possible to have austenite and martensite at RTP?
Yes, small amounts of austenite might be present
What is spheroidisation?
Steel held below A3 transformation temperature; Fe3C particles in pearlite will gradually form spheres, giving increased strength and ductility
What are the implications of a greater cooling rate?
Greater risk of failure due to residual stress or excessive distortion (thermal strain); residual stresses arise from the thermal stresses induced by differential contraction within a part
What alloying elements push nose to right, and which pushes to the left?
Mn and most TMs push nose to right, allowing some austenite to persist down to RTP; Co is abnormality and pushes to the left
Where was ferrous metallurgy originated?
40-50,000 years ago: sea levels dropped; earthquake sealed off connection between Mediterranean and Black Seas. 20,000 years ago: glacial period ends; glaciers melt; sea levels rise; Black Sea remains same level. 5,600 years ago: another earthquake dislodges what was blocking passage; forms waterfall 120m tall; Mediterranean empties into Black Sea; took 50 years for seas to equilibrate; displaces people along River Danube; form Vinca culture at the Iron Gates; were protoliterate and invented ferrous metallurgy; carried knowledge up Danube into Western Europe
What was likely the original method of extraction of ores?
Digging out of outcrops; open-cast mining; e.g. where a valley was carved out at end of last Ice Age, exposing intersected mineral bands
What other ores contained useful amounts of iron ore?
Cu or Pb ores
