I am working on high carbon steel and martensite transformation happen from cooling rate 3 c/s. . By increasing cooling rate from 3 to 5 °CS-1 in one side there is less time to growth grain which means that grains size decrease and could decrease martensite start temperature. On the other side the phase fraction of other transformed phase (perlite +Bainite) decrease significantly by increasing cooling rate and increase possibility of martensitic transformation consequently from 3 to 5 C/s consequently MS increase from 136 to 158 and afterwards decrease in cooling rate 10 C/s to 148 where no pearlite and bainite exist. After increasing cooling rate to 160 C/s the martensite start temperature increase significantly again (from 150 to 186 C). what is scientific reason for this significant increase in last step?
What is the exact chemical composition of your steel, how exactly do you measure the Ms temperature, what is the material history beginning from the ingot?
Generally speaking, change in cooling rate shifts the CCT lines either towards the right or left of the diagram. In case of faster cooling, martensite fraction increases and other phase fraction decreases and in one point you will get full martensitic structure as you mentioned. However, it very unlikely that Ms temp. is also varying with the cooling rate. I observed that the Ms temp. do not follow any particular pattern and it is arbitrarily changing. One probable reason may be the constitutional super-cooling of the alloy system. It is also very important to identify the alloying element segregation pattern which may give some light on the phenomenon. Partial alloy segregation can have an impact, if measurable, on the Ms.
Thanks for Grzegroz
The sample prepared from forged ball which I annealed them for 1 hour at 950 C to have fully Pearlie phase . The main component in my composition is 1% C,1% Mn, 0.63% Cr, 0.23 Si. I used dilatoemtry curve to identify Ms temperature and I am sure about dilatoemtry result because I repeated in 2 equipment and I used hollow samples
The most important was austenitising temperature. Here you have 0.63 Cr at high cooling rates it may remain in martensite but at slower cooling rates it may precipitate in form of carbides and decrease its amount in martix what leads to change in Ms temperature. This is just one possible way.
I am not sure that it was related to Cr because I could not see in Cr precipitation and soaking temperature and holding time would be enough high to solve element before starting quenching. I remember that for another experiment I had EDS analysis and there is no significant different for CR in different area. I suppose it should be related to affectation of cooling rate on hardenability as well as existence of other phase . For cooling rate 3 and 5 we have bainite pearlite and martensite transformation and if other transformation before martensite happen it could change martensite transformation start temperature as well.
- Badges
- Science topic
- Similar topics
- Philosophy
- Philosophy Of Science
- Reasoning