Hardenability Concepts

The  goal  of  heat  treatment  of  steel  is  very  often  to  attain  a  satisfactory  hardness.  The  important  microstructural  phase  is then  normally  martensite,  which  is  the  hardest  constituent  in  low-alloy  steels.  The  hardness  of  martensite  is  primarily dependent on its carbon content as is shown in Fig. 13.

If the microstructure is not fully martensitic, its hardness is lower. In  practical  heat  treatment,  it  is  important  to  achieve  full  hardness  to  a  certain  minimum  depth  after  cooling,  that  is,  to obtain  a  fully  martensitic  microstructure  to  a  certain  minimum  depth,  which  also  represents  a  critical  cooling  rate.  If  a given steel does not permit a martensitic structure to beformed to this depth, one has to choose another steel with a higher hardenability  (the  possibility  of  increasing  the  cooling  rate  at  the  minimum  depth  will  be  discussed  later).

There  are various  ways  to  characterize  the  hardenability  of  a  steel.  Certain  aspects  of  this  will  be  discussed  in  the  following  article in  the  Section  and  has  also  been  described  in  detail  in  previous  ASM  Handbooks,  formerly  Metals  Handbooks  (Ref  23). The  CCT  diagram  can  serve  this  purpose  if  one  knows  the  cooling  rate  at  the  minimum  depth.  The  CCT  diagrams constructedaccording to Atkinsor Thelning presented above are particularly suitable.