The precipitation behavior of cementite in low carbon steels at various heating rates from
0.3 to 100 K/s has been studied using a high-frequency induction heating apparatus. The materials
used in this study were steel platesfor welded structures: 610 and 780 MPa class steel plates with a
mixed microstructure of bainite and martensite.Cementite was observed using a carbon extraction
replica method and the hardness and toughness were also examined. When heated at the
conventional slow rate of 0.3 K/s, relatively large cementite particles with an average diameter of
72 nm precipitated at the lath boundaries, whereas when heated at a rapid rate over 3.0 K/s,
cementite precipitated both within the laths and at the lath boundaries, and the cementite was
refined down to an average diameter of 54 nm. With such refinement of the cementite, the
toughness was improved. On the other hand, the hardness was irrespective of the heating rate and
was dependent on the tempering parameter. TEM observations of the cementite precipitation
behavior during the rapid heating process revealed that cementite begins to precipitate at the lath
boundaries at about 773 K and within the laths at about 873 K. It is concluded that rapid heating
especially from 773 to 873 K contributes to the cementite refinement and consequently the
improvement in toughness. The effect of alloying elements such as chromium, molybdenum or
silicon on the cementite growth during the rapid heating and tempering treatment is also discussed.
Fatigue strength capacity of load-carrying fillet welds on ultra-high-strength steel plates subjected to out-of-plane bending