HEAT TREATMENT OF STEELS ANNEALING of STEELS • To soften the steel and to improve machinability (spherodization annealing) • To relieve internal stresses (stress relive annealing) • To improve the mechanical properties (full annealing) • To restore the ductility (recrystallization) • For steels with less than 0,9% carbon, annealing is heating to 25-50°C above the upper critical point (T °C), soaking at T °C and Furnace cooling. For higher carbon steels the temperature is 50°C above the lower critical point. • Normalising differs from annealing in that the metal is allowed to cool in still air. Soaking 20 min/cm from section thickness. Normalizing refine coarse grains, improve machinability and mechanical properties • The structure and properties produced, however, varying with the thickness of metal treated. The tensile strength, yield point, reduction of area and impact value are higher than the figures obtained by annealing. • The properties obtained subsequently depend on the coarseness of the pearlite and ferrite and their relative distribution.
Annealing and Normalizing Temperature Range HEAT TREATMENT OF STEELS
Effect of Annealing and Normalizing on Steel HEAT TREATMENT OF STEELS
Grain Size in Annealing and Normalizing HEAT TREATMENT OF STEELS Annealing, coarser and less uniform Normalizing, finer and more uniform
HEAT TREATMENT OF STEELS Mechanical Behavior of Steel pearlite + Fe3C pearlite + ferrite
HEAT TREATMENT OF STEELS Mechanical Behavior of Steel • More wt%C: hardness increases, and ductility decreases.
HEAT TREATMENT OF STEELS Mechanical Behavior of Steel • Fine Pearlite vs Martensite: Hardness: fine pearlite << martensite. Forms of carbide in micro-constituents in steel
HEAT TREATMENT OF STEELS HARDENING of STEELS Hardening is heating to 25-50°C above the upper critical point (T °C), soaking at T °C and Water cooling. The soaking time in air furnaces should be 2 min for each mm of cross-section or 0,6 min in salt or lead baths. Uneven heating, overheating and excessive scaling should be avoided. Steels with less than 0,3 % carbon cannot be hardened effectively, while the maximum effect is obtained at about 0,7 % due to an increased tendency to retain austenite in high carbon steels. Water is one of the most efficient quenching media where maximum hardness is required. The quenching velocity of oil is much less than water. To minimise distortion, long cylindrical objects should be quenched vertically, flat sections edgeways and thick sections should enter the bath first. Hardening induce high strength with good toughnness and induce wear resistance.
Hardening Temperature HEAT TREATMENT OF STEELS
HEAT TREATMENT OF STEELS Pearlite Morphology • Ttransf just below TE --Larger T: diffusion is faster --Pearlite is coarser. • Ttransf well below TE --Smaller T: diffusion is slower --Pearlite is finer. Adapted from Fig. 10.6 (a) and (b),Callister 6e.
HEAT TREATMENT OF STEELS • Spheroidite: -a crystals with spherical Fe3C -diffusion dependent. -heat bainite or pearlite for long times -reduces interfacial area (driving force) • Isothermal TTT Diagram Non-equilibrium Products in Fe-C Adapted from Fig. 10.10, Callister, 6e. Adapted from Fig. 10.9,Callister 6e.
HEAT TREATMENT OF STEELS • Martensite: --g(FCC) to Martensite (BCT) Non-equilibrium Products in Fe-C Fig. 11.11 • Isothermal Transf. Diagram Fig. 11.13 Fig. 11.12
HEAT TREATMENT OF STEELS Tempering of Martensite • reduces brittleness of martensite, • reduces internal stress caused by quenching.
HEAT TREATMENT OF STEELS CHANGES DURING TEMPERING OF STEELS The principles underlying the tempering of quenched steels have a close similarity to those of precipitation hardening. The overlapping changes, which occur when high carbon martensite is tempered, as follows: Stage 1. 50-200°C. Martensite breaks down to a transition precipitate known as c-carbide (Fe2.4C) across twins and a low carbon martensite which results in slight dispersion hardening, decrease in volume and electrical resistance. Stage 2. 205-305°C. Decomposition of retained austenite to bainite and decrease in hardness. Stage 3. 250-500°C. Conversion of the aggregate of low carbon martensite and c-carbide into ferrite and cementite precipitated along twins, which gradually coarsens to give visible particles and rapid softening. Stage 4. Carbide changes in alloy steel at 400-700°C. In steels containing one alloying addition, cementite forms first and the alloy diffuses to it. When sufficiently enriched the Fe3C transforms to an alloy carbide.
HEAT TREATMENT OF STEELS CHANGES DURING TEMPERING OF STEELS Tempering curves for 0,35 % C steel and die steel
HEAT TREATMENT OF STEELS Summary