13. Heat Treatment of Metals 13.1 Tempering of Martensite 13.2 Annealing of Metals and Alloys • •
Annealing of Non-ferrous Metals and Alloys Annealing of Ferrous Metals and Alloys
13.1 Tempering of Martensite • Tempering:
•
Tempered martensite consists of extremely small and dispersed cementite particles in a ferrite matrix (much smaller than those in spheroidite).
a-
a-Ferrite
EFFECTS OF TEMPERING •
Tempered martensite has increased ductility and toughness, while it has reduced the strength and hardness compared to martensite.
•
Tempering relieves the internal stresses that were introduced during quenching (when martensite was formed).
• faster
carbon diffusion larger cementite particles grown less ferrite – cementite phase boundary area per unit volume weaker and more ductile material
• more time for carbon diffusion larger cementite particles less ferrite – cementite phase boundary area per unit volume weaker and more ductile material
Austempering and Martempering
13.2 Annealing of Metals and Alloys •
Purposes:
– Increase softness, ductility, and toughness of coldworked materials
– Relieve internal stresses – Produce a specific microstructure •
•
Stages of annealing processes: 1. Heat the material to a desired elevated temperature ( 2. Hold the elevated temperature (“soaking”) 3. Cool to room temperature Types of annealing processes: – For non-ferrous metals and alloys: Stress relief annealing Process annealing – For ferrous alloys (steels): Spheroidizing Full annealing Normalizing
anneal )
13.2.1 Annealing of Non-ferrous Metals and Alloys
Stress relief annealing Process annealing
Effects of Reheating of Non-ferrous Metals after Cold Working
RECRYSTALLIZATION TEMPERATURE •
The recrystallization temperature: the temperature at which recrystallization just reaches completion in 1 hour
•
Recrystallization temperature is for metals.
Stress Release Annealing of Non-ferrous Metals/Alloys) • •
• •
Carried out at a temperature lower than the recrystallization temperature (Tanneal < Trecryst ). Aims to eliminate caused by: – Plastic deformation processes (machining, grinding, etc.) – Non-uniform cooling after welding or casting – Phase transformations induced during cooling wherein parent and product phases have different densities, e.g., density decreases during Austenite (FCC crystal) Martensite (BCT crystal). Involves recovery ONLY. It can annihilate dislocations, as an elevated temperature enhances atomic diffusion, which hence reduces the dislocation density in the material. Thus, mechanical properties such as strength and ductility are partially recovered to their pre-cold-worked states.
Process Annealing of Non-ferrous Metals/Alloys) •
Carried out after cold-working to:
–
Allow continuation of deformation without fracture or excessive energy consumption.
–
Increase the ductility of strain-hardened metals
•
Carried out at a temperature higher than the recrystallization temperature ( ).
•
Involves recovery, recrystallization, and grain growth.
RECRYSTALLIZATION • •
• •
Recrystallization occurs at temperature > Trecryst . In recrystallization, very fine new crystals are formed by consuming the old cold-worked crystals. The new crystals have much lower dislocation density than the cold-worked crystals, thus softening the material. All cold-worked crystals are finally consumed. Mechanical properties (strength, ductility) are almost restored to the pre-cold-worked values. 0.6 mm
33% cold worked brass
New crystals nucleate after 3s. at 580 C.
After 4s at 580 C.
GRAIN GROWTH •
Grain growth occurs in all polycrystalline materials at elevated temperature.
•
Grain growth will decrease in total grain boundary area and reduce total grain surface energy.
.
After 8s at 580C.
GRAIN GROWTH (Cont’d) 0.6 mm
0.6 mm
After 8 s, 580C
After 15 min, 580C
• An
: Initial grain diameter
grain diam. at time .
d
n
coefficient dependent exponent on material and T. typ. n = ~ 2 elapsed time n
d o Kt
increases with temperature due to higher atomic diffusion rates.
13.2.2 Annealing of Ferrous metals and alloys
Spheroidizing Full annealing Normalizing
TYPES OF Ferrous Metals and Alloys – Steels (<1.4 wt% C) • Low alloy steels containing plain Fe and C, and in some cases low levels of other alloying elements. . – Plain carbon steels: contain only C and some Mn as the alloying elements. – Other low alloy steels: contain low concentrations of alloying elements in addition to C/Mn. • High alloy steels containing high concentrations of alloying elements other than C and Mn. Example: , SS 316L: 0.03% C, 17% Cr, 12% Ni, 2.5% Mo, 2.0% Mn, <1% Si, <0.045% P, <0.03% S.
– Cast irons (> 2.5 – 4.5 wt% C)
SPHEROIDIZING •
Carried out by:
– Heating to a temperature just below the eutectoid line (727C)
– Maintaining this temperature for more than
15-24 hours to
obtain spheroidite.
– Cooling to room temp. •
Carried out usually before machining or plastic deformation to achieve greater ductility.
Spheroidite
FULL ANNEALING •
•
Carried out by: – Heating to a temperature above the eutectoid line (727C) and for a sufficient time to convert (austenitizing) . – Furnace cooling (slow cooling) to obtain Carried out usually for low- and medium-carbon steels before machining or extensive plastic deformation to: (plus a proeutectoid phase if it was – Produce present before the annealing), and – Increase ductility.
Coarse Pearlite
NORMALIZING •
Carried out by: – Heating to a high enough temperature and for a sufficient time to convert (austenitizing) . – Cooling in air ( fast cooling) to obtain
•
Carried out usually after plastic deformation to: – Decrease the average grain size by producing – Produce a more uniform size distribution of pearlite.
Fine Pearlite
