Heat Treatment of Metals Eng. Ahmed Afeefy Eng. Ibrahim Aljaish

Objectives To study the effect of the cooling rate and the amount of alloying elements on the micro structure and the mechanical properties ( Rockwell hardness ) of medium carbon steel. To know the several techniques of heat treatment.

What is Heat Treating ? Controlled Heating And Cooling of Metal to Change Its Properties and Performance. Through: Change in Microstructure Change in Chemistry or Composition It involves the use of heating, normally to extreme temperatures, to achieve a desired result such as hardening or softening of a material.

Why Heat Treat? To improve Toughness To increase Hardness To increase Ductility To improve Machinability To refine Grain Structure To remove Residual Stresses

Heat treatment techniques include Case Hardening Tempering Annealing normalizing Quenching

Case Hardening Case Hardening: is the process of hardening the surface of a metal, often a low carbon steel, by infusing elements into the material's surface, forming a thin layer of a harder alloy.

Tempering Tempering is done to "toughen" the metal by transforming brittle martensite into bainite or a combination of ferrite and cementite. Quenching introduces internal stress that has a weakening effect (the material becomes hard but brittle at the same time and can’t be worked) Tempering is performed by heating martensite to a T below eutectoid temperature (250°C-650°C) and keeping at that T for specified period of time.

Annealing Annealing: is a heat process whereby a metal is heated to a specific temperature and then allowed to cool slowly in the furnace . The steel becomes soft and ductile depending on the pearlite (ferrite + cementite )percentage means it can be cut and shaped more easily.

Full annealing is the process of slowly raising the temperature about 50 ºC above the Austenitic temperature line in the case of Hypoeutectoid steels (steels with < 0.77% Carbon) and 50 ºC into the Austenite-Cementite region in the case of Hypereutectoid steels (steels with > 0.77% Carbon). It is held at this temperature for sufficient time for all the material to transform into Austenite or Austenite- Cementite as the case may be. It is then slowly cooled at the rate of about 20 ºC/hr in a furnace to about 50 ºC into the Ferrite-Cementite range. At this point, it can be cooled in room temperature air with natural convection.

Why Annealing? Annealing is carried out to: Relieve Stress Increase softness, ductility and toughness Produce a specific microstructure

Annealing Stages The annealing process may be divided into three stages: 1.  Recovery : the relief of residual internal stresses 2. Recrystallization : there is a significant drop in tensile strength, hardness and a large increase in the ductility of the material during recrystallization. 3. Grain growth : the tensile strength and hardness continue to decrease but at a much less rate than the recrystallization stage. The major change observed during this stage is the growth of the grain boundaries and reaching the original grain size.

Process Annealing (not required in the exam) It is used to treat work-hardened parts made out of low-Carbon steels (< 0.25% Carbon). This allows the parts to be soft enough to undergo further cold working without fracturing. Process annealing is done by raising the temperature to just below the Ferrite-Austenite region line on the diagram. This temperature is about 727 ºC so heating it to about 700 ºC should suffice. This is held long enough to allow recrystallization of the ferrite phase, and then cooled in still air.

Since the material stays in the same phase through out the process, the only change that occurs is the size, shape and distribution of the grain structure. This process is cheaper than either full annealing or normalizing since the material is not heated to a very high temperature or cooled in a furnace.

Normalizing It is the process of raising the temperature to over 60 º C into the Austenite range. It is held at this temperature to fully convert the structure into Austenite, and then removed form the furnace and cooled at room temperature under natural convection. This results in a grain structure of fine Pearlite with excess of Ferrite or Cementite. The resulting material is soft; the degree of softness depends on the actual ambient conditions of cooling. This process is considerably cheaper than full annealing since there is not the added cost of controlled furnace cooling.

Quenching Quenching is an accelerated method of bringing a metal back to room temperature. Quenching can be performed with forced air convection, oil, fresh water, salt water and special purpose polymers. super-cooling causes rapid nucleation rate many nuclei produces small crystals results in a harder form of steel.

Ferrite Austenite Pearlite (Perlite) Cementite(fe3c) Known as α-iron Pure iron at room temperature Body-centered cubic structure BCC. Austenite Known as γ-iron Face-centered cubic FCC. Much softer than ferrite More easily worked Pearlite (Perlite) Most common constituent of steel Gives steel most of its strength layered mixture of ferrite and cementite Cementite(fe3c) Hard, brittle, white On the phase diagram, cementite corresponds to a vertical line at 6.7% C

The IRON–IRON Carbide(Fe–Fe3C) Phase Diagram

Procedure Prepare 4 steel specimens. Place 3 specimens in the furnace and start the heating process. On each specimen perform one of the 3 different heat treatments. Measure the hardness of each specimen and also measure the hardness of a non-treated specimen. Construct a table and plot the results. compare the hardness of the three different heat treated specimens.

Results Brinelle hardness test for the 4 specimen

Example of results EN 9 steel was used for the present work Example of results EN 9 steel was used for the present work. Hardness was determined for differently heat-treated steels.

references An Experimental Investigation on Hardness and Microstructure of Heat Treated EN 9 Steel