RETAINED AUSTENITE   ·      As we know, for ferrous alloys the austenite to martensite transformation is never practically completed and a certain amount of austenite always remains even after the completion of the process. It is the untransformed austenite. It is usually more found in Hardened and High carbon steels. ·      The amount of retained austenite depends largely on the M s and M f temperature which are the martensitic start and martensitic finish temperature respectively. Both M s and M f temperatures depend upon the carbon content  and both are inversely proportional to the amount of carbon. As the carbon content increases the M s and M f temperature  lowers down and the amount of retained austenite increases. It  is shown in the figure below.                             Figure: Effect of carbon on retained austenite. ·  ...

Ductile vs Brittle behavior The behavior of materials under loading can be classified as ductile or brittle depending on if the material undergoes gross plastic deformation. The following figure represents the stress-strain curve for ductile and brittle materials. As we can see that a brittle material almost fails at elastic limit. But brittleness is not an absolute property of a material. For example, tungsten is brittle at room temperature and is ductile at high temperatures. The three factors contributing to brittle fracture are : triaxial state of stress, low temperature and high strain rate. All these factors need not be present for a material to show brittle cleavage. A triaxial stress which is present at notch along with low temperature may lead to brittle fracture. Also high strain rate assists those two factors. For most of the engineering materials ductility is an important criteria. Thus, little amount of ductility is needed for most of the mater...

                   True Stress-Strain Curve (or) Flow Curve : The conventional engineering stress-strain curve for ductile material is based on original cross-sectional area. Thus, it doesn’t explain the deformation characteristics of a metal properly. Whereas the true stress-strain curve is based on instantaneous cross-sectional area of the material. In engineering stress-strain curve we can see that the curve drops after maximum load(necking). This is due to the fact that the stress is based on the original area. This stress is known as engineering stress and is given by : After necking the load required to deform the material decreases and area being constant the stress also decreases, thus the curve drops until fracture. But here in true stress-strain curve the stress is based on instantaneous area during the application of load. Thus, during necking we know that the area decreases and in turn the stress ...

                                                                 Engineering Stress-Strain Curve The engineering stress-strain curve gives the relationship between stress and strain for a ductile material. It is generally obtained by applying load to a tensile specimen. The curve reveal many properties of a material such as Young's Modulus(E), Yield point, UTS   etc. Stress : The stress in the engineering stress-strain diagram is given by load divided by the original area. Strain : The strain in the engineering stress-strain curve is giver by ratio of change in length / elongation to its initial length. Strain has no unit since it’s a ratio of two quantities. Proportionality limit : I...

                                                                    THE ELLINGHAM DIAGRAM The Ellingham diagram is the simplest method of representing the relationship between the free energy( Δ G) and temperature of various oxides and sulphides. In metallurgy, the Ellingham diagram is used to   find out a suitable reducing agent. In the Ellingham diagram the highly stable oxides are found at the bottom and the less stable oxides are found at the top. An element occupying a lower position in the diagram can reduce the oxides of another element present above it. For example, Mg lies below Si thus Mg can be used as a reducing agent for oxides of Si. Why Ellingham diagram has all straight lines? Reason for upward slope of line...

THE IRON-CARBON PHASE DIAGRAM Of all binary alloy systems the one that is possibly the most important for a metallurgist is the iron and carbon system. We know that both steel and cast iron play a important role in structural applications and they both are iron-carbon system. Pure iron upon heating experiences changes in its crystal structure until 1538 °C and melts there. At room temperature it is present at a stable form called ferrite(α), which has a BCC crystal structure. The ferrite transforms to austenite( ט ) at 912°C which has a FCC crystal structure. At 1394°C it again undergoes a phase transformation to                                δ-ferrite which has a BCC crystal structure. Pure iron finally melts at 1538°C. All these changes can be seen in the left vertical axis of the Fe-C phase diagram. In the composition axis ...