The three thermally active and overlapping stages during annealing are: 1. Recovery 2. Recrystallisation 3. Grain Growth. The first two stages of annealing are caused by the stored cold- worked energy. Grain growth, the third stage of annealing, occurs if annealing is continued after recrystallisation has completed. In this stage, the recrystallised grains grow in size at the expense of [...]
In this article we will discuss about the structure of recrystallisation diagram for electrolytically refined iron. It is difficult to show the effect of amount of cold work and annealing temperature (for a given time of annealing) on the grain size of the metal or alloy by simple two dimensional graphs. Thus, the recrystallisation diagrams as shown in Fig. 7.43 [...]
The most extensively used method is conventional hardening for quenching in a single medium. The disadvantage of this method, however, is that the cooling rate in the martensitic transformation range will be very high. It will differ only slightly from the rate on the upper zone of super-cooled austenite of low stability and, therefore, cracks, distortion and other defects may [...]
The various heat treatment processes are discussed as follows: 1. Annealing 2. Normalising 3. Hardening 4. Tempering 5. Surface Hardening. 1. Annealing: The objects of annealing are: (i) To soften the metals. (ii) To improve machinability. (iii) To refine grain size due to phase recrystallisation. (iv) To increase ductility of metal. (v) To prepare steel for subsequent treatment. (vi) To [...]
The various heat treatment processes are discussed as follows: 1. Annealing 2. Normalising 3. Hardening 4. Tempering 5. Surface Hardening. 1. Annealing: The objects of annealing are: (i) To soften the metals. (ii) To improve machinability. (iii) To refine grain size due to phase recrystallisation. (iv) To increase ductility of metal. (v) To prepare steel for subsequent treatment. (vi) To [...]
In this article we will discuss about:- 1. Introduction to Heat Treatment 2. Types of Phase Diagrams in Heat Treatment 3. Benefits of Alloy Hardening 4. Variables. Introduction to Heat Treatment: The properties of metals and alloys which are all so important to the engineer and the user, are intimately related to the micro-structure. Micro-structure is defined in a broad [...]
The first step in the true heat treatment cycle of steel is the austenitisation, i.e. to get a homogeneous austenite by heating it to a predetermined temperature in the austenite stability range. Normally, the initial structure of carbon steels, before this heat treatment, is ferrite + pearlite (in hypo-eutectoid steels), pearlite (eutectoid steel), or cementite + pearlite (hyper eutectoid steels). [...]
Unless special precautions are taken, heating the steel results usually in certain amounts of oxidation or scaling and may also result in decarburisation. Both are usually undesirable. To prevent oxidation and decarburisation of steels, various methods from the simplest, but not completely reliable, to complex and expensive but almost full proof, are used depending on the criticality of the part, [...]
In large number of commercial heat treatment, the transformations occur during continuous cooling of the steel, and the information from TTT diagrams is not directly useful to these heat treatments. However, TTT diagrams are the basis of certain heat treatments in which transformations occur isothermally, such as: 1. Martempering: It is a hardening process with an objective to minimise distortion [...]
In this article we will discuss about:- 1. Introduction to Annealing of Steel 2. Objectives of Normalising of Steel 3. Advantages of Normalising over Annealing. Introduction to Annealing of Steels: Annealing, in general refers to heating the material to a predetermined temperature for a definite time (i.e. soaking), and then cooling it slowly normally in a furnace by switching it [...]