Heat Treatment Essay

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Heat Treatment

OBJECT:
To perform a Jominy end-quench test in order to observe heat treatment hardening and prepare the hardenability curve for a steel bar.

THEORY:
Steel is the most important engineering and construction material; it accounts for approximately 80 % of all metals produced. Steel has attained this degree of prominence because it combines strength, ease of fabricability into many shapes, and a wide range of properties along with low cost. Also it is possible to give a wide range of mechanical properties to steels by changing the size ad shape of the grains or changing its microconstituents. This property owes to several different ways that austenite can decompose.

Fundamentally, all steels are alloys of iron and carbon. So-called plain carbon steels also generally have small but specified amounts of phosphorus and sulfur. Alloy steels are those which contain specified percentages of other elements in their chemical compositions.
[pic]
Figure 1. Iron-Carbon Equilibrium Diagram [1]

HARDENABILITY:

In general strength of a given steel is proportional to its hardness; the higher the hardness, the stronger the steel. The carbon content of a steel determines the maximum hardness attainable. The most important factor influencing the maximum hardness is mass of the metal being quenched. In a small section, the heat is extracted quickly, thus exceeding the critical cooling rate of the specific steel. The critical cooling rate is that rate of cooling which must be exceeded to prevent formation of non-martensite products.

Hardenability is the ease with which hardness may be attained. A steel that transforms rapidly from austenite to ferrite plus carbide has low hardenability because ((+carbide) is formed at the expense of martensite. Conversely a steel that transforms slowly from austenite to ferrite has greater hardenability.

For any given steel, there is a direct and consistent relationship between hardness and cooling rate. However the relationship is highly non-linear. There is a standardized test that lets us make necessary predictions of hardness. This is the Jominy end-quenched test. A round bar with a standard size is heated to form austenite and is than end-quenched with a water stream of specified flow rate and pressure. Hardness values along the bar are determined on a Rockwell harness tester and a Hardenability curve is plotted.

The quenched end is cooled very fast and therefore has the maximum possible hardness for the particular carbon content of the steel that is being tested. The cooling rates at points behind the quenched end are slower and consequently the hardness values are lower.

[pic]
Figure 2. Transformation Processes [3].

HEAT TREATMENT:

The amount of carbon present in plain carbon steel has a pronounced effect on the properties of a steel and on the selection of suitable heat treatments to attain certain desired properties. Below are some major types of heat treatment processes:

Annealing: Steel is annealed to reduce the hardness, improve machinability, facilitate cold-working, produce a desired microstructure. Full annealing is the process of softening steel by a heating and cooling cycle, so that it may be bent or cut easily. In annealing, steel is heated above the transformation temperature to form austenite, and cooled very slowly, usually in the furnace.

There are several types of annealing like black annealing, blue annealing, box annealing, bright annealing, flame annealing, intermediate annealing, isothermal annealing, process annealing, recrystallisation annealing, soft annealing, finish annealing and spheroidizing. These are practiced according to their different final product properties in the industry.

The two-stage heat treating process of quenching and tempering is designed to produce high strength steel capable of resisting shock and deformation without breaking. On the other hand, the annealing process is intended to make
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