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BIS System Of Designation Of Steels

Introduction

You might have seen material names like 40C8, 50C8. What does it mean ? . Engineering materials has various composition,types, Applications, properties. Every material have its own different mechanical properties, Bureau of Indian Standards ( BIS ). have standardised the designation method for steel and other material . These standards are mainly followed by Indian industries, other countries may be using ASME standard.

A large number of varieties of steel are used for machine components.

Designation Of Steels

Steels are designated by a group of letters or numbers indicating any one of the following three properties.
1. tensile strength;
2. carbon content; and
3. composition of alloying elements.

Steel, which are standardized on the basis of their tensile strength without detailed chemical composition, are specified by two ways- a symbol Fe followed by the minimum tensile strength in N/mm2. Another method is FeE steel followed by the yield strength in N/mm2.

For examples:
 
Fe350 - This indicates a steel with tensile strength of 250 newton per mm square.
FeE 250- yeild strength of 250 N/mm2.

Designation of Plain Carbon Steels:

This consist following three quantities:
1. figure and indicating 100 times the average percentage of carbon.
2. a letter C
3. a figure indicating 10 times the average percentage of manganese.

Example: 55C4 indicates a plain carbon steel with 0.55% carbon and 0.4 % manganese.

Designation of unalloyed free cutting steels.

This consist following quantities.
1. a figure indicating 100 times the average percentage of carbon.
2. a letter C
3. a figure indicating 10 times the average percentage of manganese.
4. a symbol S, Se, Te or Pb depending upon the element that is present and which makes the steel free cutting.
5. a figure indicating 100 times the average percentage of the above elements that makes the steel free cutting.

Example
25C12S14 indicates free cutting steel with 0.25 % carbon ,1.2 % manganese and 0.14 % sulphur.

Designation of high alloy steel

The designation of high alloy steels consist of following quantities
1. a letter 'X'
2. a figure indicating hundred times the average percentage of carbon
3. chemical symbol for allowing elements each followed by the figure for its average percentage content  rounded off to the nearest integer.
4. chemical symbol to indicate especially at that added element to attain desired properties ,if any.

Example:
X15Cr25Ni12 is a high alloy steel with your own 15 % carbon, 25 % chromium and 12 % nickel.

Designation Of Cast Steels

Components from cast steel are manufactured by pouring the molten steel into the mould.
There are two varieties of steel castings
1. carbon steel castings and
2. high tensile steel castings.
Cast steels are designated according to the tensile strength.

Example:
CS640 is a steel casting with minimum ultimate tensile strength of 640 N/mm2.

Designation of Alloy steels:

The designation of alloy steels consist of following quantities:
1. figure indicating hundred times the average percentage of carbon
2. chemical symbol for allowing elements always followed by the figure for its average percentage content multiplied by the factor. The multiplying factor depends upon the alloying element and its shown in following figure.
In alloy steels, if manganese % is more than 1 then chemical symbols and their figures are arranged in descending order of their percentage content.

Examlpe:
25Cr4Mo2 is an alloy steel having average 0.25% of carbon , 1% chromium and 0.2 % molybdenum.


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