BIS System Of Designation Of Steels

Introduction

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

Engineering materials and How they are Designated.

Steel :

It is an alloy of iron and carbon, with carbon content up to a maximum of 1.5%. The carbon occurs in the form of iron carbide, because of its ability to increase the hardness and strength of the steel. Other elements e.g. silicon, sulfur, phosphorus, and manganese are also present
to a greater or lesser amount to impart certain desired properties to it. Most of the steel produced nowadays is plain carbon steel or simply carbon steel.

Carbon steel is defined as a steel which has its properties mainly due to its carbon content and does not contain more than 0.5% of silicon and 1.5% of manganese.

The plain carbon steels varying from 0.06% carbon to 1.5% carbon are divided into the following types depending upon the carbon content.

1. Dead mild steel — up to 0.15% carbon
2. Low carbon or mild steel — 0.15% to 0.45% carbon
3. Medium carbon steel — 0.45% to 0.8% carbon
4. High carbon steel — 0.8% to 1.5% carbon

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 based on their tensile strength without detailed chemical composition, are specified in 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 a tensile strength of 250 newtons per mm square.
FeE 250- yield 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 consists 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 mold.
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 a 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 the 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.

Example:

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

Designation of Steel Castings and Forgings:

Five types of steel castings are designated as below by Bureau of Indian Standards:

(i) Unalloyed steel castings are designated by letter CS followed by minimum tensile strength in MPa (N/mm2).

(ii) Unalloyed special steel castings (high magnetic permeability) are designated by letters CSM followed by the minimum tensile strength in MPa (N/mm2).

(iii) Alloy steel castings are designated by letters CS followed by the minimum tensile strength in MPa and the chemical name and percentage of important alloying ele­ments.

(iv) Heat resistant steel castings are designated by let­ters CSH followed by minimum tensile strength in MPa and chemical name and percentage of important alloying ele­ments.

(v) Corrosion resistance steel castings are designated by letters CSC followed by the minimum tensile strength and chemical name and percentage of important alloying el­ements.

Some Examples asked in examination 

1. Identify the Material and Composition: 

a) X 10 Cr18Ni9Mo4Si2 :

Material: High alloy steel
Composition: Avg. 0.1% Carbon, 18% chromium,9% nickel, 4% Molebdenum,2% Silicon. 

b) XT 72 W 18 Cr 4 V 1 :

Material: High-speed tool steel 
Composition: average carbon content 0.72 %, tungsten 18 %, chromium 4 % and vanadium 1 %.

2. Give the composition of

(i) FeE220: Steel having a yield strength of 220 N/mm2.
(ii) 20C8 : Carbon steel containing 0.15 to 0.25 percent (0.2 percent on average) carbon and 0.60 to 0.90 percent (0.80 percent on average) manganese.

3. State the following material specifications.

(i) FeE 230 (ii) FG 200 (iii) 35C8 (iv) X20Cr18Ni12

i) FeE 230 –steel (Steel having a yield strength of 230 N/mm2) with a minimum tensile strength of 230 N/mm2
ii) FG 200- Grey cast iron with a minimum tensile strength of 200 N/mm2
iii) 35C8 Means a carbon steel containing avg. percentage of carbon is 0.35 and avg. percentage of manganese is 0.8.
iv) X20Cr18Ni12 –Means alloy steel with an average percentage of carbon is 0.20, the average percentage of chromium is 25 , the average percentage of nickel is 12

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