After steel has been designed for a specific application, the designer, or the purchaser has to select the specific steel from those available in the market. The steels are sold with standard specifications and associated notations.
Some knowledge of the specifications is thus, essential. The specifications and the notations used for the steels vary from country to country, but each specification is based on certain criterion. The chemical composition, mechanical properties, hardenability, method of manufacture, nature of applications, etc. are some of the criteria used.
The majority of the specifications are based on the chemical composition of the steels, because it indicates to an experienced engineer the probable heat treatment to be given to the steel and the corresponding mechanical properties obtainable from it.
The designer could also easily choose the right steel matching the composition he had arrived at for a specific part, or application. In India, not only the knowledge of the Indian Standard Specifications is essential, but the familiarity with American, British, etc. standards is essential as the designer, or engineer comes across the imported steels, or, he might have to import a particular steel.
Very often foreign specifications are being used by many organisations:
1. Indian Standard Specifications:
In Indian Standard Specifications, for the purpose of code designation, the steels have been classified on the basis of properties and the chemical composition, though mainly it is based on the latter.
(a) Code Designation Based on Mechanical Properties:
It uses the tensile strength, or the yield strength for designation.
Fe = minimum tensile strength in N/mm2
Fe E = minimum yield strength in N/mm2
(Code ‘St’ stands for tensile strength in kg/mm2; ‘St E’ stands for yield strength in kg/mm ). Special characteristics like deoxidation method, etc. follow these designations.
(b) Code Designation Based on Chemical Composition:
The carbon content of the steel is specified as numerical figures, called the carbon points, where one point of carbon is defined as 0.01%. Letter ‘C’, if present, is used for plain carbon steels. Letter ‘T’ is added at the start of the notation for alloy tool steels and after the numerical figure for carbon point for plain carbon tool steels. A numerical figure following the letter ‘C’ or ‘T’ indicates 10 times the average percentage of manganese content.
Elements, which improve machinability of steels, with their chemical symbols ‘S’ ‘Se’, ‘Te’, ‘Pb’ or ‘P’ indicate the free cutting steel and the numerical figure which follows the symbol indicates 100 times the respective element.
For the alloy steels too, the chemical symbols of the elements present are used. If it is a tool steel, then the code starts with letter T, otherwise the numerical figure indicates the carbon points in steel. It is then followed by the symbols of the significant elements in the descending order of their amounts present. The average or the nominal percentage of each element is indicated by the numerical figure written immediately following its chemical symbol.
If the amount of an alloying element is up to 1%, the numerical figure is expressed up to two decimal places (without indicating the decimal point in the code) but normally underlining the numerical value (the underlining may be missing in many cases), except B, N which are indicated only by the symbol of the element. When the amount of an element is between 1 and 10%, the index number is rounded to the nearest whole number (i.e. up to 0.5% is rounded down bill 0.5% and more is rounded up).
If it is possible, the decimal part may be rounded off to one digit but underlined. When the amount of the alloying element is more than 10%, the numerical figure is rounded to the nearest whole number. If two or more significant alloying elements have the same numerical figure, then their chemical symbols are put together followed by the numerical figure written only once.
2. AISI/SAE Specifications:
The American Iron and Steel Institute and Society of Automotive Engineers (London) have cooperated together, and thus have evolved similar specifications, based on chemical compositions of the steels.
The specifications normally have four numerical digits (in some cases five). One of the following letters is prefixed to indicate the method used in steel making, (SAE specifications do not have letter prefix), but the omission of the letter prefix assumes that the steel is made in open-hearth.
A = Alloy steel, basic open-hearth
B = Carbon steel, acid-bessemer
C = Carbon steel, basic open-hearth
D = Carbon steel, acid open-hearth
E = Electric furnace steel.
For example, AISI-CXXXX is a basic open-hearth carbon steel due to the letter prefix ‘C’.
Of the numerical digits, the first digit from the left indicates one of the following types of steels:
The second digit (from left) for simple steels element in steel, hut in other steels, it is some n digits indicate as usual the carbon points in the boron-vanadium steel, ‘L’ for leaded steel is fixed the digits for carbon point.
Thus, a steel:
3. British Specifications:
British specifications for steels used to be designated by the letters ‘En’ followed by a number. This number has no relationship with either the composition, or the mechanical properties of the steel.
The new British system, BS 970, uses the first three digits for the content of alloying elements, followed by a letter A, H or M which means, Analysis based, Hardenability based or, Mechanical properties based specification respectively. The last two digits after this letter is meant for carbon points.