Technological properties are those qualities which give information regarding the suitability of metals for various technological operations or processes. Such properties are highly desirable in shaping, forming and fabrication of materials.
The following technological properties will be discussed: 1. Machinability 2. Weldability 3. Castability 4. Formability 5. Malleability.
1. Machinability:
It is defined as the ease with which a given material can be cut permitting the removal of material with a satisfactory finish at lower cost. It is used to signify how well a material takes a good finish. It may also be called finish ability.
Good machinability is associated with the following:
ADVERTISEMENTS:
(i) High cutting speed.
(ii) Low power consumption.
(iii) Good surface finish.
(iv) Removal of material with moderate force.
ADVERTISEMENTS:
(v) Medium degree of tool abrasion (longer tool life).
(vi) Formation of small chips.
Machinability depends on the following factors:
(i) Chemical composition of the work piece material.
ADVERTISEMENTS:
(ii) Micro structure.
(iii) Mechanical properties.
(iv) Physical properties.
(v) Cutting conditions.
ADVERTISEMENTS:
(vi) Coolant properties.
(vii) Feed and depth of cut.
(viii) Kind and shape of cutting tool.
(ix) Size and shape of cut.
ADVERTISEMENTS:
(x) Coefficient of friction between chip and tool material.
(xi) Tool material.
(xii) Type of machine used.
(xiii) Type of machining operation.
For judging the machinability the main factors to be chosen depend on the type of operation and production requirements.
The following criteria may be considered while evaluating machinability:
(i) Value for cutting forces.
(ii) Tool life between two successive grinds.
(iii) Quality of surface finish.
(iv) Form and size of chips.
(v) Temperature of chips.
(vi) Metal removal rate.
(vii) Rate of cutting under standard force.
(viii) Cutting forces and power consumption.
The following factors increase machinability:
(i) Small undistorted grains.
(ii) Uniform microstructure.
(iii) Lamellar structure in low and medium carbon steels.
(iv) Less hardness, less ductility and less tensile strength.
(v) Cold working of low carbon steel.
(vi) Annealing, normalising and tempering operations.
(vii) Addition of small amounts of sulphur, lead, phosphorus and manganese.
Machinability may be improved by the addition of small percentages of certain elements such as lead, selenium, sulphur, manganese etc.
‘Grey cast iron’ is much more machinable than ‘white cast iron’ since the former possesses carbon in free form as graphite flakes which assist the chips to break up easily during the machining (in addition graphite lubricates the tool during cutting operation) whereas the latter (white cast iron) possesses free carbides (carbon in combined form) which are very hard constituents and are difficult to be machined.
Machinability Index:
The machinability of different metals to be machined may be compared by using the machinability index of each material which may be defined as follows:
A Standard steel has a carbon content of 0.13% max., manganese of 0.06 to 0.10% and sulphur of 0.80 to 0.03% and can be machined relatively easily; its machinability index is arbitrarily fixed as 100%.
Relative machinability of certain alloys is given below:
Excellent Machinability:
(i) Magnesium alloys.
(ii) Aluminium alloys.
(iii) Zinc alloys.
Good Machinability:
(i) Brass sheets and red brass
(ii) Gun metal
(iii) Malleable cast iron
(iv) Grey cast iron
(v) Free cutting steels
(vi) Copper aluminium alloys.
Poor Machinability:
(i) Low carbon steel
(ii) Annealed nickel
(iii) Low alloy steel.
Fair Machinability:
(i) Ingot iron and wrought iron
(ii) High speed steel
(iii) Monel metal
(iv) Sintered carbide.
Not Machinable:
(i) 18:8 stainless steel
(ii) Stellite (alloy of W, Cr, C, CO)
(iii) White cast iron.
2. Weldability:
It is defined as the capacity of a metal to be welded under the fabrication conditions imposed in a specific suitably designed structure and to perform satisfactorily in the intended service.
The real criteria in deciding on the weldability of a metal is the weld quality and the ease with which it can be obtained.
Weldability is of significant importance for fabrication of metals into various structures.
The weldability of a metal is affected by the following factors:
(i) Composition of metal
(ii) Brittleness of metal
(iii) Thermal properties
(iv) Welding technique
(v) Filler materials
(vi) Flux material
(vii) Strength of metal at high temperature
(viii) Stability of micro-constituents upto welding temperature
(ix) Affinity of oxygen and other gases before and at welding temperature
(x) Shielding atmosphere
(xi) Proper heat treatment before and after deposition of metal.
Weldability can be known by determining a metal’s behaviour under fusion and cooling, by crack and notch sensitivity, or by comparison of the heating and cooling effects which take place at the joint of the metal with the metal of known weldability.
Effects of Alloying Elements on Weldability:
The alloying elements affect the weldability in the following ways:
(i) Improve mechanical properties.
(ii) Form carbides.
(iii) Increase or decrease hardenability in the heat affected zone.
(iv) Provide grain refinement.
(v) Provide deoxidation of molten metal.
(vi) Form age hardening precipitates.
(vii) Control ductile to brittle transformation temperature.
a. To determine the suitability of iron and steel bar the following welding tests are carried out:
(i) Bending test (on a welded joint),
(ii) Bar welded into a link should stand being closed up without failure.
b. The following materials have good weldability in the ascending order:
(i) Stainless steel
(ii) Low alloy steel;
(iii) Cast iron;
(iv) Carbon steel;
(v) Iron.
3. Castability:
The ease with which a metal can be cast into form is known as castability of the metal. It is based on factors like solidification rate, gas porosity, segregation, shrinkage etc.
Following factors are favourable to castability of metal:
(i) Fluidity of metal
(ii) Low rate of shrinkage (It is the reduction in volume of a metal when it goes from a molten to a solid state).
(iii) Low or negligible segregation (Excluding alloying elements from the metal as they begin to solidify is known as ‘segregation’; segregation may be overcome by very slow cooling of the metal or subsequent heat treatment).
(iv) Low gas porosity.
4. Formability:
Formability is the ability of metals of forming into different shapes.
The various factors which govern to a large extent, the flow ability or ductility of the material are:
(i) Metal structure.
(ii) Grain size.
(iii) Hot and cold working.
(iv) Alloying elements.
(v) Softening heat treatments (Annealing and normalising).
A small grain size is recommended for shallow drawing of metals whereas for heavy drawing relatively large grains are recommended.
Hot and cold working causes distortion of grains. Generally cold worked crystals are more distorted than the hot worked and therefore the cold worked metals are usually less ductile than the hot worked.
Most alloying elements in a pure metal reduce its ductility, for example, the ductility of steel decreases as the amount of carbon in iron increases.
In softening heat treatments like annealing and normalising the ductility of metal is restored. The warped and distorted crystal are reformed and consequently the force required to cause slippage is reduced.
5. Malleability:
It is the ease with which the material undergoes too much change in shape under compressive stress without rupture.
The materials like soft steel, wrought iron, copper and aluminium have good malleability. They can be hammered or rolled into the desired shape without rupture.
The degree of malleability is measured by the thickness of leaf or foil which can be produced.