What are the different types of metal hardening process and its effects on the hardness
HARDENINGIn heat treating to harden a metal, the metal is heated to a temperature where the elements in the metal become a solution. Before doing this, defects in the crystal lattice structure of metal are the primary source of ‘give’ or plasticity. Heat treating addresses those deficiencies by bringing the metal into a reliable solution with fine particles to strengthen the metal. Once the metal is thoroughly heated to the right temperature to produce a solid solution, it is quickly quenched to trap the particles in solution.
Tempering :
Tempering is a method of heat treating used to increase the resilience of iron-based alloys like steel. Iron-based metals are very hard, but they are often too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, and strength of metal, which usually makes it easier to machine. The metal will be heated to a temperature below the critical point as lower temperatures reduce brittleness while maintaining hardness. For increased plasticity with less hardness and strength, higher temperatures are required.
Case Hardening:
Case hardening : In the process of case hardening, the external layer of metal is hardened while the interior metal remains soft. For metals with a low carbon content such as iron and steel, additional carbon has to be infused into the surface. Case hardening is a process often used as a final step after the piece has already been machined
Annealing : Annealing is a heat treatment method where a metal such as aluminum, copper, steel, silver, or brass is heated to a specific temperature, held at that temperature for some time to allow transformation to occur, and then air cooled. This process increases the metal’s ductility and decreases hardness to make the metal more workable.
Normalizing is an annealing process for steel where it is heated 150-200°F higher than in annealing and held at the critical temperature long enough for the transformation to occur. Steel treated in this way must be air cooled. The heat treating in normalization causes smaller austenitic grains, while air cooling produces more refined ferritic grains. This process improves machinability, ductility, and strength of the steel.
Carburising
This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm).
Tempering :
Tempering is a method of heat treating used to increase the resilience of iron-based alloys like steel. Iron-based metals are very hard, but they are often too brittle to be useful for most purposes. Tempering can be used to change the hardness, ductility, and strength of metal, which usually makes it easier to machine. The metal will be heated to a temperature below the critical point as lower temperatures reduce brittleness while maintaining hardness. For increased plasticity with less hardness and strength, higher temperatures are required.
Case Hardening:
Case hardening : In the process of case hardening, the external layer of metal is hardened while the interior metal remains soft. For metals with a low carbon content such as iron and steel, additional carbon has to be infused into the surface. Case hardening is a process often used as a final step after the piece has already been machined
Annealing : Annealing is a heat treatment method where a metal such as aluminum, copper, steel, silver, or brass is heated to a specific temperature, held at that temperature for some time to allow transformation to occur, and then air cooled. This process increases the metal’s ductility and decreases hardness to make the metal more workable.
Normalizing is an annealing process for steel where it is heated 150-200°F higher than in annealing and held at the critical temperature long enough for the transformation to occur. Steel treated in this way must be air cooled. The heat treating in normalization causes smaller austenitic grains, while air cooling produces more refined ferritic grains. This process improves machinability, ductility, and strength of the steel.
Carburising
This manufacturing process can be characterized by the following key points: It is applied to low-carbon workpieces; workpieces are in contact with a high-carbon gas, liquid or solid; it produces a hard workpiece surface; workpiece cores largely retain their toughness and ductility; and it produces case hardness depths of up to 0.25 inches (6.4 mm).
How does the metal metallurgy affect the grinding process .
The metallurgy of the metal will tell us major properties for grinding its brittleness, ductility and the hardness.
These properties decide which type of grinding wheel to use. Harder , brittle metal will work with Aluminium oxide and White Alumina wheels, Ductile metals will work great with White Alumina for a free porous cutting, Black Silicon Carbide for castiron , steel SS series , some types of aluminium as well. High speed steels, carbides work great with Green Silicon Carbide.
These properties decide which type of grinding wheel to use. Harder , brittle metal will work with Aluminium oxide and White Alumina wheels, Ductile metals will work great with White Alumina for a free porous cutting, Black Silicon Carbide for castiron , steel SS series , some types of aluminium as well. High speed steels, carbides work great with Green Silicon Carbide.
How do I select a grinding wheel ?
First identify the following
1. Work-piece metal grade
2. Hardness in rc , Heat treatment process
3. Amount of metal to be removed
4. Ra requirement
5. Machining tolerance
6. Machine capacity in terms of grinding power.
7. Machine's ability to change workpiece rpm and wheel rpm
8. Coolant type and concentration ( 3:1 ) recommended .
General Rule of Thumb :
Soft Metal harder wheel : O,P,Q,R,S
Hard Metals Softer wheel : J,K,L,M,N
Most common metal have hardness from 20 to 65 rc .
Choose your grit , depending upon the amount of metal to be removed and required Ra. You can still get a very good Ra with a 46 grit and still have a bad finish with a 80 grit. just because you did not choose the right hardness of the wheel.
The most optimal grinding process is the when the abrasive grain wear out and later pop out with just enough bond strength during grinding. Dressing should be just a mere operation of cleanup / opening up the wheel in ideal grinding process.
1. Work-piece metal grade
2. Hardness in rc , Heat treatment process
3. Amount of metal to be removed
4. Ra requirement
5. Machining tolerance
6. Machine capacity in terms of grinding power.
7. Machine's ability to change workpiece rpm and wheel rpm
8. Coolant type and concentration ( 3:1 ) recommended .
General Rule of Thumb :
Soft Metal harder wheel : O,P,Q,R,S
Hard Metals Softer wheel : J,K,L,M,N
Most common metal have hardness from 20 to 65 rc .
Choose your grit , depending upon the amount of metal to be removed and required Ra. You can still get a very good Ra with a 46 grit and still have a bad finish with a 80 grit. just because you did not choose the right hardness of the wheel.
The most optimal grinding process is the when the abrasive grain wear out and later pop out with just enough bond strength during grinding. Dressing should be just a mere operation of cleanup / opening up the wheel in ideal grinding process.
What are the most common coolant mistakes ?
Operators in open machines usually 33 mps hydraulic grinders , will lower the coolant flow . This has been predominantly observed in surface grinders and cylindrical grinders.
The thumb rule is that the coolant must flow much faster than the velocity of the wheel , just to flush the metal particles and most importantly the heat. At a given point in time , you need maximum possible coolant at the point of contact.
So the velocity of the coolant flow has to be greater than the velocity of the wheel. Just try increasing the pressure of the coolant pump.
The thumb rule is that the coolant must flow much faster than the velocity of the wheel , just to flush the metal particles and most importantly the heat. At a given point in time , you need maximum possible coolant at the point of contact.
So the velocity of the coolant flow has to be greater than the velocity of the wheel. Just try increasing the pressure of the coolant pump.
How do I increase the grinding output in terms of workpiece qty ?
Metal Hardness = >55 hrc use JKLM < 55 Hrc use a NOPQRS
Amount of metal = Coarse Grit 24,36,40,54 for more metal 60,80,100,120, medium to cuts , 120,220,240,280,320 super light cuts
Required Ra = 24,36,40,54 can give .8 to .3 60,80,100 between .3 to .2 .1 , 100, 120,200,240,280,320 > .1
These above rules are very much in general and will depend on the operating conditions and parameters available to change.
Amount of metal = Coarse Grit 24,36,40,54 for more metal 60,80,100,120, medium to cuts , 120,220,240,280,320 super light cuts
Required Ra = 24,36,40,54 can give .8 to .3 60,80,100 between .3 to .2 .1 , 100, 120,200,240,280,320 > .1
These above rules are very much in general and will depend on the operating conditions and parameters available to change.
