Forging Theory
HOT MECHANICAL WORKING
1. Ingost slow to solidify (Ingost can't be used for engineering purpose)
2.Its structure is disorderly,course and inhomogeneous chemistry and impurities.
3.Therefore ductility is poor and highly stressed.
4.The stresses are sufficient to repture if sudden temperature stress are induced.
Reasons for hot working :
1. Reconstruction and refinement of crystalline structure.
2. Fragmented and arranged in a desired direction.
3. Section is reduced to a desired level.
4. To improve physical properties by selecting appropriate temperature and mechanical working.
Hot working range and effects
1. Near 370C metal strength decreases and plasticity increases until at lowest temperature where the lowest melting constituent of steel starts to melt. The strength decreases and looses plasticity and tind to crumble under applied stress instead of flow.
2. Selection of temperature depends on a) Chemistry. b) Melting point of the lowest constituent.
3. Finishing temperatures. Lower the finishing temperature finer the structure of the piece but limited to mass and shape. The finishing temperature should be near the lower transformation point.
SCALING
1. Not preferred - reducing atmosphere or > 12% CO level.
2. Hammered into the surface.
3. Effects refractories of the furnace.
4. Dimension effect (reduced).
DECARBURIZATION
1. Surface chemistry alters
2. Time and temperature if higher min scaling and decarburization.
3. Lowers the strength and therefore affects forging process.
Over heating.
1. Austenite grains becomes excessively large which impairs physical properties.
2. Boundary oxidation.
BURNING
1. An extremely overheated condition that causes more fusible matter of the steel to melt.
2. Usually at the austenite boundary leads to hot shortness.
Rate of heating to forging temperature . Time of heating --> centre temperature
1. Ingost slow to solidify (Ingost can't be used for engineering purpose)
2.Its structure is disorderly,course and inhomogeneous chemistry and impurities.
3.Therefore ductility is poor and highly stressed.
4.The stresses are sufficient to repture if sudden temperature stress are induced.
Reasons for hot working :
1. Reconstruction and refinement of crystalline structure.
2. Fragmented and arranged in a desired direction.
3. Section is reduced to a desired level.
4. To improve physical properties by selecting appropriate temperature and mechanical working.
Hot working range and effects
1. Near 370C metal strength decreases and plasticity increases until at lowest temperature where the lowest melting constituent of steel starts to melt. The strength decreases and looses plasticity and tind to crumble under applied stress instead of flow.
2. Selection of temperature depends on a) Chemistry. b) Melting point of the lowest constituent.
3. Finishing temperatures. Lower the finishing temperature finer the structure of the piece but limited to mass and shape. The finishing temperature should be near the lower transformation point.
SCALING
1. Not preferred - reducing atmosphere or > 12% CO level.
2. Hammered into the surface.
3. Effects refractories of the furnace.
4. Dimension effect (reduced).
DECARBURIZATION
1. Surface chemistry alters
2. Time and temperature if higher min scaling and decarburization.
3. Lowers the strength and therefore affects forging process.
Over heating.
1. Austenite grains becomes excessively large which impairs physical properties.
2. Boundary oxidation.
BURNING
1. An extremely overheated condition that causes more fusible matter of the steel to melt.
2. Usually at the austenite boundary leads to hot shortness.
Rate of heating to forging temperature . Time of heating --> centre temperature