Causes and Prevention of Defects in Die Heat Treatment
There are soft spots on the exterior of the die
There are soft spots on the surface of the die after heat treatment, which will affect the wear resistance of the die and reduce the service life of the die.
Reasons for this:
Before heat treatment, there are oxide scales, rust spots and partial decarbonization on the surface of the die.
After quenching and heating of the die, the selection of quenching medium is inappropriate, and the impurities in quenching medium are excessive or aging.
The oxide scale and rust spots should be removed before the heat treatment of the die. The appearance of the die should be properly maintained during quenching heating. Vacuum electric furnace, salt bath furnace and maintenance atmosphere furnace should be adopted as far as possible.
When the die is cooled after quenching and heating, the suitable cooling medium should be selected. The long-term used cooling medium should be stopped filtering or changed regularly.
Poor structure of die before heat treatment
The final spheroidizing structure of the die is uneven and imperfect, and there are network, band and chain carbides in the structure. This will make the die easy to crack after quenching and form the scrap of the die.
Reasons for this:
Serious carbide segregation exists in the original structure of die steel.
Poor forging technology, such as excessive heating temperature, small deformation, high stop forging temperature and slow cooling rate after forging, makes the forging structure coarse and network, ribbon and chain carbides exist, which makes spheroidizing annealing difficult to eliminate.
Poor spheroidizing annealing process, such as too high or too low annealing temperature and short isothermal annealing time, can result in uneven microstructures or poor spheroidization of spheroidizing annealing.
In general, according to the working conditions of the die, the consumption batch and the strength and toughness of the data itself, the die steel materials with good quality should be selected as far as possible.
Improvement of forging process or normalizing preparation for heat treatment to eliminate the unevenness of network and chain carbides and carbides in the original data.
For high carbon die steels with severe carbide segregation which can not stop forging, solid solution refining and heat treatment can be stopped.
The proper standard of spheroidizing annealing process for forged billet can be established by heat treatment with quenching and tempering and rapid homogenizing spheroidizing annealing.
Reasonable charging ensures the average temperature of the billet in the furnace.
Quenching Cracks in Dies
Cracks in quenched dies are the biggest defects in the process of heat treatment of dies, which will make the processed dies scrapped and cause great losses in consumption and economy.
Reasons for this:
Serious segregation of reticulated carbides exists in die data.
There are mechanical processing or cold plastic deformation stresses in the die.
Improper heat treatment operation of die (too fast heating or cooling, improper selection of quenching cooling medium, too low cooling temperature, too long cooling time, etc.).
The complex shape, uneven thickness, sharp angle and threaded hole of the die make the thermal stress and organizational stress too large.
Overheating or over-burning occurs when the quenching heating temperature of the die is too high.
Tempering is not timely after quenching or lack of tempering holding time.
When the die is repaired and quenched, it is reheated and quenched again without intermediate annealing.
The grinding process is improper for the heat treatment of the die.
There are high tensile stress and micro-cracks in the hardened layer during EDM after thermal treatment of the die.
Strictly Control the Internal Quality of the Original Materials of Dies and Moulds
Improve forging and spheroidizing annealing process, eliminate network, band and chain carbides, improve the average of spheroidizing structure. ,
After mechanical processing or cold plastic deformation, the die should stop stress relief annealing (> 600 C) and then stop heating and quenching.
Asbestos blocked threaded holes should be used to bind the risky section and thin wall of the complex die, and graded quenching or isothermal quenching should be adopted.
Annealing or high temperature tempering should be stopped when repairing or innovating dies.
The die should be preheated during quenching and heating, and pre-cooled during cooling, and suitable quenching medium should be selected.
Temperature and time of quenching heating should be strictly controlled to avoid over-heating and over-burning of dies.
After quenching, the die should be tempered in time, the holding time should be plentiful, and the high alloy complex die should be tempered 2-3 times.
Choose the right grinding technology and suitable grinding wheel.
Improve the die EDM process and stop stress relief tempering.