Failure mechanism and life extension methods of hot cracking and cracking of wheel hub molds

Failure mechanism and life extension methods of hot cracking and cracking of wheel hub molds
The cracking of the wheel hub mold belongs to thermal fatigue failure, and the cavity is repeatedly cycled at 200-350 ℃, starting to appear after 10000 cycles.
Cracks start from the surface hot spot
The surface of the mold cavity undergoes heating and cooling once per mold, and the temperature amplitude can reach over 250 ℃. Repeated thermal stress causes microcracks to appear at grain boundaries.
The R angle between the spoke and the wheel rim transitions, with large variations in wall thickness and concentrated hot spots. The cracks here usually appear 30% -40% earlier than in flat areas.
At the beginning of the crack, it appears as a network of fine lines with a depth of less than 0.1mm. When it develops to 0.3mm or more, corresponding protrusions will appear on the surface of the casting.
Materials and heat treatment determine the starting point
The hardness of H13 type hot work die steel is commonly 45-52HRC. High hardness and decreased resistance to thermal cracking; Low, insufficient wear resistance.
The inclusion content of electroslag remelted steel is lower and the isotropy is better. Under the same conditions, the service life can be increased by 20% -30% compared to ordinary forgings.
Three rounds of tempering after quenching and tempering cannot be omitted. When the residual austenite content is higher than 5%, it will continue to transform and the dimensional stability will deteriorate during use.
Improper cooling design can accelerate cracking
The waterway is too close to the surface of the cavity, resulting in strong local cooling and a sudden drop in surface temperature. The larger the temperature gradient, the higher the thermal stress.
It is recommended that the distance between the waterway and the mold cavity be no less than 25mm. When it is less than 20mm, simulation should be used to confirm that the surface temperature is not less than 180 ℃.
Xinfeng Mold will conduct thermal balance simulation on the waterway layout of the wheel hub mold, controlling the surface temperature difference of the mold cavity within 15 ℃ to slow down thermal fatigue.
The influence of usage habits is underestimated
Uneven spraying of release agent and localized rapid cooling can result in additional thermal shock. The recommended spraying distance is 200-300mm to avoid direct spraying at fixed points.
If the preheating of the mold is not sufficient before production, the thermal shock of the first mold is the greatest. Preheating to above 200 ℃ before pouring can significantly reduce initial cracks.
If production resumes after a shutdown of more than 4 hours, preheating is required again. Directly installing the cold mold on the machine is a common cause of crack propagation.
Four measures to extend lifespan
The first is surface nitriding of the cavity, with a depth of 0.05-0.15mm and a surface hardness of over 900HV, improving both anti sticking and anti cracking properties.
The second is PVD coating, commonly AlCrN type, with a thickness of 2-5 μ m. Coating can reduce the friction coefficient and decrease the demolding force by 10% -20%.
The third is to regularly relieve stress, conducting low-temperature tempering every 10000 molds at a temperature 20-30 ℃ lower than the original tempering temperature.
The fourth is early repair and grinding of cracks. Cracks with a depth less than 0.2mm can be smoothed along the crack direction to prevent further propagation.
When should we change the mold
If the depth of the crack exceeds 0.5mm or the coverage area of the mesh crack exceeds 30% of the mold cavity, grinding cannot guarantee the appearance of the casting. It is recommended to replace the mold.
Cracks in the wheel rim can cause air tightness failure, which cannot be maintained through long-term infiltration. Replacing the insert is a more reliable choice.
The cost of replacing a single wheel rim insert in a mold with an insert structure is about 25% -35% of the cost of the entire mold, which is more economical than replacing the entire mold.
Frequently Asked Questions
Q: How long does mold cracking usually occur?
Answer: It usually appears after 10000 cycles, and the hot spot appears earlier.
Q: How deep is the crack that requires mold replacement?
Answer: It is recommended to replace if it exceeds 0.5mm or covers more than 30%.
Q: How much longer can nitriding extend its lifespan?
Answer: The common increase is 20% -40%, depending on the working conditions.
Q: What is the preheating temperature of the mold?
Answer: It is recommended to start pouring at temperatures above 200 ℃.
Q: Will the coating peel off?
Answer: Partial detachment can be repaired, but the cavity needs to be cleaned first.
Q: Do I need to preheat before resuming production?
Answer: If you stop for more than 4 hours, you need to preheat again.
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