Three Common Mold Heat Treatment Defects

What are the heat treatment defects?

Heat treatment defects refer to various problems that occur during the final stage of the mold’s heat treatment or in subsequent processes and usage.

What are the causes of heat treatment defects in molds?

In mold heat treatment, these defects can be classified into three categories: quenching crack, insufficient hardness, poor deformation. And quenching crack is the most difficult to remedy and often requires re-molding.


Common Defects in Mold Heat Treatment and Preventive Measures

Defects Causes Measures
Quenching Crack  1.Material quality: It will cause crack because of material defect or choosing the wrong steel grade. Improve the IQC system and choose the right steel grade.
2. Shape Effect: This is caused by design factors such as sharp corners, small holes, and non-uniform wall thickness. Improve the design, including small holes placement, wall thickness of molds, and reducing sharp corners.
3.Improve the design, including small holes placement, wall thickness of molds, and reducing sharp corners. Understand the cooling characteristics of the quenching medium or tempering treatment to select the appropriate coolant.
4.Tensile Stress: It is generated on the surface of the mold due to different martensitic transformation rates. Improve surface machining accuracy and relieve stress through annealing.
5.Oxidation or Decarburization: This is caused by overheating, unprotected heating in air furnaces, small machining margins, and residual decarburized layers from forging or preheating. Use controlled atmosphere heating, molten bath heating, vacuum furnace, box furnace with used carburising agent or cast iron chips. Apply anti-oxidation coating. Increase the machining allowance.
6.Grain Coarsening: The martensitic structure is coarse due to grain coarsening that caused by excessively high quenching or prolonged holding time. Cool slowly in the martensitic range (using interrupted quenching, oil quenching and martempering). Select suitable heating conditions and quenching conditions.
Insufficient Hardness 1.The quenching structure of ferrite and austenite is formed because of quenching temperature too low or holding time insufficient. This may be corrected by normalising or annealing with the proper specified procedure of hardening.
2.High quenching temperature increases the residual austenite. Correct the process temperature and overhaul the temperature control system.
3.If the cooling rate is too slow, the pearlite and bainite transformation will occur that forms an incomplete quenching structure. Understand the cooling characteristics of the quenching medium, add or cool the quenching medium if necessary.  
4.Overtempering, caused by High tempering temperature or holding time too long, will leads to the drop in hardness.   Correct the process temperature and enter the furnace at a temperature no higher than the set temperature.
5.If the heating is decarburizing atmosphere, the decrease of the surface carbon content will reduce the hardness. Heat in furnaces with reducing, neutral or protective atmosphere, heat in boxes with used carburising agent or cast iron chips, or heat in molten salt baths.
Poor Deformation
(Two types of deformation: dimensions and shape. )
1.The volume of the steel increases due to the martensitic transformation in cooling or heating. Use alloy steels that are only slightly deformed by hardening.
2.From the material processing state before heat treatment, if there is machining residual  stress during cold working and machining, it will be deformed due to the relief and elimination of these residual stresses during heat treatment. Eliminate the residual stress of mechanical processing before quenching.
3.Eliminate the residual stress of mechanical processing before quenching. Pay attention to the furnace loading method, hold the mold in the proper position when lowering it into the furnace, or heat or cool uniformly.

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