Surface Strengthening Techniques Used for Molds
Posted: 04/26/2016 04:36:06  Hits: 91
At present, molds in China have short service life, high consumption. So, improving service life of mold has already become a priority. The surface treatment on mold has great influences on its service life. Most of damages on mold are caused by improper heat treatment. According to statistics, mold failure caused by improper heat treatment accounts for more than 50% of total failure rate. For this, the heat treatment on foreign mold is more and more using vacuum furnace, half vacuum furnace and atmosphere furnace without oxidation protection. Heat treatment on mold includes matrix strengthening and surface strengthening. Matrix focuses on improving intensity and toughness of matrix, reducing rupture and deformation. So the common heat treatment should strictly follow technology. Surface strengthening refers to improve abrasive resistance, corrosion resistance and lubrication performance. There are several surface strengthening techniques, mainly including carburization, nitrizing, sulfurizing, boriding, nitrocarburizing and metal cementation. Using different surface strengthening techniques can improve service life of mold by several times or even dozens of times. In recent years, some new surface strengthening techniques come out. 

1. Ion nitriding 
To improve corrosion resistance, abrasive resistance, thermal fatigue resistance and anti-sitction, ion nitriding can be used. The most outstanding advantage of ion nitriding is to reduce time of nitriding significantly. Microstructure of nitrided layer can be controlled through regulating different constituents of gas, decreasing surface brittleness of nitrided layer and having less deformation. Hardness distribution curve of nitrided layer is also steadier, which cannot cause peeling and thermal fatigue. Permeable matrix materials are more than those of gas nitriding. Moreover, ion nitriding has no toxicity and it also cannot explode, ensuring production security. However, in terms of complicated mold, even heating and nitriding layer are acquired difficultly. And nitriding layer is shallower and transition layer is steeper. Measurement and homogeneity of temperature still need to be figured out. 

The temperature of ion nitriding ranging from 450 to 520℃ is proper. After being processed for 6 to 9 hours, the depth of nitriding layer is about 0.2~0.3mm. If the temperature is over low, nitriding layer is too thin. If the temperature is over high, porous layer can be caused easily on surface, weakening anti-sticking. The thickness of nitriding layer with 0.2~0.3mm is suitable. Worn ion nitrided mold can be used again after being repaired and ion nitrided again, high improving service life of mold. 

2. Nitrocarburizing
The temperature of nitrocarburizing is lower (560~570℃), having less deformation. The surface hardness of mold steel after being treated can reach as high as 900~1000HV, having good abrasive resistance, corrosion resistance and higher high temperature hardness. It can be used for die casting mold, cold punching mold, cold extrusion mold, hot extrusion mold, high speed forging mold and plastic mold, which can improve service life of molds by 1~9 times. However, molds usually deform after being gas nitrocarburized. Swell capacity accounts for about 25% of thickness of compounds. It is not suitable for precise mold. Annealing should be carried out and remains should be eliminated before processing.

For example, in terms of leaf spring hole punching mold made of Cr12MoV, the service life of mold is improved by 3 times after being treated with gas nitrocarburizing and borate bath vanadatizing. Cold heading screws punch made of 60Si2 uses advance nitriding, short term carbonitriding, direct oil quenching, low temperature quenching and high temperature tempering, which can improve toughness of center, extending service life of high cold upsetting punch by more than 2 times.

3. Carbonnitrohoriding
Carbonnitrohoriding is carried out in nitriding furnace. Permeating agents are organic impregnation agents containing boron and ammonia. The proportion is 1:7. The temperature of carbonnitrohoriding is 600℃. The time of carbonnitrohoriding is 4 hours. The thickness of compound in carbonnitrohorided layer is 3~4μm and the depth of diffusion layer is 0.23mm. The surface hardness is HV011050. The service life of mold is improved significantly after being carbonnitrohorided.
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