Requirements on the die of wood pellet mill

A qualified wood pellet mill has strict requirements on the selection of materials, especially the die of wood pellet mill. While many manufacturers pay less attention to the die when making it that they use the common iron material to make the die rather than alloy or stainless steel. However, the die made in iron would be broken in a very short period of time under high pressure, which would not only affect the yield of pellet machine, but also bring unpredictable influence on the whole pellet plant. Then, what material is the right choice for the die of wood pellet mill?


 
It is difficult to make a complete definition for the relationship between the material properties of the die and its material. At present, all kinds of die materials of wood pellet mill sold on the market and their performance and shortcomings are to be tested. Currently, the material used in the production of die can be divided into three categories: alloy, chromium or hardened stainless steel, carburized stainless steel. The above three types of materials have many varieties and their physical properties and chemical properties are not identical. The property of each kind of material can be used in the manufacturing process through special heat treatment.
 
1. Alloy die: most pellet plants of the United States use alloy die. The die of wood pellet mill is made of carburizing steel with high hardness, the best degree of abrasion resistance and toughness.
The carburizing and hardening (hardening of the surface) of die is a kind of heat treatment, that is to heat the die to high temperature, and then mixed with plenty of carbon gas. In this way, the outer layer of steel would absorb a lot of carbon. The carbon makes the steel surface more hard so as to increase the abrasion resistance on the surface of the die. Core material of low carbon is soft that it has high impact toughness. This type of die is not easy to crack. Because the surface of die is hard carbide layer and the core is in toughness, thus the die is more wear-resisting. But this kind of die has poor corrosion resistance. The material used for the production of this kind of die should be determined by the process characteristics of the cost and the heat treatment of die. In general, the different content of nickel should also be noticed when choosing materials. Because of the high nickel content, it can enhance the toughness and increase the quantity of holes of the die, but the cost of materials would increase as well.
 
2. Sometimes, the high chromium die is also called stainless steel die. Generally, 4Cr13 is used for making the die of wood pellet mill through hardening or vacuum hardening.
This kind of material has carbon content of about 0.4% ~ 0.5% and chromium of 12% ~ 14%. This kind of die can prevent the pitting of raw material, but the abrasion resistance and toughness are poor than the die of carburizing steel. Its outside surface is not hard, but its core is harder than the die of carburizing steel. The crystal structure of standard neutral quenching hard chromium die is quite uniform from surface to core. Different from the decreasing hardness of carburizing steel die, the hardness change of neutral hardened chromium die is small from the surface to the core. Die of high chromium has very high corrosion resistance. In Europe, due to the high corrosivity of feed, the chromium content of die is higher. While the feed mills in the United States do not need to use this kind of die.
 
3. The carburized stainless steel die: this kind of die is better. Carburized stainless steel die has similar hardened surface as alloy die, and because the carburized layer has a large number of chromium carbide with strong wear resistance, but its hardness decreases gradually from surface to the core as carburizing alloy.

The chromium carbide on the surface of die presents many spots when observing through a microscope. These hard particles dispersed in the entire carburized layer so as to increase the wearability of die in wood pellet mill. Due to the excellent abrasion resistance of die, it can be applied for many times, thus the consumption of die for tons of pellet is lower. The die is more resistant to corrosion than alloy die, but it is inferior than high chromium die.

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