Comparison between vacuum heat treatment and traditional heat treatment

- Jun 14, 2019-

Comparison between vacuum heat treatment and traditional heat treatment


Vacuum heat treatment refers to the comprehensive technology combining vacuum technology and heat treatment, which means that all and part of the heat treatment process is carried out in the vacuum state. Vacuum heat treatment can realize almost all heat treatment processes, such as quenching, annealing, tempering, carburizing, chromizing, nitriding. In the quenching process, gas quenching, oil quenching, nitrate quenching, water quenching, etc., which has the following advantages compared with ordinary heat treatment.


1, no oxidation, no decarbonization, no carbon on the workpiece internal and surface has a good protection


Oxidation causes the metal surface to lose its luster, increase the surface roughness, and decrease the precision. Moreover, the oxide skin on the steel surface is often the source of quenching soft point and quenching cracking. Oxidation reduces the strength of steel pieces and other mechanical properties. Decarbonization (see typical decarbonization diagram of steel in figure 1) refers to the phenomenon that the surface carbon content of steel decreases when it is heated.


Figure 1 typical decarbonization of steel


Generally, the oxidation and decarburization of steel is carried out at the same time. As the decarburization layer is oxidized, the reduction of carbon content will significantly reduce the hardening hardness, wear resistance and fatigue properties of steel, while the decarburization of high-speed steel will reduce the red hardness (see figure 2).


Figure 2 W6Mo5Cr4V2 furnace quenching diagram has decarbonization phenomenon, the depth is 0.23mm


Since the metal is heated under a certain vacuum degree, the workpiece avoids contact with oxygen. The workpiece has no oxidation and no decarbonization, so it can obtain a bright surface and better heat treatment quality. At the same time, no reduction reaction will occur in the vacuum state, and no carbon increase will occur, as shown in FIG. 3.



In figure 3, W6Mo5Cr4V2 vacuum quenching figure, the carbide distribution is uniform, and the grain size can be rated as grade 10.5


The advantages of vacuum heat treatment can be clearly seen by comparing the following surface of workpiece with the metallographic diagram of vacuum quenching after adding protective agent. As shown in figure 4 and figure 5:


Fig.4 quenching and tempering after 40Cr carburizing, martensite grade 3, residual austenite grade 2, a small amount of granular carbide grade 1 surface and sub-surface martensite structure thickness is different



Figure 5. The vacuum quenching structure of 40Cr is medium carbon martensite and a small amount of black strip tempered martensite. Uniform heating, surface no oxidation decarbonization


2, improve the overall mechanical properties, degassing and promote the purification of the metal surface


Vacuum has obvious degassing effect on liquid metal, and also has good degassing effect on dissolved gas in solid metal. The most harmful gas in metals is hydrogen. When using vacuum heating, hydrogen in metal and alloy can be quickly reduced to the minimum degree, eliminating hydrogen embrittlement, thus improving the plasticity, toughness and fatigue strength of materials, and improving the overall mechanical properties of the workpiece. When metals and alloys are heated in vacuum, if the vacuum degree is lower than the decomposition pressure of corresponding oxides, such oxides will decompose, and the free oxygen formed will be immediately discharged from the vacuum chamber, which will further improve the surface quality of the metal, and even make the surface to reach the activation state, thus playing a role of purification.


3, the workpiece deformation is small


In general, be processed workpiece by thermal radiation heat within the furnace, the internal and external temperature difference is small, the thermal stress is small, thus determines the components of the vacuum heat treatment processing deformation is small, at the same time under the vacuum heating and quenching under vacuum, automatically, to avoid the workpiece under the thermal state in the air handling (salt bath treatment and the protection of the atmosphere is heating in anaerobic environment, but it is still in the air or complete quenching) containing oxygen quenching medium, reduces the machining deformation. For example, the rolling silk wheel made of Cr12MoV material is respectively treated with vacuum heat treatment and salt bath. The deformation amount of vacuum heat treatment is 70% lower than that of salt bath treatment, and the product qualification rate is high.


4, can reduce the volatility of gold elements in the workpiece


Vacuum heat treatment can be heated and quenched in a controlled atmosphere, thus reducing the volatility of gold elements in the workpiece and ensuring the heat treatment quality of the workpiece.


5. Others

Vacuum heat treatment furnace has high thermal efficiency, which can realize rapid heating and cooling. Good stability and repeatability. Good working environment, safe operation, no pollution and public hazards.


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