The carburizing of titanium alloy generates TiC phase on the surface, which has very high hardness. However, the bonding force between the TiC layer and the substrate is poor, which prevents practical use. Too high temperature will accelerate the growth of titanium carbide grains:
- 1. Sintering temperature. The final sintering temperature of titanium carbide high manganese steel cemented carbide is generally 1420 ℃. The sintering temperature should not be too high. Even making the binder phase into liquid phase metal loss, so that the hard phase adjacent, aggregate and grow, forming a source of fragmentation. This is why the bonding phase between the hard phase grains analyzed earlier becomes less. Of course, the sintering temperature should not be too low, otherwise the alloy will be under-fired. Especially in the three stages of degumming, reduction and liquid phase sintering.
- 2. Heating rate during sintering. Such alloys should not be heated faster when sintered. Strictly control the heating rate and holding time. Because in the process of low temperature degumming, the green compact releases the pressing stress and the volatilization process of the forming agent. If the heating rate is fast, the forming agent will not evaporate and become liquefied and become steam, which will cause the green compact to burst or microcrack; In the reduction stage, the compact should be given enough time to remove the volatiles and oxygen in the raw material powder (such as Mn2Fe master alloy); when entering the liquid phase sintering stage, the heating rate should be slowed down to fully alloy the compact.
Titanium will react with oxygen, nitrogen and other gases at high temperature to cause hardening. At high temperature (800-900 degrees), nitriding treatment will make the surface Vickers hardness up to 700 or more; through surfacing welding, the same in argon Appropriate amount of nitrogen or oxygen can increase the surface hardness by 2-3 times; through ion plating, a layer of titanium nitride is formed on the surface, the thickness is about 5 microns, and the surface Vickers hardness is as high as 16000-20000; chrome plating. Various areas may be formed during nitriding. If the oxygen content is not high, an outer area composed of titanium nitride is formed, which has a golden yellow color and a hardness of 14000-17000 MPa, but this titanium nitride layer is very low. At the nitriding temperature or subsequent high temperature heating (annealing), the nitrogen is completely dissolved into the titanium solid solution on the metal surface, and the titanium layer no longer increases or disappears in a certain heat treatment process. In the case of the titanium layer, the titanium solid solution layer is dissolved in nitrogen. This layer also has high hardness, but the core hardness decreases. When ammonia gas is used for nitriding, additional tissue changes occur due to the effect of hydrogen penetration. Titanium nitride is hard and conductive titanium nitride generates more heat than all titanium oxides. Therefore, we must also pay attention to the nitriding treatment under the condition of complete deoxygenation. Titanium and nitrogen undergo surface reactions according to the parabolic law over time. Therefore, the nitriding speed decreases as the nitriding time increases. Because the diffusion rate of nitrogen in the outer titanium nitride layer is lower than the diffusion rate of the underlying titanium solid solution region, it is impossible to form a thick nitride layer, and nitrogen or ammonia must have higher purity. Because oxygen not only hinders the formation of the nitride layer, but also causes the surface layer to remove scale at higher temperatures, the moisture content (humidity) must be at least this level, even if it reaches the melting point.
Titanium surface is boronized to form TiB2 phase with high hardness. According to reports in the literature, the pickled titanium parts are embedded in a mixed powder of amorphous boron powder and A1203 powder in half (including 0.75% -1.0% NH4F * HF), and kept at 1010 degrees for 1 hour, that is TiB2 layer can be generated. Under the above conditions, the thickness of the coating varies with the alloy. The thickness of the coating produced on industrial pure titanium is 25p, the thickness of the TC4 titanium alloy is 20um, and the hardness is in the range of HV2800-3450. The temperature requirement of boronizing is high, which limits its application. If iron is plated on the titanium plate first, and then boronized, the boronization temperature can be reduced to 870 degrees, the thickness of the coating can reach 40um, and the hardness can reach HV2300. Since titanium also reacts with nitrogen, argon must be used as a carrier. If an oxygen / nitrogen mixed gas (air) is used as an oxygen source, sufficient nitrogen will be formed at the oxygen diffusion temperature (about 850 ° C), which will reduce the oxygen diffusion. In order to optimize the depth and distribution of the oxygen diffusion layer, the oxygen concentration needs to be high enough to produce the maximum diffusion rate. But it cannot be so high as to form a continuous surface oxide film, and it is reported to block diffusion.
The purpose of surface hardening is to improve wear resistance and eliminate the risk of parts adhering to each other working under friction conditions. As the hardness increases, the corrosion resistance and fatigue strength may also increase. Here we first focus on the improvement of surface hardness, the process itself and its effect on the improvement of surface hardness. Surface hardening should be performed in a furnace with a pressure-protected atmosphere and well controlled. It can easily change the gas composition at the end of the treatment to produce a uniform pore-free rutile layer. The result is similar to the TO process. In this way, it is processed in a one-step manner, and does not require three steps like BDO / TO combined processing, which significantly saves energy. This process uses only completely inert gases-argon and oxygen, so it is very environmentally friendly, non-toxic, and will not cause the global greenhouse effect. Although the process is very good, the vacuum treatment is expensive, and there are obvious control problems in the two-step oxidation / diffusion treatment. Even if the diffusion time in the vacuum is fixed, a small change in the oxide content formed in the first step will cause a significant difference in its final hardness distribution.
