The TiC phase is formed on the surface by carburizing titanium alloy material, which has very high hardness. However, the poor adhesion between TiC layer and matrix hinders practical use. Excessive temperature will accelerate the growth of titanium carbide grains:
1. Sintering temperature
Titanium carbide high manganese steel bonded carbide final sintering temperature generally take 1420 ℃ more appropriate. Sintering temperature should not be too high. Even the bond phase into liquid phase metal loss, so that the hard phase adjacent, aggregation and grow up, forming a source of fragmentation. This is why the previously analyzed hard phase grains have less binding phase between them. Of course, the sintering temperature should not be too low, otherwise the alloy will underburn. Especially in the three stages of degumming, reduction and liquid phase sintering.
2. Heating rate during sintering
The heating rate of this kind of alloy should not be fast during sintering. The heating speed and holding time should be strictly controlled. Because in the low temperature degumming stage, the compaction releases the pressing stress and the forming agent volatilizes the process, if the heating rate is fast, the forming agent has no time to volatilize and becomes vapor after liquefaction, so that the compaction occurs burst or micro-crack phenomenon; Above 900 ℃ reduction stage, to make compact have enough time to take off all use the raw material powder (such as Mn2Fe intermediate alloy) in volatiles and oxygen; When entering the liquid phase sintering stage, it is necessary to slow down the heating rate to make the compaction fully alloying.
Titanium will react with oxygen, nitrogen and other gases at high temperature, causing hardening, nitriding treatment at high temperature (800-900 degrees), making its surface vickers hardness up to more than 700; By surfacing welding, an appropriate amount of nitrogen or oxygen is injected into argon to increase the surface hardness by 2-3 times. Through ion electroplating, a layer of titanium nitride is generated on the surface. The thickness is about 5 microns, and the surface vickers hardness is as high as 16000-20000. Chrome plating, etc. When nitriding may form a variety of different area, if you don't high oxygen content, form the outer zone of titanium nitride, with golden color and hardness of 14000-17000 mpa, but this kind of titanium nitride layer is fu due to the low temperature or nitride and high temperature heated (annealing), nitrogen is completely dissolved into the titanium metal on the surface of the solid solution, titanium intensified layer no longer increases or disappear in a heat treatment process, therefore, the discovery of titanium nitride layer when ti solid solution have dissolved into nitrogen the layer with high hardness, but the core hardness decrease. When ammonia gas is used for nitriding, additional tissue changes occur due to hydrogen percolation. The heat of formation of titanium nitride exceeds that of all titanium oxides. Therefore, attention must be paid to nitriding treatment under the condition of complete removal of oxygen. Titanium reacts with nitrogen on the surface in a parabolic pattern over time. Therefore, the rate of nitriding decreases with the increase of nitriding time. Because the diffusion velocity of nitrogen in the outer titanium nitride layer is lower than that in the solid titanium solution region below, it is impossible to form a thick nitride layer, and nitrogen or ammonia must have high purity. Since oxygen not only inhibits the formation of the nitride layer, but also causes the surface layer to remove the scale at higher temperatures, the moisture content (humidity) must be at least to such a degree, even if it reaches the melting point.
The surface of titanium is boronizing to form TiB2 phase, and the hardness is very high. It has been reported in the literature that the titanium parts after pickling are embedded in the mixed powder of amorphous boron powder and A1203 powder (0.75-1.0% of NH4F*HF is added), and the TiB2 layer can be generated after 1 hour of heat preservation at 1010 degrees. Under the above conditions, the thickness of the coating varies according to the alloy. The coating thickness on the industrial pure titanium is 25p, and the coating thickness on the TC4 titanium alloy is 20um, and the hardness is in the range of hv2800-3450. The high temperature requirement of boronizing limits its application. If the titanium plate is first plated with iron and then borided, the boriding temperature can be reduced to 870 degrees, the coating thickness can be up to 40um, and the hardness can be up to HV2300. Since titanium also reacts with nitrogen, argon must be used as the carrier. If use oxygen/nitrogen mixture gas (air) as a source of oxygen, the oxygen in the diffusion of temperature (850 ℃) can form nitrides enough it will reduce the diffusion of oxygen. 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 high enough to form a continuous surface oxide film, and it has been reported to block diffusion.
The purpose of surface hardening is to improve wear resistance and eliminate the risk of adhesion between parts working under friction conditions. With the improvement of hardness, the corrosion resistance and fatigue strength may also be improved. Here we first focus on the improvement of surface hardness, focusing on the process itself and its impact on the improvement of surface hardness. The surface hardening shall be carried out and well controlled in a furnace with a pressure-protected atmosphere, which conveniently changes the composition of the gas at the end of the treatment to produce a uniform rutile layer without porosity. The result is similar TO that of TO process. In this way, it is double-processed in a one-step manner, and does not need the three-step BDO/TO combined treatment, thus significantly saving energy. This process only USES completely inert gas -- argon and oxygen, so it is very environmentally friendly and gas-free, and will not cause 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 a vacuum remains constant, a small change in the oxide content formed in the first step will cause a significant difference in the final hardness distribution.