Key words: gas hard nitriding, gas nitrocarburizing, H13 die steel, aluminum extrusion
H13 die steel contains high content of carbon and vanadium, good wear resistance, relatively weak toughness, good heat resistance, good strength and hardness at higher temperatures, high wear resistance and toughness , excellent comprehensive mechanical properties and high resistance to tempering, is currently the most widely used and most representative hot work die steel. H13 die steel can be obtained by heat treatment such as quenching + tempering before use to obtain excellent comprehensive properties.
When the extrusion die works, it directly contacts the high temperature ingot, and at the same time withstands the effects of high temperature, high pressure, severe friction, etc., and the working conditions are extremely bad, so that the mold is easily broken due to wear and fatigue, which will significantly reduce the service life of the mold. Surface modification of H13 steel molds is the key to comprehensively improving mold life. The use of surface nitriding technology to improve the surface quality of mold materials is a low-cost, convenient and practical treatment method. At present, the most commonly used nitriding methods are gas nitriding, liquid nitriding, and glow ion nitriding. Among them, gas nitriding is divided into hard nitriding and soft nitriding.
2. Gas hard nitriding
Gas hard nitriding, that is, gas nitriding, refers to a chemical heat treatment process in which a nitrogen atom penetrates into the surface layer of a steel material at a certain temperature. In order to separate it from the gas nitrocarburizing zone, it is also called gas hard nitriding. . When the gas is nitriding, the workpiece is placed in the furnace, and the NH3 gas is directly input into the nitriding furnace at 500-560 ° C. The NH 3 gas is thermally decomposed at a temperature higher than 480 ° C as follows: 2NH3 → 2 [N] + 3 H2
Most of the decomposed nitrogen atoms are formed as N2 gas, and a small portion of the active nitrogen atoms are absorbed by the surface of the workpiece and then diffused into the interior of the workpiece. After a period of time, a certain depth of nitride layer is obtained on the surface of the workpiece. The nitride layer includes a chemical layer of the outer layer (also known as a white bright layer) and a diffusion layer (also referred to as a dark layer) adjacent thereto. White bright layers play a decisive role in wear.
After gas nitriding, the structure is mainly ε phase and γ′ phase. When the treatment is not proper, the ξ phase is easy to occur, and the ξ phase should be avoided as much as possible. The ε phase and the γ′ phase have high hardness, and the compound layer (white bright layer) has a dense structure and good wear resistance; the ε phase has a high electrode potential and good corrosion resistance; thus, the wear resistance of the workpiece is improved. And corrosion resistance. In the matrix, iron and alloying elements (Mo, V, Cr, etc.) have a strong affinity with nitrogen, forming many alloy nitrides with nitrogen atoms, significantly increasing the hardness of the nitrided layer; and forming the specific volume of various nitrides. More than iron, after nitriding, the surface of the material forms a large residual compressive stress, which can offset the external tensile stress, which significantly improves the high cycle fatigue resistance of the steel.
2.2, nitridation law
The main process parameters of gas nitriding are nitriding temperature, nitriding time and ammonia decomposition rate, which have great effects on nitriding speed, nitride layer depth, nitride layer hardness and wear resistance, corrosion resistance and toughness. Impact.
When the nitriding time and the ammonia decomposition rate are the same, as the temperature increases, the nitrided layer thickens and the nitriding dispersion decreases. When the temperature rises above 580 ° C, the surface hardness decreases. When the nitriding temperature is lowered, the diffusion rate of the nitrogen atom is slowed down, the infiltration layer is shallow, the surface hardness is high, and the brittleness is large. The nitriding temperature is generally controlled at 520-540 °C.
The ammonia decomposition rate is generally controlled within the range of 20% to 60%, usually about 40%, depending on the temperature. The decomposition rate of ammonia gas is more than 60%, which will reduce the hardness and wear resistance of the nitrided layer; after more than 70%, the depth of the nitride layer will drop sharply.
The nitriding holding time depends on the nitriding temperature and the depth requirement of the nitride layer. The longer the holding time, the deeper the nitride layer, but the hardness of the layer will decrease.
3. Gas soft nitriding
Gas nitrocarburizing, that is, gas nitrocarburizing, refers to low temperature nitrocarburizing, which is mainly based on gas nitriding and supplemented by carburizing. Common medium has 50% ammonia + 50% endothermic gas (Nitemper method); 35%-50% ammonia + 50-60% exothermic gas (Nitroc method) and instilling ethanol or formamide when ammonia is used Several kinds. In soft nitriding, since the solubility of carbon atoms in the ε phase is high, the surface layer of soft nitriding is a compound common to carbon and nitrogen, and the compound has good toughness and wear resistance.
In the process of gas nitrocarburizing, since the solubility of carbon atoms is extremely low, it quickly reaches a saturated state, and many ultra-microscopic cementite particles are precipitated. These cementite particles act as the core of the nitride crystals, promoting the formation of nitrides. When the surface nitrogen concentration reaches a certain level, the ε phase is formed, and the ε phase has a high carbon solubility, which in turn accelerates the dissolution of carbon.
After soft nitriding, the structure consists of ε phase, γ' phase and nitrogen-containing cementite Fe3(C,N). Carbon will reduce the diffusion rate of nitrogen, so thermal stress and microstructure stress are harder than nitriding. The layer is thinner. At the same time, since the soft nitrided layer does not have a germanium phase, the toughness of the nitrided layer is better than that after hard nitriding.
3.2, process parameters
The main process parameters of gas nitrocarburizing are nitriding temperature, nitriding time, and nitriding atmosphere.
The gas soft nitriding temperature is usually 560-570 ° C, because the hardness of the nitride layer is the highest at this temperature. The nitridation time is usually 3-4 hours because the hardness of the compound layer reaches a maximum at 2-3 hours of co-infiltration, and the depth of the nitride layer increases slowly with time. The nitriding atmosphere is determined by the ammonia decomposition rate and the dropping rate of the carbon infiltrant.
4, comparative analysis
4.1, test content
Take the same batch of H13 die steel a total of 4 pieces, of which 2 are gas hard nitriding treatment, and the other 2 are gas nitrocarburizing treatment.
The gas hard nitriding process is as follows: nitriding temperature 530 ± 10 ° C, nitriding time 15 hours, ammonia gas decomposition rate 25% -30%. The sample number is 1-2.
Gas soft nitriding uses ammonia gas and formamide as co-infiltration medium. The nitriding process is as follows: nitriding temperature is 570±10°C, ammonia gas decomposition rate is controlled at 25%-30% during holding period, and formamide dosage is 70-80. Drip/min, holding time 3-4 hours; ammonia gas decomposition rate during purification period is 65-80%, time 1 hour. The sample number is 3-4.
4.2, test data and analysis
Figure 1 is a comparison of the hardness gradient of the nitrided layer of the obtained sample. It can be seen from the above that the surface hardness and hardness gradient curves of the four specimens are similar, indicating that the hardness of the nitrided layer of H13 die steel after gas hard nitriding and gas soft nitriding is not much different. The hardness of the surface layer reaches 1000-1200 HV, which can meet the needs of hot extrusion of aluminum profiles. The steeper characteristics of the gas soft nitriding hardness curve are not shown, which may be the reason why the measured depth of the layer is not enough.
Fig. 2 to Fig. 5 are the metallographic micrographs of the white bright layer after nitriding of the sample No. 1-4, and it can be found that the thickness difference of the white bright layer of the four samples is small.
After H13 die steel gas hard nitriding and gas soft nitriding, the hardness and white bright layer have a small difference, which can better adapt to the working environment of aluminum profile hot extrusion. In comparison, gas nitrocarburizing is more efficient, but the process is relatively complex and requires more operators. Aluminum profile manufacturers should choose the nitriding method suitable for the company according to their own needs and actual conditions.