Nitriding is a chemical heat treatment process to infiltrate nitrogen atoms into the surface of the workpiece at a certain temperature in a certain temperature. The process of nitriding is mainly to use the nitriding furnace, put the workpiece into the material frame, and then use the crane to hoist the workpiece to be heated at the bottom of the furnace. Also can be made to strengthen mouth bearing design nitriding furnace, by lifting and hanging tool vertical hanging workpiece heating. The cover of the nitriding furnace is usually driven by the motor reducer, and the electric automatically rises and falls. After the cover of the furnace is closed, the bolt is also pressed to ensure the vacuum sealing of the nitriding furnace. The vacuum is first pumped and then nitrogen is added to the nitriding heat treatment.
Cast stainless steel is used for gas damper units in the material, is based on the chemical composition of foreign materials were successfully 10X14H5M2 a material localization. The dampers require surface nitriding and nitriding of ZG10Cr14Ni5Mo2 steel at present is the first production in China. In order to master the nitriding characteristics of the steel, the process tests and physical and chemical tests at different temperatures and times were carried out. A lot of data were obtained, and the best nitriding process parameters were worked out.
ZG10Cr14Ni5Mo2 steel has high chromium content. When the steel is contacted with air, it will form a dense protective oxide film (passivation film) on the surface of steel, which impede the penetration of nitrogen atoms. Therefore, the removal of passivation film is one of the key factors for the nitriding of this steel. At present, domestic and foreign to the removal of the passivation film of stainless steel surface produced by nitriding has done a lot of research work, the pill method has advantages of simple operation, the operator of the personal injury of small, low cost, nitriding quality and stable quality of products, is the most suitable methods on the new international.
Experimental material: ZG10Cr14Ni5Mo2 steel; nitriding technology requirements: Nitriding depth 0.30~0.50mm, the surface hardness is greater than or equal to 700HV. The test materials for casting, chemical composition of the master alloy (mass fraction): C (0.12-0.16%); Si = 0.75%; Mn = 0.9%; Cr (13.0-15.0%); Ni (4.5-5.5%); Mo (2.3-2.8%); S = 0.025%; P = 0.025%, the sample size is 10mm * 10mm * 50mm.
Sample blank before nitriding heat treatment annealing: the first 780 DEG +/-10 DEG 6~6.5h, insulation and air cooling; second annealing 650 DEG +/-10 DEG, insulation 4~4.5h air cooling; normalizing 1120 DEG +/-10 DEG 3~3.5h, insulation and air cooling; at -70 deg. 2H air cooling cold treatment; tempering 620 DEG +/-20 DEG, air cooling insulation 2h.
The gas nitriding furnace with a power of 75kW was used for nitriding of samples at 560, 580 and 600 degrees centigrade. The time of nitriding are 30h, 36h, 42h, 48h, 52h, 58h, 64H. Taking ammonia as nitriding agent, ammonia decomposition rate of 40~60%, and adding ammonium chloride as catalyst; Determination of nitriding depth by the metallographic method. Metallography is to determine the depth of the nitrided layer with nitride layer microstructure and core microstructure corrosion resistance of different. The nitriding sample was eroded with 5% CuSO4 solution; the depth of the nitriding layer was measured by an electron microscope, and the hardness of the nitriding layer was measured by a Vivtorinox hardness tester.
When nitriding, the amount of ammonium chloride has a great influence on the surface hardness and depth of the nitriding layer. The excessive amount of workpiece surface pitting corrosion, nitriding tank, pipeline blocking; the amount is too small, in addition to passivation effect is not good, the nitride layer will thin and uneven.
Through the experiment, the following results are summarized.
(1) the nitriding process parameters of ZG10Cr14Ni5Mo2 steel are as follows: the nitriding temperature is 600 C and +/-5 C, the heat preservation time is 48~60h, and the decomposition rate of ammonia is 40~60%. The ammonia decomposition rate is regulated by the flow of ammonia.
(2) the gas nitriding of ZG10Cr14Ni5Mo2 steel, the depth and holding time of the nitriding layer conforms to the parabolic rule of the diffusion controlled.
(3) there is a clear boundary between the nitriding layer and the matrix of ZG10Cr14Ni5Mo2 steel, and there is a clear transition layer.
Nitriding can also be divided into soft nitriding, which is often referred to as nitrocarburizing. This is not a single carburizing or nitriding. It is a surface treatment technology that mainly infiltrate N and has C permeation. The temperature of soft nitriding is lower than that of carburizing, but it is higher than nitriding, usually about 570 degrees. Besides, it needs to reheat and quench after carburizing, but directly quenching after heat preservation, which saves a heating process. The depth of hardened layer of soft nitriding treatment is equivalent to carburizing, but higher than nitridation. This method has the advantages of the two, which is better than a single process.
Nitriding furnace is used for the heat treatment of gas nitriding in all kinds of steel mechanical parts, automobile glaze, gear, motorcycle brake pad, mould and so on. In the nonferrous metals industry, can be applied to copper, copper, copper, copper and copper in the protective atmosphere of bright annealing without oxidation. It is also used in the vacuum nitrogen free heat treatment process for the vacuum nitrogen protection of seamless thin walled steel pipe, long shaft and various metal parts.