Analysis of heat treatment technology development and heat treatment industry market (5) Vacuum acetylene carburizing technology: Vacuum carburizing has been used in industry for nearly 25 years. Originally, propane was used as carburizing agent, but it was greatly restricted. When the temperature is higher than 600 ° C, propane is easily decomposed into carbon, hydrogen and methane, which will produce carbon black on the surface of the part and cause tar in the furnace cavity and pump tube, which increases the maintenance difficulty of the furnace.

Later, acetylene vacuum carburizing, or low pressure carburizing, was developed. The decomposition of acetylene gives a very high carbon potential: C2H2 = 2C + H2, which gives a uniform carburized layer for densely packed and elongated blind hole parts. Acetylene decomposes two free carbon atoms to participate in carburization, which is more capable of carburizing than other hydrocarbon atmospheres. The carbon atoms decomposed by acetylene can achieve uniform carburization under very low pressure (10-1000 Pa or 0.1-10 mbar), that is, only a small amount of raw material gas is needed, and carbon black formation is greatly reduced. By integrating the area of ​​the carbon concentration distribution curve, the carbon flux of carburizing for 10 min at 1000 °C and 10 mbar can be obtained, and the C2H2 can reach 150 g/m2. h, and 2 g/m2 for CH4, C3H8 and C2H4, respectively? h, 120 g/m2? h and 130 g/m2? h. The comparison of the depths of the three layers of the process under the same carburizing conditions is shown in Table 2 below. The superiority of low pressure carburization using acetylene is very obvious. Moreover, the uniformity of carburization is significantly higher than that of propane. For the blind holes of 3×90 mm deep, the penetration depths of the low-pressure carburized holes of acetylene and propane were 90 mm and 27 mm, respectively, and the two results of measuring the hardness and the effective hardened layer depth were similar.

Table 2 Comparison of the depth of the three layers of the same process under the same carburizing conditions

The main advantages of acetylene vacuum carburizing are: (1) high carbon transfer rate; (2) shortened treatment cycle; (3) no carbon deposit and tar, reducing furnace maintenance; (4) uniform carburizing and high quality carburizing; (5) furnace can be used to close the workpiece, Improve productivity; (6) overcome the oxidative defects in the workpiece (hard to overcome in a controlled atmosphere furnace); (7) clean and bright air-quenched parts; (8) improve working environment and pollution. Controllable high carbon monoxide gas carburizing: This method has been well developed and developed in Japan since 1999. It can shorten the carburizing time by 10 to 40%. Under certain conditions, it is equivalent to vacuum carburizing, and it has attracted more and more people's attention ("Mechanical Workers (Hot Processing)"), which has not been reported in China.

For the carburizing atmosphere containing CO and H 2 as the main carbon transport, the main application of CO adsorption and oxygen desorption reaction is explained. Because oxygen is involved in the reaction, oxide forming elements contained in the steel are generated. Oxidation problems of (Si, Cr, Mn, etc.), thus causing internal oxidation of steel (as mentioned above, can only be avoided under vacuum carburizing, it is generally considered that the internal oxidation depth is less than 3 μm, which has little effect on the performance of the workpiece. ).

High carbon monoxide (CO) gas carburization is carried out under atmospheric pressure. With the increase of CO amount, the carbon transfer rate is significantly increased and the carburization time is shortened under the high carbon potential CP. This method is called URX method. The hot atmosphere is referred to as RX method). For this method, the carburization reaction is: 2CO = [C] + CO 2

When the reaction reaches equilibrium, the upper equilibrium equilibrium constant is where ac is the activity of carbon in austenite, fc is the activity coefficient, [%C] is the carbon concentration in the surface of the steel at equilibrium, Pco2 and Pco It is the partial pressure of CO2 and CO in the furnace gas.

One can easily understand that the CO concentration increases (that is, the Pco increases), the contact frequency of CO with the surface of the steel increases, and the adsorption of CO on the surface of the steel increases, which is beneficial to carburization. The faster the carbon content on the surface of the steel increases, but the actual In the operation, it is also necessary to prevent the precipitation of carbon black. The results of effective carburization depths of URX (CP1.35) and RX (CP1.00) applied to r18×50 mm samples of SCr420H (corresponding to China's 20CrH steel) at 950 °C are shown in Table 3 below. The corresponding URX and RX furnace gas compositions are: CO 47.00%, CO2 0.40%, H2 51.20% and CH41.30%; CO 23.40%, CO2 0.14%, H2 33.80%, CH4 0.32% and the balance N2. Table 3 shows that the carburization time can be shortened by about 40% when the effective layer is 0.5 to 0.9 mm. Generally, it should be pointed out that the carbon content of the steel is reduced, the shortening rate is increased, and when the depth of the layer is increased, the shortening rate is lowered.

Table 3 Carburizing time and time reduction rate in URX and RX atmospheres

Effective hardening depth

0.5 mm

0.7 mm

0.9 mm

URX CP 1.35

62 points

113 points

180 points

RX CP 1.0

106 points

190 points

300 points

Shortening rate %




High CO gas carburizing equipment: Adding a high CO furnace in front of a general continuous or periodic furnace. This high CO atmosphere generally only passes into the carburizing furnace section on the continuous furnace, and the other furnace sections still use an endothermic atmosphere. . Regarding the RX atmosphere, the principle of the high CO gas generator can be further explained. Corresponding equipment features: (1) low power consumption; (2) simple adjustment of CO concentration; (3) stable atmosphere composition. The test furnace of Japan's Sino-foreign Furnace Technology Research Institute can provide user test results as required. Solid solution nitriding of stainless steel:

The stainless steel is nitrided by a pure nitrogen gas with a pressure of PN2 in a vacuum furnace at 1050 to 1150 ° C to obtain a nitriding layer of 2.5 mm deep, which also has excellent corrosion resistance, which Ipsen calls solid solution. Nitriding treatment, labeled SOLNIT. There are two kinds of SOLNIT-M and SOLNIT-A. The former refers to nitriding low-carbon martensitic stainless steel, and obtains a high-hardness martensite surface layer (about 0.5% N) by cooling above the critical cooling rate; Nitriding of austenitic or two-phase stainless steel results in a high-strength austenitic surface layer (about 0.9% N). Stainless steel nitriding can generally be carried out by using ion nitriding and salt bath nitriding (Sursulf or Tenifer) at about 500 to 600 ° C. However, because of the precipitation of nitride, Cr is depleted, resulting in a decrease in corrosion resistance.

The principle of solid solution seepage is to consider the balance of steel nitriding surface, nitrogen diffusion and precipitation of brittle nitride. This paper has been translated and published in Mechanical Workers (Hot Processing) 2005.

Process parameters of solid solution nitriding treatment: temperature 1050 ~ 1150 ° C, time can be up to 24h, nitrogen partial pressure PN2 = 0.01 ~ 0.3MPa (0.1 ~ 3bar). The depth of the nitrided layer can be up to 2.5 mm after treatment. Cooling after solid solution nitriding: using liquid quenching agent or high pressure gas quenching. Note that because the nitriding temperature is higher, it will cause the core and the layer to grow. Generally, for martensite, the second phase precipitation can prevent grain growth; the austenite nitride layer is refined by cold working (such as shot peening) after nitriding.

Previous Next

Steel Pressed Spool

Steel Pressed Spool,High Quality Steel Pressed Spool,Steel Pressed Spool Details, Ningbo Beilun Tiaoyue Machine Co., Ltd.

plastic spools model Co., Ltd. ,

Posted on