CN1540029A

CN1540029A - Method for eliminating structure of internal oxidation in gas carbonitriding heat treatment for gear wheels

Info

Publication number: CN1540029A
Application number: CNA2003101108316A
Authority: CN
Inventors: 飚王; 王飚; 王宇栋; 张自华
Original assignee: Kunming University of Science and Technology
Current assignee: Kunming University of Science and Technology
Priority date: 2003-10-28
Filing date: 2003-10-28
Publication date: 2004-10-27
Anticipated expiration: 2023-10-28
Also published as: CN1234903C

Abstract

A method for eliminating the internal oxide structure in the gas-phase carbon-nitrogen osmosis of gear features that the CCL4 is added to the kensene as carburizer and then is decomposed to generate Cl2 for reducing the oxide, the ammonia gas as nitrizing agen passes through the cylinder of silica gel or granular calcium chloride for adsorbing residual water vapour and then through the cylinder of 5A molecular sieve for adsorbing residual O2, and the resistance wire is wound on igniting pipe at furnace top to prevent the NH4Cl in exhausted gas from being condensed.

Description

Internal oxidation structure eliminating method in gear gas carbonitriding heat treatment

The technical field is as follows:

the invention relates to a method for eliminating internal oxidation structure in gear gas carbonitriding heat treatment, belonging to the technical field of surface chemical heat treatment of metal materials

Background art:

after the gear or other machine parts are treated by gas CN co-permeation, the hardness of the surface layer is raised, and the abrasion resistance and fatigue life are greatly increased, but the gear made of alloy steel such as 20CrMnTi or 20CrMo, etc. contains residual water vapor or residual oxygen due to the permeation agent (ammonia gas, kerosene) entering the furnace or oxygen pressure (P) of the furnace atmosphere_CO2，P_H2O) Too high a content of Cr, Ti, Mn, etc. in the infiltrated layer will have affinity to oxygenHigh internal oxidation of the alloying elements, which causes the appearance of a brittle black band in the structure of the infiltrated layer, which, once present, causes the "weakening" of the structure of the CN co-infiltrated layer, reducing the wear resistance and fatigue-resistant continuous working life of the gear or other machine part, for example in the literature [1]]The test data provided (Table 1) show that internal oxidation, once present, has a detrimental effect on CN co-carburized gear bending fatigue life.

TABLE 1 Effect of internal Oxidation textures (Black Net and Black bands) on CN Co-penetration Gear bending fatigue Life

Number of black bands	Number of black screen levels	Average bending fatigue life	The service life is reduced by%
Number of black bands	Number of black screen levels	Average bending fatigue life	The service life is reduced by%	0	0	1.5×10⁵	0％
0～2	6～7	1.01×10⁵	51％	0	0	1.5×10⁵	0％
0～2	6～7	1.01×10⁵	51％	3～4	4～7	(0.5～0.39)×10⁵	67～74％

The test results in table 1 show that the occurrence of a small amount of internal oxidation structure has a significant harmful effect on the continuous working life of the CN co-carburized gear, so that the bending fatigue life of the gear is reduced by 51-74%.

In addition, at home and abroad, a method of grinding after co-infiltration is adopted toremove an infiltration layer with internal oxidation so as to eliminate the influence of an internal oxidation layer^[2、3]However, this method is not preferable because the internal oxide layer is ground off and the carburized layer is also ground off, so that the main part of the carburized layer is thinned, and the advantageous effects of the carburized layer are not fully exerted, and in addition, grinding after co-carburization increases the processing cost of gears and other co-carburized parts.

So far, the public reports of the same technology as the present invention are not found.

Reference to the literature

[1]Changchun institute of automotive research: "Heat treatment of metals", No. 7 in 1980

[2]"M i OM", No.12 in 1971 (Russian)

[3]"Met.Trans." (A). No.11 of 1979

[4]Zhanggen, Caizixin, etc.: "thermodynamic analysis of internal oxidation during carbonitriding", Heat treatment of metals, No.5 in 2001

The invention content is as follows:

the invention aims to overcome the defects of the prior art and provide a method for eliminating internal oxidation structures in the gear gas carbonitriding heat treatment.

Aiming at the reason that the internal oxidation structure appears in the gas CN co-permeation is caused by overhigh oxygen pressure in the atmosphere in the co-permeation furnace, the invention adopts the following measures to eliminate the internal oxidation structure:

(1) and the ammonia gas used for the gas CN co-permeation is filtered before entering the furnace as follows:

in the filtering treatment process, the silica gel and the calcium chloride mainly play a role in filtering water in the ammonia gas, and the 5A molecular sieve (purchased in the market) mainly plays a role in filtering residual oxygen in the ammonia gas;

(2) in order to completely prevent the occurrence of the internal oxidation structure, C, N co-doping is performed in a co-doping furnace, and additionally a reducing atmosphere is also performed to reduce the formed internal oxides of the alloying elements. For this purpose, a reducing agent CCl is added to the carburizing agent (kerosene)₄The addition amount is 6-8%, CCl is adopted₄The advantage of using as a reducing agent is that at the high temperature of the cementation furnace, CCl₄Not only active chlorine can be dissociated to reduce internal oxides, but also CCl₄The carbon dissociated from the above step can also be used as a carburizing agent. CCl₄When present, the chemical reactions that occur in the cementation furnace are as follows:

……(3)

……(5)

……(6)

……(7)

……(8)

if the oxygen pressure is too high during the co-cementation process, internal oxides of the alloying elements are formed as in reactions (1) and (2), then CCl is present₄And NH₃Presence of (C) CCl₄And NH₃The new reducing agent Cl is decomposed according to the reaction formulas (3), (4) and (5)₂And HCl, the internal oxide formed will be converted to chloride according to reactions (6) and (7), and the chloride is converted to the nitride of the alloy element according to reaction (8).

(3) 6 to 8 percent of CCl is added into kerosene₄In the meantime, even if the kerosene dropping port is separated from the ammonia gas inlet, part of NH which is not completely decomposed is remained at the temperature (860 to 880 ℃) in the furnace₃This remains NH₃Combining with HCl gas produced according to reaction (5) to produce NH₄Cl gas, NH formed₄Cl is gaseous at the high temperatures in the furnace, but as the co-penetrating exhaust gases leave the furnace chamber and enter the ignition tubes, part of the NH is produced as a result of the temperature drop₄The Cl gas condenses into solid powder, which blocks the squib (exhaust pipe). To overcome the problem, 5 circles of resistance wires sleeved with small ceramic rings are wound on the ignition tube part close to the furnace top (the ceramic rings are used for insulating the resistance wires from the ignition tube), and NH can be prevented by heating the ignition tube by the resistance wires₄The Cl gas condenses to a solid within the ignition tube.

Compared with the prior art, the method has the advantages of eliminating internal oxidation tissues in the gas CN co-permeation process on the premise of not increasing the co-permeation cost, along with convenient operation, higher co-permeation speed and the like.

Description of the drawings:

FIG. 1 is a diagram of the process and operational parameters for co-permeation of gear gas CN.

FIG. 2 is an observation (400 times magnification) of the absence of internal oxidation in a co-infiltrated layer obtained after CN co-infiltration according to the method of the invention in example 1.

FIG. 3 is an observation (400 times magnification) of a co-carburized layer of a 20CrMnTi gear obtained without CN co-carburization according to the present invention.

FIG. 4 is an observation (400 times magnification) of the absence of internal oxidation in the co-infiltrated layer obtained after CN co-infiltration according to the method of the invention in example 2.

The specific implementation mode is as follows:

example 1:

in a 30KW well-type gas carburizing furnace, a 20CrMnTi steel gear with the weight of 60 kilograms is loaded, the chemical treatment of gas CN co-carburization surface is carried out, in order to prevent the internal oxide of Cr, Mn and Ti alloy elements from being generated in the carburized layer, 8 percent of industrial pure CCl is added into the carburizing agent (kerosene)₄. The ammonia gas entering the shaft furnace firstly passes through a silica gel barrel (10 kg of silica gel is filled in) and granular CaCl₂Bucket (with industrial CaCl inside)₂8 kg) to remove water vapour from the ammoniagas. Then, the gas was filtered through a 5A molecular sieve drum (containing 8 kg of 5A molecular sieve) to remove residual oxygen from the gas. To prevent from being caused by CCl₄Is added to generate NH₄Cl condensation solid blocks the ignition tube, and 5 heating resistance wires sleeved with ceramic rings are wound on the ignition tube. And carrying out the co-permeation treatment according to a gas CN co-permeation process diagram shown in figure 1. The whole co-permeation process is divided into five processes of temperature rise, 1-hour exhaust, 4-hour co-permeation, 1-hour diffusion and temperature reduction soaking, and the temperature and kerosene (containing CCl) controlled in each process₄) And the amount of ammonia added are shown in FIG. 1. After the heat is uniformly heated, the temperature is reduced and the speed is increased, the ammonia gas is stopped to be fed, the furnace cover is opened,by hangingThe gear charging hopper is taken out of the car, immersed in mixed oil (50% diesel oil and 50% engine oil) for quenching, and tempered for 1.5 hours at 180 ℃ in a box furnace after quenching. Tempering, cutting from the tooth root after the gear is cooled, taking out a metallographic analysis sample, and carrying out analysis and test on the depth of a permeable layer and the internal oxidation condition, wherein the test result shows that the depth of the permeable layer reaches 0.4-0 and is 5mm, and no internal oxidation tissue is in the permeable layer as shown in figure 2. For comparison, operating according to the co-cementation process of FIG. 1, but without the method of the present invention, the results of observing the oxidation structure in the tooth root of the resulting co-carburized gear are shown in FIG. 3. from FIG. 3, it can be seen that in the 0.5mm CN co-carburized layer, the 0.1 to 0.15mm skin layer is filled with coherent oxidation structures (i.e., black bands and dots in FIG. 3).

Example 2:

in 30KW well type gas carburization, 80 kg of 20CrMo steel gear is loaded to carry out gas CN co-carburization operation, in order to prevent internal oxidation tissue in carburization layer of gear, 6% industrial CCl is added into carburizing agent₄The ammonia gas introduced into the furnace was passed through a silica gel tank and granular CaCl as in example 1₂The barrel is filtered to remove water vapor, and the gas is filtered by a 5A molecular sieve barrel to remove residual oxygen. To prevent the addition of CCl due to kerosene₄To generate NH₄Cl solid powder blocks an ignition tube, 5 resistance wires sleeved with ceramic rings are wound at the position of the ignition tube close to the upper part of the furnace top, and the heating power of the resistance wires is 1.5 KW. The CN co-permeation operation process is carried out according to the figure 1, namely, the temperature adopted by heating, exhausting, co-permeation, diffusion, soaking and temperature reduction, and kerosene (containing CCl)₄) The addition of ammonia gas and the quenching and tempering were performed in the same manner as in example 1, except that the time for exhausting was 1 hour, the time for co-infiltrating was 5 hours, and the time for diffusing was 1 hour. The obtained result is that the depth of the CN co-permeation layer reaches 0.5-0.6 mm, and no obvious internal oxidation tissue is shown in figure 4.

Claims

1. The method for eliminating the internal oxidation structure in the gear gas carbonitriding heat treatment is characterized by adopting the following measures:

1.1 adding 6 into gas carburizing agent keroseneCCl of 8% to₄CCl in a co-infiltration furnace₄Decomposition of nascent Cl₂Reducing the formed oxides in the alloy elements;

1.2 before the carburizing agent ammonia enters the co-carburizing furnace, filtering the carburizing agent ammonia through a silica gel bottle or barrel and a granular calcium chloride bottle or barrel to adsorb residual water vapor in the ammonia, and adsorbing residual oxygen in the ammonia through a 5A molecular sieve bottle or barrel;

1.3 winding a resistance wire with a ceramic ring around the ignition tube near the top of the co-infiltration furnace, heating by the auxiliary belt of the resistance wire to make NH in the exhaust gas₄The Cl does not condense and cake.