CN117144288B - Chemical heat treatment process for surface modified layer steel - Google Patents
Chemical heat treatment process for surface modified layer steel Download PDFInfo
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- CN117144288B CN117144288B CN202311378770.5A CN202311378770A CN117144288B CN 117144288 B CN117144288 B CN 117144288B CN 202311378770 A CN202311378770 A CN 202311378770A CN 117144288 B CN117144288 B CN 117144288B
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 50
- 239000010959 steel Substances 0.000 title claims abstract description 50
- 238000000034 method Methods 0.000 title claims abstract description 46
- 238000010438 heat treatment Methods 0.000 title claims abstract description 44
- 230000008569 process Effects 0.000 title claims abstract description 39
- 239000000126 substance Substances 0.000 title claims abstract description 17
- 239000010410 layer Substances 0.000 claims abstract description 81
- 238000005121 nitriding Methods 0.000 claims abstract description 38
- 238000004321 preservation Methods 0.000 claims abstract description 23
- 238000005496 tempering Methods 0.000 claims abstract description 22
- 238000005255 carburizing Methods 0.000 claims abstract description 21
- 238000010791 quenching Methods 0.000 claims abstract description 20
- 230000000171 quenching effect Effects 0.000 claims abstract description 19
- 229910001566 austenite Inorganic materials 0.000 claims abstract description 13
- 239000002344 surface layer Substances 0.000 claims abstract description 13
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 8
- 238000007254 oxidation reaction Methods 0.000 claims abstract description 6
- 230000003647 oxidation Effects 0.000 claims abstract description 5
- 238000001816 cooling Methods 0.000 claims description 24
- 238000009792 diffusion process Methods 0.000 claims description 11
- 230000001590 oxidative effect Effects 0.000 claims description 3
- 238000010792 warming Methods 0.000 claims description 3
- 229910052799 carbon Inorganic materials 0.000 abstract description 28
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 abstract description 22
- 230000007704 transition Effects 0.000 abstract description 4
- 239000000463 material Substances 0.000 description 23
- 230000035882 stress Effects 0.000 description 17
- 230000032683 aging Effects 0.000 description 9
- 239000011651 chromium Substances 0.000 description 8
- 239000011159 matrix material Substances 0.000 description 7
- 230000004048 modification Effects 0.000 description 7
- 238000012986 modification Methods 0.000 description 7
- 238000005336 cracking Methods 0.000 description 6
- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 5
- 238000005516 engineering process Methods 0.000 description 5
- 230000008595 infiltration Effects 0.000 description 5
- 238000001764 infiltration Methods 0.000 description 5
- 238000004140 cleaning Methods 0.000 description 4
- 230000007797 corrosion Effects 0.000 description 4
- 238000005260 corrosion Methods 0.000 description 4
- 230000000717 retained effect Effects 0.000 description 4
- 229910001567 cementite Inorganic materials 0.000 description 3
- 239000013078 crystal Substances 0.000 description 3
- 230000000694 effects Effects 0.000 description 3
- KSOKAHYVTMZFBJ-UHFFFAOYSA-N iron;methane Chemical compound C.[Fe].[Fe].[Fe] KSOKAHYVTMZFBJ-UHFFFAOYSA-N 0.000 description 3
- 229910052758 niobium Inorganic materials 0.000 description 3
- 125000004433 nitrogen atom Chemical group N* 0.000 description 3
- 238000010583 slow cooling Methods 0.000 description 3
- 229910052720 vanadium Inorganic materials 0.000 description 3
- FBPFZTCFMRRESA-JGWLITMVSA-N D-glucitol Chemical compound OC[C@H](O)[C@@H](O)[C@H](O)[C@H](O)CO FBPFZTCFMRRESA-JGWLITMVSA-N 0.000 description 2
- 229910045601 alloy Inorganic materials 0.000 description 2
- 239000000956 alloy Substances 0.000 description 2
- 230000008859 change Effects 0.000 description 2
- 230000007547 defect Effects 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 238000011161 development Methods 0.000 description 2
- 230000018109 developmental process Effects 0.000 description 2
- 238000009826 distribution Methods 0.000 description 2
- 239000012535 impurity Substances 0.000 description 2
- JEIPFZHSYJVQDO-UHFFFAOYSA-N iron(III) oxide Inorganic materials O=[Fe]O[Fe]=O JEIPFZHSYJVQDO-UHFFFAOYSA-N 0.000 description 2
- 239000000203 mixture Substances 0.000 description 2
- 229910052759 nickel Inorganic materials 0.000 description 2
- 230000008439 repair process Effects 0.000 description 2
- 239000010935 stainless steel Substances 0.000 description 2
- 229910001220 stainless steel Inorganic materials 0.000 description 2
- 229920001187 thermosetting polymer Polymers 0.000 description 2
- VYZAMTAEIAYCRO-UHFFFAOYSA-N Chromium Chemical compound [Cr] VYZAMTAEIAYCRO-UHFFFAOYSA-N 0.000 description 1
- 230000009471 action Effects 0.000 description 1
- 238000004026 adhesive bonding Methods 0.000 description 1
- 238000000137 annealing Methods 0.000 description 1
- 238000005452 bending Methods 0.000 description 1
- 230000009286 beneficial effect Effects 0.000 description 1
- 150000001721 carbon Chemical group 0.000 description 1
- 125000004432 carbon atom Chemical group C* 0.000 description 1
- 229910052804 chromium Inorganic materials 0.000 description 1
- 239000002131 composite material Substances 0.000 description 1
- 238000005261 decarburization Methods 0.000 description 1
- 230000006866 deterioration Effects 0.000 description 1
- 230000008030 elimination Effects 0.000 description 1
- 238000003379 elimination reaction Methods 0.000 description 1
- 229910052742 iron Inorganic materials 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000005012 migration Effects 0.000 description 1
- 238000013508 migration Methods 0.000 description 1
- 229910052750 molybdenum Inorganic materials 0.000 description 1
- 238000007709 nanocrystallization Methods 0.000 description 1
- 150000004767 nitrides Chemical class 0.000 description 1
- 238000009659 non-destructive testing Methods 0.000 description 1
- 239000002244 precipitate Substances 0.000 description 1
- 238000002360 preparation method Methods 0.000 description 1
- 230000002035 prolonged effect Effects 0.000 description 1
- 238000007670 refining Methods 0.000 description 1
- 230000001105 regulatory effect Effects 0.000 description 1
- 238000011160 research Methods 0.000 description 1
- 230000000630 rising effect Effects 0.000 description 1
- 238000004904 shortening Methods 0.000 description 1
- 238000009751 slip forming Methods 0.000 description 1
- 238000005728 strengthening Methods 0.000 description 1
- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
- 229910052721 tungsten Inorganic materials 0.000 description 1
- 239000010937 tungsten Substances 0.000 description 1
Classifications
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- C—CHEMISTRY; METALLURGY
- C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
- C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
- C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
- C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
- C23C8/34—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in more than one step
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
- C21D1/18—Hardening; Quenching with or without subsequent tempering
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
-
- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D6/00—Heat treatment of ferrous alloys
- C21D6/007—Heat treatment of ferrous alloys containing Co
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C38/00—Ferrous alloys, e.g. steel alloys
- C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
- C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
- C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P10/00—Technologies related to metal processing
- Y02P10/20—Recycling
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Physics & Mathematics (AREA)
- Thermal Sciences (AREA)
- Crystallography & Structural Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Solid-Phase Diffusion Into Metallic Material Surfaces (AREA)
Abstract
The invention discloses a chemical heat treatment process of surface modified layer steel, which comprises the following steps: s1, pre-oxidation treatment; s2, carburizing treatment, wherein a hardening layer with the depth range of 1.4 mm-2.5 mm is formed on the surface of the workpiece; s3, heating and quenching treatment; s4, sequentially performing cryogenic treatment and intermediate temperature tempering on the quenched workpiece, and repeating for a plurality of times to obtain an intermediate piece, wherein the intermediate temperature tempering is at 460-540 ℃, and the heat preservation time is 1-4 h; s5, nitriding the intermediate piece to convert the residual austenite in the hardening layer into stable martensite, so as to obtain a finished piece. Also discloses a surface modified layer steel and application thereof to precision workpieces. The invention realizes the stable transition from the surface layer of the workpiece to the carbon content of the core, and the hardness of the core is improved to a certain extent, so that the strength and hardness of the whole workpiece are improved, the internal stress is reduced, and the toughness is improved.
Description
Technical Field
The invention relates to the field of heat treatment processes of steel, in particular to a chemical heat treatment process of steel with a surface modified layer.
Background
The surface modified layer steel is a steel material subjected to surface working or heat treatment, and is characterized in that the surface layer of the steel material from the outer surface to a certain depth has properties different from those of the matrix, and the aspects and the influence degree included in the properties are different from those of the matrix according to the surface working or heat treatment process. The indexes of bending fatigue performance, contact fatigue performance, gluing performance and the like of the surface modified layer steel are closely related to the bearing capacity, service life and reliability of the formed workpiece.
Along with the development of research, the element composition of the steel is continuously optimized in the prior art, and the steel can realize good high-temperature performance and toughness through the combination of elements and the combination of surface chemical treatment, so that the development of the surface modification layer steel capable of bearing high Hertz stress, high fatigue strength, high temperature resistance and corrosion resistance is urgent.
The steel obtained by the heat treatment process in the prior art adopts a plurality of composite elements, namely tungsten element, chromium element, nickel element and the like to obtain a material with grain refinement, the heat treatment process for the formed workpiece comprises oil quenching at 1000-1150 ℃, cryogenic treatment at a temperature lower than-85 ℃, tempering treatment at 500-540 ℃, repeated cryogenic treatment and repeated 2-3 times and the like.
However, practice shows that when the steel is subjected to heat treatment, a strong infiltration and diffusion technology is adopted, carbon enriched on the surface of the steel is diffused to the center for a long time under pulse current, so that the carbon content of the outermost layer is reduced by about 5-10 mu m, martensite is not formed after quenching, retained austenite is reserved, and a softening layer appears on the surface within the range of 5-10 mu m; meanwhile, under the combined action of the carbon concentration and the cooling speed, the cooling speed is insufficient to ensure that martensite is completely transformed in the boundary area (namely, the depth range of 1.5 mm-2.5 mm) of the carburized layer and the matrix, so that more residual austenite is reserved in the area to form a soft point (concave point) in hardness. The occurrence of the softening layer and pits can lead to the peeling of a hardening layer of the workpiece in the service process, thereby seriously affecting the fatigue performance and the friction and wear performance and shortening the service time of the workpiece.
In order to eliminate soft spots, the prior art adopts a single aging annealing method, which can harden the core, but can lead to decarburization of the surface of the steel body, further deepen the softened layer of the outermost layer, further reduce the hardness and further deteriorate the effect of the surface layer as the aging time is longer.
In order to eliminate a softening layer on the surface of steel, the prior art can adopt surface nanocrystallization treatment to increase deformed grains on the surface of the steel for refining, and the technology can increase the hardness of the surface of a workpiece, but the material core is not supplemented in advance, so that the problem of soft spots cannot be solved.
Therefore, it is difficult to effectively solve the problems of both the inner layer soft spot and the surface softening layer through the heat treatment process in the prior art.
Disclosure of Invention
The invention aims to provide a chemical heat treatment process of surface modification layer steel, which aims to solve the technical problem that the softening layer defect and the soft point defect of the surface modification layer steel in the prior art cannot be eliminated at the same time.
In order to solve the technical problems, the invention specifically provides the following technical scheme:
the invention provides a chemical heat treatment process of surface modified layer steel, which comprises the following steps:
s1, pre-oxidizing treatment, namely enabling the surface of a workpiece with a compact oxide film to continuously form Fe oxide under the high-temperature condition so as to enable the compact oxide film to be loose and porous;
s2, carburizing treatment, wherein a hardening layer with the depth range of 1.4 mm-2.5 mm is formed on the surface of the workpiece;
s3, heating and quenching, namely directly heating the carburized workpiece from the carburization temperature to the austenitization temperature, and directly performing rapid cooling and quenching after heat preservation;
s4, sequentially performing cryogenic treatment and intermediate temperature tempering on the quenched workpiece, and repeating for a plurality of times to obtain an intermediate piece, wherein the intermediate temperature tempering is at 460-540 ℃, and the heat preservation time is 1-4 h;
s5, nitriding the intermediate piece to convert the austenite remained in the hardening layer into stable martensite, so as to obtain a finished piece.
Preferably, the temperature range of the pre-oxidation treatment is 800 ℃ to 950 ℃.
Preferably, the dense oxide film is mainly a Cr-containing dense oxide film.
Preferably, the carburizing treatment is pulse vacuum carburizing;
the specific conditions of the carburizing treatment are as follows: the temperature range is 930 ℃ to 980 ℃, and the heat preservation time is 10h to 40h.
Preferably, in the carburizing treatment process, the strong-permeability stage and the diffusion stage are sequentially repeated, and the time ratio of the strong-permeability stage to the diffusion stage is 1:2-1:3.
Preferably, the depth of the hardened layer of the carburization treatment is 1.4 mm-2.5 mm, and the depth of the hardened layer refers to the vertical distance from the surface of the part to the position with the Vickers hardness of 550Hv according to the national standard GB/T9450-2005.
Preferably, the carburized workpiece is directly heated from the carburization temperature to the austenitization temperature in a vacuum environment;
the austenitizing temperature is 1000-1150 ℃;
the requirements of the heat preservation time are as follows: and the internal and external temperatures of the workpiece are uniform, so that the workpiece is completely austenitized, and the end of the austenite grains is ensured not to grow up.
Preferably, the cryogenic treatment comprises the steps of:
the temperature of the workpiece is reduced to minus 120 ℃ to minus 190 ℃ at a cooling speed of 2.5-6.0 ℃/min, and after heat preservation is carried out for 2-4 hours, the workpiece is warmed to room temperature at a warming speed of 2.0-10.0 ℃/min.
Preferably, the nitriding treatment adopts vacuum nitriding, and the following conditions are satisfied:
nitriding depth is 0.2 mm-0.6 mm;
temperature range: 460-520 ℃;
the heat preservation time is as follows: 10-30 h.
Compared with the prior art, the invention has the following beneficial effects:
1. according to the invention, the surface of the workpiece is formed into the hardening layer with quite high thickness through carburization, after carburization, nitriding treatment is adopted to repair the workpiece, and long-time aging treatment is carried out on the center of the workpiece, so that carbide in the hardening layer is diffused into the workpiece, the surface hardening layer of the workpiece is prevented from falling off, the hardness of the workpiece is improved, and the surface hardness of the surface modified layer steel reaches 66-70 HRC; and more carbide is separated out from the core part of the workpiece, the hardness of the core part is further improved, the internal stress of the workpiece is reduced, the toughness of the material is improved, and meanwhile, a softening layer and a soft point are eliminated.
2. The method adopts the strong infiltration stage and the diffusion stage to be sequentially repeated, accurately controls the carbon potential, improves the depth of a hardening layer on the surface of steel, improves the hardness of the surface of a further formed workpiece, and simultaneously improves the friction and wear resistance and the thermosetting property of the surface modified layer steel.
3. According to the invention, a mode of direct heating and quenching in a vacuum furnace is adopted without slow cooling after carburization, and the workpiece with high internal stress after carburization is directly heated to austenitizing temperature for heat preservation and then quenched, so that the internal stress can be effectively relieved, and the workpiece is prevented from cracking; meanwhile, the process flow is simplified, and the cost is reduced.
4. The invention further limits the treatment process during heat treatment by medium-temperature tempering, so that the surface hardening layer of the surface modified layer steel is smoothly transited to the transition zone, thereby eliminating hardness pits of the material during the pretreatment process, improving the corrosion resistance and high temperature resistance of the surface modified layer steel, improving the wear resistance and prolonging the service life.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below. It will be apparent to those of ordinary skill in the art that the drawings in the following description are exemplary only and that other implementations can be obtained from the extensions of the drawings provided without inventive effort.
FIG. 1 is a schematic flow chart of a chemical heat treatment process for providing a surface modified layer steel according to the present invention;
FIG. 2 is a graph showing the comparison of core hardness to skin hardness between example 1 and the control group according to the present invention.
Detailed Description
The following description of the embodiments of the present invention will be made clearly and completely with reference to the accompanying drawings, in which it is apparent that the embodiments described are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
As shown in fig. 1, the present invention provides a chemical heat treatment process for surface modified steel, which aims at the problems that a softening layer exists on the surface of a material treated by the existing heat treatment process for surface modified steel and soft spots (i.e. hardness pits) exist in a certain depth, and comprises the following steps:
s1, pre-oxidizing treatment, namely enabling the surface of a workpiece with a compact oxide film to continuously form Fe oxide under the high-temperature condition so as to enable the compact oxide film to be loose and porous;
s2, carburizing treatment, wherein a hardening layer with the depth range of 1.4 mm-2.5 mm is formed on the surface of the workpiece;
s3, heating and quenching, namely directly heating the carburized workpiece from the carburization temperature to the austenitization temperature, and directly performing rapid cooling and quenching after heat preservation;
s4, sequentially performing cryogenic treatment and intermediate temperature tempering on the quenched workpiece, and repeating for a plurality of times to obtain an intermediate piece, wherein the intermediate temperature tempering is at 460-540 ℃, and the heat preservation time is 1-4 h;
s5, nitriding the intermediate piece to convert the austenite remained in the hardening layer into stable martensite, so as to obtain a finished piece.
The main technical problems to be solved by the invention are as follows: the original austenite structure is not completely transformed into martensite during the quenching and tempering processes, and more retained austenite is retained in the two partial areas, so that the hardness is not required, and soft spots and pits are formed.
The thinking of the invention to solve the technical problems is as follows: the surface of the workpiece is formed into a hardening layer with quite high thickness through carburizing treatment, and after the carburizing treatment, nitriding aging treatment is adopted to repair the workpiece, on one hand, nitriding process is equivalent to aging treatment on the workpiece for a long time, so that unstable retained austenite in hardness pits and a softening layer can be converted into martensite, carbide in the hardening layer can be diffused into the workpiece, more carbide is simultaneously separated out from the center of the workpiece, the hardness of the center is jointly improved by the two, the surface deterioration layer steel is smoothly transited from the surface hardening layer to the tissue of the center, the surface hardening layer of the workpiece is prevented from falling off, and the toughness and comprehensive mechanical property of the whole workpiece (from the surface layer to the center) are improved; on the other hand, nitriding can infiltrate nitrogen atoms into the surface layer softening region to form nitride with higher hardness, and the surface softening layer (5-10 mu m) is far smaller than the minimum nitriding depth (200 mu m). Therefore, the softening layer and the hardness pits can be eliminated at the same time no matter the thickness of the hardening layer and the nitriding thickness are out of the scope of the invention as long as the nitriding aging temperature and the nitriding time are reasonably controlled.
In addition, the cracking problem caused by severe lattice distortion and high internal stress caused by a large amount of precipitated carbide after carburization is avoided by the invention adopts the following two methods: the first method is to eliminate stress through heat treatment, directly heat to austenitizing temperature after carburizing treatment, and the heat preservation time is required to be uniform to the inner and outer temperature of the workpiece so as to ensure that the workpiece is completely austenitized, ensure that austenite grains do not grow up, and directly cool and quench after heat preservation, wherein carbide caused by carburization and carbon atoms in lattice gaps are completely dissolved into austenite lattices, and the inner grains are rearranged so as to thoroughly eliminate internal stress. In the prior art, the workpiece is prevented from cracking by slow cooling along with the furnace, and the reticular carbide is easy to separate out along the grain boundary in the slow cooling process, which is an important origin for generating cracks; the second method for eliminating internal stress is a cooling method, in which after the work is subjected to a heating quenching treatment, it is necessary to immediately perform a deep cooling treatment, and internal stress caused by lattice distortion can be eliminated by rearranging internal grains by rapidly cooling the work.
In the deep cooling treatment, the deep cooling part is generally cooled to room temperature and then discharged from the deep cooling treatment equipment, and the deep cooling part can also be directly discharged from the deep cooling equipment according to the workpiece requirement and naturally cooled in the air. The post-treatment of the cryogenic part should remove the condensed moisture on the surface of the workpiece, and in addition, it is notable that the cryogenic part should be tempered in time, and the tempering process and parameters are determined according to the performance requirements of the workpiece.
Preferably, the cryogenic treatment comprises the steps of reducing the temperature of a workpiece to below 100 ℃ below zero, preserving heat, recovering to room temperature after heat preservation, wherein different cooling and heating speeds can influence the elimination of internal stress of the workpiece, the internal non-uniformity of a material is easily caused by the too fast temperature change, the cracking of the material is caused, the internal stress in the material is difficult to remove due to the too slow temperature change, so that the cooling speed of the cryogenic treatment is limited to be 2.5-6.0 ℃/min, the temperature of the workpiece is reduced to-120 ℃ to-190 ℃, cooling is stopped, and the temperature is raised after heat preservation for 2-4 hours. The temperature rising speed is limited to be 2.0-10.0 ℃/min, so that the workpiece is warmed to the room temperature.
The specific temperature and the heat preservation time of the cooling are related to the material and the size of the workpiece, and can be adjusted according to actual production indexes.
In addition, the pre-oxidation treatment is a necessary condition for successful carburizing treatment. Aiming at the problem that a compact oxide film containing Cr influences the carburization effect, the temperature range of pre-oxidation treatment is 800-950 ℃, and Fe oxide is continuously formed on the surface of a workpiece with the compact oxide film so as to loosen the compact oxide film, thereby facilitating the infiltration of carbon elements during carburization treatment.
In order to improve the carburization efficiency, shorten the carburization time and meet the workpiece carburization requirements of different carburized layers, the effective carburized layer thickness of the carburization treatment is 1.4-2.5 mm; the specific conditions of the carburizing treatment are as follows: the temperature range is 930 ℃ to 980 ℃, and the heat preservation time is 10h to 40h. In the carburizing treatment process, the strong infiltration stage and the diffusion stage are sequentially repeated, the time ratio of the strong infiltration stage to the diffusion stage is 1:2-1:3, and the carbon enriched on the surface is diffused to the core for a long time under the pulse current, so that the main purpose is to improve the depth of a hardening layer on the surface of steel and the hardness of the surface of a further formed workpiece, and further improve the friction and wear resistance and the hot hardness of the surface modified steel.
Because the material is infiltrated with carbon with very high concentration, a great amount of carbon is infiltrated on the surface of the material to generate serious lattice distortion, so that great internal stress is caused, and the material is easy to crack. If the tempering temperature is low, the energy is insufficient to reduce the internal stress and stabilize the structure, if the tempering temperature is high, carbon in more matrixes is easily separated out from the crystal boundary to cause growth of carbide, and the carbide is connected into net-shaped carbide at the crystal boundary to cause crystal edge cracking.
Therefore, in the invention, the temperature of the medium temperature tempering is strictly controlled at 460-540 ℃ for 1-4 h, the overall (from the surface to the center) hardness is improved, and the cracking of the material is prevented. Meanwhile, after quenching, the tempering further improves the overall (from the surface to the core) hardness, improves the toughness and improves the comprehensive mechanical property.
The quenching process is repeated for a plurality of times, namely, the deep cooling treatment and the medium temperature tempering process are sequentially carried out on the quenched workpiece, and the quenching process has very important effect on solving the problems of a softening layer and pits, and is the same as the nitriding treatment principle.
Nitriding treatment technology is also mentioned in the prior art, but in order to improve the hardness of the surface of a material, carburization treatment is firstly carried out, and after high-concentration carburization, the existing nitriding technology is not easy to infiltrate, because carbon on the surface layer can obstruct diffusion of nitrogen atoms, the depth and the concentration of nitriding are improved on the basis of the existing nitriding technology, and nitriding parameters are regulated so that N atoms can infiltrate.
In order to solve the problem of carbon atom obstruction, the nitriding treatment adopts vacuum nitriding, and the following conditions are satisfied: nitriding depth is 0.2 mm-0.6 mm; the temperature is 460-520 ℃.
Preferably, the heat preservation time of nitriding treatment is 10-30 hours.
As is clear from the above, one of nitriding aging treatment is a process of converting a high-energy unstable state into a low-energy stable state, and after a workpiece is subjected to nitriding heating for a long time, the high-concentration carbon in the hardened layer is high due to severe lattice deformation energy, and at this time, the nitriding heating can lead to a driving force for migration, so that the carbon in the hardened layer can diffuse to a place with low carbon concentration in the center, and the deformation energy is reduced, and the state is more stable. The carbon and alloy elements originally exist in the matrix at the center, the matrix is continuously heated to obtain driving force, and the alloy elements such as C, mo, cr and the like are separated out from the matrix, so that the distortion of the matrix can be reduced, the energy of the whole workpiece is reduced, and the workpiece is in a stable state.
The method not only can improve the hardness of the surface layer of the material, but also can reduce the carbon content of the core, is economical and practical, saves the cost and is easy to realize.
The invention also generally includes the additional steps of:
1) Carrying out preliminary heat treatment before carburizing treatment, wherein gears with higher distortion control requirements are preferably tempered, the tempered structure is tempered sorbite, and cementite is spherulites and the structure is uniform and fine;
2) Preparing a workpiece after preliminary heat treatment, ensuring the surface of the workpiece to be clean before carburization, removing oxide scales, rust spots, oil scales and stains, performing nondestructive detection, ensuring no crack before carburization, and adopting necessary anti-seepage protection measures at anti-seepage parts;
3) The deep cooling treatment and the medium temperature tempering are repeated for two times, the times of the deep cooling and the tempering can be determined according to the shape and the size of the workpiece, generally 2-4 times, the tempering can stabilize the structure, eliminate the internal stress, and simultaneously precipitate carbide to play a role in strengthening;
4) And cleaning and detecting the surface of the workpiece before nitriding treatment, namely timely cleaning oil stains on the surface of the workpiece, detecting the hardness distribution and the surface carbon content from the surface to the center of the workpiece, and obtaining that the mass fraction of the carbon on the surface of the workpiece is within the range of 0.7% -1.3%, namely meeting the requirements.
Specific example 1 is provided below:
1. selecting materials:
the Cr-containing stainless steel workpiece comprises the following chemical components of C, cr-containing, mo, V, ni, co and Nb elements and incidental impurity elements, wherein the content of the C element is 0.16%, the content of the Cr element is 13.5%, the content of the Mo element is 4.5%, the content of the Co element is 13.0%, the content of the Ni element is 2.2%, the content of the V element is 0.58% and the content of the Nb element is 0.03%.
2. Heat treatment process
1. Preparation
The gear workpiece with higher distortion control requirement is firstly quenched and tempered, the tempered structure is tempered sorbite, cementite is prepared, the cementite is spherulites, and the structure is uniform and fine;
before carburization, the surface of the workpiece is ensured to be clean, and oxide skin, rust spots, oil dirt and stains are removed;
nondestructive testing is carried out to ensure no crack before carburization;
and adopting anti-seepage protection measures at the anti-seepage parts.
2. Pre-oxidation treatment
Pre-oxidizing the workpiece at 850 ℃ to continuously form Fe oxide on the surface of the workpiece with the compact oxide film under the high-temperature condition so as to make the compact oxide film loose and porous, wherein the compact oxide film is mainly a compact oxide film containing Cr;
3. carburizing
Performing pulse vacuum carburization on a workpiece, wherein in the carburization treatment process, the carburization temperature is 960 ℃, and the carburization time is 28h;
the strong cementation stage and the diffusion stage are sequentially repeated, the time ratio of the strong cementation stage to the diffusion stage is 1:2, the surface of the workpiece is gradually hardened, and the effective cementation layer thickness of the obtained carburization treatment is 1.5mm.
4. Quenching by heating
In a vacuum furnace, the carburized workpiece is directly heated from the carburization temperature to the austenitization temperature, namely 1080 ℃, and is kept until the internal and external temperatures of the workpiece are uniform, and then the workpiece is directly subjected to rapid cooling quenching treatment by quenching oil.
5. Cryogenic treatment
After quenching is finished, the temperature of the workpiece is reduced to-120 ℃ at a cooling speed of 4 ℃/min, and the workpiece is kept for 3 hours;
the workpiece was warmed to room temperature (25 ℃) at a warming rate of 6 ℃/min.
6. Intermediate temperature tempering
And (3) heating the workpiece subjected to the cryogenic treatment to 480 ℃ again, preserving heat for 2.5 hours, repeating the step 5 and the step 6 for three times to obtain an intermediate piece, and carrying out the next step.
8. Cleaning and detection
Cleaning the surface greasy dirt of the intermediate piece in time, detecting the hardness distribution and the surface carbon content of the intermediate piece from the surface to the core, wherein the mass fraction of the obtained intermediate piece surface carbon is 1.2%, and the requirement is met.
9. Nitriding treatment
And (3) carrying out vacuum nitriding treatment on the intermediate piece at the temperature of 490 ℃, preserving heat for 35 hours, and carrying out nitriding thickness of 0.5mm to finally obtain the steel, namely the surface modified layer steel.
Detecting the element content in the surface modification layer steel, wherein the surface modification layer steel at least comprises the following components in percentage by mass:
0.13% C, 13.3% Cr-containing, 4.3% Mo, 0.58% V, 2.1% Ni, 13.0% Co, 0.025% Nb.
Control groups are provided below:
1. selecting materials:
the Cr-containing stainless steel workpiece comprises the following chemical components of C, cr-containing, mo, V, ni, co and Nb elements and incidental impurity elements, wherein the content of the C element is 0.16%, the content of the Cr element is 13.5%, the content of the Mo element is 4.5%, the content of the Co element is 13.0%, the content of the Ni element is 2.2%, the content of the V element is 0.58% and the content of the Nb element is 0.03%. The materials were chosen as in example 1.
2. Heat treatment process
Nitriding treatment was performed in the same manner as in example 1.
Conclusion: according to the comparison of example 1 according to the present invention with the control group, i.e., as shown in fig. 2, the arrows on fig. 2 indicate that the control group contains a softening layer, and the hardness pits contained in the control group are marked in the rectangular frame. Whereas in example 1 no softening layer and no hard pits were seen.
After the workpiece is carburized to obtain a carbonized layer, the carbonized layer is optimized by nitriding, so that the depth of the carbonized layer can be deepened, the smooth transition from the surface layer to the core carbon content is realized, meanwhile, carbon elements in the core are separated out towards the carbonized layer, the hardness of the core is improved to a certain extent, the strength and hardness of the whole workpiece are improved, the internal stress is reduced, and the toughness is also improved.
According to the nitriding aging treatment disclosed by the invention, carburizing treatment is optimized, and long-time aging treatment is performed on the center of the workpiece, so that carbide in a hardening layer is diffused into the workpiece, the surface hardening layer of the workpiece is prevented from falling off, the hardness of the workpiece is improved, and the surface hardness of the surface modified layer steel reaches 66-70 HRC; and more carbide is separated out from the core part of the workpiece, so that the hardness of the core part is further improved, the internal stress of the workpiece is reduced, the toughness of the material is improved, and meanwhile, a softening layer and hardness pits possibly generated in the material are eliminated.
The invention optimizes the element composition of the workpiece, the heat treatment step, the temperature and the treatment time, eliminates the internal stress of the workpiece due to carburization treatment, improves the depth of a hardening layer on the surface of steel, improves the hardness of the surface of a further formed workpiece, and simultaneously improves the friction and wear resistance and the thermosetting property of the surface modified layer steel;
the invention precisely controls the temperature and time of medium temperature tempering, and further limits the treatment process during heat treatment, so that the surface hardening layer of the surface modified layer steel is smoothly and excessively transferred to the transition zone, thereby eliminating hardness pits of the material during the pretreatment process, improving the corrosion resistance and high temperature resistance of the surface modified layer steel, improving the wear resistance and prolonging the service life.
According to the above, the surface modified layer steel obtained by the regulation and control of the surface chemical heat treatment methods such as carburization, nitridation and the like has a tiny, uniform and excessive structure from the surface layer to the core, so that the surface modified layer steel has the advantages of super-strong and super-hard mechanical properties and no soft spots, the high temperature resistance and corrosion resistance are correspondingly improved, the good application of the surface modified layer steel in high-speed heavy load and high-temperature environments is realized, the wear resistance is improved, and the service life is prolonged.
The above embodiments are only exemplary embodiments of the present application and are not intended to limit the present application, the scope of which is defined by the claims. Various modifications and equivalent arrangements may be made to the present application by those skilled in the art, which modifications and equivalents are also considered to be within the scope of the present application.
Claims (5)
1. The chemical heat treatment process of the surface modified layer steel is characterized by comprising the following steps of:
s1, pre-oxidizing treatment, namely enabling the surface of a workpiece with a compact oxide film to continuously form Fe oxide under the high-temperature condition so as to enable the compact oxide film to be loose and porous;
s2, carburizing treatment, wherein a hardening layer with the depth range of 1.4 mm-2.5 mm is formed on the surface of the workpiece;
s3, heating and quenching, namely directly heating the carburized workpiece from the carburization temperature to the austenitization temperature, and directly performing rapid cooling and quenching after heat preservation;
the austenitizing temperature is 1000-1150 ℃;
the carburizing treatment is pulse vacuum carburizing;
the specific conditions of the carburizing treatment are as follows: the temperature range is 930 ℃ to 980 ℃, and the heat preservation time is 10h to 40h;
the requirements of the heat preservation time are as follows: the internal and external temperatures of the workpiece are uniform, so that the workpiece is completely austenitized, and the end of the austenite grains which do not grow up is ensured;
s4, sequentially performing cryogenic treatment and intermediate temperature tempering on the quenched workpiece, and repeating for a plurality of times to obtain an intermediate piece, wherein the intermediate temperature tempering is at 460-540 ℃, and the heat preservation time is 1-4 h;
s5, nitriding the intermediate piece to convert residual austenite in the hardening layer into stable martensite so as to obtain a finished piece;
wherein, the nitriding treatment adopts vacuum nitriding, satisfies the following conditions:
nitriding depth is 0.2 mm-0.6 mm;
temperature range: 460-520 ℃;
the heat preservation time is as follows: 10-30 h.
2. A chemical heat treatment process for a steel having a modified surface layer according to claim 1, wherein,
the temperature range of the pre-oxidation treatment is 800-950 ℃.
3. A chemical heat treatment process for a steel having a modified surface layer according to claim 1, wherein,
the dense oxide film is mainly a dense oxide film containing Cr.
4. A chemical heat treatment process for a steel having a modified surface layer according to claim 1, wherein,
in the carburizing treatment process, a strong-permeability stage and a diffusion stage are sequentially repeated, and the time ratio of the strong-permeability stage to the diffusion stage is 1:2-1:3.
5. A chemical heat treatment process for a steel having a modified surface layer according to claim 1, wherein,
the cryogenic treatment comprises the following steps:
the temperature of the workpiece is reduced to minus 120 ℃ to minus 190 ℃ at a cooling speed of 2.5-6.0 ℃/min, and after heat preservation is carried out for 2-4 hours, the workpiece is warmed to room temperature at a warming speed of 2.0-10.0 ℃/min.
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