Application of NM treatment to eliminate surface hypereutectoid carbide rickets
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Using the NM treatment process, we have produced hundreds of thousands of shearing blades and a small amount of lime beater hammers since 1974. Some units in Inner Mongolia are used for military heavy-duty gears and certain aerospace product parts, and they have achieved satisfactory results.
Cutting machine
2N.M. processing to eliminate K segregation of shearing blades
In the early 1960s, China began to produce shears. At that time, the sheep were mostly grazing in desert or semi-desert grasslands. The amount of sand contained in the wool was extremely high. A pair of blades each 2-3 sheep must remove the sharpening knife, each pair of blades cut tens of sheep on the wear and tear, severely affected the promotion of mechanized shearing. All factories have used domestically produced T12A, Cr06, and Soviet Union X05 (w% C1.25—1.40, Cr0.4—0.6) steels are not effective. In 1968, the former Ministry of Agricultural Machinery uniformly imported Japanese SKS8 (w%C1.30—1.50, Cr0.20—0.50, W2.0—2.5) steel for use by each plant, and no improvement was seen.
From 1974 to l978, the author participated in the quality control of the shearing blade and conducted a comprehensive analysis of the blade material. He found that the raw materials of SKS8, Cr06, and X05 all contained excessive carbon, and the metallographic structure was thick and severely meshed and banded. (See Figure 1a, b heart). Fennep, a famous former Soviet Union tool steel expert, pointed out that when the carbon content in carbon steel and low-combination steel is >1.20%, K segregation almost inevitably occurs [1]. In the research, we successfully developed the domestic blade steel and new heat treatment process together with the metallurgical department and completed the task of overtaking the standard in advance [2]. In the face of a large number of SKS8 and Cr06 steels that were overstocked in various plants, many tests were also conducted. New heat treatment processes for eliminating K defects were found and these steels were reused.
2.1 High temperature normalizing
The blade manufacturing plant used these conventional steels for high-temperature normalizing to eliminate K segregation. Since the carbon content of SKS8 and Cr06 steel is ≥1.30%, Acm≥950°C, the normalizing test starts from 950°C×30min (salt bath heating). The upper blade blank is 1.5mm cold rolled steel strip and the lower blade blank. 4.5mm hot rolled strip). The results showed that K could not completely dissolve even at 240°C for 240 min. Later used 1070 °C × 30min air-cooled. However, after high-temperature normalizing, the microstructure is very coarse. The secondary K precipitates in a network along the coarse grain boundaries (about 2-3 grain sizes), and has needle-like Weishi structure (Fig. 2). The hardness is as high as 34-39HRc. In order to eliminate the secondary K network, it was supplemented with 920°C oil quench and 730°C temper softening. Finally, the pretreatment process adopted by the plant is: 1070°C×30min air cooling +920°C×10min oil quenching +730°C×60min tempering, hardness 18-22HRc.
2.2NM treatment process
2.2.1 Defining blade made of steel with severe K segregation, solid-solution nitriding in well gas carburizing furnace (the same effect in salt solution containing active N, but no pollution) The oil is quenched and tempered at low temperatures to be the final heat treatment of the blade. This is the "nitrogen-containing Martensitic treatment", referred to as "NM treatment." It has two meanings: a. Nitriding treatment, ie, solid solution nitriding of the workpiece to obtain nitrogen-containing austenite—A(N); b. Martensitic treatment, ie quenching A(N) into Nitrogen-containing martensite-MN) plus appropriate nitrogen-containing retained austenite-AR(N). Due to the different treatment processes, some excess K may still remain in the penetrating layer, which may improve the wear resistance of certain parts, while the sharp cutting tools such as the shearing blades do not want to exist. N can promote the penetration of K in A, so even if the permeability layer still retains some excess K, its morphology will also change: refinement, spheroidization, mesh stripe weakening. In summary, it contributes to the improvement of fatigue strength, wear resistance and toughness. Measuring tools and other precision parts can greatly improve the finish after processing. The NM treatment does not allow the emergence of a new N-layer due to the infiltration N, otherwise it is not a solid solution N, it is not NM treatment. There is no doubt that M and AR in the penetrating layer also contain carbon, even a small amount of alloying elements. But they are all brought by the steel itself and have nothing to do with this process. Therefore, this process is still named as nitrogenous martensite treatment. The C atoms in the permeation layer may partially transfer to the inner layer when N atoms infiltrate, and even decarburize outwards (for example, ammonia or urea as the osmotic agent, the carbon potential in the furnace is lower), as long as the hardness and the wear resistance are not reduced. This is permissible and it is not necessary to deliberately add kerosene or benzene to maintain a high carbon potential.
2.2.2 Treatment process Blades are loaded into the furnace at high temperature, and the infiltration agent (triethanolamine, formamide, and urea are all available, the nitrogen potential of the three is increased in turn, and the carbon potential is reduced in sequence). carbon. The processing temperature is 760°C-850°C, which can be combined with the normal quenching temperature of each grade of tool steel, and quenched out of the direct oil. Below 750°C, the center of the blade is not quenched and has a thorium. Above 860°C, the M(N) in the osmotic layer is too thick and the AR is too much to affect the performance. The processing time is 15-60min, the surface can get 0.03-0.10mm K dissolving layer (different alcohol 5min before discharge, the cyanide gas in the furnace is exhausted), less than 15minK dissolution layer is not obvious, more than 60min surface may appear nitride. For carbon steel and low-combination steel, the hardness of the compound is sometimes not as high as M(N) due to the lack of an element that forms a hard nitride.
2.2.3 Infiltration layer composition and organization selection thickness 0.1mm, composition and blade steel similar Cr03 steel foil (w% C1.23, Cr0.32) after co-seepage with the furnace, the determination of steel foil composition w%C1.17 -1.30, N0.39-0.74. Some studies have shown that T8 steel is CN-permeated in a benzene-plus ammonia atmosphere at 800° C. The N content in the first hour can quickly reach 0.8%, and the time extension gradually decreases, while the C content keeps rising. Our experiment also confirmed this.
The normal NM treatment layer organization should consist of tiny M(N)+AR(N), and for the shearing blade, no excess K is required, which is based on the results of our research on blade wear mechanism. For other parts, they can be treated differently according to their service conditions and failure modes. However, elimination of surface coarse K and bandlike segregation is the purpose that NM processing must achieve.
2.2.4 Shearing effect The NM treatment eliminates blade scratches on the surface of the blade, prevents chipping, and greatly increases the blade life (represented by the number of fine wool shears per blade after sharpening each blade). Under the same conditions, the results of large-scale shearing inspection of tens of thousands of sheep, compared with ordinary oil quenching, NM treatment increased the SKS8 test blade from 3.7 heads/time to 16.2 heads/time; making SKS8 batch production blades from 2 heads/time Increased to 13.9 heads/time. This data is slightly higher than the number of blades (13.4 heads/times) that were imported and had an international ace blade. The new material T12J blades we developed are 21 heads/times, and the total number of scrapped heads used per blade is more than 1,000.
2.2.5 Analysis and Discussion Nitrogen is an element that strongly expands the gamma region and can greatly reduce the alpha-gamma transition. From the Fe-C-N ternary alloy 700 °C, 800 °C, 900 °C isothermal cross-section phase diagram (Figure 3) can be seen, with the increase of temperature, the gamma zone continues to expand, 800 °C when the C content of 0.5 ~ 1.10% The carbon steel requires only a slight penetration of N to obtain a single γ (N) and quench it to obtain M (N).
Figure 4 is an air-cooled photograph after NM treatment (sample is 4.5mm Cr06 hot rolled steel strip). It clearly reflects the changing process of NM processing. In the figure, the four layers A, B, C, and D are divided by the table and the inside. The topmost layer A is an oxide layer produced by exhausting the exhaust gas at a high-temperature furnace and discharging the furnace air, and the oxide layer is formed in M(N) and AR(N). Black mesh with internal oxidation, hardness is very low, not allowed during normal processing. Parts to be ground after quenching are otherwise.
The surface tissue after normal NM treatment should be layer B in the figure. This layer N penetrates into and dissolves in A in the most, because N causes K to dissolve into A, contains high C, N A, air cooling can quench, so get M (N) and partial AR (N), there is no excess K , hardness 800-840HV0.1. As N continued to deepen, its concentration gradually decreased and it was not enough to dissolve K when it reached the C layer. However, A, which has infiltrated N, can be hardened in air cooling and has less AR. Therefore, the C layer is composed of M (N) plus K and a small amount of AR. The highest hardness is 825-880 HV0.1. This layer is the most wear resistant. The inward D layer did not seep N, and it was not hardened when air cooled. It was only normalizing and consisted of sorbite plus original K with a hardness of 284-368 HV0.1. If the oil is quenched after NM treatment, the D layer should be the normal quenched structure of the tool steel.
If the metallographic sample is corroded just right, it can be seen that from the surface to the C layer, the M(N) is all grayish white, which is clearly distinguishable from the D layer. The white base K is also distributed on the gray base of the C layer, and its morphology is basically the same as that of the D layer. At the same time, the thicknesses of the C and B layers are comparable, indicating that the thickness of the N infiltration layer is approximately twice that of the K completely dissolved layer.
Since the segregation of the penetrating layer K is eliminated and more AR is contained, the surface is ductile fracture. The edge of the NM-treated blade is not chipped by the abrasive particles, but plastic flow, which effectively consumes abrasive energy, delays the wear and damage of the cutting edge (Figure 5).
3N, M treatment to eliminate K segregation of CN osmotic layer
3.1 Parts for Aerospace Products
A part material is 20CrMnTi steel forgings, requires CN co-osmotic quenching, penetration depth ≥1.30mm, long time of infiltration, thick and dense massive network-like CN compounds often appear in the infiltration layer, high brittleness, and it is easy to produce during subsequent machining. Cracks, often caused by decarburization during secondary heat quenching, have low hardness problems and cannot be resolved for a long time. After being inspired by the NM treatment, since solid solution N can eliminate the K surface flaws in steel, it should also improve the microstructure and properties of the osmotic layer. In the late 1980s, the NM treatment was used successfully instead of the original secondary heat quenching to ensure that the organization and performance were qualified, and the production quality was always stable.
3.1.1 The original process CN co-permeation was carried out in a well-type gas infiltration C furnace. The seepage agent was triethanolamine + ethanol, the temperature was 880°C, the furnace was slowly cooled, the infiltration layer was ≥ 1.30mm, and the secondary heat quenching after machining was performed. In order to stabilize the size, it needs to be cooled after quenching and finally tempered at low temperature.
3.1.2 New Process The CN co-permeation process is unchanged, and the machined NM treatment is performed in the same well-type carburizing furnace. In order to avoid viscous triethanolamine blocking the pipe, ammonia is used instead of a small amount of ethanol to prevent decarburization. The temperature is 870°C. The time depends on the size of the workpiece. The oil is directly quenched and then cooled and tempered according to the original process.
3.1.3 N.M. treatment effect After the NM treatment, the surface layer is given a very fine M(N) plus AR (technical requirements 1-3, practically all 1). The surface CN compounds are substantially dissolved, and even after the cohesive mass fractions of grades 5-6, the dense CN compounds become fine particles of grades 2 and 3 (requirements for grades 1-4 are acceptable). Hardness is 61-65HRC (requires 60-66HRC). Microhardness test, even after the co-permeation of the compound of the 5-6 grade, after NM treatment and tempering, the hardness gradient of the penetrating layer is also very gentle. The NM treatment is simple, does not increase the process, does not need to add equipment, the cost is not high, and the quality is stable.
3.2 for military high precision heavy load gear
3.2.1 new and old technology Inner Mongolia machine factory 20Cr2Ni4A steel, modulus 7-9 high-precision heavy-duty gear, the original process is 820 °C ammonia plus kerosene CN co-permeation air cooling - finishing - quenching - low temperature tempering. Due to the deep penetration and the long period of co-penetration, a large-sheet CN compound or a coarse hook-like CN compound and a coarse-grain M are often present in the infiltrated layer, and the fatigue resistance of the tooth surface is poor and the service life is low. Li Donggui et al. elaborately designed a new process: gears were first 920°C high-temperature CN co-permeation to quickly obtain the required thickness of the penetration layer, and then CN-permeation of 570°C×(3-4h) was performed with the furnace cooling. Obtain a high N content, and then with the furnace temperature to 760 °C -880 °C heating A, baked quenching. The three steps of CN co-permeation, NC co-permeation and A-chemistry can be carried out continuously in the carburizing furnace, or after the CN co-permeating after tooth finishing, NC co-permeation and overall heat quenching. The gears treated in this way have a fine dislocation M and a highly dispersed fine round CN compound, have higher static bending strength and contact fatigue strength, and have better wear resistance and seizure resistance than general CN penetration Or oozing C quenched high.
3.2.2 process analysis module 7-9 gear, the thickness of the permeation layer are more than 1.30mm, with ammonia plus kerosene as a seepage agent, 820 °C CN co-permeability, the inevitable long time, CN compound thick and mesh belt Segregation is inevitable. The new process is co-permeation at 920°C, the infiltration rate is greatly increased, and the thickness of the infiltration layer can be reached relatively quickly. However, at 920°C, the ammonia decomposition rate is high, the N-potential in the furnace is low, and the main reason is carburizing. Nitrogen infiltration is very limited. Therefore, the subsequent 570°CNC co-permeation can greatly increase the surface N content, resulting in ε+γ compounds (confirmed by X-ray diffraction), followed by quenching heating ε+γ decomposition, N dissolved in A, forming a high N containing A, quenching Later changed to M(N).
The increase of N content in A also promotes the dissolution or refinement of the coarse network-like CN compounds produced by high-temperature co-sorption, eliminating the segregation of compounds.
The co-permeability at 570°C has two functions: a. It can preheat the 920°C co-infiltrated and then quenched gears, which can reduce the stress and reduce the deformation. b. Even after the 20Cr2Ni4A steel is air-cooled after 920°C, The structure is also twinned M and AR and K. The NC co-permeability at 570°C also plays a role of high-temperature tempering, which precipitates fine granular CN compounds in M ​​and AR, and prevents grain growth during quenching and heating; on the other hand, It also reduces the CN content in the matrix. In the second heat quenching, this portion A may be converted into a fine dislocation M. Therefore, this process design is reasonable and provides valuable reference for people.
4 Conclusion
(1) Infiltration of nitrogen can expand the steel γ zone and reduce the A1 and A3 (Acm) points. The hypereutectoid steel is combined with quenching and heating, and at the same time solid solution nitriding, the surface carbides can be dissolved into A, and after quenching, a K-free M(N) surface layer is obtained;
(2) The nitrogen-containing martensitic treatment (NM treatment) can be used to eliminate the defects such as particle coarseness and reticulation, strip segregation and other defects in tool steel or carburizing, carbonitriding parts K and CN compounds, and improve the parts Performance and longevity;
(3) Compared with the traditional high-temperature normalizing, NM treatment is the most simple and most energy-saving new process to eliminate K-eruption of hypereutectoid steel.