Ginger block sorbitic treatment of the hottest low

2022-08-15
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Ginger block sorbitic treatment of low alloy cutting tool steel

Abstract: This paper introduces a process that tool steel is heated for a short time at a temperature slightly lower than ACL, so that the flake sorbitic is not completely spheroidized and ginger block carbide is formed. It is used for the pretreatment of shearer blades instead of spheroidizing annealing, and the service life of the blades after quenching is greatly improved. In this paper, the formation mechanism of ginger carbide is explained from the spatial morphology, and the reason why ginger carbide improves the wear resistance of blade is analyzed from the microscopic mechanism. The research shows that fine carbides can strengthen the cutting edge, but can not prevent the scratch of sand particles pressed into the cutting edge; However, the dispersion strengthening effect of coarse carbides is poor, but it can prevent the scratch of sand particles, but it will aggravate the edge collapse. Ginger carbide has the advantages of both coarse-grained and fine-grained carbide, taking into account the different performance requirements of the cutting edge and the cutting surface, so it is the ideal structure of the blade

key words: low alloy cutting tool steel; Sorbitic treatment; Carbide morphology; Shearer blade; Abrasive wear

study on new heat treatment for ginger like

sorbite of low alloy cutlery steels

huang Jianhong (Inner Mongolia Research Institute of

agricultural machinery, Hohhot, 010020)

liu Dongyu (Department of material engineering,

inner Mongolia Polytechnic University, Hohhot)hhot, 010062)

[Abstract] The new heat treatment process for ginger-like sorbite of low alloy cutlery steels was given in this paper.The characteristic of the new process is heating under Acl temperature for suitable time to change lamellar Sorbite to ginger-like Sobite.The life of the sheep shearing machine cutter after quenching is improved largely by using the new process instead of traditional spheroidizing annea ling.The ginger-like Sorbite forming mechanism and the high wear-resistance of ginger-like carbide cutter blade were studied in this paper.From research the following conclusion can be got:Small size carbide can strengthen the cutting edge but cannot resist the scratching of abrasive particles.Large size carbide can resist the scratching of abrasive particles but cause the edge tipping. The lumpy ginger like carbide having the advantage of small and large size carbide is the ideal microstructure of cutter blade.

key words:low alloy cutter steels, sorbite treating, carbide morphology, sheet shearing machine cutter, wear with abrasive particle

1 preface

in the process of completing the project of "comprehensive research on the wear mechanism of agricultural machinery blades", When we have a better understanding of the wear mechanism of the blade, we have discussed that the traditional spheroidizing annealing can obtain fine and round carbide (k). Is it the best original structure of tool steel in any case? At this time, among the new steel grades tested, there is a kind of hot-rolled air-cooled unannealed strip and a kind of annealed strip that is seriously overheated. We adopt annealing slightly lower than the temperature of ACL to make K incomplete spheroidization, form ginger block sorbite, and process it into a shearer blade. Good results are obtained in the test

2 test materials and test methods

2.1 test materials

the composition of the new steel used for the test is No.8 (1.17% C, 1.04% Si, 1.12% Mn, 0.28% v) and No.1 (1.23% C, 0.23% Mo). Both steels are melted in an alkaline medium frequency induction furnace, poured into 270Kg square ingots, and cut into 90mm on a 2T steam hammer × 90mm square billet, then rolled into 15mm × 90mm flat billet, acid pickling after air cooling, surface cleaning, hot rolling to 4.5mm × 90mm strip steel. Hot rolling process: heating at 950 ~ 1000 ℃, rolling at 940 ℃, final rolling at 850 ~ 800 ℃, spreading air cooling

2.2 test method

optical metallographic analysis is carried out on a neohot-i metallographic microscope. The worn blade was observed and photographed under the S-530 scanning electron microscope, and the accelerating voltage was 20kV. In order to find a fresh fracture, the whole blade is cut in  φ Conduct full blade scanning on the 120mm large sample table to find out typical photos. When observing the cutting edge, the sample table rotates by 30 °

2.3 blade life assessment

put forward four higher and more comprehensive indicators than JB/t7881.4-95 "wool shears. Test methods", and compared with a foreign blade known as a trump blade under the same conditions: ① take the number of sheep shears per blade and pure shearing time after each grinding as the basic assessment data; ② Count the probability of high head number and low head number, indicating the stability of blade quality; ③ The total number of sheep sheared from each blade to scrap; ④ The tooth breaking rate of the blade under normal shearing conditions

the experiment was carried out in combination with shearing production in pasture. The shears are Xinjiang fine wool sheep. In order to eliminate the impact of personal technical differences of shearers, each blade is handed over to all shearers in turn. A total of more than 3800 sheep were sheared with foreign blades. After removing the irregular data such as teeth interruption in the shearing process, machine failure and midway knife change, a total of 3227 sheep were effectively sheared

3 ginger block sorbitic treatment process

no.8 hot rolled strip is not annealed, the structure is very fine lamellar sorbite, and the carbide is also very fine and uniform, as shown in Figure 1. The hardness is 302 ~ 326hbs, which is difficult to cut and need annealing

Figure 1 hot rolled unanealed structure of No.8 steel × 500

3.1 blanking

in order to improve the plasticity of the material, the strip steel is heated to 650 ~ 680 ℃, and the blanking is carried out with a 60t punch. The washed blanks are stacked for air cooling, and the microstructure and hardness are checked for no change

3.2 ginger block sorbitic treatment

the measured ACL of the two steels is 726 ℃ and 729 ℃ respectively. Therefore, the sorbitic temperature is determined to be 715 ± 5 ℃. In order to make the heating uniform and the temperature control accurate, heat it in the salt bath for 3h, cool it to 680 ℃ with the furnace, and then discharge it for air cooling. The hardness is 263 ~ 283hbs, and the finish of the machined surface is very high

metallographic analysis shows that the microstructure of No.8 steel is basically the same relative to the rolling direction, both longitudinal and transverse sections, as shown in Figure 2. Due to the extremely fine tissue, the 500 times photograph is clear. When the oil lens is used for photography at a magnification of more than 1000 times, the photograph is not clear because of the small vertical depth of field. Therefore, figure 2 is made by using 500 times negative to enlarge 4.5 times when making photos, which is equivalent to 2250 times

Figure 2 ginger block sorbite structure of No.8 steel × 2250

3.3 No.8 steel blade final heat treatment process

heating in salt bath 780 ℃ × 5min oil quenching, (150 ~ 170) ℃ × 3H tempering. The hardness of the whole batch is 63.5 ~ 65hrc, mostly 64hrc, which is extremely uniform, and the hardness difference at each point of each blade is ≤ 0.6hrc

4 shearing effect and description

see the following table for shearing results. It can be seen from the table that the service life of the test blade is significantly higher than that of foreign famous brand blades. ① The number of sheep heads and pure shearing time per grinding and the total life of each blade are higher than those of foreign blades; ② The probability of stabilizing at the high head number is significantly higher than that of foreign blades, and there is no low index lower than 11 heads/time, while the lowest number of foreign blades is only 1 head/time, and 47.42% is less than 11 heads/time; ③ If the test blade is normal in the above 1 cutting, the teeth will not be cut under normal shearing conditions; ④ After checking the blades replaced after shearing, it is found that the wear of No.8 blade itself is the lightest among all kinds of blades. When sharpening, as long as a thin layer is removed, the edge can be restored to sharpness. Therefore, each pair of blades can be sharpened many times, and the total number of sheep shears is also high. This is the result of high blade hardness and uniform carbide distribution. Shearing data sheet of test blade and foreign blade

note: ① at the beginning, 18 sets of blades are cast for each kind of blade, and 2 sets of teeth are broken for foreign blades in the midway, and 2 sets are added, a total of 20 sets are cast. Foreign blades were tested 97 times. Due to the long service life of No.8 and No.1 steel blades, the sheep have been cut after 47 and 45 determinations respectively. ② Among the three kinds of blades, the highest is 21 sheep/time on average, and the lowest is 11 sheep/time for domestic blades. Therefore, taking ≥ 21 and < 11 as the indicators of high and low life, the probability of high and low head numbers of various blades is counted respectively. It is not very simple to calculate a value ③ of the 97 determinations, 22 ≥ 21 heads/time, 22/97 = 22.7%. ④ 20 pairs of blade breaking teeth, 2 pieces, 2/20 = 10%

group 3 in the table is the data of another new steel No.1 blade. When the raw materials enter the plant, it is found that the cold-rolled strip used for the upper blade of No.1 has good spheroidization, while the hot-rolled strip used for the lower blade is seriously overheated after annealing, and K is very coarse (Fig. 3). Therefore, the coarse carbides (38 ~ 44hrc) were eliminated by normalizing at 920 ℃, and then ginger block sorbitic treatment (211 ~ 251hbs) was carried out with No.8 steel at 715 ℃. The metallographic structure is similar to that in Figure 2, but there are a few coarse undissolved carbides, which are greatly improved. After being processed into a lower blade, it is oil quenched at 800 ℃ and tempered at 150 ~ 170 ℃. Because the structure of No.1 steel upper blade is normal, it is treated with other new processes. Shearing results the average number of shearing heads of this group of blades is the highest. Although it is related to the special treatment of the upper blade, as the lower blade, there are serious defects in the raw material structure. After normalizing and ginger block sorbitic treatment, excellent results can still be achieved, which once again proves that this kind of structure is suitable

Figure 3 No.1 steel lower blade raw material annealing overheating structure × 500

5 Analysis and discussion

it is well known that steel holding at a temperature slightly lower than ACL for a long time can spheroidize carbides, but it is rarely used in industry because it takes too long. The process time we use is short, and we get incomplete spheroidization effect, but the principle is the same

in literature [1, 2], when heating below the temperature of ACL, the description of the transformation process of carbides from sheet to ball is that spherical carbides have the smallest surface energy, so the transformation from sheet to ball, which reduces the free energy of the system, is a spontaneous process. According to the colloidal equilibrium theory, the solubility of the second phase particles is related to the radius of curvature R. ferrite (f) with small R at the sharp corner of the nearest carbide (k) has a higher carbon concentration, while f with large r near the plane has a lower carbon concentration, which causes the diffusion of carbon, leads to the dissolution of K at the sharp corner, and precipitates K at the plane, and finally forms spherical K with similar r in all parts. In addition, there are dislocation, sub grain boundary and other crystal defects in k-sheet. Both sides of the defect are like sharp corners with small R, which will also cause the dissolution of K, precipitate to the plane part through the diffusion of carbon, and finally realize spheroidization. The above process can be illustrated schematically in Figure 4: there is a sharp corner or crystal defect a with small R in the K sheet; As a result of dissolution diffusion precipitation, k-piece breaks B; And further dissolved into short rod-shaped C; As long as there is enough time, it will eventually become spherical D

it should be noted that the spheroidization process of carbide flakes is generally illustrated in the book with a cross-sectional diagram, such as 3 Regularly lubricate the mechanical part a ~ d in Figure 4. Let's sketch the change process of K's spatial morphology, such as a '~ C'. In a k-piece of pearlite or sorbite, there are not only sharp corners or sub grain boundaries on the section, but also such defects a 'on the surface of k-piece; At an appropriate temperature, K pieces dissolve and become several lobes, on which there are grooves B 'in different directions; As time goes on, the groove widens and the sharp corners become rounded, and the k-piece will become a ginger like k-piece composed of several fragments. Some parts of it have been broken, and even some small fragments have been spheroidized, but most of them are still connected. Making its section from different angles is like slicing ginger. Its section is circular, elliptical or short rod-shaped, as shown in Figure 4C, but its spatial morphology is mostly connected branches and blocks, which is a bit like ginger, as shown in Figure C '. We tried to extract it and observe it under scanning electron microscope, but it was unsuccessful. Because the k-piece is very thin before treatment, some have been disconnected after treatment, and the connection is also very thin

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