Drilling of high manganese steel ZGMn13

As a typical difficult-to-machine material, high manganese steel ZGMn13 is widely used in parts subject to greater impact due to its excellent wear resistance and low price. This article introduces the basic properties and drilling of high manganese steel ZGMn13 On the basis of sex. The tool structure and materials that can be used for ZGMn13 drilling in practice are described.

Keywords: ZGMn13; drilling; work hardening; cemented carbide; drill

1. introduction

Since 1882, the British Hadfield (Hadfield) invented high manganese steel. It has been used for more than one hundred years. It has become a traditional wear-resistant material and has been widely used. Such as buckets of excavators, jaws and linings of crushers, track plates of tanks, driving wheels, driven wheels and track support rollers, railway frogs, etc. The wear resistance of ZGMn13 steel is unmatched by any other steel grade. However, it has good working performance and at the same time shows the difficulty of machining when cutting it. It is a typical difficult-to-machine material, especially in hole processing, because drilling itself is a semi-closed processing, making the drilling of ZGMn13 steel more difficult.

2. Basic properties of ZGMn13 steel

The main components of ZGMn13 steel are: 1.30%C, 0.30%～0.80%Si, 11.0%～14.0%Mn (Mn/C=10～12). ZGMn13 steel castings are hard and brittle in nature. Machining is difficult and cannot be applied in practice. The reason is the presence of carbides in the as-cast structure. Practice has proved that ZGMn13 steel exhibits the best toughness and wear resistance only when all austenitic structures are obtained. In order to make ZGMn13 steel get a good pure austenite structure, it must be “water toughening treatment”. Water toughening treatment is the operation of quenching treatment. The method is to heat the steel to above the critical point temperature (about 1000-1100℃) for a period of time, so that all the carbides in the steel can be dissolved into austenite, and then quickly immersed and quenched Cool in water. Because the cooling rate is very fast, the carbides are too late to precipitate from the austenite, thus maintaining a uniform austenite state. The original hardness of ZGMn13 is similar to that of No. 45 steel (normalized), about HB180-220. When ZGMn13 steel receives a strong impact. The hardness of the surface layer is rapidly increased to HB450-550, and the work hardened layer can reach 0.3mm. Therefore, it has high abrasion resistance, while the core part still maintains the original toughness. When the hardened layer is worn off by external forces, a new hardened layer can be produced.

3. Drilling workability of ZGMn13 steel

• 3.1 Severe work hardening. ZGMn13 forms a hardened layer on the processed surface. It is difficult to process again. Therefore, it is strictly forbidden to stop during the actual drilling process, and it is also required to use one pass as much as possible during the process setting. In the middle of drilling, if the drill is damaged and cannot continue cutting, you can hear the piercing scream when you change the drill again. This is the effect of cutting on the hardened layer. Because the chisel edge of the drill has a large negative rake angle, it is a wedge-split scraping during the drilling process, and the work hardening process increases the axial force, which is easy to wear or chip the chisel edge.
• 3.2 The thermal conductivity is small. The thermal conductivity of ZGMn13 is 13W·m-1·k-1, which is 1/4 of that of No. 45 steel. During the cutting process, the heat cannot be conducted quickly, resulting in high cutting temperature and aggravating the wear of the drill bit. During the drilling of the middle 31.5 holes of the ZGMn13 railway frog, the drill bit became red, which seriously reduced the durability of the drill bit.
• 3.3 High toughness and high plasticity. The impact toughness value of ZGMn13 is 2.9 to 4.9 J·cm2, which is 6 to 10 times that of No. 45 steel, and the elongation (plasticity) is 50% to 80%, which is 3 to 5 times that of No. 45 steel.
  This not only makes the cutting easy to deform and increase the cutting force, but also the chips are tough and not easy to break. The chips are mostly wrinkled and long bands. It is very easy to wrap around the shank during drilling.
• 3.4 The linear expansion coefficient is large. The coefficient of linear expansion of ZGMn13 steel is about 20 10-6°C-1, which is almost the same as that of brass. Under the action of cutting heat and cutting temperature, the part of the workpiece rapidly thermally expands and deforms, thereby affecting the machining accuracy.

4. Reasonable drilling conditions for ZGMn13 steel

• 4.1 Tool material. With the continuous development and maturity of powder metallurgy technology, cemented carbide tools are widely used in cutting difficult-to-machine materials. However, there is still a big gap between China’s cemented carbide manufacturing level and foreign countries. Still need to keep improving. At present, the commonly used carbide tool materials for drilling ZGMn13 steel are: YG8, YG6X, YS2, YW2, Y1v767 and Y1v798. Although YG cemented carbide is more commonly used, it is not suitable for high-speed cutting. Because when the rotation speed is high, the wear at the turning point of the outer edge of the drill bit is faster, and the wear of the flank surface is accelerated, and the tool durability is reduced. When the rotation speed is high and the cutting process is relatively stable, it can be considered to choose Ⅵ’type cemented carbide. Adding an appropriate amount of TaC or NbC (usually 0.5% to 3%) to YG cemented carbide can increase its hardness and wear resistance without reducing its toughness. As the amount of cobalt in the cemented carbide increases. These advantages are more significant. Therefore, the general-purpose cemented carbide with TaC and NbC as additives is also suitable for drilling high manganese steel. The coating technology of cemented carbide has developed a single coating into a composite coating technology, which has solved the contradiction between the hardness and toughness of cemented carbide.
• 4.2 Tool structure. Most of the drill bits used for ZGMn13 steel are welded carbide blade structures, and there are also occasions where indexable shallow drills are used. The welding structure is relatively simple and the manufacturing cost is low. The E2 type cemented carbide blade is inlaid and welded in the slot, and the welding methods mainly include oxygen-acetylene flame brazing and high-frequency induction brazing. However, since the linear expansion coefficient of the carbide blade is much smaller than that of the body steel, there are more residual stresses after welding, and welding cracks are prone to occur especially on the larger brazing surface. During the drilling of the ZGMn13 steel railway frog, the failure of the drill bit due to welding cracks reached more than 10%. The drill body with indexable structure is more complicated to manufacture, and the blades are generally made of carbide-coated blades, so the cost of the drill bit is relatively high.

5. Several common carbide drill bits for drilling ZGMn13 steel

• 5.1 General-purpose carbide drill bits. This kind of drill bit adopts the insert welding form. Since about 57% of the drilling force is generated on the chisel edge when the drill is drilling, the axial force is reduced by grinding the chisel edge, and the rake face of the chisel edge is sharpened to increase the rake angle, which can further reduce the axial force. And torque to improve the durability of the drill. Reduce the helix angle of the cutter body by 13 to 20. , Increase the front angle 2 to 140. , In order to facilitate chip evacuation. Or grind into a double front angle to improve heat dissipation conditions. When drilling shallow holes, straight flute drills can be used, which not only enhances the strength of the drill core, but also has simple manufacturing and low cost. In order to keep the bit sharp, when the wear size of the outer edge corner is less than 1mm, it should be reground in time. Cooling should be sufficient when drilling, and cooling should not be interrupted. The production of dry drilling actually shows that the durability of the drill is low.
• 5.2 Cemented carbide group drill. The structure and manufacture of this cemented carbide drill bit are the same as the general cemented carbide drill bit. The body of the drill bit should have good rigidity and strength, generally made of 40Cr, and the length should be as short as possible. The material of the blade can be YG8 or YW2. If the crown bit is selected, the drilling effect will be better. The geometric parameters and angles of the cutting part are basically similar to those of cast iron group drills, except that the tip height is increased to 0.08d (d is the diameter of the drill bit), and the arc radius of the arc blade is increased to 0.4d to increase The strength of the tool tip improves the heat dissipation conditions and plays a role of chip separation. At the same time, the double front angle is ground at the outer edge, and the negative front angle is ground, and the rear angle at the outer edge is increased to 20. . After the drill is sharpened, it should be carefully polished with oilstone to improve the smoothness of the blade surface, and the blade must not have serrations. During the drilling process, the cooling should be sufficient, and the conditions permit, the entire workpiece can be immersed in the cutting fluid for drilling. Appropriate selection of cutting parameters can control the cutting temperature within the range of about 600°C. At this time, the processing conditions are more favorable. At the same time, it can be observed that when the workpiece is thin and drilled to the exit, the material reaches a dark red degree.The blunt standard of group drills is equally important, the same as that of general-purpose carbide drills. However, the structure of the cutting edge of the group drill is complex, and the sharpening requirements are high, and the technical level of the staff is very high.
• 5.3 No chisel welding type cemented carbide shallow hole drill. The significant difference between this kind of drill and the general carbide drill is that there is no chisel edge. It is replaced by a gap with a width of △ and a depth of several millimeters in the core part, thereby forming a non-cutting zone in the core part. In the non-cutting area, a small core with a diameter of △ will be produced during the cutting process. Due to its low strength, it will be repeatedly generated and broken in the gap, and the broken part will be taken away with the chips. Since the drill has no chisel edge, the axial force during drilling is small, which can be reduced by 30-50%. Controlling △ is the key to making drill bits, and it is generally taken as about 0.8mm. In order to facilitate the self-breaking of the small core, the greater the toughness of the processed material, the smaller the width of the gap △ should be.
• 5.4 Shallow hole drill can be indexed. The tool angle of the indexable shallow hole drill is composed of two parts: the slot and the blade, so the machining accuracy of the tool body is relatively high. Since the cutting edge of the drill is composed of two blades, the outer edge and the inner edge, they are not symmetrical about the axis of the drill. In order to balance the radial force during drilling, the outer blade and inner blade must be configured according to the principle of mechanical balance during design. The drill is well centered during drilling, cutting smoothly, and high machining accuracy. Convex triangular carbide-coated inserts are used, and the chip breaker on the insert forms 10 pieces. The front corners of the left and right sides. The back angle is 7. , The cutting edge is sharper, which can reduce the work hardening of ZGMn13 steel. The bit angle is 160. , The chip deformation is small, and the flow direction is close to the direction of the chip removal groove, which is beneficial to chip removal. It must be noted during use that when the hole is drilled, a thin sheet will fly out, and it must be protected safely.

6. Heating process applied to ZGMn13 drilling processing

Heating methods mainly include acetylene-oxygen flame and plasma heating method. The former is simple and practical, and the latter has done experiments in turning ZGMn13. The former is used when drilling ZGMn13 steel plates. The effect is remarkable. After the water toughening treatment, ZGMn13 is reheated to above 350℃. Carbides will re-precipitate. And the formation of troostite or tempered martensite. The plasticity and toughness of steel will drop sharply. But when the temperature exceeds 600″C, the plasticity and toughness increase rapidly. Therefore, when drilling in this temperature range, it is stable, labor-saving, smooth processing, and no chipping. It not only ensures the processing quality, but also improves the production efficiency. Moreover, the service life of the tool is prolonged.

When drilling using this simple heating method. Align the acetylene-oxygen gas welding torch at the center of the drill hole and heat it, the range is about twice the diameter of the hole, and heat it to about 450°C, that is, after the steel plate sees red, go down immediately after leaving the heating. High speed and large feed can be used. It has been proved that acetylene-oxygen heating and drilling of high manganese steel is a simple, practical and efficient machining method.

7. in conclusion

In the drilling process of ZGMn13 steel. The main thing is to solve the problems of work hardening and cutting heat. Any new drill type and processing method are carried out around these two problems. As long as the chip deformation is small, the chip evacuation is smooth. The work hardening is small, and the cutting heat is also reduced accordingly. In addition to traditional drilling processing forms. Special processing methods can also be applied, such as using the low-temperature brittleness of high-manganese steel to perform low-temperature drilling. At present, low-temperature turning experiments have achieved results. In addition, magnetic drilling can also be used. The continuous update and appearance of tool materials provide a guarantee for solving the problem. With the continuous deepening of research, the difficult drilling problem of ZGMn13 will surely be greatly improved.

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