On 2018-08-08 14:48:54
Tool material heat treatment process needs to be personalized
One hundred years ago, American mechanical engineer f. W Taylor and metallurgical engineer m. White, after extensive and systematic cutting tests, established the optimal composition of high-speed steel for cutting, W18Cr4V(C 0.75%, W 18%, Cr 4.0%, V 1.0%), when cutting medium carbon steel at a speed of 30m/min, more than ten times higher than before.
This achievement led to an epoch-making revolution in machining, which led to the extraordinary development of cutting tool materials in the 19th century, creating unprecedented glory.
Over the past 100 years, despite the continuous emergence of various new cutting tool materials, high-speed steel has not been eliminated by history, has been used and developed, and even occupied the dominant position for a long time.
Today, we re-examine the development of high speed steel, may bring some useful inspiration for the further development of tool materials.
Many people may be familiar with the general principles of tool heat treatment, but the research on tool and tool material differential heat treatment is not deep. I have been engaged in heat treatment process for nearly 40 years, and have accumulated many successful experience and failure lessons. I am deeply aware of the importance of identifying the differences between tools and tool materials and conducting personalized operation.
The following is a brief introduction to the heat treatment of different tools:
According to the standard of GB4211, turning tool can be divided into square and rectangle. Turning tool working conditions are relatively harsh, generally without coolant, dry cutting, heavy cutting, intermittent cutting, high-speed cutting, etc., turning tool should have high wear resistance and high red hardness.
In view of the above situation, the heat treatment process is: 1230 ~ 1240℃ high temperature heating austenitization, the metallographic level up to 8 ~ 9, then tempering 4 times, overheating 3 ~ 4 levels (rectangular turning tool overheating 1 ~ 2 levels), the final tissue hardness of 65 ~ 67HRC. It has been proved that the turning tool is durable only when it is overheated. In 1988, I tested the life of the 12×12 square turning tool with grade 5 overheated tool.
Hob belongs to the more expensive metal cutting tools, requires a high red hardness, wear resistance, so the quenching temperature is higher, but slightly lower than the turning tool, often in 1225 ~ 1230℃, 580 ~ 620℃ classification, microstructure metallographic grade 9.5 ~ 10. The first tempering should be treated with 350 ~ 380℃ or secondary bainite, followed by three times of high temperature tempering at 550℃×1h. The temperature is 1 ~ 2, or even 3, and the final tissue hardness can reach more than 65HRC.
Although the hob is easy to handle the tool, but for the module 6 or more of the large hob, because of its hardness is not high and easy to crack the keyway characteristics, must be strictly guarded.
Side and side milling cutter with straight teeth
Usually this kind of products because of its fast cutting speed, large amount of cutting knife, wear resistance, toughness requirements are quite high, high hardness but can not break the edge.
The author has dealt with such a quality problem many times: the user response to the hardness of 64 ~ 65HRC milling cutter is not wear-resistant, the use of hardness 66HRC milling cutter, service life doubled. The same composition of the material, different heat treatment products can have such a big difference. The heat treatment process adopted by the straight tooth side milling cutter is quenching at 1220 ~ 1230℃, metallography level 9.5 ~ 10.5, tempering 3 ~ 4 times, overheating 1 level, and the final tissue hardness is 65 ~ 67HRC.
Medium tooth saw blade milling cutter
GB6120 standards apply to 32 ~ 315 mm diameter, thickness of 0.3 ~ 6.0 mm in tooth saw blade milling cutter, milling cutter of particularly large thin heat treatment process should be individualized, usually on the performance requirements of the tooth saw blade milling cutter is toughness first, but not too low hardness, generally during 1205 ~ 1215 ℃ quenching, quenching organization metallographic level grade 10 ~ 11, then grading isothermal, hot clip, tempering 4 times, tempering process prohibit overheating, eventually organizations hardness 64 ~ 66.5 HRC. In the heat treatment process of this kind of tool, it is easy to appear the key slot crack and plane out of tolerance.
Big thin blade
Due to the decoration and the rapid development of construction industry, the market for Φ 250 ~ 500 x 2 mm thin blade in great demand. The main requirement of this product is to highlight toughness, followed by wear resistance. The corresponding heat treatment process is 1180 ~ 1200℃ high temperature heating, 260 ~ 280℃ isothermal, the metallographic level up to 11 ~ 12, then temper 4 times, not allowed to overheat, hardness up to 62 ~ 65HRC. The result is better if the finished product is treated with steam or oxygen nitriding.
Integral straight shank twist drill
Drill is the most used hole tool, working conditions and other tools are different. When drilling, the cutting part is deeply embedded in the metal, cutting conditions are harsh, requires the drill has a high hardness and toughness, compared with the two, usually the latter is more important; For dry cutting, due to the high hardness of the workpiece, is required to be hard first. The heat treatment process generally adopted by the drill is as follows: first, quench at 1210 ~ 1225℃, the metallography level of the quenched tissue can reach 10.5 ~ 11, then temper for 3 times, overheating for 2 levels, the tempered microstructure with hardness of 63 ~ 66.5HRC can be obtained.
Due to the increasing number of machining centers and CNC machine tools, the difficult materials are increasing day by day, and the requirements for drill bits are getting higher and higher. Similarly, drill bits, different usages, heat treatment process is completely different, enough to illustrate the importance of heat treatment process personalized.
Broaching processing is widely used in automotive, aviation and other manufacturing industries. The broach is arranged with many teeth, because broaching is very wide, the tooth increment gradually increases, even in the case of a small amount of feed, its resistance is quite large.
Because of this, broaching speed is generally relatively slow, in the broaching process, the axial bear a great deal of tension, at the same time, the cutting teeth due to the friction of the workpiece and generate heat, so the broach in addition to the high hardness of the general tool, more important is to have a high strength and toughness. The commonly used heat treatment process of broach is oil quenching at 1205 ~ 1215℃, two or three times of graded cooling, straightening while hot, and the metallographic grade of sample quenching structure is 10.5 ~ 11.5. Each tempering must be straightened, hardness up to 64 ~ 66.5HRC.
At present, the main problem of heat treatment is low hardness and non-wear resistance, followed by fracture, the corresponding heat treatment process should be formulated according to the specific type of broach.
In the process of cutting, the main resistance of the tap is torsional torque, the axial force in each thread has been offset. Taps do not need red hardness, toughness is the first index, so the quenching temperature is relatively low, grain size can be controlled very fine, but the hardness can not be low, must meet the national standards.
Now because of the adoption of a new ductile low temperature quenching process, compared with the traditional process, the quenching temperature reduced 15 ~ 20℃, thoroughly solve the tap overheating, brittle and other defects, is actually a good thing.
My personal experience is: hardness is the surface phenomenon, metallographic organization is the essence of things; We pursue high hardness, but not only high hardness; We require high hardness to achieve a good match between hardness and metallography on the premise of ideal (but not qualified) microstructure.
Here are a few key questions to keep in mind:
1) first of all, no mixing, including mixing furnace number, mixing steel number, mixing specifications. Tool factory must be managed by furnace number, if not, it can not do a good job in heat treatment, such as the composition of qualified M2 steel, if the use of 1220℃ heating and quenching, may appear 9 ~ 12 grain number 6.
2) second, different smelting methods affect the heat treatment process. The heat treatment process of steel smelted with intermediate frequency and electroslag remelted with electric arc furnace should be different from that of M2 steel. The quenching temperature of the latter should be appropriately higher, and the former is prone to overheating and other problems.
3) the third is that forging affects the heat treatment process. The same specification, the same composition of the M2 steel manufacturing of the same specification of cutting tools, forging and not forging, heat treatment process should not be the same, through the forging, the quenching temperature should be slightly lower. These in the formulation of heat treatment process should be paid attention to.
4) fourth, the quenching temperature of rework parts should be 8 ~ 15℃ lower than that of conventional quenching. As a result of a variety of reasons, a few rework is inevitable, so before quenching must find out what causes the rework, and then the annealing situation, so as to be appropriate.
5) fifth, because the M2 steel quenching temperature belt is relatively wide, from 1180℃ to 1240℃ span of 60℃, and different tools require different performance, so the heat treatment workers must find out what the most important performance of the tool processing object, the implementation of personalized indicators in the heat treatment process to fully tap the material potential.