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Cutting processing of high temperature alloy

Classification of high-temperature alloys

In terms of composition, commonly used materials are iron, nickel, and diamond based high-temperature alloys. Traditionally, they are classified as iron-based high-temperature alloys (also known as heat-resistant steels). This type of alloy is mainly composed of iron, so it is called heat-resistant steel. In terms of metallographic structure, there are pearlite alloys (such as GH34) and austenitic alloys (such as GH2036, GH2132).

Iron nickel based high-temperature alloys generally have a nickel content of 30-45%, with typical representatives such as GH130, GH2135, and GH1140.

Nickel based high-temperature alloys have excellent high-temperature performance, such as 

ъ 800 ° C>20kg/mm2 or ъ 1000 ° C>14kg/mm2

100 hours                        100 hours

 it also has good oxidation resistance and gas corrosion resistance under high temperature conditions, and is widely used in gas turbine engines. The nickel content of this type of alloy is mostly above 50%, and its typical representatives are GH30, GH4033, GH4037, and GH4049.

Cobalt based high-temperature alloy. Cobalt is an expensive element and a scarce mineral resource in China, so it is rarely used. Its typical representative is K10, which has also been replaced by other high-temperature alloys.

1. Cutting characteristics of high-temperature alloys

The metallographic structure of high-temperature alloys is complex, with multiple alloy components, some of which have more than 20 different elements. For example, nickel based alloys are mostly alloys with six, eight, or ten or more components. It has good strengthening effect and high alloy performance, therefore, its cutting performance is poor. The machinability of cutting 45 steel is 100%, while the relative machinability of high-temperature alloys is only about 20% of it. According to the properties of various high-temperature alloys, the order of their machinability from easy to difficult is:

wrought High Temperature Alloy ->Casting High Temperature Alloy

The order of wrought of high-temperature alloys is:

GH34→GH2036→GH2132→GH2135→GH1140→GH30→GH4033→GH37→GH4049→GH33A

The sequence for casting high-temperature alloys is:

K11→K214→K1→K6→K10

The following table lists the physical and mechanical properties of several high-temperature alloys:

The main reasons why high-temperature alloys are difficult to cut are as follows:

High cutting force. Various high-temperature alloys, most of which have a certain degree of plasticity, some of which have good plasticity. Like GH140

The room temperature elongation rate is as high as 40%. Most of the elements in high-temperature alloys are Cr, Co, Mo, W, V, Nb, Ta, Hf. These elements have high melting points, high activation energies, high atomic energies, and stable atomic bonding. To break free from equilibrium, a large amount of energy is required, resulting in high deformation resistance. In addition, the alloy elements Ti, A1, C and the matrix metal form compounds dispersed in the matrix, which not only increases the plastic deformation resistance of the alloy, but also causes severe distortion of the lattice in the deformation zone, thus greatly improving its strength and hardness, and significantly increasing the cutting force, which is 2-3 times greater than that of general steel.

The cutting temperature is high. Due to the significant plastic deformation of high-temperature alloys during cutting, there is strong friction between the tool, workpiece, and chips, resulting in a large amount of cutting heat. At the same time, high-temperature alloys have low thermal conductivity and difficult heat transfer, resulting in a high concentration of cutting heat near the cutting edge, causing cutting temperatures to reach around 1000 ° C. For example, when turning GH1131, the cutting temperature exceeds 900 ° C, while under the same conditions, the cutting temperature of 45 # steel is only around 640 ° C. This temperature not only softens the tool material, but also significantly exacerbates the diffusion and bonding wear of the tool, greatly reducing its lifespan.

The phenomenon of work hardening is severe. Due to the high cutting temperature of high-temperature alloys, the strengthening phases in the alloy separate from the solid solution and are uniformly distributed as extremely fine dispersed particles in the matrix, further increasing the strengthening ability of the alloy and improving its hardness. For example, when cutting high-temperature alloys, the surface hardness of the processed surface is 50-100% higher. In addition, all high-temperature alloys contain hard phases such as carbides, nitrides, borides, etc., which cause strong wear on cutting tools. At the same time, the cutting tool may also experience plastic deformation and chipping, which further increases the difficulty of cutting. Due to the severe hardening phenomenon during the processing of high-temperature alloys, in addition to normal wear, cutting tools may also experience boundary wear and groove wear.

2. Selection of cutting tool materials

High temperature alloys have excellent high-temperature comprehensive performance and low thermal conductivity, so when cutting, hard alloy cutting tools are required to have high high-temperature strength, high-temperature hardness, and sufficient toughness. originally, YG8N, YG6A, YW1, YW2 and other alloys were used as tool materials for cutting high-temperature alloys. Although these alloys can be used for cutting, their cutting efficiency and tool life are extremely short, and the surface quality of the processed products is poor. In recent years, due to the development of the hard alloy industry, there has also been progress in cutting high-temperature alloys. Especially after people paid attention to the difficulty of cutting such alloys, careful research was conducted on the selection of tool materials. After research and analysis, it is recommended to use hard alloys that do not contain titanium carbide or contain very little titanium carbide, and add tantalum carbide (niobium) and other alloying elements when cutting high-temperature alloys. Because these alloys have good high-temperature performance and excellent thermal conductivity, they achieve significant results during the cutting process. Practice has proven that:

Rough rough cutting at low speeds, intermittent cutting of nickel based high-temperature and casting high-temperature alloys, recommended for use

YS2T（YG10HT）

When cutting continuously, it is recommended to use

YS8、YS10

3. Selection of geometric angles for cutting tools

When cutting high-temperature alloys, the selection method of the rake angle also follows general rules, and its size mainly depends on the type of high-temperature alloy and the requirements of workpiece accuracy and surface quality. When rough machining, it is more suitable to use a hard alloy tool with a diameter of 00. When precision machining, the rake angle can be slightly increased. It is worth noting that the cutting edge should be sharpened and there should be no serrated defects. The smoothness of the rake surface should be kept above ▽ 9, which can reduce chip deformation and cutting force. The cutting edge is generally not chamfered. If chamfering is necessary to increase the strength of the blade, its width should also be as small as possible.

The phenomenon of work hardening in high-temperature alloys is severe, which can easily cause wear between the cutting surface and the machined surface. Therefore, it is advisable to choose a larger back angle. In general, ao=60-80, and the upper limit value can be taken during precision machining.

From the perspective of improving the strength of the cutting edge and enhancing heat dissipation conditions, the main deviation angle is usually set below 75 ° to improve tool durability. However, this must be done within the limits of the machine's power and process rigidity, as the absence of this premise can cause vibration and other phenomena. The inclination angle of the ruler is generally around λ s -4 °.

4. Selection of cutting parameters

When cutting high-temperature alloys, there is an optimal cutting speed, which means selecting a reasonable cutting speed within the optimal cutting temperature range. According to experiments, the optimal cutting temperature range for cutting high-temperature alloys with hard alloy cutting tools is 750 ° C to 1000 ° C, corresponding to an optimal cutting speed of approximately 10-30m/min for rough cutting and around 50m/min for fine cutting. Experimental results have shown that only at such cutting temperatures and speeds, the relative wear of the tool is small, and the durability and cutting distance are maximized. Deviation from this value significantly increases tool wear.

The feed rate is an important parameter in the cutting process, especially with a significant impact on the cutting temperature. Its increase causes the cutting temperature to rise in a straight line, exacerbating tool wear. At the same time, it is also a sensitive value that affects the optimal cutting speed, and the feed rate of high-temperature alloys is preferably less than 0.3mm/r.

The recommended cutting depth is generally ap=3-5mm (coarse lathe)

Ap=0.2~0.5mm (precision machining)

Another point to note is that due to the high cutting temperature when cutting high-temperature alloys, tool wear is faster. To solve this problem, it is recommended to use cutting fluid when cutting high-temperature alloys with hard alloys. This not only reduces cutting temperature and increases cutting speed, but also improves the surface quality of the workpiece.