7 Types of Annealing Process

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Various energy-saving annealing processes such as low-temperature annealing, water-blast annealing, rapid softening, and isothermal spheroidizing annealing performed by domestic counterparts, compared with traditional high-temperature and long-term annealing processes, the new annealing process saves more than 30% of energy.

As long as the phase transformation of high-speed steel parts occurs after heat treatment, they generally need to be annealed before cutting and preparing for the final heat treatment. In addition, the high speed steel tools and dies after quenching and tempering must be annealed before final heat treatment, otherwise, it is very likely to cause naphthalene fracture and scrap.

The conventional annealing process of high speed steel has a long cycle, generally more than 36 hours (from furnace to furnace), low efficiency, high energy consumption, and the uniform distribution of carbides is not ideal. As shown in the figure is the typical process curve of w18 steel forging annealing in box type electric furnace. The hardness should be less than 255hbw after annealing.

In order to change the disadvantages of traditional annealing process, the tool industry has done a lot of experiments, bold reform and innovation, to transfer positive energy for energy conservation and emission reduction. The following is a brief introduction of several energy-saving annealing processes.

1. Low temperature rapid quenching annealing process

Low temperature rapid quenching annealing is to heat the workpiece to 20 ~ 30 ℃ below AC1 for a certain period of time (1.5 ~ 2H in salt bath furnace and 3 ~ 4H in electric furnace), complete the transformation of spherical pearlite, and then quickly oil cool to obtain low hardness of 32 ~ 36hrc, which is convenient for cutting.

This method is suitable for the rework annealing of small high speed steel cutting tools. It should be noted that the heating temperature should not be too high and should not exceed the AC1 line, otherwise it will cause partial austenite transformation and cause high hardness after annealing. Compared with the traditional process, the new technology saves electricity by 2 times, simultaneous interpreting time by 3 times, and reducing the cost by 1.5 times.

2. A new rapid softening process — water annealing

Water quenching is a common hardening method for carbon steel heat treatment, which is well known, but water annealing is rare. Water annealing is a special form of incomplete annealing. It is a fast and convenient softening process when it is eager to work or soften to achieve the purpose of cutting.

The key point is that the annealing temperature is about 100 ℃ lower than the phase transformation point AC1, holding for more than 10 minutes (in salt bath), and then quenching in water for 1 ~ 2 minutes, the hardness can be reduced to below 30hrc (even if the original hardness is ≥ 60HRC). This process is very suitable for small batch production. The heating furnace should be salt bath furnace and cooled by running water.

3. Rapid cyclic annealing

The high speed steel workpiece to be annealed is heated to 20 ~ 30 ℃ below AC1 in a salt bath furnace for 10 ~ 15min, and then quickly transferred to another bath furnace at AR1 – (20 ~ 30) ℃ for 10 ~ 15min, repeated for 3 ~ 4 times, with a total time of 6 ~ 7h. Air cooling after each cycle can obtain lower hardness and spheroidizing effect.

According to the test, after one cycle, the hardness decreased from above 60HRC to 36 ~ 42hrc, after two cycles, to 30 ~ 35hrc, and after three cycles, the hardness decreased to 28 ~ 32hrc. The more the number of cycles, the greater the decrease of hardness. When the cycle to 5 times, the hardness will drop to below 25hrc, fully meet the cutting requirements, there is no need to recycle.

It has been proved that the decrease of annealing hardness is related to the composition of the steel and the pretreatment process. For example, the more overheated the workpiece is, the more difficult it is to be annealed.

Compared with the traditional annealing process, the rapid cycle annealing process can save 90% ~ 100% electricity and increase the work efficiency by 2 times. It is especially suitable for single piece, urgent piece and small batch production.

4. Fast spheroidizing annealing

After high-speed steel is forged, slow cooling followed by reheating annealing process is adopted in general units. There are two kinds of annealing methods: ordinary annealing and isothermal annealing. Some units adopt the rapid spheroidizing thermomechanical treatment, that is, the organic combination of forging and heat treatment. The new process simplifies the process, shortens the production cycle, saves 70% ~ 90% electricity, reduces the cost, improves the working conditions, improves the forging quality, and is conducive to mechanized operation.

The high speed steel workpiece after rolling and die forging isothermal processing does not need to use the conventional annealing process, but can also use a, B, C three processes as shown in Figure 2. The process cycle is short, only 5 ~ 6h, only 1 / 4 ~ 1 / 5 of the conventional process, and the energy saving effect is remarkable.

5. Water bursting during annealing of high speed tool steel wire rod

The high-speed steel wire rod after normal annealing is heated to a certain temperature below AC1, and then cooled to room temperature by rapid water explosion after holding for a period of time. It is reported that the cold drawing plasticity of high-speed steel wire rod can be obviously improved by increasing water explosion after normal annealing, which is conducive to the smooth drawing. The process adopted in a steel plant is: M2 steel wire rod 880 ℃× 2H → furnace cooling 760 ℃× 5h → water explosion. The results show that the tensile strength and hardness of the wire rod treated by water explosion are similar to those of the conventional isothermal spheroidizing annealing, but the plasticity is greatly improved, and the reduction of area is increased by 37%.

6. Low temperature annealing process of high speed steel

For manufacturing twist drills, center drills, machine taps and other rod cutters, they are usually punched with high-speed steel wire rod, and the milling cutter blanks made of steel plate are also punched. Low temperature annealing process should be adopted: 650 ~ 680 ℃× 4 ~ 5h furnace cooling to below 300 ℃ furnace air cooling, energy saving, power saving and stable quality.

7. A new process for improving high speed chipping performance boiling water quenching

The hardness of high speed steel is 207 ~ 255hbw, and it is difficult to reach the surface roughness of RA ≤ 3.2 μ m when the hardness is so low. In order to improve the shoveling performance, most tool factories adopt the quenching and tempering process of incomplete quenching and high temperature tempering. The process curve is shown in the figure, and the hardness after quenching and tempering is 36 ~ 42hrc. There are some difficulties in implementing this process: first, the hardness is uneven, and the residual salt adhering to the workpiece affects the cooling effect and produces soft spots; second, it is difficult to control the hardness within the process range; third, the cleaning is time-consuming and the production cost is high. It has been proved by experiments that boiling water quenching has the advantages of fast speed, low cost and stable quality, which can completely replace the original time-consuming and power consuming quenching and tempering process.

In order to carry out water explosion process, the billet should be of the same heat number, and the heating temperature and water explosion time should be strictly controlled. The results show that the composition of the material with the same furnace number is the same, the phase transformation is the same, and the hardness after water explosion is basically the same; the higher the heating temperature is, the more carbides dissolve into austenite, the higher the hardness after water explosion is, the lower the heating temperature is, and the hardness after water explosion is insufficient; the water explosion time is 2 ~ 4S. The longer the water explosion time, the higher the hardness.

Boiling water quenching is energy-saving and environment-friendly, the water temperature is constant at 100 ℃, the cooling speed is the same everywhere, and the surface hardness is the same after water explosion; because the cooling speed of boiling water is slower than that of oil, there is no need to worry about quenching crack; after water waterfall, it uses its own waste heat to self temper, so there is no need to re temper. Compared with traditional quenching and tempering and low temperature annealing, boiling water quenching is superior.

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