How to Realize Induction Heating Quenching of High Speed Steel?

1. Metallographic problems of induction hardening

(1) Rapid heating critical point rise

The heating rate of induction heating ranges from tens to hundreds of degrees per second, and the pulse quenching reaches thousands of degrees per second (2000-3000 ℃ / s).

Because the heating speed is fast and the duration is short, the quenching temperature is higher than the general salt bath quenching temperature, so that the structure can be transformed into austenite and homogenized.

Table 1 shows the relevant data that the AC1 point of T10 steel and GCr15 steel increases with the heating speed during rapid heating.

Table 1 Relationship between induction heating speed and critical point AC1

Steel grade

Heating speed / (℃ / s)

Original state

10

50

100

150

200

300

T10

annealing

745

760

765

760

765

765

quench

735

745

755

755

760

765

GCr15

annealing

770

810

825

830

835

830

quench

740

750

785

800

815

810

From practice, we know that the quenching temperature of induction heating is 80 ~ 150 ℃ higher than that of conventional quenching.

Table 2 is the recommended heating temperature for high-frequency quenching of common steel.

Table 2 heating temperature of common steel for high frequency quenching

Steel gradeHeating temperature / ℃
45860~920
50860~900
40Cr940~980
T7, T7A880~960
T8, T8A860~960
Steel gradeHeating temperature / ℃
T10, T10A850~960
GCr15920~1020
GCr9900~1000
CrWMn850~960
9SiCr880~1000

The specific power of induction heating is much larger than that of furnace heating, so the heating speed is faster, and the time required to promote the temperature rise of pearlite to austenite is shorter.

The original structure of steel has a great influence on the nucleation, growth and homogenization of austenite during rapid heating, so it also significantly affects the temperature of induction quenching and the microstructure and properties of quenching.

Fig. 1 shows the relationship between the critical point of T8 steel with various original structures and the heating speed.

Flake pearlite is easier to complete the structure transformation process during heating than spherical pearlite.

Therefore, the induction quenching temperature of the same steel with different original structure must be: t quenching (annealing state) > t quenching (normalizing state) > t quenching [quenching and tempering (annealing + high temperature tempering)].

The physical meaning of α0 in the figure:

How to Realize Induction Heating Quenching of High Speed Steel? 1

For pearlite, it represents half of the distance between two adjacent cementites;

For free ferrite, it represents half the distance between the nodes of the dislocation network.

Fig. 1

During rapid heating, the AC3 point also increases with the increase of heating temperature.

Fig. 2 shows the temperature required for complete quenching of hypoeutectoid steel at different heating speeds.

How to Realize Induction Heating Quenching of High Speed Steel? 2

Fig. 2

(2) Rapid heating can make the steel obtain fine grain or ultrafine grain

In the range of low heating speed, the austenite initial grain formed just after austenitizing decreases significantly with the increase of heating speed, but at high heating speed, the austenite initial grain hardly decreases with the increase of heating speed.

It has been proved that under the actual conditions of induction heating, the heating speed is very high, and the initial grain obtained is very small, and has nothing to do with the heating speed.

However, the growth of the formed austenite grains is related to the heating rate.

When heating continues to a certain temperature, the smaller the heating speed, the larger the actual austenite grain formed, as shown in Fig. 3.

Therefore, as long as the heating temperature and heating time are properly controlled, induction heating will not produce overheating.

How to Realize Induction Heating Quenching of High Speed Steel? 3

Fig. 3

2. Various phenomena of rapid heating of high speed steel

(1) Rapid heating of high speed steel blade

As early as 1923-1924, vologgin of the former Soviet Union began to study the high-frequency quenching of high-speed steel tools, but it was not successful.

The reason for the failure is that the tools made of high-speed steel must be completely quenched, or the obtained hardened layer with high thermal hardness and high strength is relatively thick.

At the same time, it is also worried that the poor dissolution of high-frequency quenching carbides will affect other properties.

However, this is a superficial perception, and induction quenching has not been studied in depth.

It was not until 1952 that a breakthrough was made.

Gedeberge and others finally succeeded in quenching the W18Cr4V (P18) blade with a size of 3-10mm.

Unfortunately, it did not go to industrial production, but it is enough to show that high-speed steel tools can be induction hardened.

(2) Rapid heating of high speed steel weldment

Rod shaped tools such as high-speed steel taper shank drills and end mills, whether flash welding or friction welding, are models of rapid heating, which can heat steel parts to more than 1000 ℃ in a few seconds.

(3) Rapid heating of high speed steel forgings

The author advocates that the φ60mm high speed steel billet should be directly heated in the high temperature zone, that is, the cold material should be directly heated in the 1150-1200 ℃ zone without preheating.

It has been put into production for many years and the forging quality is stable.

(4) Application of quenching parameter formula of high speed steel tool

There is a quenching parameter formula in the heat treatment of high speed steel tools

That is,

P = t (37 + lg τ)

Where

  • P – quenching parameter;
  • t — quenching heating temperature;
  • τ—— Quenching heating time.

P in the formula represents the comprehensive effect of quenching heating temperature and heating time.

In the quenching process, no matter how the quenching heating temperature and heating time change, as long as the quenching parameters are the same as the result of the two actions, the degree of austenitizing should be equal.

It means that the quenching quality of the tool is the same as long as P is the same at high temperature for a short time (rapid heating) and low temperature for a long time.

(5) Fast heating and semi fast heating in high speed steel tool furnace

At the end of the 1950s, with the help of the heat treatment experts of the Soviet Union, Beijing, Tianjin, Shanghai and other places promoted the new technology of rapid heating and energy saving for heat treatment, and achieved many successful experiences.

Unfortunately, there are not many data left.

The author only has the data of rapid heating of φ14mm taper shank drill and slot milling cutter made of W18Cr4V Steel of Shanghai tool factory.

It is reported that the quenching heating temperature of W18Cr4V Steel is increased from 1270 ~ 1280 ℃ to 1300 ~ 1310 ℃, and the heating coefficient is reduced from 10 ~ 12s / mm to 5 ~ 6S / mm, and the tool life is slightly increased instead of decreasing.

(6) Surface modification of high speed steel cutting tools with high energy density such as laser and electron beam

In recent years, there have been continuous reports on the surface modification of high-speed steel by laser, pointing out that high-speed steel can be heated rapidly.

The technical method is to apply plasma with high energy density to the surface of M42 steel at high speed, so that local rapid temperature rise and rapid cooling occur on the surface of the material.

The temperature rise and cooling speed can reach 104-108k ° / s.

Therefore, the crystal structure modification layer can be formed on the surface of the workpiece to improve the material performance.

(7) Rapid heating of high speed steel has a long history

Over the past 100 years since the advent of high-speed steel, people have never stopped innovating and reforming its heat treatment process.

Some people in the former Soviet Union said that steel could be heated at any speed.

Limited to the conditions at that time, it was only limited to the salt bath furnace and high-frequency heating, and the quenched parts were no longer simple rods or pieces, which did not have universality.

The rapid heating application of high-speed steel forging billet is relatively successful.

Most people think that the heating speed of high-speed steel material after pressure processing and annealing can be unrestricted before forging.

With the emergence of new technologies and processes such as laser and electron beam, there are many reports on the surface modification of high-speed steel by rapid heating, which indicates that the rapid heating of high-speed steel has entered a substantial application stage.

3. Application of induction heating quenching in high speed steel mechanical blades

High speed steel has good hardenability and can be quenched in air, so it is called “wind steel”.

Its hardenability is good, and it can be quenched to above 64HRC in air, so it can grind very sharp edges, so it is also called “blade steel”.

Induction heating quenching of high-speed steel belongs to self cooling quenching, which is energy-saving and environmental friendly and has high production efficiency.

No matter what kind of steel is quenched, there are two basic conditions: first, it must be austenitized; second, it must be cooled immediately.

The cooling rate should be greater than the critical cooling rate of steel (V).

The characteristic of induction heating is that the surface of the workpiece is heated.

If the heating is stopped immediately after the surface layer is austenitized, and the adjacent unheated metal can quickly conduct the heat of the heating layer, and its cooling speed is greater than V, the surface will be hardened.

It is not cooled by spraying quenching liquid on the surface, but is cooled by the cold metal inside.

This special quenching process can only be realized under the condition of high-energy density heating.

Induction heating is one of the high energy density heating methods.

Because the power density is extremely high and the heating time is extremely short, it is also called pulse heating.

The temperature of induction heating can be measured by an infrared photoelectric pyrometer or an optical pyrometer, or the quenching heating temperature can be judged by visual inspection (according to the color of the heated workpiece).

During induction heating, the heat generated by the eddy current on the workpiece is mainly used in the surface layer required for heating.

However, there are two kinds of heat emitted from the workpiece during this process.

The first is radiated from the heating surface to the air, which is called radiant heat;

The second is conducted from the heating layer of the workpiece to the center, which is called heat conduction.

These two heat losses, especially the effect of internal heat conduction, deepen the theoretical depth of the heating layer.

depth = 0.2How to Realize Induction Heating Quenching of High Speed Steel? 4  (mm), where t is the heating time (s).

The loss increases with the decrease of power density and the extension of heating time.

If the workpiece is relatively thin, the heat conduction will soon be transmitted from the surface to the core, and the entire section will be fully hot.

The high-speed steel is a self hardening material, and it will be hardened immediately after the heating is stopped.

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