Tempering Methods and Common Defects of Induction Heat Treatment

01 Preface

Generally speaking, we all know that tempering (note: tempering is also called “matching”) must be done after quenching. But sometimes we observe on-site, why some workpieces do not seem to be tempered after induction hardening. In fact, are such workpieces really not tempered? The answer is definitely no. A specific method of tempering is used, see the following.

02Tempering of induction hardened parts

Generally, only low-temperature tempering is carried out after induction quenching. The purpose is to reduce residual stress and brittleness without reducing hardness but should try to maintain high hardness and high surface residual compressive stress.​​

Commonly used tempering methods include furnace tempering, self-tempering, and induction heating tempering.

1 Furnace tempering

Induction hardened cold-through workpieces, immersion quenched workpieces after continuous quenching, and thin-walled and complex-shaped workpieces are usually tempered in an air furnace or an oil bath furnace;
In order to retain high residual compressive stress on the surface of the workpiece after high-frequency surface quenching, the tempering temperature is lower than that of ordinary heating and quenching, generally not higher than 200 °C.​​

2 Self-tempering

Self-tempering is a short-term tempering achieved by cooling the residual heat by induction hardening.​​
Self-tempering not only simplifies the process but is also effective in preventing quenching cracks in high carbon steel and some high alloy steels. The main disadvantage of self-tempering is that the process is not easy to master, and the elimination of quenching stress is not as good as furnace tempering.​​

When self-tempering is used, the temperature, spray cooling time, and spray pressure of the cooling medium should be strictly controlled. Specific operating specifications should be determined by testing specific workpieces.


3 Induction heating and tempering
​​Induction Heat Treatment

For long shafts or other parts that are continuously induction hardened, it is sometimes convenient to use induction heating and tempering.​​

This tempering method can be carried out immediately after quenching. Since the tempering temperature is lower than the magnetic transition temperature, the penetration depth of the current is small. On the other hand, in order to reduce the residual tensile stress in the transition layer of the surface hardened part, the depth of the induction heating layer for tempering should be deeper than that of the quenching layer to achieve the purpose of tempering.​​
Therefore, induction heating and tempering should use a very low frequency or a small specific power, prolong the heating time, and use heat conduction to thicken the heating layer. When using the simultaneous heating method, the heating layer can be thickened by the continuous heating method.​​

The biggest feature of induction heating tempering is the short tempering time. Therefore, to achieve the same hardness and other properties as tempering in the furnace, the tempering temperature should be increased accordingly.​​

In addition, using induction heating and tempering, due to the short heating time, the obtained microstructure has great dispersion, and the wear resistance and impact toughness after tempering are higher than those in furnace tempering, and it is easy to arrange on the assembly line. . Induction tempering requires a heating rate of less than 15-20°C/s.

 

03 Common quality problems and repairs of induction hardening

​​
1 Common problem of induction hardening

Common quality problems in induction heating heat treatment include cracking, too high or too low hardness, uneven hardness, too deep or too shallow hardened layer, etc.​​

Defect 1: Cracking

Cause:

The heating temperature is too high and the temperature is uneven; the cooling is too rapid and uneven; the quenching medium and temperature are improperly selected; the tempering is not timely and tempering is insufficient; Unreasonable design of parts, improper technical requirements, etc.


Defect 2: The hardened layer is too deep / too shallow

Cause:

The heating power is too large or too small, the current frequency is too low or too high, the heating time is too long or too short, the hardenability of the material is too low or too high, the temperature/pressure/composition of the quenching medium is improper, etc.​​

Defect 3: Surface hardness is too high / too low

Cause:

The high or low carbon content of the material, surface decarburization, low heating temperature, improper tempering temperature or holding time, improper quenching medium composition/pressure/composition, etc.​​

Defect 4: uneven surface hardness

Cause:

The structure of the inductor is unreasonable, the heating is uneven, the cooling is uneven, and the material organization is poor (band-like tissue segregation, local decarburization), etc.​​

Defect 5: Surface melting

Cause:

The structure of the sensor is unreasonable, the parts have sharp corners, holes, grooves, etc., the heating time is too long, and there are cracks on the surface of the material.

2 Rework for unqualified induction hardening

Induction heating heat treatment parts are allowed to be repaired in the following cases.​​

If the hardness is low or there are large soft spots, the extent of the hardened area and the depth of the hardened layer do not meet the technical requirements, and the metallographic structure is unqualified due to insufficient heating temperature, the processing methods of the repaired parts can be carried out according to the following two methods:

method 1

After induction heating to 700-750 ℃, the repaired parts are cooled in air, and then the second quenching is carried out according to the quenching specifications of the parts.​​

Method 2

The repaired parts are heated to 550-600 ℃ in the furnace and kept for 60-90 minutes. Then cool in water or air, and then perform a second quenching according to the original quenching specification.

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