Surface heat treatment methods, characteristics and applications

June 12, 2019

Surface heat treatment methods, characteristics and applications (Flame surface quenching and induction heating surface quenching)


Surface heat treatment is by changing the surface structure of parts to obtain high hardness of martensite, and retain the core toughness and plasticity (surface quenching), or change the chemical composition of the surface at the same time, in order to obtain corrosion resistance, acid resistance, alkali resistance, and surface hardness higher than the former (chemical heat treatment) method.

Flame surface quenching: the flame burning with a mixture of acetylene - oxygen or gas - oxygen is sprayed onto the surface of the part for rapid heating. When the quenching temperature is reached, water is sprayed or cooled with emulsion immediately.The depth of the quenching layer is generally 2-6mm, and the excessive depth often leads to severe overheating on the surface of parts, which is prone to quenching cracks.Surface hardness: up to HRC65 for steel, hrc40-48 for gray cast iron, hrc43-52 for alloy cast iron.This method is simple and does not need special equipment, but it is easy to overheat and the quenching effect is not stable, so its application is limited.Suitable for single or small batch production of large parts and the need for local quenching tools or parts, such as large shaft, large modulus gear.

Commonly used steel is medium carbon steel, such as 35,45 and medium carbon alloy structure steel (alloy element <3%), such as 40Cr, 65Mn, can also be used for gray cast iron, alloy cast iron.Carbon content is too low, the hardness after quenching is low, and carbon and alloy elements are too high, it is easy to fracture, therefore, carbon steel with carbon content between 0.35-0.5% is the most suitable.

Induction heating surface quenching: the workpiece is put into the inductor to generate induction current on the surface of the workpiece. After heating to the quenching temperature in a very short time, water is immediately sprayed to cool the surface of the workpiece, so as to obtain very small acicular martensite structure.According to the current frequency, induction heating surface quenching can be divided into: high-frequency quenching;100-1000khz. Medium frequency quenching;1-10khz. Power frequency quenching;50 hz

1. The surface hardness is 2-3hrc higher than ordinary quenching and has lower brittleness:

2. Fatigue strength and impact toughness are improved, and the general workpiece can be increased by 20-30% :

3. Small deformation:

4 quenching depth is easy to control:

5 it is not easy to oxidize and decarburize during quenching:

6 less expensive low hardenability steels are available:

Easy to realize mechanization and automation, high productivity

The higher the current frequency, the thinner the quenching layer.High-frequency quenching generally 1-2mm;Medium frequency quenching generally 3-5mm;Power frequency quenching can reach >=10-15mm high frequency induction heating: the current frequency is 100 ~ 500 kHz (kHz), and the effective hardening depth is 0.5 ~ 2 mm (mm), which is mainly used for medium and small parts requiring a thin quenching layer, such as small modulus gear, small and medium-sized shaft, etc. Medium frequency induction heating: the current frequency is 500 ~ 10000 Hz (Hz), and the effective hardening depth is 2 ~ 10 mm (mm). It is mainly used for parts requiring a deeper quenching layer, such as gears with medium modulus, gears with large modulus and shafts with large diameter.The intermediate heat treatment of surface quenching parts is quenching surface.

Disadvantages: it is more difficult to deal with complex parts than carburizing: medium carbon steel (0.4-0.5%C) and medium carbon alloy structural steel are commonly used, high carbon tool steel and low alloy structural steel are also available, and cast iron.When the penetration depth of general parts is about 1/10 of the radius, the best combination of strength, fatigue resistance and toughness can be obtained.For parts with a small diameter of 10-20mm, it is recommended to use a deeper penetration layer depth, which can reach 1/5 of the radius;For parts with large cross sections, it is advisable to have a shallow penetration depth, that is, less than 1/10 of the radius.

1. Parts working under friction conditions, such as pinion, shaft, 45, 40Cr, 42MnVB;High-frequency quenching, depth of 1.5-2mm

2. Parts subject to distortion and pressure load, such as crankshaft, large gear, grinder spindle, 45, 40Cr, 65Mn, 9Mn2V, ductile iron;Medium frequency quenching, quenching depth 3-5mm

3. Large parts subject to distortion and pressure load, such as cold rolls, 9Cr2Mo, 9Cr2W;Power frequency quenching, quenching depth >=10-15mm.