Ensure that there is no defect crack after quenching the steel bar

What should I do if the workpiece exhibits defective cracks after quenching? Haituo Electromechanical teaches you how to do it, no longer need to worry, the defects after quenching.
After the steel bar is quenched by the medium frequency induction steel bar heat treatment equipment, in order to eliminate the residual stress generated during quenching, improve the plasticity and toughness of the material, and obtain good comprehensive mechanical properties, a certain degree of tempering is required.

 

First, the mechanical properties of the steel bar after tempering
1. Strength and plasticity The strength of carbon steel after tempering at a lower temperature is slightly improved, the plasticity is basically unchanged, and the strength is decreased and the plasticity is increased when the tempering temperature is further increased.
 
2. The hardness of the hardness steel decreases as the tempering temperature rises. Carbon steel with a high carbon content has a slight increase in hardness when carbides are precipitated. Alloy steel containing a strong carbide forming element undergoes "secondary hardening" when a special carbide is formed, and the hardness increases. The high-alloy steel such as high-speed steel has a large amount of retained austenite and is very stable. Some of the retained austenite is not fully decomposed after tempering, but after cooling, it is transformed into martensite, which increases the hardness of the steel.
 
3. Toughness carbon steel and a chrome-nickel steel decrease in toughness after tempering at 250-400 °C. The brittleness at this time is called the first type of temper brittleness or tempered martensite brittleness, and the chromium-nickel steel is 450- The toughness decreases again when tempered at 600 ° C, and the resulting brittleness is called the second type of temper brittleness or high temperature temper brittleness.
 
Second, the common defects after tempering are mainly
(1) Distortion, mainly due to the redistribution of quenching stress during the tempering process. Therefore, the flat and slender parts should be compensated by means of pressurized tempering or heat-alignment.
 
(2) The hardness is unqualified, the hardness of the workpiece is high or low after tempering in the heat treatment furnace of the steel bar, or the hardness is not uniform, which is mainly caused by uneven temperature of the equipment and incorrect tempering temperature.

 

(3) Reticulated cracks. If the surface is decarburized in high-speed steel or high-carbon steel, the specific volume change of the inner layer during tempering is greater than that of the surface layer, and multi-directional tensile stress is formed on the surface to form a network crack. At the same time, due to the superheating of the surface during tempering, the surface layer is quickly preferentially tempered to form a multi-directional tensile stress, and a network crack is also formed.
 
For complex tools, molds and high-cold rolls made of high-carbon, high-alloy steel, due to the large quenching stress, if there is no tempering in time after quenching, there is a danger of cracking at any time.
 
Important parts need to be tempered after quenching, so it is necessary to understand the performance and defects of the workpiece after tempering. Haituo electromechanical personnel have rich experience in quenching and tempering parts. If you have any questions, you can consult Haituo Electromechanical technicians.