Vacuum Heat Treatment Overheat & Hardening Defects

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Vacuum heat treatment often presents the question of overheat or underheat, so what is overheat, what is underheat ?

Shanghai Gehang Vacuum Technology Co., Ltd. is a manufacturer with 20 years of production, research and development, design of vacuum furnace, today let’s understand what is the vacuum heat treatment of overheating, underheat and quenching defects.

The microscopic arrangement of overheating after quenching can be investigated from the rough mouth of the bearing parts of the roller fittings.However, it is necessary to investigate the microscopic arrangement in order to determine the degree of overheating.If there is coarse acicular martensite in the quenching arrangement of GCr15 steel, it is quenching superheat arrangement.The reason may be that the quenching heating temperature is too high or the heating and holding time is too long.It may also be due to the original arrangement of banded carbides, in the low carbon zone between the two zones constitute part of martensite acicular thick, formed part of the overheating.The residual austenite increases and the scale stability decreases in the superheating arrangement.Because the quenching arrangement is overheated, the crystal of the steel is thick, which will cause the tolerance of the parts to decline, the impact resistance function to decline, and the life of the bearing to decline.Serious overheating will even form a quenching crack.

Quenching temperature is too low or poor cooling will occur in the microscopic arrangement beyond the specification of the torstenite arrangement, known as the underheat arrangement, it makes the hardness decreased, wear resistance decreased sharply, affecting the bearing life of roller accessories.

Quenching crack defects are common quenching, the reasons are various, due to the defects of heat treatment begins with product design, so the work should prevent crack from the product design, vacuum heat treatment processing Suggestions to correctly choose materials, reasonable structure design, put forward the appropriate heat treatment technical requirements, properly arrange processing route,Choose reasonable heating temperature, holding time, heating medium, cooling medium, cooling method and operation mode, etc.

1. Carbon is an important factor affecting the tendency of quenching.With the increase of carbon content, MS point decreases and quenching tendency increases.Therefore, under the condition of meeting the basic properties such as hardness and strength, the lower carbon content should be selected as far as possible to ensure that it is not easy to quench.

2. The influence of alloying elements on the quenching tendency is mainly reflected in the influence on the hardenability, MS point, grain size growth tendency and decarburization. The influence of alloying elements on the hardenability affects the tendency of quenching.Generally speaking, the hardenability increases, the hardenability increases, but at the same time, the quenching medium with weak cooling capacity can be used to reduce the quenching deformation to prevent the deformation and crack of complex parts.Therefore, for parts with complex shapes, it is a better solution to choose steel with good hardenability and quenching medium with weak cooling capacity in order to avoid quenching cracks.

Alloying elements have a great influence on the MS point. Generally speaking, the lower the MS point is, the greater the tendency of quenching is. When the MS point is high, the martensite generated by phase transformation may be self-tempered immediately, thus eliminating part of the phase transformation stress and avoiding quenching.Therefore, when the carbon content is determined, a small amount of alloying elements should be selected, or steel with elements that have little influence on MS point.

3. When choosing steel, overheating sensitivity should be considered.Overheat is more sensitive to the steel, easy to crack, so in the choice of materials should be paid attention to.

Editing by Frank Lee
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