29
2022
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09
What are the common defects of Hastelloy tubes during heat treatment?
The common defects in the heat treatment of Hastelloy tubes are realized by means of heating, heat preservation and cooling. If these three methods are not well understood, the following common problems will occur:
1. Overheating - the retained austenite in the overheated structure increases and the dimensional stability decreases. Because the quenching structure is overheated and the steel crystal is coarse, the toughness and impact resistance of the parts will be reduced, and the life of the bearing will also be reduced. Severe overheating may even cause quenching cracks.
2. Underheating - low quenching temperature or poor cooling will produce more than standard troostite structure in the microstructure, which is called underheating structure. It will reduce the hardness, wear resistance and material life.
3. Quenching crack - the reasons for this kind of crack are: because the quenching heating temperature is too high or the cooling is too fast, the thermal stress and the structural stress when the metal mass volume changes are greater than the breaking strength of the steel; The original defects of the working surface (such as surface micro cracks or scratches) or the internal defects of the steel (such as slag inclusion, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) form stress concentration during quenching; Serious surface decarburization and carbide segregation; Insufficient tempering or delayed tempering of parts after quenching; Excessive cold punching stress, forging folding, deep turning tool marks, sharp edges and corners of oil grooves caused by the previous process. In a word, quenching cracks may be caused by one or more of the above factors, and the existence of internal stress is the main reason for forming quenching cracks. The quenching crack is characterized by no decarburization on both sides of the crack, which is obviously different from the forging crack and material crack.
4. Heat treatment deformation - during heat treatment, there are thermal stress and organizational stress. This kind of internal stress can overlap or partially offset each other, which is complex and changeable. Because it can change with the change of heating temperature, heating speed, cooling method, cooling speed, part shape and size, heat treatment deformation is inevitable.
5. Surface decarburization - in the heat treatment process, if it is heated in an oxidizing medium, the surface will be oxidized to reduce the mass fraction of carbon on the part surface, causing surface decarburization. Parts will be scrapped if the depth of surface decarburization layer exceeds the allowance for post-processing. Metallographic method and microhardness method can be used to determine the depth of surface decarburization layer in metallographic examination. The method of measuring microhardness distribution curve of surface layer shall prevail and can be used as arbitration criterion.
6. Soft spot - the phenomenon of insufficient local surface hardness caused by insufficient heating, poor cooling, improper quenching operation, etc. is called quenching soft spot. Like surface decarburization, it can seriously reduce the surface wear resistance and fatigue strength.
Shandong Titanium Nickel Special Steel Co., Ltd
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