Common defects in the heat treatment process of Hastelloy tube UNS N08810 (Incoloy 800H), heat treatment is achieved through heating, heat preservation and cooling. If these three methods are not well grasped, the following common problems will occur:
Overheating-the retained austenite in the overheated UNS N08810 (Incoloy 800H) structure increases and the dimensional stability decreases. Due to the overheating of the quenched structure, the crystals of UNSN08810 (Incoloy 800H) steel are coarse, which will lead to the decrease of the toughness of the parts, the impact resistance and the life of the bearing. Severe overheating may even cause quenching cracks.
Underheating-low quenching temperature or poor cooling will produce troostite structure in the microstructure that exceeds the standard, called underheated structure, which reduces the hardness of UNSN08810 (Incoloy 800H) and sharply reduces wear resistance, which affects UNS N08810 (Incoloy 800H) Material life.
Quenching cracks-the reasons for this kind of cracks are: due to the excessively high quenching heating temperature or the rapid cooling, the thermal stress and the structural stress of the metal mass volume change are greater than the fracture strength of the UNS N08810 (Incoloy 800H) steel; the original defects of the working surface (Such as surface micro cracks or scratches) or internal defects of UNS N08810 (Incoloy 800H) steel (such as slag inclusions, serious non-metallic inclusions, white spots, shrinkage cavity residues, etc.) stress concentration during quenching; severe surface decarburization and carbides Segregation; insufficient tempering of parts after quenching or not timely tempering; excessive cold punching stress caused by the previous process, forging folding, deep turning tool marks, sharp edges and corners of the oil groove, etc. In short, the cause of quenching cracks may be one or more of the above factors, and the existence of internal stress is the main reason for quenching cracks. The structural feature of quenching crack is that there is no decarburization on both sides of the crack, which is obviously different from forging crack and material crack.
Heat treatment deformation—UNS N08810 (Incoloy 800H) has thermal stress and structural stress during heat treatment. This internal stress can be superimposed or partially offset each other, which is complex and changeable because it can vary with heating temperature, heating speed, cooling method, The cooling rate, part shape and size change, so UNS N08810 (Incoloy 800H) heat treatment deformation is inevitable.
Surface decarburization-UNS N08810 (Incoloy 800H) in the heat treatment process, if it is heated in an oxidizing medium, oxidation will occur on the surface to reduce the mass fraction of carbon on the surface of the part, resulting in surface decarburization. The depth of the surface decarburization layer exceeds the remaining amount of post-processing will cause the parts to be scrapped. UNS N08810 (Incoloy 800H) surface decarburization layer depth can be measured in metallographic inspection by metallographic method and microhardness method. The surface layer microhardness distribution curve measurement method shall prevail, which can be used as the arbitration criterion.
Soft point-due to insufficient heating, poor cooling, improper quenching operation, etc., the phenomenon of insufficient surface hardness of UNS N08810 (Incoloy 800H) is called quenching soft point. It can cause the surface UNS N08810 (Incoloy 800H) wear resistance and fatigue strength to seriously decrease like surface decarburization.
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