On 2018-08-08 14:48:54
Common problems with heat treatment
Heat treatment refers to the material in the solid state, through the means of heating, insulation and cooling, in order to obtain the expected structure and performance of a metal thermal processing process.
The microstructure overheating after quenching can be observed from the rough edge of bearing parts. But to determine exactly how hot it is, you have to look at the microstructure. If the rough needle martensite appears in the quenched structure of GCr15 steel, it is the quenched superheated structure. The formation reason may be that the quenching heating temperature is too high or the heating insulation time is too long caused by the overall overheating; It may also be due to the serious banding carbide in the original tissue, forming local martensitic acicular thickening in the low carbon zone between the two bands, resulting in local overheating. The residual austenite in superheated tissues increased and the dimensional stability decreased. Due to the overheating of the quenching structure, the thick steel crystal, will lead to the decline of the toughness of the parts, the impact resistance performance, bearing life is also reduced. Severe overheating can even cause quenching cracks.
Low quenching temperature or poor cooling will produce more than the standard in the microstructure of the tortolite, known as the underheat tissue, it makes the hardness decline, wear resistance sharply reduced, affect the bearing life of idler accessories.
The cracks formed by internal stress in the process of quenching and cooling of bearing parts are called quenching cracks. The reasons for this crack include: because the quenching temperature is too high or the cooling is too fast, the thermal stress and the microstructure stress when the metal mass volume changes are greater than the fracture strength of the steel; The original defects on the working surface (such as surface micro-cracks or scratches) or internal defects in the steel (such as slag, serious non-metallic inclusions, white spots, shrinkage cavity residual, etc.) form stress concentration during quenching; Severe surface decarbonization and carbide segregation; Parts are not tempered enough or not tempered in time after quenching; The previous process caused too much cold impact stress, forging folding, deep turning tool marks, sharp edges and corners of oil groove, etc. In short, the cause of quenching crack may be one or more of the above factors, the existence of internal stress is the main cause of forming quenching crack. Quenching crack deep and slender, straight fracture, broken section without oxidation color. It is usually longitudinal flat crack or ring crack on the bearing ring; The shape on the bearing steel ball is S - shaped, T - shaped or ring - shaped. The microstructure characteristic 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
During the heat treatment of NACHI bearing parts, there are thermal stress and tissue stress, this kind of internal stress can be superposed or partially offset each other, is complex and changeable, because it can change with the heating temperature, heating speed, cooling mode, cooling speed, part shape and size changes, so the heat treatment deformation is inevitable. Understanding and mastering its changing rules can make the deformation of bearing parts (such as the ellipse of the ring, the size increases, etc.) in a controllable range, which is conducive to the production. Of course, mechanical collision in the process of heat treatment will also cause deformation of parts, but this deformation can be reduced and avoided by improved operation.
In the process of heat treatment, if the bearing parts are heated in oxidizing medium, the surface will oxidize to reduce the mass fraction of carbon on the surface of the parts, resulting in surface decarburization. The depth of the decarbonization layer on the surface exceeds the final machining allowance, which will cause the parts to be scrapped. The depth of surface decarburization layer can be determined by metallographic method and microhardness method. The measurement method of microhardness distribution curve of surface layer is the criterion for arbitration.
The phenomenon of insufficient local hardness on the surface of roller bearing parts caused by insufficient heating, poor cooling and improper quenching operation is called quenching soft point. Like surface decarbonization, it can cause a severe decrease in surface wear resistance and fatigue strength.