常见的质量缺陷 bearing parts after heat treatment are: quenching microstructure Quality Defects overheating, underheating, quenching cracks, 硬度不足, 热处理变形, surface decarburization, soft spots and so on.
The microstructure overheating after quenching can be observed from the rough mouth of bearing parts. Quality Defects However, the microstructure must be observed to determine the degree of overheating.
If coarse acicular martensite appears in the quenched structure of GCr15 steel, 为淬火过热结构. The cause may be the Quality Defects overall overheating caused by too high quenching heating temperature or too long heating and holding time;
It may also be due to the serious banded carbide in the original Quality Defects structure and the formation of local martensite acicular coarseness in the low-carbon region between the two bands, 导致局部过热. 过热组织中的残余奥氏体增加，尺寸稳定性降低.
Due to the overheating of quenching structure and the coarseness of steel crystal, the toughness of parts will be reduced, the impact resistance will be reduced, and the service life of Quality Defects bearings will also be reduced. 严重的过热甚至可能导致淬火裂纹.
Low quenching temperature or poor cooling will produce troostite Quality Defects structure exceeding the standard in the microstructure, 称为欠热结构. It will reduce the hardness, sharply reduce the wear resistance and affect the service life of the bearing.
The crack formed by internal stress during quenching and cooling of bearing parts is called quenching crack. The reasons for this kind of crack are as follows: because the Quality Defects quenching heating temperature is too high or the cooling is too urgent,
the structural stress when the thermal stress and metal mass volume change is greater than the fracture strength of the steel; The original Quality Defects defects on the working surface (such as surface micro cracks or scratches) 或钢的内部缺陷
(比如夹渣, serious non-metallic inclusions, 白点, Quality Defects shrinkage cavity residue, 等等) 淬火时形成应力集中; 严重的表面脱碳和碳化物偏析; The tempering of parts after quenching is insufficient or not tempered in time;
Excessive cold punching stress caused by the previous process, 锻造折叠, deep turning tool marks, sharp edges and corners of oil groove, ETC. In a word, the cause of quenching crack may be one or more of the above factors, and the existence of internal stress is the main cause of quenching crack.
The quenching crack is deep and slender, the fracture is straight, and the fracture surface has no oxidation color. It is often a longitudinal straight crack or annular crack on the bearing ring; The shape on the bearing steel ball can be 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.
During the heat treatment of bearing parts, there are thermal stress and structural stress. 这种内应力可以相互叠加或部分抵消, 这是复杂多变的. 因为它会随着加热温度的变化而变化, 加热速度,
cooling mode, 冷却速度, 零件形状和尺寸, 热处理变形不可避免. Understanding and mastering its change law can put the deformation of bearing parts (such as the ellipse of ferrule, 尺寸增加, 等等) 在可控范围内, which is conducive to production.
当然, 热处理时的机械碰撞也会引起零件变形, 但这种变形可以通过改进操作来减少和避免.
During the heat treatment of bearing parts, 如果它们在氧化介质中加热, the surface will be oxidized, reducing the mass fraction of carbon on the surface of parts and causing surface decarburization. 如果表面脱碳层深度超过后加工余量, 零件将被报废.
表面脱碳层深度可用金相法和显微硬度法测量. The measurement method of microhardness distribution curve of surface layer shall prevail and can be used as arbitration criterion.
The phenomenon of insufficient local hardness on the surface of bearing parts caused by insufficient heating, poor cooling and improper quenching operation is called quenching soft spot. Like surface decarburization, it can cause a serious decline in surface wear resistance and fatigue strength.