The generation, classification and removal method of residual stress

Causes of residual stress

A component is affected by internal and external influences such as uneven stress field, strain field, temperature field, and inhomogeneity of material structure during production, processing and processing, and the stress that remains after the component is completed. Without the action of external factors, the internal stress of the component is kept in balance.

 When a component bears a load that exceeds the elastic limit range, the residual stress in the component after the load is removed.

Classification of the causes of residual stress

⑴ Residual stress generated by heat treatment

⑵Residual stress caused by surface treatment

⑶Residual stress caused by cutting and grinding

⑷ Residual stress caused by cold working

⑸ Residual stress caused by casting and forging

⑹Residual stress caused by electroplating

⑺Residual stress caused by welding

The residual stress analyzer can quickly and easily analyze the residual stresses generated by gears, bearings, rolls, crankshafts, camshafts, pressure vessel pipes and some other parts in the process of heat treatment, machining, welding, shot peening, and rolling. Effectively avoid harmful residual stress from reducing the fatigue strength and corrosion resistance of the workpiece, prolong the service life of the workpiece, and avoid major accidents. And some parts introduce beneficial residual stress, such as rolling, shot peening, etc., which can improve the surface performance of the workpiece. Therefore, accurate measurement of residual stress becomes very necessary.

According to the mutual influence and range of residual stress, it can be divided into three types:

⑴ Macroscopic stress: the stress caused by the unevenness of the macroscopic deformation of different parts of the component, or the stress caused by the uneven external load, that is, the volumetric stress (larger material area), the balance within the macroscopic size range.

⑵ Microscopic stress: it is caused by the unevenness of deformation between grains or sub-grains, that is, the internal stress of the structure or the residual stress of the structure (within the smaller range of the material), and the balance within the grain size.

⑶ Lattice distortion stress: it is caused by a large number of lattice defects formed in the plastic deformation of the component, or the uneven residual stress in each crystal grain, the uneven deformation stress caused by dislocation (very small material Within the region), the balance within a single unit cell.

The distortion and cracks formed during the heat treatment of the workpiece are the result of the combined effect of these internal stresses. At the same time, under the action of heat treatment stress, sometimes one part of the workpiece is in a state of tensile stress, while another part is in a state of compressive stress, which may sometimes make the distribution of the stress state of each part of the workpiece very complicated. In this regard, it should be analyzed according to the actual situation.

Removal and adjustment of residual stress

1. Thermal aging method

⑴ Tempering processing deformation→welding→quenching→temperature→time→speed

⑵ Resistance method Direct heating → local

2. Mechanical Law

⑴Static load method natural aging

⑵Vibration aging method Resonance and harmonics→excitation→control and detection

⑶Rolling method Compressive stress → fatigue strength → smoothness → cracking, life

⑷Shot peening method Compressive stress→anti-fatigue→improving hardness→corrosion resistance