III. Common defects causing metal failure
Common metal forging and rolling parts defects
1 Forging defects
Large grain
Causes: the beginning of the forging temperature is too high and the degree of deformation is insufficient, the final forging temperature is too high or the degree of deformation falls into the critical deformation zone. Aluminium alloy deformation degree is too large to form a structure, high temperature alloy deformation temperature is too low to form a mixture of deformation organization may also cause coarse grain.
Impact: coarse grain will make the plasticity and toughness of forgings reduced, fatigue performance decreased significantly.
Uneven grain
Causes: uneven deformation of the billet in various places so that the degree of grain fragmentation, or the degree of deformation of the local area falls into the critical deformation zone, or high-temperature alloys, local work hardening, or Quenching and heating of the local coarse grains.
Impact: uneven grain will make the forging lasting performance, fatigue performance decreased significantly.
Cold hardness
Causes: deformation temperature is low or deformation speed is too fast, as well as forging cooling too fast, so that the softening caused by recrystallisation can not keep up with the deformation caused by the strengthening (hardening), so that after hot forging forgings are still partially retained within the cold deformation of the organisation.
Impact: improve the strength and hardness of forgings, but reduce the plasticity and toughness, serious cold hardening phenomenon may cause forging crack.
Crack
Causes: Large tensile, shear or additional tensile stresses exist during forging. Cracks occur in the part of the billet is usually the greatest stress, the thinnest part of the thickness. If the billet surface and internal micro-cracks, or billet organisational defects, or improper heat processing temperature so that the plasticity of the material is reduced, or the deformation rate is too fast, the degree of deformation is too large, more than the material allows the plasticity of the pointer, etc., may be in the forging process to produce cracks.
Impact: cracks will seriously affect the quality and performance of the forging, and even lead to the scrapping of the forging.
Folding
Causes: metal deformation process has been oxidised surface metal confluence and formed. It is related to the shape of raw materials and billets, the design of moulds, the arrangement of forming process, the lubrication situation and the actual operation of forging.
Impact: Folding not only reduces the bearing area of the part, but also tends to be a source of fatigue due to the stress concentration here when working.
Through the flow
Cause: a form of improper distribution of flow. In the flow-through area, the original into a certain angle distribution of flow lines converge together to form a flow-through, and may make the flow-through area, outside the grain size difference is relatively large.
Impact: through the flow so that the mechanical properties of forgings reduced, especially when the flow through the belt on both sides of the grain difference is more disparate, performance reduction is more obvious.
2,Rolled parts defects
Iron oxide pressed into
Cause: hot rolling process, the slab heating produced serious iron oxide skin, not removed before rolling clean, rolling pressed into the plate.
Impact: iron oxide pressed into the defect will directly affect the surface quality of rolled parts, and even affect the subsequent processing and performance.
Numbness
Cause: hot rolling process, the surface of the work roll rough or rolling surface of thicker iron oxide, resulting in iron broken to form pockmarks.
Impact: pitting defects will reduce the surface finish of rolled parts, affecting the appearance of quality and performance.
Hole
Cause: In the rolling process, due to the side of the steel plate impact on the side guide plate of the finishing mill unit, the impact splash falls into the surface of the steel plate, and in the subsequent rack pressed into the steel plate.
Impact: the hole defect will seriously affect the strength and toughness of the rolled parts, reducing its use of performance.
Scratch
Causes: rolling process, the work rolls or coiler pinch rolls sticking to the foreign body, or rolling parts and mechanical equipment components relative to the movement of scratches.
Impact: scratches defects will destroy the surface integrity of the rolled parts, affecting its appearance and performance.
Preventive measures
For the above defects, the following prevention and control measures can be taken:
Strictly control the process parameters during forging and rolling, such as temperature, speed, degree of deformation, etc., to ensure that processing is carried out under suitable process conditions.
Strengthen the quality control of raw materials and billets to ensure that there are no defects or the defects are within the permissible range.
Optimise mould design and forming process arrangement to ensure even metal flow and no defects such as folding and flow-through.
Strengthen the maintenance and upkeep of equipment to ensure good surface quality of key components such as work rolls.
Strengthen quality inspection and control to detect and deal with defects in a timely manner.
