The defects of raw materials mainly include surface cracks, surface folds, surface decarburization, surface coarse crystal rings, residual shrinkage cavities and inclusion defects.
Surface crack defects of raw materials
Cracks exist on the surface of raw materials. Cracks that are not taken out after fastener processing or that are caused to expand during the forming process will cause cracks on the surface of fasteners, as shown in the following figure:
1. Longitudinal crack of bar
The fastener raw material bar, due to the improper rolling process of the steel mill, caused the longitudinal crack extending along the rolling direction on the bar surface.
This type of crack is generally linear, and the crack direction is basically consistent with the main deformation direction of rolling.
There are many reasons for the longitudinal cracks of raw materials, such as the scratch type defects on the surface of rolled billets, which crack along the scratch when cooling due to stress concentration.
Another example is that the internal defects of raw materials deform along the streamline direction of rolling and are exposed on the surface of bars, cracking under stress.
The figure below shows the longitudinal crack morphology of the raw material surface of the superalloy bar.
2. Transverse crack of bar
The raw material for fasteners is bar. Due to improper head cutting process in the steel plant or improper extrusion process parameters, the surface of the head of the bar has transverse cracks. The following figure shows the transverse equilines on the surface of the raw material of the superalloy bar.
Transverse surface cracks are more harmful than longitudinal cracks.
3. Internal crack
The internal crack of raw materials is also a common defect. Generally, the cracks are zigzag, the tail is thin, and there are local bifurcations.
If decarburization exists on both sides of the crack and there is oxide layer in the crack, it indicates that the crack has existed before heat treatment.
The following figure shows the metallographic appearance of the transverse and longitudinal section of the internal crack of the bolt raw material.
The internal cracks are mostly related to the internal defects of the material, such as looseness and inclusion.
Especially in the process of extruding the bar, because the deformation of the centerline is zero theoretically, the original defects inside the material are deformed incongruously due to the extrusion stress of the outer surface is much larger than the center, resulting in further cracking.
Folding of raw material surface
Alloy bars for fasteners, due to improper production process in the steel plant, cause folding or even cracks on the surface of the bars.
For fasteners, the folding is mainly caused by the inaccurate sizing of groove on the roll or the burr generated on the wear surface of groove being involved in the rolling process of metal billet, which forms folding at a certain angle with the material surface.
The folding cracks on the surface of raw materials of nuclear materials for fasteners are shown in the figure below.
Decarbonization of raw material surface
The decarburization on the surface of raw materials is mainly caused by improper annealing of raw materials during drawing in the steel plant.
The decarburization layer on the surface of raw materials is distributed on the surface of fastener materials, and there is no decarburization on the machined end faces at both ends.
In use, decarburization will not only reduce the overall strength of fasteners, especially because the decarburized layer on the surface has low strength, which is easy to produce surface cracks in the decarburized layer, causing early fracture failure of fasteners.
Especially for threads, surface decarburization leads to a significant reduction in thread strength, which is prone to tooth loss, tripping and other failures.
The decarburization structure on the screw surface is shown in the figure below.
Coarse crystal ring on the surface of raw material
Coarse crystal ring on the surface of raw materials is a defect that usually occurs on aluminum alloy and magnesium alloy extruded bars, and also occurs in steel and other materials.
The thickness of the coarse crystal ring increases gradually from the beginning to the end of the extrusion.
If the lubrication conditions during extrusion are good, the existence of coarse crystal rings can be reduced or avoided after heat treatment.
When the coarse crystal ring on the surface of the raw material causes the coarse crystal ring on the surface of the fastener, the performance of the fastener will be significantly reduced, especially on the interface between the coarse crystal ring and the fine crystal due to the difference in strength, resulting in incompatible deformation and cracks.
The coarse crystal ring on the surface of a bolt rod is shown in the figure below.
It can be seen that the interface between the coarse crystal ring and the normal tissue area is very obvious, and it is very easy to crack on this interface.
When coarse grains exist on the surface of raw materials, cold extrusion or pier forming will also lead to the poor surface roughness of fasteners, which will show orange peel characteristics in severe cases.
This is most obvious in fasteners made of low strength materials such as aluminum alloy or magnesium alloy.
Orange peel is essentially a network of micro cracks on the surface along the grain boundary. When in use, the micro cracks expand and become macro cracks.
The main reason for the formation of coarse crystal ring is the improper cold drawing deformation of raw materials.
For example, for high temperature alloy cold drawn bars, the deformation of the last cold drawing before the finished product is not properly controlled, so that the deformation of the material surface falls in the critical deformation zone, and the grains in the critical deformation zone grow up to form coarse crystal rings after solution heat treatment.
Residual shrinkage cavity of raw materials
The residual shrinkage cavity of raw materials is mainly caused by the steel mill’s incomplete removal of the ingot cap, which remains in the raw materials during opening and rolling.
In areas with dense residual shrinkage cavities, inclusions, porosity, segregation and other defects will generally occur.
After deformation or processing of fasteners, raw materials with residual shrinkage cavities will make the center of fasteners appear through holes and irregular internal cracks from beginning to end.
The crack propagation is easy to cause the failure of fasteners. TB3 titanium alloy screws have been broken for many times during assembly.
The analysis shows that the screw fracture is due to the residual shrinkage cavity of the raw material is not removed completely, and it is not welded during the forging process.
Due to repeated forging deformation, the defect cracks along the original β grain boundary. When the screw is subjected to axial assembly stress, the fracture occurs.
The fracture morphology of the broken screw is shown in the figure below (a), and the shrinkage cavity in the center is obviously seen.
The free surface characteristics of the enlarged shrinkage cavity surface are shown in the figure below (b).
Inclusion defects of raw materials
The impurity elements in raw materials mainly come from the raw materials used in the material smelting process, the pollution of the atmosphere, furnace wall or crucible materials in the smelting process, the pollution of alloy ingots or master alloys and the pollution in the casting process of parts.
When the inclusions are clustered or distributed in a chain along the deformation direction during the forming process, it is easy to cause the fasteners to crack along the inclusions during processing and use.
The following figure shows the longitudinal crack of the GH2036 superalloy nut after processing due to inclusion.
When observing the crack fracture, a large number of inclusion particles were found on the fracture, as shown in the figure below.