In our daily steel wire production, oil-quenched steel wires occupy a large part. Mastering the method of judging the quality of steel wires is very important for us to make a good spring.
The use of spring steel wire to judge the problem of oil quenching currently only stops on the surface of the wire and the entire shape. It is not easy to judge the deep interior. Today we will gradually analyze the surface and internal problems of the wire.
First, the surface quality defect
1. Surface crack: The linear cracks on the steel surface should generally be consistent with the forging or rolling direction.
Reasons for formation:Mainly because of wrinkles and thermal cracks during processing (forging, rolling, heat treatment and tempering) due to surface over-burning, decarburization, loosening, deformation and excessive internal stress, as well as high levels of sulfur and phosphorus impurities on the surface And cold cracks.
Surface cracks can be detected by visual inspection, pickling, magnetic particle inspection, color inspection and gold equivalent methods. When confirming cracks, we must pay attention to distinguish the cracks on the surface of the steel itself that are brittle and loose, and cracks that appear after slight bending, but the steel itself does not have cracks.
2. Heavy skin and folds: "Tongue" or "scaly" metal flakes adhered to the surface of the steel, forming overlaps on local surfaces with obvious folds.
Reason for formation: During the hot working process, due to burrs, depressions, inclusions, subcutaneous pores, and surface looseness on the billet, the metal rheologically changes during thermal deformation, and heavy skin and folds are formed on the surface.
3, Ear: The surface of the steel material extends along the rolling direction.
Reasons for formation: The gap between the rolling holes of the rolling mill is too large, which causes the surface of the steel to form protrusions along the pores.
4. Scratch: The surface of the steel is a linear or arc-shaped groove mark under the action of external force (the groove bottom can be seen).
Second, internal defects
1. Segregation: In fact, it is a general term for the uneven distribution of chemical components in steel. On the acid leaching sample, when the segregation is an erodible substance or gas inclusions, it is dark and dark in color, irregular in shape, slightly concave, flat at the bottom, and there are many dense microporous spots. If it is a resist element, it is light in color, irregular in shape, and relatively smooth, slightly concave spots. According to the location and shape of segregation, they are generally classified into the following categories:
① Center segregation: dark and dark spots with irregular shapes appearing in the center.
② Ingot-type segregation: Dark-spotted spots concentrated on a closed band with different widths， which have the cross-sectional shape of the original steel ingot (usually square), so ingot-type segregation is also called box segregation.
③Spotted segregation: The spots are generally large, showing darker, slightly concave shapes, oval or melon seeds. Generally distributed, called general point segregation: those distributed on the edge of the steel are called edge point segregation.
Reason for formation: Segregation is the aggregation of certain elements caused by selective crystallization and diffusion during the ingot casting and solidification process. Segregation is unavoidable under normal production conditions.
2. Looseness: The pores in the steel are generally irregular polygons on low magnification samples, and the sharp pits at the bottom usually appear in the segregated spots. In severe cases, there is a tendency to become spongy. According to the situation of loose distribution, they are divided into two categories: central loose and general loose:
① Loose center: there are concentrated voids and dark spots in the center of the low magnification sample. There is a slight interlayer on the longitudinal fracture. Under the microscope, you can see that pearlite increases in the center looseness, indicating that the carbon content in the center looseness increases.
②Generally loose: The tissue is dense on the low magnification sample, showing scattered small pores and small black spots. The pores are mostly irregular polygons or graphics, which are distributed on the entire cross section except the edge part.
Central loosening generally occurs at the head and middle of the steel ingot. The difference from ordinary loosening is that it is distributed on the steel section and the central part instead of the entire section. Generally, the higher the carbon content of the steel, the more severe the center porosity.
Reason for formation: During the solidification process of the steel ingot, due to the solidification and contraction of the low-melting-point material in the intergranular part and the release of gas, voids were generated, and the welded pipe was not equipped during the hot processing.
In steel, slight segregation and higher levels of porosity are allowed.
3. Inclusions: Inclusions include metal inclusions and non-metallic inclusions.
①Metal inclusions: Mainly due to defects caused by metal bars, sheets, and blocks falling into the ingot mold or iron alloy blocks added at the end of the smelting process during the casting process, which are not melted, and the edges are mostly clear. The color is significantly different from the surrounding geometry.
② Non-metallic inclusions: During the pouring process, there is no time to float out of the slag or the furnace lining peeled into the molten steel and refractory materials on the inner wall of the pouring system. After peeling off, leaving small round holes.
4. Shrinkage: On the low magnification sample, the shrinkage hole is located in the center, and there are often segregation, inclusions, or dense places around it. Sometimes, you can see caves or cracks before corrosion. After the corrosion, the pores became dark and irregularly wrinkled.
Reason for formation: When the ingot is poured, the last solidified part (heart) of the molten steel cannot be filled after the solidification and shrinkage, and the macroscopic holes left behind are not filled. The shrinkage holes are mainly formed at the ingot head (cap end).
5. Bubbles: On the low magnification sample, there are cracks that are approximately perpendicular to the surface, and there is a slight oxidation and decarburization phenomenon nearby. Below the surface, there are called subcutaneous bubbles, and deeper subcutaneous bubbles are called pinholes.
Causes: Defects caused by the gas produced during the ingot casting process and the gas released.
6. Crack: On the low magnification sample, the axis position cracks along the intergranular, forming a spider web, and when severe, it cracks radially.
Reasons for formation: There are mainly two types. One is internal tearing of the ingot due to some reason during solidification and cooling, which fails to weld during forging and rolling; the other is internal cracking due to improper forging. .
7. White point: On the low magnification sample, there are short cracks, which are generally concentrated in the steel, and there is almost no surface layer with a thickness of 20-30mm. Because cracks are not easy to distinguish, a fracture test should be added to verify. White dots appear as coarse silvery white dots on the fracture.
Reason for formation: It is generally believed that the effect of hydrogen and tissue stress is the small cracks caused by the combination of local pressure and the internal stress generated by the steel when the hydrogen is released and concentrated in the loose micropores to generate huge pressure.
Third, the external size defects
1, dimensional difference: including the length, diameter, thickness, positive and negative tolerances, grinding depth, width and other dimensions of the steel does not meet the requirements of the order standard.
2. Curvature: The steel is not straight in the length and width directions. The curvature of different materials has different names. Profiles are expressed by curvature; plates and strips are expressed by sickle, wave, and flutter.
3. Twisting: The steel bar is twisted into a spiral in the axial direction.
Common defects of steel wire
1. Scratching: Generally, the quality of phosphating is poor during the drawing process, and the wire drawing die is damaged and scratches the surface of the wire.
2. Local cracking: Generally belongs to local non-metallic inclusions or localized hardened phase structure in raw wire rods, subsequent drawing is not conducive to extension.
3. Crack: Generally belongs to the inheritance of raw material wire rods or the compression ratio of fine gauge steel wire is too large, which causes the internal stress of the steel wire to be too large.
4. Bending: Generally belongs to artificial handling and hanging bending
5. Ellipse: caused by drawing die ellipse
6. Decarbonization: caused by serious oxidation during genetic or heat treatment of raw materials
Common problems in spring production
1. Winding fracture:
① Local defects
② The spring winding ratio is small, and the hardness of the wire is too large
③ The hardened structure appears locally, and the material has poor ductility, which is generally related to the local segregation of the material
2. There is a sound of winding: the surface of the steel wire is not sufficiently lubricated, the surface oxide film is uneven or the material hardness is high
3. The spring diameter is unstable and the height is different: the performance of the wire rod is not good, or the setting of the winding machine is abnormal.
4. The fatigue life cannot meet the requirements:
① There are pits, scratches, abrasions, cracks, micro-cracks (improper shot peening) on the surface of the material, non-metallic inclusions on the surface, folding, decarburization, or severe segregation
② Improper spring design, uneven ends, stress concentration should at a certain point during fatigue test .
③ coarse material structure
④ Insufficient shot peening on the surface.
⑤ Insufficient tempering after spring winding, stress is not completely removed
This product is strictly forbidden from contact with acid and is easy to be brittle （hydrogen embrittlement）.
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