A batch of 35K materials in September caused cracks in the blanks after cold heading. After a series of tests, it was found that near the cracks, inclusions formed into pieces, clusters, inclusions, and the number of inclusions exceeded the standard range, seriously affecting the material. Continuity, Consistency. In the case of cold heading, under the combined effect of external forces and internal defects, the product is cracked.
The causes of cracking in cold heading products are various:
The increase in the carbon and alloying elements of the material, and the high hardness and strength of the raw material can cause cracking of the product;
The raw material materials are poorly organized, and there is segregation of materials, poor annealing texture, difficulty in cold deformation of the product, and increased possibility of product cracking;
The increase in the complexity of the shape of fasteners will also bring certain difficulties to the deformation and increase the risk of product cracking;
In addition, equipment, mold mismatch, the possibility of product cracking will increase;
Including the increase of inclusions in the material and the increase of individual inclusions exceed the requirements of the standard grade range, which is one of the most important causes of cold heading cracking.
First, nut cracking conditions:
A group of 35K materials entered by the company in September 2015 had cracks when cold-flanged flange nut blanks, and the mold consumption was relatively large. Although the percentage of cracked blanks was about 3%, it brought about the production of the company. A lot of troubles occurred. Not only cracks appeared in the blanks, but a small amount of nuts that had slipped through the net were processed through various processes. After the finished product, a small amount of cracking nuts was found, which caused a lot of trouble for the company. (See Fig. 1A for cracking of the blanks. See the nut cracking. Figure IB).
Cracking of the blank affects the quality and yield of the product; cracking nuts that are not picked up not only cause problems for the next process, but also some cracked nuts are not picked out (see Fig. 1B) and are issued to the user, bringing about product quality. A great deal of hidden danger will directly affect the reputation of the company. The crack nut must be completely eliminated before the finished product leaves the factory. Otherwise, the outflow of the company will lead to endless troubles.
Second, routine detection analysis:
1. Component analysis:
From the material composition point of view, this batch of wire meets the composition requirements of 35K materials.
2. Hardness analysis:
The average hardness of the company's wire rod raw material is HRB 84.3, which meets the company's specification requirements for the hardness average ≤ HRB85; the average value of the hardness of the nut blank is HRB 98.2, which meets the technical specifications of the hardness average ≤ HRB100.
3. Metallographic picture of the material organization.
From the picture of the metallurgical microstructure annealed in Fig. 2, the pearlite in the organization is relatively small, disperse and uniform, and the remaining phase ferrite is not formed into pieces. The formation of agglomerates is an ideal annealing structure. The grade is evaluated as annealing. Five levels of organization, normal raw material organization.
Third, crack analysis:
1. Macro analysis:
The cracks of the individual flange nuts are severe. The cracking of the blank begins at the edge of the opening. The clearance at the opening is large, forming a bell mouth, which cracks from the edge of the flange to the inner edge of the flange.
2. Crack metallographic analysis:
Crack status analysis:
(1) The cracks of the cracked blank in Fig. 4 are divided into four areas A, B, C, and D. The four areas are rotated 90° to the left, and after enlargement, the distribution of the inclusions and their shapes can be seen clearly.
(2) It can be seen from the enlargement of cracks in the cracking of Figure 4: Along the cracked thread edge, it can be seen that there are many irregularities along the line, there are many pits, irregular irregularities, and curves; the shape of the notches is an irregular semicircle and The arc is formed by inclusions.
(3) These spots are areas where the inclusions are densely distributed in the material. They are concentrated areas of material inclusions. The plasticity of the material is reduced and the continuity is broken. Under the condition of increasing external force, the inclusions concentrate in the area and cannot stand the deformation of the product. Deformation squeeze force, the area along the concentrated inclusions first tear, resulting in cracks, many string areas, the formation of potholes curve, leading to cracking of the blank. Inclusion concentrated area is the weak link of raw materials, is the formation of cracks in the area.
(4) Analyze the cracks in each segment from Figure 4 above (see Figure 5):
1 In Fig. 5A, it can be seen that along the fault line, an irregular curve and an arc line are formed, and a series of inclusions are closely distributed below the edge line of the crack.
2 Figure 5B. Figure 2 B. Two sides of the crack line. Inclusions are arranged in series. The extent of pits along the crack is more serious. The curvature of the right side of the crack line is especially obvious. The distribution of inclusions below the right side of the crack line is more dense.
3 Figure 5C Both sides of the crack line are irregular, irregular, with cracks and lines, and the degree of tortuosity is very serious. Among them, there are elliptical bends and dendritic crucibles in the crack line, which are typical areas in which the inclusions continuously distribute.
4 Figure 5 D is below the C area, similar in shape to Fig. 5C. The continuous distribution of inclusions is disorderly and branched.
(5) Analysis summary:
Non-metallic inclusions in steel are numerous, strange in shape, dense in distribution, large in size, distributed in clusters, and branching of inclusions, which are connected in series with spherical balls, which is a major cause of cracking of fasteners. .
In particular, the non-metallic inclusions D (spherical oxides) and Ds (spheroidal oxides) inclusions are densely distributed, disorganized, in a string of beads, blocking the consistency of raw materials, continuity, and internal organization of materials. Blocking and plasticity decrease, and the continuity of the material is destroyed. Under the action of external force, it is easy to produce internal and external interactions, resulting in product fracture phenomenon; and inclusions are close to the surface of the wire rod, which is extremely hazardous; in the technical standards of cold heading Requires all types of inclusions; Class B and Class D inclusions are within Class 2; Ds Inclusion Levels are generally controlled to a Class 2 or less standard. This not only reduces the risk of cold heading cracking, but also reduces heat treated quench cracks. The possibility of achieving a preventative cracking effect.
At present, the degree of concentration of inclusions in cracked products is similar to that of Class D. At the same time, the particle size exceeds the Ds class. This is not allowed by the standard, and the continuity of the material is severely blocked, resulting in cracking during cold heading. The above picture clearly shows that the inclusions around the crack are densely packed, with open pits, and the cracking of the product caused by excessive inclusions under cold heading is verified.
Let us look at the size of the inclusions: Figure 6 Figure 1 The inclusions with large particles are clearly present in the crack-slug area. Eight specific inclusions in the picture were measured. The diameters of the inclusions were: (1) 35.20 μm; (2) 55.46 μm; (3) 60.98 μm;
(4) 32.94 μm; (5) Length: 111.24 μm Width: 44.68 μm; (6) Length: 124.42 μm Width: 51.64 μm; (7) 52.27 μm; (8) 85.39 μm. For details, see right figure B2. The largest inclusions: length: 124.42μm width: 51.64 μm; the smallest inclusions also reach: 32.94μm.
According to GB/T10561-2005 “Standard Test Method for Non-metallic Inclusion Content in Steel”, the highest single-inclusion grade is 3. The maximum diameter of inclusions is 76μm. Figure 6(5) 111.24μm, (6 ) 124.42μm, (8) 85.39μm, all exceed the requirements of the highest class 3 in the standard; the smallest inclusions are 32.94μm, also reach the level 2 standard; and the distribution density also exceeds the D-type (spherical oxide) inclusions The standard requirements.
3. After the same batch of cracking nuts are manufactured:
(1) Cracked nut shape of the finished product: The same batch of material, after the nut was cold-headed, was not picked out by the nut blank. After quenching and tempering, the surface cracks were expanded (see Figure 7).
(2) Metallurgy of cracking nuts (see figure 8):
In accordance with the requirements of the Qualifying Organization (GB/T13320-2007), despite the presence of cracks, the quenched and tempered structure is in good condition. It is basically the remaining phase in the tempered thoracic body + tempered schistosomiasis-ferrite. Body tissues, distributed evenly, small, no large pieces, exist in pieces. Overall, the organization is in good condition, and the conditioning group is rated as level 2. The organization is ideal. There are no problems with the composition of the materials and other processes.
(3) Finished product hardness after quenching and tempering: The average value reaches HRC29.1, which meets the standard of class 10 nuts, and the hardness is in the range of lower deviation, so the cracking probability of the product is logically low (see the table below).
(4) Cracks and inclusions in the same batch of cracked finished nuts (Figure 9):
The nuts that cracked after the finished product leaked through the mesh, and the inclusions in the low-intensity tissue cracks also had the problem that the distribution of inclusions was disordered, the number was large, the particles were large, and the cracks were concentrated; at the same time, irregularities were observed in the crack area. Arc, semi-circular (Figure 9); the remaining areas of many inclusions, cracking conditions and cold heading exactly the same.
Fourth, analysis of conclusions:
1. Cracks are caused by poor materials, mainly due to the wide distribution of inclusions in the material, large particles, and dense distribution, all exceeding the standards required by the technology.
2. These spots are densely distributed areas of inclusions in the material. Inclusions concentrate in the area of the material, breaking the inherent inherent consistency of the material, continuity, material plasticity is seriously reduced, when the external force increases, inclusions concentrated area, accept It cannot be a huge extrusion force when the product is deformed. The continuous destruction of the inclusions along the line, the stress concentration area—the weak link of the material, first tears, and the finished product cracks.
3. Excessive inclusions in the material are the cause of cracking.
references
1, GB/T13320 - 2007 "Steel Forgings Metallographic Organization Rating Chart and Evaluation Method"
2, GB/T10561-2005 "Determination of non-metallic inclusions in steel content standard rating chart microscopy
Inspection method◆
Author: Yan Zhen of China Fastener Info consultants, Shanghai Detroit Precision Fastener Co., Ltd. Cheng Ping
This article was originally published in the 44th issue of Golden Spider "Fasteners"
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