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Volume 39, No. 5 Metallographic examination of metallurgical fastener bolts and quenching and tempering Zhang Xianming (Hunan Lengshuijiang Tianbao Industrial Co., Ltd., Lengshuijiang, Hunan 417500) and the reasons. In the actual production application, the metallographic test can be used to determine whether the finished product quality meets the standard or determine the distribution and nature of various defects, analyze the cause of the defects and the influence of various process factors on the quality, and provide a basis for improving the process.
High-strength bolts have good mechanical properties and high toughness after quenching and tempering (quenching and high temperature tempering). Commonly used steels are low and medium carbon high quality carbon steel and low alloy steel, such as 10B21, SWRCH35K, ML35, 35, 45 (production class 12.9 bolts) and other grades.
1 quenched and tempered metallographic examination and tempering is to heat the sub-eutectoid steel over Ac; above 30 ~ 50C, when the workpiece quenching temperature is normal, the holding time is sufficient, and the cooling rate is also high, the supercooled austenite is not in the quenching process When decomposition occurs, the microstructure obtained after quenching should be lath martensite and needle martensite. During the medium-high temperature tempering, carbides are precipitated in the martensite to obtain tempered sorbite or tempered troostite.
1.1 Quenching and tempering structure and evaluation In order to ensure the high-strength bolt quenching, the austenitization is sufficient, the quenching structure is uniform, and there is no undissolved ferrite and non-martensitic structure. The metallographic examination of the quenched structure must be fully considered. For high strength bolts of class 10.9 and above, the uniformity of the quenched structure is particularly important. Foreign high-strength bolt heat treatment attaches great importance to the full austenitization of steel, ensuring the uniformity of its structure, to obtain the best toughness and coordination, and to ensure the safety and reliability of bolts in service. Domestic high-strength bolt manufacturers have not paid enough attention to this. The common problem is that the bolt quenching heat insulation is insufficient and the austenitization is insufficient.
The quality of the quenching and tempering treatment is generally based on the metallographic organization rating chart and evaluation method of GB/T 133202007 steel die forgings.
This standard is applicable to structural steel parts that have been quenched and tempered and is not suitable for the evaluation of tissues such as decarburization, overheating, and over-burning. The standard requires that the sample be mechanically prepared in the cold state; if hot cutting is used, the heat affected zone must be completely removed. During the sample preparation process, there should be no tissue changes due to heat. After the sample is polished, it is eroded with a volume percentage of 2% to 5% nitric acid solution.
~8 level assessment, level 1 organization is the best, level 8 organization is the worst. The third group of rating charts of this standard is applicable to structural steel quenching and tempering parts, especially the quenching and tempering test of high-strength bolts. When the assessed tempering organization is between two levels, the following level is the judgment level. For example, if it is greater than 3 levels and less than 4 levels, it is judged as level 4.
The metallurgical structure analysis of quenched and tempered steel was observed under the optical microscope with 500 times. The qualified level was negotiated by both the supplier and the buyer, and the level 1 to 4 was not agreed. Production practice shows that the bolts that are served in a low temperature environment, grades 1 to 3.5 are acceptance criteria. If there is a dispute at the time of rating, the result of the mechanical property test can be judged.
1.2 Bolt quenching and tempering and organization nails and studs emphasize that the material requirements for products of grade 8.8 and above should have sufficient hardenability to ensure that the core of the bolt thread section has a volume fraction of about 90 before hardening and tempering. % martensite. In order to ensure good hardenability, bolts of class 8.8 and thread diameters exceeding 20 mm shall be quenched and tempered by standard alloy steel materials.
The thickness of martensite after quenching can be evaluated according to the martensitic grade of carbon steel and medium carbon alloy structural steel in B/T 92112008.
Martensite morphology and size are different due to different austenitizing temperatures.
Grade 1 belongs to a low austenitizing temperature, and the quenched structure is crypto-joule martensite, fine-needle martensite and ferrite with a volume fraction of not more than 5%; and grade 8 is an overheated structure, which is a thick slat. Martensite + coarse needle martensite. Normal quenching is controlled at grades 3 to 5. The structure is fine lath martensite + sheet martensite. Grade 6 has high impact toughness, yield strength and tensile strength. It is suitable for larger specifications and requirements. Highly hardenable bolts. It is a 40Cr steel strip and a needle-shaped martensite grade 4, which is 45 steel strip and needle martensite 5 grade, and is SCM435 steel strip and needle martensite 4 grade, all of which are normal quenched structures.
The slat-like and acicular martensite (grade 40, 4) x1 tempered sorbite structure is actually a very small granular carbide distributed on the a-phase matrix, as shown. The tempering temperature is generally between 450 and 600 °C according to the high-strength bolt grade. The chemical composition of the steel varies according to the specific temperature range. Since the addition of alloying elements slows down the decomposition of martensite, the precipitation and aggregation of carbides, and the transformation of retained austenite, the tempering temperature will shift to a higher level.
2 tempered defect structure 2.1 quenching overheated structure quenching heating temperature is high, causing austenite grain growth, after quenching to obtain a coarse martensite structure, as shown. Once coarse acicular martensite is present, even if a reasonable tempering temperature is used to return the coarse martensite structure (42CrMo, grade 8) x500 sample to 920C quenching, the crystal grains grow sharply and are parallel in different grains. The martensitic orientation is different, the austenitization is correspondingly increased, and the retained austenite is relatively more after quenching. The coarse martensite structure has poor overall performance and is also easy to be quenched.
2.2 Quenching underheated structure The high-strength bolts obtained after normal quenching should be lath martensite and needle-shaped martensite. The quenching underheating is that the quenching heating temperature is too low or the insulation is insufficient, the austenite is not homogenized, and the microstructure after quenching is martensite and undissolved ferrite, as shown. Even by tempering, ferrite cannot be eliminated.
~ Also 3 insulation, ferrite remains in the matrix. At the same time, due to the low quenching temperature and short holding time, the austenite is poorly homogenized, and the troostite transformation occurs locally during cooling.
2.3 The quenching temperature of the under-quenched structure is normal and the holding time is sufficient, but the cooling rate is not enough to be hardened. As a result, different microstructures will be obtained along the cross-section of the workpiece. Even if the surface is martensite, non-Martens will gradually appear in the center. Body organization. Non-martensitic structures include troostite, bainite, etc., and the core is a structure such as troostite and ferrite. The non-martensitic structure that appears in low alloy steels is generally not troostite, but upper bainite, as shown by zero. The microstructure is a small amount of bainite distributed on the martensite matrix, which is easily detected by the metallographic method.
0 core structure tempering sorbite + upper bainite medium carbon steel quenched and tempered microstructure depends on quenched structure, insufficient ferrite remains in the martensite structure of the bulk ferrite, or insufficient cooling in the crystal The precipitation of reticulated or semi-reticular ferrite at the boundary is harmful.
It is worth noting that the undissolved ferrite caused by insufficient heating is different in morphology from the precipitation of ferrite first. The former is a blunt block or a thick and uneven discontinuous network (1), and then The newly formed pro-eutectoid ferrite is relatively fine in morphology and distributed on the austenite grain boundary (2).
3 common heat treatment quenching defects bolt quenching and heat treatment process quenching defects are the most common, such as insufficient hardness, deformation, cracking and so on. There are many reasons for defects, which need to be analyzed from various aspects. Metallographic examination is a common method.
3.1 Quenching cracks The internal stress caused by the bolts in the quenching is the root cause of deformation and open 2 quenching and cooling of the tissue (40Cr) X500. When the internal stress exceeds the yield strength of the material, it causes deformation; when the internal stress exceeds the breaking strength of the material, it causes cracking. Only tensile stress is a necessary condition for crack initiation and expansion. The cause of quenching crack can be considered from two aspects. First, what factors cause large stress; second, there are defects in the material, resulting in lower strength and toughness.
3.1.1 Quenching crack characteristics In most cases, the crack propagates from the surface to the core, and the macroscopic shape is relatively straight.
3 Severe decarburization on both sides of the crack 3.1.2 The internal stress increase leads to the unreasonable design of the quenching crack of the normal structure. If there is a sharp angle and a sudden change in the section, it is easy to cause stress concentration.
The cooling is too strong. If the quenching oil should be used, the aqueous solution should be selected, and it should not be cold and cold when it is cold.
The cooling method was improper during quenching, and it was not tempered in time after quenching.
3.1.3 Tissue defects cause quenching crack quenching temperature is high, austenite grains are coarse, and coarse martensite is formed after quenching, which is easy to crack. In particular, coarse high-carbon martensite is often accompanied by microcracks.
The steel has a brittle phase such as reticulated carbide, which is easily cracked along the brittle carbide network during quenching. There is a carbide network distributed along the grain boundaries at the grain boundaries, which is also prone to cracking during grinding.
Steel has defects such as folding or coarse inclusions, and it is easy to form cracks along this defect during quenching.
There is severe segregation in the steel, the structure is not uniform after quenching, the internal stress is large and uneven, and it is easy to crack.
Due to the decarburization of the surface of the fastener, the volume expansion of the surface layer during quenching is small, and the corresponding force is easily formed to form a crack.
3.2 Hardening hardness is insufficient. The heating temperature is insufficient. When the quenched body is cooled, the hardness does not change significantly, but the metallographic structure is easy to identify.
The quenching cooling rate is insufficient, and the quenched structure has a troostite or bainite structure in addition to the Martens. The more troostite or bainite, the lower the hardness.
Surface decarburization, martensite is not easily formed during quenching, or low carbon martensite is formed, as shown in 4.
After quenching and overheating, the martensite of the superheated structure is coarse, the amount of retained austenite is obviously increased, and the hardness is also lowered.
4 After the surface decarburization ferrite and low carbon martensite X200 bolts are quenched and tempered, the performance is not up to the technical requirements, and the influencing factors are various. The quality analysis is a complicated process. Here, it is only emphasized that the metallographic examination should pay attention to the adverse factors affecting the toughness, such as coarse grains, non-metallic inclusions, reticulated cementite, reticular ferrite, non-martensitic structures in quenched structures, and Tissue unevenness caused by microsegregation in low-power detection.
4 The purpose of the metallographic test is to determine whether the quality of the finished bolts meets the relevant standards; on the other hand, to determine the distribution and nature of various defects, to analyze the causes of the defects and the quality of various process factors. The impact of providing data for improved process and experimental research is an indispensable tool.
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