Nondestructive testing in fastener manufacturing process

Fastener is actually a kind of connector (connector), but because of its wide variety and wide application, it is only one kind in practical use, even referred to as standard parts. At present, the standard parts in the market are mainly made of carbon steel, stainless steel, brass and aluminum alloy.

Fasteners are widely used in automobile, aerospace, construction, shipping, medicine and health and other fields, and play a very important role in people's production and life. With the development of the times, the quality requirements of fasteners are constantly improving. More and more aerospace standards directly take nondestructive testing as a necessary process of fastener manufacturing.

Fastener defects are mainly divided into defects in raw materials and defects in manufacturing process.

The defects in raw materials (mainly bars, wires and pipes) include folding, delamination, cracks and non-metallic inclusions. The unqualified raw materials can be removed by non-destructive testing.

In the manufacturing process, upsetting (cold upsetting, hot upsetting, stamping, etc.) may lead to such defects as stretch marks, cracks, folds, etc., grinding may lead to grinding cracks, rolling thread may lead to folds, and heat treatment may lead to quenching cracks.

The non-destructive testing method can be used to find the unqualified products in fastener products as soon as possible, reduce the cost, improve the manufacturing process and improve the reliability of products.

Nondestructive testing in fastener manufacturing process

Common nondestructive testing methods for fasteners

Magnetic particle testing

According to GJB 2028a-2007 magnetic particle testing, surface and near surface defects (such as cracks, inclusions, hairlines, folds, porosity, etc.) can be found by magnetic particle testing, and the shape, position and size of defects can be visually displayed, and the nature of defects can be roughly judged. It has high detection sensitivity, and the minimum width of defects that can be detected is about 1 μ M. As long as the appropriate magnetization method is adopted, almost all parts of the fastener surface can be detected, which is not only fast, but also easy to operate and low cost. It is especially suitable for detecting various surface and near surface longitudinal defects of ferromagnetic fasteners.

The main limitation of magnetic particle testing is that it can only detect fasteners made of ferromagnetic metal materials, but can not detect fasteners made of austenitic stainless steel? Aluminum? Copper? Magnesium? Titanium and other non-magnetic materials. The sensitivity of magnetic particle testing is closely related to the magnetization direction. If the direction of the defect is parallel to the magnetization direction, or the angle between the defect and the fastener surface is less than 20 degrees, it is difficult to show the defect, and it is difficult to detect the shallow scratch on the surface and the deep hole.

In addition, if there is a covering layer on the surface of the fastener, such as a paint layer, it will have an adverse effect on the detection sensitivity. The thicker the covering layer is, the greater the effect is; if the surface of the fastener is shot peened, it will also have an adverse effect on the detection sensitivity.

Penetrant testing

According to GJB 2367a-2005 "penetrant testing" standard, penetrant testing can detect surface opening defects of various non porous materials, and the detection results are not affected by the shape, size, microstructure, chemical composition and defect orientation of fasteners. The defect display is intuitive, and the detection sensitivity is high (it can detect defects with width of 0.5 μ m? Depth of 10 μ m? Length of about 1mm), and defects in various directions can be detected at one time; Due to the high speed of penetration testing, a large number of fasteners can be inspected in batches at the same time, so as to achieve 100% inspection; it is suitable for detecting various surface opening defects of fasteners.

The main limitation of penetrant testing is that it can only detect the defects of fastener surface opening, not suitable for detecting parts made of porous or loose materials and fasteners with rough surface; it can only detect the surface distribution of defects, and it is difficult to determine the actual depth of defects, so it is difficult to make quantitative evaluation of defects; the test results are also greatly affected by the operator. The shallow and wide surface defects of fasteners are easy to be missed, and carbon steel? Alloy steel is easy to rust in the detection process.

Eddy current testing

The main advantages of eddy current testing are fast testing speed, no direct contact between coil and fastener, no coupling agent, no pollution and damage to surface, easy to realize modern automatic testing; especially suitable for tube and bar testing, generally tens of meters per minute.

In the manufacturing process of fasteners, eddy current testing is mainly used for the re inspection of the surface and near surface quality of raw materials. In addition to the detection of surface and near surface defects of raw materials, it can also be used for the separation of different materials (mixture identification).

The main limitation of eddy current testing is that it can only be used for conductive materials, and it is difficult to inspect fasteners with complex shapes. Due to skin effect, eddy current testing can only check the surface and near surface parts of thin fasteners or thick fasteners; the testing results are not intuitive, and it is difficult to judge the shape, size and specific location of defects; the correctness of testing results depends on the level of testing personnel.

When the outer through coil is used for detection, the coil covers a length of circumference on the bar and pipe, and the information obtained is the cumulative result of the influencing factors on the whole ring, so the specific position of the circumference at the defect can not be judged.

Ultrasonic testing

Ultrasonic testing is mainly used in the defect detection of plates, bars, pipes, forgings, weldments and castings. It is suitable for the detection of surface defects with a certain size, such as delamination, cracks, lack of fusion, lack of penetration, etc. When the extended surface of the defect is perpendicular to the ultrasonic beam, it is most conducive to the ultrasonic testing.

Ultrasonic testing is suitable for the detection of large thickness workpiece, which can detect the internal cracks, delamination, hollow and other defects of fastener raw materials. In order to apply ultrasonic to fasteners and receive the ultrasonic signal, it is necessary to adopt appropriate coupling mode. Therefore, the surface roughness of fasteners should be limited to a certain range.

The conventional ultrasonic testing can judge the defect by the waveform, but it can only give the equivalent size of the defect, and it is difficult to judge the nature of the defect, so it is not suitable for the testing of finished fasteners.

Nondestructive testing in fastener manufacturing process

Determination of nondestructive testing time

The manufacturing process of fasteners is: raw materials → upsetting (cold upsetting, hot upsetting, stamping, etc.) → cutting (turning, grinding, milling, etc.) → thread forming (rolling, turning, tapping, etc.) → heat treatment → surface treatment → finished products

According to the manufacturing process and working experience of fasteners, it is generally selected to set the nondestructive testing process before the use of raw materials and surface treatment.

Nondestructive testing of raw materials

The raw materials of fasteners are mainly bars, wires and tubes, which are processed by ingots through rolling and drawing processes. The defects such as folding, delamination, cracks and non-metallic inclusions are easy to appear on the raw materials. The quality of raw materials will directly affect the manufacturing process of fasteners. Generally, nondestructive testing is carried out before the use of raw materials to eliminate unqualified raw materials.

Nondestructive testing in manufacturing process

In the process of fastener manufacturing, some defects are inevitable due to the influence of raw materials, equipment, personnel and process parameters.

Due to the influence of material plasticity, deformation, die structure, lubrication state and other factors, surface folding, pull marks and cracks often appear in upsetting (cold upsetting, hot upsetting, stamping, etc.) process; due to the influence of grinding speed, cooling and lubrication, grinding wheel material and other factors, the heat generated in grinding process may not be released in time, resulting in grinding burn and grinding crack defects. Due to the defects of raw materials, irregular shape of fasteners, too fast cooling speed, delayed tempering after quenching, transfer speed and other factors, cracks often appear in the heat treatment process.

The NDT in the manufacturing process should be arranged after the process that may cause or expose defects. Therefore, in the manufacturing process of fasteners, the NDT should be arranged after upsetting, cutting, heat treatment and other processes that may cause defects, and before coating, plating, anodizing, bluing, phosphating and other surface treatment. If the coating may have defects (such as electroplating cracks), nondestructive testing should be carried out before and after the electroplating process, so as to identify the link of defects.

Nondestructive testing in fastener manufacturing process

Selection of nondestructive testing methods

Nondestructive testing of raw materials

Nondestructive testing of fastener raw materials mainly includes ultrasonic testing and eddy current testing. Fastener manufacturers generally use eddy current testing for raw material defects, while raw material manufacturers mainly use ultrasonic testing.

The diameter of fastener raw materials is small, which is limited by the manufacturing process of raw materials. The defects are mainly concentrated on the surface and near surface, and the main types of defects are folding, delamination, cracks and non-metallic inclusions.

Eddy current testing can find surface and near surface defects. It has the advantages of fast testing speed, no direct contact between coil and specimen, no coupling agent, no pollution and damage to surface, and easy to realize modern automatic testing. Therefore, fastener manufacturers generally use eddy current testing for raw material defects.

Ultrasonic testing is applicable to the detection of internal cracks, delamination, hollow and other surface defects of fastener raw materials, which is mainly used by raw material manufacturers.

Nondestructive testing methods in manufacturing process

The size of the fastener is small, and the defects are mainly concentrated on the surface and near surface. Among the five conventional nondestructive testing methods, penetrant testing, magnetic particle testing and eddy current testing are mainly used to detect surface and near surface defects. Because eddy current testing is not suitable for complex shape specimens, and the test results are not intuitive, it is difficult to judge the shape, size and specific location of defects, so magnetic particle testing and penetrant testing are generally selected for nondestructive testing of fasteners.

According to the material category, fasteners are divided into ferromagnetic material fasteners and non ferromagnetic material fasteners. Magnetic particle testing is used for fasteners of ferromagnetic materials, and penetrant testing is used for non ferromagnetic materials.

In the process of fastener magnetic particle testing, in order to find different sizes of surface and near surface defects, fluorescent magnetic particle testing should be preferred. Because the sensitivity of fluorescent magnetic particle testing is much higher than that of non fluorescent magnetic particle testing, the testing speed is also fast. For the fasteners with large length diameter ratio such as screw, screw pile and pin shaft, the direct energization method shall be adopted for the longitudinal defects, and the coil longitudinal magnetization method shall be adopted for the circumferential defects. For hollow fasteners such as nuts, bushes, pipe joints, ring parts and bearing rings, the central conductor method should be adopted. Because the central conductor method adopts induction magnetization, the axial defects on the inner and outer surfaces of the fasteners and the radial defects on the two end faces can be found.

In the process of fastener penetration testing, according to the type and location of defects and the size and shape of the tested fastener, the hydrophilic post emulsion fluorescent penetrant combined with dry powder imaging agent detection technology or the water washing fluorescent penetrant combined with dry powder imaging agent detection technology should be preferred. According to the requirements of different sensitivity levels, fluorescent penetrant with different sensitivity levels is selected. The cost of water washing type penetrant testing is low, the detection speed is fast, but the detection sensitivity is low, and the wide and shallow defects are easy to be missed; the post emulsion type penetrant testing has high sensitivity, which can detect the wide and shallow defects, but the cost is high.

Nondestructive testing in fastener manufacturing process

summary

(1) The application of non-destructive testing technology in fastener manufacturing can find out the unqualified products as soon as possible, reduce the cost, improve the manufacturing process and improve the reliability of product quality.

(2) According to the manufacturing process and defect formation mechanism of fasteners, it is generally selected to set nondestructive testing procedures before using raw materials and before surface treatment.

(3) According to the defect types, fastener characteristics and applicability of NDT methods, the economical and efficient NDT methods should be flexibly selected on the premise of ensuring the sensitivity.

Hebei Shengxin Fastener Manufacturing Co., Ltd