Chemical composition check
Generally, the chemical composition of forgings is not checked. The chemical composition is based on the analysis of samples taken before the furnace during smelting. However, for important or suspicious forgings, some chips can be cut from the forgings and chemical analysis or spectral analysis methods can be used to test the chemical composition.
Size and shape inspection
Appearance visual inspection
This is a commonly used method for inspection between processes. Use the naked eye to observe whether there are folds, cracks, bruises, scars, surface overburning and other defects on the surface of the forging. Deep defects are hidden on the surface of forgings, which are often observed after heat treatment and cleaning.
Dimensional measurement inspection includes
Length, width and height dimensions, geometric tolerances, misalignment, etc. The dimensions of forgings are inspected regularly, and the tools used for measurement are divided into general and special ones. Common measuring tools include calipers, calipers, vernier calipers, etc. When the production volume is large, special tools should be used, such as snap gauges, plug gauges, templates, special fixtures, etc. Inspection instruments controlled and analyzed by computers can also be used, or optical comparators and optical projectors can be used for relevant dimensional inspections.
Mold structure inspection
Before the final forging die and pre-forging die chamber of the new mold (or after renovation) are put into production, lead samples or mixed metal salt castings should be poured to check the shape of the sample, flash state, etc., and judge Whether the mold cavity design is reasonable. The first batch of forgings after pickling should be checked for fullness and folding to determine the correctness of the design of the forging die fillets, flanges, ribs, etc. In addition, the trimming quality of forgings should also be checked to identify problems with trimming die design or installation.
Mechanical property testing
Mechanical property samples must be cut from forgings taken out of the same smelting furnace and heat treatment furnace. Sample cutting, sample shape and size, and inspection methods are all carried out in accordance with national standards.
Hardness test: This is commonly used in production and the simplest method to judge the mechanical properties of forgings. The purpose of the hardness test is to ensure normal cutting performance during machining of forgings; to determine the decarburization of the forging surface and to understand the unevenness of the internal structure of the forging. The hardness test is generally performed after the forging is heat treated. Brinell hardness or Rockwell hardness can be used. Hardness tester, etc.
Tensile test: Determine the yield limit σs, strength limit σ, elongation δ and cross-sectional shrinkage ψ under unidirectional static tension.
Impact test: used to test the toughness of materials or forgings. Parts that are subject to impact loads and vibration loads during operation, or work at high temperatures and high speeds, such as turbine disks, turbine blades, etc., generally require impact testing.
Other tests: For some important or large die forgings, or forgings working under special conditions, tests such as fatigue, bending, torsion, high temperature creep and lasting strength should be carried out as needed.
Surface flaw detection inspection
Magnetic particle inspection: It can detect minor defects on the surface of forgings, such as cracks, folds, inclusions, etc. It can only be used for ferromagnetic materials, and the forging surface must be flat and smooth.
The principle of magnetic particle inspection is: place the forging between two magnetic poles, and the magnetic lines of force should pass through it evenly. If there are cracks, pores, non-magnetic inclusions, etc. in the forging, the magnetic lines of force will bypass these defects and bend. If the defect is on the surface, the magnetic field lines will leak into the air, bypass the defect, and return to the forging. This magnetic leakage phenomenon produces a local magnetic pole at the magnetic flux leakage site. When the external magnetic pole is removed, the local magnetic pole remains for a long time. If magnetic powder is sprinkled on the surface of a forging, the magnetic powder will be attracted to the magnetic flux leakage and accumulate into traces similar in size and shape to the defects.
Fluorescence inspection: used to inspect surface defects of non-magnetic metal forgings. The principle is to use fluorescent liquid to penetrate into the cracks of the forging, and with the help of the display agent, under the irradiation of ultraviolet rays, the defects of the forging will emit clear fluorescence. It can display surface cracks with a width less than 0.005mm that are invisible to the naked eye, and is suitable for various metal materials and forgings of different sizes.
Ultrasonic inspection
Ultrasonic inspection can quickly and accurately detect macroscopic defects inside forgings, such as cracks, inclusions, shrinkage cavities, white spots, and the shape, location, and size of bubbles. However, it is difficult to judge the nature of defects, and it must be inferred by using standard inspection specimens or accumulating experience.
The principle of ultrasonic inspection is to convert electrical energy into high-frequency pulse ultrasonic waves through a quartz converter and inject them into the interior of the forging. Since ultrasonic waves cannot penetrate the solid-gas interface, when the ultrasonic waves encounter internal defects, they will be reflected back. If the ultrasonic wave does not encounter defects, it will not be reflected back until it reaches the bottom of the forging. The reflected ultrasonic pulse signal is received by the flaw detector, and is transmitted to the fluorescent screen of the oscilloscope through conversion, amplification, and detection, and is displayed in the form of amplitude, as shown in the figure.
When the ultrasonic wave first enters the surface of the forging, the initial wave peak A will appear. When the ultrasonic wave encounters a defect inside, a defect peak B will appear. When the ultrasonic wave is reflected from the bottom surface, a bottom wave peak C will appear. Therefore, when moving the probe for detection, if there are no defects inside the forging, there will only be the initial wave and the bottom wave on the fluorescent screen. Once the ultrasonic wave encounters a defect, a defect wave will appear between the initial wave and the bottom wave. According to the distance between the defect wave, the initial wave and the bottom wave, the relative position of the defect can be determined. The size of the defect peak will show the size of the defect.
Low magnification tissue inspection
It is to observe the macrostructure of the forging surface or cross-section with the naked eye or with the help of a low-power magnifying glass.
Acid etching inspection: This is the use of acid to display the macrostructure. It can inspect the flow lines, segregation, shrinkage cavities, voids, white spots, inclusions, cracks, etc. of forgings.
For small and medium-sized forgings, transverse samples are generally taken to check the quality of the entire section; longitudinal samples can be taken to check defects such as streamline distribution and non-metallic inclusions; if surface cracks and quenching soft spots are to be tested, the outer surface of the forging should be retained Conduct acid etching inspection; large forgings need to be inspected non-destructively. You can use cement as a retaining wall to form an acid pool on the surface of the corner area of the forging, and then conduct acid etching inspection.
Acid etching methods are divided into hot acid etching and cold acid etching. Hot acid etching is suitable for small and medium-sized forgings. Steel forgings generally use 1:1 industrial concentrated hydrochloric acid aqueous solution, the working temperature is 65~80°C, and the etching time is 10~30 minutes. Cold acid etching is mostly used for large forgings. Use cotton yarn dipped in cold acid etching agent to wipe the surface of the forgings until the macrostructure is revealed. For general steel forgings, the cold acid etching agent is 5% to 10% nitric acid aqueous solution, or 10% to 20% ferric persulfate aqueous solution; for stainless steel, high chromium alloy steel, and high chromium nickel alloy steel, 1 part nitric acid and 2 parts hydrochloric acid can be used Mixed acid (aqua regia).
Fracture inspection: Defects in forgings caused by raw material problems or heating, forging, and quenching heat treatments can be inspected. Such as naphthalene-shaped fractures, stone-shaped fractures, layered structures, white spots, internal cracks, non-metallic inclusions, etc. The fracture of the lamellar structure is dull and has a wood-layered appearance, which is mainly caused by defects in the raw materials, often at the axis of the steel, and only appears after hot rolling and quenching.
Sulfur print inspection: This uses the function of photographic paper and sulfides to check sulfide impurities and their distribution in forgings. It can also indirectly determine the distribution of other elements in steel. Sulfur print inspection is only performed when the sulfur content of the raw materials is too high or the sulfide segregation is serious.
High magnification metallographic structure inspection
This is to observe the metallographic structure and various microscopic defects of forging sliced samples under an optical microscope, showing segregation and the distribution of some compounds, and rating of grain size and non-metallic inclusions. Microscopic examination helps to understand the relationship between microstructure and forging properties.
The location and direction of cutting the sample are related to the purpose of the inspection: to test the metallographic structure, inclusions, and elongation or breakage of the band structure, longitudinal samples can be cut; to test decarburization, folding, over-burning, surface hardenability, and permeability. To determine the depth of the carbon layer, etc., transverse samples should be cut; to test the grain size, samples can be cut according to regulations. It is advisable to use mechanical cutting methods such as sawing and planing to cut samples, and operate in strict accordance with the requirements for metallographic sample preparation, so that the observation part of the sample can maintain its original organizational state.
Residual stress inspection
If there is excessive residual stress inside the forging, during machining, the residual stress will be out of balance, causing the part to deform and affect the assembly; during the use of the part, the superposition of residual stress and working stress will also cause the part to fail, or even Damage the entire machine. Therefore, some important forgings (such as forgings for aviation, aerospace, military industry, and power stations) have strict regulations on residual stress in the technical conditions. Methods for measuring residual stress include X-ray method, boring method and ring cutting method.
