Inspection methods for forgings

What are forgings?

A forged part is a metal part that is shaped and processed using a hammer, press, or other machinery. The forging process creates parts with enhanced strength, durability, and structural integrity. Ensuring the quality of forgings is critical because forgings are often used in critical applications such as aerospace, automotive, and heavy machinery. Inspection methods for forgings can be broadly classified as visual, destructive, and non-destructive inspection. In this article, we will discuss these methods in detail and help you understand how they can be used to ensure the quality of forged parts.

What is the purpose of the forging inspection method?

The purpose of the forging inspection method is to ensure that forged parts meet the required quality standards and are free from defects that could affect their function or cause failure during use.

Inspection Methods for Forgings

Forgings are an essential part of various industries, from aerospace to automotive. Ensuring their quality and integrity is crucial to guarantee the safety and performance of the final product. In this article, we will discuss various inspection methods used to assess forgings, including: visual, destructive, and non-destructive, and how to select the most suitable method for your application.

Visual Inspection

Surface Inspection

The first step in assessing forgings is the visual inspection of their surface. This method involves checking for defects such as cracks, folds, laps, and inclusions that may affect the forging’s structural integrity. Surface inspection can be done manually or through automated systems using cameras and image processing software.

Dimensional Inspection

Dimensional inspection verifies that the forgings meet the required specifications in terms of size, shape, and tolerances. It can be performed using traditional measuring instruments like calipers, micrometers, and height gauges, or advanced technologies like coordinate measuring machines (CMMs) and laser scanners.

Destructive Test Method

Macrostructural Examination Method

The macrostructural examination provides information on the overall structure of the forging, such as grain flow and the presence of defects. This method requires cutting a sample from the forging, polishing, and etching its surface to reveal its structure. It is then observed using a low-power microscope or a stereoscope.

Microscopic Examination Method

Microscopic examination is used to study the microstructure of the forging material, including grain size, phase distribution, and possible defects. This method involves preparing a thin section of the forging, polishing, and etching it to reveal its microstructure. A high-power microscope is then used to examine the sample.

Acid etching inspection

Cutting a sample from the section of a stainless-steel forging that needs to be inspected and undergoing acid corrosion can display the macroscopic structure and defects on the section.

Sulfur print test

Sample cutting and inspection surface processing are the same as acid etching inspection. It uses the action of photographic paper and sulfides to inspect the distribution of sulfides in steel parts. It can also indirectly determine the distribution of other elements in the steel grade.

Fracture inspection

Defects in the raw material of stainless-steel forgings can be found, or defects caused by heating, forging, heat treatment, or fatigue cracks caused by using parts.

Determination of Performance and Composition

Destructive tests, such as tensile, compression, impact, and fatigue tests, can be conducted to evaluate the forging’s mechanical properties. Chemical analysis, like spectroscopy or X-ray fluorescence, is used to determine the material composition and verify that it meets the required specifications.

Non-Destructive Testing Method

Magnetic Particle Inspection

Magnetic particle inspection (MPI) is a widely used non-destructive testing method for detecting surface and near-surface defects in ferromagnetic materials. It involves magnetizing the forging and applying magnetic particles to the surface. The particles accumulate around any defects, making them visible under ultraviolet or white light.

Penetrant Testing Method

Penetrant testing is suitable for detecting surface defects in non-porous materials. The forging is cleaned, and a liquid penetrant is applied to its surface. The penetrant seeps into any defects, and after a dwell time, the 
excess penetrant is removed. A developer is then applied, which draws the penetrant out of the defects, making them visible under ultraviolet or white light.

Eddy Current Inspection Method

Eddy current inspection is a non-destructive testing method used to detect surface and near-surface defects in conductive materials. It works by inducing eddy currents in the forging, which are then measured by a sensor. Any changes in the eddy currents indicate the presence of defects or variations in material properties.

Ultrasonic Inspection Method

Ultrasonic inspection is a non-destructive testing method that uses high-frequency sound waves to detect internal defects in materials. A transducer sends ultrasonic waves into the forging, and the reflected waves are analyzed to identify any discontinuities in the material, such as cracks or inclusions.

Radiographic Testing

Radiographic testing involves using X-rays or gamma rays to create an image of the internal structure of the forging. The radiation passes through the material, and the amount absorbed or transmitted depends on the material’s thickness and density. Defects in the forging will cause variations in the radiation pattern, which can be detected on a radiographic film or digital imaging system.

Non-destructive testing methods for forgings VS Non-destructive testing methods for forgings

NDT methods used for quality inspection of forgings are generally: magnetic particle inspection, penetration inspection, eddy current inspection, ultrasonic inspection, etc.
Whether macrostructure inspection method or microstructure inspection method or performance and composition determination method, are destructive test methods, for some important, large forgings destructive methods have been unable to fully adapt to the requirements of quality inspection, on the one hand, because it is too uneconomical, on the other hand, mainly to avoid the one-sidedness of destructive inspection. The development of non-destructive testing technology for forging quality inspection provides a more advanced and better means.
The magnetic particle inspection method is widely used to check the surface or near-surface defects of ferromagnetic metal or alloy forgings, such as cracks, hairlines, white spots, non-metallic inclusions, delamination, folding, carbide or ferrite bands, etc. The method is only applicable to the inspection of ferromagnetic materials forgings, forgings made of austenitic steel are not suitable for the use of the method.
Penetration inspection method can check the magnetic material forgings, but also check the surface defects of non-ferromagnetic material forgings, such as cracks, loosening, folding, etc., generally only used to check the surface defects of non-ferromagnetic material forgings, cannot find the defects hidden below the surface.
Eddy’s current inspection method is used to check the surface of conductive materials or defects near the surface.
The ultrasonic inspection method is used to check the internal defects of forgings such as shrinkage, white spots, heart cracks, slag, etc. Although the method is easy to operate, fast, and economical, the nature of the defect is difficult to accurately determine.
With the development of nondestructive testing technology, there are now new nondestructive testing methods such as the acoustic vibration method, acoustic emission method, laser holography method, CT method, etc. The emergence of these new methods and their application in forging inspection will certainly make the level of forging quality inspection to be greatly improved.

The choice of inspection methods for forgings

It is worth proposing that the accuracy of the quality inspection results of forgings depends on the correct test methods and testing techniques, but also on the correct analysis and judgment. Only the correct test method, without accurate analysis and judgment, will not come to an appropriate conclusion. Therefore, the analysis of forging quality is actually the comprehensive application of various test methods and the comprehensive analysis of each test result. For large and complex forgings, the problem cannot simply rely on a certain method, from this point on it can be said that various test methods are complementary in the analysis process, the organic cooperation of various test methods, and the comprehensive analysis of their respective test results, in order to reach the correct conclusion. At the same time, in terms of the purpose of forging quality analysis, in addition to the correct inspection, the necessary process tests should be conducted to find out the real causes of quality problems and put forward successful improvement measures and preventive measures.

In practice, the selection of those testing methods, and the use of what testing means should be based on the type of forgings and the specified testing items to carry out. In the selection of test methods and test methods, both advanced and practical, economic, not simply the pursuit of advanced, can use a means to solve the problem do not use two or more, the choice of test methods should be an accurate determination of the nature of the defect and the exact cause of the defect as a starting point, sometimes the choice of too advanced test methods will lead to unnecessary consequences to Sometimes, too advanced testing methods can lead to unnecessary consequences and undue losses.

When should I choose destructive testing over non-destructive testing?

Destructive testing is often chosen when you need to verify the properties and composition of the material and when it is acceptable to sacrifice the sample for testing. Nondestructive testing is preferred when you want to evaluate the entire forged part without causing damage.

How to ensure the quality of forgings?

The equipment for free forging is divided into two categories: forging hammers and hydraulic presses. The forging hammers used in production are air hammers and steam-air hammers. Hydraulic presses are used to deform the billet by the static pressure generated by liquid and are the preferred way to produce large forgings. The basic processes of free forging include upsetting, drawing, punching, cutting, bending, twisting, shifting, and forging joints. Also known as extension, it is a forging process that reduces the cross-sectional area of the billet and increases its length. The extension is often used to forge rods and shafts. Forging quality in a sense, on the one hand, is the quality of the forgings that have been made gate-keeping, large forgings factory, on the other hand, to the forging process to point out the direction of improvement, so as to ensure that the quality of free forging forgings in line with the requirements of the technical standards of forgings, and to meet the design, processing, use of the requirements.
The requirements for forgings include bearing mode, load size, and loading state.
The load-bearing mode has a one-way load, divided into tensile load and compression load or alternating load in the working state. Multidirectional or composite loads are divided into tensile, compression, shear, bending, torsion, and support loads. They are either parallel to the central axis or at an angle to it, in the shape of the change of parts should be used to uniform transition of the adapter round orphan, in order to reduce stress concentration. Where stress concentration is unavoidable, the notch toughness of the forging material is usually an important factor in the selection of materials. The periodic load can have a high circumferential load and a low circumferential load.
Load size and loading state: size; loading speed incremental or shock; forging temperature should be determined in that temperature accumulated in the longer working time; environment including forging in the cycle of atmospheric condensation, the chemical composition of the environment, and corrosion, wear, erosion or other friction conditions forgings, forgings are good or bad about the overall operation.

Conclusion

Inspection methods for forgings play a crucial role in ensuring the quality and integrity of the final product. Visual inspection, destructive testing, and non-destructive testing methods, such as magnetic particle inspection, penetrant testing, eddy current inspection, ultrasonic inspection, and radiographic testing, are all effective ways of detecting defects and verifying the material’s properties. Selecting the most appropriate method depends on various factors, including the material, defect type, and cost and time constraints.

Source: China Forgings Manufacturer: www.epowermetals.com

FAQ:

What are the most common defects in forgings?

Common defects in forgings include cracks, laps, folds, inclusions, and porosity. These defects can compromise the structural integrity and performance of a forged part.

Can one inspection method detect all types of defects in forgings?

There is no single inspection method that can detect all types of defects in every material. It is critical to select the appropriate method based on the material and specific defects to be inspected.

Are NDT methods always more expensive than destructive testing methods?

Not necessarily. The cost of a testing method depends on factors such as equipment, labor, and the complexity of the inspection process. In some cases, NDT methods may be more cost-effective, especially when evaluating large numbers of forgings.

What is the importance of grain flow in forgings?

Grain flow refers to the arrangement of metal grains during the forging process. Proper grain flow can improve the mechanical properties of a forged part, such as strength, ductility, and resistance to fatigue and impact. Inspection methods such as macrostructural inspection help to assess grain flow and ensure the quality of forged parts.