Have you ever encountered situations during procurement where castings had to be scrapped due to internal defects such as porosity or cracks during machining or pressure testing, significantly increasing your costs? This is because the manufacturer failed to perform non-destructive testing(NDT) on the castings. This article will provide a detailed explanation of non-destructive testing in stainless steel investment casting, helping you avoid financial losses.
NDT Methods for Investment Casting
There are many different methods of non-destructive testing. Understanding these methods will give you a clearer picture of each method and will help you take the technical initiative when making purchasing decisions.
Liquid Penetrant Testing(PT)
This inspection method is the most cost-effective and is suitable for detecting surface defects in castings, such as surface porosity and cracks. It primarily relies on capillary action to apply a dye solution, such as a colored dye or fluorescent penetrant, to the casting surface. These liquids would penetrate microscopic surface defects. Once penetration is complete, the surface is cleaned, and the developer is sprayed, revealing any cracks on the casting surface.
Radiographic Testing / X-Ray(RT)
This inspection method is the most expensive and is suitable for detecting internal defects, such as cavities caused by casting shrinkage, as well as certain internal cracks and gas porosity. It primarily relies on the principle of radiography: since the density is lower at defect sites within the casting and absorbs less radiation, these internal voids cast deeper shadows on the digital image plate when X-rays and gamma rays pass through the metal.
Magnetic Particle Testing(MT)
Magnetic particle inspection is suitable for detecting tiny cracks on the surface of castings and up to 1–2 mm below the surface. The process involves magnetizing the casting and then evenly applying magnetic particles to its surface. If there are cracks on or near the surface, the magnetic particles will adhere to them, forming visible traces.
Ultrasonic Testing(UT)
This method is suitable for detecting large-area internal defects in castings, such as delamination and cracks. It primarily relies on the reflection of sound waves to locate defects. By emitting ultrasonic waves into the casting, if there is a defect, the waves will produce a reflected echo; the location of defects can then be pinpointed based on the time of arrival and the wavelength of the reflected signal.
Eddy Current Testing(ET)
This method is suitable for detecting surface and near-surface defects in conductive metals. It primarily relies on electromagnetic induction, that is, bringing an energized coil close to the casting to induce eddy currents on its surface. If a defect is present, the distribution of the eddy currents and the impedance will change.
Selecting the Right NDT Method for Different Stainless Steel Grades
The above section describes five methods of non-destructive testing; however, due to the unique properties of stainless steel, not all are suitable for stainless steel castings. Understanding these differences will help you select the appropriate non-destructive testing method for your components, thereby reducing the time and financial costs associated with choosing the wrong testing method.
NDT for Austenitic Stainless Steel
Austenitic stainless steel is suitable for liquid penetrant and radiographic testing, but not for magnetic particle, eddy current, or ultrasonic testing.
- PT & RT
Because austenitic stainless steel exhibits an extremely high surface finish after investment casting, typically ranging from Ra 3.2 μm to 6.3 μm, it is easily cleaned of liquid penetrant. Furthermore, the significant density difference between internal defects and the surrounding stainless steel, which allows for the production of clear radiographic images on digital images. As a result, it is highly suitable for PT and RT.
- MT
Since austenitic stainless steel is typically non-magnetic, castings such as 304 and 316 castings are not suitable for magnetic particle inspection. This is because magnetic particle inspection relies on the casting being magnetic; if MT is applied to such castings, nothing will happen, resulting in a wasted inspection cost.
It is worth noting that when austenitic stainless steel castings undergo severe cold working, their internal microstructure transforms into martensite, resulting in weak magnetic properties. If magnetic particle inspection is performed under these conditions, the results will be completely unreliable, leading to misjudgments and significantly increasing the costs associated with subsequent re-inspection.
- ET
Most austenitic stainless steel castings have complex internal structures and extreme inhomogeneities in electrical and magnetic conductivity, such as intricate flow paths and curved surfaces. However, eddy current testing requires the metal coil to maintain a stable distance from the metal surface. These complex structures would cause fluctuations in the distance, making it impossible to accurately detect the presence of defects.
- UT
Because coarse columnar grains would form inside stainless steel castings after investment casting, ultrasonic waves entering the casting could produce numerous reflections from these grains, making it very difficult to identify genuine defect reflections.
NDT for Martensitic Stainless Steel
Due to its strong magnetic properties and fine internal grain structure, martensitic and precipitation-hardening stainless steels are suitable for most inspection methods. However, because of its strong magnetic properties and extreme inhomogeneities in electrical and magnetic conductivity, it is not suitable for eddy current testing.
NDT for duplex stainless steel castings
Since duplex stainless steel contains 50% austenite and 50% ferrite, the inspection methods applicable to it are largely the same as austenitic and ferrite; it is suitable for liquid penetrant testing and radiographic testing, but not for the remaining three methods. This is because it possesses both austenitic and ferrite structures, resulting in uneven magnetic properties. This characteristic causes magnetic particles to disperse unevenly on the surface of castings, thereby missing actual cracks. Furthermore, like austenitic stainless steel, it has coarse columnar crystals and extreme inhomogeneities in electrical and magnetic conductivity. These factors also make it unsuitable for ultrasonic testing and eddy current testing.
NDT Applications
Stainless steel castings are subjected to varying levels of pressure under different operating conditions, which means requiring varying levels of NDT coverage. Understanding NDT Applications will help you select the appropriate non-destructive testing method, ensuring that your components do not fail due to internal cracks or porosity and minimizing your procurement costs.
Standard Industrial Castings
Typical industrial castings include architectural hardware, food machinery brackets, and non-load-bearing automotive structural components, all of which are used in low-pressure, ambient-temperature environments. For castings subjected to minimal stress, it is recommended to perform a 100% visual inspection first, which simply means checking for obvious surface cracks with the naked eye. Additionally, 5% to 10% of the castings should be randomly selected for surface liquid penetrant testing.
Pressure-Retaining Castings
Pressure-retaining castings include industrial valve body castings, chemical pump castings, and other castings that must withstand high pressure, high temperatures, and highly corrosive media. For these castings, which require extremely high airtightness, 100% liquid penetrant testing must be performed to ensure that every casting surface is free of cracks. This is because, under operating conditions with high pressure, even a microscopic crack can rapidly spread until the entire casting cracks.
Then, 5%–20% of the castings should be selected for radiographic testing of their core pressure walls, flow channel corners, and areas prone to cracking, to ensure internal integrity. It is worth noting that every first casting produced in each batch must undergo radiographic testing. This is done to verify the perfection of the gating system design and its ability to provide adequate feeding, thereby preventing internal defects in the entire batch caused by improper gating design.
High Precision Castings
High-precision components, such as aerospace impeller castings, medical castings, pump housing castings, and other castings that require extremely high precision, high-pressure withstand, and absolute structural integrity. For this category of castings, 100% inspection is required. First, 100% liquid penetrant testing is performed to inspect surface quality, or high-sensitivity fluorescent penetrant testing is used to ensure that the casting surface is free of even the slightest crack.
This is followed by 100% radiographic testing, with a separate report for each casting to ensure there are no internal shrinkage cavities. To ensure the absence of any subsurface porosity, a secondary liquid penetrant test is conducted after the casting has been machined.
Optimizing Component Design for Successful NDT Inspection
Adhering to the design for non-destructive testing rules is essential. Some factories would discover numerous blind spots during non-destructive testing, but the casting surface looks perfect. This is often due to design flaws, such as wall thickness uniformity, complex structure, and so on. Understanding the causes of these non-destructive testing issues could allow you to address them during the design phase, thereby avoiding the need for repeated testing later on and significantly reducing costs.
Wall Thickness Uniformity
The uniformity of wall thickness greatly affects the accuracy of radiographic testing. Uneven wall thickness can lead to “overexposure” or “underexposure.” This is because in radiographic testing, X-rays must penetrate the metal and allow the film to detect internal voids in the casting. If the wall thickness suddenly changes from thin to thick, the X-ray dose must be adjusted.
If the radiation dose is increased to examine the thick-walled areas, the thin-walled regions will be overexposed, appearing as solid black on the film. However, if the dose remains unchanged, the thick-walled areas will not be penetrated, resulting in underexposure and failure to form a clear image on the film.
When this occurs, the factory has to perform multiple X-ray inspections on various sections of the casting with different wall thicknesses, which will cause higher non-destructive testing costs.
Therefore, when designing castings, it is best to maintain uniform wall thickness to ensure smooth X-ray inspection and reduce non-destructive testing costs.
Complex Structure
Due to investment casting, many stainless steel castings have complex structures, including curved runners or deep cavities. However, some designers, in an effort to ensure the casting’s strength, may design the casting to be overly complex. This would create numerous blind spots that cannot be detected during non-destructive testing, significantly reducing the success rate.
This issue is particularly severe for radiographic testing. If the internal structure of a casting is overly complex, the X-ray image will appear as a jumble of overlapping shadows, making it impossible for inspectors to determine whether these shadows are caused by internal porosity or by the overlapping complex structure. Unless expensive industrial CT methods are employed, it is difficult to determine.
Furthermore, overly complex structures also have accessibility limitations. This is particularly difficult for successful liquid penetrant testing. If the casting surface features curved or intricate internal channels, it is difficult to apply the penetrant evenly, and completely cleaning afterward is also challenging. If residual penetrant remains inside the casting’s internal passages, it can actually affect the stainless steel’s corrosion resistance.
The Importance of NDT in Stainless Steel Casting
NDT is extremely important and offers numerous benefits for you. Understanding these benefits will help you place greater emphasis on performing non-destructive testing on castings.
Cost Reduction
One of the most obvious benefits of non-destructive testing is cost reduction. The earlier cracks are detected, the greater the cost savings. If you perform non-destructive testing immediately after a casting is poured, you will undoubtedly maximize cost savings. However, if you only discover that there are numerous internal porosities or cracks through NDT after machining, this will significantly increase your costs.
If you put a casting into service without performing NDT, and it fractures due to cracks during operation, the entire operating system could be destroyed, even resulting in personal injury. The resulting losses would be incalculable.
Preserving Part Integrity
NDT allows for the inspection of the internal structure of an entire batch of parts without damaging them, thereby significantly preserving their integrity. It prevents parts from being destroyed by destructive testing, which would otherwise require you to pay for two or more additional castings.
Process Improvement
NDT could also guide factories in optimizing their processes. If a specific location on a casting repeatedly shows shadows on the radiographic film during radiographic testing, the factory can identify the underlying cause quickly. For example, if the film consistently shows shadows of shrinkage cavities at a corner of a valve body, factory technicians can immediately recognize that this is due to a problem with the shrinkage feeding of the gating system. This allows them to address the issue promptly, modify the design, and reduce subsequent problems arising from this defect.
The Standards of NDT
Understanding the acceptance criteria for NDT could allow you to directly specify requirements to the foundry during procurement, thereby ensuring the high quality of your castings.
Liquid Penetrant Testing Standards
The standard for liquid penetrant testing is ASTM E1417, which strictly specifies the grade of the penetrant, ambient light intensity, cleaning time, and application specifications.
Radiographic Testing Standards
Since different foundries employ varying processes, their radiographic testing operational standards are also different. If traditional film-based radiography is used, they must follow ASTM E1742, which includes rigid indices for radiation source energy, focal length, and film density. If digital radiography is used, the standards ASTM E2422 and ASTM E2033 must be followed.
The above outlines the operational standards for radiographic testing. In addition, there are standards for the film itself. The most critical film standard for stainless steel castings is ASTM E446, which applies to castings with wall thicknesses of 2 inches (approximately 50 mm) or less.
Final Thoughts
We trust you already have a comprehensive understanding of the benefits of non-destructive testing for stainless steel castings. This process ensures your parts are free from quality risks while maintaining their structural integrity. If you are looking for a stainless steel investment casting manufacturer capable of performing standard non-destructive testing, please contact us. With our excellent quality control and one-stop service, Pingheng Machinery can provide you with castings that perfectly meet your requirements.
FAQ
Q1. If a casting fails the liquid penetrant testing, can it be repaired by welding?
This depends on the casting material and its intended application. For common austenitic stainless steels used in industry, such as CF8 (304) or CF8M (316), surface defects detected by PT are typically allowed to be thoroughly removed by grinding, followed by localized repair welding.
However, the casting must undergo solution heat treatment again. After heat treatment, a mandatory secondary PT re-inspection must be performed to ensure there are no residual cracks.
Instead, for precision stainless steel castings used in aerospace or medical applications that must withstand high pressure, any form of welding repair is generally strictly prohibited.
Q2. Will adding non-destructive testing significantly increase our total costs?
No. Although non-destructive testing increases initial inspection costs, it also reduces the remanufacturing costs that would otherwise arise later due to the lack of such testing. Therefore, overall, adding non-destructive testing can significantly reduce hidden costs in the later stage.
Q3. What are the potential risks to practical use if the liquid penetrant remains on the surface of stainless steel castings?
It will significantly affect the corrosion resistance of the stainless steel castings and may even directly cause pitting corrosion. This is because liquid penetrants typically contain chloride ions. If cleaning is not complete, these chloride ions will destroy the passivation film on the stainless steel surface in high-temperature, high-pressure, and highly corrosive operating conditions, directly causing pitting corrosion or even cracking. To address this issue, we perform ultrasonic cleaning on castings after testing to ensure that no liquid residue remains on the surface.
Q4. When should we employ non-destructive testing on castings?
For stainless steel castings, the optimal time for non-destructive testing is after heat treatment and before machining. This is because the water quenching process during heat treatment can cause cracks in castings, so testing after this stage is highly effective in detecting such cracks. Machining is costly; performing testing before machining allows for the timely detection of cracks, thereby preventing unnecessary additional costs later on.
Q5. How to prevent common internal defects to reduce the high recasting rate during non-destructive testing?
Common defects include casting shrinkage, casting porosity, hot tears, cold cracks, and so on. If you would like to learn more about them, please click “What is Casting Shrinkage and How to Control It?”、“What is Casting Porosity and How to Prevent It?”, and “The Biggest Casting Defects: Hot Tears vs. Cold Cracks & How to Prevent Them.”
