Have you ever encountered situations during procurement where stainless steel castings suffered brittle fracture under low-temperature or high-pressure conditions? This is caused by the lack of impact testing, a critical test for verifying if a casting’s toughness meets the required standards. This article will provide everything you want to know about impact testing, delivering a detailed understanding of its importance and how it can help you save costs.
Types of Impact Testing
There are many types of impact tests. Determining which one is appropriate for your stainless steel castings can help you save time in the early stages and prevent an unsuitable test from producing unreliable data that could lead to casting fractures.
Charpy V-Notch Impact Testing
Charpy impact testing is the most common impact test of stainless steel investment casting. In particular, Charpy V-notch impact testing perfectly simulates real operating conditions, allowing you to thoroughly evaluate the impact resistance of your stainless steel castings under high-pressure and extremely low-temperature environments. This helps prevent low-temperature brittle fracture in your castings, extends maintenance intervals, and reduces maintenance costs.
- Specimen Preparation
Proper specimen preparation lays the foundation for subsequent impact testing, ensuring that it accurately reflects the toughness of stainless steel castings. This effectively ensures the accuracy of the results and ensures that your castings possess good toughness. Selecting standard test specimens is the first step.
The test specimens were machined from Y-blocks cast in the same furnace as the castings. They must be cast-on with the stainless steel castings, which ensures they undergo the same heat treatment and possess the same properties.
- Overall Machining
For impact testing, test specimens must be machined into rectangular prisms measuring 55 mm × 10 mm × 10 mm, with height and width tolerances of ±0.075 mm and length tolerance of ±0.6 mm, to ensure they can be securely clamped in the machine.
- V-Notch Machining
After cutting the test specimen into a cuboid, the manufacturer must use a machining tool to create a 45° angle, 2 mm deep V-notch at the center of a 55 mm length. The root radius of the notch must be equal to 0.25 mm ± 0.025 mm. This step is critical for impact testing because the strict requirements for the root radius and angle ensure that the point of mechanical stress is fixed. Furthermore, the presence of the V-notch realistically simulates potential stress concentration points that may occur in stainless steel castings under operating conditions.
- Test Temperature Selection
There are many different types of stainless steel, and depending on their specific operating environments, different grades must undergo impact testing at different temperatures. For example, austenitic stainless steel, specifically 304 and 316 stainless steel castings, must undergo impact testing at -101°C or -196°C when used in cryogenic valves for liquefied natural gas or in marine applications.
- Maintaining Test Temperature
When preparing a specimen for testing, ensure that both the surface and the core reach the operating temperature. Taking austenitic stainless steel under cryogenic operation as an example, since it must operate at an ultra-low temperature of -196°C, the specimen must be immersed in a cryogenic bath for at least 10 minutes before testing.
After immersion, the operator must transfer the specimen to the Charpy impact testing machine and immediately release the heavy pendulum to break it within 5 seconds, ensuring that the notch faces away from the pendulum.
If the transfer and impact are not completed within 5 seconds, the specimen will absorb ambient heat, compromising test accuracy and failing to reflect the casting’s true cryogenic performance. This could lead to your castings fracturing under real conditions with low temperatures, thereby increasing your costs.
- Test Analysis
Once the pendulum fractures the specimen, the system will automatically record and display the absorbed energy in Joules. In addition, it is necessary to test the percent shear fracture and the lateral expansion in the specimen’s fracture surface upon impact, that is, to determine whether the fracture is a gray ductile fracture or a bright brittle fracture; in this way, you can directly assess whether the solution heat treatment was performed properly. Determining whether your castings meet the toughness standard and reducing the risk of brittle fracture could give you greater peace of mind during use.
Izod Impact Testing
Izod impact testing is another method for impact testing, but due to its procedure, it is very rarely used on stainless steel castings.
Izod impact testing uses a cantilever beam configuration, which means the specimen must be placed vertically in the fixture and secured with mechanical screws. If the test is for austenitic stainless steel castings that need to be used under cryogenic conditions, the process is too slow, and the surface temperature has already risen, resulting in measurement data that does not accurately reflect the casting’s actual performance.
If your castings need to operate at temperatures below -196°C, Izod impact testing may indicate good toughness at -100°C; however, there is still a high probability of fracture during actual operation.
Factors Affecting Impact Testing
Many factors influence impact testing; some are issues in the process of investment casting, while others result from errors in steps of the impact test. Understanding these factors will help you effectively trace the process, accurately determine what affected the test specimen’s toughness, and reduce the costs associated with repeated testing.
Chemical Factors
For austenitic stainless steel, to prevent hot cracks during the cooling period after pouring, the internal structure must retain 3%–8% ferrite. If the spectrometer analysis is not performed before casting, resulting in excessively high chromium content, the ferrite content may exceed 12%. In this case, if the factory fails to detect excessive ferrite content in metallographic analysis, it will cause sensitization, and the casting will transition from ductile to brittle at low temperatures, causing a precipitous drop in the Joule value when subjected to impact.
Heat Treatment Process
Heat treatment has a decisive impact on the impact toughness of stainless steel castings. Solution heat treatment can effectively dissolve the carbides precipitated on the casting surface, improving impact toughness. However, if the holding time is insufficient or the water-quenching transfer time is too slow, carbides will precipitate again at the grain boundaries, which would affect impact toughness. As a result, when the specimen is subjected to impact, cracks will instantly tear along the grain boundaries, causing sensitization and making the measured impact toughness meaningless.
Casting Defects
If the pouring system is improperly designed during the investment casting process, it can lead to uneven shrinkage and air entrapment in the test specimen, resulting in internal defects such as shrinkage and porosity. These internal defects can significantly affect the impact toughness of the test specimen. If these defects are located beneath the V-notch, they will cause the test specimen to fracture prematurely when struck by the pendulum, resulting in the measured joule value being far lower than the true value.
Machining Precision
Because the Charpy impact test requires high precision, including strict control over specimen dimensions and the V-notch root radius, any machining error can affect the result. If the broach becomes worn during machining, the bottom radius of the notch may become too sharp, resulting in severe stress concentration. Consequently, castings that were originally up to standard may fail due to issues with the test specimen, requiring re-heat treatment or even recasting. The complete re-cast and heat treatment cycle can delay your project by at least 2–4 weeks and bring unnecessary costs to you.
The Advantages of Using Impact Testing
Impact testing offers many benefits for your stainless steel castings. Not only does it ensure that your castings meet the required toughness standards, but it also eliminates the risk of fracture and reduces your maintenance costs.
Verifying Compliance with Solution Heat Treatment
The most direct benefit of impact testing is that it reveals whether the solution heat treatment was performed correctly. Although the chemical composition meets specifications, if the holding time is insufficient or water quenching is too slow, the joule value measured during impact testing will drop instantly. If the impact test yields normal Joule values, it indicates that every casting has undergone a flawless solution treatment, with the internal microstructure achieving an optimal metallographic state, thereby ensuring compliance with castings’ material quality.
Eliminating the Risk of Low-Temperature Brittle Fracture
Austenitic stainless steels, such as 316L stainless steel, if they have not undergone impact testing, are prone to brittle fracture when subjected to sudden impact in extremely low-temperature conditions.
Impact testing effectively eliminates this risk. After undergoing this test, it can be guaranteed that castings will retain sufficient impact toughness even under extreme conditions where they may be subjected to impact at any time, ensuring they will not fracture suddenly. This eliminates the risk of leaks and fractures in pipe and valve castings under low-temperature or high-pressure environments, which could lead to increased costs and project delays.
Optimizing casting design
After impact testing, you’ll have precise and traceable joule values. With this accurate data, engineers can optimize the wall thickness design of castings while ensuring safety. Furthermore, under extreme operating conditions, there’s no need to use more expensive, higher-toughness alloys; only stainless steel can meet the strength requirements. This directly helps you reduce costs and improve efficiency, achieving the best results at the lowest cost.
Critical Industries Requiring Impact-Tested Castings
Not all stainless steel castings require impact testing; it is only for those used in extreme conditions, such as extremely low temperatures, high pressure, or situations that affect personal safety. Knowing which industries require impact testing will help you determine whether it is necessary for your stainless steel castings, thereby avoiding unnecessary costs.
Liquefied Natural Gas Industry
Liquefied natural gas is typically transported at temperatures ranging from approximately -162°C to -196°C. Under these cryogenic conditions, pump housing and valve body castings made of stainless steel must possess excellent impact resistance to prevent low-temperature brittle fracture and leaks. Therefore, impact testing must be conducted to ensure that these stainless steel castings fully meet the required standards for impact resistance and toughness under cryogenic conditions.
Petrochemical Industry
The petrochemical industry often requires operations in deep-sea or Arctic regions. Stainless steel castings used in these areas must not only withstand high pressure but also endure extreme cold and continuous impact. If brittle fracture occurs, it could result in a catastrophic oil spill. Therefore, impact testing is very important for stainless steels used in this industry, primarily duplex and super-austenitic stainless steel.
Marine Engineering
In marine engineering, stainless steel castings such as high-pressure seawater desalination pumps and ship propellers must not only resist pitting corrosion but also withstand long-term exposure to marine cyclic stresses and the low-temperature environment of the deep sea. Therefore, stainless steel castings used in this industry, particularly 316L stainless steel castings, must not only be corrosion-resistant but also undergo impact resistance testing to guarantee they possess extremely high fatigue and cryogenic impact properties.
International Standards of Impact Testing
For impact testing, particularly Charpy impact testing, there are international standards that address it. Knowing which specific standards apply will help you use them as a basis for requiring manufacturers to conduct impact testing during procurement.
ASTM A370
The ASTM A370 standard is the core standard of stainless steel inspection. It not only specifies tensile and hardness testing but also sets out specifications for Charpy V-notch impact testing of stainless steel castings. For example, it specifies that the V-notch angle must be 45° ± 1°, the notch depth must be 2 mm ± 0.025 mm, and the supporting thickness below the notch must be exactly 8 mm. In addition to these dimensional tolerances, this standard specifies that the surface roughness at the notch root must be less than 2 μm.
ASTM E23
ASTM E23 focuses on specifying the equipment machine such as testing machines and pendulums. For example, the linear velocity of the pendulum when it strikes the specimen at its lowest point must be maintained between 4.5 and 6.0 m/s, and the testing machine base must be sufficiently sturdy, with the total mass of the base being at least 12 times that of the pendulum to prevent the machine from vibrating and absorbing energy during the impact, which would result in lower test results.
ASTM A923 Method B
This standard is specifically for duplex stainless steel. It requires that Charpy impact testing at -40°C or -46°C be used to determine whether sigma-phase precipitation occurs in duplex stainless steel, that is, whether the duplex stainless steel Charpy V-notch energy in joules meets the required standard. For the standard duplex steel S32205, the minimum impact energy requirement is ≥ 54 J.
Final Thought
We believe you now have a general understanding of stainless steel impact testing, which is a crucial test for ensuring the toughness and impact resistance of castings. If you are looking for a reliable stainless steel investment casting manufacturer, please contact us. Pingheng Machinery, with its professional impact testing equipment, expert operations, and one-stop customization capabilities, can provide you with perfect castings that meet your requirements.
FAQ
Q1. If my stainless steel casting has already passed a tensile test, is it still necessary to perform an impact test?
This mainly depends on the operating conditions of your casting. If it is used only for hardware components at room temperature, a tensile test alone is enough. However, if your casting is used under extreme conditions, an impact test is required.
This is because tensile testing primarily measures the yield strength and elongation of a casting under normal operating conditions. Impact testing, on the other hand, primarily measures whether a stainless steel casting can maintain its toughness under a sudden, high-intensity impact in severe environments, and whether that toughness will transition to brittleness. The two tests have different focuses but are complementary.
Q2. How do you ensure your impact tests pass 100%?
We strictly adhere to the ASTM A370 standard. Before conducting impact tests, we use a spectrometer to analyze the chromium and nickel content and perform metallographic analysis to determine the ferrite content. After confirming that these levels are within specifications, we proceed with heat treatment. By optimizing the gating system and controlling the heating process, we eliminate carbides from the casting surface. Then we will use CNC machines to precisely machine the test specimens, ensuring that the impact test accurately reflects the toughness of your castings under extreme conditions. And we will offer a stainless steel casting inspection report of the impact test for you.
Q3. What’s the difference between Charpy impact testing and Izod impact testing?
The main differences between the two lie in how the test specimens are positioned and the materials they are used on.
In the Charpy impact test, the test specimen is placed horizontally on support blocks, and the pendulum strikes the back surfaces of the V-notch, allowing for rapid testing within 5 seconds. In contrast, in the Izod impact test, the specimen is positioned vertically, with its lower half securely clamped, and the pendulum strikes the upper half.
Because of this specimen orientation in the Izod impact test, it is not suitable for assessing the toughness of austenitic stainless steel under cryogenic conditions. Therefore, unlike the Charpy impact test, which is commonly used to evaluate the toughness of stainless steel, the Izod impact test is primarily used for plastics or metals used at room temperature.
Q4. What does “Percent Shear Fracture” tell us about casting quality?
The percent shear fracture reflects the ratio of ductile fracture to brittle fracture, that is, the percent of gray surface to bright surface. If there is a high percentage of gray, rough surfaces, it indicates that the solution heat treatment of the casting was thoroughly performed, that carbides at the grain boundaries have been fully dissolved, and that the internal structure will not transition from ductile to brittle.
Q5. What if the casting wall is too thin to machine a standard 10×10mm Charpy specimen directly from the part?
According to ASTM A370 Charpy V-notch acceptance criteria, you can use Sub-size specimens, such as 10 mm × 7.5 mm, 10 mm × 5 mm, or 10 mm × 2.5 mm, with the required joule value adjusted proportionally.
