Have you ever encountered a situation where stainless steel castings fractured after only a few days of operation? This is often caused by improper tensile testing. Tensile testing is a critical process for determining a casting’s tensile strength, yield strength, and elongation. This article will explain everything you need to know about tensile testing in stainless steel investment casting, ensuring your castings meet quality standards and helping you avoid the increased costs resulting from substandard casting strength.
The Process of Tensile Testing for Investment Casting
Tensile testing is a critical test for ensuring the mechanical properties of stainless steel castings. Generally speaking, it involves applying tensile force to a test specimen until it breaks, but each step must be conducted with rigor. Knowing the specific procedures for tensile testing helps you gain technical advantages during procurement, ensuring the manufacturer’s processes are compliant and traceable, thereby eliminating the risks of subsequent casting fractures, system downtime, and increased costs for you.
Prepare the Test Specimens
First, test specimens must be prepared; however, not any stainless steel casting can be used, as the test specimen needs to meet the precision requirements. Rigorous specimen preparation ensures that test data is 100% representative of the component’s actual performance and reduces the risk of inaccurate data caused by non-compliant specimens.
- Cast-on Test Bars
Test specimens used in tensile testing must be cast-on test bars to prevent variations in temperature and internal shrinkage rates from stainless steel castings.
- Collect Location
The test specimen must be taken from the thickest part of the casting, as this part best reflects the casting’s mechanical strength.
- Precision Machining
To ensure that fracture occurs within a measurable area, tensile test specimens must be machined into a dog-bone shape to get reliable data.
- Surface Roughness
When machining the test specimens, surface roughness must be controlled to Ra ≤ 0.8 μm, and tool marks must be ≤ 0.01 mm. Otherwise, the specimen may fracture prematurely at the tool marks due to stress concentration.
Measure the Dimension
Once the test specimens are prepared, specialized measuring tools must be used to record their initial data, including the original diameter and gauge length, which is necessary for subsequent calculations of tensile strength and elongation. Accurately recording the initial dimensions eliminates any human calculation errors and provides you with a traceable report.
- Measuring the original diameter
Using a high-precision micrometer, take multiple measurements at different points along the parallel section of the test specimen, that is, the middle section of the dog-bone specimen where the diameter is smallest. After measuring, the minimum value is used to calculate the original cross-sectional area, which determines the extent of deformation in the test specimen.
- Measuring the original gauge length
Use a dedicated gauge length marking machine to mark the standard length on the central section of the test specimen. After the test specimen breaks due to being unable to withstand the tensile force, the length by which the test specimen was elongated can be calculated based on the gauge length.
Mounting the Tensile Test Specimen
Mount the prepared test specimen on the Universal Testing Machine. During mounting, the manufacturer should ensure that the centerline of the test specimen aligns perfectly with the machine’s tensile axis when it is on the fixture to avoid the influence of lateral eccentric forces. Additionally, attaching a high-precision electronic extensometer to the specimen surface is necessary because it could capture minute elastic deformations in time and accurately calculate the yield strength.
Control the Rate
After starting the testing machine, when conducting tensile testing, it is necessary to control the machine’s speed. Before the test specimen undergoes slight deformation, the test must be conducted at a slow, steady speed between 0.005 and 0.015 min⁻¹ to ensure accurate measurement of the yield strength. Once the test specimen begins to deform, the machine’s speed can be increased. Strictly controlling the tensile speed ensures that the measured yield strength is accurate and reliable, thereby preventing product fracture caused by insufficient strength.
Collect the Data
As the machine speed increases, the tensile force applied to the test specimen gradually increases. Once necking occurs and the test specimen breaks, the two broken sections can be taken down, the fracture surfaces can be carefully joined together, and the final length between the marked points can be measured with a vernier caliper to calculate the elongation. Next, the tensile strength is calculated by dividing the load sustained by the specimen before fracture by its original cross-sectional area.
Testing Equipment for Tensile Testing
Tensile testing involves specialized machines, which provide you with the most accurate and reliable data. Knowing these machines will help you understand where your procurement costs are allocated, what benefits they offer, and ultimately how they help reduce your overall costs.
Main Testing System
The most essential equipment in tensile testing is the computer-controlled universal testing machine. It can generate the stress-strain curves in real time, which are directly linked to the material test report and uploaded to a computer, ensuring that the data is absolutely accurate.
Precision Sensors
To get the most accurate data, electronic extensometers and high-precision digital micrometers are used to capture minute deformations of stainless steel test specimens under tensile stress, measure yield strength, and prevent quality issues caused by data errors.
Specimen Preparation Equipment
Since test specimens must be machined into a dog-bone shape for tensile testing, CNC machines are used to machine them. This ensures surface precision and prevents stress concentrations caused by surface roughness, which could lead to premature fracture of the test specimens and affect measurement data.
Key Factors that Influence Tensile Testing Data
In the stainless steel investment casting process, many factors can influence tensile test data. Even a minor error can lead to inaccurate tensile test results. Understanding these key influencing factors would help you effectively trace the process and accurately determine what has affected the tensile strength of the test specimens, thereby reducing the costs associated with repeated testing later on.
Heat Treatment
Heat treatment significantly influences the tensile strength, yield strength, and elongation of stainless steel. Taking austenitic stainless steel as an example, if the solution treatment temperature is insufficient or the cooling rate is too slow, carbides will precipitate at the grain boundaries, causing intergranular corrosion. This leads the overall strength of the stainless steel to decrease, elongation to drop, and tensile strength to decline significantly.
Wall Thickness
If the wall thickness of a stainless steel casting is uneven, the cooling rate will also be affected. Excessively thick walls would result in slow cooling, increasing the grain size of the stainless steel, which in turn reduces the casting’s yield strength and elongation. If there are sharp internal transitions in wall thickness, they can generate residual internal stresses during cooling, leading to a significant decrease in tensile strength and premature fracture of the test specimen.
Internal Casting Defects
If there are microscopic casting defects, such as casting shrinkage, casting porosity, hot tears, and cold cracks due to an improperly designed gating system and uneven wall thickness, the defects on the test specimen surface will act as stress concentration points during tensile testing, resulting in a significant decrease in tensile strength and elongation.
Advantages of Using Tensile Testing
There are many benefits of using tensile tests, the most obvious of which is that they help you assess the quality of castings. Understanding the advantages of tensile testing will help you pay more attention to this test, thereby preventing sudden fractures in castings during operation that could result in unnecessary costs for you.
Ensuring Casting Quality
Using tensile testing is the best way to ensure that your stainless steel castings meet the required mechanical properties, meaning no internal defects, and that the heat treatment complies with standards. This helps you prevent casting quality issues that could lead to large-scale returns and financial losses.
Reducing Raw Material Costs
When designing castings, particularly pump and valve castings, designers often make parts larger and heavier to ensure quality, which directly increases raw material costs. However, once rigorous tensile testing has been conducted to determine precise yield strength and elongation values, designers can take more innovative designs while ensuring quality compliance. This includes adopting lightweight and thin-walled designs, thereby reducing your raw material costs and helping you get higher profit margins.
Common Defects of Improper Tensile Testing
Proper tensile testing offers many benefits, but improper tensile testing can also cause many problems. Understanding the common defects of improper tensile testing will help you quickly identify the root cause, reduce downtime, and minimize losses.
Catastrophic Failures
If the tensile testing machine’s speed is not properly controlled, or if the fixtures jam due to excessive friction, this can result in artificially inflated tensile strength and yield strength. While the data may appear perfect, the castings themselves do not actually possess such high strength. Once installed in high-pressure valves, chemical processing equipment, or such load-bearing equipment, they may fracture quickly due to insufficient mechanical strength, leading to high additional costs for you.
Delivery Delays
If the depth of tool marks and surface roughness are not properly controlled during the machining of test specimens, or if the centerline of the specimen is not perfectly aligned with the tensile axis of the testing machine when the specimen is mounted, the test specimen may fracture prematurely due to stress concentration or the effects of lateral eccentric forces. However, the casting itself is actually no problem. Incorrect data will cause the factory to spend more time finding the root causes, including re-performing heat treatment and re-machining, which will take at least 1–2 weeks and significantly delay your project.
Tensile Testing Mechanical Behavior of Common Stainless Steel Grades
Different types of stainless steel exhibit different mechanical behaviors in tensile testing. Understanding these differences will help you directly compare the manufacturer’s tensile test data to determine whether the ratio of elongation to yield strength for the castings is normal when purchasing.
| Stainless Steel Type | Typical Grades | Mechanical Behavior | Fracture Characteristics |
| Austenitic Stainless Steel | 304(CF8) 316(CF8M） 316L(CF3M) | High tensile strength High elongation Lower yield strength Excellent ductility. | The neck noticeably necks down, and the fracture surface has a distinct cup-and-cone fracture with fibrous shear lips and a central fibrous zone. |
| Martensitic Stainless Steel | 410, 431, 440C | Extremely high tensile High yield strength Poor elongation Easily brittle | Virtually no necking down; the fracture surface is smooth, and exhibits a crystalline appearance with a metallic luster.
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| Ferritic Stainless Steel | 430, 446 | Moderate properties, strength, and ductility between austenitic and martensitic | Moderate necking down; the fracture surface exhibits mixed fracture characteristics. |
| Duplex Stainless Steel | 2205, 2507 | High yield and high elongation, combining the best of austenitic and martensitic | The fracture surface exhibits both necking down and tough tear cracks.
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Standards of Tensile Testing
There are strict international standards for verifying compliance with tensile testing methods and for determining whether the tensile strength, yield strength, and elongation of stainless steel meet standards. Understanding these standards will allow you to intuitively determine whether the castings comply with specifications when reviewing the tensile test reports.
ASTM E8
This standard specifies the tensile test speed and how yield strength data should be calculated.
- Tensile Speed
This standard explicitly requires that tensile testing speed must be divided into two phases, typically controlled by strain rate. Before the test specimen begins to deform, the strain rate should be controlled between 0.005 and 0.015 min⁻¹, meaning the specimen elongates by 0.5% to 1.5% per minute; after the test specimen begins to deform, the strain rate should be increased to 0.05–0.5 min⁻¹
- Data Calculation
This standard specifies that the yield strength of the test specimen must be calculated by drawing a parallel line on the stress-strain curve using the 0.2% offset method, and it requires a minimum of 2 test specimens for each tensile test.
ASTM A351
This standard requires mechanical property values for stainless steel grades in tensile testing. For 316 stainless steel castings, after tensile testing, the tensile strength must be ≥485 MPa, the yield strength must be ≥205 MPa, and the elongation after fracture must be ≥30%. More importantly, it requires a minimum of 2 test specimens for each tensile test.
Final Thought
We believe you now have a general understanding of tensile testing for stainless steel castings, a critical step ensuring casting quality. If you’re looking for a reliable stainless steel investment casting manufacturer with in-house tensile testing, please contact us. Pingheng Machinery provides you with perfect stainless steel castings through our rigorous tensile testing procedures, precision CNC machining, and customized one-stop services.
FAQ
Q1. What happens if a tensile test specimen fails?
If a test specimen fails, we first determine whether the issue is due to improper testing procedures. If the problem stems from excessively deep tool marks or fixture issues, we will remanufacture a test specimen from the same casting batch for retesting.
If the issue is not due to improper operation, we will conduct a metallographic analysis to determine whether it is due to an improper heat treatment issue or an internal defect. If it is a heat treatment issue, we will subject the test specimen and the entire batch of castings to solution heat treatment again; if there are internal porosity defects, we will recast the components to ensure the quality of your castings.
Q2. If there is a problem with one of the test specimens in my order, how can I be sure which one it is?
We mark each batch of test specimens with heat numbers. If a problem arises with one of them, we can trace it back to the original tensile test records and retained samples by heat number, quickly determine the cause, and resolve it immediately.
Q3. Can test specimens cast separately be used for tensile testing?
No, because the test specimens must be the same as your castings in every aspect. Only cast-on test bars are consistent in pouring temperature, chemical composition, and cooling rate. However, if the test specimens are cast separately, the internal structures of the specimens and the castings will be completely different, even if the temperature is only 1° higher.
Q4. Why does a casting pass the chemical composition test but fail the tensile test?
Meeting chemical composition standards is only the baseline; it indicates that stainless steel castings, such as 316L stainless steel castings, meet corrosion resistance requirements. However, tensile testing is used to verify compliance for subsequent heat treatment and machining processes, as well as to detect internal defects in the castings. Both tests are essential.
Q5. Can we use hardness testing to replace tensile testing?
No. Although there is a relationship between hardness and tensile strength, hardness testing cannot measure yield strength or elongation and serve as a standard for determining the mechanical strength of stainless steel castings. For example, if heat treatment does not meet the standard, the casting may still meet the standard on a hardness test, but it could fracture immediately under pressure.
