Incoming factory inspection of raw materials for metal stamping parts: standard procedures, quality control points and equipment configuration for tensile strength testing

In the manufacturing chain of metal stamping parts, the stability of raw material quality directly determines the stamping quality and product service safety.If the mechanical properties such as yield strength, tensile strength and elongation of a piece of cold-rolled steel plate, a roll of stainless steel strip or an aluminum alloy plate deviate from the design requirements, it may lead to a series of quality problems such as stamping cracking, excessive forming rebound, and insufficient product strength.

However, the incoming raw material inspection of many hardware stamping factories still remains at the stage of "checking the material list + visual inspection of appearance", lacking actual measurement verification of mechanical properties.The material sheets provided by suppliers are often "batch representative values", and there is even a risk of "passing off quality products and passing them off".Establishing a standardized tensile strength testing process for raw materials entering the factory is a key measure for hardware stamping companies to ensure quality is the first step.

1. Why is it necessary to conduct tensile testing of raw materials entering the factory?

1. The material list does not equal the actual quality.The material certificate (warranty) provided by the supplier is usually based on the sampling test results of the heat number or batch, and represents the "typical performance of the batch of material", rather than the actual performance of the roll/material you receive.During the transportation, slitting, and flattening process, the material may be deformed, mixed, and other problems may occur.What's more, some unscrupulous suppliers will provide material sheets that are inconsistent with the actual products.Actual testing in the factory is the only reliable way to verify the authenticity of materials.

2. The stamping process is highly sensitive to mechanical properties.The core processes of metal stamping - stretching, bending, flanging, and bulging - have strict requirements on the material's yield strength, tensile strength, elongation, yield ratio (yield strength/tensile strength) and other parameters.Taking deep drawing as an example, the lower the yield ratio (usually required to be ≤0.65), the better the formability of the material and the less likely it is to crack; the higher the elongation, the higher the forming limit.If the mechanical properties of raw materials fluctuate greatly, even if the mold and process parameters remain unchanged, there will be a problem of "this batch can be pulled out, but the next batch will crack".

3. The need for cost control.Stamping scrap caused by unqualified mechanical properties of raw materials, the loss is not only the material itself, but also includes mold loss, wasted working hours, and delivery delays.Establishing an incoming factory inspection mechanism can detect problems before putting them into production, return and exchange goods in a timely manner, and avoid unqualified materials from flowing into the production process and causing greater losses.

4. Customer factory audit and system certification requirements.Customers in the automotive, electronics, home appliance and other industries usually require inspection records of raw materials entering the factory during supplier audits, of which mechanical property testing is a core item.Quality management systems such as IATF 16949 and ISO 9001 also explicitly require verification of raw materials.

2. Standard procedure for tensile strength testing

Commonly used raw materials for metal stamping parts include cold-rolled steel plates (SPCC, DC01~DC06), hot-rolled steel plates, stainless steel strips (304, 316, 430, etc.), aluminum alloy plates (5052, 6061, etc.), copper alloy strips, etc.The testing standards corresponding to different materials are slightly different, but the core processes are basically the same.

Step One: Sampling

Sampling location:Take samples from the end of each roll/sheet of raw material, avoiding the edge defect area (usually ≥50mm from the edge) and the unstable section of the head and tail (usually remove 1~2 meters from the head and tail).For steel coils, it is recommended to take samples at fixed locations on the uncoiling line to ensure sampling consistency and traceability.

Sampling quantity:It is recommended to take at least 1 set of samples (3 parallel samples) from each batch (same heat number, same specification, same arrival batch).For critical products or new suppliers, this may be increased to 5 parallel samples per set.

Sampling direction:The length direction of the tensile specimen should be consistent with the rolling direction of the material (longitudinal specimen).For materials with obvious anisotropy (such as deep-drawn steel plates), longitudinal and transverse specimens can be taken simultaneously to evaluate the plastic strain ratio (r value) and anisotropy.

Sampling method:It is recommended to use wire cutting or milling machines to process samples to avoid work hardening and heat affected zones caused by shearing or flame cutting.After sampling, the sample number, material batch, sampling direction and other information should be marked to ensure traceability.

Step 2: Sample preparation

Sample shape:The tensile test of metal sheets usually uses rectangular cross-section specimens (dumbbell-shaped), which comply with the requirements of GB/T 228.1.Commonly used dimensions are: gauge length 50mm, parallel section width 12.5mm or 20mm, transition arc radius ≥15mm, and total length ≥200mm.For thin plates less than 3 mm thick, proportional specimens (gauge length = 5.65√cross-sectional area) may also be used.

Processing requirements:

• The edges on both sides of the specimen should be smooth and free of burrs, scratches and work-hardened layers.It is recommended that in the last step, you use sandpaper or a file to lightly polish the edges along the length direction to eliminate machining knife marks.
• The original thickness of the raw material should be maintained in the thickness direction and should not be thinned by polishing.
• Sample size tolerance: width ±0.1mm, thickness measurement accurate to 0.01mm.
• There should be no rust, oil stain, oxide scale and other defects on the surface of the specimen that would affect the test.

Step 3: Preparation before testing

Dimensions:Use a vernier caliper or micrometer to measure the width and thickness of the gauge section of the specimen. Measure each dimension three times and take the average value to record the original cross-sectional area.For thin strips less than 0.5mm thick, it is recommended to use a spiral micrometer with an accuracy of 0.001mm.

Equipment inspection:

• Confirm that the tensile testing machine has been powered on and preheated (usually ≥15 minutes), and the force value display has returned to zero.
• Confirm that the equipment is within the calibration validity period and the accuracy level meets the requirements (level 0.5 or level 1 is recommended for metal material testing).
• Choose the appropriate clamp: For thin plate stretching, it is recommended to use a wedge-shaped clamp with a flat tooth clamp surface. For thin strips with a thickness of ≤1mm, a pneumatic flat clamping clamp can be used.
• Install an extensometer: If it is necessary to measure yield strength and elongation, an electronic extensometer or video extensometer should be installed, and the gauge length should be consistent with the gauge length of the specimen.

Environmental conditions:The test should be conducted at room temperature (10~35℃). For tests with high accuracy requirements, it is recommended to control it at 23±5℃.Specimens should be placed in the test environment for sufficient time to achieve temperature equilibrium.

Step 4: Clamping and Centering

Clamping operation:

• First, clamp the upper end of the specimen into the upper clamp. The clamping position should be at the center of the specimen clamping section, ensuring that the gauge length section is in the middle of the clamp.
• After clamping the upper clamp, gently pull the lower end of the sample into the lower clamp so that the sample is naturally vertical without twisting or bending.
• Clamp the lower clamp, and the clamping force should be uniform to ensure that the specimen does not slip during the test.For stainless steel belts with smooth surfaces, toothed clamps can be used or sandpaper can be inserted to increase friction.

Alignment check:After clamping is completed, observe whether the specimen coincides with the loading axis of the testing machine.This can be checked by the equipment's alignment indicator or visually.Poor alignment will cause the specimen to bear additional bending stress, resulting in low test results and large data dispersion.

Preload:After clamping is completed, apply a small pretension force (usually 1% to 2% of the expected yield force) to eliminate clamp gaps and specimen looseness, and then reset the force values and displacements to zero.

Step 5: Test loading

Loading rate:According to the requirements of GB/T 228.1:

• Before yielding (elastic section and yielding section): Strain rate control is adopted, and the rate is 0.00025/s~0.0025/s (corresponding to a 50mm gauge specimen, the beam speed is approximately 0.75~7.5mm/min).
• After yielding (strengthening section and necking section): It can be switched to displacement control, and the rate does not exceed 1mm/min (or the strain rate does not exceed 0.0067/s).
• The entire test process should be continuously loaded without any pauses or speed changes.

Data collection:The testing software should continuously record force value, displacement, and deformation (extensometer) data with a sampling frequency of no less than 10Hz to ensure that a complete force-displacement curve and force-deformation curve are obtained.

Extensometer removal:For tests requiring measurement of post-break elongation, the extensometer should be removed after the specimen yields but before necking (usually at the end of the uniform deformation phase) to avoid damage to the extensometer when the specimen breaks.After removal, the deformation was continued to be recorded using displacement data until fracture.

Step Six: Data Processing and Determination

Automatic calculation:After the test is completed, the software should automatically calculate the following parameters:

• Yield strength (ReL or Rp0.2)
• Tensile strength (Rm)
• Elongation after break (A)
• Modulus of elasticity (E, if extensometer data available)
• Quantity ratio (Re/Rm)

Parallel sample data processing:Three parallel samples were taken from each group, and the arithmetic mean was taken as the final result.If a certain data deviates significantly (the deviation exceeds 10%), check whether it is an abnormal value and make additional samples if necessary.

Qualification judgment:Compare the test results with the mechanical property requirements in raw material standards (such as GB/T 700, GB/T 5213, GB/T 3280, etc.) or customer technical agreements. If all indicators are within the specified range, it will be judged as qualified, otherwise it will be deemed unqualified.

Step 7: Sample retention and recording

Fracture specimen retention:The fracture specimens after testing should be retained for at least 3 months (or as required by the customer/system) and marked with the specimen number and test date for review and dispute resolution.

Records and reporting:Each batch of tests should generate a complete test record, including: material batch, supplier, arrival date, sample number, sample size, test date, ambient temperature and humidity, equipment number, tester, test data (force-displacement curve, various performance parameters), and qualification conclusion.Inspection records should be archived to facilitate traceability and customer review.

3. Key points of quality control: common problems and solutions

In actual operation, tensile testing of raw materials entering the factory often encounters the following problems, which need to be focused on:

Problem 1: The specimen broke at the clamp.

• Reason:The clamp teeth damage the specimen surface, the clamping force is too large, the centering is poor, and there is stress concentration at the clamp mouth.
• Solution:① Use appropriate clamping surfaces (flat teeth for thin plates, coarse teeth for thick plates); ② The clamping force is moderate, as long as it does not slip; ③ Ensure good alignment; ④ Insert a gasket similar to the sample material at the clamp; ⑤ If the clamp breaks repeatedly, consider switching to a specimen type with wider shoulders.

Problem 2: The test data has large dispersion.

• Reason:The sample processing quality is inconsistent, the clamping conditions are unstable, the material itself is uneven, and the centering deviation is inconsistent.
• Solution:① Standardize the sample preparation process and use special sample preparation equipment or molds; ② Unify clamping operations, use the same clamp and the same clamping force; ③ Increase the number of parallel specimens (from 3 to 5); ④ Regularly check and calibrate the fixture alignment accuracy.

Problem 3: The yield strength is not measured accurately or cannot be measured.

• Reason:The loading rate is too fast, the extensometer is improperly installed, the material has no obvious yield platform (Rp0.2 needs to be measured), and the equipment force value accuracy is insufficient.
• Solution:① Strictly control the strain rate before yielding; ② Install the extensometer correctly to ensure accurate gauge length and knife edge fit; ③ For materials without obvious yield, measure according to the Rp0.2 method specified in the standard; ④ Use 0.5-level precision equipment to ensure measurement accuracy in the small force value section.

Problem 4: The thin strip sample is slipping.

• Reason:The friction force of the clamping surface is insufficient, the clamping force is insufficient, and there is oil stain on the surface of the sample.
• Solution:① Use toothed clamps or put fine sandpaper on the clamps; ② Increase the clamping force (the pneumatic clamp can adjust the air pressure); ③ Use alcohol or acetone to clean the oil on the surface of the sample before testing; ④ For extremely thin strips (<0.2mm), it is recommended to use a winding clamp.

4. Equipment configuration plan

In response to the demand for incoming factory inspection of raw materials for metal stamping parts, the following equipment configuration solutions are recommended:

Basic configuration (meeting routine testing requirements):

• Host: 50kN electronic universal testing machine (double-column structure, accuracy level 0.5 or level 1)
• Clamp: Wedge-shaped stretching clamp (clamping range 0~10mm), two clamping surfaces: trim teeth and coarse teeth
• Extensometer: electronic extensometer (gauge length 25mm/50mm switchable)
• Software: Built-in metal tensile standards such as GB/T 228.1, automatic calculation and report generation
• Auxiliary equipment: sample preparation mold or wire cutting machine, vernier caliper/micrometer

Advanced configuration (to meet high-precision and multi-material testing requirements):

• Add to the basic configuration:
• Small range sensor (1kN or 5kN) for small force testing of thin strips and soft materials
• Pneumatic flat clamping fixture, used for stainless steel strips, copper strips and other thin strips with a thickness of ≤1mm
• Video extensometer, non-contact measurement, avoids damage to thin specimens by the extensometer knife edge
• High and low temperature environmental chamber, used for mechanical property testing in special environments
• Multi-sample automatic feeding device improves batch testing efficiency

Our solution advantages:

• Wedge fixture optimized for testing metal sheets and strips, with centering accuracy ≤0.3mm and low jaw breakage rate
• Supports high-precision measurement from 0.4% to 100% of full scale, and the data in the small force value segment is equally reliable
• The testing software has built-in mainstream metal tensile standards such as GB/T 228.1, ASTM E8, ISO 6892, etc., and can call standard methods with one click.
• Special report templates for incoming raw material inspection can be customized, including batch information, qualification determination, trend statistics and other functions
• Provide complete solutions from sample preparation to testing, as well as operator training and follow-up technical support

5. Suggestions on establishing an incoming factory inspection system

Finally, for metal stamping companies that plan to establish or improve the tensile testing system for incoming raw materials, the following suggestions are given:

1. Clarify the scope and frequency of testing.According to the importance of the material and the quality stability of the supplier, the raw materials are divided into three categories: A/B/C: Category A (key materials, new suppliers) must be inspected in every batch; Category B (important materials, qualified suppliers) are randomly inspected in every batch or every other batch; Category C (general materials, long-term stable suppliers) are inspected regularly.
2. Establish a profile of qualified suppliers.Record the testing data of each batch of each supplier, and regularly count the pass rate and performance fluctuations as the basis for supplier evaluation and elimination.
3. Set internal warning lines.Based on the qualified range stipulated in the material standards, set a more stringent internal warning line (for example, the standard requires a yield strength of 150~220MPa, and the internal warning line is set to 160~210MPa). When approaching the warning line, communicate with the supplier in advance to avoid unqualified batches.
4. Regular comparisons and proficiency testing.Conduct comparison tests with third-party testing agencies at least once a year to verify the accuracy of internal testing data.Actively participate in proficiency testing activities organized by industry organizations and continue to improve testing levels.
5. Data information management.Integrate raw material testing data into the quality management information system (QMS) to realize automatic data statistics, trend analysis and abnormal warning, and provide data support for purchasing decisions and process optimization.

Raw materials are the source of product quality. Controlling the incoming factory inspection is a basic investment for hardware stamping companies to improve product quality, reduce quality costs, and enhance customer trust.A standardized tensile strength testing process, coupled with appropriate testing equipment and strict management systems, can protect the quality of the company.

If you are planning a raw material incoming testing laboratory, or need to upgrade your existing tensile testing equipment, please contact us for detailed equipment configuration plans and investment return analysis.We have been deeply involved in the field of mechanical testing for many years and have provided professional testing solutions to hundreds of metal stamping, metal processing, and auto parts companies.