Spring Steel Plate Testing

Spring steel plate inspection mainly conducts scientific testing and analysis on the mechanical property testing, hardness testing, metallographic structure analysis, chemical composition analysis, surface quality and dimension precision testing and fatigue property testing, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Mechanical property testing: This is the core of the spring steel plate testing, mainly including the determination of the yield strength, tensile strength, elongation and reduction of area. Through these data, the ability of the steel plate to resist the deformation and fracture under the loads can be evaluated, ensuring that it can work normally under the repeated alternating loads. It is the key basis for judging whether the material is qualified.

Hardness testing: The hardness is the indicator measuring the ability of the material surface to resist the local plastic deformation. The Brinell hardness (HBW), Rockwell hardness (HRC) or Vickers hardness (HV) are commonly used for the testing. The hardness values directly affect the wear resistance, fatigue strength and elastic limit of the spring, and are the important parameters of the production process control and finished product acceptance.

Metallographic structure analysis: The microstructures of the material, such as the morphologies, distributions and uniformity of the quenched and tempered structures including the martensite, bainite and troostite, are observed through the microscope. The grain sizes, non-metallic inclusion grades and decarburized layer depths are analysed. These micro-structures are the fundamental factors determining the macro mechanical properties and fatigue lives of the spring steel plate.

Chemical composition analysis: The contents of the main alloying elements such as the carbon (C), silicon (Si), manganese (Mn), chromium (Cr) and vanadium (V), as well as the harmful elements such as the phosphorus (P) and sulphur (S) in the steel plate are precisely determined. The chemical composition is the basis of the formulation of the material heat treatment processes, directly related to the final properties, and needs to strictly conform to the relevant material standards.

Surface quality and dimension precision testing: Checks whether there are the defects such as the cracks, folds, scars, oxide scales and rust on the steel plate surface. At the same time, the dimension and shape deviations such as the thickness, width, length, flatness and camber of the steel plate are measured, ensuring that they meet the precision requirements of the downstream processing and assembly.

Fatigue property testing: Simulates the working state of the spring under the long-term alternating stresses, and determines its fatigue limits (such as the maximum stress without fracture at 10⁷ cycles) or draws the S-N curves through the fatigue testing machines. This testing directly verifies the service lives and reliability of the spring steel plate under the actual working conditions.

Residual stress testing: The harmful residual stresses may be produced in the spring steel plate after the rolling, heat treatment and forming. The X-ray diffraction method or hole-drilling method is usually adopted for the measurement. The reasonable residual stress distributions help to improve the fatigue strength, while the uneven residual stresses may lead to the early failures or shape instability.

Testing Scope

Incoming raw material inspection: The incoming inspection of the spring steel plate coils or plates mainly checks the quality certificates, verifies the designations, carries out the chemical composition spectrometry screening, and checks the surfaces and dimensions. It is the first gate of controlling the product quality, preventing the unqualified materials from entering the production process.

Heat treatment process monitoring: The testing is carried out before and after the key heat treatment processes such as the quenching and tempering, such as testing the hardness after the quenching and the hardness and metallographic structures after the tempering. The purpose is to verify the effectiveness and stability of the process parameters, ensuring the consistency and traceability of the performances of each batch of products.

Finished product ex-factory inspection: The comprehensive inspection carried out before the delivery of the spring steel plate finished products to the customers, covering all the key performance indicators, including the mechanical properties, hardness, dimensions and surface quality, and issuing the test reports conforming to the standard requirements. It is the final proof of the product conformity.

Process research and development and failure analysis: In the stages of the new material development and new process testing, the compositions and processes are optimized through the systematic testing and analysis. When the springs have the failures such as the fractures and plastic deformations in the testing or use, the causes are traced through the testing, providing the data support for improving the designs, materials or processes.

Spring steels of the different material types: The testing scope covers the various types of spring steels, such as the carbon spring steels (such as 65Mn), alloy spring steels (such as 60Si2Mn and 50CrVA), stainless spring steels (such as 3Cr13 and SUS304) and the new high-strength spring steels, and the testing is carried out for their specific performance requirements.

Steel plates of the different processing states: Including the spring steel plates of the different processing and heat treatment states such as the hot-rolled, cold-rolled, annealed and quenched and tempered states. The testing emphases are different in the different states. For example, the hot-rolled plates pay more attention to the decarburized layers, while the quenched and tempered plates focus on the comprehensive mechanical properties and structures.

Testing Methods

Tensile testing method: In accordance with the standards such as GB/T 228.1, ISO 6892-1 and ASTM A370, carried out on a universal material testing machine. The standard tensile specimens are prepared, and the axial tensile forces are continuously applied, recording the load-displacement curves, thereby precisely calculating the key mechanical property parameters such as the yield strength, tensile strength and elongation after fracture.

Hardness testing method: The Brinell hardness (GB/T 231.1) is applicable to the softer or medium-hardness materials and thicker specimens; the Rockwell hardness (GB/T 230.1) has the fast operation, suitable for the harder spring steels after the quenching and tempering; the Vickers hardness (GB/T 4340.1) has the high precision, suitable for the detection of the thin layers or small areas, such as the hardness gradient testing of the decarburized layers.

Metallographic examination method: In accordance with the standards such as GB/T 13298 and GB/T 10561. After the sampling, mounting, grinding, polishing and etching, the observation is carried out under a metallographic microscope. The grain sizes (GB/T 6394), microstructures, non-metallic inclusions (such as the types A, B, C and D) and the depths of the surface total decarburized layers (GB/T 224) can be rated.

Chemical analysis method: Including the traditional wet chemical analysis and modern instrumental analysis. The spark discharge atomic emission spectrometry (GB/T 4336) is used for the fast furnace-front analysis; the inductively coupled plasma atomic emission spectrometry (ICP-OES) and carbon-sulphur analysers are used for the precise determination of the major and trace elements; the energy dispersive spectrometers (EDS) can be combined with the electron microscopes for the micro-area composition analysis.

Ultrasonic flaw detection method: Utilizes the principle that the high-frequency sound waves reflect when encountering the defects during the propagation in the material, detecting the defects such as the inclusions, delaminations and cracks inside the steel plate. This method is non-destructive with the strong penetration, commonly used for the internal quality inspection of the spring steel plates for the important uses (such as the automobile suspension leaf springs).

Fatigue testing method: Usually carried out on the hydraulic servo fatigue testing machines or high-frequency resonance fatigue testing machines. The specimens bear the alternating loads such as the tension-tension, tension-compression or bending, and the number of cycles and stress levels leading to the specimen failures are recorded, used to evaluate the durability of the material under the dynamic loads.

Dimension and geometric tolerance measurement method: The micrometers, vernier callipers and plate thickness micrometers are used to measure the thicknesses and widths; the surface plates, feeler gauges and right-angle rulers are used to measure the flatness and camber; for the high-precision requirements, the laser scanners or image measuring instruments can be adopted for the automated and non-contact measurement.

Testing Standards

Domestic Standards:

1. GB/T 228.1-2021, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

2. GB/T 231.1, Metallic materials — Brinell hardness test — Part 1: Test method

3. GB/T 230.1-2018, Metallic materials — Rockwell hardness test — Part 1: Test method

4. GB/T 4340.1-2009, Metallic materials — Vickers hardness test — Part 1: Test method

5. GB/T 224, Determination of decarburized layer depth of steel

6. GB/T 4336, Carbon and low-alloy steel — Determination of multi-element contents — Spark discharge atomic emission spectrometric method

7. GB/T 6394, Determination of estimating the average grain size of metal

8. GB/T 10561-2023, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standards diagrams

9. GB/T 13298, Inspection methods of microstructure for metals

International Standards:

1. ASTM A370, Standard test methods and definitions for mechanical testing of steel products

2. ISO 6892-1, Metallic materials — Tensile testing — Part 1: Method of test at room temperature

3. ISO 9227, Corrosion tests in artificial atmospheres — Salt spray tests

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

Universal material testing machine: The core equipment for the mechanical property tests such as the tensile, compression and bending. The modern models are mostly the microcomputer-controlled electro-hydraulic servo or electronic types, with the high-precision load cells and extensometers, can automatically acquire the data and generate the test reports, meeting the requirements of the various metal material testing standards.

Hardness tester series: Including the Brinell hardness testers (needing to be equipped with the indentation measuring microscopes), Rockwell hardness testers (configured with the different indenters and test forces) and Vickers/micro-hardness testers. The automatic turret type hardness testers can realize the fast switching of the different scales, and the digital display type hardness testers have the accurate and convenient readings. Some models can also be connected to the computers for the data management.

Metallographic microscope and sample preparation equipment: The metallographic microscope (with the functions such as the bright field, dark field and polarization) is used to observe the structures. The supporting equipment includes the cutting machines, mounting machines, grinding and polishing machines (automatic or manual) and etching devices, used to prepare the high-quality metallographic specimens. The image analysis systems can carry out the quantitative analysis of the structures.

Spectrometer: The spark direct-reading spectrometer is the standard equipment of the fast chemical composition analysis in the iron and steel industry, and can simultaneously determine the contents of more than ten elements in the steel within dozens of seconds. The inductively coupled plasma spectrometers (ICP) and carbon-sulphur analysers are used for the more precise quantitative analysis and the determination of the low carbon and sulphur.

Non-destructive testing equipment: The ultrasonic flaw detectors are the main equipment for detecting the internal defects, used with the probes of the various frequencies and angles. In addition, the magnetic particle flaw detectors are used to detect the surface and near-surface cracks; the eddy current flaw detectors can be used for the fast scanning of the surface defects and the material sorting.

Fatigue testing machine: Mainly including the electro-hydraulic servo fatigue testing machines (with the large loads, wide frequency ranges and complex waveform loading) and high-frequency fatigue testing machines (with the frequencies up to hundreds of hertz and high test efficiencies). They are all equipped with the precise load control systems and environmental chambers (such as the high and low temperature chambers), to simulate the actual working conditions.

Precision dimension measuring instruments: The traditional tools such as the outer diameter micrometers, digital callipers and dial indicators. The high-end equipment includes the coordinate measuring machines (CMM), which can precisely measure the complex shapes and position tolerances; the laser thickness gauges and profilometers can realize the on-line real-time monitoring of the steel plate thicknesses and plate shapes on the production lines.