Non-quenched and Tempered Structural Steel Testing

Non-quenched and tempered structural steel inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, macroscopic and microscopic structure examination, hardness testing, and macrostructure and defect examination, 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

Chemical composition analysis: This is the basic link of the non-quenched and tempered structural steel inspection, aiming to precisely determine the contents of the five conventional elements C, Si, Mn, P and S in the steel, as well as the micro-alloying elements such as the V, Ti, Nb and N. The chemical composition directly determines the strength, toughness and welding performance of the steel, and is the primary basis for judging whether it meets the requirements of the specific designations. The analysis usually follows the standards such as GB/T 4336 and GB/T 20123.

Tensile property testing: Used to evaluate the mechanical behaviour of the non-quenched and tempered steel under the uniaxial static tensile load. The main determination items include the yield strength (ReH or Rp0.2), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z). These data are the key indicators for evaluating the load-bearing capacity and plastic deformation ability of the material, ensuring that it does not produce the excessive deformation or fracture during the service.

Impact toughness testing: Mainly evaluates the ability of the material to resist the brittle fracture under the dynamic loads or low-temperature environments. The Charpy (V-notch) impact testing is usually adopted, and the impact absorbed energy (KV2 or KU2) is determined at the specified temperatures (such as the room temperature, -20 ℃ and -40 ℃). The excellent impact toughness is an important premise for ensuring the safe service of the parts under the impact loads or harsh environments.

Macroscopic and microscopic structure examination: The microstructure of the steel, such as the morphology, distribution and grain size of the ferrite and pearlite, as well as the precipitation state of the micro-alloy carbonitrides, is observed through a metallographic microscope or scanning electron microscope. The structure analysis is the bridge connecting the chemical composition and mechanical properties, used to optimize the production process and diagnose the material defects such as the banded structures and abnormal structures.

Hardness testing: As a fast and simple indirect evaluation method of the mechanical properties, the hardness testing (Brinell HBW, Rockwell HRC and Vickers HV) is widely used for the quality control and process verification of the non-quenched and tempered steel. There is a certain conversion relationship between the hardness value and the strength and wear resistance of the material, and it can be used to monitor the consistency of the production batches and the effect of the heat treatment (such as the normalizing).

Macrostructure and defect examination: Through the acid etching or sulphur printing methods, the cross-sections or longitudinal sections of the steel are inspected to reveal the macroscopic defects inside, such as the porosity, segregation, residual shrinkage cavities, flakes and cracks. This testing is crucial for evaluating the density, uniformity and metallurgical quality of the material.

Testing Scope

Incoming acceptance testing of the raw materials: When the steel plants or parts manufacturers procure the raw materials such as the non-quenched and tempered steel bars and forged blanks, the strict incoming re-inspection must be carried out. The testing items usually include the chemical composition, tensile properties, impact toughness, hardness and macrostructure, to ensure that the incoming materials meet the requirements of the procurement technical agreements or the relevant national standards (such as GB/T 15712).

Forging or hot working process monitoring testing: The performance of the non-quenched and tempered steel is highly dependent on its hot working (forging and rolling) process. During this process, the key process parameters (such as the final forging temperature and cooling rate) need to be monitored, and the microstructure and hardness of the specimens need to be sampled and tested, to verify the stability of the process and ensure the expected strengthening and toughening effects.

Quality judgement testing of the finished parts: The comprehensive performance testing is carried out on the final products that have completed all the machining processes, such as the key safety parts like the automotive crankshafts, connecting rods and steering knuckles. In addition to the routine mechanical properties, the fatigue testing and bench testing may also be required, to simulate the actual working conditions and comprehensively judge their service reliability and life.

Failure analysis and quality arbitration testing: When the early failures (such as the fracture and abnormal wear) of the non-quenched and tempered steel parts occur during the processing, assembly or use, the systematic failure analysis needs to be carried out. The testing scope covers the fracture morphology, crack source analysis, composition segregation and abnormal structure identification, aiming to find out the root causes of the failure, and provide the technical evidence for the quality dispute arbitration and design improvement.

New product development and process research testing: When developing the new non-quenched and tempered steels or optimizing the heat treatment and deformation processes of the existing materials, the systematic and in-depth testing and analysis need to be carried out. The testing scope is wide, including the transformation point determination, continuous cooling transformation (CCT) curve mapping and precipitation kinetics research, providing the data support for the core technology research and development.

Compliance certification testing of the export products: For the non-quenched and tempered steel products exported to the markets such as the European Union, North America and Japan, in addition to meeting the domestic standards, they usually also need to meet the technical regulations and standard requirements of the destination markets, such as the EN, ASTM and JIS standards. The testing scope needs to be customized according to the specific certification requirements of the target markets.

Testing Methods

Inductively coupled plasma optical emission spectrometry method (ICP-OES): This method uses the high-temperature plasma to excite the atoms in the sample, and quantitatively analyses the element contents by measuring the intensities of the characteristic spectral lines. For the precise determination of the micro-alloying and trace elements such as the V, Ti, Nb and Al in the non-quenched and tempered steel, the ICP-OES has the significant advantages of the high sensitivity, wide linear range and simultaneous multi-element analysis.

Infrared absorption method and thermal conductivity method: These are the classic methods for determining the carbon and sulphur contents in the steel. The carbon determination usually adopts the infrared absorption method, and the sulphur determination can adopt the infrared absorption method or thermal conductivity method. The specimen is burned in the high-temperature oxygen flow, and the generated CO₂ and SO₂ gases are detected by the corresponding detectors, thereby calculating the percentage contents of C and S, with the high accuracy and fast analysis speed.

Room-temperature tensile testing method: In accordance with the standard GB/T 228.1, a universal material testing machine is used to apply the axial tensile force to the tensile specimens prepared according to the standards until the fracture. Through the force-displacement curves or stress-strain curves, the core mechanical property parameters such as the yield strength, tensile strength and elongation after fracture are automatically calculated or manually determined.

Charpy pendulum impact testing method: In accordance with the standard GB/T 229, the standard specimens with the V-notch or U-notch are placed on the supports of the impact testing machine, and the pendulum released from the specified height strikes them once to fracture. By measuring the angle at which the pendulum swings up after breaking the specimen, the impact energy absorbed by the specimen is calculated, thereby evaluating the toughness of the material.

Metallographic microscope examination method: In accordance with the standards such as GB/T 13298, the specimens are cut from the tested samples, and after grinding, polishing and etching (commonly with the 4% nitric acid alcohol solution), the metallographic specimens are made. Under the metallographic microscope, the microstructure types, grain size (according to GB/T 6394) and non-metallic inclusions (according to GB/T 10561) are observed and recorded, and the quantitative or semi-quantitative rating is carried out.

Brinell hardness testing method: In accordance with the standard GB/T 231.1, a cemented carbide ball indenter of a certain diameter is used to press into the specimen surface under the specified test force, maintained for the specified time and then the test force is removed. The indentation diameter on the specimen surface is measured, and the Brinell hardness value (HBW) is obtained through the table lookup or calculation. This method has the large indentation area, can better reflect the average hardness of the material, and is particularly applicable to the materials with the non-uniform structures.

Testing Standards

Domestic Standards:

1. GB/T 15712, Non-quenched and tempered mechanical structural steels

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

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

4. GB/T 229-2020, Metallic materials — Charpy pendulum impact test method

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

6. GB/T 6394-2017, Determination of estimating the average grain size of metal

International Standards:

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

2. 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

Direct-reading optical emission spectrometer (OES): As the core equipment for the rapid analysis before the furnace and the incoming material re-inspection in the steel industry, the direct-reading optical emission spectrometer can simultaneously determine the contents of up to dozens of elements in the non-quenched and tempered steel within 1-2 minutes. Its principle is that the arc or spark excites the sample to generate the characteristic spectra, which are analysed by the photomultiplier tubes or CCD detectors, with the characteristics of the fast speed, high precision and simple operation.

Universal material testing machine: The key equipment for carrying out the static mechanical property tests such as the tensile, compression and bending. The modern electronic universal testing machines adopt the servo motor drive and computer control, can precisely control the loading rate, automatically record the load-displacement curves, and calculate and output the various mechanical property parameters, with the accurate and reliable test results.

Pendulum impact testing machine: The equipment specially used for the Charpy impact testing. Its core component is a heavy pendulum that can swing around a fixed axis, and the notched specimen is struck once by releasing the pendulum. The equipment needs to be regularly verified for its energy loss and the position of the striking centre, to ensure the accuracy of the impact absorbed energy measurement values.

Metallographic microscope system: This system usually includes the inverted or upright metallographic microscopes, image acquisition cards, CCD cameras and computer analysis software. In addition to the traditional microstructure observation, the modern systems can also realize the quantitative metallographic analysis functions such as the automatic grain size rating, phase area fraction measurement and inclusion statistical analysis, greatly improving the detection efficiency and objectivity.

Brinell/Rockwell/Vickers hardness testers: The appropriate hardness testers are selected according to the different standards and application scenarios. The Brinell hardness tester is suitable for the softer or non-uniform materials; the Rockwell hardness tester has the high testing efficiency and is suitable for the batch detection; the Vickers hardness tester has the small indentations and can be used to test the thin layers, small parts or carry out the micro-hardness testing. The equipment needs to be regularly calibrated with the standard hardness blocks.

Carbon-sulphur analyser and nitrogen-oxygen-hydrogen analyser: The carbon-sulphur analyser is usually based on the infrared absorption principle, specially used for the precise determination of the carbon and sulphur contents in the steel. The nitrogen-oxygen-hydrogen analyser mostly adopts the pulse heating inert gas fusion-thermal conductivity/infrared detection principle, used for the precise determination of the trace nitrogen, oxygen and hydrogen elements in the non-quenched and tempered steel. These gas elements have the significant influence on the toughness and fatigue performance of the steel.