Hot-rolled Wire Rod Testing

Hot-rolled wire rod inspection mainly conducts scientific testing and analysis on the chemical composition analysis, mechanical property testing, macro and micro metallographic inspection, dimension and shape testing, surface quality testing and process 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

Chemical composition analysis: This is the basis of the hot-rolled wire rod quality evaluation. The tested elements mainly include the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S), as well as the possible alloying elements such as the chromium (Cr) and nickel (Ni). Their contents directly affect the mechanical properties, welding properties and subsequent cold working properties of the wire rod. For example, the carbon content determines the strength and hardness of the wire rod, and as the harmful elements, the sulfur and phosphorus contents must be strictly controlled to guarantee the toughness.

Mechanical property testing: Used to evaluate the behavior of the wire rod under the applied forces. The core items include the tensile testing (determining the tensile strength, yield strength and elongation after fracture), reduction of area testing, as well as the hardness testing required by some specifications (such as the Brinell hardness). These indicators are directly related to whether the wire rod can meet the subsequent deep processing requirements such as the drawing and cold heading, and are the key to judging its service properties.

Macro and micro metallographic inspection: The macro inspection mainly observes the low-power structure defects of the wire rod, such as the central porosity, shrinkage cavity remnants, subcutaneous bubbles and segregation. The micro metallographic inspection is carried out under the microscopes, evaluating the austenite grain sizes, types, grades, morphologies and distributions of the non-metallic inclusions, as well as the decarburized layer depths. These structure characteristics have the decisive effects on the fatigue lives and cold working cracking tendencies of the wire rod.

Dimension and shape testing: Including the tolerance measurement of the wire rod diameters and out-of-roundness (ellipticity), as well as the inspection of the surface defects. The precise dimensions are the premise of guaranteeing the subsequent drawing die lives and wire dimension precision. The shape inspection needs to confirm that the wire rod has no surface defects harmful to the use such as the fins, folds, cracks, scars and shells.

Surface quality testing: Through the methods such as the visual inspection, eddy current flaw detection or ultrasonic flaw detection, the continuous defects on the surfaces and near the surfaces of the wire rod are systematically checked, such as the cracks, hairline cracks and laps. These defects are extremely easy to expand in the subsequent cold deformation processes, causing the product fractures or scrap, so they are the items that must be strictly screened for the high-quality wire rods.

Process property testing: Simulating the subsequent processing processes of the wire rod to evaluate its applicability. The common tests include the reverse bend testing, torsion testing (determining the torsion cycles and observing the surface cracking conditions), upsetting testing (checking the cold heading properties) and wrapping testing. These tests can intuitively reflect the plasticity, toughness and work hardening tendencies of the wire rod.

Testing Scope

Carbon steel hot-rolled wire rods: Mainly applied to manufacturing the ordinary standard parts, welding wires, hardware products and building wires, etc. The testing emphases are the chemical compositions, stability of the mechanical properties and surface qualities, ensuring that they meet the grade requirements for the different drawing and cold upsetting uses in the national standards such as GB/T 14981.

Alloy steel hot-rolled wire rods: Referring to the wire rods containing one or more alloying elements (such as Cr, Ni, Mo and V), used to manufacture the high-strength fasteners, bearings, springs and special-purpose steel wires. In addition to the routine items, the testing needs to particularly pay attention to the alloying element segregation, special carbide morphologies and the higher purity (low inclusion) requirements.

Stainless steel hot-rolled wire rods: Including the different series such as the austenitic, martensitic and ferritic. The testing core is to verify that their chemical compositions meet the corresponding designation standards (such as GB/T 4356). At the same time, the special items such as the intergranular corrosion susceptibility testing and ferrite content determination need to be carried out, to ensure their corrosion resistance and processing properties.

Specific-purpose dedicated wire rods: Such as the wire rods for the prestressing steel wire and strand, wire rods for the tire cord, cold heading steel wire rods and spring steel wire rods. The testing scope of this type of wire rod strictly follows the dedicated standards (such as YB/T 146 and GB/T 24242). In addition to the general items, there are extremely strict specific indicators for the non-metallic inclusions, decarburized layers, micro-structures and sorbite ratios, etc.

Full-process monitoring of the wire rod products: The testing scope covers from the steelmaking raw materials, continuous casting billets to the hot-rolled finished products, and even the re-inspection before the users' use. Including the low-power defect inspection of the cast billets, online dimension and surface monitoring during the rolling processes, and comprehensive inspection before the finished product warehousing, forming the closed-loop quality control throughout the whole process.

Testing Methods

Spectroscopic analysis method: Used for the fast and accurate quantitative analysis of the chemical compositions. Usually, the direct-reading spectrometers (OES) or X-ray fluorescence spectrometers (XRF) are adopted. Among them, the spark discharge direct-reading spectrometry is the mainstream method of the iron and steel industry, which can complete the simultaneous determination of more than ten elements such as C, Si, Mn, P and S within 1-2 minutes, with the high precision, suitable for the fast inspection and process control on the production sites.

Tensile testing method: According to the GB/T 228.1 standard, carried out on the universal material testing machines. The wire rod specimens are machined into the specified shapes, and the axial tensile forces are applied at the uniform speeds until the fracture. Through the recorded force-displacement curves, the key mechanical parameters such as the tensile strength, upper/lower yield strengths and elongation after fracture can be calculated. The test environment temperature, specimen clamping and extensometer use need to be strictly standardized.

Metallographic inspection method: According to the standard series such as GB/T 13298. After the sampling, the metallographic specimens are made through the mounting, grinding, polishing and chemical etching (such as the 4% nital solution). The macro inspection is carried out under the magnifying glasses or stereomicroscopes; the micro-structures, grain sizes and inclusion ratings are compared and determined under the optical microscopes with reference to the standard charts such as GB/T 6394 and GB/T 10561.

Non-destructive testing method: Mainly including the eddy current flaw detection and ultrasonic flaw detection. The eddy current flaw detection (according to GB/T 17990) uses the electromagnetic induction principle, is sensitive to the defects such as the cracks and folds on the surfaces and near the surfaces of the wire rod, and can realize the high-speed online detection. The ultrasonic flaw detection (usually adopting the through-type probes) can detect the internal defects such as the central porosity and inclusions. The combination of the two can realize the comprehensive screening of the wire rod defects.

Dimension and morphology measurement method: The dimension parameters such as the diameters and out-of-roundness can be measured with the tools such as the micrometers and laser diameter gauges. The depths and morphologies of the surface defects can be quantitatively evaluated through the portable microscopes or profilometers. For the decarburized layer depths, the thicknesses of the complete decarburized layers and total decarburized layers need to be measured on the metallographic specimens from the surfaces toward the cores.

Process property testing method: Such as the reverse bend testing (GB/T 238), torsion testing (GB/T 239.1) and wrapping testing. These methods all simulate the actual processing conditions: the bend testing evaluates the plasticity, the torsion testing evaluates the uniform plastic deformation ability and surface qualities, and the wrapping testing inspects the surface defects and coating bonding forces. The test results are usually judged by "no fracture", "no cracks" or the specific cycles.

Testing Standards

Domestic Standards:

1. GB/T 14981, Hot-rolled wire rods for drawing — Dimensions and tolerances

2. GB/T 4356, Stainless steel wire rods for drawing

3. YB/T 146, Wire rods for prestressing steel wire and strand

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

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

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

7. GB/T 10561, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standard diagrams

8. GB/T 17990, Round steel — Method for eddy current testing (point probe method)

9. GB/T 238, Metallic materials — Wire — Reverse bend test

10. GB/T 239.1, Metallic materials — Wire — Part 1: Torsion test

International Standards:

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

2. ISO 643, Steels — Micrographic determination of the apparent grain size

3. ISO 4967, Steel — Determination of content of nonmetallic inclusions — Micrographic method using standard diagrams

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 spectrometer: The core equipment of the chemical composition analysis. Composed of the excitation light sources, optical systems, photometric systems and data processing systems. Its working principle is that the samples generate the characteristic spectra under the spark discharge excitation, and the element contents are determined by measuring the intensities of the spectral lines at the specific wavelengths. The equipment needs to be regularly calibrated with the standard samples, to ensure the accuracy of the analysis results.

Universal material testing machine: The key equipment of carrying out the mechanical testing such as the tensile, compression and bending. The modern testing machines are mostly the microcomputer-controlled electro-hydraulic servo or electronic types, with the high-precision load transducers and displacement measurement systems, which can automatically draw the stress-strain curves and calculate the various performance parameters. Equipped with the high and low temperature environmental chambers, the mechanical property testing at the different temperatures can also be carried out.

Metallographic microscope system: Including the inverted or upright optical microscopes, image acquisition systems and analysis software. Used to observe and analyse the micro-structures, grain sizes and non-metallic inclusions. The advanced systems are equipped with the automatic stages and image analysis software, which can realize the automatic scanning, statistics and rating of the inclusions, improving the testing efficiency and objectivity.

Non-destructive flaw detection equipment: The eddy current flaw detectors are composed of the oscillators, probes, signal processing units and alarm devices, often used for the online high-speed detection. The ultrasonic flaw detectors include the pulse generators, transducers (probes), receiving amplifiers and display units, used for the offline or online detection of the internal defects. Both need to use the standard sample rods with the artificial defects for the sensitivity calibration.

Dimension and morphology measurement equipment: Including the high-precision digital micrometers, laser scanning diameter gauges (used for the online real-time monitoring of the diameters and ellipticity), profilometers and portable video microscopes. The laser diameter gauges are non-contact and have the high precision, and are the key equipment of the modern rolling line quality control. The profilometers can precisely measure the depths and profile shapes of the surface defects.

Hardness testers and process testing machines: The Brinell, Rockwell or Vickers hardness testers are used to determine the wire rod hardness. The dedicated testing machines such as the torsion testing machines (can record the torque-angle curves), reverse bend testing machines and wire wrapping testing machines are used to carry out the specific process property testing. These equipment have the dedicated structures, and the operations need to strictly follow the corresponding test method standards.