Round Steel Material Testing

Round steel material inspection mainly conducts scientific testing and analysis on the chemical composition analysis, tensile property testing, impact toughness testing, hardness testing, metallographic structure examination and dimension and shape 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 round steel material inspection, used to determine the contents of the main elements such as the carbon (C), silicon (Si), manganese (Mn), phosphorus (P) and sulfur (S) and the alloying elements in the material. The results directly determine the mechanical properties, machinability and final uses of the round steel. The testing basis usually refers to the standards such as GB/T 4336, GB/T 223 series or ASTM E415.

Tensile property testing: Used to determine the mechanical property indicators of the round steel under the uniaxial static tensile loads, and is the key to evaluating its load-bearing capacity. The core indicators include the yield strength (Rp0.2), tensile strength (Rm), elongation after fracture (A) and reduction of area (Z), providing the direct strength and plasticity basis for the structural design, following the standards such as GB/T 228.1 or ASTM A370.

Impact toughness testing: Mainly evaluates the ability of the round steel to resist the brittle fracture under the dynamic loads or low-temperature environments. Usually adopting the Charpy V-notch impact testing, the impact absorbed energy (KV2) is determined at the specific temperatures (such as the room temperature, -20℃ and -40℃). This testing is crucial for evaluating the round steel used in the low-temperature or dynamic-load environments, carried out according to GB/T 229 or ASTM A370.

Hardness testing: Through the measurement of the ability of the round steel surfaces to resist the hard object indentation, its strength, wear resistance and heat treatment states are indirectly reflected. The common methods include the Brinell hardness (HBW), Rockwell hardness (HRC/HRB) and Vickers hardness (HV). The hardness testing is fast, non-destructive or micro-destructive, and is often used for the fast on-site evaluation and batch product quality control.

Metallographic structure examination: The micro-structures of the round steel, such as the grain sizes, pearlite and ferrite proportions, banded structures, and the types, grades and distributions of the non-metallic inclusions, are observed through the optical microscopes or scanning electron microscopes. This item is the bridge connecting the chemical compositions, heat treatment processes and final mechanical properties of the material, and has the guiding significance for analysing the material failure causes and improving the production processes.

Dimension and shape testing: The tools such as the callipers, micrometers and laser diameter gauges are used to precisely measure the geometric parameters such as the nominal diameters, out-of-roundness, lengths and straightness of the round steel. The strict dimension tolerance control is the prerequisite for ensuring that the round steel can be smoothly used in the subsequent processing and assembly, and the relevant requirements are usually clearly specified in the product standards (such as GB/T 702).

Surface quality inspection: Through the visual inspection or with the aid of the magnifying glasses, magnetic particle flaw detection and penetrant flaw detection, whether the round steel surfaces have the defects such as the cracks, folds, scars, pits and scratches is checked. The surface defects are often the stress concentration points and may become the origins of the workpiece failures, so they need to be strictly controlled. The testing standards are such as GB/T 5777 (ultrasonic flaw detection).

Testing Scope

Carbon structural steel round bars: Such as Q235 and 45# steel, widely applied in the building structures, mechanical parts manufacturing, etc. The testing emphases are the chemical compositions, tensile properties, cold bending properties and dimension tolerances, ensuring that they meet the basic strength, plasticity and weldability requirements.

Alloy structural steel round bars: Such as 40Cr, 42CrMo and 20CrMnTi, often used to manufacture the important parts such as the shafts and gears bearing the heavy loads or impacts. In addition to the routine items, the testing particularly pays attention to the hardenability, impact toughness, metallographic structures and non-metallic inclusion control.

Bearing steel round bars: Such as GCr15, used to manufacture the rings and rolling elements of the rolling bearings. The testing requirements are extremely strict. The core items include the extremely high cleanliness (non-metallic inclusion ratings), uniform and fine carbide distributions, strict hardness and dimension stability, as well as the decarburized layer depth control.

Stainless steel round bars: Including the types such as the austenitic (such as 304), martensitic (such as 420) and ferritic (such as 430). The testing emphases are the chemical compositions (especially the contents of the corrosion-resistant elements such as Cr, Ni and Mo), corrosion resistance (such as the intergranular corrosion testing), mechanical properties and surface finish.

Tool steel and die steel round bars: Such as T10, Cr12MoV and H13, etc. The testing focuses on the accuracy of the high-carbon high-alloy compositions, the high hardness and red hardness after the heat treatment, the good wear resistance and the extremely low retained austenite contents. The metallographic structure examination is the key.

Special purpose round bars: Such as the marine anchor chain steel, petroleum drill pipe steel and high-pressure boiler steel, etc. The testing scope needs to add the special items according to their service environments, such as the low-temperature impact, fatigue properties, hydrogen induced cracking (HIC) resistance testing and non-destructive flaw detection (UT, MT and PT), etc.

Testing Methods

Spectroscopic analysis method: Mainly used for the fast quantitative analysis of the chemical compositions. The common ones include the spark discharge atomic emission spectrometry (OES) and X-ray fluorescence spectrometry (XRF). The OES has the high precision and is the mainstream method of the furnace front analysis and laboratory testing; the XRF is usually used for the fast screening and non-destructive analysis.

Mechanical property testing method: On the universal material testing machines, the tensile, compression or bending loads are applied to the standard specimens, and the load-displacement curves are recorded through the transducers, thereby calculating the indicators such as the strength and plasticity. The impact testing is carried out on the pendulum impact testing machines, determining the energies absorbed by the specimen fractures.

Hardness testing method: The Brinell hardness (HBW) adopts the cemented carbide ball indenters, suitable for the softer or medium-hard materials; the Rockwell hardness (HRC/HRB) uses the diamond cone or steel ball indenters, with the simple and fast operation; the Vickers hardness (HV) adopts the diamond pyramid indenters, suitable for the hardness measurement of the thin layers or tiny areas.

Metallographic analysis method: The specimens are cut from the round steel, and the metallographic specimens are prepared through the processes such as the mounting, grinding, polishing and etching (such as with the 4% nital solutions), and then observed and photographed under the metallographic microscopes, and the qualitative and quantitative ratings of the structures are carried out according to the standards such as GB/T 13298 and GB/T 10561.

Non-destructive testing method: Probes the internal and surface defects under the premise of not destroying the material. The ultrasonic flaw detection (UT) uses the high-frequency sound waves to probe the internal defects; the magnetic particle flaw detection (MT) is used to detect the surface and near-surface defects of the ferromagnetic materials; the penetrant flaw detection (PT) is used to detect the surface open defects of the non-porous materials.

Macro examination method: Through the acid etching (such as the hot acid etching) testing, under the low-power microscopes or naked eyes, the low-power structure defects such as the central porosity, ingot segregation, white spots and cracks of the round steel cross-sections are checked, carried out according to the GB/T 226 standard.

Testing Standards

Domestic Standards:

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

2. GB/T 702, Dimension, shape, weight and tolerance of hot-rolled steel bars

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 226, Method of etching test for macrostructure and defect of steel

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

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

International Standards:

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

2. ISO 148-1, Metallic materials — Charpy pendulum impact test — Part 1: Test method

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

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. The sample atoms are excited through the sparks generated by the electrodes, and the quantitative analysis is carried out by measuring the characteristic spectral line intensities. It has the fast analysis speed, high precision and simultaneous determination of the multiple elements, and is the standard instrument of the modern iron and steel enterprise laboratories.

Universal material testing machine: The main equipment of carrying out the mechanical property testing such as the tensile, compression and bending. Equipped with the high-precision load transducers and extensometers, the computer control systems automatically collect the data and generate the test reports, ensuring that the test processes meet the requirements of the loading rates and control modes in the standards such as GB/T 228.1.

Pendulum impact testing machine: The dedicated equipment of determining the impact absorbed energy of the material. The testing machine has the pendulums with the adjustable heights. After the release, the standard notched specimens installed on the supports are impacted and broken, and the consumed energy values (joules) are read through the pointer or digital display systems.

Hardness tester: Including the various types such as the Brinell, Rockwell and Vickers. The modern hardness testers mostly adopt the electronic loading and automatic measurement of the indentation diagonals or depths, digitally displaying the results, reducing the human reading errors. Some models can also directly transmit the data to the computers.

Metallographic microscope and sample preparation equipment: The metallographic analysis system includes the cutting machines, mounting machines, pre-grinding machines, polishing machines, etching devices and optical microscopes. The advanced systems are equipped with the image analysis software, which can automatically measure the grain sizes, phase proportions and inclusion sizes, realizing the quantitative metallographic analysis.

Non-destructive flaw detection equipment: The ultrasonic flaw detectors (with the probes of the various frequencies and angles), magnetic particle flaw detectors (including the yokes, coils and magnetic suspension spraying systems) and penetrant flaw detection agents (cleaners, penetrants and developers). These equipment are used for the comprehensive defect screening of the round steel on the production lines or in the laboratories.

Precision measuring instruments: Such as the digital micrometers, laser diameter gauges, large vernier callipers and straightness measuring instruments. Used to precisely control the shape dimensions such as the diameter tolerances, ovality, lengths and straightness of the round steel, and are the key tools of guaranteeing that the product geometric precision meets the standards.