What are the standards and methods for hardness testing of precision steel pipe parts

Hardness is one of the core mechanical performance indicators of precision steel pipe parts, directly affecting their wear resistance, compressive strength, and service life. Its test results provide crucial basis for production process optimization, product quality acceptance, and application adaptation. The following details the core standards and practical methods for hardness testing, considering the high precision and multi-scenario applications of precision steel pipe parts.

First, the core testing standards for precision steel pipes.

Hardness testing of precision steel pipe parts must follow internationally accepted standards to ensure the accuracy, comparability, and authority of the test results. Different testing methods correspond to specific standard systems, as follows:

(I) Domestic Standards (GB/T)

GB/T 231.1-2022: "Metallic Materials—Brinell Hardness Test—Part 1: Test Methods," applicable to macroscopic hardness testing of medium- and low-hardness precision steel pipes, specifies the hard alloy ball indenter, load application specifications, and indentation measurement methods. It is the basic standard for Brinell hardness testing in China. GB/T 230.1-2022: Metallic Materials—Rockwell Hardness Testing—Part 1: Test Methods, equivalent to international standards, specifies the indenter type, load parameters, and reading rules for each Rockwell hardness scale (HRA, HRB, HRC), applicable to rapid hardness grading of finished precision steel pipes.

GB/T 4340.1-2022: Metallic Materials—Vickers Hardness Testing—Part 1: Test Methods, for thin-walled, small-section, and high-precision steel pipe parts, standardizes the use of diamond square pyramid indenters and the indentation diagonal measurement procedure, ensuring accuracy in microscopic hardness testing.

GB/T 17394-1998: Metallic Materials—Leib Hardness Testing Methods, guides the application of portable Leeb hardness testers in field and large precision steel pipe part testing, clarifying hardness value conversion rules and instrument calibration requirements.

GB/T 3639-2023, "Cold-drawn or Cold-rolled Precision Seamless Steel Tubes," as a product standard, clarifies the acceptance range of hardness indicators for precision steel tubes and the corresponding requirements for selecting testing methods.

(II) International and Industry Standards

ASTM series standards: ASTM E10-23 (Brinell hardness), ASTM E18-24 (Rockwell hardness), ASTM E384-22 (Vickers hardness), and ASTM A956-22 (Riekel hardness) provide hardness testing guidelines for precision steel tube parts circulating in the international market, standardizing test conditions and result interpretation in different scenarios.

ISO series standards: ISO 6506-1:2023 (Brinell), ISO 6508-1:2023 (Rockwell), and ISO 6507-1:2023 (Vickers) unify the technical specifications for hardness testing of precision steel tubes globally, improving the comparability of cross-regional test data. Specialized industry standards, such as DIN 2391 (German standard for cold-drawn precision seamless steel tubes) and ASTM A519 (standard for seamless carbon and alloy steel tubes for machinery), provide additional specifications for the hardness requirements and testing methods of precision steel tubes for specific applications.

Second, mainstream testing methods and practical points for precision steel tubes.

Hardness testing of precision steel tube parts requires selecting an appropriate method based on the material's hardness range, wall thickness, surface finish, testing scenario, and batch requirements. The core methods include Brinell, Rockwell, Vickers, and Leeb, with specific characteristics as follows:

(I) Brinell Hardness Testing (HB)

(a) Applicable Scenarios: Suitable for mild steel and low-alloy steel precision steel tubes with a thickness ≥ 6mm and a hardness ≤ 450HB. Especially suitable for testing semi-finished products before heat treatment/after annealing, reflecting the overall hardness characteristics of the material. Not suitable for thin-walled, high-hardness, and finished parts (large indentations can easily affect appearance and performance).

(b) Operational Points: Use a cemented carbide ball indenter, apply a load of 3000 kg, hold for 10-30 seconds, then remove the load. Measure the average diameter of the indentation using a microscope and calculate the Brinell hardness value using a conversion table. For example, 120HBS10/1000130 means: using a 10 mm diameter steel ball under a 1000 kgf load for 30 seconds, the hardness value is measured to be 120 MPa.

(c) Advantages and Disadvantages: Advantages include stable results, good repeatability, and the ability to reflect the macroscopic uniformity of material hardness; disadvantages include large indentation size (typically 2-6 mm), low testing efficiency, and generally low requirements for surface finish (Ra ≤ 1.6 μm is sufficient).

(II) Rockwell Hardness Testing (HR)

(a) Applicable Scenarios: Suitable for medium-to-high hardness materials such as finished precision steel pipes, quenched steel, and alloy steel with a thickness ≥ 1.5 mm. It is the most widely used testing method in mass production (accounting for over 70% of total hardness testing), and can quickly complete hardness grading.

(b) Operational Points: Select the appropriate scale based on the material's hardness. Commonly used scales and parameters are as follows: HRA (60kg load + diamond cone tip, suitable for cemented carbide steel pipes), HRB (100kg load + 1.5875mm hardened steel ball, suitable for low-carbon steel and annealed steel pipes), HRC (150kg load + diamond cone tip, suitable for quenched and hardened steel pipes). During testing, apply the initial load first, then the total load. After removing the main load, measure the increment of the residual indentation depth and calculate the hardness value using the formula (A/C scale: HR=100-e; B scale: HR=130-e, where e is the depth increment in 0.002mm).

(c) Advantages and Disadvantages: Advantages include small indentation size (diameter < 1mm), quick operation, and direct reading, making it suitable for finished part inspection. Disadvantages include the small indentation size leading to weaker representativeness of the results, high requirements for surface finish, and the need to remove oxide scale and impurities beforehand.

(III) Vickers Hardness Testing (HV)

(a) Applicable Scenarios: Thin-walled, small-section, surface-hardened, and high-precision steel pipe parts, such as precision steel pipes for aerospace and hydraulic systems. It can also be used for precise laboratory testing and special needs scenarios.

(b) Operational Points: A diamond square pyramid indenter is used. An adjustable load of 1-120 kg is applied, maintained for a specified time, and then unloaded. The lengths of the two diagonals of the indentation are measured using a microscopic system. The hardness value is calculated using the formula (HV=0.1891×F/d², where F is the load and d is the average length of the diagonals). During testing, the surface roughness must be approximately mirror-like to avoid affecting indentation observation.

(c) Advantages and Disadvantages: Advantages include a wide measurement range, high accuracy, and extremely small indentation size, suitable for testing both soft and hard materials and microscopic areas. Disadvantages include complex operation, slow testing speed, and high cost, making it unsuitable for batch on-site testing.

(IV) Leeb Hardness Testing (HL)

(a) Applicable Scenarios: Suitable for large, immovable precision steel pipe parts, such as integrated steel pipe components and on-site pipeline inspection, enabling non-destructive and rapid screening.

(b) Operational Points: Use a portable Leeb hardness tester, select the "Forged Steel" setting, impact the steel pipe surface with an impactor, measure the rebound velocity of the impactor to calculate the Leeb hardness value, and then convert it to HB/HR/HV values according to GB/T 17394 standard. The instrument must be calibrated with a standard hardness block before testing. The impact direction should be perpendicular to the test surface. Each measurement point should be tested at least 3 times, and the data dispersion should not exceed ±15HL of the average value.

(c) Advantages and Disadvantages: Advantages include portability, speed, no workpiece damage, and high testing efficiency; disadvantages include slightly lower accuracy, greater susceptibility to surface condition, vibration, and workpiece thickness, and suitability only for screening and non-arbitration testing.

Third, General Operating Procedures and Precautions for Precision Steel Pipes

(I) Standard Operating Procedures for Precision Steel Pipes

(a) Sample Pretreatment: Remove oxide scale, oil, and rust from the steel pipe surface. Grind until the surface is smooth and clean, with a roughness Ra≤1.6μm. For curved steel pipes, grind to create a 5mm×50mm or 20mm×50mm flat surface to avoid affecting the indentation accuracy. Do not damage the pipe wall thickness during grinding.

(b) Method Selection: Determine the method based on the thickness, hardness range, surface accuracy, and testing scenario of the precision steel pipe. For example, Vickers hardness tester is selected for thin-walled precision parts, Rockwell hardness tester is selected for batch finished products, and Leeb hardness tester is selected for large parts in the field.

(c) Instrument Calibration: Before testing, calibrate the hardness tester with the corresponding standard hardness block, check the wear of the indenter, the accuracy of the load, and the accuracy of the measurement system to ensure the instrument is in a qualified condition.

(d) Testing Procedure: Set the load, indenter, and holding time according to the selected method. Measure at key locations such as both ends and the middle of the steel pipe, testing at least 3-5 points at each location. The distance between any two indentations should be ≥3mm, and the center of the indentation should be ≥5mm from the edge of the part.

(e) Data Processing: Remove outliers and take the average of the remaining data as the final hardness value. Record the testing method, instrument parameters, test locations, and results in detail to generate a test report.

(II) Key Precautions for Precision Steel Pipes

(a) Surface Condition Control: Avoid surface defects such as cracks, inclusions, and scratches. Secondary grinding may be necessary. The test surface must be clean and dry to avoid oil and dust affecting indenter contact and measurement accuracy.

(b) Load and Wall Thickness Matching: High load testing is prohibited for steel pipes with a wall thickness ≤5mm to prevent distorted results due to elastic deformation of the pipe wall. Small-diameter steel pipes with an outer diameter <45mm are not suitable for portable Brinell hardness testers and require sampling and laboratory testing.

(c) Environment and Operating Procedures: The laboratory testing temperature should be maintained at 20±5℃, avoiding vibration and dust interference. During on-site testing, ensure the instrument is stably supported, the impact direction is perpendicular to the testing surface, and there is no relative shaking between the operator and the instrument.

(d) Instrument Maintenance and Calibration: Regularly inspect the indenter, calibrate the load system and measuring device periodically, and retain calibration records for future reference.

(e) Result Judgment and Arbitration: When there is a dispute over the test results, the Vickers hardness test should be used as the preferred arbitration method. For batch testing, the standard sampling ratio must be followed to ensure sample representativeness.

In summary, the hardness testing of precision steel pipe parts must strictly adhere to the corresponding standards, scientifically select testing methods based on the characteristics of the parts, standardize operating procedures, and control key details to ensure accurate and reliable test results, providing a guarantee for product quality control and application safety.