【Keywords】Roll hardness compliance, national standard benchmarking testing, precise surface hardness verification, uniform hardness quality control, roll wear resistance enhancement, non-destructive hardness testing, roll surface performance calibration, industrial roll quality control, hardness error management, roll life extension
I. Core Standards for Roll Surface Hardness (National Standard Benchmarking Testing)
Currently, domestic roll production and quality inspection strictly adhere to two core national standards: GB/T 1503-2024 "Cast Steel Rolls" and GB/T 13313-2008 "Shore and Leeb Hardness Test Methods for Rolls." Simultaneously, it incorporates supporting testing standards such as GB/T 231.1 and GB/T 4340.1, forming a complete national standard benchmarking testing system. The hardness indicators for rolls of different materials and under different working conditions are clearly differentiated, adapting to various rolling scenarios.
According to national standards, the surface hardness of rolling mill rolls is measured using Shore hardness (HSD) and Rockwell hardness (HRC) as the core measurement standards. Specific requirements are also set for hardness uniformity quality control to prevent excessive localized hardness deviations on the roll surface. The specific grading standards are as follows:
1. Strip continuous rolling mill rolls: Surface hardness uniformity deviation ≤ 4HSD, with a stable overall hardness range suitable for high-speed continuous rolling conditions, ensuring stable plate rolling accuracy;
2. Medium and heavy plate rolling mill rolls: Large rolls with a roll length greater than 4100mm, hardness uniformity deviation ≤ 5HSD, suitable for heavy-duty, high-load rolling operations;
3. Finishing work rolls: Conventional hardness range controlled between 58-65HRC, with a neck hardness of 35-45HRC, balancing roll surface wear resistance and roll body toughness;
4. Vertical rolls and auxiliary rolls: Surface hardness conventional range 45-60HRC, with test point spacing not less than 20mm, fully covering the roll surface testing range.
The new standard adds hardness specifications for five new types of roll materials, including high-chromium steel and alloy tool steel, and improves the hardness index system for 12 materials, achieving standardized calibration of roll surface performance for all types of rolls and adapting to the quality control needs of modern high-end rolling production lines.
II. Mainstream Roll Surface Hardness Testing Methods (Non-destructive Hardness Testing)
Given the large size, curved surface structure, and precise working conditions of rolls, the industry mainstream adopts non-destructive hardness testing technology, balancing testing accuracy, efficiency, and product integrity. The core testing methods are divided into the following four categories, adaptable to different production and quality inspection scenarios, and can accurately complete surface hardness verification.
1. Shore Hardness Testing Method (Industry-Common Core Method)
This method is the roll-specific testing method promoted by GB/T 13313-2008. It relies on a Shore hardness tester to impact the roll surface with a free-falling hammer, calculating the hardness value based on the rebound height. It has the advantages of fast testing speed, non-destructive testing, and adaptability to curved roll surfaces, making it the preferred method for roll factory quality inspection and on-site inspection. This method is mainly used for detecting the overall hardness uniformity of rolls, quickly identifying localized hardness anomalies on the roll surface. It is suitable for daily quality control of various cast steel and alloy rolls, providing efficient data support for industrial roll quality control.
2. Rockwell Hardness Testing Method (Precise Quantitative Testing)
Following ASTM E18 and GB/T 231.1 standards, this method uses an HRC scale and applies a constant test force through a diamond indenter. The hardness value is determined based on the indentation depth, resulting in extremely high accuracy. It is mainly used for precise testing of finishing rolls and high-end alloy rolls. It can accurately quantify the hardness of the hardened layer on the roll surface, precisely control the quality of the roll quenching and tempering processes, effectively manage hardness errors, and prevent hardness non-compliance caused by process defects. However, this method is relatively slow and is mostly used for finished product sampling and re-inspection of disputed items.
3. Leeb Hardness Testing Method (Portable On-Site Testing)
Also included in the GB/T 13313 national standard, this method uses a portable Leeb hardness tester. No fixed workstation is required; testing can be performed directly on-site, with equipment in operation. It is suitable for rapid testing of large, heavy-duty rolls and rolls already in use. This method is easy to operate, highly adaptable, and the data can be quickly compared to national standards. It is suitable for daily maintenance and inspection of production lines, promptly identifying hardness decay issues after long-term roll operation and contributing to extending roll life.
4. Vickers Hardness Testing Method (Microscopic Precision Testing)
Based on the GB/T 4340.1 standard, this method is applicable to the hardness testing of the hardened layer and micro-area of roll surfaces. It can accurately measure shallow hardness changes on the roll surface, identifying microscopic defects such as uneven quenching and surface decarburization. It is widely used for in-depth quality inspection of high-precision rolls and imported rolls, ensuring roll surface performance stability at the microscopic level, further enhancing roll wear resistance, and preventing early wear caused by minute hardness defects.
III. Inspection Operation Standards and Error Control Key Points
The accuracy of roll surface hardness test data directly affects product quality inspection results and production quality control decisions. Daily inspections must strictly adhere to standardized operating procedures and comprehensively implement hardness error control to avoid inspection deviations caused by human error, equipment malfunctions, and environmental factors.
In the pre-inspection preparation phase, the roll surface inspection area must be thoroughly cleaned of oxide scale, oil, rust, and wear burrs to ensure a smooth and clean surface, preventing surface impurities from affecting indentation and springback data. Simultaneously, the testing equipment must be calibrated to ensure the hardness tester's measurement accuracy meets national standards, and equipment verification must be completed regularly. Inspection points must be evenly distributed across the roll's working surface, avoiding roll ends, welds, and defect areas. The spacing between test points must strictly adhere to the standard requirement of ≥20mm to prevent overlapping points and missed areas.
During the inspection process, the equipment must be kept perpendicular to the roll surface, and the test force must be applied stably to avoid shaking or shifting that could lead to data deviations. Large rolls require segmented, multi-point inspections to comprehensively verify the uniformity of roll hardness, strictly adhering to national standard deviation limits. After the test is completed, the ambient temperature, test points, and equipment parameters are recorded simultaneously. Abnormal data are retested in a timely manner to ensure that each set of data is true and valid, and to achieve standardized quality control throughout the entire process.
