This article describes the method of roll hardness testing, analyzes the influencing factors and proposes countermeasures.
Key words: roll; hardness test; influencing factors; test method
Roll is an indispensable tool in steel rolling production. In addition to the material of the roll itself, hardness is the most important item in the technical index of the roll. For the same type of material, its hardness usually has a certain corresponding relationship with strength, toughness, etc., especially for rolls. The hardness of rolls of the same or similar material is usually related to the microstructure and residual stress level of the roll. , wear resistance, thermal crack resistance, and fatigue resistance are closely related. Compared with other testing methods, hardness testing has the characteristics of fast speed, low cost, and common testing instruments. Therefore, both manufacturers and users regard the roll hardness value as an important indicator for predicting the performance of rolls, and hope to achieve the best rolling performance by using rolls with the most suitable hardness.
Hardness testing runs through the entire process of roll production and use. The manufacturer conducts testing during the production process and the finished product state to ensure that the roll hardness meets the technical requirements. The user checks the roll hardness when the new roll enters the factory and during use. and monitoring to avoid rolling accidents caused by unqualified roll hardness or affecting the quality of rolled products.
Affected by factors such as “people, machine, material, method, and environment”, that is, the skill level of the inspector, the model of the inspection instrument, the physical state of the roll, the inspection standard and method, and the external environment, it is very difficult to obtain an accurate absolute value of hardness. Yes, it often happens that manufacturers and users have obvious differences in the hardness test results of the same roll. Therefore, based on the roll manufacturing and testing experience, the reasons for the deviation of the testing results are analyzed.
Commonly used roll hardness testing methods
Hardness refers to the ability of a material to locally resist hard objects being pressed into its surface. Hardness is usually divided into three categories: scratch hardness, indentation hardness, and rebound hardness.
Whether it is the scratch hardness testing method represented by Mohs hardness, or the indentation hardness testing method represented by Brinell hardness, Rockwell hardness and Vickers hardness, or the hardness testing method represented by Shore hardness and Leeb hardness. Springback hardness testing methods can be considered as non-destructive testing for most parts testing, because these testing methods leave very little marks on the part surface. But for rolls, such marks often cause damage to the roll body. For example, scratches on the roll body will cause the eddy current test on the surface of the roll body to fail. Larger scratches may also cause cracks in the roll service process. , eventually causing the roll to break or peel off. Therefore, for large rolls, scratch hardness is generally not used for testing.
Rolls are large-scale casting and forging products. Rolls commonly used in the metallurgical industry usually weigh more than several tons, and the largest back-up rolls of wide and heavy plate mills even weigh 240 tons. Rolls of such specifications obviously cannot enter the laboratory for hardness testing on conventional Rockwell, Brinell or Vickers hardness testing machines, so the hardness testing of large rolls can only use rebound Shore hardness or Leeb Hardness testing.
Shore hardness is the use of a punch of a specific material and weight, which falls freely onto the part from a specific height, and then the punch bounces back, and the hardness of the part is determined according to the bounce height.
C-type Shore hardness tester: drop a diamond punch with a mass of 2.5 g and a top spherical radius of 1.0 mm from a height of 254 mm onto a quenched high-carbon steel standard sample. When the average rebound height is 165.1 mm, The scale of the hardness tester is set at 100 HSC at this height, and the scale extending down to the surface of the sample is 0 HSC. The portion of the height difference between 0 HSC and 100 HSC is divided into 100 equal parts, and each part is divided into 1 HSC.
Type D Shore Hardness Tester: Drop a diamond punch with a mass of 36.2 g and a top spherical radius of 1.0 mm onto the standard sample from a height of 19 mm. When the rebound height is 13.57 mm, the hardness tester will be scaled at this height It is 100 HSD, and the scale extending down to the surface of the sample is 0 HSD. Make 100 equal divisions for the height difference between 0 HSD and 100 HSD, each division is 1 HSD, and then scale up to 140 HSD.
Taking the Shore hardness tester used by the quality inspection team of a large roll manufacturing company as an example, there are obvious differences between HSC and HSD hardness testers. HSC is read by detecting the rebound height of the punch with the naked eye, and HSD is read by reading the data indicated by the pointer on the dial.
The Leeb hardness is also a rebound hardness, which is basically similar to the principle of the Shore hardness test, and a punch of a specified quality is also used to impact the surface of the sample.
D-type Leeb hardness is commonly used. A tungsten carbide punch with a mass of 5.5 g and a spherical radius of 1.5 mm at the top impacts the surface of the sample at a certain speed under the action of elastic force. The ratio of the speed is used to calculate the hardness value. The formula for calculating the D-type Leeb hardness HLD is:
HLD=1 000×(rebound speed Vr/impact speed Va)
There are two main differences with the Shore hardness: one is that the Leeb hardness punch is impacted at a certain initial speed, and the source of the initial speed is a certain force given by the impact spring; the other is that the Leeb hardness does not pass The hardness is calculated by measuring the rebound height, and the hardness value is calculated by measuring the ratio of the rebound velocity of the punch at a distance of 1 mm from the sample surface to the impact velocity.
Unlike the Shore hardness tester, which must ensure that the hardness tester is vertical, the Leeb hardness tester can adjust the impact angle at any time according to the different detection positions. Therefore, the use of the Leeb hardness tester does not require high skill level of the testing personnel. Most personnel only need to pass Simple familiarization allows quick detection. In addition, almost all Leeb hardness testers currently use digital display, which has a higher degree of intelligence, and the reading of detection values can completely avoid the influence of human subjective factors.
Main factors affecting hardness test results
Accurate hardness is difficult to obtain. The hardness testing process is affected by many factors. It is precisely because of these factors that the hardness testing results of the same roll are different between the manufacturer and the user.
Effect of hardness conversion
The difference in hardness testing equipment directly leads to the difference in the results of the two hardness tests. The diversity of hardness types and hardness testers brings a big problem to the hardness testing of rolls, that is, the conversion between various types of hardness. In hardness testing, a hardness conversion table is required to convert Leeb hardness to Shore hardness, Vickers hardness or Rockwell hardness.
Most hardness conversion tables are formulated by roll manufacturers based on big data measurement. The standards of each country and region are different, and even the conversion standards of different materials are also different. In GB/T 13313-2008, the forging Conversion table between HSD and HLD for steel, cast steel and cast iron. Table 1 lists the differences between different hardness conversion tables taking forged steel material as an example.
Although there are many conversion tables in the roll manufacturing industry and the hardness tester manufacturing industry, none of the table developers, including standards organizations, has explained how this table was obtained. These tables may be obtained through testing samples one by one in the laboratory, or may be obtained through the measurement of large data of actual roll testing, and it is more likely to be obtained through a small amount of measurement-assisted mathematical regression calculation.
Different testing equipment has different built-in hardness conversion tables, which leads to differences in testing results. Most disputes between roll producers and roll users about hardness deviations are caused by hardness conversion.
Influence of testing apparatus and test block
(1) Accuracy of hardness tester
The precision of the testing equipment itself determines the accuracy of the testing results. The accuracy of the hardness testers produced by different manufacturers is quite different, and even the same type of hardness testers produced by the same manufacturer still have some deviations. It is precisely because of these possible detection errors that the hardness tester must be calibrated with a standard test block before the roll detection.
(2) Calibration block hardness range
The hardness value range of the standard test block also has a great influence on the actual hardness test results, as shown in Table 2. Using the same hardness tester to test standard test blocks with different ranges, the deviation results obtained are different. Taking the 2# hardness tester as an example, its standard deviation value Δ is -0.5 HSD when the 60 HSD standard block is calibrated, but its standard deviation value Δ reaches -1.5 HSD when the 90HSD standard block is calibrated. Taking this set of data as an example, if the Δ value at 60 HSD calibration is used to calibrate samples within the hardness range of 90 HSD, the final result will be 1 HSD lower than the actual value.
|Standard block hardness||1#Hardness Tester||2#Hardness Tester|
|Measurements||Calibration bias∆||Measurements||Calibration bias∆|
Based on the above analysis, when measuring samples with different hardness ranges, the standard test block with the closest hardness to the sample to be tested must be selected for calibration. The corrected value after standard deviation Δ correction may deviate from the actual hardness value.
For roll production enterprises, since the range of roll hardness to be tested is very wide, many standard blocks with different hardness ranges are equipped. However, for general rolling lines, there are usually only one or two standard test blocks in the grinding roll room. Most operators perform hardness testing on different types of rolls and different parts of the same roll after one calibration. Taking tandem cold rolling as an example, the hardness of the working roll body can reach more than 95 HSD, but the hardness of the supporting roll body used with it is usually only 65~70 HSD, and the hardness of the neck part of these rolls will be as low as 35~45 HSD, in the case of such a large hardness span, only rely on the calibration results of one test block for measurement and correction, which will increase the possibility of hardness detection deviation.
(3) The cleanliness of the hardness tester punch
Regardless of the Shore hardness or the Leeb hardness, it is necessary to use a specific punch to impact the surface of the sample for testing. If there are stains on the surface of the punch, adhesion of foreign matter, etc., when impacting the sample, part of the impact energy must be absorbed by the deformation of the stain or foreign matter, which directly affects the rebound speed and height of the punch, and the hardness of the sample will be slightly lower. lower than the actual value.
In the actual roll inspection process, experienced operators will use a clean silk cloth to wipe the punch after each inspection to keep it clean and reduce inspection errors. The staff of a rolling line in a steel mill once reported that a large number of rolls had low hardness on site. The staff of the roll company carried another hardness tester for on-site re-inspection and found that the hardness of the rolls was not low. After detailed investigation, the personnel of both parties found that there was a small piece of stain on the punch of the hardness tester. After cleaning the punch, the hardness of the rolls was re-inspected and all were qualified.
Influence of roll surface state
When testing the hardness of the roll, there are requirements for the condition of the roll surface to be clean, non-magnetic, oil-free, scale-free, anti-rust oil or any other kind of coating, and the roughness Ra of the surface to be tested is also required to be less than or equal to 70 in the hardness value. When HSD is not greater than 3.2, when it is greater than 70 HSD, it is not greater than 1.6. In the actual testing process, there are many deviations in the hardness value caused by the problem of the roll surface, and the common types are as follows.
(1) Oil residue on the roller surface
The friction coefficient between the punch of the hardness tester and air and oil is different. The friction force received in the oil is large, the impact energy loss is increased, and the rebound speed and height will be low. As shown in Table 3, the actual detection of different types of rolls shows that the detection hardness is low when there is oil on the roll surface. Among them, point A is the point with oil stain, and points B and C are the clean area around the oil stain point. The measured data is the average value of 5 times.
|serial number||material||standard hardness||A||B||C|
|1||chilled cast iron||77~83||76.2(Soft film anti-rust oil residue stains)||78.3||79.4|
|2||chilled cast iron||70~76||70.4(Soft film anti-rust oil is not removed)||74.5||75.2|
|3||High chromium cast iron||75~80||74.3(Grinding fluid residues)||76.2||76.7|
|4||Cr3 forged steel||60~65||52.2(Dura anti-rust oil is not removed)||61.2||61.8|
|5||Cr5 forged steel||65~70||66.4(Flaw detection couplant stains)||67.2||67.0|
Rolls, like other steel parts, must be coated with anti-rust oil on the surface after processing. There are yellow or tan, soft film anti-rust oil with a certain thickness, and transparent, firmly attached to the surface of the roll. Very thin hard film anti-rust oil. A roll manufacturing company once received complaints from customers about the low hardness of new rolls entering the factory. After the roll company personnel arrived at the scene, they found that the on-site personnel only used a rag to wipe individual points on the roll surface and then carried out the hardness test. The anti-rust oil affected the test results, and after careful cleaning, the hardness was re-tested to pass.
(2) Influence of roll residual magnetism
When affected by an external magnetic field, the rolls will produce a certain degree of magnetism, which has little effect when using a Shore hardness tester to test these rolls, but may cause large deviations when using a Leeb hardness tester. The Shore hardness tester relies on the rebound height to convert the hardness, and the slight magnetic field distribution has little effect on the rebound height; the Leeb hardness tester relies on the rebound speed to convert the hardness, and the measurement speed depends on some electronic components , when these components work in a strong magnetic field, there may be a certain measurement deviation, which will eventually affect the measured value.
(3) Influence of surface roughness
Roll users of some wire and bar mills and section steel mills often complain to the roll manufacturer about the low hardness of the new rolls. Most of the time, the problem is caused by the roughness of the roll surface. The delivery status of the rolls used in wire bar mills and section steel mills and the rolls used in plate and strip mills is different. The roll surface of the former is usually in the turning state, while the roll surface of the latter must be in the grinding state, and its roughness is very large. difference.
For the roll delivered in the turning state, the roll manufacturer usually polishes the light band or bright spot on the roll surface during the inspection to ensure that the roughness requirements of the inspection are met. However, roll users usually perform hardness testing in the turning state, and the roll surface is relatively rough, so the measured data is low.
Measures to eliminate hardness differences
(1) When formulating the roll production process, in addition to clarifying the hardness required by the technical conditions, the roll producer should have a detailed understanding of the hardness tester model or conversion table used in the rolling line, and qualified producers should choose the same hardness tester as the site. To avoid detection differences caused by different hardness conversion tables.
(2) Whether it is a roll manufacturer or a user, it is necessary to select a suitable test block for hardness tester calibration before testing the roll hardness. When the indication value of the hardness tester fluctuates greatly, the parts such as the punch of the hardness tester should be cleaned to avoid the influence of foreign objects on the test results.
(3) Before testing the rolls, they should be cleaned in strict accordance with the standard requirements to ensure that there is no oil or rust. If a Leeb hardness tester is used for detection, it should be kept away from magnetic field interference, and the roll should be demagnetized if necessary. When testing the turned roller surface, the light band or bright spot should be polished to ensure that the surface roughness meets the requirements.
(1) During roll hardness testing, testing equipment, calibration blocks, roll surface roughness, and roll surface cleanliness all affect the measurement of hardness values. These factors are the difference between the test results of the same roll hardness between the roll manufacturer and the user. the main reason.
(2) Roll manufacturers should not only consider the type of hardness tester and hardness conversion table used by themselves when preparing the process, but should calibrate and compare the hardness tester actually used by the customer’s rolling line with the hardness tester used on the roll production site. Determine the deviation value between the two. Convert the required hardness value at the production site according to the deviation value, and determine the roll composition and heat treatment process based on this, so as to ensure the hardness detection deviation caused by the difference in the hardness tester or hardness conversion table.