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Various problems in the Rockwell hardness testing machine

Aug August, 2018

Although the Rockwell hardness testing machine is simple in structure and convenient to operate, if the long-term operation is improper, the hardness is inaccurate, which will greatly affect the quality of the product and bring about adverse consequences. Now we will introduce some common errors and treatment methods in our verification as follows:

1. Human error

(1) The operator is not skilled enough and has poor practical experience and should be used by personnel familiar with the hardness tester;

(2) The loading is too fast, the holding time is short, the hardness of the low hardness part is high, and the loading is too slow, the holding time is long, the hardness is low, and the loading should be flat during operation to maintain a certain loading time.

2. The factors affected by the measured parts

(1) Different surface finishes exhibit different effects in the Rockwell hardness test. The lower the surface finish, the higher the hardness of the high hardness test. On the contrary, the lower the hardness, the rough surface with the knife mark, the first cooling at the time of quenching, or the hard surface layer, the hardness value is high. On the contrary, when the tempering part is tempered at a high temperature, the surface layer of the knife mark is first transformed, the ability to resist tempering is small, and the hardness value is low. When testing parts with a surface finish of Δ7 or less, it must be ground with a scrap wheel, smoothed with a trowel, or polished with a fine hand wheel, and then wiped clean.

(2) There are salt, sand and other materials on the surface of the heat-treated parts. When the load is applied, the parts will slip. If there is greasy, the diamond head will be lubricated when pressed, reducing friction and increasing the depth of pressure. These two reasons make the measured hardness value low. The hardness of the thin layer of the oxidized skin and vegetable layer is reduced in the part tested, and the hardness value of the dense layer of the scale is increased. Parts that are to be tested for hardness must be descaled, wiped clean, and free from dirt.

(3) The error of the bevel (or taper), spherical and cylindrical parts on the hardness test is larger than the plane. When the indenter is pressed into the surface of such a part, the resistance around the press-in portion is smaller than the plane, and even the phenomenon of deviation and slippage is increased, the depth of pressure is increased, and the hardness is lowered. The smaller the radius of curvature, the larger the slope, and the more significant the reduction in hardness. Diamond indenters are also susceptible to damage. A special workbench is designed for such parts to make the workbench and the indenter concentric.

3. The impact of the indenter

(1) The diamond indenter does not meet the technical requirements or wears after a period of use. If the operator cannot judge whether the diamond is good or bad, it can be verified by the measuring and testing mechanism.

(2) The strength and hardness of the steel ball indenter are not enough, and it is easy to cause deformation. The steel ball is flattened and deformed to be elliptical. When the short axis is perpendicular to the surface of the part, the indentation is shallow and the indication value is high. When the long axis is perpendicular to the surface of the part, the indentation is deepened, the indication value is reduced, and the steel ball tolerance is 0.002. Mm.

4. The load

(1) Initial load: There is friction between the spring and the main shaft, the lever and the dial indicator, resulting in an increase or decrease of 100N. Adjust the screw loose, adjust the block to move, the ejector pin is not in the proper position. There is a difference in the starting line, causing the initial load to be incorrect. If the initial load is wrong, adjust the spring, spindle, lever, dial indicator, etc. After the position of the adjustment block is moved properly, tighten the adjusting screw and tighten the position of the jack. The tolerance of the initial load should be less than ±2%.

(2) Main load: The leverage ratio is wrong, the weight of the boom and weight is inaccurate; the main shaft, the lever and the weight are skewed, which will cause errors in the main load. The leverage ratio is wrong and should be adjusted. If the blade is worn, it should be repaired or replaced, and the spindle deformation should be straightened. The spindle, lever and weight skew should be positive. The tolerance of the main load of various scales is less than ±0.5%.

5. The hardness tester is not properly placed

The hardness tester is not in a horizontal position, and its value is low when the hardness is tested. Level the level with a spirit level and then level the hardness tester.

6. The surface of a certain test part of the part is in poor contact with the workbench, or the support point is not stable

This will cause slippage, rolling, lifting, etc., which will not only make the results inaccurate, but also damage the instrument. The appropriate workbench should be designed according to the geometry of the part.

7. The impact of the surrounding environment

The hardness tester used in factory production is often affected by the vibration of the surrounding environment, resulting in loose instrument structure and unstable indication. The hardness tester should be installed in a place where there is no vibration or is far from the source.

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