1. Proposal of the problem According to GB/T1348-2009 "Spheroidal Cast Iron Castings", the surface roughness of the mechanical properties of the ductile iron castings is not required. Therefore, the cost factor is taken into account in the processing of the specimens, and the specimen Lc is not The area is ground, and the surface roughness of the sample car after processing is generally Ra1.6~3.2. At present, the super-new GB/T228.1-2010 "Metal material tensile test Part 1: room temperature test method" also does not put specific requirements on the surface roughness of tensile specimens; The roughness of the sample Lc region was Ra3.2. Taking QT400-18AL material as an example, the tensile strength requirement is ≥360MPa, the surface roughness Ra3.2 of the sample car after processing, and the tensile displacement rate is 10mm/min. Although the sample processing meets the standard requirements, the tensile test is found in the retest. The intensity detection value is often at 360~380 MPa, just at the critical value.
2. Analysis of the problem A large number of studies have shown that the surface roughness of the machined sample is related to the tensile test results, especially for the poor plasticity or brittle material, generally with the increase of roughness, the strength of the material and the extension of the material. The elongation rate will decrease. When the roughness is less than 0.8 μm, the tensile strength and elongation after fracture tend to be stable.
3. Solving the problem In order to study the influence of roughness on strength and plasticity, the Ra3.2 sample with the original tensile strength of 360 MPa was resampled, and the Lc area of the sample was ground with a roughness of Ra0.8 and stretched. The displacement rate is 10mm/min, and the test data is shown in Figure 2~3 and Table 1.
Sample Rm Rp0.2 A Z Ra3.2 360 230 23.5 23 Ra0.8 380 260 24.5 25
According to the experimental results, the surface roughness of the tensile specimens is required to reach Ra0.8. A large number of test data show that the tensile strength of QT400-18AL material is generally increased to 370~395MPa, and the tensile strength of Ra0.8 is increased relative to Ra3.2. Near 20 MPa, the measured performance of the sample Lc region Ra0.8 can better meet the actual performance of the material.