The mold heat treatment inspection should strictly inspect the inspection items specified in the process documents or technical standards in accordance with the procedures specified in the national standards, industry standards or enterprise internal control standards, and supervise whether the on-site process discipline is implemented in accordance with the process to prevent and reduce waste and rework The production of goods. Quality inspection is a very important link in mold heat treatment, and it is a database of information and digital management.
1 Hot work mold heat treatment quality inspection
The heat treatment inspection of hot work molds mainly includes 4 aspects: appearance, deformation, hardness and metallography.
(1) Visual inspection. There should be no visible cracks on any part of the mold surface, and the key parts should be examined carefully with a magnifying glass of 5-10 times. There should be no obvious bumps on the surface, especially the working surface.
(2) Deformation inspection. Mold deformation generally refers to the change of mold flatness. Use a knife-edge ruler to check the flatness and measure it with a feeler gauge. The allowable deformation of the hammer forging die surface and the die surface of the press, as shown in Table 1 and Table 2, the allowable deformation of the die-casting die plane is ≤0.15mm. If there is no requirement, the deformation should be less than 1/3~1/2 of the margin.
(3) Hardness inspection. Firstly, the parts to be inspected are ground or polished. Generally, it is verified with Rockwell hardness tester C. According to the situation, Vickers, Shore and Leeb hardness testers can also be used for inspection. Rockwell A standard HRA can also be used. Many people object to using it. The HRC standard inspection claims that using HRC to detect deep indentation may become the source of cracks in the later mold, but it is not absolute. If the measured hardness value is extremely high, several points should be measured at different parts as accurately as possible. According to the indicated value, make a decision whether to increase the tempering temperature; if the measured hardness value is too low, it should be re-tested near the original hardness. Polish and continue testing. If it is still low, spark identification or spectral analysis should be performed. The real reason for the low hardness must be found to provide a theoretical basis for rework heat treatment.
Table 1 Allowable deformation of die face of hammer forging die (mm)
Table 2 Allowable deformation of die surface of press die (mm)
(4) Metallographic inspection. Since the 1990s, the author has repeatedly emphasized the view that “hardness is a surface phenomenon, and metallographic structure is the essence” in many papers. To achieve a certain hardness value, it is only handwork, but to achieve the ideal metallographic structure, so that the heat treatment counterparts are troubled. Practice has proved that for a certain range of hardness values, at least three different heat treatment processes can be implemented. On the surface, the hardness values are almost the same, but the metallographic structure is somewhat different, or even very different. Don’t be busy hitting the hardness all day, but also take a closer look at the metallography. Several professional heat treatment factories in Taizhou, Zhejiang, have inspected the hardness of molds and parts after heat treatment sent by customers, and attached metallographic inspection reports. Customers are very satisfied. The metallographic inspection of hot work molds can be performed in accordance with the JB/T8420-1996 “Hot work mold microstructure evaluation” standard. The microstructure characteristics and maximum martensite length of several commonly used die steels are shown in Table 3.
Mold heat treatment metallographic diagram
Note: 1. Observe under 500 times magnifying glass, the maximum length in the table should be ×500;
2. Generally, the qualified grade of hot work die martensite is 2 to 4;
3. The metallographic structure of all hot work molds can be rated according to Table 3;
4. The sample etchant is 4% nitric acid alcohol solution.
Table 3 Microstructure characteristics of hot work die steel and maximum martensite needle length (mm)
2 Heat treatment quality inspection of cold work die
The heat treatment quality inspection of cold work die is also divided into 4 aspects: appearance, deformation, hardness, metallographic inspection.
(1) Visual inspection. There should be no bumps, scratches, burns, pitting, serious oxidation, decarburization, and corrosion on the surface of the mold. There must be no cracks under naked eye observation, the surface must be smooth, mud and salt stains must not be blocked in holes, especially blind holes, and attachments such as tied iron wires must be removed.
(2) Deformation inspection. The deformation of the mold after heat treatment shall not exceed 1/3 to 1/2 of the remaining wear. The allowable deformation range and center hole pitch deformation range refer to Table 4.
(3) Hardness inspection. After the heat treatment of the mold, 100% of the hardness should be checked. The batch size is too large to be inspected, but the amount of random inspection should be increased. Those who cannot use the instrument inspection should use standard files to inspect, and never let one unqualified.
The hardness of the punching die and cold die within 5mm from the cutting edge must meet the technical requirements, and there must be no soft spots. For cold heading, cold extrusion, bending and deep drawing molds, the hardness of the main bearing surface and R must meet the technical requirements. The hardness of the fixed part of the tail of the small punch of carbon tool steel should be controlled within 30～40HRC, and the remaining parts should be checked and accepted according to the drawings.
It is recommended to use Vickers, Shore, Richter or HRA light-duty scales to check the hardness. Try to avoid HRC inspections, because there are precedents of cracks in the indentation, so be careful.
(4) Metallographic inspection. The qualified level of metallographic structure of cold-work die steel after annealing is checked according to Table 5.
Table 4 Allowable deformation of cold work die after heat treatment (mm)
Table 5 Metallographic structure of cold work die steel after annealing
Table 6 Metallographic inspection after heat treatment of cold work die
Cr12 steel carbides are rated according to JB/T7713; large carbides in high-speed steel such as W18Cr4V are rated according to GB/T4462; quenching and tempering metallography is rated according to the internal control standard of the enterprise; the martensite grade after quenching of die steel is evaluated according to Table 6. The microstructure of high-carbon alloy steel refrigeration molds can be evaluated with reference to JB/T7713-2007 standard.
3 Surface strengthening mold quality inspection
The potential of heating and quenching in salt bath furnaces and box furnaces to increase the life of the mold is deeply tapped. It is difficult to further improve it. Therefore, people make a lot of superficial articles and try to use surface strengthening to achieve the purpose of extending the life of the mold.
Box heat treatment furnace
3.1 Quality inspection of blackening and phosphating molds
Focus on checking appearance, color and corrosion resistance. For blackening, refer to the industry standard inspection issued by the former Ministry of Machinery Industry; phosphating refer to the standard inspection of the former Ministry of Aviation. For all kinds of molds with HRC>55, hydrogen embrittlement should be prevented during the blackening and phosphating process.
3.2 Quality inspection of carbonitriding, nitrocarburizing, and nitriding molds
(1) Visual inspection. The surface should be uniform in color, without variegation, rust, and bruising.
(2) Deformation inspection Mold bending, wing curvature, distortion, roundness and expansion and contraction should meet the technical requirements.
(3) Micro-hardness inspection Use small load Vickers hardness or micro-hardness to test the hardness of the infiltrated layer of the same furnace sample. The small module can be directly tested on the machine, and its hardness value should meet the requirements of Table 7.
Table 7 Depth and hardness of gas soft nitriding layer of mold
(4) Infiltration layer tissue inspection. According to the national standard GB11354 “Evaluation of Nitriding Layer Depth and Metallographic Structure Inspection Standards for Steel Parts”, the nitriding layer structure should not have reticular nitride and fishbone nitride, and a small amount of vein-like structure is allowed.
After nitriding, grinding is generally not performed, and the thickness of the compound should not exceed 0.03mm.
(5) Corrosion resistance inspection The workpiece or sample is titrated with 6% to 10% CuSO4 aqueous solution, and no copper is precipitated within 5 minutes as qualified. It can also be tested by salt spray test.
(6) Quality inspection of oxynitride molds. Refer to JB/T3912-1999 “High-speed steel tool steam treatment, oxynitriding quality inspection” standard inspection.
(7) Quality inspection of TiN coating mold. Refer to JB/T8365-1996 high-speed steel tool TiN coating technology standard for acceptance.
(8) Quality inspection of boronizing mold. The quality inspection of the boronizing mold is carried out in accordance with the standards of JB/T4215-1996 “Boronizing” and JB/T7709-1995 “Method for Measuring the Microstructure, Hardness and Layer Depth of Boronized Layer”.
(9) Others. In recent years, the surface strengthening technology of molds has developed rapidly, such as metal infiltration. It should be inspected according to national or industry standards. If there is no standard, the supplier and the demander shall negotiate and draft a simple and practical inspection standard.