Vernier caliper is an important measuring tool for measurement and inspection in the production and processing of mechanical parts. The workpiece material is T8 steel. The technical requirements for heat treatment of vernier calipers are: the hardness of the scale body is 43 - 53HRC, the hardness of the measuring surface is ≥58HRC, the hardness of the front and rear surfaces is ≥40HRC; the metallographic structure, the martensite level of the measuring surface is ≤2.5.
During production, it was found that the quality of the vernier calipers treated with the original process was unstable. The hardness of the inner and outer caliper measurement surfaces often appeared to be seriously low. The hardness of the ruler body dropped to 25-35HRC, and the local hardness of the front end surface was only about 25HRC; The body level reaches level 5. This not only seriously damages the quality of the vernier caliper product, but also significantly reduces its service life, making it prone to breakage, failure, damage, and production accidents.
The key point of process improvement is to heat the measuring tape to 780-790℃ in a salt bath and then put it into 150-180℃ nitrate salt for cooling (isothermal), and then immediately immerse the workpiece in a 390℃ nitrate salt tank for partial tempering treatment . The metallographic examination is as follows: the body structure is tempered troostite + residual carbide, the hardness is 732 - 804HVO.2, and the martensite level is ≤2.5; the transition zone structure is tempered troostite + sorbite + a small amount Carbide, the hardness is 450-510HVO.2; the measuring surface structure is tempered martensite + residual carbide, the hardness is 350HVO.2; the narrow surface structure is tempered trotenite + a small amount of carbide + a trace of sorbite, Hardness is 510 -548HVO.2.
After many tests and improvements, a new quenching hanger and a special fixture for tempering were designed. During production, hang the workpiece in the hanger in Figure 6-36, and quench it in stages in nitrate salt at 180-220°C for 1 min. After taking it out, place it on the fixture in Figure 6-37 and clamp it, and put it in 390 Temper in nitrate solution at 400°C for 20-30 minutes, then cool to room temperature in air. During air cooling, the cooler does not need to be removed, but the water can be cut off; during tempering, the cooler is cooled by water. After air cooling, clean, and then temper at 160~180℃ for 3 - 4h0. After the 150mm measuring ruler is processed by the above process and improved hanging fixture, measure the hardness of the measuring ruler.
After adopting the new quenching hanger, the workpiece is easy to load and unload. The spacer is used to reduce the quenching and cooling speed in the transition zone. The structure formed is a mixed structure of fine pearlite + a small amount of martensite. The hardness of the workpiece after tempering is 37-49HRC, and the effect is good. . The advantage of using a tempering fixture is that the deformation of the caliper can be controlled and corrected, the flatness error of the ruler body can be controlled within 0.20mm, and the straightness error of the side of the ruler body can be controlled within 0.10mm. The new process of the vernier caliper adopts one-time heating quenching, thermal straightening, and local tempering with temperature, which is beneficial to eliminating quenching stress and quenching deformation; and the inner and outer measuring surfaces are close to the cooler, so the amount of retained austenite can be produced without increasing the return of the cooler. When there is a fire, water is used for cooling, so that the accuracy of the vernier caliper can be maintained stable for a long time.
The above process improvements are also applicable to the heat treatment of 200mm and 300mm vernier caliper gauges, and only the process parameters need to be adjusted accordingly.
After adopting the improved technology, the quality of the vernier caliper is excellent, the workpiece structure level, arsenic eagle index and deformation tolerance all meet the technical requirements, the technical and economic benefits are good, and the production runs normally.
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GS-ZP-1200
