Machine tool castings crack how to prevent

With the development of our country's economy, closely related to machine tool demand will conduct a field scale, with depth adjustment and reform. Under this development trend, on the performance of machine tool castings have higher requirements. Blank manufacturers to ensure the performance of machine tool castings, such as strength, hardness, with content of carbon, silicon, and speed up the cooling rate during aging treatment method, resulting in production or appeared frequently during aging treatment of metal casting deformation and crack, even fracture phenomenon. According to the above phenomenon, casting botou city airport star content measuring tool co., LTD. In practice, summed up several measures to prevent machine tool castings from the crack.

1. Chemical composition (navigation star cast)

(1) using the chemical composition of high carbon and low silicon recommended machine tool castings as shown in table 1.

(2) high carbon equivalent, when choosing carbon equivalent as far as possible with high carbon equivalent, is good to prevent machine tool castings from the crack. Carbon equivalent machine tool castings at home and abroad for such as shown in table 2.

(3) high silicon carbon than control the silicon/carbon ratio in 0.57 ~ 0.62, high carbon and low silicon, machine tool castings is not easy to crack. Machine tool castings silicon carbon than at home and abroad such as shown in table 3.

(4) control than in producing gray iron, manganese, sulphur manganese sulfur ratio should be considered as Mn = 1.71 S + (0.2 ~ 0.2). The manganese sulfide in the process of actual production ratio increased slightly, especially on HT250, HT300 and other materials.

(5) low alloying iron liquid adding alloying elements to increase the number of austenite dendrite, increase and refine pearlite, strengthening ferrite, refine graphite and eutectic, improve section sensitivity. General wCu = 0.4% ~ 0.6% or wCr = 0.2% ~ 0.35%, and can also add other alloying elements such as Sb, Sn, Mo, etc. But seldom used an element in the process of production, are combined, such as Cu and Cr, Cu and Sb, Cu and Sn, etc., in use process should fully consider, can we truly have the effect of alloying.

2. The ingredients (cast iron plate)

Good control of furnace charge ratio, reduce machine casting crack tendency, also can ensure the internal quality of casting. Electric furnace melting in particular, should pay more attention to burden ratio. Ingredients are in the process of production: pig iron, steel scrap and back to the furnace charge + recarburizer (according to the ratio of mature join).

3. The inoculation (cast iron box)

Inoculation can promote graphitization, can reduce machine casting chilling and improve the uniformity of section, control the graphite morphology, reduce other graphite and ferrite formation, in order to obtain the good characteristics of flake graphite, etc. Inoculation method choice is very important, however, different inoculation methods, different graphite length and its thickness can be obtained, which play a decisive role on performance of machine tool castings. Microstructure is whether the machine tool castings produced the most essential reason of crack. At present, has changed to the following several inoculation methods: instantaneous inoculation (special) funnel, pouring basin and floating silicon breeds, etc., have made obvious effect, as shown in figure 1 and figure 2 for the use of the kinds of inoculant and inoculation quantity are the same, just different metallographic images acquired way.

4. Aging treatment

Domestic casting aging treatment have heat aging and vibration aging and natural aging of three ways, which is given priority to with thermal aging. In the process of thermal aging, often appear machine casting defects such as cracks, deformation. Is mainly due to the heat treatment process is not reasonable or man-made causes of operation, has 18 t stage in the process of heat treatment from the rupture in the middle of the case, in the final analysis is mainly hot aging treatment of metal caused by the improper casting fracture. In order to prevent the casting in the process of heat treatment does not appear defects such as cracks, deformation, the following points should be paid attention to.

(1) outfit furnace temperature of furnace temperature should be less than 150 ℃.

(2) the temperature not too fast heating speed is generally take 30 ~ 100 ℃ / h, for complex control below 20 ℃ / h.

(3) the deficiency of heat preservation time and high temperature heat preservation time is caused by casting stress relieving poor, too high, resulting in decreased strength and hardness. Heat preservation temperature generally is 500 ~ 600 ℃, holding time is usually 2 ~ 6 h, holding time can be calculated according to the wall thickness of 25 mm/h.

The highest temperature of aging treatment of castings can be press type options:

T = 480 + 0.4 sigma b

Sigma in type b - diameter of 30 mm standard test bar tensile strength (MPa).

Casting time of heat preservation and the highest temperature should be comprehensive consideration of casting wall thickness, size, structure, material factors, such as is not a wall thickness 100 mm insulation casting 4 h is the best.

(4) cooling fast cooling speed, stress relieving effect is poor. Along with the furnace cooling speed should be controlled under 30 ℃, it is concluded that the data as shown in table 4.

In addition, the extension of the machine tool castings in the type of cooling time, to prevent the machine tool castings was beneficial to crack.

5. Using the heat treatment process

In our production, we adopt the heat treatment process is shown in figure 3.

conclusion

By the chemical composition of the machine tool casting, ratio of furnace charge, inoculation method, the control of aging treatment, has achieved significant effect. At present, 156 products have been produced (820 t), found no machine casting crack defects, for the production of high strength and high hardness of large machine tool castings laid a solid foundation, has solved the difficult problem in many years.