Main factors for reducing sand hole and air hole defects of large steel castings and quality improvement scheme

1.How to control the pouring temperature?

Since lowering the pouring temperature has so many advantages, how can we ensure smooth pouring under the condition of reducing the pouring temperature? How high is the pouring temperature suitable?

Molten steel refineries outside the furnace improve the fluidity of molten steel to achieve the purpose of low-temperature pouring. The molten steel smelting equipment of large-scale cast steel enterprises is relatively advanced. It is equipped with high-power or ultra-high-power large capacity electric arc furnace, ladle refining furnace, VD vacuum smelting furnace, etc. these means can effectively improve the quality and fluidity of molten steel and realize low-temperature pouring. What about small and medium-sized enterprises without these means? Recommend a simple and easy solution (also the simplest means of refining outside the furnace), namely “hydrogen blowing at the bottom of the ladle”. “Hydrogen blowing at the bottom of the ladle” can float the inclusions and gas in the molten steel, so as to purify the molten steel, uniform the temperature, effectively improve the fluidity and create necessary conditions for reducing the pouring temperature.

Determination principle of pouring temperature: when the measure of “ammonia blowing at the bottom of the ladle” is adopted, the pouring temperature is 30-50 ℃ higher than the melting point; Without “hydrogen blowing at the bottom of the ladle” and other refining measures outside the furnace, the pouring temperature is 40-60 ℃ higher than the melting point. The auxiliary judgment method of whether the pouring temperature is appropriate is: under normal pouring conditions, turn the ladle immediately after pouring, and leave “flower ladle bottom” or “small ladle bottom”, that is, the temperature is appropriate.

2.In the later stage of pouring, flow control pouring shall be carried out to avoid slag.

At the later stage of pouring, the liquid level in the ladle is low, which is easy to make the steel slag enter the mold cavity with the steel flow, and the flow control pouring must be carried out. Some people think that the specific gravity of steel slag is smaller than that of molten steel and will not enter the cavity with the steel flow unless the molten steel is poured out. In actual production, there are many examples of steel slag entering the cavity with the steel flow. You may as well do an experiment: plug the drain hole of the pool, fill the pool with water, sprinkle some floating debris on the water surface, and then open the drain hole. You will see that when the water surface is still high enough, the floating objects fall down with the center of the water flow. Similarly, the steel slag can enter the cavity with the steel flow, especially when the nozzle diameter is large and the molten steel level is low in the later stage of pouring.

3.During double ladle or multi ladle combined pouring, the reasonable control of steel water volume in one pouring is another important link to improve the quality of castings.

When double ladle or multi ladle combined pouring is required, it is often caused by large casting weight, insufficient molten steel organization capacity, or limited lifting capacity. This pouring method is very beneficial to improve the pouring speed, but a basic principle must be followed: the flow of body pouring shall not be cut off, let alone stop flow and change ladle pouring before pouring to the parting surface. Otherwise, it will cause major quality hidden dangers and even scrap castings. As mentioned earlier, the “radiation heat transfer intensity” of molten steel to the cavity during pouring is closely related to factors such as “radiation time, radiation source temperature and radiation distance”. If a radiation condition of “close distance, long time and high temperature” is artificially created, nine out of ten castings poured out will be waste products.

4.Riser maintenance after pouring.

This link is very important, but it is often ignored. Therefore, not only the yield of the process is reduced, but also shrinkage and porosity defects occur from time to time. Someone has done a special study. When using an ordinary sand riser, the heat lost on the top of the riser is more than 60% of the total heat lost by the riser. What’s more, we now use a thermal insulation riser! More attention should be paid to the role of covering agent. Therefore, the following two links should be well controlled:

① Select the riser covering agent suitable for large and medium-sized steel castings. The covering agent with ideal effect is selected, which is equivalent to completing 60 ~ 70% of the task. When selecting the covering agent, the “thermal insulation performance” of the covering agent must be fully considered according to the characteristics of large riser size and long solidification time of large and medium-sized steel castings. In other words, when selecting the covering agent, the heating function of large and medium-sized steel castings should be considered in a secondary position.

② Add enough covering agent immediately after pouring, constantly observe the changes of riser top surface, and add a certain amount of covering agent in time to keep the riser top surface from exposing red for a considerable period of time (especially within 30 minutes).

5.Control of riser cutting process

Compared with small steel castings, large and medium-sized steel castings have relatively large added value of technical capacity, added value of equipment capacity, added value of quality risk and relatively high price level. Many enterprises have joined their production ranks.

However, due to the limitation of smelting capacity and lifting capacity, we have to overload steelmaking, overload lifting, and even have to cut the riser in the as cast state before heat treatment, which greatly increases the difficulty of quality control. Under the specific conditions of overload production, how to reduce quality risk and stabilize casting quality has become one of the key links in the production process of large and medium-sized steel castings.

Generally, steel castings shall be cooled slowly in the mold for a proper time after pouring, so as to open the box and drop sand when the surface temperature of the casting is reduced to a lower temperature, and cut the riser after the initial heat treatment. For large and medium-sized steel castings produced under overload, double or multiple ladle pouring can be carried out during pouring, which alleviates the lifting load. The lifting of castings (especially those with risers) has become a restrictive link, and they can only be lifted after the risers are cut at the modeling site. In order to reduce quality risk, the following points should be noted:

(1) it must be cut at a sufficiently high temperature;

(2) the cutting speed shall be as fast as possible, and remember not to cut or stop;

(3) the riser after cutting shall not be lifted away from the casting immediately, and the original position shall be reserved to cool the cutting position of the casting slowly.

(4) initial heat treatment shall be carried out as soon as possible after riser cutting.

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