Analysis on forming mechanism of slag inclusion defect of steel castings in epc

1 The prevalence of slag inclusion defects in steel castings with epc

It is very difficult to produce steel castings with lost mold. At present, most of them are wear-resisting, heat-resisting corrosion-resisting castings without processing less processing, some other thin-wall castings.  The main reasons for the defects of low carbon steel castings are uneven carburization slag inclusion defects of thick large parts.  For the steel castings with a certain thickness most of the low carbon steel castings, the proportion of carburization, slag inclusion porosity defects is more than 60%, which makes the low carbon steel thick thick steel castings become the difficult problem of the lost mold casting process, even considered that the lost mold casting process is suitable for steel castings.

1.1 Defect forms of epc steel castings

The defects of epc steel casting are slag inclusion, porosity carburization.  The shape of defects is regular, the defect edge is irregular, the defect density is very dispersed, which is manifested in different color shades on the metallographic diagram.  The accumulation form of defects is mostly cluster shape with fuzzy boundary scattered color, which is difficult to be removed by processing.

1.2 Proportion of defects in lost mold casting steel parts

The proportion of defects in epc steel castings is very high.  Including wear -, heat - corrosion-resistant castings, other thin - thick-walled steel castings, without without machining.  For thin-wall steel castings, the defects are mostly pores slag holes at the root of gate riser.  For thick wall steel castings, the defects are mostly subcutaneous slag defects.  For low carbon steel castings, the defects are mostly surface uneven carburization defects.

1.3 Parts prone to defects of epc steel castings

The wall thickness carbon content of epc steel castings are different in the parts defects are easy to occur.  For thin wall three resistant castings, mainly appear in the casting gate riser connected parts.  Parts that are connected to the casting casting filling process, the flow for a long time, to keep the heat time is longer, molten steel overheat the mold material, mold material, partial melting absorb more gas in the liquid steel slag accumulation by blocked, molten steel cooling solidification shrinkage, easy to cause these parts after cooling solidification form hole, shrinkage porosity, slag mixed defects.

2. Particularity of mold filling of epc cast steel

Casting defects are formed at the moment of casting filling solidification process, generally the filling time of small medium castings is very short, the filling time of large castings is also short.  Different ordinary cavity casting, the particularity of mold filling of epc casting is the main reason of slag inclusion defect of epc steel casting.

2.1 Filling form of epc steel castings

As for the liquid metal filling process of epc, most of the researches are based on the filling process of epc for aluminum alloy, most of them are filled without negative pressure.  Under such conditions, the shape of liquid metal filling is that after entering the casting "cavity" the inner gate, the liquid metal front pushes forward in a fan-shaped shape. Under the action of gravity, the liquid metal filling front deforms downward, but the general trend is to push away the inner gate until the "cavity" is filled.  The boundary shape of the contact between liquid metal shape is related to the temperature of liquid metal, the properties of shape material the filling speed. If the temperature of liquid metal is higher, the shape density is smaller the filling speed is faster, the overall advancing speed of liquid metal is faster.  It varies with alloy type, pouring temperature, sprue area, pouring speed, appearance density, high temperature air permeability of coating negative pressure.  For aluminum alloy without negative pressure pouring, the interface between liquid metal shape can be divided into four models according to different conditions: contact mode, clearance mode, collapse mode involvement mode.

2.2 Turbulent morphology wall attachment effect of liquid metal filling

In the mold in the production of cast steel, cast iron pieces, Chinese enterprises are in the process of casting impose negative pressure on dry sand casting, to tighten of dry sand mould, make the mold with sufficient strength rigidity, to resist the impact of liquid metal buoyancy, ensure complete pouring solidification in the process effectively, to obtain a complete structure of castings.  The dry sand mold has enough strength stiffness without increasing the height of sand box. It plays a key role in the development of lost mode casting technology.

3 Analysis of the source thermodynamics kinetics of slag inclusion in molten steel

There are several sources of slag gas in molten steel, including the residue gas of pyrolysis products like gasification, the residue gas produced in the smelting process of molten steel, the oxide residue formed by oxidation of molten steel, the dissolution of some gases by high temperature molten steel.  Due to the small density of these dregs gases, they will float upward slowly in the filling process liquid cooling process before solidification, float toward the lower transverse pressure under the action of negative pressure.

4 Ways suggestions to reduce slag inclusion of steel parts with lost mold casting

4.1 Directly reduce the original inclusions in molten steel

Reducing the inclusions in molten steel before pouring is one of the main ways to reduce the slag inclusion defects in lost mold castings.  There are many ways to purify molten steel, such as using slagging material, relying on the adsorption of the purifying agent on the inclusion, adsorbed the small particles of the inclusion on the large particles of the added purifying agent, forming the larger volume of inclusion particles, which is beneficial to improve the dynamic conditions of floating.

4.2 Reduce inclusions in molten steel through technological measures strengthen the discharge of inclusions

(1) Reasonable design of pouring riser system.  As far as possible with less than one box casting, as far as possible to reduce the existence of molten steel in the pouring system time, that is, reduce cancel the runner;  More than one box casting will inevitably make the pouring system too long. When molten steel is filled by the pouring system, it is easy to produce turbulence splash in the multi-bend variable section channel of the pouring system, which reduces the temperature of molten steel, leads to molten steel oxidation, scours the side wall of the sprue, increases the original inclusions in molten steel.

(2) reduce the appearance of adhesive joints.  Too much shape bonding gap, easy to cause the gap with too much change in glue, resulting in adhesive joints convex concave.  Due to the higher density of the convex adhesive, the gas residue generated after gasification are more, resulting in the increase of the total slag amount;  The concave adhesion glue forms a gap, when coating coating, the coating with extremely strong permeability easily enters the concave gap.

(3) Appropriate reduction of negative pressure.  Negative pressure is an important reason for increasing turbulence caused by molten steel filling.  Increased turbulence causes molten steel to scour the pouring system the "cavity" wall, the molten steel to splash more, forming a flow vortex, easily involved in inclusions gases.  The appropriate way is to meet the appropriate strength stiffness of dry sand casting ensure that the casting does collapse in the process of pouring filling, the lower the negative pressure, the better.