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 and corrosion-resisting castings without processing or less processing, or some other thin-wall castings.  The main reasons for the defects of low carbon steel castings are uneven carburization and slag inclusion defects of thick and large parts.  For the steel castings with a certain thickness and most of the low carbon steel castings, the proportion of carburization, slag inclusion or porosity defects is more than 60%, which makes the low carbon steel and thick thick steel castings become the difficult problem of the lost mold casting process, and even considered that the lost mold casting process is not suitable for steel castings.

 

1.1 Defect forms of epc steel castings

 

The defects of epc steel casting are slag inclusion, porosity and carburization.  The shape of defects is not regular, the defect edge is irregular, and 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 and 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 – and corrosion-resistant castings, or other thin – and thick-walled steel castings, without or without machining.  For thin-wall steel castings, the defects are mostly pores and slag holes at the root of gate or 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 and carbon content of epc steel castings are different in the parts where defects are easy to occur.  For thin wall three resistant castings, mainly appear in the casting and gate or 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 and slag accumulation by blocked, molten steel cooling and 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 and medium castings is very short, and the filling time of large castings is also short.  Different from 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, and most of them are filled without negative pressure.  Under such conditions, the shape of liquid metal filling is that after entering the casting “cavity” from 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 from the inner gate until the “cavity” is filled.  The boundary shape of the contact between liquid metal and shape is related to the temperature of liquid metal, the properties of shape material and the filling speed. If the temperature of liquid metal is higher, the shape density is smaller and 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 and negative pressure.  For aluminum alloy without negative pressure pouring, the interface between liquid metal and shape can be divided into four models according to different conditions: contact mode, clearance mode, collapse mode and involvement mode.

 

2.2 Turbulent morphology and 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 and rigidity, to resist the impact of liquid metal and buoyancy, ensure complete pouring and solidification in the process effectively, to obtain a complete structure of castings.  The dry sand mold has enough strength and 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 and thermodynamics and kinetics of slag inclusion in molten steel

 

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

 

4 Ways and 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 and 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 or 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 and splash in the multi-bend and 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, and 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 or concave.  Due to the higher density of the convex adhesive, the gas and 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 and the “cavity” wall, and the molten steel to splash more, forming a flow vortex, easily involved in inclusions and gases.  The appropriate way is to meet the appropriate strength and stiffness of dry sand casting and ensure that the casting does not collapse in the process of pouring filling, the lower the negative pressure, the better.

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Post time: Sep-24-2021