Macro Defects in Steel - Common Casting Defects

Common Casting Defects

Casting is a forming process which converts liquid steel into a solid product. In foundries liquid steel is cast into complex shapes by pouring of liquid steel into a mould in which it sets to the required shape. In steel plants, liquid steel is normally continuously cast in the form of slab (either thick or thin), bloom or billets. Casting defects are defined as those characteristics which create a deficiency or imperfection exceeding quality limits imposed by design and service requirements.

There are in general three broad categories of defects in the foundry cast steel products. These are (i) the major or most severe defects which result in scraping or rejection of castings, (ii) intermediate defects which permit salvaging of castings through necessary repairs, and (iii) minor defects which can be easily repaired. Common defects which generally occur in castings are given below.

Porosity – It consists of the spherical holes of varying size, with bright walls, usually evenly distributed and formed due to the gases in the liquid steel. The larger holes are tend to be found in the heavier section (i.e. last to solidify). If the gas content of liquid steel is low prior to casting then the pinhole type of porosity appears due to absorption of hydrogen (H2) from steam in the mould.

Blowholes – Blowholes are mainly found in three forms namely (i) elongated cavities with smooth walls, found on or just below the surface of the top most part of castings and are caused by the entrapped air, (ii) round shaped cavities with smooth bright walls which are caused by mould or core gases, coupled with insufficient permeability or venting, and (iii) small cavities immediately below the ‘skin’ of the casting surface which are formed by the reaction of the liquid steel with the moisture in the moulding sand.

Piping – This is the defect encountered in risers or within the casting proper.

Inclusions – These are material discontinuities formed by the inclusion of oxides, dross, and/or slag in a casting. They are due to either careless skimming and pouring, or the use of a dirty ladle, or turbulence because of improper gating methods. Faulty closing of moulds can cause ‘crush’ and loose pieces of sand becoming incorporated in the casting.

Sponginess – it is a defect which occurs during the early stages of solidification of a casting and has the appearance, as the name imply, of a sponge. It is usually local or general in extent. The major cause is failure to obtain directional solidification of the casting towards the desired heat centres, such as risers and ingates, insufficiently high pouring temperature and placing of ingates adjacent to heavy sections.

Shrinkage – It is a casting defect which occurs during the middle and later stages of solidification of the cast steel. It has a branching formation. It is readily distinguishable from that of sponginess and is in form of void.

Hot tears – These are discontinuities which result from stresses developed close to the solidification temperature while the steel is still weak. These are attributed to the resistance of the mould and the core, which hinder contraction of the casting, causing thermal stress. Hot tears resemble ragged cracks.

Crack – Crack is well defined and normally straight. Cracks are formed after complete solidification of the liquid steel. Quite large stresses are needed to cause fracture, and the walls of such cracks are discoloured according to the temperature of the casting when the cracks are formed. Bad casting design coupled with restriction of contraction by the mould, core, or box bars contribute to the cracking.

Cold shuts – These are discontinuities (a form of lack of fusion) caused by the failure of a stream of liquid steel to unite with another stream of liquid steel, or with a solid steel section such as a chaplet. They are linear in appearance, with perhaps a curling effect at the ends. A cold shut is caused by the fluidity of the liquid steel being too low (i.e. too cold surfaces) or perhaps unsatisfactory methods of feeding the liquid steel.

Unfused chaplet – A chaplet is often used to support a section of a mould or a core within a mould and when the liquid steel is poured in the chaplets, they are to fuse into the casting. When unfused, the chaplet causes a discontinuity in the casting.

Misplaced core – It is an irregularity of wall thickness, e.g. one wall thicker than the other, It is caused by core out-of- alignment, careless coring-up and closing of mould, or rough handling after the mould is closed.

Segregation – Segregation is a condition resulting from the local concentration of any of the constituents of the steel. The segregation can be ‘general’ extending over a considerable part of a casting, ‘local’ when only the shrinkage voids or hot tears are wholly or partially filled with a constituent of low melting point or ‘banded’ which is mainly associated with centrifugal castings but can also occasionally occur in static castings.

Reference: Website http://ispatguru.com/metallurgical-processes-and-defects-in-steel-products, 2017.