Casting Defects in Steels

Extrait:

Metal casters try to produce perfect castings. Few castings, however, are completely free of defects. Modern foundries have sophisticated inspection equipment can detect small differences in size and a wide variety of external and even internal defects.
For example, slight shrinkage on the back of a decorative wall plaque is acceptable whereas similar shrinkage on a position cannot be tolerated. No matter what the intended use, however, the goal of modern foundries is zero defects in all castings.

Metal casters try to produce perfect castings. Few castings, however, are completely free of defects. Modern foundries have sophisticated inspection equipment can detect small differences in size and a wide variety of external and even internal defects.

For example, slight shrinkage on the back of a decorative wall plaque is acceptable whereas similar shrinkage on a position cannot be tolerated. No matter what the intended use, however, the goal of modern foundries is zero defects in all castings.

Scrap castings cause much concern. In industry, scrap results in smaller profits for the company and ultimately affects individual wages. Scrap meetings are held daily. Managers of all the major departments attend these meeting. They gather a castings that have been identified as scrap by in inspector. The defect(s) is circled with chalk. An effort is made to analyze the cause of the defect, and the manager whose department was responsible for it is directed to take corrective action to eliminate that specific defect in future castings.

There are so many variables in the production of a metal casting that the cause is often a combination of several factors rather than a single one. All pertinent data related to the production of the casting (sand and core properties, pouring temperature) must be known in order to identify the defect correctly. After the defect is identified you should attempt to eliminate the defect by taking appropriate corrective action.

The system used here for classifying defects is one based on a physical description of the defect under consideration. It is intended to permit an identification to be made either by direct observation of the defective casting or from a precise description of the defect, involving only the criteria of shape, appearance, location and dimensions. This unique system of classification, based upon the morphology of the defects, is more logical than one based upon causes since it requires no prior assumptions to be made.

Seven basic categories of defects have been established, as listed below and for each basic category only one typical defect is being presented here.

1. Metallic Projections

Joint flash or fins. Flat projection of irregular thickness, often with lacy edges, perpendicular to one of the faces of the casting. It occurs along the joint or parting line of the mold, at a core print, or wherever two elements of the mold intersect.

Possible Causes

  • Clearance between two elements of the mold or between mold and core;
  • Poorly fit mold joint.

Remedies

  • Care in pattern making, molding and core making;
  • Control of their dimensions;
  • Care in core setting and mold assembly;
  • Sealing of joints where possible.

2. Cavities

Blowholes, pinholes. Smooth-walled cavities, essentially spherical, often not contacting the external casting surface (blowholes). The largest cavities are most often isolated; the smallest (pinholes) appear in groups of varying dimensions. In specific cases, the casting section can be strewn with blowholes of pinholes. The interior walls of blowholes and pinholes can be shiny, more or less oxidized or, in the case of cast iron, can be covered with a thin layer of graphite. The defect can appear in all regions of the casting.

Possible Causes

Blowholes and pinholes are produced because of gas entrapped in the metal during the course of solidification:
  • Excessive gas content in metal bath (charge materials, melting method, atmosphere, etc.); Dissolved gases are released during solidification;
  • In the case of steel and cast irons: formation of carbon monoxide by the reaction of carbon and oxygen, presents as a gas or in oxide form. Blowholes from carbon monoxide may increase in size by diffusion of hydrogen or, less often, nitrogen;
  • Excessive moisture in molds or cores;
  • Core binders which liberate large amounts of gas;
  • Excessive amounts of additives containing hydrocarbons;
  • Blacking and washes which tend to liberate too much gas;
  • Insufficient evacuation of air and gas from the mold cavity; -insufficient mold and core permeability;
  • Entrainment of air due to turbulence in the runner system.

Remedies

  • Make adequate provision for evacuation of air and gas from the mold cavity;
  • Increase permeability of mold and cores;
  • Avoid improper gating systems;
  • Assure adequate baking of dry sand molds;
  • Control moisture levels in green sand molding;
  • Reduce amounts of binders and additives used or change to other types; -use blackings and washes, which provide a reducing atmosphere; -keep the spree filled and reduce pouring height;
  • Increase static pressure by enlarging runner height.

3. Discontinuities

Hot cracking. A crack often scarcely visible because the casting in general has not separated into fragments. The fracture surfaces may be discolored because of oxidation. The design of the casting is such that the crack would not be expected to result from constraints during cooling.

Possible Causes

Damage to the casting while hot due to rough handling or excessive temperature at shakeout.

Remedies

  • Care in shakeout and in handling the casting while it is still hot;
  • Sufficient cooling of the casting in the mold;
  • For metallic molds; delay knockout, assure mold alignment, use ejector pins.

4. Defective Surface

Flow marks. On the surfaces of otherwise sound castings, the defect appears as lines which trace the flow of the streams of liquid metal.

Possible Causes

Oxide films which lodge at the surface, partially marking the paths of metal flow through the mold.

Remedies

  • Increase mold temperature;
  • Lower the pouring temperature;
  • Modify gate size and location (for permanent molding by gravity or low pressure);
  • Tilt the mold during pouring;
  • In die casting: vapor blast or sand blast mold surfaces which are perpendicular, or nearly perpendicular, to the mold parting line.

5. Incomplete Casting

Poured short. The upper portion of the casting is missing. The edges adjacent to the missing section are slightly rounded, all other contours conform to the pattern. The spree, risers and lateral vents are filled only to the same height above the parting line, as is the casting (contrary to what is observed in the case of defect).

Possible Causes

  • Insufficient quantity of liquid metal in the ladle;
  • Premature interruption of pouring due to workman’s error.

Remedies

  • Have sufficient metal in the ladle to fill the mold;
  • Check the gating system;
  • Instruct pouring crew and supervise pouring practice.

6. Incorrect Dimensions or Shape

Distorted casting. Inadequate thickness, extending over large areas of the cope or drag surfaces at the time the mold is rammed.

Possible Causes

Rigidity of the pattern or pattern plate is not sufficient to withstand the ramming pressure applied to the sand. The result is an elastic deformation of the pattern and a corresponding, permanent deformation of the mold cavity. In diagnosing the condition, the compare the surfaces of the pattern with those of the mold itself.

Remedy

Assure adequate rigidity of patterns and pattern plates, especially when squeeze pressures are being increased.

7. Inclusions or Structural Anomalies

Metallic Inclusions. Metallic or intermetallic inclusions of various sizes which are distinctly different in structure and color from the base material, and most especially different in properties. These defects most often appear after machining.

Possible Causes

  • Combinations formed as intermetallics between the melt and metallic impurities (foreign impurities);
  • Charge materials or alloy additions which have not completely dissolved in the melt;
  • Exposed core wires or rods;
  • During solidification, insoluble intermetallic compounds form and segregate, concentrating in the residual liquid.

Remedies

  • Assure that charge materials are clean; eliminate foreign metals;
  • Use small pieces of alloying material and master alloys in making up the charge;
  • Be sure that the bath is hot enough when making the additions;
  • Do not make addition too near to the time of pouring;
  • For nonferrous alloys, protect cast iron crucibles with a suitable wash coating.

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