by clicking the arrows at the side of the page, or by using the toolbar.
by clicking anywhere on the page.
by dragging the page around when zoomed in.
by clicking anywhere on the page when zoomed in.
web sites or send emails by clicking on hyperlinks.
Email this page to a friend
Search this issue
Index - jump to page or section
Archive - view past issues
button in toolbar for more information.
Metal Casting Technologies : June 2007
52 Back to the www.metals.rala.com.au D A Foundryman s Guide for Analysing Casting Defects in Iron Castings (Compiled and edited by Prof. J. Hermes D. Bautista, Technical Consultant, PMAI) efects in steel castings differ slightly from those in iron castings as to nature and causes. The very first step in analyzing defective steel castings is to correctly identify the nature of the defect. Only correct identification will lead to the proper solution of the problem. A good foundry should be equipped with the proper sand laboratory and inspection equipment to effectively control casting defects. It will be assumed that the molten steel is properly deoxidized, thus minimizing, but not entirely eliminating, defects coming from this source. Compared to cast irons, steel castings are much more sensitive to incorrect molding and pouring conditions, such as molding sand properties, core sand properties, and pouring practice. Guesswork will prove to be very expensive in terms of high rejection rates and unreliable castings. A good foundry producing steel castings should have a rejection rate of no more than 3% as cast at the worst; average would be approximately 0-1% considering casting repairs, as steel castings are easily repaired by welding. Although reparable by welding, repaired steel castings are not the "pride" of a good steel foundryman. Steel castings are usually heat treated -- normalized to relieve stresses, annealed to soften, quenched and tempered to harden -- to improve their mechanical properties which is one of their great advantages, but usually pinholes appear after this treatment. Molten steel is more viscous than molten iron, thus it has lower fluidity to facilitate its filling up the mold properly. This should be taken into consideration in the design of steel castings. Steel also has a higher contraction index than iron, and thus requires the provision of bigger risers to feed the castings better. Being a metal reservoir to feed the casting during solidification, the design of risers should be such that the neck of the riser solidifies last to facilitate the proper feeding of the voids in the castings attendant to the solidification process, and the risers should be properly, and correctly, located to ensure solid castings. The gating of steel castings is best done through the bottom in order to ensure the filling up of the mold as "quietly" as possible. Turbulence should be avoided during the filling up process. Note also that steel melted by the acid process (or acid steel) can be poured into molds made of silica sand; but steel melted by the basic process (or basic steel) is best poured only into molds made of chromite sand, or any other neutral or basic mold material. The author has successfully produced austenitic manganese steel (Hadfield's steel) and stainless steel castings in silica sand molds coated with either a magnesite wash or a zircon wash. The basic washes made possible the use of silica sand (an acidic sand) that is the cheapest sand available. NAME & DESCRIPTION OF DEFECT CAUSES Blow • Mold or core gases displace the molten metal Usually looks like a shallow depression in the casting surface, generally with a • Gas originates from organic binders. smooth surface and edge, though in some cases a ragged edge exists. • Gas originates from organic binders. Sometimes the depression is slightly discolored. (A small, round, and rather deep • Excessive moisture in the mold. cavity, especially at reentrant angles, should not be mistaken for a "blow." • Low permeability of the sand. This defect is usually caused by an atmospheric-pressure break through the • Improper venting of the mold. casting surface into a hot spot and is associated with "shrinkage." Buckle • The defect results from radiated heat from A depression in the casting surface of an irregular shape. May be of large, the rising metal causing a thin layer lateral extent and is usually found on the cope side. It is similar to a "rat tail" of the mold surface to expand. or "seam," but is larger. • It results also from high hot shrinkage of sand fines and bonds. • Excessive sand strength from high moisture. • Sand grains have high expansion characteristics. Casting Overweight • Too soft ramming of molds. The casting weighs more than the normal limits allowed. • Shifting or misplacement of cores.