Designing a Simple Corner for Ferrous Castings.

Editor's Note: In the second installment of a new column, modern casting provides basic casting design information to help increase knowledge and facilitate further communication between foundries and their customers.

A discussion of design and shape with the idea of increasing casting complexity must begin with the simplest shape--the corner. Experience indicates that corners are the cause of many severe failures in casting fatigue strength if not designed properly.

All corner failures show a duplex fracture. A starting rupture at the corner is discolored by corrosion and usually displays the oyster shell markings of a fatigue crack. The remainder of the rupture shows the crystalline appearance of a tensile or shear fracture under extreme stress concentration. This fracture is common to all metals whether cast or wrought, and it occurs without any evidence of metallurgical or processing defects as the cause. The assumption is that unexpected bending or torsional stress high enough to cause fatigue failure occurred.

Rounded Corners

The simplest solution to corner fatigue from a metallurgical and manufacturing perspective is to round (slightly) external corners for cast components. The sharper the corner, the more subject it is to crack-like ruptures due to cooling stresses or mechanical nicking, and, ultimately, failure in fatigue. In addition, high temperature gradients and stresses are present in any sharp corner during cooling. If the metal hardens on rapid cooling (as ferrous metals do), the problem is compounded.

The temperature gradients in a sharp corner of a steel section are shown in Fig. 1. The metal in the extreme corner cools at a rate four times that of the metal back from the corner. Rounding corners to radii 10% of the section thickness (0.1t) eliminates this highly stressed hardened metal. It also is a simpler and more foolproof solution than processing precautions to minimize cooling stresses and hardening of such corners.

During the component design stages, rounded corners are not always considered because of the difficulty and cost of manufacturing this aspect in processes other than casting. However, with cast components, rounded corners can be designed and manufactured with little to no extra cost. The only issue with rounded corners can arise when the corner is on the parting line of the mold or part of the core print. Both situations can be accommodated if specified before tooling is made. The parting line can be shifted away from the corners and the core prints can be enlarged to round the external corners formed by the core (Fig. 2).

When developing a design for a cast component, it is best that highly stressed components incorporate a notation that corners may be rounded to 0.1t radii unless otherwise specified, Even a 0.15t radius will "clean up" on machining with normal machine finish stock. If possible, the cast radius should be increased to keep a rounded corner after machining. All evidence indicates that rounded corners with cast surfaces are less susceptible to fatigue failure than sharp machined corners.

Designing Corners

When designing gray iron castings, another advantage to rounding external corners is controlled chilled edges. This problem increases in severity as the strength of the iron increases. Although alloying, inoculation and heat treatment is used to ensure freedom from chilled edges and the resultant machinability difficulties, the best solution is the elimination of the rapidly cooled metal that is the cause of the chilled edge. In gray iron design, rounding external corners by as much as 0.5t is desirable.

When designing all ferrous castings, an objection often is raised when shifting a parting line away from a corner or enlarging core prints to round a corner. The belief that mold/core shifts and mismatches will result is incorrect.

These design modifications allow for an easier inspection for mold/core shifts and mismatches. The valve casting example in Fig. 3 shows how a core that forms sharp external corners (Fig. 3) would require machining or a layout inspection to determine a shift or mismatch. A core that forms rounded corners with an enlarged core print (Fig. 3) would easily display this defect on the outer surface of the casting.

Benefits

The simple step of rounding corners to 10% of the section thickness (0.1t) has four main advantages:

1. A potential area of stress concentration in end-use is eliminated.

2. An area of high thermal stress and potential thermal cracking is eliminated.

3. An area susceptible to chilled edges (in gray iron) is eliminated.

4. Increased dimensional accuracy can result from easier visual inspections with enlarged core prints.

This article was adapted from Design of Ferrous Castings by J.B. Caine.