Part for Construction Need


• ultimate tensile strength: 550 MPa (80 ksi);


• tensile yield strength: 345 MPa (50 ksi); • percent elongation: 22% in 50 mm; • reduction in area: 35%. The steel used for the Cast Connex


connector was required to have Charpy impact strength of 27 J at -20C and had to be weldable, with the fi nished weld having Charpy impact strength of 27 J at -30C. The surface fi nish requirement for the part was 450 RMS. The qual- ity assurance requirements also were extensive, covering both casting pro- cess parameters and comprehensive mechanical testing and nondestructive evaluation of the castings.


The Casting Design Issues Cast Connex selected Pacifi c Steel


Casting Co., Berkeley, Calif., to produce its steel connectors. The casting design team (Cast Connex design engineers and Pacific Steel casting engineers) focused on three imperatives: •Design for Performance •Design for Production/Castability •Design for Cost The requirements for performance,


casting production and cost were closely interconnected. Three casting design issues played a major role in meeting the three design imperatives: 1. Select a steel alloy that met the me-


chanical requirements and reliably produced the four different size castings in sound condition.


2. Select a molding method that pro- duced the required dimensional tolerances and surface fi nish in a cost-effective manner.


3. Develop a casting and mold design that produced flaw-free, within- tolerance connectors at the best cost.


1. Steel Alloy The Cast Connex design engi-


neers originally considered three different steel casting alloys, as specified by ASTM standards: A27 Grade 70-40, A148 Grade 80-50 and A958 Grade 8620. A27 Grade 70-40 steel is the stron-


gest grade of A27, but the designers recognized it does not meet the me- chanical property requirements for the steel connector. The alloy’s ultimate tensile strength of 70 ksi was below the 80 ksi design goal, and its tensile yield strength of 40 ksi was below the 50 ksi goal. The elongation of the alloy met the 22% requirement, but the reduction in area value failed to meet the 35% target. In addition, the chemistry and heat treat specifi cations were not tight enough to permit precise control of the alloy for this demanding application. A148 Grade 80-50 steel met all the mechanical property requirements for


the steel connector, with an ultimate tensile strength of 80 ksi, tensile yield strength of 50 ksi, elongation of 22% and reduction in area of 35%. But the broad chemistry and open heat treat specifi cations were not tight enough to permit precise control of the alloy for the application. A958 Grade 8620 steel met all the mechanical property requirements for the steel connector, with an ulti- mate tensile strength of 80 ksi, tensile yield strength of 50 ksi, elongation of 22% and reduction in area of 35%. The alloy’s full maximum/minimum specifi cation for chemistry (0.18/0.23 carbon, 0.60/1.0 manganese, 0.3/0.6 silicon, 0.35 maximum phosphorous, 0.04 maximum sulfur, 0.4/0.7 nickel, 0.4/0.60 chromium and 0.15/0.25 molybdenum) and heat treatment (austenitize at 1,650F, air quench and temper at 1,150F) ensured that the casting would meet specifi cations. So, the design team selected A958 Grade 8620 steel for the connector. 2. Molding Method In addition to selecting the appropri-


ate alloy, the casting design team worked together to determine the best molding method, which affects the surface fi nish and dimensional tolerances of the fi n- ished casting. The three most critical di- mensional features in the connector were:


Fig. 1. The cast steel connector, designed by Cast Connex Corp., Toronto, Canada, is used as a structural brace in the construction of buildings. Above, the cast component is shown bolted in place.


JANUARY/FEBRUARY 2010 METAL CASTING DESIGN AND PURCHASING 31


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