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


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 sized castings in sound condition.


2. Select a molding method that pro- duced the required dimensional tolerances and surface finish 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 strongest


grade of A27, but the designers recog- nized it does not meet the mechanical 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 specifications 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


32


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 specifications 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 specification 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 specifications. So, the design team selected A958 Grade 8620 steel for the connector. 2. Molding Method In addition to selecting the appropriate


alloy, the casting design team worked together to determine the best molding method, which affects the surface finish and dimensional tolerances of the finished casting. The three most critical dimen- sional features in the connector were: • The uniformity and the +/- tolerance of the center slot opening. If it were too narrow, the gusset wouldn’t fit into the slot; too wide and the bolted plates wouldn’t have surface contact with the gusset.


• The position, alignment and diameter of the bolt holes on the two plates.


• The angle, depth and uniformity of the weld shoulder on the cylinder. (The dimensional tolerances for the


castings were specified by the ISO 8062 Grade CT8. The A grade specifies the tol- erances per the basic casting dimension.) Cast Connex and Pacific Steel engi-


neers considered two types of molding methods to meet the requirements for the steel connector: green sand and nobake sand molding. The designers weighed the relative capabilities, ad- vantages and costs of the two molding methods and attempted to choose the one that took into account the as-cast tolerance and finish requirements and level of detail while achieving the low- est possible cost. Green sand molding can meet the


baseline requirements for the connector for surface finish, detail level and cost. But for the longer 27-in. heavy connec- tor, green sand molding has a larger dimensional tolerance (0.12 in.) than the target tolerance requirement of 0.1 in. Nobake sand molding also can meet


the baseline requirements and offers improved surface finish. In addition, nobake molding can hold tighter di- mensional tolerances at larger dimen- sions than green sand. It meets the target tolerance of 0.1 in. for the larger dimension, so the designers determined it was the best choice for the connector. 3. Casting Design and Mold Orientation One of the advantages of the casting


process is that detailed features can be produced in a near-net shape configu-


MODERN CASTING / September 2010


Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52  |  Page 53  |  Page 54  |  Page 55  |  Page 56  |  Page 57  |  Page 58  |  Page 59  |  Page 60  |  Page 61  |  Page 62