the design so the proper material selec- tion can be made. Stress, strain, fatigue, impact, wear, creep and corrosion all are common conditions that can impose design limits on steel castings.


Stress Stress results from a mechanical load


carried by a component. The strength of a steel casting is the measure of its load- carrying ability. Loads can be applied and removed without deformation if they are small enough (elasticity). When a large enough is applied, the material will stretch and deform permanently (plasticity). Plas- tic deformation starts to occur when the yield strength is exceeded. The maximum load that can be applied before the materi- al fractures is the ultimate tensile strength.


Designers need to make certain the part will not break or permanently deform, so designing a casting for stress levels below the yield strength or in the elastic region of the part is important. Typically, one-half the yield strength is used for safety, but two-thirds can be utilized if the design is thoroughly evaluated.


The strength of steel depends


on the composition and heat treatment. Steel is iron-based and alloyed mainly with carbon. Other alloying elements add strength and are important in de- termining how effectively the steel grade will respond to heat treatment, which rearranges the crystal structure of the iron and the distribu- tion of carbon. During the cooling process, slow cool- ing rates produce coarse microstructures, which have lower strength. Sever al


cooling


This carbon steel connection tube for the consumer industrial market was produced via investment casting.


Fig. 1. This graph shows cast low-alloy steel properties at room temperature. NT denotes normalize and temper. QT denotes quench and temper.


2010 Casting sourCe DireCtory


Fig. 2. Cast steel offers mechanical properties similar to wrought steel. Typically, fatigue will gradually increase during stress cycles for either wrought or cast grades, as shown here.


Metal Casting Design & PurChasing 21


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