ficiently. Creep rate is the rate of stretching at a particular load and temperature, and stress rupture is the time to failure under a given load at a particular temperature. When selecting a creep-resistant material, oxidation or other high-temperature corrosion must be considered because they might limit the service life of the component.
Corrosion Corrosion is a chemical attack that
removes material from an exposed surface. It can be a general loss of material or a lo- calized condition like pitting, cracking or selective attack. Standard tests for corrosion exist, but they rarely replicate service con- ditions or the nature of the environment. Corrosive conditions, such as temperature, pH, oxygen, chlorine and other variables, must be taken into account when selecting the proper cast materials.
Alloy Selection Cast and wrought materials differ in
their structural-direction. The structure of sections of rolled steel is elongated in the direction of rolling. The strength and ductility is improved in that direction but reduced across the rolling direction. The cold rolling of steel also can strengthen the steel but reduces ductility and toughness.
Table 2. Common Steel Alloys for Casting ASTM A915 ASTM A915 Minimum Low Strength
(Grade)
SC1020 SC1030 SC1040 SC8620 SC8630 SC4130 SC4140 SC4330 SC4340
Tensile (ksi) Yield (ksi)
Elongation (%) 65-35-24 65-35-24 70-36-22 80-50-22 80-50-22 80-50-22 90-60-20 90-60-20 90-60-20
Contrarily, the lack of a rolling direction in steel castings gives them uniform prop- erties in all directions. Cast steel grades achieve the same tradeoff through alloy- ing and heat treating. Therefore, casting grades with similar mechanical properties to wrought grades are designated with a different name (ASTM A216 grade WCB is the cast counterpart to wrought 1020). ASTM A915 and A958 use grade names similar to their wrought counterparts. When selecting a steel alloy, the de-
sign limit should be understood first. It is essential to know how and where failures may occur, and selecting a mate-
Minimum High Strength Tensile (ksi) Yield (ksi)
Elongation (%) 70-36-22 80-50-22 90-60-20 115-95-14 150-135-7 150-135-7 165-150-5 210-180-4 210-180-4
Median
Composition Ideal Critical
Diameter (inches) 0.4 0.7 0.9 2.0 3.5 3.0 5.5 6.0 8.0
rial can help address the design limit. Al- loying elements can be added to improve performance; thus, the best practice is starting with a carbon steel and building from there. A material selection guide for five major design applications is shown in Fig. 4. The chart is meant to provide some initial guidance, but it also is important to consult with a metalcast- ing facility to select the right material for each application. This is especially true of higher-alloyed materials (outer rings in the figure). Other alloys may be better, and the alloy and heat treatment can be tailored for specific conditions. Following are descriptions of the common uses for steel castings.
Structural Applications Structural applications are those that
are controlled by strength, deflection and fatigue. Strength and ductility still are the main properties used to designate avail- able steel grades. Increasing the strength of steel is easily achieved through more severe heat treatments or increases in the alloy content. The addition of elements not only increases the strength that an alloy can achieve but also increases the section size of the part, which can be ef- fectively heat treated. The grades in Table 2 are common
alloys of steel available for casting ASTM A915 or A958. The first column is the alloy designation, the second is the minimum requirements for the low- est strength grade commonly available from that alloy, the third column is the minimum requirements for the highest strength grade, and the last column is the calculated largest section that can be effectively heat treated through the sec- tion. Higher-strength alloys have lower ductility but can be heat treated more effectively in larger sections. Higher strength alloys also require more ex- tensive weld procedures and may crack in heat treatment, especially at high carbon contents.
24 Metal Casting Design & PurChasing 2010 Casting sourCe DireCtory
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