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• Vanadium increases harden- ability and is used at low levels. It is used to provide additional creep resistance to chromium and chromium-molybdenum steels.


in increasing hardenability. Chromium and molybde- num are often used together to produce alloys with good high-temperature properties, including creep resistance. Alloys containing 0.15-0.3% molybde- num have a minimum suscepti- bility to temper embrittlement.


• Aluminum (Al) is added to steel as a strong deoxidizer and grain refiner (for greater yield strengths). Ranges of 0.02-0.08% aluminum content are normal, but heavy sections require aluminum to remain below 0.05% to prevent alumi- num nitride formation.


This axle for a transportation vehicle is 6 ft. long and weighs 300 lbs. The compo- nent is a quenched and tempered low alloy steel (chrome-molybdenum type) casting manufactured via the green sand process.


and heat treatment options provide a wide range of proper- ties are available to designers. Choosing an alloy composition and heat treatment to improve one property may result in the


reduction of another property. For example, higher hardness, lower toughness and lower ductility values are associated with higher


• Boron (B) is added to steel at low levels to increase harden- ability. Boron additions are not as detrimental to weldability as other alloy additions but must be limited to less than 0.005% because of a propensity for boron steels to form brittle grain boundary precipitates. T ese precipitates coat the grain with a glass-like material, making it unable to absorb sudden impacts and greatly reducing its ductility. Following are the undesirable


elements. • Phosporus (P) increases the


• Sulfur (S) has detrimental eff ects on the toughness and ductility of steel, and its level usually is kept as low as possible. Phosphorus and sulfur separate during solidifi cation and can contribute to under-riser cracking, making the casting more likely to need welding repairs.


• Hydrogen (H) in excess of 4 parts per million (ppm) can cause poor ductility, although it has little effect on tough- ness. Hydrogen can be reduced by heat treatment, although


strength and hardenability of steel but dramatically reduces toughness and ductility. It has been identifi ed as a major con- tributor to temper embrittlement.


• T e oxygen (O) content of steel should be below 100 ppm to avoid gas porosity and poor sulfi de shape. Oxygen is tied up by deoxidizers such as aluminum, silicon, manganese, titanium, calcium or complex mixtures of these and other elements.


increased section sizes require more rigorous treatments for reduction. Hydrogen also is known to cause a defect called “cold cracking” in quenched and tempered steels, which makes the casting more likely to need welding repairs.


• Nitrogen (N) content must be kept below 100 ppm to avoid gas porosity in castings. Also, aluminum nitrides can form in steel castings, resulting in inter- granular failures in heavy sections.


The Selection Process T e variety of steel composition





strength values. The choice of alloy composition and heat treatment will depend on many factors, but cost and avail- ability are critical. Unless a designer has overriding reasons to specify a particular alloy composition, consid- eration should be given to grades of steel with appropriate hardenabili- ties that are already in production at the casting supplier’s facility. This is especially important to accom- modate small order quantities and to facilitate on-time deliveries. In order to select an appropri- ate steel alloy composition and heat


This housing for a directional thrust revers- er featured in the hydraulic fi tting for a KC 135 military aircraft is best produced out of investment cast steel so it can easily meet the 150-170 ksi strength requirements.


In carbon and low alloy steels, chemical composition largely determines hardenability, which dictates mechanical properties. Carbon is present in every steel type, and its effect on hardenability must be considered.


Jul/Aug 2012 | METAL CASTING DESIGN & PURCHASING | 31


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