Understanding Material Specifications for Steel Castings
A number of considerations affect the way a steel part should be specified for a metalcasting facility, including material properties, tolerance limits, inspection and certification. To make things simple, the Steel Founders’ Society of America, Crystal Lake, Ill., recommends end-users deliver the following to their casting supplier:
- Shape of the metal casting(drawing or pattern).
- Number of parts.
- Supplementary requirements (test methods and acceptance criteria).
- Material specification and grade.
For many designers, the final piece of this puzzle can be the most difficult. What material specifications are appropriate for casting alloys, and how should they be delivered to your casting supplier?
According to materials information society ASM International, Materials Park, Ohio, carbon and low alloy steels generally are classified based on composition. The higher-alloy steels (stainless, heat-resistant, wear-resistant steels, etc.) can be classified according to composition, microstructure, application or specification.
Carbon steels contain only carbon as the principal alloying element, but other elements, such as silicon and manganese may be added for deoxidization. Low alloy steels contain other alloying elements up to a total of 8% alloy content in addition to carbon.
Of the three major classification systems used in the U.S., two employ chemical composition as their basis:
- The American Iron and Steel Institute (AISI) and Society of Automotive Engineers (SAE) System. The AISI/SAE system uses a four- or five-digit code designation (e.g. AISI/SAE 1040), where the last two or three digits indicate carbon content (three digits are used for steels with 1.00% carbon or more), and the first two digits refer to compositional class. AISI/SAE 1040 refers to carbon steel with 0.4% carbon. (See Table 1 for a full list of compositional classes.)
- The Unified Numbering System (UNS). UNS was developed by technical societies and government agencies to clear up the confusion surrounding steel material specification. The system uses a letter followed by five digits to cover all steel types. The system incorporates the AISI/SAE system (AISI/SAE 1040 is G10400) and includes a number of additional classifications. (The letter “G” is used for plain carbon and alloy steels; cast steels are designated by the letter “F” or “J.”) However, the system is not yet widely used.
- The American Society for Testing and Materials (ASTM) system is based on the steel product and application, rather than composition. The specifications, which might be for railroad rails, boiler tubes, plate or bolts, contain within them composition, mechanical properties and other required characteristics. According to the Steel Founders’ Society of America, the steel casting industry most commonly uses the ASTM system.
The Big Four
In the metalcasting arena, many grades of carbon and low alloy steel meet specific end-use requirements, such as structural strength, wear resistance, high temperature properties and corrosion resistance. While such material is generally ordered to ASTM requirements for steel castings, other specifications may be used. The most common ASTM specifications are:
- ASTM A 781/A 781M-97—Covers mandatory requirements for steel and alloy castings in general industrial use.
- ASTM A 703/A 703M-97—Covers common requirements that apply to steel castings for pressure-containing parts.
- ASTM A 957-96—Covers requirements for steel and alloy castings produced via the investment casting process.
- ASTM A 985-98—Covers common requirements for steel investment castings used in pressure-containing applications.
The process and procedures most commonly specified by the customer in addition to material are heat treatment (which may enhance the properties of specific alloys) and welding. These processes have the greatest effect on casting metallurgical quality and serviceability, and both are routinely performed by steel casting facilities. The temperature-control and recording equipment on heat treatment furnaces should be calibrated at specified intervals. The metalcasting facility can be required to submit procedures and qualifications for review before starting an order.
Understanding the effects of different alloying elements on steel can help designers select a steel composition that meets the component’s requirements. Elements that can be added to influence the properties of steel are divided into two groups: desirable elements and undesirable elements. Following are the desirable elements.
Carbon (C) has the largest influence on determining steel alloy properties, including the hardness and strength levels that can be achieved. A higher carbon content increases the tensile strength and hardness levels that can be achieved while decreasing ductility and weldability. When considering alloys, it is important to remember that carbon content determines the strength of the steel, but higher carbon content also makes heat treatment more difficult. Medium- to high-carbon steels (more than 0.3% by weight) can develop cracks during the heat treating or welding processes. Because of this, most non-wear resistant applications use steel with carbon levels less than 0.3%.
Manganese (Mn) is a significant contributor to hardness, strength and hardenability (but to a lesser degree than carbon). Increasing manganese content somewhat reduces steel’s ductility and weldability. Manganese contents greater than 1.5% can increase susceptibility to cracking. To avoid this problem, molybdenum (Mo) is often added in conjunction with manganese to produce manganese-molybdenum steels.
Silicon (Si) increases hardenability and strength but decreases ductility and toughness. Silicon has less influence than carbon or manganese. Silicon is often required for steel castability.
Nickel (Ni) is used primarily to enhance the toughness of steels, especially in low-temperature uses. It increases hardenability slightly and yields small boosts in strength and hardness without decreasing ductility or toughness.
Chromium (Cr) is added to increase hardenability and improve high-temperature strength, abrasion resistance, corrosion resistance and creep resistance (when a material deforms at high temperature over a long time). Ductility and toughness usually are reduced when chromium is added to the alloy, in conjunction with increases in strength. Improved toughness can be gained by using nickel with chromium. Chromium also makes steel more susceptible to temper embrittlement (brittleness that occurs when cooling after tempering).
Molybdenum can be added to reduce this effect. Molybdenum has a marked effect on the heat treatment response of steels and is a powerful element in increasing hardenability. Chromium and molybdenum are often used together to produce alloys with good high-temperature properties, including creep resistance. Alloys containing 0.15-0.3% molybdenum have a minimum susceptibility to temper embrittlement.
Vanadium increases hardenability and is used at low levels. It is used to provide additional creep resistance to chromium and chromium-molybdenum steels.
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 aluminum nitride formation.
Boron (B) is added to steel at low levels to increase hardenability. 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. These 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 strength and hardenability of steel but dramatically reduces toughness and ductility. It has been identified as a major contributor to temper embrittlement.
Sulfur (S) has detrimental effects on the toughness and ductility of steel, and its level usually is kept as low as possible. Phosphorus and sulfur separate during solidification 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 toughness. Hydrogen can be reduced by heat treatment, although 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.
The oxygen (O) content of steel should be below 100 ppm to avoid gas porosity and poor sulfide shape. Oxygen is tied up by deoxidizers such as aluminum, silicon, manganese, titanium, calcium or complex mixtures of these and other elements.
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 intergranular failures in heavy sections.
The Selection Process
The variety of steel composition and heat treatment options provide a wide range of properties 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 strength values.
The choice of alloy composition and heat treatment will depend on many factors, but cost and availability are critical. Unless a designer has overriding reasons to specify a particular alloy composition, consideration should be given to grades of steel with appropriate hardenabilities that are already in production at the casting supplier’s facility. This is especially important to accommodate small order quantities and to facilitate on-time deliveries.
In order to select an appropriate steel alloy composition and heat treatment for a particular application, the design engineer must be clear on which properties are required. If the required properties and section size are known, the steel alloy composition’s response to heat treatment can be evaluated using hardenability. The minimum ideal critical diameter (DI) is a single number often used to describe the hardenability of an alloy composition.
Casting geometry and section size is important in determining the effectiveness of the heat treatment and hardenability. If the casting under consideration is more plate-shaped than spherical or cylindrical, the DI needs to be larger because plates cool more slowly than cylinders. This is especially important since more applications for castings will approximate plates than cylinders. The hardenability requirement for a plate section can be estimated by multiplying the calculated DI by 1.5. Tempering curves are useful in approximating hardness at various locations after quenching and tempering.
In carbon and low alloy steels, chemical composition largely determines hardenability, which in turn dictates the mechanical properties of steel. Carbon is one element that is present in every steel type, and its effect on hardenability must be considered. Increasing carbon increases hardenability. When selecting an alloy’s composition, start with carbon, which should be as low as possible while still meeting the established objectives. The higher the carbon, the more prone the steel will be to quench cracking and welding difficulties. It also has been determined that moderate amounts of several alloying elements are more effective in attaining a desired hardenability than large amounts of one or two elements.