Steel castings are used in a variety of end-use applications, particularly those that require heavy-duty components. The castings are used in railroad cars, pumps and valves, heavy trucks, construction and mining equipment, and power generation equipment. A good steel casting application can provide strength while utilizing the flexible geometry inherent in the metalcasting process. Steel castings offer high mechanical properties over a wide range of operating temperatures.
Further, cast steel offers the mechanical properties of wrought steel and can be welded to produce multi-piece parts, as well as large structures (Figure 1). Green sand molding is the most common molding process for steel castings, which can range from a few ounces to hundreds of tons.
Several alloying elements are critical to the formation and properties of cast steel. Carbon steel is considered steel in which carbon is the principal alloying element. Other elements that are present and that must be reported are manganese, silicon, phosphorus and sulfur. In a sense, all of these elements are residuals from the raw materials used in the manufacture of the steel, although the addition of manganese is often made during the steelmaking process to counter the deleterious effect of sulfur, and silicon is added to aid in deoxidation.
A minimum amount of silicon provides the necessary fluidity to properly fill the mold during casting. Other elements, particularly those not easily oxidized, such as copper and nickel, will be recovered from the scrap charge. The amounts of these two elements and others, such as chromium, molybdenum and vanadium, may or may not have to be reported by the manufacturer, depending upon the alloy specification.
Alloy steel grades are considered those to which elements (other than carbon) are added to improve mechanical properties, physical properties and/or corrosion resistance. (Mechanical properties are measured by plastically deforming or destructively testing the material. Physical properties are those that may be measured without plastically deforming the metal.)
Keys to Steel Use
When purchasing steel parts, the part design should be completed with material and property requirements and the material should be designated with a specification and grade (e.g. ASTM A 27/A 27M – 95 Grade 60-30 Class 1). Requirements should call out both the test method and acceptance criteria specifications. Mechanical properties are typically obtained from separately cast test bars.
There are three keys to selecting the right steel casting alloy for optimized performance and cost. One, utilize the geometry of the steel casting to uniformly carry the loading. Two, start with carbon steel for most applications, modify the heat treatment and then add alloying elements to improve properties. Three, know the design limit for the application and work with a metalcasting facility to design the part and select a material.
Designers frequently ask for the highest strength material available, but often toughness and fatigue resistance are what is really needed. By using different alloy compositions, designers have a wide variety of robust steel properties available that can be optimized for specific applications.
Different alloying elements influence the performance of carbon and low alloy steel and their response to heat treatment in different ways. The effect of steel composition on its ability to be heat treated generally is called hardenability.
Hardness is the resistance to penetration. It is measured with a hardness test. Hardenability is a measurement of the section size of steel that can be effectively heat treated. It is a measure of how thick a steel section of a given composition can be quenched to form martensite. Quenching uses gaseous or liquid media to rapidly cool castings, increasing hardenability. Martensite is a hard, brittle steel structure that is unusable without being tempered (reheating steel to a temperature below the critical range to soften it and improve impact strength). For many applications, the best set of steel properties is obtained by tempering and quenching the steel to a desired hardness. The common measurement of hardenability is the distance below the quenched casting’s surface at which a certain level of hardness is exhibited.
Brinell, Rockwell and other standard tests are used to determine hardness. Hardness correlates well with the tensile strength of steel. The maximum attainable hardness of steel depends mostly on its carbon content. Steel grades that exhibit high hardness at great depths have high hardenability, while those that exhibit shallow hardness have low hardenability.
While considering hardenability, understanding the effects of different alloying elements on steel helps designers to select a steel composition that meets the component’s requirements. Elements that can be added to influence the properties of steel are categorized as desirable elements and undesirable elements. The desirable elements are:
Meanwhile, the following are the undesirable elements in steel:
Designing with Steel
The metalcasting process offers freedom of geometry, allowing component design to play a key role in mechanical performance. Sections of a cast part subject to higher stress can be enhanced, while low-stress regions can be reduced. This flexibility can help cast a part with optimum performance and reduced weight, both of which minimize cost. It is feasible to cast any geometry, but this may increase cost.
To develop a good casting, first reduce the number of isolated heavy sections. Junctions within a casting should be designed not to add mass. When working with metalcasters, datum points should be stated, and machine stock should be added to required locations. Section thickness in a casting should be changed through smooth, easy transitions, which can be achieved by adding draft and large radii. Draft should be added to the design dimensions, but metal thickness must be maintained. The amount of draft recommended under normal conditions is 1.5 degrees. Further, reducing undercuts and internal geometry helps minimize cost.
The metalcaster and customer also should agree on tolerances because specifying as-cast tolerances is important in minimizing cost. Other post-processing details, such as machining and how the part will be held in a fixture, also influence the final cost of the part.
Beyond green sand casting, other processes can be used. Shell molding is more suited for smaller than large castings, and investment casting is a better fit for smaller than larger components.
A wide range of properties are available to designers through different steel compositions and heat treatments. Choosing an alloy composition and heat treatment to improve one property may result in the reduction of another. 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 two of the more important ones. 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 metalcasting facility that will be producing the castings. This is especially important to accommodate small order quantities and 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.
The casting geometry and section size are 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 may be estimated by multiplying the calculated DI by 1.5. Tempering curves are useful in approximating hardness at various locations after quenching and tempering (Figure 2).
In carbon and low alloy steel, chemical composition largely determines hardenability, which in turn dictates the mechanical properties of steel. Carbon is one element that is present in every steel grade, and its effect on hardenability must be considered. Increasing carbon increases hardenability. When selecting an alloy’s composition, start with carbon. The carbon level should be as low as possible while still meeting the established objectives. The higher the carbon level, the more prone the alloy will be to quench cracking and welding difficulties. Moderate amounts of several alloying elements are more effective in attaining a desired hardenability than large amounts of one or two elements.
When choosing a material to fulfill your casting needs, the following basic guidelines can help to simplify the process:
- Choose a material that can develop the necessary strength for your application, but remember to consider other beneficial characteristics as well.
- Make sure the material has sufficient hardenability to allow good heat treat response.
- Keep carbon and alloying additions to a minimum to improve part castability and weldability.
Design requirements for cast steel typically are determined in terms of strength or maximum stress. The design is commonly constrained by modulus, fatigue, toughness or ductility.
Increasing the strength of steel normally reduces the ductility, toughness and weldability. Therefore, it often is more desirable in steel casting design to use a low-strength grade and increase the section size or modify the shape. The design freedom makes castings an attractive way to obtain the best material performance, as well as the needed component stiffness and strength. When designing a part, it is important to understand the limits of the design so the proper material selection can be made. Stress, strain, fatigue, impact, wear, creep and corrosion all are common conditions that can impose design limits on steel castings.
This article is an excerpt of Designing & Purchasing Metal Castings published by the American Foundry Society.
Click here to see this story as it appears in the January-February 2019 issue of MCDP.