When a company places an order with a foundry, the correct material specification must be identified in order for the desired casting to have the qualities required for best performance. Orders for castings are usually made to ASTM specifications, which classifies steel and other materials based on mechanical properties, chemical analysis, and intended use. ASTM International, formerly known as the American Society for Testing and Materials (ASTM), is a globally recognized standards organization.
Depending on the mechanical and chemical properties required, steel (element Fe) will contain varying amounts of foreign elements (alloys) in addition to carbon (element C). ASTM A27 is a specification developed for carbon steel castings for general application. Often referred to as "mild steel" or "plain carbon steel," the balance of cost versus strength of this common form of cast steel make it desirable for a wide range of uses.
- This specification covers carbon steel castings for general applications that require up to 70 ksi (485 MPa) minimum tensile strength
- At room temperature, steel castings conform to the A27 standard and offers a good combination of strength and ductility
- ASTM A27 provides several grades and two classes of steel castings that can be specified depending on the chemical and/or strength requirements of the design
- ASTM A27 provides several supplementary requirements that can be specified if castings require unique inspections for soundness and adherence to other design needs
ASTM A27 steel grades and classes
There are several grades and two classes of steel castings covered under ASTM A27, and each grade varies based on the chemical composition and types of heat treating needed to affect changes in mechanical properties.
Supplemental tests can be applied to specifications when testing beyond chemical and tensile analyses are demanded. Such supplemental testing requires producers to conduct from a selection of additional tests:
- Surface inspections (e.g. Magnetic particle examination) looking for casting cracks or discontinuities
- Internal testing for soundness (e.g. Radiographic examination) looking for internal voids
- Strength testing (e.g. Charpy impact testing) looking to verify temperature, dependent ductile, and brittle transition
Selecting a steel grade
Higher grades and additional tests will typically demand higher costs. Therefore, when choosing your steel grade, it is advisable to choose the one that fulfills your actual requirements rather than a grade that—perhaps with several supplemental requirements being checked—would fulfill conditions that are never expected. If superfluous testing or acceptance criteria beyond actual need is called for, expense is added without necessarily increasing serviceability of the casting. The amount of testing demanded and the required acceptance criteria should follow design and service requirements.
Selecting a steel class
When choosing your steel class, there are two separate classes to consider:
- Class 1: Post-weld heat treatment is required—the foundry must heat treat, or re-heat treat castings if any welding is performed on the castings
- Class 2: Post-weld heat treatment is not required—the foundry may conduct some welding on the castings without necessitating heat treating or re-heat treating. This is normal in cases on very small, cosmetic welds
ASTM A27 heat treatment
Heat treating is a process used to alter, repair, or improve the physical properties of metal. Heat treating in a foundry involves the use of heating and chilling in heat treat ovens—often to extreme temperatures—to reduce stress in a cast part, to homogenize the physical properties of the casting, and/or to modify the physical properties of the metal.
Except for Grades N-1 and U-60-30, all A27 steel castings are to be heat treated by full annealing, normalizing and tempering, or quenching and tempering. Heat treatments shall be performed after castings have been allowed to fully cool after casting.
- Grade N-1: Chemical analysis only
- Grade N-2: Heat treated but not mechanically tested
- Grade U-60-30: Mechanically tested but not heat treated
- Grades 60-30, 65-35, 70-36, and 70-40: Heat treated and mechanically tested
ASTM A27 strength testing
Tests are normally performed on representative steel bars, called coupons, rather than on sections taken from actual castings. Tests made on these representative steel bars have the same composition of the casting, as they are poured and heat treated at the same time as the castings. Therefore, the results show the steel quality, but not necessarily the properties of the actual casting. For example, when a heat treatment is administered, solidification conditions and rate of cooling have an impact—rate of cooling is affected by the size, shape, and thickness of the casting.
Steel specifications generally refer to the tensile and yield strength. Except for Grades N-1 and N-2, all A27 steel must undergo a tension test and conform to the following mechanical requirements: tensile strength, yield point, elongation, and reduction of area. To understand testing basics, you will need a few definitions:
- Tensile Strength (also referred to as ultimate tensile strength) is the strength of a material measured by the greatest stress it can undergo while being stretched before breaking
- Yield Point is the tension a material can undergo before it starts to deform plastically
- Elongation (%)is the percentage of the initial length that a tensile test piece stretches before breaking or fracturing (length at breaking minus initial length, divided by the initial length)
- Reduction of area (%) is the difference between the original cross-sectional area of a tensile specimen and the smallest area after rupture, expressed as a percentage of the original area
Tensile strength vs. hardness
Tensile strength is not the same as hardness. Hardness testing such as Brinell testing (the oldest form of hardness testing) is covered under ASTM E10 and ISO 6506. Hardness tests are used to roughly or routinely confirm product physical properties without using destructive testing. In a perfect world, all castings will have test coupons poured along with them—as opposed to having to destroy the casting to verify its actual physical properties. That test coupon would be subject to physical testing to determine the tensile properties (tensile strength, yield point, elongation, and so on) of the casting. If the test bar is approved and meets minimum physical property standards, it is assumed that the casting also possesses the same, acceptable properties. Hardness testing provides a cheap, easy, and systematic way of checking for physical properties without having to perform unique tensile tests. Although it is not a perfect test, it can provide a rapid, cost-effective method of quickly confirming physical property assumptions.