Setting Expectations Through Specifications
Defining and communicating expectations to the casting supplier is critical in receiving delivery of quality parts on time. This is done through the specifying process of sourcing cast components.
With all the existing steel casting and metallurgical specifications, it can be difficult to explain exactly what a casting customer needs and wants. Various standards can help fill in the details of the specification part of a casting order.
The main goal of any specification sheet is to set out the details of how, exactly, a certain product or structure is designed to operate. Most of the time, the information included is very technical and extensive. It is not uncommon for specifications to span multiple pages and feature charts, diagrams, and data sets.
Details that can be specified by the casting buyer include:
• Chemical analysis.
• Mechanical properties (hardness, tensile strength, etc.).
• Dimensions, weight, tolerance, number of parts.
• Property requirements.
• Inspection methods.
• Testing methods.
• Reference various standards.
Many organizations offer technical standards and codes that casting buyers might use in their specifications. Two of the main organizations are ASTM International and ASME.
ASTM International is a global developer of international voluntary consensus standards. Approximately 12,000 ASTM standards are used around the world to improve product quality, enhance safety, facilitate market access and trade, and build consumer confidence. ASTM’s steel standards (as well as those for other alloys) are instrumental in classifying, evaluating, and specifying the materials, chemical, mechanical, and metallurgical properties. They help guide labs and refineries, product manufacturers, and end-users of steel in their proper application to ensure quality towards safe use.
ASME is an international developer of codes and standards associated with the practice of mechanical engineering. ASME develops voluntary standards that enhance public safety, health, and quality of life as well as facilitate innovation, trade, and competitiveness.
Looking Out for Defects
A specification sheet is a tool to ensure parts perform the way they are supposed to in their application and are defect-free. Defects are conditions in a casting that must be corrected or the casting will be rejected. Whether an anomaly is considered a “defect” is subjective to the terms of the casting buyer and vary depending on the ultimate intended use of the part. Specifications provide clearly communicated quantitative ranges to either accept or reject/repair a casting.
"The International Atlas of Casting Defects,” published by AFS, provides a categorization system for identifying defects and their root causes. Once a defect has been correctly identified, then scientific methods can be used to determine corrective action.
Checking for Quality
Foundries use a variety of inspection methods to ensure the quality of their castings, and those inspection methods and parameters—such as frequency and acceptable ranges—can be detailed in the specification.
Inspection methods can be destructive or nondestructive. Destructive tests can test for visual, microstructure, and mechanical properties through the inspection of a sliced section of the casting. But this will obviously destroy the casting in the process of the inspection, and the procedures may also miss the flaw, or the sample may not represent the behavior of the entire lot. Nondestructive tests are usually preferred because the casting being tested remains serviceable. Nondestructive inspection methods include:
• Die penetrant.
• Magnetic particle.
• Pressure/leak testing.
Metalcasting facilities also perform chemical and mechanical tests in process to ensure specifications are met. For example, chemical composition testing is used to identify/confirm the metallurgical properties of the casting. When a part requires a specific chemical requirement, a sample is taken from the furnace and sent for a chemical analysis. Once the properties are determined, a type of formula or recipe can be created to bring the melted metal in the furnace to a particular chemical specification. In steel casting, this is typically done by adding various alloys to the furnace.
Samples are also taken later in the process to verify the steel’s composition. A final test is performed on a sample piece after it is poured. This sample is the basis of the certification the foundry will send to the customer.
Chemical composition can be further affected by minor alloying elements added to the material. Casting alloys are typically specified according to ASTM, SAE, and AMS standards. Depending on how susceptible an alloy is to variation of its chemical compositions, chemical analysis may be required to verify the proper composition is present.
Chemical analysis often involves evaluating a sample by spectrographic atomic absorption or X-ray fluorescence analysis.
Many foundries check the chemical composition of the alloys they are pouring throughout the course of a day and shop personnel will make required adjustments to the alloy composition as needed.
Mechanical testing reveals the elastic and inelastic behavior of a material when force is applied. It shows whether the material is suitable for its intended mechanical applications by measuring elasticity, tensile strength, elongation, hardness, fracture toughness, impact resistance, stress rupture, and fatigue limit.
Typical mechanical tests include hardness, tensile, impact, and service load testing. Customers often drive the type of mechanical test performed based on the goal functions of the casting in its end-use application.
Standards and specifications are important to setting and managing expectations between the casting buyer and foundry—as is communicating effectively. A clear specification sheet may mean the difference between failure and success. If necessary, throughout the relationship the foundry may ask to revisit requirements, specifications, standards, etc., so expectations stay manageable. This open communication will help both partners achieve their goals. CS
Note: This article was based on material from the AFS Institute Steel 101 class. A Live Online Steel 101 class will next be held October 6-7. You can find more information and register for the course at https://bit.ly/3Qa0QXS.