Chemical composition
Each heat is analysed before pouring, normally by optical emission spectrometry on a chilled sample. The result is compared with the specification and reported on the inspection certificate. Carbon, sulphur, nitrogen and oxygen are determined by combustion or fusion analysers where required precision exceeds that of the spectrometer.
Positive material identification (PMI) with portable X-ray fluorescence or optical emission instruments is used to confirm the alloy of finished parts and to prevent mix-ups. Handheld XRF does not determine carbon, so it cannot distinguish low-carbon from standard grades.
Mechanical tests
- Tensile test (ISO 6892-1, ASTM E8 or A370): yield or proof strength, tensile strength, elongation and reduction of area.
- Hardness test: Brinell for general castings, Rockwell and Vickers for hardened parts and surface layers. Hardness is also the usual check of heat-treatment response on the parts themselves.
- Impact test (ISO 148-1, ASTM E23): Charpy absorbed energy at a specified temperature, required for low-temperature grades and many duplex and pressure-equipment specifications.
Test pieces
Material standards define where test pieces come from: separately cast test bars poured from the same heat, test bars attached to the casting, or specimens cut from a casting. Separately cast bars characterise the heat and its heat treatment; they do not necessarily represent the properties in every section of a casting, which depend on local solidification conditions. If properties in the casting itself are required, this must be specified, together with the location of the specimens. Test bars must accompany the castings they represent through heat treatment.
Metallography
Polished and etched sections examined under the microscope show microstructure, grain size, phase distribution, carbides, non-metallic inclusions, surface layers such as decarburisation and micro-porosity. Metallography is used for process qualification, for specified checks such as intergranular precipitation, and for failure analysis.
Alloy-specific verification
- Ferrite content in austenitic and duplex stainless castings, measured magnetically or by image analysis.
- Intergranular corrosion tests for austenitic stainless steels, for example the practices of ASTM A262.
- Pitting corrosion and intermetallic phase tests for duplex stainless steels, as required by the purchaser's specification.
- Hardness limits for service in sour environments.
- Case depth and surface hardness for case-hardened steels.
Heat-treatment verification
Heat treatment is verified by furnace records (temperature, time, cooling medium), by the calibration and temperature uniformity of the furnace, and by tests on the load: hardness on parts and mechanical tests on accompanying test bars. Records identify the furnace load so that results can be traced to the castings.
Laboratories
Routine analysis and mechanical testing are performed by the manufacturer's laboratory or by external laboratories. Where a contract requires tests from a laboratory accredited to ISO/IEC 17025, this is arranged for the tests concerned; see quality assurance.